PP33C-1561
The Mid-Pleistocene Climate Transition in the Southern Hemisphere: Evidences From ODP Site 1123
One of the most intriguing aspects of the Pleistocene climate is the development of quasi-periodic (ca.100 kyr), high-amplitude glacial variability during the middle Pleistocene. A number of causes have been suggested for this Mid-Pleistocene Transition (MPT), but there is no consensus yet on this matter despite more than two decades of research. Most hypotheses for the origin of the MPT invoke either a response to a long-term cooling, possibly induced by decreasing atmospheric pCO2, or changes in internal ice sheet dynamics. Marine records from the North Atlantic and tropical-ocean upwelling regions have already provided support to the first hypothesis, documenting decreases of sea surface temperatures during the MPT. In contrast, very little is known about how deep-water temperatures evolved during this climate transition, and how it correlated to the surface water variability. Here we present records of Mg/Ca and stable isotopes on planktonic (Globigerina bulloides, Globorotalia inflata) and benthic (Uvigerina spp.) foraminifera over the time interval 0.35-1 Ma from a marine sediment core recovered in the mid Southern latitudes (ODP Site 1123, Southwest Pacific Ocean). The Mg/Ca-based temperature estimates allow us to test the hypothesis of a global cooling associated with the MPT in the Southern Hemisphere and, paired with the foraminiferal δ18O record, derive the δ18O of the water in which the foraminifer calcified, the latter of which combines a global glacioeustatic signal with local hydrographic effects. These results permit preliminary discussion of the magnitude of the surface- and deep-water temperature changes during glacial/interglacial transitions and the interglacials themselves. The phase relationship between surface- and deep-water signals is tentatively assessed. Finally, the phasing of variations in the marine record will be considered with respect to other component of the climate system, and a comparison with greenhouse gases and atmospheric temperature from EPICA Dome C will be attempted, after evaluation of methods for precise synchronization of these records.
PP33C-1562
A 35 Myr Higher Plant n-Alkane Stable Carbon and Hydrogen Isotope Record from the Gulf of Mexico: Implications to North American C4 Grasslands and Hydrologic Cycle Dynamics
To assess the relationship between Cenozoic North American climate and floral change, particularly C4 grass expansion, we developed δ13C, δD and compound distribution records of higher-plant n-alkanes from Gulf of Mexico sediments (DSDP Site 94). Beginning in the earliest Oligocene, n-alkane δ13C values become increasingly depleted in 13C and remain stable throughout the early Miocene. The Middle Miocene n-alkane record is relatively enriched in 13C compared to the Early Miocene/Oligocene and culminates with the most positive values during the Middle Miocene Climatic Optimum. In the latest Miocene, alkane values become gradually more 13C-enriched into the Pleistocene by ~3°. N-alkane δ13C values reflect factors other than water stress alone, including the isotopic composition of atmospheric CO2, plant community dynamics, and atmospheric pCO2. Accordingly, alkane δ13C values corrected for variations in δ13CCO2 indicate North American C4 inputs during the Early Miocene, a sharp spike in C4 input during the Middle Miocene Climatic Optimum, and a steady increase during the Late Miocene to the Pleistocene consistent with other North American records. Normal alkane δD values are relatively stable between 33-7 Ma. Beginning at 7 Ma, δD values become depletion in D of 25 ‰ and remains stable until the Miocene/Pliocene boundary. Further, Pliocene/Pleistocene δD values become increasingly deplete in D by 30 ‰ from Miocene/Pliocene values. Changes in plant community cannot account for the observed variations in δD alone. Furthermore, average chain length of n-alkanes remains remarkably constant for 33-4 Ma; however, a gradual trend towards longer chain length occurs during the last 4 million years, indicating an increase in aridity. We suggest the depleted δD signal and increased chain length reflects changes in moisture source region and seasonality of moisture delivery associated with the shallowing of the Panamanian Seaway at ~7 Ma, amplification of North Atlantic Deep Water Production, and increased meridional winds. Further, we propose the southerly movement of the Intertropical Convergence Zone near 4 Ma allowed for a near-modern pressure/storm track system to develop, driving increased aridity within the Great Plains. Together, coupled δD and δ13C along with detailed compound distribution data paint a dynamic picture of the North American expansion of C4 grasslands.
PP33C-1563
Upper Water Column Structure During Marine Isotope Stage 11.3 and the Holocene on the Portuguese Margin
Most paleoceanographic studies have focused on reconstructing past surface water conditions by applying global trace metal calibrations of surface dwelling planktonic foraminifera (PF) from tropical - subtropical regions, and little attention has been paid to deep dwelling species and trace metal calibrations that include coastal upwelling samples. We have generated regional core-top PF trace element ratio (Mg/Ca, Ba/Ca, Cd/Ca) and stable isotope (δ18O, δ13C) calibrations for the Portuguese margin upper water column temperature and nutrient content. Three species with different depth habitats were are explored in this seasonal upwelling region: G. bulloides an upper water column species; G. ruber (white), a surface dwelling species; and G. inflata, a deep dwelling species. Mg/Ca and δ18O values, and the derived sea surface temperature (SST), of the three PF species mirror seasonal temperatures in the water column as well as different depth habitats: (1) G. ruber (white) reflects surface conditions (~10 m water depth) in the warm winter-time Portugal Coastal Countercurrent (PCCC); (2) G. inflata indicates the winter mixed layer but is also present in the PCCC; (3) G. bulloides records the summer upwelling species lives in the upper 100 m water depth, but appears to migrate during the upwelling into waters closer to the surface (~10 m). The Ba/Ca, Cd/Ca and δ13C imply a complex relationship between nutrient tracers and the nutrient content of water masses on the Portuguese margin. Taken at face value, each of the nutrient proxies and the three foraminifera species seem to record different nutrient information. G. bulloides data, for example, suggest higher nutrients related with the upwelling waters whereas G. inflata imply higher nutrients at the base of the thermocline during winter. The insights gained from this study are being applied to reconstruct the upper water column structure on this seasonal upwelling margin during Marine Isotope Stage 11.3 and the Holocene, the most similar interglacial periods in terms of orbital parameters and greenhouse gas concentrations, with the MD03-2699 core (recovered off Lisbon: 39° N; 10.7° W). Additionally, these shell geochemistry data are compared with alkenone abundance and derived SST data.
PP33C-1564
Reconstructing Radiocarbon Reservoir Ages North of Iceland During the Last Millennium: Implications for Ocean Circulation and Climate Change
In this study we provide radiocarbon reservoir ages for the north Icelandic shelf waters for intervals during the last millennium based on annually banded molluscs. The master shell chronology is based on long-lived bivalves (Arctica islandica L.), which were live-caught in 2006 in 80 m water depth. We have successfully cross-matched fossil A. islandica shells with the modern chronology establishing a continuous shell chronology for most of the last millennium. Based on these data, we calculate Delta R values (deviation from the modelled global mean surface ocean reservoir age of about 40514C years) to infer the relative position of the Polar Front. The oceanic Polar Front is now situated in the Denmark Strait between Greenland and Iceland where the relatively warm and saline waters from the North Atlantic flow clockwise around Iceland as the Irminger Current and mix with the relatively cold and low saline waters of the East Icelandic Current. The position of the Polar Front is intimately linked to North Atlantic Deep Water convection sites and the North Atlantic meridional overturning circulation, and these strong oceanic and atmospheric gradients make this region particularly sensitive to climate change. Our initial results compare well with nearby sediment- based Delta R records, and the combined data set suggests that the Polar Front location was highly dynamic from AD 1000 to 1300, and during the most recent period (AD 1800 to 1950). From AD 1300 to 1800, during the Little Ice Age, the Delta R values were stable and showed little variability, suggesting that the Polar Front moved southward of the north Icelandic shelf during this interval. The implications of these findings are discussed with respect to the interpreted palaeoceanographic evolution of the north Icelandic shelf during the last millennium.
PP33C-1565
The Younger Dryas from VM27-232 in the Eastern Equatorial Atlantic
Mg/Ca values of planktonic foraminifer Globigerinoides ruber (pink, 20 individuals from the 355-425micron size fraction) from deep-sea sediment core VM27-232 (from the Gulf of Guinea at latitude 0.23degS, longitude 7.55degE from a water depth of 2615m off the coast of Gabon) indicate a decrease in upper ocean temperatures during the Younger Dryas period between 12.9 and 11.6ka. This result is slightly different from those of a nearby location recently published by Weldeab et al. (2005, 2007). A decrease in Gulf of Guinea sea surface temperatures is consistent with other paleoclimate records from other regions of the Atlantic basin which suggest the mechanism of the Younger Dryas event involved changes in deep-ocean circulation, intensification of the trade winds, and increases in equatorial Atlantic upwelling.
PP33C-1566
Tracing the Influence of Mediterranean Outflow Waters on the Mid-depth Portuguese Margin Between Marine Isotope Stages 9 and 13
Calypso piston core MD03-2699 was retrieved from the Estremadura promontory north of Lisbon from a water depth of 1895 m. Nowadays, this site is bathed by Northeast Atlantic Deep Water (NEADW), whose physical properties are modified by diffusive mixing with the overlying Mediterranean Outflow Water (MOW; 700-1400 m). During the last glacial maximum the MOW became denser and settled deeper in the water column and its lower core's flow strength increased on millennial time scales during the Greenland stadials of the last 50 ka. In order to reconstruct deep-water variations on the mid-depth Portuguese margin during the mid-Brunhes we generated benthic stable isotope and trace element records and measured the mean grain size <63µm for the interval from 300 to 510 ka. Because of the strong MOW derived salinity overprint on the benthic Mg/ Ca data we currently use the western Mediterranean equation (Cacho et al., 2006) to calculate bottom water temperatures (BWT). During the MIS 10 glacial inception, BWT and grain size records reveal millennial-scale oscillation in deep-water conditions with warmer MOW waters (8-10°C) bathing the site during stadials and NEADW (5-7°C) during interstadials. The lower MOW core was the dominant water mass throughout glacial MIS 10 and 12 and NEADW during interglacial MIS 9.5 and 11.31. During MIS 13.1, on the other hand, strong MOW influence on the BWT is observed nearly throughout with NEADW-level BWT occurring only between 493 and 497 ka. During termination IV the MOW/ NEADW boundary shifted upwards right at the onset of the termination, but during termination V the lower MOW core settled further up in the water column only after 408 ka. The Cd/ Ca data indicates that the glacial and stadial MOW was enriched in nutrients either by exporting nutrients from the Mediterranean Sea or by mixing with southern source waters. Overall, our records reveal that deep-water dynamics on the mid-depth Portuguese margin were very variable during the mid-Brunhes, experienced millennial-scale oscillations similar to the last glacial cycle and are driven by the density and thus settling depth of the MOW.
PP33C-1567
Meridional Overturning Circulation Climate Feedbacks During the Marine Isotope Stage 11 Interglacial
The Atlantic Meridional Overturning Circulation (MOC) is thought to have played a key role in the climatic evolution of the Marine Isotope Stage 11 interglacial, but variations in the return limb of surface waters flowing northwards into the Atlantic Ocean is poorly understood. New data from Southeast Atlantic site ODP- 1085 shows that periods of intensified northwards surface water flux after the end of the interglacial optimum coincided with threshold changes in the configuration of northern and southern deepwater masses. It is hypothesised that these short periods of intensified MOC fuelled steps in ice-sheet growth, which in turn altered the state of the Atlantic deep ocean circulation through sea-ice, sea-surface temperature and salinity feedbacks. These changes also forced a progressively shallower chemocline between high CO2 southern source deepwaters and low CO2 northern source deepwaters, amplifying the climatic deterioration through changes in atmospheric CO2. A prominent surface salinity maximum during the termination 5 deglaciation also supports models that emphasise the role of increased Agulhas Leakage in forcing the MOC back into an interglacial mode. Understanding the exact sequencing of these events requires better age-control on the detailed stratigraphic sequences currently available from the global oceans.
PP33C-1568
Reconstructing the oceanic circulation and sediment source-to-sink patterns in the East China Sea during the past 30ka
In the present study we try to reconstruct the oceanic circulation and sediment source-to-sink patterns during the past 30 ka in the East China Sea and adjoining delta area, based on organic, elemental and Sr-Nd isotopic geochemical compositions in the sediments from Core DGKS9604 from the middle Okinawa Trough and Core CM from the Changjiang Delta. The data show that the Changjiang-derived sediments played an important role in the formation of sedimentary strata in the marginal sea, albeit with variable influences during the Marine Isotope Stage 3, Last Glacial Maximum and early Holocene. The Taiwan Island-derived terrestrial sediments transported by the Kuroshio Current might dominate the middle Okinawa Trough during the mid-late Holocene. Organic matters accumulated in the Changjiang Delta are primarily derived from terrestrial C3 plant, whereas marine organic matter dominated in the middle Okinawa Trough. Geochemical compositions also revealed the rapid climate changes over the past 30ka, which is comparable with other sedimentary and palaeoceanographic records elsewhere. In particular, the combined influences of natural climatic variability, sea-level changes and anthropogenic activities on the production, transport, deposition and preservation of terrestrial sediment and organic matter in the Changjiang Delta and adjoining East China Sea were recognized. The changes of the Kuroshio Current mainstream over the past 30 ka greatly regulated the dispersal patterns of terrestrial sediments sourced from the surrounding landmasses, producing unique sedimentary architecture in the delta area and marginal sea. It should be cautious to make a comprehensive comparison of paleoenvironmental records in the delta and East China Sea due to the intense land-sea interaction and widespread discontinuity in the late Quaternary sedimentary strata of the Changjiang Delta. The present study shed new light on the paleoenvironmental reconstruction of East Asian marginal seas, especially in relation to the land-sea interactions during the late Quaternary.
PP33C-1569
Extending the SST gradient into the Arctic: Paleoclimate evidence and future climate implications
The most recent geologic interval characterized by surface temperatures similar to those projected for the end of this century occurred about 3.3 to 3.0 Ma, during the mid-Piacenzian Age of the Pliocene Epoch. Climate reconstructions of this warm period are integral to understanding both past warm climate equilibria and to predicting responses to today's transient climate. The Arctic Ocean is of particular interest because in this region climate proxies are rare, and climate models struggle to predict climate sensitivity and the response of sea ice. In order to provide the first quantitative climate data from this region during this interval, we estimated sea surface temperatures (SST) from Ocean Drilling Program Sites 907 and 909 in the Nordic Seas and from Site 911 in the Arctic Ocean based on Mg/Ca of Neogloboquadrina pachyderma (sinistrally coiled) and alkenone unsaturation indices. SST estimates indicate much warmer than modern conditions in the Arctic Ocean with temperatures as high as 18°C. SST anomalies (mid-Piacenzian minus modern) increase with latitude across the North Atlantic and into the Arctic, extending and confirming the reduced mid- Piacenzian pole-to-equator temperature gradient. The agreement between proxies and with previously documented qualitative assessments of intense warming in this region corroborate a poleward transport of heat and at least seasonally ice-free Arctic, conditions that may serve as a possible analog to future climate if the current rate of Arctic sea-ice reduction continues.
PP33C-1570
Mg/Ca in the Benthic Foraminifera Elphidium Williamsoni: Evaluating pH and Calcification Rate Effects
We cultured the benthic foraminifera, Elphidium williamsoni, in a physicochemically stable, large volume (400 l) recirculating seawater system over a range of pH (7.6, 8.1 and 8.3) for a period of 8 weeks. We used calcein staining to identify the foraminifera chambers deposited in the experimental system. The foraminifera typically deposited 1-3 new chambers during the culturing period which were analysed by electron probe. Mg/Ca varied significantly within single chambers. Mean Mg/Ca varied significantly (by as much as x 2) between individuals of similar size and calcification rate (numbers of chambers deposited over the experimental period), cultured under the same conditions. E. williamsoni is an epibenthic species and individuals were visible on the silica substrate surface during culturing. Mg/Ca variations between specimens do not reflect changes in ambient water chemistry but must reflect biological variability between individual foraminifera e.g. dependent on genotype or phenotype. Variations in test chemistry may reflect variability in the composition of the calcifying fluid used for test construction or variations in the relative proportions of high and low Mg calcites incorporated in each test. Within each of the pH treatments we observed no relationship between calcification rate and test Mg/Ca, within each culture pH. Foraminifera cultured at low pH produced less dense chamber walls but we observed no relationship between culture pH and test Mg/Ca. Our study indicates that any effect of pH or calcification rate on test Mg/Ca is effectively swamped by chemistry differences between individual specimens. Foraminifera test Mg/Ca is a commonly used paleothermometer. Our study shows that variations in the relative proportions of individual foraminifera, of different geochemistry, that make up a sample for chemical analysis may affect bulk determinations of test chemistry and subsequent estimates of past climate.
PP33C-1571
Mg/Ca Paleothermometry In High Salinity Environments
Planktonic foraminiferal Mg/Ca ratios are a fundamental temperature proxy in past climate reconstructions. However, in the highly evaporative seas of the tropics and subtropics, anomalously high planktonic foraminiferal Mg/Ca ratios arise, possibly linked to high salinities. Here, we present new Mg/Ca data from the highly saline (>40 psu) Red Sea, using a combination of analytical techniques, including conventional analysis as well as time resolved flow through leaching and electron microprobe analysis. The latter two techniques separate pure foraminiferal calcite from other phases. The results show that high Mg/Ca ratios (7 to 13 mmol/mol), found by conventional analysis of planktonic foraminifera from a Red Sea sediment core, may not be caused by increased Mg uptake into foraminiferal calcite in a high salinity setting, but instead result from secondary high Mg-calcite overgrowths. The overgrowths likely formed near the sediment- seawater interface, from CaCO3 supersatured interstitial seawater.
PP33C-1572
An evaluation of temperature control on Mg/Ca, Sr/Ca and B/Ca in surface-dwelling species, Globigerinoides ruber: Comparison of sediment trap and core top systematics from the South China Sea transect
Trace element ratios in planktonic foraminifera are robust proxies for reconstruction of physical and chemical changes in the past ocean. Reliable temperature and carbonate chemistry are essential for studying heat distribution and carbon cycle globally, particularly of seawater carbonate ion and atmospheric CO2 variability. This study examines systematically relationships between trace element ratios and seawater temperature for a surface-dwelling foraminiferal species, Globigerinoides ruber (white), collected from sediment traps and core top specimens in the South China Sea (SCS). Mg/Ca ratios on G. ruber (white) agree well with published data, following an equation of Mg/Ca= 0.32 (±0.04) exp (0.090×T), R2= 0.81. Partial dissolution artifacts, however, play a significant role to modify foraminiferal Mg/Ca. For the Sr/Ca ratio, it has been shown to covary consistently with δ18O-calcification temperature and Mg/Ca-based temperature, and can be described by a linear relationship: Sr/Ca= (0.020 (±0.001)×T) +0.915 (±0.010), R2= 0.78. Previous studies have demonstrated that foraminiferal Sr/Ca ratios in the surface dwellers have no temperature dependence. However, a rather large Sr/Ca in the core-top tests can be attributed to stronger dissolution effect compared with sediment trap specimens in the SCS. B/Ca ratio in G. ruber is strongly influenced by ambient temperature, B/Ca= 4.65 (±0.02) exp (0.120 (±0.018)×T), R2= 0.86, and show no conspicuous effect of partial dissolution. There is a significant difference in the relationship of shell B/Ca and seawater temperature at different ocean basins. This emphasizes the complexity of using B/Ca for oceanic pH proxy due to biological and/or environmental variations. Thus this study highlights the need for in situ empirical calibration for different hydrographic conditions before applications of down-core reconstruction of carbonate chemistry can be realized.
PP33C-1573
Interpretation of the seasonal variations in nitrogen isotopic signals of settling particles in the western subarctic Pacific with an ecosystem model including nitrogen isotopes
Settling particles were collected by time-series sediment trap moored at station KNOT (44N 155E, water depth 770m) during periods from October 25th 1999 to June 20th 2001, and from June 25th 2002 to May 11th 2006. Particulate nitrogen contents and its isotope ratios (d15N) in the collected samples were measured. The results of d15N showed the general variations that are lower during the spring bloom periods and higher during winter periods. In order to interpret the processes controlling such variations quantitatively, we developed an ecosystem model including nitrogen isotopes on the basis of the recent ecosystem model of Fujii et al. (Deep Sea Research ¢º, vol. 49, pp. 5441-5461, 2002), which successfully simulated the observed seasonal cycles of ecosystem dynamics at station KNOT. In our model, we took it into consideration that the isotope fractionations by nitrate and ammonium assimilation by phytoplankton, excretion and egestion by zooplankton, nitrification, remineralization of particulate and dissolved organic nitrogen (PON, DON) and decomposition of PON, which influence the d15N values of settling particles. This model was validated using an actual data set and successfully reproduced the seasonal variations in d15N of settling particles. Simulated lower d15N values in spring bloom period were mainly caused by phytoplankton assimilation of nitrate with the lowest d15N in a year. Simulated higher d15N values in winter were mainly determined by the two processes: 1) enhanced d15N values of ammonium by nitrification and following assimilation of ammonium by phytoplankton, and 2) increase of relative proportion of zooplankton, with higher d15N values compared to phytoplankton, in settling particles. Of the two processes, the former had much larger impacts on d15N values of settling particles than the latter did. In our model, we investigated the sensitivity of isotopic fractionation effect of each process to the seasonal variations in d15N values of settling particles. As a result, we found that the ƒÔ15N values of settling particles can be influenced considerably by the difference between isotopic fractionation effects of ammonium assimilation by phytoplankton and nitrification, in addition to isotopic fractionation effect associated with nitrate assimilation by phytoplankton. If the difference were found to be variable over spatial and/or temporal scales, then the variations in the difference might play an important role in the paleoceanographic record of d15N data.
PP33C-1574
The Fate of Barite in Sediments Under a Coastal Upwelling System.
Marine barite has been associated with decaying organic matter that carries particulate Ba to the seafloor and consequently it has been directly related to marine organic carbon flux. Ba normalized to Al otherwise known as bariumexcess is often used to demonstrate past changes in oceanic productivity. However, it is well known that barite is extremely susceptible to dissolution in suboxic conditions. The solubility of barite in deep ocean waters remains low, until depletion in SO42- begins to occur. At which point dissolution of barite occurs leading to increased dissolved barium in the sediment porewaters. ODP Hole 1017E (34°32'N, 121°6'W) on the Southern California Margin provides an ideal location to examine the relative contribution of productivity and redox history to the production and preservation of barite in sediments. This site lies beneath an upwelling cell and has yielded high-resolution records of paleoproductivity. The location of the site within the lower oxygen minimum zone (OMZ) along with high organic carbon rain results in changes in pore water oxygen concentration. Presently the OMZ is a significant feature along the California Margin. However, during the last glacial, large changes in porewater oxygen have been documented suggesting the waxing and waning of the OMZ in close concert with rapid climate change in the North Atlantic. Multiproxy results from ODP Hole 1017E have allowed researchers to determine both the history of past productivity changes as well as understand the redox chemistry of the sediments. Here the record of bariumexcess is presented demonstrating a history of marine barite delivery to the sediments as well as changes in the shallow sulphate reduction depth. Significant barite enrichment occurs as a diagenetic front at the modern sulphate reduction boundary. This boundary also intersects a sediment facies containing fine grained-organic rich sediments deposited during Interstadial Event 7 (35 Ka) of the last glacial. Changes in diffusion rates associated with sediment facies change produce a strong but misleading correlation between barite enrichment and rapid climate change. Thus within the bariumexcess record at ODP Hole 1017E is a history of the shallow (upper tens of cm) redox conditions and an indication of modern deep redox chemistry.
PP33C-1575
Eastern Equatorial Pacific Dust Provenance on Deglacial Timescales
Changing patterns of eolian dust deposition preserved in deep-sea sediments have the potential to provide us with a better understanding of changes in past atmospheric circulation. One way in which to determine the provenance of dust in deep-sea sediments is to use radiogenic isotopic tracers which can fingerprint potential dust sources. Models (e.g., [1]) suggest that sources of dust to the Eastern Equatorial Pacific (EEP) are from areas as diverse as Asia, North, Central, and South America, and, perhaps, even Africa. Here, we investigate spatial and temporal changes in the provenance of the eolian component in the EEP by measuring Pb, Sr, and Nd isotope ratios in dust extracted from sediments along a transect at 110oW from 7oN to 3oS (ODP sites 853 - 848). In this region, although fluxes of dust were higher during the last glacial maximum (LGM) than those in the Holocene by up to 100%, the glacial flux of dust displayed a shallower meridional gradient [2]. However, it is unclear whether this shallower gradient is due to a mean southerly displacement of the Intertropical Convergence Zone (ITCZ). Most of the dust trying to pass through the ITCZ will be scavenged and rained out at the ITCZ. Along the meridional gradient, therefore, temporal variations in the Pb, Sr, and Nd isotopic fingerprints of the distinct dust sources will determine the extent to which the position of the ITCZ changes on deglacial timescales. [1] Mahowald et al., 2005, Global Biogeochemical Cycles 19, GB4025. [2] McGee et al., 2007, EPSL 257, 215-230.
PP33C-1576
Paleoproductivity Variations in the Eastern Equatorial Pacific Over Glacial Timescales
Filippelli at al. (2007, DSR II) proposed that changes in oceanic phosphorus (P) mass balances caused by glacial sea level changes may be a potential driver for productivity variations. To test this hypothesis, a broader suite of sampling locations including additional oceanic paleoproductivity proxy data collection is necessary. The eastern equatorial Pacific (EEP) is an ideal site for productivity studies due to its high levels of nutrients and importance in terms of burial production. This research examines nutrient proxies (Ba/Ti and P/Ti ratios, isotopic indicators) of four EEP sites while also comparing the sites' glacial and interglacial productivity variations to the geochemistry and productivity results of an independent central equatorial Pacific site. Phosphorus and other elemental data were collected from Sites 845, 848, 849, and 853 (ODP Leg 138). Our initial analysis of Ba/Ti and P/Ti ratios (an "excess" P proxy) revealed distinct productivity variations during glacial and interglacial periods. The observed variations agree with a pattern of increased productivity during glacial periods and lower productivity during interglacial periods, with a lag observed perhaps reflecting the rearrangements in oceanic nutrient mass balances posited by Filippelli et al. Central equatorial Pacific cores RR0603-03TC and RR0603-03JC (IODP site survey cruise for Proposal 626) have been used as a reference for geochemical concentration parameters, as well as a comparison tool for productivity variations among the central and eastern sites. The central equatorial geochemistry results provided support for sea level changes driving paleoproductivity variations. The similar variation patterns displayed by the EEP's geochemical data in this research could provide additional support for this hypothesis.
PP33C-1577
Biological Productivity in the Eastern Equatorial Pacific Through the Pliocene-Pleistocene Transition
The Pliocene-Pleistocene transition is significant in paleoclimate studies because it represents a major change from a warm global climate to a cooling climate. The Early to mid Pliocene warm period was followed by a transition to a cooling climate, with the eventual onset of the Northern Hemisphere Glaciation and latest ice age during the Pleistocene. Ocean sediment records from this period provide accessible data from a major climate transition, leading to reconstructions of past climate conditions via biogeochemical proxies. The Eastern Equatorial Pacific is of particular interest for biogeochemical studies because it is a region with strong nutrient upwelling patterns and high biological activity. Previous studies indicate the presence of an El-Nino like SST pattern across the Equatorial Pacific during this time, as well as a shoaling of the thermocline, accompanied by concomitant changes in ocean circulation and heat transport patterns. Such changes in physical oceanic and atmospheric properties are inextricably linked to chemical and biological changes. In this study, we investigate paleoproductivity in the Eastern Equatorial Pacific using a variety of biogeochemical proxies. The samples analyzed are from ODP Leg 138, Site 853 (7 12.655'N, 109 45.078'W). P/Ti, Ba/Ti, and nitrogen isotope data will be used to explore nutrient availability, biological productivity levels, and nutrient utilization/partitioning. The goal of this work is to compare several nutrient proxies at one site, and in light of existing micropaleontology evidence, attempt to gain new insights into productivity variations in this region. The intent is to combine this biogeochemical information with established knowledge of sea surface temperatures and other physical ocean parameters, in order to explore how nutrient cycling patterns respond to climate change over the Pliocene-Pleistocene time scale.
PP33C-1578
Evaluating the Controls on Lipid D/H Fractionation in Globally Distributed Saline Systems
Several recent studies indicate that the hydrogen isotopic composition of lipids from aquatic microorganisms is influenced by salinity and therefore contains information about the moisture regime of the growth environment. Specifically, data from culture studies and isolated environmental settings have shown that hydrogen isotope fractionation in algal lipids decreases with increasing salinity. Understanding the precise nature of the salinity-driven isotope response requires an evaluation of the hydrogen isotopic composition of such compounds from a variety of organisms across a range of modern environmental settings. To this end we present hydrogen isotope data from bulk lipid extracts and individual lipids of modern lake and lagoon sediment, suspended particles, aquatic vegetation, algae, and microbial mat deposits. Our sample set of more than 60 locations and sample types represents lakes and lagoons of varying ionic composition, with salinities ranging from 0 to >300 ppt. Geographic extent of the sample set spans a wide range of marine and continental settings spanning an array of altitudes, and latitudes. Results suggest that although salinity may be correlated with hydrogen isotope composition, other factors such as specific conductance or some other as yet undiscovered physical property that varies with salinity may be the primary control. Our data illustrate the importance of isolating specific compounds for isotope analysis, in spite of the fact that the isotopic composition of total lipid extracts demonstrates a salinity effect in systems with relatively few input sources. These findings also suggest that the salinity-isotope response may be non-linear over the range of samples studied, but such conclusions require further analyses to confirm. Ultimately, however, the most promising result is the suggestion that the salinity related isotope effect recorded in lipids may be globally applicable as a new tool for reconstructing past precipitation regimes.
PP33C-1579
Integrating Magnetic and Modelling Approaches to Reconstruct Ocean Circulation at the LGM
Formation of N. Atlantic Deep Water (NADW) is an important component of the ocean thermohaline circulation but debate exists over the ocean circulation state during glacial stages. Some geological and modelling studies suggest decreased NADW and increased formation of Southern Ocean deep water during the Last Glacial Maximum (LGM); others indicate similar, or higher, rates of NADW advection. Here, we use an innovative, integrated approach combining ocean and general circulation model results with ground- truthing measurements of sediment magnetic properties, in order to test two different potential LGM ocean states. We compare modelled iceberg trajectories produced by each of the stable modeled ocean states with magnetically-mapped patterns and sources of LGM ice-rafted debris (IRD). One LGM state is characterised by vigorous NADW formation; the other by dominant deep water production in the Southern Ocean. Cluster analysis of sediment magnetic properties was used to characterise N. Atlantic IRD patterns and sources, which match most closely iceberg trajectories arising from some combination of the 'southern-sinking' and 'intermediate' ocean circulation states. The magnetic data indicate two major IRD sources, Fennoscandia and Greenland/Iceland, and one minor source, the St Lawrence region. The model and magnetic data suggest that the LGM N. Atlantic circulation was dominated by a cyclonic central N. Atlantic gyre, separated from the N. Atlantic Current which was displaced south of ~ 42 oN. This approach can provide discriminatory evidence and validation of past ocean circulation states, on an ocean-wide and non-sample destructive basis.
PP33C-1580
A Nd Isotopic Study Along a Depth Transect at Ceara Rise
We present Nd isotopic records constructed from fossil fish teeth and debris along a depth transect at the Ceara Rise in the tropical South Atlantic for the early Miocene to Pliocene. This location is ideally situated to monitor the interplay between northern and southern sourced deep waters as a result of changing rates of deep water production. The three ODP sites used for this study are Sites 925 (3,040 m), 926 (3,600 m) and 929 (4,360 m). From 18 to 8 Ma the two shallowest sites record values of ~-15 to -16, while the deepest site records values of ~-13. These values are less radiogenic than typical Northern Component Water (NCW) or Southern Component Water (SCW) throughout the Cenozoic. At 8 Ma, εNd values at all three sites increase rapidly to ~-11.5, a value similar to contemporaneous NCW. Values then decrease gradually to ~-13 from 8 to 4 Ma, when all three sites record a smaller increase to ~-11, where they remain until 2 Ma. The dramatic positive shifts at 8 and 4.5 Ma coincide with documented uplift events of the Andes Mountains and associated changes in clay mineralogy at Ceara Rise (Harris and Mix, 2002) that reflect a shift in terrigenous sources from the Amazonian lowlands (South American shield material) to the Andean highlands (volcanic arc material). A positive shift in the εNd value of weathering products transported by the Amazon river would be expected with this change. Two lines of evidence argue that our εNd values from teeth and debris do not reflect a purely diagenetic signal of this source change: 1) REE patterns preserved in fossil fish teeth and debris at all three sites are typical of seawater rather than continental material, and 2) the deepest site (929) records a much smaller εNd change. However, εNd values of -14 to –17 suggest that the water column was influenced to some extent by the continental signal, possibly through reverse scavenging, which would be consistent with the larger shift documented at the shallower sites. Although part of the shift at 8 Ma may reflect changes in the source material available for reverse scavenging, it appears that there is also a component of circulation within this signal because values recorded at ROM fracture zone, Walvis Ridge, and Ceara Rise all converge on the North Atlantic Deep Water (NADW) value determined from ferromanganese crusts in the North Atlantic. The timing of this convergence also coincides with increased production of NADW calculated by Poore et al. (2006). The combination of enhanced NADW production and convergence of εNd values throughout the Atlantic indicates that the εNd record at the Ceara Rise is a mix of continental inputs and circulation and that the deep Atlantic was well mixed at this time. There is also no evidence that SCW ever extended as far north as the Ceara Rise at these depth over the past 18 My. Reference: Harris, S. E. and Mix, A. C., (2002) Geology, vol. 30, p. 447-450. Poore, H.R., Samworth, R., White, N.J., Jones, S.M., and McCave, I.N., 2006, Geochem., Geophys., Geosys., v. 7, DOI:10.1029/2005GC001085
PP33C-1581
South Atlantic Nd isotope distribution and water mass geometry during the Holocene and LGM
The climate system and the meridional overturning circulation (MOC) are closely linked. The detailed nature
of this coupling, however, is still elusive. Paleo-records of ocean circulation and climate can yield information
on past changes and provide powerful constraints on ocean-climate interactions. Documenting past MOC
changes is therefore essential for a detailed understanding of the ocean-climate system.
Neodymium
isotope ratios behave quasi-conservatively in the ocean that is, they approximately reflect the mixture of
different water masses without being affected by changes nutrient cycling or carbonate chemistry. Their
application to paleoceanography therefore facilitates direct tracing of water masses in the past ocean and
has the potential to give insight into changes in volume transport of the MOC. In order to improve the basis
for the use of Nd isotopes as paleo-water mass tracer and for robust quantifications of past circulation
changes, we have analyzed Nd isotopes on the authigenic ferromanganese oxide fraction of core-top
sediments from the Atlantic Ocean with a focus on the Southern Hemisphere part of the basin. We collected
samples from intermediate to bottom water depths and from 43°S to 30°N in the Atlantic,
significantly adding to existing data from the South Atlantic from Piotrowski et al. (EPSL, 2008). Eighty
percent of our new authigenic Nd isotope data match the seawater isotopic distribution well. This confirms the
value of Nd isotopes as a water mass tracer and strongly supports its application and merit for the
reconstruction of past ocean circulation changes. Nd isotope ratios diverge from the seawater isotopic
composition at some margin sites, where the data suggest overprints from local terrigenous Nd sources, and
at some sites in the Southern Ocean, where the seawater integrity of Nd isotopes is compromised by Nd from
volcanic material. Nd isotope values of last glacial maximum (LGM) Atlantic sediments show a northward
invasion of Southern Ocean-like Nd isotope signatures at depth, consistent with the increased northward
expansion of Antarctic Bottom Water and the reduction of southward export of North Atlantic Deep Water at
this time.
PP33C-1582
Southern Ocean Evidence for Reduced Export of North Atlantic Water During Heinrich Event 1
Deep-sea corals form unique, high-resolution archives of ocean circulation that can be dated using the decay of uranium to thorium. They are abundant in the Southern Ocean, and can provide unprecedented insights into ocean circulation and ocean chemistry on sub-millennial time-scales in areas where traditional paleoceanographic proxies are fraught with difficulties. Here we present the first coupled neodymium (Nd) isotope and radiocarbon data from deep-sea corals in the Drake Passage (Southern Ocean) adding new constraints on ocean circulation during the last Heinrich event (H-1; 16,700 years ago). The modern day Drake Passage water column is homogeneous with respect to Nd isotopes (expressed in epsilon units; εNd). The seawater value of close to -9.0 is largely controlled by the mixture of North Atlantic Deep Water and Pacific waters. The aragonite of modern Drake Passage corals reflects this water-column value. In contrast, a fossil coral from H-1 is significantly higher at -6.4 ±0.4. We interpret this ~2.5 epsilon unit shift as a reduction in the influence of North Atlantic-sourced Nd in the Southern Ocean during H1. This interpretation is supported by a series of radiocarbon analyses on the same sample, and is consistent with a two-fold or greater slow down in export of North Atlantic waters from the Atlantic Basin. This shift has important implications for the evaluation of lower latitude paleo-εNd reconstructions that have been used to assess the mixing ratio of northern to southern waters in the past.
PP33C-1583
North Pacific Intermediate Water Circulation During Marine Isotope Stage 3: Southern California Margin
Pleistocene-age sediments on the California margin preserve a detailed history of fluctuations in the intensity of the oxygen minimum zone (OMZ). Of particular interest is the pattern of millennial-scale (stadial- interstadial) changes in seafloor oxygen levels that characterize Marine Isotope Stage 3. Ocean Drilling Program Sites 893 and 1017 (595m and 955m) indicate a weakened OMZ during stadials (greater seafloor oxygenation) and a strong OMZ during interstadials (dysoxic seafloor conditions). Recent studies identified two hypotheses to explain the changes in seafloor oxygenation. The first invokes local increases in primary productivity. The alternative hypothesis suggests that changes in seafloor oxygenation resulted from switching the source of the water at intermediate depths. The inferred source of intermediate waters during interstadials is an older, oxygen-poor water mass from the southern high latitudes. During stadials the source switched to a young, oxygenated water mass derived from the north Pacific. However, traditional techniques are unable to distinguish between these two mechanisms (productivity vs. ocean circulation). To address the circulation hypothesis we generated a neodymium isotope record (143Nd/144Nd expressed as ε Nd) to reconstruct the composition of waters that reached the California margin. In the Pacific, waters from the north bear ε Nd values of ~-2, whereas waters from the south have values ~-7 to -9. We focus on the interval from 37-52ka, spanning interstadials 8-14. Samples from Sites 893 and 1017 were taken ~500 and ~750 years apart, respectively, to capture all interstadials and stadials. The ε Nd of the water mass bathing Sites 893 and 1017 did not change on a stadial-interstadial basis. Both sites also have values representative of invariant, southern sourced water masses (ε Nd = -9 and -7, respectively). This finding suggests that ocean circulation did not change on a millennial- scales supporting the hypothesis that changes in primary productivity exert major control over seafloor oxygenation on the California margin. The different ε Nd values at 1017 and 893 suggest that the two sites may be bathed in different water masses.
PP33C-1584
A comparison of Nd isotopes in seawater and authigenic sediments from the South African Margin
The neodymium isotopic composition of marine archives is an exciting paleocirculation tracer with the potential to provide information on changes in ocean circulation during periods of drastically different climate. An outstanding question that this tracer may help to answer is how the vertical structure of the ocean has changed through time. With this goal in mind we analyzed seawater and leachates of authigenic core-tops sediments from the South African margin, essentially creating depth profiles for seawater and modern sediments for this margin, which can be directly compared. We use this to evaluate the suitability of using leachates of authigenic sediments to recreate the water mass distribution along this margin, where NADW is exported out of the Atlantic system, through time. We report Nd isotopes in two seawater depth profiles as well as in authigenic leachates of multiple sediment core-tops collected from depths ranging between 1010m and 3706m. The core locations span nearly the entire South African margin, from near Durbin to near Cape Town, and the locations of the seawater profiles fall within this span. The seawater profiles show low εNd values (εNd ~ - 13.5) in the near surface Agulhas Current waters, then trend toward higher values (εNd ~ -9) at intermediate depths where waters represent mainly AAIW. The εNd values in deep waters show a minimum (εNd ~ -12) at depths of the core of NADW (depth ~2500m) then trend toward higher values, with a maximum of -10.5 in the deepest sample (~3600m), which is a NADW-AABW mixture. Some simple calculations show that these profiles are consistent with mixtures of the major end-member water masses present at this location. The Nd isotope ratios of the core-top leachates range in εNd from -14 to -9. Some core-top leachates do not match seawater measurements from the same depth, with a maximum deviation of nearly 4 ε-units at a depth of 1000m near the Tugela River cone. However, when sediment cores located near major rivers and submarine canyons are excluded, the trend from intermediate to deep waters (high to low εNd values) and from deep to bottom waters (low to high eNd) are both reproduced. This suggests that the sediments from the South African margin record the Nd isotope composition of the ambient waters and this provides a pathway to recreate the water mass distribution of waters along the South African margin in the past.
PP33C-1585
Lead and Neodynium isotopic stratigraphy constrains paleocurrents and Himalyan erosion Rate for the past 40 My in the Northern Indian Ocean
During the last 40 My, an important reorganization of the Indian Ocean circulation occurred in response to tectonic plates reorganization. The Australian block moved northward closing the Indonesian Passage and opening the Tasman Strait in the south. The Thethys disappeared closing the connection of the Indian Ocean and the Atlantic at low latitude. The collision of the Indian and Asian blocks yielded the rise of the Himalayan Chain and the Tibetan Plateau. The Indian Ocean circulation and chemistry were both largely affected by the modification of the connection with the other oceans and also by the changes of the Nd sources and their erosion intensities. Nd isotope stratigraphy performed on carbonated sediments from the ODP Sites 707, 757 and 758 in the Equatorial Indian Ocean indicates that a major reorganization occurred in the Indian Ocean at ≈14 Ma with the initiation of the MIOJet (Miocene Indian Ocean Equatorial Jet), a strong westerly oceanic current that durably linked the eastern and the western Indian Ocean. (Goulan et al., 2008). We also analyzed the lead isotopic composition of the carbonated sediment of the three ODP Sites using a specific leaching technique. This Pb isotope stratigraphy shows a more complex pattern induced by the fact that lead has a much shorter residence time than Nd and also because hydrothermal activity at ocean ridges contributes to the Pb budget in the ocean and not to the Nd budget. However despite this difficulty, lead has the advantage of providing 3 isotopic ratios, which carry different information. A second advantage is that binary mixings are straight lines in the Pb-Pb diagrams. Using Pb isotopes in conjunction with Nd, it is possible to reconstruct the variation of the oceanic currents flux, both southward and westward. In addition to the MIOJet that can also be observed from the Pb records, the influence of the Himalayan surrection also appears over the past 30 My. By contrast with the Nd records, lead allows us to distinguish between the different phases and timing of the Himalayan rise.
PP33C-1586
Effect of ocean gateway changes under past equable warm climates
The role of tectonic Southern Ocean gateway changes in driving Antarctic climate change at the Eocene/ Oligocene boundary remains a topic of debate. Here, we find a significantly greater sensitivity of Antarctic temperatures to Southern Ocean gateway changes when atmospheric CO2 concentrations are high. In particular, the closure of the Drake Passage (DP) gap is a necessary condition for the existence of ice-free Antarctic conditions at high CO2 concentrations in our coupled climate model. The absence of the Antarctic Circumpolar Current (ACC) is particularly conducive to warm Eocene Antarctic conditions at higher CO2 concentrations, markedly different to previous simulations conducted under present-day CO2 conditions. Antarctic sea surface temperature and surface air temperature warming due to a closed DP gap reach values around ~5C and ~7C respectively for high concentrations of CO2 (above 1250 ppm). In other words, we find a significantly greater sensitivity of Antarctic temperatures to atmospheric CO2 concentration when the DP is closed. The thermal isolation of Antarctica arising from the development of the ACC inhibits a return to the warmer Antarctic and deep ocean conditions resembling the Eocene, even under enhanced atmospheric greenhouse gas concentrations.
PP33C-1587
The response of the Mediterranean thermohaline circulation to changes in the ocean gateway
Geological records of the Neogene suggest that Mediterranean hydrography experienced severe changes in response to both global climate change and the regional tectonic evolution. We examine the question of the effect of Late Miocene Paleogeography on the Mediterranean thermohaline circulation through the use of an Oceanic General Circulation Model. In order to isolate as much as possible the effect of gateway geometry we use an idealized basin shape and bathymetry and impose simplified atmospheric forcing. A series of sensitivity tests is performed in which we vary the depth of the sill of the ocean gateway. The model is able to simulate accurately the main features of the present-day Mediterranean thermohaline circulation as inferred from observations and models. The impact of changes in sill depth is expressed on the salinity and temperature structure which is clearly controlled by the transport and water mass exchange through the gateway. Since the model forcing is held constant for all experiments, we find that the Mediterranean thermohaline circulation depends strongly on the sill-depth variation. As the sill-depth decreases the upper thermohaline cell is linearly reduced, while - contrary to what has been thought - the deep cell intensifies showing non-linear behaviour. In fact, the shoaling of the sill-depth induces a "blocking effect" on the outflow waters which creates a strong recirculation in the deep layers. Nevertheless, deep-water formation is reduced and, as a consequence, the ventilation of the deep layers diminishes. Our results are consistent with the geological record and paleoceanography proxies of the Mediterranean and may be useful to understand the behaviour of other land-locked basins, both extant and from the geological past. This work is supported by Utrecht Center of Geosciences.
PP33C-1588
Climatic and Oceanographic Changes Across the Eocene-Oligocene Boundary: Results From South Atlantic ODP Site 1090
The Eocene-Oligocene (E-O) boundary is marked by one of the largest climate transitions of the past 50 Ma. The Oi-1 event is characterized by a 1.0‰ increase in benthic foraminiferal δ 18O that occurred at ca. 33.5 Ma. This δ 18O shift has mainly been attributed to large-scale Antarctic ice- sheet growth, whereas deep-water temperature change during Oi-1 remains unresolved. However, recent studies have suggested that this event and its precursor (EOT-1) require the presence of significant ice sheets in Antarctica and the Northern Hemisphere, a eustatic lowering of ~150 meters, and ice-volume storage that is ~2.0 to 2.5 times that of modern ice sheets. We examined the δ 18O and δ 13C of the benthic foraminifera Cibicidoides from South Atlantic ODP site 1090 during Oi-1 at a 10 ka resolution. The benthic δ 18O record from site 1090 shows an overall ~1.5‰ step-wise increase composed primarily of a 0.5‰ shift at ca. 33.8 Ma followed by an even larger 1.0‰ shift at ca. 33.5 Ma, similar to the punctuated shift across the E-O transition documented at sites 1218, 522, and St. Stephens Quarry. Our benthic foraminiferal δ 13C record for site 1090 displays a 1.0‰ increase associated with Oi-1 that is coeval with the δ 18O change. In addition, we analyzed planktonic foraminiferal δ 18O and δ 13C (Subbotina) across this interval. Our planktonic δ 18O data show a transient (<200 kyr) increase of 1.5‰ at 34.0 Ma that we attribute to a change in the structure of the thermocline. A planktonic foraminiferal δ 13C decrease at ca. 34.1 Ma supports the interpretation of a stronger thermocline with a coeval drop in the vertical carbon isotope gradient between the thermocline-dwelling planktonic and deep-dwelling benthic foraminifera. Additional planktonic and benthic foraminiferal stable isotope data from South Atlantic ODP site 1265 across the E-O interval should help determine the cause of the planktonic foraminiferal δ 18O and δ 13C changes. We also investigated changes in deep water circulation patterns during Oi-1 at site 1090 using Nd isotope ratios from fossil fish teeth, which display a trend towards less radiogenic values during the E-O transition, culminating at Oi-1 (~33.5 Ma). Lower 143Nd/144Nd isotopic ratios in seawater are consistent with Nd from an ancient continental source. Thus, these results may reflect a change in the Southern Ocean endmember 143Nd/144Nd value in response to the exhumation and erosion of ancient east Antarctic craton(s) during ice sheet growth. The Nd isotope results from ODP Site 1090 corroborate findings from Kerguelan Plateau and Maud Rise sediment cores.
PP33C-1589
Re-organization of Pacific intermediate and deep water circulation during middle Miocene stepwise onset of
We use benthic and planktonic foraminiferal δ13C and δ18O, planktonic Mg/Ca together with Nd isotopes and deep-water ventilation proxies (benthic foraminiferal accumulation rates, proportion of coarse fraction and XRF Fe) in ODP cores from the NW and SE Pacific to track circulation changes across the Pacific and to elucidate the role of the tropics during middle Miocene global climate change (16.5-12.5 Ma). Our high resolution (3-4 kyr) planktonic δ18O record from South China Sea Site 1146 exhibits high amplitude variability with significant power in the obliquity and precessional bands, suggesting that a monsoonal regime was established in SE Asia by the middle Miocene. Following episodes of ice expansion at 14.6, 14.2 and 13.9 Ma, planktonic δ18O values strikingly decreased and SST estimates increased, suggesting intensification of summer monsoon coupled with northwards shifts of the ITCZ (close to present day position). Nd isotopes and ventilation proxies in SE Pacific Sites 1236 and 1237 also reveal that Antarctic glaciation events were associated with deepening of the Calcite Compensation Depth and major re- organization of intermediate and deep water circulation in the Pacific. Thus, our results indicate a tight coupling of tropical and southern hemisphere climate as well as heightened sensitivity to obliquity and precessional forcing during the stepwise onset of icehouse conditions in the middle Miocene.
PP33C-1590
Extreme Warmth in the Late Miocene North Atlantic
Climate in the late Miocene was unquestionably much warmer and less glaciated than the following Pliocene. This is implied by benthic oxygen isotope records, but as they necessarily combine the effects of temperature and ice volume, a precise climate history leading up to the onset of Northern Hemisphere glaciation is difficult to resolve. Unambiguous, continuous, and well-dated sea surface temperature (SST) records are required to characterize a pre-glacial world. Ideal sites lie in the sensitive high-latitudes, physically close to the growing ice sheets. ODP Site 907 (69°N, 12°E) is situated at a sensitively balanced location with respect to the world's ocean dynamics. A temperature record from this location may help to constrain the history of northern-sourced deep water, which plays a pivotal role in global climate. Located at the conflux of three major surface currents, Site 907 experienced the effects of migrating wind fronts, sea-ice cover, and current migration. Magnetostratigraphy demonstrates continuous sedimentation at the level of polarity zonation, and provides an age model to 14 ma. However, lack of abundant microfossils has prevented traditional isotope or faunal assemblage-based climate analysis from this Site. We present an alkenone-based SST and productivity (Total C37 alkenones/gram sediment = C37 total) record from this location, at moderate resolution (~50k between samples) covering the past ~14 ma. SSTs were as high as 23°C at ~14 ma, cooling gradually (~1.6°/my) to around 10°C by 6 ma. A small increase in C37 total indicates a response to the 5 ma global biogenic bloom event observed in other records, but the most distinctive feature of the productivity record is a crash in haptophyte productivity at ~3.5 ma. This crash is echoed by biogenic opal records, and presumably total productivity in the high-latitude North Atlantic was equally affected. The long-term trends (e.g. the Miocene cooling trend) of global benthic δ18O records correlate well with this alkenone-derived local record. The dramatic late Miocene cooling and the mid-Pliocene productivity crash, however, hint at precursor adjustments to the climate system that significantly preceded the establishment of permanent ice sheets in the northern hemisphere.
PP33C-1591
Changing Ocean Surface Conditions in the Late Pliocene North Atlantic: Insights From a Latitudinal Transect
The Plio-Pleistocene transition was marked by global cooling and the rapid expansion of ice across continents in the Northern Hemisphere. Here, we present late Pliocene (2-4 Ma) alkenone sea surface temperature (SST) and ocean productivity time series from a latitudinal transect of four sites in the Atlantic Ocean (ODP 907, ODP 982, DSDP 607, and ODP 662). Our records, spanning the latitudinal zone from the equator to 69°N, document a long-term surface ocean cooling, but show that rates of cooling were quite variable (ranging from ~0.6°C/Myr to ~2.5°C/Myr) with greater cooling rates occurring in mid to high latitude regions. While the total amplitude of each record varies from site to site, the remarkable similarity in overall structure of our SST records suggests coordination by a large-scale forcing mechanism, such as a change in atmospheric CO2. Spectral analysis of our time series indicates that obliquity played a dominant role in cyclic variations of both SST and productivity prior to and during the intensification of Northern Hemisphere glaciation. Our records of alkenone abundance and biogenic opal indicate a pronounced crash in productivity in high latitude regions of the North Atlantic mirroring changes observed in the productivity pattern in other high latitude regions in association with the Plio-Pleistocene transition. Our new records suggest that at least in the North Atlantic Ocean this productivity crash was asynchronous, with the crash occurring earliest at the highest latitude site. Alkenone abundance and biogenic opal records from mid to low latitude regions reflect a pattern inverse to that observed at high latitudes with productivity in these regions increasing toward the present. These observed changes in productivity suggest that a major shift in nutrient availability in the surface ocean occurred in association with the Plio-Pleistocene transition.
PP33C-1592
Milankovitch-Band Evolution of Tropical Sea Surface Temperatures Over the Plio- Pleistocene
Benthic foraminiferal oxygen isotope records indicate a transition from relatively warm and weakly variable high latitude conditions during the early and middle Pliocene (3-5 Ma) to increasingly cold, more variable, and more intensely glaciated conditions throughout the late Pliocene and Pleistocene (0-3 Ma). Here, we complement this high latitude perspective on Plio-Pleistocene climate evolution through analysis of three long, continuous, and highly resolved records of sea surface temperature (SST) changes in the tropics. We find that the eastern equatorial Pacific, eastern equatorial Atlantic, and Arabian Sea all record very similar SST histories beginning 2.7 Ma, coincident with the intensification of northern hemisphere glaciation. Amplified ice albedo and carbon cycle feedbacks associated with northern hemisphere glaciations were likely responsible for coupling SST responses throughout the tropics from this time forward. Intensification of the 41 kyr band SST response in the eastern equatorial Pacific beginning ~3 Ma may have provided the trigger for northern hemisphere glaciation, through feedbacks involving poleward heat export. Detailed analysis of the tropical surface ocean response across the mid-Pleistocene transition suggests that tropical SSTs evolved largely in tandem with the benthic foraminiferal oxygen isotope record over this interval.
PP33C-1593
Thallium isotopes in ferromanganese crusts as a proxy for marine productivity
We have constructed the first high resolution thallium (Tl) isotope records in two ferromanganese crusts (Fe- Mn crusts), CD29-2 and D11-1 from the central Pacific Ocean, which show pronounced systematic changes. Applying an osmium isotope age model both crusts display coherent virtually identical Tl isotope variations over the entire Cenozoic. The most prominent variations occur in the time interval between about 55 Ma and 45 Ma. The Tl isotope composition increases smoothly from ε205Tl = +6.0 at 55 Ma to ε205Tl = +10.5 at 45 Ma (ε205Tl represents the deviation of the 205Tl/203Tl isotope ratio of a sample from NIST SRM 997 Tl in parts per 104). The remainder of the crusts displays relatively limited Tl isotope variations. These variations are unlikely to reflect diagenetic overprinting or changes in isotope fractionation between seawater and Fe-Mn crusts. Most probably, the Fe-Mn crusts track the Tl isotope composition of seawater over time. Using a simple box model we show that the Tl isotope composition of seawater depends almost exclusively on the ratio between the two principal output fluxes of marine Tl. These output fluxes are removal of Tl from seawater via scavenging by authigenic Fe-Mn oxyhydroxide precipitation and uptake of Tl during low temperature alteration of oceanic crust. However, the low Tl isotope composition that occurred during the Paleocene is consistent only with a more than four-fold higher sequestration of Tl by Fe-Mn oxyhydroxides compared with the present day. The seawater curves of Tl and S isotopes show close synchronicity through the entire Cenozoic and we conclude that both systems must have responded to the same change in the marine environment. We propose that this change was most likely a marked and permanent increase in marine productivity, which caused an increase in carbon export that led to higher pyrite burial rates and also significantly reduced the amount of Fe-Mn oxides precipitated because of biological uptake of Fe and Mn.
PP33C-1594
Developing a Proxy for Intermediate/Deep Seawater Phosphorus Concentrations Using Bamboo Coral From California Coastal Seamounts
This study investigates whether bamboo coral (family Isididae, order Alycyonacea), a cosmopolitan gorgonian octocoral, can be used to trace phosphorus (P) concentrations in intermediate/deep seawater. Developing such a P proxy would elucidate patterns of circulation in intermediate water masses, which are sources of nutrient rich upwelling. The bamboo coral samples were collected from 4 relict seamounts in the upwelling zone off the California coast in 2004 and 2007. A hand drill was used to excavate 1mm width samples from the edge and interior of coral skeletons in order to obtain 1mg coral powder samples. The phosphorus to calcium (P/Ca) ratios of dissolved coral samples were measured by an inductively coupled plasma mass spectrometer and ranged from 0.28 to 0.56 mmol/mol, consistent with previous studies of other deep sea coral species. P/Ca ratios vary by as much as 0.1 mmol/mol in samples taken from the edge to the core of the coral sections. These P/Ca values were compared to in situ water samples and phosphate concentration data from Levitus et al. (1994). Based on preliminary P/Ca values, the distribution coefficient for P in corals relative to the Levitus et al. (1994) dataset is approximately 1.3. While initial data show a poor correlation between P/Ca in bamboo corals and phosphate concentration, ongoing work aims to further elucidate the controls on P/Ca in bamboo corals and the source of P/Ca variability over time.
PP33C-1595
Coral Calcifying Fluid pH (Indicated by δ11B) and Skeletal Sr/Ca in Coral Skeletons
We have produced a high spatial resolution SIMS record of Sr/Ca and δ11B across ~ 1 year of a modern Porites lobata coral from Hawaii. We used skeletal δ11B to estimate the calcifying fluid pH at the time of calcification and have correlated δ11B with Sr/Ca. Our data indicate that Sr/Ca and calcification site pH are inversely correlated at high pH. This is consistent with the model for the role of the enzyme Ca-ATPase in calcification. During periods of rapid calcification e.g. under optimal light conditions, Ca-ATPase activity is high, Ca2+ is rapidly pumped to the calcification site and H+ is extruded resulting in a high pH in the calcifying fluid. Sr is either not transported by the enzyme or is transported less efficiently than Ca and the Sr/Ca of the calcifying fluid is effectively reduced. However the relationship between Sr/Ca and δ11B is not observed over the lower pH range. It is reasonable to assume that calcification is slower at these low pHs and it is possible that the contribution of Ca-ATPase to Ca transport is negligible under these conditions. We observe significant Sr/Ca heterogeneity in regions of the skeletons deposited at both high and low pH and observe a large range in Sr/Ca (>0.5mmol mol-1) deposited at any single pH value. This indicates that skeletal Sr/Ca heterogeneity is generated by other processes besides the activity of Ca-ATPase. The role of other Ca (and Sr) transport processes in generating this heterogeneity remains to be resolved.
PP33C-1596
Modern Deep-sea Sponges as Recorders of Bottom Water Silicon Isotopes
Major zones of opal accumulation in the world oceans have experienced geographical shifts during the Cenozoic coincident with times of transition in oceanic circulation and climate. The global marine silica cycle is likely to respond to various large-scale changes including the distillation of Si and other nutrients in ocean basins; weathering and continental inputs; and biological productivity in surface waters. These processes could potentially be distinguished by their impact on the isotopic composition of dissolved silica in the world oceans. Although diatoms dominate uptake of silica in surface waters, box-modelling (de la Rocha and Bickle, 2005) suggests that sponges spicules have a greater potential to reflect whole ocean changes in the silica cycle, by recording deep-water silicon isotopes. Here, we introduce a new calibration study of modern deep- sea sponges collected on a transect cruise across the Drake Passage, in the Southern Ocean, from a range of depths and seawater silicic acid concentrations. Sponges were collected by benthic trawling, and dried immediately. The spicules were later isolated from cellular material and cleaned for surface contaminants, before dissolution and analysis by NuPlasma HR MC-ICP-MS in medium resolution mode. We discuss our preliminary data, the extent to which inter and intraspecies variations reflect environmental conditions, and the implications for palaeoreconstructions of the marine silicon cycle. de la Rocha, C. and M. Bickle (2005). Sensitivity of silicon isotopes to whole-ocean changes in the silica cycle. Marine Geology 217, 267-282.