PP41D-1475
Determination of Boron Isotope Ratios in Coral Skeletons Using Multiple Collector ICP- MS
Boron isotopic compositions in biogenic carbonates are useful proxy for oceanic pH in the past. This information is essential for a better understanding of possible mechanisms that control environmental and climate change. A high-precision analytical technique was developed for isotopic determination of boron in coral skeletons using high-resolution multiple collector ICP-MS (MC-ICP-MS, Neptune, Thermo-Fisher). The mass discrimination was corrected by the sample-standard bracketing method. The main advantage of this technique is that both of sample throughput and analytical precision are significantly higher than P-TIMS and N-TIMS. The accuracy of this technique is further examined by replicated analyses of the international coral standard, Jcp-1, and in-house coral standard using both MC-ICP-MS and P-TIMS techniques. The long-lived massive coral skeletons (Porites lobata) from Lanyu Island offshore the southeastern Taiwan show annual δ 11B variation of around 2‰, 23.81 to 25.86‰ (n=39) during 1994 to 1996 A.D., which is similar to recent results from Flinders Reef in the Coral Sea. In combination with Sr/Ca-based thermometry, the intra-annual pH record shows clear seasonal cycles with high pH at cold period. B/Ca and U/Ca ratios also show a good correlation with Sr/Ca derived SSTs. Further in-depth δ 11B applications would help us to understand relationships between the global climate change and response in the surface ocean.
PP41D-1476
Boron Isotopic Composition Variation During Early-Bajocian δ13Cmin Positive Excursion
Early Bajocian is a period of sea level rise and platform drowning during the mid-Jurassic greenhouse world. This period is geochemically characterized by a positive excursion of inorganic δ13C signal as recorded in Western Thetys sections. This signal, concomitant with an increased biosiliceous sedimentation, is meant to reflect both a eutrophication event and a carbonate production crisis (Bartolini et al. 1996 ; Bartolini and Cecca 1999). High atmospheric CO2 level is assumed for this period, linked with the birth of the Pacific Plate and a faster sea-floor spreading (Bartolini and Larson 2001). Opening of the Liguro- Piemontese ocean may have led to rearrangement of oceanic current circulation patterns. These global conditions, potentially leading to oceanic eutrophication and carbonate saturation state modification, have been suggested as a trigger for Early-Bajocian events. Atmospheric CO2, carbon cycle and seawater pH are connected through the seawater carbonate system and boron isotopes in carbonates are a paleopH proxy. Geochemical analyses including δ11B were performed on bajocian carbonates from Terminilletto section, Italy, one of the rare carbonate section spanning this period. A new extraction process combined with a new direct injection method for MC-ICP-MS (d-DIHEN) helped to improve analyses reproducibilty (Louvat et al. in prep). The results show clearly a variation of the isotopic signal. This variation can be explained by a rise of seawater pH, occuring just before the carbonate production crisis and connected to the eutrophication. Modelisation will thus be performed to reproduce pH variation and reconstitute carbon cycle perturbation at this time. Bartolini A., Baumgartner P. O., and Hunziker J. (1996) Middle and Late Jurassic carbon stable-isotope stratigraphy and radiolarite sedimentation of the Umbria- Marche Basin (Central Italy). Eclogae geol. Helv. 89(2), 811-844. Bartolini A. and Cecca F. (1999) 20 My hiatus in the Jurassic of Umbria-Marche Apennines (Italy: carbonate crisis due to eutrophication. Comptes Rendus de l'Académie des Sciences 329, 587-595. Bartolini A. and Larson R. (2001) The pacific microplate and Pangea supercontinent in the Early -Middle Jurassic. Geology 29(8), 735-738. Louvat P., Bouchez, J., and Paris, G. (in prep for Chem. Geol.) Boron isotopes measurements by MC-ICP-MS and direct injection nebulization (d-DIHEN)
PP41D-1477
Heavy boron isotopic composition of Pennsylvanian brachiopod calcite: implications for pH, CO2 and secular variation of seawater δ11B
The Late Carboniferous (Pennsylvanian) icehouse is thought to have been a period of low, rapidly fluctuating CO2 concentrations coincident with major episodes of glaciation, as evidenced by proxy measurements, carbon budget models, and the temporal distribution of glacial deposits and cyclothems. The B-isotope paleo-pH proxy has proven to be a valuable tool for estimating paleo-atmospheric pCO2. However, applications of the proxy to deep time are complicated by a probable long-term secular variation in the B-isotopic composition of seawater, and potential diagenetic alteration of ancient samples. Our data show that very low δ11B values for Mississippian and Permian brachiopods appear to require significant secular variation in the B isotope composition of seawater. We measured δ11B of Late Pennsylvanian brachiopod calcite with negative thermal ionization mass spectrometry (NTIMS). Museum specimens were screened for diagenetic alteration using scanning electron microscopy and cathodoluminescence microscopy. The Pennsylvanian values are higher than Neogene values, consistent with high pH and low CO2, if changes in seawater δ11B are ignored. While we cannot directly calculate a pH (pCO2) from these results, they appear to require a higher pH than today's ocean and/or a high δ11B of Pennsylvanian seawater. Some models have suggested secular variability related to hydrothermal alteration at mid-ocean ridges while others call on variation of river run-off as the dominant control. We suggest another control may be exerted by aqueous B speciation. During periods of low pCO2 (high pH), we expect that seawater δ11B will increase due to the relatively higher abundance of the isotopically lighter B(OH)4-, which is chemically reactive and sorbs to clays and carbonates, thus removing isotopically light boron. On the other hand, during periods of high pCO2 (low pH), the relative increase in the abundance of the uncharged B(OH)3 species will result in less adsorption and removal of B, resulting in a decrease in seawater δ11B, approaching river input values. Therefore measurements of δ11B cannot be used to reconstruct absolute values for ocean pH and atmospheric CO2 concentration without knowing the secular variation reconstruction of seawater pH, but relative shifts can provide important paleo-climate information.
PP41D-1478
Shell Weight Variability from the Cariaco Basin: Insights into Carbon Dioxide Concentrations During the Last Glacial
Here we present decadal scale records of sediment color reflectance, planktonic foraminiferal shell weight, and planktonic foraminiferal δ13C from the Cariaco Basin during the Marine Isotope Stage 3 ~30-55kyrBP and demonstrate that the temporal variability in each record is associated with stadial/interstadial climate oscillations. Sediment trap results from the Cariaco Basin suggest that the shell weight variability is driven, in part, by changes in the surface water carbonate ion concentration. Hence, we use the shell weight record to estimate paleo-carbonate ion concentrations during the last glacial. The paleo-carbonate ion concentration, in combination with estimates of past seawater temperature and salinity, is used to assess past changes in atmospheric carbon dioxide. Calculated pCO2 values during interstadial events are higher than that estimated for stadial events, despite higher primary productivity. While enhanced productivity would be expected to drawdown atmospheric CO2, we attribute the higher pCO2 values during interstadials to decreased pH and alkalinity due to increased river input and enhanced calcite production. We conclude that the Cariaco Basin was a source for atmospheric carbon dioxide during this time interval and hence did not play a role in the drawdown of atmospheric carbon dioxide during the last glacial period.
PP41D-1479
A boron isotopic study of Cibicidoides wuellerstorfi: Implication for bottom-water pH, pCO2 and [CO3=] variations in the Caribbean Sea over the last 21ka
The δ11B of foraminifera Cibicidoides wuellerstorfi over the past 21ka extracted from a Caribbean core, TT9108-1GC, have been measured in order to reconstruct pH value of paleo-bottom-water. Our results indicate that the pH value near the bottom water at the Caribbean was similar between the LGM and the present, and was ca 0.6 pH unit higher during the Termination I. The Caribbean Sea bottom- water-pCO2 and [CO3=] reconstructed using combined pH record and theoretical seawater total alkalinity derived from sea level change show significant fluctuations between 12ka to 16ka. However, there is no corresponding fluctuation observed in the sedimentary record (Broecker and Clark, 2002), and which can be attributed to influence by sea-surface-productivity and pore water chemistry. Furthermore, the Caribbean Sea bottom-water-pCO2 shows an inverse pattern with sea-surface-pCO2 of the western Pacific and the Caribbean Sea during the last 21ka, supporting the suggestion of Foster (2008) that Caribbean Sea-surface carbonate system is controlled by Brazil Current which passed through Eastern Equatorial Atlantic upwelling zone rather than coastal upwelling.
PP41D-1480
Boron Isotopes in Benthic Foraminifera by MC-ICPMS: Unlocking the Ocean's Carbon Cycle
The cause of glacial-interglacial CO2 cycles has been described as the "holy grail" of climate science. All models currently proposed invoke changes in deep ocean carbon storage, but the mechanisms by which this took place remain unclear. Proxies for two components of the ocean carbonate system would allow us to fully reconstruct ocean carbonate equilibria and trace the spatial and temporal pattern of glacial carbon storage, providing valuable constraints on the causal mechanisms of atmospheric CO2 change. The theory behind the boron isotope pH proxy is well understood, but its reliability has been questioned, primarily due to uncertainty in the fractionation factor between boron species in seawater, and analytical difficulties associated with negative thermal ionisation (NTIMS) measurements. We have developed a new technique for boron isotopic analysis by multicollector inductively coupled plasma mass spectrometry (MC- ICPMS), which overcomes many of the problems associated with NTIMS measurements. Our method is precise (better than 0.25%, or ~0.02 pH units, on full procedural replicates at 95% confidence), rapid (allowing duplicate measurement of 10-20 samples per analytical session), and has small sample size requirements of ~10 ng boron (~0.5 mg foraminiferal tests). As MC-ICPMS analysis requires separation of boron prior to measurement, any bias between samples and standards with different matrices is also removed. Recent experimental work has also improved uncertainty in the isotopic fractionation factor (now measured at 1.0272 ±0.0006 [1]), providing a powerful independent means to test the behaviour of the foram-based δ11B proxy, and its ability to provide absolute pH values. We have measured δ11B in several species of benthic foraminifera from a range of core-top samples. In contrast to previous studies, we find a very close match between foraminiferal δ11B values and the δ11B of seawater B(OH)4- - predicted using the recently determined fractionation factor of [1] - allowing a ready determination of deep water pH. Here we combine this with measurements of B/Ca ratios, a proxy for carbonate ion saturation [2], to determine all components of the ocean's carbonate system. We will present results from several key locations in order to gain insights into the mechanisms responsible for glacial-interglacial pCO2 change. [1] K. Klochko, A. J. Kaufman, W. Yao, R. H. Byrne, and J. A. Tossell (2006), Earth and Planetary Science Letters, 248, 261-270. [2] J. Yu and H. Elderfield (2007), Earth and Planetary Science Letters, 258, 73-86.
PP41D-1481
Carbonate Dissolution in the Eastern Equatorial Pacific During the Pleistocene: A Preliminary Study From ODP Site 1241
Recent studies of Pacific cores support the view that changes in carbonate preservation, rather than productivity, have controlled Pleistocene fluctuations in sediment CaCO3. Records of shell fragmentation and percent coarse fraction (>150μm, %CF) have been used as proxies for dissolution. Shell fragmentation records are considered the more direct measure of CaCO3 dissolution because they are based solely on the percentage of broken foraminiferal tests, which increases as dissolution progresses. Unlike shell fragmentation, %CF can be influenced by volcanic debris and terrigenous input. We show that agreement between the %CF and shell fragmentation records suggest that dissolution is the primary factor dominating %CF changes in the eastern equatorial Pacific (EEP). We present a collection of new records of %CF and shell fragmentation from the EEP in order to examine the variability of carbonate dissolution in the region and to compare the results to published dissolution records in the EEP, central, and western Pacific. Samples from EEP core ODP Site 1241 (5° 50N, 86° 26W; 2027m water depth) are being used to generate a new and continuous benthic foraminiferal (U. auberiana) O18 record extending back to ~1 Ma. The core was sampled at 5 cm intervals providing roughly ~4-5 ky resolution to ~360 ky and sampled at 10 cm intervals, for a resolution of ~8- 10 ky from ~360 ky to 1 Ma. In addition to the O18 record, a %CF and shell fragmentation record are in development and will serve as proxies of CaCO3 dissolution over the last ~360 ky. Comparison of the %CF record from ODP 1241 over the last ~360 ky to previously published records (TR163-22, 31P) and new records (TR163-18, 19, 20B, 23, 25) from the EEP shows a general trend of increasing preservation towards the present followed by a sharp drop at the core tops (~40-60% drop in coarse fraction). Preservation is generally greatest during glacial to interglacial transitions, as has been found in previous studies. Trends in %CF appear similar throughout the EEP, even over areas where productivity patterns have undoubtedly varied, suggesting Pacific-wide dissolution changes have controlled %CF.
PP41D-1482
Modelled reconstructions of the oceanic carbonate system for different histories of atmospheric carbon dioxide during the last 20 million years
A box model was built and used to estimate values for oceanic carbonate system parameters (total dissolved inorganic carbon, alkalinity, CO2(aq), CO32-, HCO3-, and pH) in the surface and deep ocean over the past 20 Ma. Geological data were used to constrain model boundary conditions including the carbonate compensation depth (CCD) and the temperature, salinity, and ionic composition of the ocean. Sensitivity tests were performed to evaluate potential errors arising from uncertainties in model boundary conditions. We modelled values of surface water pH assuming four different histories of atmospheric CO2, and compared our model results with proxy-based estimates of pH. We found that multiple CO2 histories could be invoked to explain the same pH trend, depending on what assumptions were made regarding ocean overturning and biological productivity. The two CO2 histories most consistent with existing proxy constraints on pH are the least consistent with our understanding of global climate during the last 20 Ma.
PP41D-1483
Atmospheric CO2 Changes During the Pliocene Epoch Revealed by Foraminiferal Boron Isotope Ratios.
In order to better understand climate sensitivities and to improve estimates of future climate change, reconstructing past atmospheric CO2 levels has become a high priority in paleoclimate studies. The Pliocene Epoch is particularly interesting, as despite warmer temperatures, the northern hemisphere became increasingly glaciated from 3.6 Ma to 2.5 Ma, finally leading to "Quaternary-style" climate. One of the hypotheses proposed to explain the onset of northern hemisphere glaciation is the impact of the closure of the Panamanian Isthmus (at 4.2 Ma) on the increased export of moisture from the North Atlantic to the northern high latitudes (Keigwin et al., 1982; Driscoll and Haug, 1998). An alternative scenario suggests that a decrease in atmospheric CO2, combined with reduced solar insolation, could have triggered the glaciation (Li et al., 1998). This scenario has been supported by a recent modeling study (Lunt et al., 2008), which also suggests that a decrease in pCO2 from the Pliocene to the Quaternary is the most efficient way to build an ice-sheet at that time. To differentiate between the two scenarios, we measured boron isotope ratios (δ11B) in the planktic foraminifer Globigerinoides sacculifer, which has been proposed as a proxy for past surface seawater-pH. In combination with a second parameter of the marine carbon system, such as carbonate ion concentration, the pH data allow for estimation of past PCO2. Samples were selected from ODP Site 999 (Caribbean Sea) and cover the interval 4.7-2 Ma with a time resolution of ~70 kyr. Our data show a gradual decrease over the entire period, possibly indicating a decrease in atmospheric pCO2 along with the global cooling of the Late Pliocene. If undergoing investigations at Site 806B (West Equatorial Pacific) confirm our results, a gradual decrease in pCO2 did occur between 4.7 and 2.0 Ma and support the idea that decreasing greenhouse forcing drove the onset of Pliocene glaciation.
PP41D-1484
Plio-Pleistocene pCO2 – a Multiproxy Approach Using Alkenone and Boron Based Carbonate System Proxies
The recent rapid rise in atmospheric CO2 is unprecedented in Earths history, and the current level (385 ppm) is higher than previously experienced for at least the last 650 kyr. Therefore in order to better understand the link between climate and CO2 it is desirable to examine times in the past that experienced similar or higher levels of CO2 compared to today. The Pliocene (2.6 to 5.3 Ma) is the most recent warm period and hence offers such an opportunity. Detailed reconstructions show that global temperatures were 2- 3°C higher ([1], 6°C at high latitudes) and the polar ice sheets were considerably smaller (sea level was 15-20 m higher; [2]) yet other boundary conditions, such as continental configuration, were similar at this time. Earlier studies, typically with low temporal resolution, have shown that at c.3 Ma pCO2 concentrations were in the range 300-400 ppm. A common assumption is that pCO2 dropped from this high value to pre-industrial values at a time coincident with the intensification of northern hemisphere glaciation (NHG) that occurred at c.2.7 Ma, although this has yet to be demonstrated. Here we present a continuous pCO2 record recovered from ocean sediments using a multiproxy approach based on the boron and alkenone carbonate system proxies. This new data allows both a determination of the magnitude of Pliocene pCO2 and for the fist time the Plio-Pleistocene evolution of pCO2 that accompanied the intensification of NHG. We developed continuous records of alkenone based εp values and foraminiferal δ11B and B/Ca ratios from ODP Sites 999 and 1000 in the Caribbean Sea and Site 1241 on the western side of the Panama Isthmus in the Eastern Equatorial Pacific spanning the last 5.3 Ma. Following a correction of the alkenone records for coccolith size, and accounting for changing growth rate where appropriate, the alkenone based εp record can be used to estimate [CO2]aq and hence pCO2 at the two sites. Similarly, using the core top calibration of [3] the B/Ca and δ11B measurements of the mixed layer foram species G. ruber can be used to estimate [CO32-] and pH and so solve for [CO2]aq and also provide an estimate pCO2. The pCO2 records for both techniques and for both Sites are similar and suggest, in agreement with previous studies, that pCO2 during the Mid-Pliocene was 340-400 ppm. The decline from this high to values similar to the pre-industrial Holocene was relatively rapid and occurred between 2.7 and 3 Ma coincident with the intensification of NHG. This contribution serves to highlight the link between atmospheric CO2 and climate and further we suggest that the Mid-Pliocene is potentially a good analogue for Earths future climate state. [1] N. J. Shackleton, M. A. Hall, and D. Pate, Proceedings of the Ocean Drilling Program, Scientific results, Vol. 138, pp. 337 (1995). [2] H. J. Dowsett, in Deep-time perspectives on climate change: Marrying the signal from computer models and biological proxies, The Micropalaeontological Society Special Publication, London, pp. 459 (2007). [3] G. L. Foster, Earth and Planetary Science Letters 271 (1-4), 254 (2008).
PP41D-1485
Reconstruction of Late Pleistocene Surface Water pCO2 in the Eastern Equatorial Atlantic Based on Alkenone and Boron Isotopes
Reconstructing the long-term history of the partial pressure of atmospheric carbon dioxide (pCO2) is a big challenge in paleoclimatic and paleoceanographic research. Hence, a number of proxies have been proposed to estimate paleo-CO2 concentrations at time periods prior to those covered by ice cores. Two of the most prominent proxy approaches are (a) the carbon isotopic fractionation of C37-alkenones produced by certain haptophyte algae and (b) the boron isotopic composition in planktonic foraminifer shells. Both techniques have limitations and uncertainties due to their specific methodology. In past paleoceanographic studies, either one or the other approach has been applied individually. Here, we present a study where we directly compare alkenone carbon isotopic fractionation over several late Pleistocene glacial cycles with published boron isotope data at the same location (ODP site 668B, Sierra Leone Rise). The site is presently characterised by an air-sea equilibrium in CO2 as well as low productivity. We show that over the last 800,000 years alkenone isotope data are not primarily controlled by atmospheric pCO2 levels. We further demonstrate that the species assemblage of alkenone producing algae has no influence on their isotopic signal. In contrast, the boron isotope proxy has been shown to have a great potential for estimating atmospheric pCO2 at this location.
PP41D-1486
Late Ordovician land plant spore 13C fractionation records atmospheric CO2 and climate change
Molecular systematics and spore wall ultrastructure studies indicate that late Ordovician diad and triad fossil spores were likely produced by plants most closely related to liverworts. Here, we report the first δ13C estimates of Ordovician fossil land plant spores, which were obtained using a spooling wire micro-combustion device interfaced with an isotope-ratio mass spectrometer (Sessions et al., 2005, Analytical Chemistry, 77, 6519). The spores all originate from Saudi Arabia on the west of Gondwana and date to before (Cardadoc, ca. 460 Ma), during (443Ma) and after (Llandovery, ca. 440Ma) the Hirnantian glaciation. We use these numbers along with marine carbonate δ13C records to estimate atmospheric CO2 by implementing a theoretical model that captures the strong CO2-dependency of 13C fractionation in non-vascular land plants (Fletcher et al., 2008, Nature Geoscience, 1, 43). Although provisional at this stage, reconstructed CO2 changes are consistent with the Kump et al. (2008) (Paleo. Paleo. Paleo. 152, 173) 'weathering hypothesis' whereby pre-Hirnantian cooling is caused by relatively low CO2 (ca. 700ppm) related to enhanced weathering of young basaltic rocks during the early phase of the Taconic uplift, with background values subsequently rising to around double this value by the earliest Silurian. Further analyses will better constrain atmospheric CO2 change during the late Ordovician climatic perturbation and address controversial hypotheses concerning the causes and timing of the Earth system transition into an icehouse state.
PP41D-1487
Timing and Significance of a Global Deep-sea Dissolution Event during the Eocene- Oligocene Transition
In deep-sea records spanning the Eocene-Oligocene (E-O) boundary, a two-stage positive shift in marine carbonate δ18O values has been shown to occur in tandem with a two-step global deepening (by ≥1 km) of the calcite compensation depth (CCD). The oxygen isotope shift and interpreted de- acidification of the deep waters are intricately linked to major global cooling and expansion of polar ice sheets, representing the most significant step towards 'icehouse' climates of the Paleogene time interval. The initial or precursor phase of major global climate changes at the E-O boundary, however, has previously received little attention. New and currently available foraminiferal stable isotope and carbonate concentration records indicate that the initiation interval (at ~34.0 Ma) is characterized by both a negative carbon isotope excursion and a strong carbonate dissolution horizon at several deep-sea sites around the globe. Correlation of carbonate concentration records reveals reduced or no carbonate accumulation within this interval at sites situated below ~3200 m paleo-water depth, inclusive of sites in the equatorial Pacific (ODP Site 1218), equatorial Indian (ODP Site 711), mid-latitude South Atlantic (DSDP Site 523), and Southern Ocean (ODP Sites 699 and 1090). We hypothesize that this dissolution event reflects a rapid, but brief, shoaling of the CCD due to a decrease in carbonate ion saturation (acidification) of deep waters. This shoaling episode occurs immediately prior to the well-documented CCD deepening and indicates that CCD changes through the E-O transition are more complex than previously thought. At Site 1090, cyclical variations in physical property and proxy parameter curves suggest that the duration of the shoaling event is between 50-200 kyr, possibly around 100 kyr as indicated by the climatic precession cycles present in the records. On-going cyclostratigraphic work and development of additional records seek to constrain the magnitude and duration of this dissolution episode in different basins. This information, in conjunction with high-resolution foraminiferal carbon isotope records, will be used to constrain possible mechanisms causing the CCD variation, ultimately providing a better understanding of the complex series of events and changes in carbon cycling that led to the major shift in global climate across the E-O transition.
PP41D-1488
On the Sensitivity of the Marine Carbon Cycle to Changes in the Ocean Circulation During the Paleocene-Eocene Thermal Maximum
The Paleocene-Eocene Thermal Maximum (55 Mya) is regarded as a suitable analog to future climate
change and uptake of carbon in the ocean. For this time, significant changes in climate and geochemistry
have been inferred from temperature proxies and stable carbon isotope ratios. Climate simulations with the
community climate system model CCSM including a marine carbon cycle model have been used to explore
effects of climate changes on the carbon cycle. Deepwater formation in the North Pacific and Southern
Ocean is associated with high productivity. Analysis with climate models reveals that the vertical mixing and
productivity in the Eocene Pacific is sensitive to freshwater pulses from the Arctic region. Simulated Paleo-
Atlantic deep-sea circulation indicates a southward transport into the Southern Ocean in agreement with
stable carbon isotope reconstructions. Comparison of recent temperature reconstructions from oxygen
isotopes, TEX86, and Mg/Ca data with a PETM 8xCO2 climate simulation indicates a considerable bias for
the high latitudes suggesting that even higher levels of greenhouse gases may have been present at
Paleocene-Eocene boundary.
http://www.uta.edu/faculty/awinguth/PETM-Home.html
PP41D-1489
Decoupling between the carbon cycle and climate during Pleistocene glaciations
We present a Pacific benthic δ13C stack of the last 800 kyr and compare it with atmospheric CO2 concentration [Lüthi et al., 2008], Antarctic temperature [Jouzel et al., 2007], and benthic δ18O [Lisiecki and Raymo, 2005]. We find that carbon cycle proxies δ13C and CO2 share some features which consistently differ from records of Antarctic temperature and benthic δ18O. For example, the carbon cycle proxies reach their most extreme glacial values more than 10 kyr before the glacial maxima of MIS 6, 10 and 16. Analysis of these relationships, observed in both marine and ice core records, may shed light on links between the ocean carbonate system and atmospheric pCO2 as well as their role in 100-kyr glacial cycles. Additionally, similarities between benthic δ13C and CO2 suggest that some apparent differences between the EDC3 [Parrenin et al., 2007] and LR04 [Lisiecki and Raymo, 2005] age scales (e.g., the MIS 13/14 and 17/18 transitions) are accurate and not the result of age model error.
PP41D-1490
Variation of carbon isotopic composition of seawater DIC in Western Tethys for the Middle Jurassic
An accurate chronostratigraphic calibration of carbon-isotope stratigraphy is necessary to clarify the relationship between the paleocarbon cycle, ocean carbon reservoirs and the faunal evolutionary processes. Marine carbonate sequences with high macro and microfossil content coupled with carbon isotope investigations permit biostratigraphic dating at the subzone level, and thus achieve an accurate chronostratigraphic calibration. Mid-Jurassic age rhythmic marl and marly limestone sequences located in the Southern Iberian paleomargin (Spain) represent ideal sections to link the stable carbon isotope curves directly to ammonite zones and subzones in the western Tethys. From the Upper Toarcian to the late Bathonian δ13C values of marine carbonates on the Iberian paleomargin are consistent with variations of carbon isotope compositions of seawater DIC in western Tethys. Positive δ13C variations are associated with high abundances of eutrophic, calcareous nannofossil taxa and strong increases in radiolarian abundance suggesting a period of high biological productivity. The positive δ13C excursions are compatible with enhanced biological productivity due to the preferential assimilation of 12C by marine organisms, thus enriching the remaining inorganic carbon with 13C. A slight offset is observed between the sections in the Iberian paleomargin and sections from the northern and southern margins of the western Tethys. This drift reflects variations in carbon isotopic composition of seawater DIC associated with changes in the oxidation rate of organic carbon in different oceanic areas of the western Tethys. Lower δ13C values in the Iberian paleomargin were probably associated with higher oxidation rates of continentally derived organic carbon than in the Umbrian-Marche-Sabina basin, which is characterized by carbonate stratigraphic sequences far from the influence of terrestrial sediments. During the Upper Aalenian, Subbetic and Apennines marine carbonates have similar δ13C values suggesting a mixing of DIC seawater reservoirs in the western Tethys, probably related to a change in oceanic circulation and a better mixing of seawater masses which contribute to increased nutrient availability and therefore an increase in marine productivity. The carbon isotope values of carbonates are not linearly correlated with extinction rates and ammonite diversity. However, the main faunal turnovers follow minimum δ13C values in which extinct taxa are replaced by new ones. Therefore, radiation episodes are associated with increasing δ13C values. Detailed analyses of faunal turnovers may be used as a proxy for identifying major paleoenvironmental and ecosystems crises in the Middle Jurassic.
PP41D-1491
Sea Ice Effect on Atmospheric pCO2 Change During the Glacial–interglacial Cycles
Sea ice is a key component in the climate system. Change in sea ice coverage is proposed to partially account for the ~80 ppmv atmospheric pCO2 variations during the Pleistocene glacial/interglacial cycles. However, it is still under debate as to how and how much sea ice actually influences atmospheric pCO2. Here, we use a three-dimensional, intermediate-complexity, biogeochemical climate model (MESMO) to determine the sensitivity of atmospheric CO2 concentration to changes in the sea ice coverage, driven by prescribed changes in planetary emissivity. Three aspects of sea ice expansion are examined: air-sea gas exchange, stratification and vertical mixing, and biological production. When a lower emissivity increases global sea ice by ~20% relative to today, gas exchange is reduced, stratification is increased, and biological production is reduced. These changes combined reduce atmospheric pCO2 by 11 ppmv relative to the control run. Individually, the three contributions are about equal in magnitude in terms of atmospheric pCO2 with change in the biological production raising pCO2, and the other two changes lowering it. Early box model studies only considered gas exchange and argued that sea ice lowers pCO2. A recent study only considered gas exchange and biological production, whose effects on pCO2 largely canceled out. By considering the additional effect of stratification and vertical mixing, which would likely accompany sea ice expansion, we show that total effect is to lower atmospheric pCO2. Thus our end result is consistent with early box model studies in showing that sea ice expansion reduces atmospheric pCO2 but for a very different reason.
PP41D-1492
Ocean-atmosphere redox, pCO2, and the global CaCO3 cycle
The abundance of carbonate minerals in marine sedimentary rocks since 3.43 Ga suggests that precipitation of CaCO3 has been an important sink of alkalinity and CO2 over much of Earth history. Reconstructions of ancient and modern carbonate depositional systems indicate that the large-scale features of shallow-water carbonate sedimentation (paleogeography, platform architecture, and sequence stratigraphy) have changed little over the last 3 billion years, suggesting similar controls on global patterns of CaCO3 deposition (e.g. climate, weathering, sedimentary accommodation and accumulation). Importantly, the continuity of CaCO3 deposition has been maintained in spite of large changes in the source of carbonate sediments in shelf and slope environments. During Archean and Paleoproterozoic time, significant CaCO3 precipitation occurred directly on (and in) the seafloor as crystal fans, microdigitate stromatolites, and isopachous cements. Seafloor precipitates declined during the Meso- and Neoproterozoic in favor of carbonate muds (micrite) and trap-and-bind stromatolites. In latest Ediacaran seas, CaCO3 - producing eukaryotes evolved, and later become the dominant component of the global CaCO3 flux. Since then, carbonate sediments have been composed primarily of skeletal CaCO3, with the exception of brief (millions of years) intervals associated with anoxia and mass extinction where authigenic seafloor precipitates return as a significant CaCO3 sink. We propose that the observed decline in CaCO3 precipitation on and in the seafloor is due largely to a local decline in CaCO3 saturation related to the increasing oxidation of the ocean-atmosphere system and a progressive decline in the size of the marine DIC reservoir since Archean time. We show that oxic respiration of organic carbon produces large gradients in CaCO3 saturation within the ocean and sediments. In contrast, anoxic respiration of organic carbon leads to a more homogeneous distribution of CaCO3 saturation, suppressing CaCO3 dissolution and promoting CaCO3 precipitation in sediments at shallow and intermediate water depths. Gradients in CaCO3 saturation are also suppressed by a large marine DIC reservoir (high pCO2). We develop a simple model of CaCO3 saturation in the ocean and use it to examine how the CaCO3 cycle depends on the distribution of CaCO3 saturation in the ocean and sediments. Secular trends in CaCO3 precipitation on the seafloor in the Precambrian and the reappearance of seafloor precipitates during episodes of widespread anoxia in the Phanerozoic may represent the response of the global CaCO3 cycle to the growth or contraction of gradients in CaCO3 saturation between the ocean and sediments associated with changes ocean-atmosphere redox and DIC.
PP41D-1493
Production of Carbon Dioxide From Sub-aerially Exposed Continental Shelves and Oceanic Islands During Glacial Periods Since the Middle Pleistocene Climatic Transition
The EPICA Dome C ice core has yielded an 800,000-year record of atmospheric carbon dioxide composition from the Middle Pleistocene climatic transition to the present day. In this record, there is a sharp increase in carbon dioxide immediately following the glacial maxima during the glacial periods which to date remains difficult to explain. We will present evidence to show that sub-aerially exposed continental shelves and oceanic islands may be at least partly responsible for the production of the missing carbon dioxide. In exposed siliciclastic-dominated shelves and oceanic islands, acid-sulphate soil development would lead to the release of carbon dioxide. On the other hand, in exposed carbonate-dominated shelves and oceanic islands, karstification would also lead to the release of carbon dioxide. Selected cores from continental shelves and oceanic islands will be used to support this claim. Further studies on cores obtained from other continental shelves and oceanic islands would facilitate the estimation of carbon dioxide loss through comparison between Holocene marine deposits and their pre-Holocene counterparts. Additionally, information on the vegetation history of exposed shelves and oceanic islands during glacial periods may be obtainable to supplement our knowledge gap on past changes in the biological pump.
PP41D-1494
Glacial-Interglacial Changes in Export Production in the Subarctic North Pacific Ocean
The subarctic North Pacific Ocean is a key region for studying the linkages between circulation, dust, marine productivity, and carbon cycling, because it encompasses a high-nutrient low-chlorophyll region where iron addition experiments and direct observation of dust events have demonstrated its potential to drawdown atmospheric CO2. Furthermore, the deep North Pacific holds a large CO2 reservoir that is currently isolated from the atmosphere by a low-salinity layer, and it has recently been hypothesized that the re-organization of these high-CO2 waters may have played a crucial role in the degassing of carbon dioxide to the atmosphere during the last deglaciation. It is highly likely that this reorganization would leave some imprint on paleo-productivity records. Several studies of deep-sea cores from the North Pacific ocean have demonstrated that export production was greatly reduced during glacial periods. We used 230Th- normalization to reconstruct particle flux to an intermediate depth sediment core. RC10-196 (54.7°N, 177.1°E, 1007 m). This core has sedimentation rates ranging from 3-6 cm/1000 yr during the last 30,000 years. Lithogenic fluxes peak during the height of the last glacial period, while opal and carbonate fluxes peak abruptly on the deglaciation (at approximately 15,000 years ago). The timing of these increases in biogenic fluxes are consistent with those observed in deeper North Pacific cores, and are coincident with abrupt increases in North Pacific temperatures observed at 15,000 years ago. This correlation with temperature and the lack of correspondence between biogenic and lithogenic fluxes suggest that physical conditions may override the role of dust inputs in controlling productivity changes during the last glacial- interglacial transition. These data will be placed in the context of other records generated for the North Pacific Ocean, as part of a regional biogenic flux data synthesis.
PP41D-1495
Decreased oxygen concentration in the glacial abyssal subarctic Pacific – evidence for enhanced oceanic carbon sequestration during cold periods?
Measurements of benthic foraminiferal Cd/Ca have indicated that the glacial-interglacial change in deep North Pacific PO4 concentration was minimal, which has been taken by some workers as a sign that the biological pump did not store more carbon in the deep glacial ocean. Here we present sedimentary redox- sensitive trace metal (Mn, Mo, U) records from ODP Site 882 (NW subarctic Pacific, water depth 3,244 m) to make inferences about changes in deep North Pacific oxygenation – and thus respired carbon storage - across glacial Terminations I and II. These observations are complemented with 230Th-normalized biogenic barium and opal measurements as indicators for past organic carbon export to separate the influences of deep-water oxygen concentration and sedimentary organic carbon respiration on the redox state of the sediment. Our results suggest that the deep subarctic Pacific water mass was depleted in oxygen during glacial maxima, though it was not anoxic. We reconcile our results with the existing benthic foraminiferal Cd/Ca by invoking a decrease in the fraction of the deep ocean nutrient inventory that was preformed, rather than remineralized. This change would have corresponded to an increase in the deep Pacific storage of respired carbon, which would have lowered atmospheric carbon dioxide (CO2) by sequestering CO2 away from the atmosphere and by increasing ocean alkalinity through a transient dissolution event in the deep sea. Preliminary calculations show that the magnitude of change in preformed nutrients suggested by the North Pacific data would have accounted for a substantial portion of the observed decrease in glacial atmospheric pCO2.
PP41D-1496
The Influence of LGM Iron Inputs on the Marine Biological Pump
Dust deposition to Antarctica was substantially higher at the Last Glacial Maximum (LGM), and model results suggest that globally dust deposition to the oceans increased approximately fourfold, with larger increases at high southern latitudes. Thus, atmospheric iron inputs to the oceans were greatly increased. However sedimentary iron inputs decreased due to the lower sea level. We examine the impacts of glacial iron inputs from both sources on marine biogeochemical cycling at the LGM, through simulations with the Biogeochemical Elemental Cycling (BEC) ocean model. The BEC model includes several key phytoplankton functional groups (diatoms, diazotrophs, picophytoplankton, and coccolithophores) and multiple potentially growth-limiting nutrients (nitrate, ammonium, phosphate, silicate, and dissolved iron). The increased LGM iron inputs led to increased production and export in the classic High Nitrate, Low Chlorophyll (HNLC) regions known to be iron-limited today. In the simulations there is also an indirect response to increased iron inputs in some low-nutrient, subtropical regions, where iron is the limiting nutrient for the diazotrophs. Increased iron fuels additional nitrogen fixation by this group, reducing the nitrogen stress of the larger phytoplankton community, and increasing production and export. The strengthening of the biological pump due to the direct (HNLC) and indirect (N fixation) pathways substantially lowers atmospheric CO2 concentrations.
PP41D-1497
Carbon-13 Observations Necessitate Reduced Ocean Ventilation at the Last Glacial Maximum, but Biogeochemical Processes Explain Most of the Atmospheric Carbon Dioxide Decrease
While the driver of the 80ppm decrease in atmospheric CO2 (pCO2atm) at the last glacial maximum (LGM) is likely oceanic, its attribution remains challenging. Viable hypotheses must explain a ~50ppm decline; since subsequent deep ocean carbonate compensation reduces pCO2atm further. Additionally, numerous observations have highlighted a 50% increase in the surface-deep gradient in the isotopic composition of oceanic dissolved inorganic carbon (δ13CDIC) at the LGM. As such, the widely measured variability in δ13CDIC is an important constraint on hypotheses that seek to explain LGM pCO2atm. Here we show that by using a state-of-the-art global ocean general circulation and biogeochemical model with different LGM circulation schemes (generated by a fully coupled ocean-atmosphere model under LGM forcing) and LGM dust input, only reduced ocean ventilation can be reconciled with 133 LGM δ13CDIC observations. However, while ocean circulation provides an explanation for a variety of paleoceanographic proxies, it explains less than 10% of LGM pCO2atm. Nevertheless, the impact of increased dust deposition, as well as changes in phytoplankton stoichiometry, are amplified by a favourable circulation to reduce pCO2atm by up to 25ppm, or one-half of the required 50ppm glacial-interglacial variability (prior to carbonate compensation). An analysis of our results that is constrained by two observational datasets (pCO2atm and δ13CDIC) suggests that although two-thirds of the change in LGM δ13CDIC gradient is controlled by ocean circulation, over 90% of the pCO2atm decline is biogeochemically driven.
PP41D-1498
Changes in the Antarctic Oceans biological pump determined from changes in the accumulation rates of deep- and shallow-living radiolarians through the last glacial cycle.
Past accumulation rate changes of deep- and shallow-living radiolarians, signal changes in the flux of organic matter to the deep high latitude oceans (above 45 degrees) of both hemispheres. Glacial age sediments are characterized by high accumulation rates of deep (greater than 200m) relative to shallow-living (less than 200m) radiolarians suggesting a more efficient biological pump, while the reverse is true for Holocene sediments. Today only beneath the highly stratified waters of the Sea of Okhotsk do accumulation rates similar to the high latitude glacial ocean occur. In the Sea of Okhotsk these unusual accumulation rates are caused by a permanently cold upper layer, between 10 and 150m, with low concentrations of radiolarians while higher concentrations of deep-living species are found in warmer waters below. This sea is probably the best modern analogue for the temperature and salinity structure of the glacial Antarctic Ocean. In a central Indian Ocean core, south of the Antarctic Polar Front, accumulation rates of shallow-living species increase by a factor of five across the Pleistocene-Holocene boundary while the accumulation rates of deep-living radiolarians are reduced to a quarter, indicating a reduction in the efficiency of the biological pump. Within the last glacial cycle (130K yrs.) multiple shallow-living species accumulation rate maxima (low stratification relatively inefficient biological pump) correlate with temperature maxima north of the Polar Front, have a pronounced 'saw toothed' pattern and can be correlated with Vostok ice core temperature maxima. These changes of shallow-living radiolarian accumulation rates most likely reflect changes in surface water stratification and stability for all are cold water, polar species. The establishment of a well stratified, nutrient depleted, cold surface layer in the glacial Antarctic, similar to that in the modern Sea of Okhotsk would have reduced shallow species production, increased deep-species production and contributed to the glacial drawdown of atmospheric carbon dioxide. If so then important to the process were nonlinearities of both seawater's density increase and the metabolic rates of heterotrophic organisms, as the freezing point of seawater is approached.
PP41D-1499
Long-term variability of iron supply, marine export production, and sea surface temperature in the subantarctic Atlantic, implications for atmospheric CO2
Paleoclimatic reconstructions have provided a unique dataset to test the sensitivity of climate system to changes in atmospheric CO2 concentrations. However, the mechanisms behind glacial/interglacial (G/IG) variations in atmospheric CO2 concentrations observed in the Antarctic ice cores over the last 800 ky are still not completely understood. Here we present a multiproxy dataset of sea surface temperatures (SST), dust and iron supply, and marine export production, from the marine sediment core PS2489-2/ODP Site 1090 located in the subantarctic Atlantic (SA). This dataset allows us to evaluate various hypotheses focussing on the role of the Southern Ocean (SO) in modulating atmospheric CO2 over the last 800ky, and provides new information on SST, dust, and export production back to the Pliocene. The close correlation observed between iron inputs and marine export production in our record suggests that the process of iron fertilization has been a recurrent process operating in the SA over the G/IG cycles of the last 1.1 My. However, our data indicates that marine productivity in the present Subantarctic Zone can only explain a fraction of atmospheric CO2 changes occurring at glacial maxima in each glacial stage. Moreover, the good correlation of our SST to the EPICA Dome C records (EDC) temperature reconstruction over the last 800ka, suggest that physical processes, possibly related to changes in Antarctic sea-ice extent, surface water stratification and westerly winds position have also played an important role in modulating atmospheric CO2 over the last 800ky. On the long-term, our paleo-SST record reveals a major cooling event around 1.2-1.5 Ma that may have caused a profound impact on atmospheric CO2 and hence in the transition to a 100 kyr world during the Middle Pleistocene Climatic Transition.
PP41D-1500
Ocean Carbon Transport and the Sensitivity of Atmospheric pCO2 to the Biological Pump
Due to the high concentration of unused nutrients in the Southern Ocean, considerable emphasis has been placed on the role of high-latitude biology in regulating atmospheric CO2. Increased biological production in the Southern Ocean, due perhaps to increased aeolian iron input, has been hypothesized to be responsible for part of the observed draw-down of atmospheric CO2 during glacial periods. Simple box models can reproduce glacial-CO2 concentrations by increasing the efficiency of nutrient utilization in the polar box, but OGCMs have had less success in reproducing glacial CO2 with similar experiments. In trying to understand the different high- versus low-latitude sensitivities of box models and OGCMs it has often been assumed that a strong sensitivity for the solubility pump translates into a correspondingly strong sensitivity for the biological pump. Here we show that because sinking particles deliver regenerated carbon to the ocean interior with a spatial distribution that is different from the one associated with the delivery of preformed carbon by the circulation, the regenerated and preformed carbon pools have different mean residence times in the ocean interior. Consequently different aspects of the global ocean circulation control the strength and sensitivity of the solubility and biological pumps. We have developed Green functions for partitioning the ocean's carbon inventory in terms of whether it entered the interior from high- or low-latitudes and whether it was transported into the ocean interior as preformed carbon, particulate organic carbon (POC) or particulate inorganic carbon (PIC). Using these Green functions we have developed a diagnostic formula that allows us to identify and quantify the separate processes that govern the sensitivity of atmospheric pCO2 to perturbations in the high- latitude biological production. These mechanisms include: (i) the direct changes in the flux of PIC and POC associated with a change in high-latitude biological production, (ii) the changes in low-latitude biological production in response to the global redistribution of nutrients, (iii) changes in the air-sea carbon flux caused by changes in the sea surface chemistry associated with the redistribution of carbon and alkalinity, and (iv) changes in the air-sea pCO2 disequilibrium. Experiments with a three-box model and an OGCM suggest that while both models can have similar overall sensitivities, the relative contributions of different mechanisms can be very different for each type of model.
PP41D-1501
Replumbing of the Biological Pump caused by Millennial Climate Variability
It has been hypothesized that millennial-timescale variability in the biological pump was a critical instigator of glacial-interglacial cycles. However, even in the absence of changes in ecosystem function (e.g. due to iron fertilization), determining the mechanisms by which physical climate variability alters the biological pump is not simple. Changes in upper ocean circulation and deep water formation have previously been shown to alter both the downward flux of organic matter and the mass of respired carbon in the ocean interior, often in non- intuitive ways. For example, a reduced upward flux of nutrients at the global scale will decrease the global rate of export production, but it could either increase or decrease the respired carbon content of the ocean interior, depending on where the reduced upward flux of nutrients occurs. Furthermore, viable candidates for physical climate forcing are numerous, including changes in the westerly winds, changes in the depth of the thermocline, and changes in the formation rate of North Atlantic Deep Water, among others. We use a simple, prognostic, light-and temperature-dependent model of biogeochemical cycling within a state-of-the- art global coupled ocean-atmosphere model to examine the response of the biological pump to changes in the coupled Earth system over multiple centuries. The biogeochemical model explicitly distinguishes respired carbon from preformed and saturation carbon, allowing the activity of the biological pump to be clearly quantified. Changes are forced in the model by altering the background climate state, and by manipulating the flux of freshwater to the North Atlantic region. We show how these changes in the physical state of the coupled ocean-atmosphere system impact the distribution and mass of respired carbon in the ocean interior, and the relationship these changes bear to global patterns of export production via the redistribution of nutrients.
PP41D-1502
Foraminifera-Bound Nitrogen Isotope Evidence for Reduced Nitrogen Fixation in the Atlantic Ocean During the Last Ice Age
In the organic matter bound within planktonic foraminifera shells in Caribbean Sea sediments, the 15N/14N ratio decreases from the last ice age to the current interglacial, while bulk sediment 15N/14N shows minimal glacial/interglacial change. The foraminiferal data are best explained by less nitrogen (N) fixation in the Atlantic during the last ice age, leading to higher nitrate 15N/14N in the Caribbean thermocline. In addition, during the last ice age, coherent 15N/14N differences among species with different depth preferences appear to record a sharper depth gradient in particulate N 15N/14N, also consistent with less N fixation. The reconstructed increase in N fixation at the end of the last ice age is most likely a response to the previously recognized deglacial increase in global denitrification, in the sense that would have worked to balance the ocean N budget and reduce variability in the ocean N reservoir, global productivity, and atmospheric carbon dioxide.
PP41D-1503
Inter-ocean switching in tropical thermocline nutrient supply: another way to vary the efficiency of the marine biological pump
Variations in atmospheric carbon dioxide content have been associated with glacial-interglacial cycles, and could help drive them. The marine biological pump has been implicated as causing up to one half of the carbon dioxide variations observed. Different mechanisms have been proposed to account for a positive feedback link between marine biological production and climate change, mediated through regulation of atmospheric carbon dioxide content. The more recent generation of these mechanisms focuses on the efficiency of export to the deep ocean, and on the flux ratio. In modeling exercises, increasing efficiency and the ratio (organic carbon to carbonate) can account for glacial period draw-down of atmosphere CO2 concentrations. While much of the thinking on the biotic pump has been focused on the polar regions, modeling of export and the flux ratio show that variation in tropical production can be important to the draw- down process (e.g. silicate leakage hypothesis). We present a new mechanism which could influence overall oceanic export and flux ratio. Combining records of carbon isotopes from the tropical thermocline and deep ocean labile organic carbon flux, we show that over the glacial cycle the supply of nutrients to the tropical Atlantic and Pacific Oceans varied inversely, with increases/decreases to the Atlantic/Pacific during the colder periods and the reverse during warmings. This, combined with greater export efficiency in the Atlantic (as well as Ice Age increases in the flux ratio in both oceans), yielded greater CO2 drawdown during glacials than interglacials. The timing of thermocline shifts in nutrient content, and tropical productivity, matches with that required to account for observed changes in atmospheric CO2. It seems likely that combined changes in a number of productivity regulating factors during glacials created the positive feedback to climate provided by the biotic pump.