PP54A-01 16:00h
Evidence for an Open Drake Passage in the Late Middle Eocene
Circulation of the Antarctic Circumpolar Current (ACC) appears to require an unimpeded circumpolar path, thus arguments for initiation of the ACC hinge on the timing of tectonic gateway development in the Southern Ocean. However, timing of the onset of seawater exchange between the Pacific and Atlantic oceans through the Drake Passage is widely debated, with estimates ranging from the late Eocene to the early Miocene. Hiatuses and productivity increases in South Atlantic sediment records imply that ocean currents accelerated during the Eocene and Oligocene, though there is not yet independent and unequivocal evidence for Pacific seawater entering the Atlantic Ocean during the Paleogene. Nd isotopes in fossil fish teeth reflect water mass mixing and are unaffected by changes in temperature, salinity, nutrients, and productivity. We present the first Nd isotope record from the Atlantic sector of the Southern Ocean that covers the entire range of estimates for the opening of the Drake Passage; from ODP site 1090. The record indicates that a water mass with a Nd isotopic composition typical of Pacific seawater appeared in the Atlantic sector at 39 Ma, as illustrated by a rapid transition to radiogenic $\epsilon$$_N$$_d$ values ($\epsilon$$_N$$_d$ = -5.8). Pacific seawater was the only volumetrically significant water mass with such a radiogenic Nd signature during the Paleogene. A second transition to radiogenic $\epsilon$$_N$$_d$ values occurs at 28.5 Ma. Pronounced peaks in productivity at site 1090 coincide with the $\epsilon$$_N$$_d$ transition at 39 Ma. The data support the opening of the Drake Passage to shallow depths at 39 Ma. Moreover, the association of productivity peaks with this event implies that a shallow opening may have enabled a precursor to the modern ACC. The data also support establishment of a deep-water connection at 28.5 Ma. The interpretations imply that tectonically induced changes in ocean circulation preceded large shifts in global climate during the Paleogene. $\epsilon$$_N$$_d$ values during the late Oligocene to early Miocene covary with benthic $\delta$$^1$$^3$C values at this site, confirming the capability of Nd isotopes as a tracer of past changes in ocean circulation.
PP54A-02 INVITED 16:15h
Exploring the Carbonate Production/Dissolution Paradox in the Mid-Bruhnes of the Southern Ocean Using Coupled Records of Biological and Chemical Dynamics
The Mid-Brunhes event, an interval including MIS 9 to 11, is considered one of the warmest during the Pleistocene. This interval is also characterized by generally high carbonate concentrations (and presumably high productivity) with punctuated intervals of high dissolution in the deep ocean. The paradox of high carbonate production and high dissolution in certain intervals might be linked to nutrient available, ocean geochemistry, and upwelling. To explore this situation in the Southern Ocean, we examined calcareous nannofossil and geochemical proxies in order to estimate the degree of dissolution in the calcareous nannofossil assemblages, and examine primary production of these organisms and their relationship with surface water dynamics. In ODP Site 1089 (Cape Basin, Atlantic Ocean) the MIS 9 to 11 is characterized, as in the whole ocean, by the dominance of Gephyrocapsa caribbeanica (a small but robust and well-calcified placolith). Although the record of calcareous nannofossil is continuous, we observed fluctuations in the degree of preservation in the calcareous nannofossil assemblages, particularly periods of high to moderate dissolution during interglacial periods. Conversely, glacials are characterized by moderate to good calcareous nannofossil preservation. During Terminations IV and V we observed maxima in calcareous nannofossil accumulation, but with moderate preservation, indicating that maxima in calcareous nannofossil production occur during Terminations. One potential explanation is related to ocean geochemical mass balances and upwelling dynamics. The geochemical data from this site show that the P/Ti ratio, a paleoproductivity and P mass balance proxy, and Sr concentrations, a nannofossil production proxy, also show peaks during Terminations IV and V-thus, high production from nutrients coincides with high carbonate dissolution. In a coupled biological, chemical, and oceanographic model, low sea levels during glacials result in a basin dominance of the shelf-basin fractionation, with generally high nutrient delivery to the deep sea and increased carbonate production. During termination, the shelf reservoir of nutrients and carbonates begins to increase, but perhaps ocean stratification is diminished with higher upwelling causing greater productivity, with high dissolution the result of a switch in alkalinity with the termination. Basin production then decreases with the increase of nutrient apportionment to the shelves during higher interglacial sea levels.
PP54A-03 INVITED 16:35h
Nutrient Dynamics in the Glacial Southern Ocean
The Southern Ocean (SO) was likely a key contributor to glacial/interglacial climate change resulting from variability either in biogeochemical cycles or ocean stratification and CO2 degassing. Many of the hypotheses to explain the interglacial to glacial difference in atmospheric CO2 suggest that higher glacial dust fluxes led to Fe fertilization of surface waters and increased export production in the SO because the modern-day Southern Ocean is co-limited by both Fe and light availability. Documented Fe sources include upwelled Upper Circumpolar Deep Water, eolian deposition, and melting sea-ice. However, the influence of these sources is variable with latitude and position relative to major frontal zones. Presumably these same Fe sources were important during glacial times albeit at potentially different rates and magnitudes. To examine this effect, we have compared sedimentary Fe fluxes with records of dust deposition. We have found that Fe fluxes are higher than can be explained by eolian deposition, supporting an additional hemipelagic source of Fe to the deep ocean during glacial intervals. Furthermore, different proxies used to evaluate export production and nutrient utilization during glacial intervals yield different and seemingly contradictory results-for example, different studies have concluded that net productivity increased, decreased, and/or remained constant in the SO. Results from phosphorus geochemistry suggest that maxima in export production actually occur at terminations rather than either full glacial or interglacial conditions adding yet another possibility. The focus here will be to try to reconcile the nutrient, export production, and Fe data into a coherent view of nutrient utilization and export production in the glacial SO.
PP54A-04 16:55h
Two Highly-Resolved Geochemical Records of Holocene Variability: A Comparison Between West and East Antarctica
Based on results from ODP Site 1098 in the Palmer Deep, we have a highly resolved record of West Antarctic Holocene climate evolution as traced by terrigenous provenance and accumulation, nutrient utilization, and surface and export production. To date, there are few comparable records from East Antarctica despite its critical role in deep ocean circulation. Here we compare results from a 25 m core recovered from Iceberg Alley on the MacRobertson Shelf of East Antarctica with the results from the Palmer Deep (PD), West Antarctica. Both cores are characterized by laminated diatomaceous muds comprising a two-component system of biogenic opal and terrigenous material. Both of these sedimentary sequences span the Holocene and capture the termination of the deglaciation event at 10kyr. The geochemical parameters we used include XRF scanned and discrete ICP-ES major and trace element analyses, biogenic opal, and carbon and nitrogen isotopes. The PD geochemical records are strongly delineated between the Holocene Climate Optimum (HCO) and the Neoglacial. Both surface and export production are elevated through the HCO, which is indicative if a warmer period. Additionally, the terrigenous provenance record of Al/Ti shows that the source of sediment during the HCO was different than during the Neoglacial. This is likely the result of water mass reorganization as a function of Westerly wind strength. The major events seen in the PD record have also been observed worldwide. In contrast, the Iceberg Alley record does not show a difference between the HCO and the Neoglacial. Rather, there are high-frequency changes that appear to vary over an unchanged average throughout the Holocene. There is, however, a large change in both the absolute values and the amplitude of the signal at 10kyr that signifies the termination of the deglaciation event. Why the Iceberg Alley records do not reflect the same structure exhibited in the PD records remains to be determined. As a first approximation, it appears that the climatic conditions differed between these two sites during the Holocene.
PP54A-05 17:10h
Paleoceanography and Paleoclimatology of the Southern Ocean: A Synthesis of Three Decades of Scientific Ocean Drilling
A Workshop on " Paleoceanography and Paleoclimatology of the Southern Ocean: A Synthesis of Three Decades of Scientific Ocean Drilling" Jan. 21-23, 2005 Boulder, CO Co-Convenors: D. Warnke, G. Filippelli, J.-A. Flores, T. Marchitto One of the greatest successes of the Ocean Drilling Program has been the concerted drilling efforts and exciting results recovered from the Southern Ocean (SO), which has been the focus of ten DSDP/ODP drilling legs. The SO is a critical component in the development and persistence of Antarctic glaciation, is a sensitive mixing pool of global water masses, a locus of high biological sedimentation, and contains high resolution records of climate forcing and response. As such, it is one of the most important oceanographic regions in the world. It is now an important time to mine the rich results from scientific ocean drilling over the past several decades and develop a scientific framework for future ocean drilling in this region. The focus of this Synthesis Workshop will be on the biogeochemical history of the SO, including: * Productivity proxies, rates, records, variations, and role of climate * Sedimentary records of organic carbon, calcium, silica, nutrients, and biogenic proxies: The role of the SO as a biogeochemical sink * Development and dynamics of the APFZ * Thermal structure and evolution of the SO * The role of limiting nutrients The overall goal to integrate the various proxies into a coherent paleoceanographic picture. Such a goal will help to synthesize several decades of scientific ocean drilling in the SO, and will likely bring to the forefront the as-yet-unanswered questions about the biogeochemical history of this important oceanic system. With this goal in mind, workshop participants will submit of a short (~250 word) abstract as the Workshop application, commit to presenting a poster at the workshop based on this abstract, and contribute to one or more manuscripts that will be published after the workshop, likely as a Geological Society of America Monograph. The workshop is open to both U.S. and international scientists. Individuals interested in participating should apply by emailing a brief statement of interest and attaching an abstract (sent as a word document) of the research to be presented to gfilippe@iupui.edu. Although the deadline for submitting abstracts will have passed at the time of the meeting, interested persons should contact gfilippe@iupui.edu immediately to see whether space is still available. Participation will be limited to optimize workshop goals.
PP54A-06 17:25h
The George V Land Continental Margin (East Antarctica): new Insights Into Bottom Water Production and Quaternary Glacial Processes from the WEGA project
The George Vth Land represents the ending of one of the largest subglacial basin (Wilkes Basin) of the East Antarctic Ice Sheet (EAIS). Furthermore, its coastal areas are zone of significant production of High Salinity Shelf Water (HSSW). Piston and gravity cores and high resolution echo-sounding (3.5 kHz) and Chirp profiles collected in the frame of the joint Australian and Italian WEGA (WilkEs Basin GlAcial History) project provide new insights into the Quaternary history of the EAIS and the HSSW across this margin: from the sediment record filling and draping valleys and banks along the continental shelf, to the continuous sedimentary section of the mound-channel system on the continental rise. The discovery of a current-lain sediment drift (Mertz Drift, MD) provides clues to understanding the age of the last glacial erosive events, as well as to infer flow-pathways of bottom-water masses changes. The MD shows disrupted, fluted reflectors due to glacial advance during the LGM (Last Glacial Maximum) in shallow water, while undisturbed sediment drift deposited at greater water depth, indicates that during the LGM the ice shelf was floating over the deep sector of the basin. The main sedimentary environment characterising the modern conditions of the continental rise is dominated by the turbiditic processes with a minor contribution of contour currents action. Nevertheless, some areas (WEGA Channel) are currently characterised by transport and settling of sediment through HSSW, originating in the shelf area. This particular environment likely persisted since pre-LGM times. It could indicate a continuous supply of sedimentary material from HSSW during the most recent both glacial and interglacial cycles. This would be consistent with the results obtained in the continental shelf suggesting that the Ice Sheet was not grounding over some parts of the continental shelf. Furthermore, the comparison of the studied area with other Antarctic margins indicate that, contrary to what happens on the Antarctic Peninsula margin, the relation between the Quaternary sedimentation and the glacial - interglacial cycles are less evident in the lithofacies observed on the continental rise area. This characteristic suggests a different glacial dynamic along the Wilkes Land continental margin that is less sensitive to the small climatic changes, with respect to the western (Antarctic Peninsula) margin.
PP54A-07 17:40h
Luminescence Dating Tests of Detrital Grains From Sediment Traps, Andvord Bay and Brialmont Cove, Antarctic Peninsula
Difficulties (e.g., large, diverse, erratic reservoir corrections) with radiocarbon dating have prompted photonic dating tests on Antarctic glaciomarine sediments. This technique (clock zeroing is based on the last exposure to daylight of quartz and feldspar grains) was applied to modern-age sediments collected in two well-characterized fjords on the western side of the Antarctic Peninsula. Our deployed sediment traps collected approximately 14 months of deposition between January 2002 and March 2003. We conducted a series of multi-aliquot infrared-stimulated luminescence tests on polymineral silt-sized grains to evaluate the extent of the clock-zeroing process in this setting, to provide a foundation for applications elsewhere in the region. Comparisons were made between samples on the basis of seasonal differences and trap depth. Samples were also characterized by SEM-EDX data. Had the sediments been exposed to sufficient sunlight at deposition, sediment grains collected by these traps would exhibit very low equivalent-dose (DE) values. Experiments done thus far have produced high (and variable) DE values for both Andvord Bay and Brialmont cove, which yield erroneously old ages, rather than zero. Apparent ages for Andvord Bay ranged between 42 $\pm$ 9 ka and 60 $\pm$ 11 ka (DE = 63 - 90 Gy), while Brialmont Cove DE values ranged between 14 $\pm$ 3 ka and 39 $\pm$ 8 ka ( DE = 21 - 58 Gy), depending on season and depth of deposition. This indicates that these glaciomarine sediments were unevenly and poorly bleached at deposition. Variations in luminescence traits between seasons and with trap depth may, however, be helpful for studying sedimentation processes in these fjords.