PP41F-01
Milankovitch Forcing of Antarctic Climate in the Ross Sea Sector during the Quaternary
Antarctica is a primary driver of global climate, but to date, there has been no clear evidence that the climate of the Ross Sea exhibits Milankovitich cyclicity. We have been studying the paleomagnetic and environmental magnetic properties of Eltanin 27-21, an 16-meter long piston core collected in the Ross Sea near Cape Adare in 1968 by the USNS Eltanin as part of Operation Deep Freeze. The magnetic polarity of 682 discrete samples from the core was used to develop an age model extending from the top of the Brunhes Chron to the Reunion Subchron. When converted to the time scale, the downcore variations of several environmental magnetic parameters exhibit features consistent with 40-ky cyclicity prior to the mid-Pleistocene transition and 100-ky cyclicity afterward. In order to better understand the nature of these features, we determined whether the environmental magnetic parameters had high or low values at each magnetic polarity boundary and compared that result with the marine oxygen isotope stage associated with the boundary. In general, high magnetic concentrations and finer magnetic grain-sizes occurred in interglacials while low concentrations and coarser grain-sizes occurred in glacials. These conclusions are consistent with findings from environmental magnetic studies of older (pre-Quaternary) sediments from other cores from the Ross Sea, such as CIROS-1 and the Cape Roberts Project. In those cores, increases in magnetic concentration and decreases in magnetic grain-size were associated with warmer, more humid conditions. Our results demonstrate for the first time that during the Quaternary, the climate of the Ross Sea sector was subject to Milankovitch forcing. They also provide a distal record of environmental magnetic paleoclimate proxies in the Ross Sea that can be compared with the proximal records being obtained from the ANDRILL project.
PP41F-02
A New Stable Isotope Record From the Subantarctic Southeastern Pacific
Few stable isotope records exist from the southeast Pacific Ocean due to its remote location, low sedimentation rates, and shallow carbonate compensation depth (CCD). The CCD in the southeast Pacific, however, has been found to be unusually deep (~4750 m), thereby allowing for the preservation of Neogene carbonates at abyssal depths. Herein we present stable isotope results from MV0502-4JC, which was recently recovered from the subantarctic region of the Southwest Pacific (50°20S, 148°08W, 4286 m). Dating the record with radiolarian biostratigraphy, we have generated a benthic stable isotope record back to the Middle Miocene from Cibicidoides spp. and a planktic record from Globigerina bulloides into the Late Pliocene. These stable isotope records, in conjunction with carbonate contents and counts of ice-rafted debris (IRD) and manganese micronodules show the effect of global ice sheet build up on this poorly understood region. A prominent negative δ13C shift of ~1‰ (from ~1 to ~0.2‰) in the benthic record occurs at ~15.3 MBSF (Middle Miocene) as carbonate contents in the core decrease from ~80% to ~20%, and significant changes in the bottom water are indicated by changes in the dominant Cibicidoides spp. at the site from C. robertsonianus to C. wuellerstorfi. Trace amounts of IRD and abundant manganese micronodules also appear at the site at this time. A significant hiatus (from the Late Miocene to the Middle Pliocene) occurs somewhere between 11.4 and 9.8 MBSF. From 9.8 to 8 MBSF (Late Pliocene) benthic δ18O increases and δ13C decreases by ~1‰. Also, IRD increases, manganese micronodules decrease, and there is a dramatic increase in the preservation of planktic foraminifera at the site. A planktic δ13C shift of >1‰ (from 0 to 1‰) occurs at ~5 MBSF (Pleistocene) coincident with a 20% increase in carbonate concentration. After this interval, the variability in both benthic δ18O and δ13C and planktic δ18O increases significantly. We interpret the negative δ13C shift at 15.3 MBSF as the end of the Monterey Carbon Excursion (~13.5 Ma), and the decrease in carbonate observed in MV0502-4JC following this shift may correspond with a widespread hiatus (NH3) associated with the increased presence Antarctic Bottom Water (AABW) in the Pacific and shoaling of the lysocline. The significant hiatus in the Late Miocene-Early Pliocene may also indicate increased AABW formation and Antarctic Circumpolar Current strength during key periods of ice sheet expansion on Antarctica. The ~1‰ increase in benthic δ18O in the Late Pliocene is interpreted as the onset of Northern Hemisphere Glaciation, although the planktic δ18O record oddly does not show this trend. Increased lysocline depth during the Late Pliocene preceded the high (~30 cm kyr-1) sedimentation rates of the Early Pleistocene that may have resulted from reduced bottom water currents and/or increased productivity. The Late Pleistocene was associated with lower sedimentation rates perhaps associated with increased non-carbonate productivity and increased AABW production during glacial intervals.
PP41F-03
An 800 kyr Record of Weddell Sea Paleochemistry from Trace Metal Foraminiferal Proxies
The Southern Ocean has long been recognised as the major player in driving the climate oscillations of the Pleistocene glacial-interglacial cycles. The surface waters of the modern Southern Ocean act as a source of CO2 to the atmosphere because of rapid mixing with nutrient, CO2-rich deepwaters, and incomplete biological utilisation of major nutrients in the surface waters. Hypotheses to account for glacially reduced atmospheric carbon dioxide have focussed on increased stratification of the water column or enhanced nutrient utilisation in the surface waters of the Southern Ocean. A recent modelling study suggests that the critical parameter which determines the oceanic sink of atmospheric CO2 is the preformed nutrient content of the major deepwater mass (AABW), sourced from surface waters south of the Polar front. The lower the preformed nutrients in AABW, the more efficient the biological pump and the oceanic carbon sequestration. Yet the lack of carbonate preservation in this area has largely prevented probing of the paleochemistry of the Southern Ocean with traditional foraminiferal proxies. Here we present an 800 kyr record of trace metal contents from both benthic and planktonic foraminifera from PS1506 (67.8oS 5.8oW, 2426m, Weddell Sea) to investigate the evolution of the water column chemistry and structure during Pleistocene glacial-interglacial cycles. Our preliminary results indicate that at these extreme low temperatures, the downcore variability in all trace metals in foraminifera is largely controlled by variations in the carbonate ion content of the ambient water. For instance, benthic B/Ca and Mg/Ca correlate closely and show a strong 100 kyr cyclicity but with higher values during the glacial periods. The glacial-interglacial amplitude of variation in carbonate ion derived from benthic B/Ca, for the last four glacial cycles is consistent with the 90 ppmv CO2 oscillations, but intriguingly, does not parallel the diminished amplitude of the CO2 oscillations of the previous two glacial cycles as documented in the EPICA ice core.
PP41F-04 INVITED
Sedimentary iron records from the Cape Basin
The Southern Ocean potentially has a significant role in the modulation of CO2 and the global C cycle on glacial/interglacial time scales resulting from variations in primary productivity and nutrient fluxes. For example, the ‘‘Iron Hypothesis'' suggests dust Fe deposited in the Southern Ocean during glacial intervals fueled primary productivity and led to CO2 drawdown. Ice-core records provide clear evidence that the glacial atmosphere was dustier than during interglacial times. There is also unequivocal proof from Fe fertilization experiments that primary productivity in the modern-day Southern Ocean is limited by Fe. Complicating the situation in the Southern Ocean are the substantial hemipelagic sources and the effects of sediment focusing, which dilutes and obscures the eolian signal. As a result, the relationships between Fe fluxes, terrigenous provenance, and any subsequent biological response during glacial intervals remains poorly understood. Here we present records of Fe concentrations (total and reducible) and Fe fluxes to highlight the importance of hemipelagic Fe sources at ODP Leg 177, Site 1089 in the Cape Basin as well as indicators of terrigenous provenance. We find that on average only 7% of the Fe flux to Site 1089 can be accounted for by particle settling, indicating that sediment focusing is responsible for an order of magnitude more Fe than particulate scavenging and settling processes, distribution by surface currents, or eolian deposition. We also find that on average 7% of the Fe content is easily reducible, and we speculate that a significant source of dissolved Fe to bottom waters may come from re- suspended and re-worked bottom sediments.
PP41F-05
Holocene sediment accumulation rates in fjords and bays of Chilean Patagonia and the Antarctic Peninsula
Beginning in 2005 and ending in May of 2007, we completed a series of four research cruises in the fjords and bays of Chilean Patagonia and the Antarctic Peninsula aimed, in part, at determining the style and rate of Holocene sediment accumulation in a range of glacial settings. Our original hypothesis stated that rates of glacial erosion are a function of sliding speed, and are therefore expected to diminish sharply as basal temperatures drop below the melting point. To test this hypothesis, we measured sediment accumulation in tidewater glacier fjords ranging from fast-moving temperate glaciers in Patagonia to slower moving polar glaciers on the Antarctic Peninsula. Four fjords were surveyed in Patagonia ranging from San Rafael fjord in the Northern Patagonia ice field to Marinelli fjord in Tierra del Fuego. The cruises to the Antarctic Peninsula included two SHALDRIL legs during which drill cores recovered 108 m and 80 m of Holocene sediment in Maxwell Bay, South Shetland Islands, and the Firth of Tay in the northwestern Weddell Sea, respectively. An additional nine fjords across the northern peninsula were surveyed with kasten and jumbo piston cores. To date, we have completed nearly 100 radiocarbon dates from fossil carbonate material, both shells and foraminifera, extracted from the sediment cores obtained in these fjords. Our initial results highlight the complexity of the controls on sediment yields and the extreme variability in sediment accumulation amongst fjords. While climate may be the first order control on glacier erosion rates and sediment transport to bays and fjords, several other factors must significantly influence these processes and may mask the broader signal. Our ongoing work with this newly acquired comprehensive dataset is examining additional controls including drainage basin size, precipitation gradient, altitude of the glaciers, and glacial substrate.
PP41F-06
High resolution record of glacial retreat in an Antarctic fjord, Maxwell Bay, South Shetland Islands
Core and seismic data integration in Maxwell Bay, South Shetland Islands documents a minimum age for glacial retreat from the outer fjord of 13100 ± 60 BP or 13860 ± 140 cal BP at 488 m sub-sea. Glacio-marine suspension deposits described as dark greenish gray silty mud with abundant diatoms and occasional very fine sand laminations comprise the majority of the 107.3 m (87% recovery) core. The outer min-basin is divided into10 litho-units and 7 distinct seismic units. The litho-units are defined by sedimentation rate variations, as well as magnetic susceptibility and bulk stable carbon and nitrogen isotope zonations. Sedimentation rates vary by two orders of magnitude from 3 mm/yr to ~ 100 mm/yr. The highest sedimentation corresponds to an interval in the core which contains abundant sand laminations and pebbly mud intervals and likely represents the melt-out phase or period of rapid glacial retreat ~8000 cal BP. Subsequent sedimentation rates range from 3 to 9 mm/yr and reflect continuous high-resolution sedimentation record. The period between approximately 5500 calBP to 2800 calBP, also known as the Mid-Holocene climatic optimum, is marked by higher than average productivity.
PP41F-07
Antarctic Climate-Cryosphere Response to Extreme Orbital Forcing During Marine Isotope Stage 31
Sedimentary records from the Antarctic Margin and Southern Ocean suggest MIS-31 was several degrees warmer than today in high southern latitudes, with evidence of open water in the Ross Sea coeval with an orbital configuration producing anomalously warm austral summers ~1.082 Ma (Scherer et al., submitted). The lack of a buttressing ice shelf during MIS-31 and perhaps other periods of Southern Hemisphere summer warmth has important implications for the ice streams that drain Antarctic Ice Sheets into the Ross Sea. Here, we test the response of the Antarctic climate-cryosphere system (with an emphasis on the Ross Ice Shelf and West Antarctic Ice Sheet) to MIS-31 orbital forcing, using a coupled GCM-ice sheet model with extensions including explicit representations of the coupled sheet-shelf-sediment system and grounding lines. GCM simulations of MIS-31 have already shown that orbital forcing alone (in the absence of CO2 feedback) is capable of raising mean annual surface temperatures by 2-5 deg. C around the Antarctic margin. The simulated (possibly nonlinear) response of grounding lines and the West Antarctic Ice Sheet to warmer sea surface temperatures, the loss of the Ross Ice Shelf, and rising sea levels are compared with well-dated proximal records of MIS-31 from the Ross Sea obtained by the Cape Roberts Project and ANDRILL sedimentary drilling programs.
PP41F-08
Interpreting Si isotope records of diatomaceous opal
In the polar oceans, sinking of diatom frustules can be the main export pathway of nutrient and diatoms can dominate the primary productivity over the annual cycle. As little of the exported organic matter is recorded in the high latitudes, we are left to try and reconstruct change in the polar surface oceans from the biogenic opal record. Whilst organic matter within the diatom frustules can provide carbon and nitrogen stable isotope records, the opal itself records the silicon isotope composition that is a proxy for silicic acid utilization. Carbon, nitrogen and silicon isotopes proxies for paleoproductivity actually record the degree of nutrient utilization (with no estimate of the total fluxes), and often give conflicting results. Here I will present a model for our present understanding of the links between upwelling, nutrient uptake by diatoms, and silicic acid utilization to interpret the Si isotope records of diatomaceous opal from polar regions. A strong interbasinal fractionation of silicic acid concentrations and Si isotope compositions is driven by southern ocean circulation, and further studies of Si isotopes should help us to understand changes in this circulation pattern, even if it does not provide a robust paleoproductivity proxy.