PP44B-01 INVITED
Greenland and Antarctic ice core records of millennial scale variability
Greenland ice cores drilled at Summit and NorthGRIP provide high resolution records of north Atlantic millennial scale variability spanning the last glacial-interglacial cycle. We will review the Greenland temperature records of millennial scale variability and discuss the magnitude and pacing of the Dansgaard-Oeschger (D/O) events, using records of water stable isotopes and firn gas fractionation method estimates. Owing to the rapid methane fluctuations associated with D/O events, ice core records from Antarctica can be synchronised with Greenland records. High resolution analyses conducted on the EPICA Dronning Maud Land ice core have indeed revealed that each D/O event has an Antarctic counterpart (EPICA comm.. members, Nature, 2006). We will use the longest available Antarctic record obtained from the EPICA Dome C ice core to discuss the Antarctic stable isotope and methane evidence for D/O type events over the past 800 ka.
PP44B-02
Deepening of the mixed layer during ice-rafting events: implication for the Meridional Overturning Circulation
Proxies from Greenland ice cores and North Atlantic marine sediment cores document repeated abrupt climate swings of a few decades to millennia, including Heinrich massive ice-rafting and Dansgaard-Oeschger ice- rafting events, which disrupted ocean circulation, resulting in severe cooling. In this study, we used sediment samples from cores CHN82-20 (43.31°N, 29.52°W; 3020 m wd) and Integrated Ocean Drilling Program Expedition 306 Site U1313 (41°N, 32.95°W; 3426 m wd) from the subtropical North Atlantic Ocean to document the impact of these events in the upper water masses. We have measured δ 18O in four planktonic foraminiferal taxa Globigerinoides ruber (white), Globigerina bulloides, and Neogloboquadrina pachyderma (dextral and sinistral) which dwell in the mixed-layer and thermocline waters. We found similar δ 18O values from mixed-layer and thermocline-dwelling planktonic foraminifera during Heinrich events and large amplitude ice-rafted detritus (IRD)-events in Marine Isotope Stage 12 suggesting the deepening of the mixed-layer. These data further suggest that the planktonic foraminiferal taxa calcified their shells at similar temperatures in a homogenized water column. Similar deepening of the upper water masses can be seen from the northern margin of the IRD-belt, implying that this homogenization of water masses was widespread in the North Atlantic. We suggest that an increase in storminess during Heinrich ice-rafting and large IRD-events intensify vertical mixing of meltwater from ice-rafting in the upper ocean.
PP44B-03
Threshold Behaviour of Millennial-Scale Variability in North Atlantic Deep Water Hydrography During the Last 1.1 Ma: Evidence From XRF Core Scanning at IODP Sites U1313 and U1314
IODP Sites U1313 (41.0°N, 32.9°W, 3425 m wd) and U1314 (56.4°N, 27.9°W, 2820 m wd) provide new deep water sedimentary archives that allow to determine the long-term evolution of millennial- scale variability in ice sheet stability and thermohaline circulation. For both sites age models are derived by orbital tuning of high resolution physical property records. XRF - core scanning measurements of major elements (e.g. Al, K, Ca, Ti, Fe, Sr) performed at 1-cm resolution are used to track changes in biogenic and terrigenous sedimentation during the Pleistocene. Site U1314 (southern Gardar Drift) exhibits pronounced sub-orbital scale variations in CaCO3 content and terrigenous provenance throughout the last 1.1 Ma. The amplitude of these millennial-scale changes is often ~40 to 70% of the maximum glacial/interglacial range. Low K/Ti ratios are typical for warmer intervals with sediment delivery mainly through the ISOW from the Icelandic basaltic province. On the other hand glacials and stadials are characterized by higher K/Ti indicating a dominance of acidic sediment sources which were likely transported by enhanced NEADW/LDW flow. Enhanced millennial scale variability in siliciclastic supply and deep hydrography at Site U1314 occured during ice growth phases when global benthic δ18O is within a threshold range of ~4.15 to 4.65 ‰. On the other hand peak glacial and interglacial intervals reveal very low variance in the sub-Milankovitch frequency band. A significant reduction in sub-orbital deep water variability occurred at ~450 kyr, a time when oceanographic boundary conditions underwent a fundamental change to more vigorous NADW production. At Site U1313 (northwest of the Azores) sedimentation rates are lower (~4.7 cm/kyr) then at Site U1314 but the sedimentation is very uniform such that spectral analyses are less biased by changing sedimentation rates. Preliminary spectral analyses reveal that significant suborbital variance with periods typical for the last 100 kyr can be detected with confidence for older intervals also at this location.
PP44B-04 INVITED
Millennial-scale climate variability in the North Atlantic beyond the last glacial cycle inferred from IODP Site U1308 (re-occupation of DSDP Site 609)
DSDP Site 609 has featured prominently in studies of millennial-scale climate variability of the last glacial cycle, including the recognition of Heinrich events and their correlation to Greenland Ice cores. During IODP Exp 303, Site 609 was re-occupied and a complete composite section was obtained to 247 mcd at Site U1308 (corresponding to ~3.1 Ma). Core logging and scanning XRF data at Site U1308 were compared with existing high-resolution petrologic counts of ice-rafted detritus (IRD) at DSDP Site 609 (measured by Gerard Bond for the last glacial period) to develop rapid, non-destructive proxies for tracking various components of IRD. We found that magnetic susceptibility is a good proxy for total lithic grains per gram in the >150 um sediment fraction (i.e., IRD) and Ca/Sr is a sensitive indicator of detrital carbonate. A time scale was constructed for the last ~1.4 Ma at Site U1308 by correlating the benthic oxygen isotope signal in the upper 100 mcd to the stacked record of Lisiecki and Raymo (2005). IRD events rich in detrital carbonate (i.e., Heinrich-like events) first appeared in the sedimentary record of Site U1308 at ~650 ka during Marine Isotope Stage (MIS) 16. Wavelet analysis of the isotopic and elemental data reveals an abrupt increase in the amplitude of the 100-ka cycle at this time, suggesting that detrital-carbonate-rich IRD events were limited to glacial periods of the "100-k world" when continental ice volume was large. At site U1308, detrital-carbonate-rich IRD events tend to occur towards the latter part of glacial stages and almost always occur at glacial terminations. MIS 6 and 14 are exceptional in that they are the only two glacial stages where detrital carbonate events are absent at Site U1308. IRD events also occurred in glacial periods prior to 650 kyrs but the lithic fraction did not contain large amounts of detrital carbonate as during Heinrich events. We observe that most of the major peaks in IRD (magnetic susceptibility) at Site U1308 coincide with a minimum values of benthic d13C, supporting a link between iceberg discharge and weakening of meridional circulation. The long Pleistocene record from Site U1308 generally supports the hypothesis that as Northern Hemisphere ice sheets grew larger in the late Pleistocene (especially after 650 kyrs), they became more prone to instability. The meltwater produced from the production of abundant ice bergs decreased sea surface salinity, enhanced stratification and stability of surface waters, and diminsihed rates of meridional overturning circulation. As a result, the climate system of the North Atlantic became more prone to abrupt change and variability in the suborbital band increased.
PP44B-05
The Pleistocene Labrador Sea, northwest North Atlantic (IODP Sites 1302/03, 1305 and ODP Site 646) with highlights on warm interglacials
Sites cored during expeditions 105 and 303 off southern Greenland (IODP 1305, ODP 646) and eastern Canada (IODP 1302/1303) are ideally located for documenting the climatic and glacial history of adjacent lands in addition of providing information on thermohaline conditions in the northwest North Atlantic, notably with regard to convection in the Labrador Sea. Multiproxy studies (stable isotopes, pollen, dinocysts, clay minerals, radiogenic isotopes, U-series) in Pleistocene sequences at Sites 646 and 1305 show that different conditions prevailed during each interglacial with respect to sea-surface conditions, deep-current, and/or terrestrial vegetation on adjacent lands. For example, clay mineral assemblages and Nd isotopes indicate that sedimentary supplies during the recent interglacials (marine isotope stages (MIS) 1, 5e, 7 and 9, notably) were very different than those of previous interglacial stages, which were characterized by a sedimentary source not yet identified, possibly related to soils developed over volcanic rocks from a largely deglaciated Eastern Greenland. Pollen analyses also suggest that dense vegetation with distinct characteristics developed over Greenland during some interglacials of the mid-Pleistocene when the Greenland ice sheet volume was significantly reduced. In particular, rich pollen assemblages dominated by spruce are recovered in sediments of MIS 11, which suggest the development of boreal forest vegetation on southern Greenland, then ice-free. In addition to analyses of long time series, special attention was paid to the last interglacial (MIS 5e) at Sites 1302/1303 and 1305, during which particular high sea-surface temperatures were recorded (up to 5° C warmer than present) in spite of the absence of Labrador Sea water formation. In addition, sedimentation rates during during MIS 5e and 11, when compared to those of MIS 1, as well as the corresponding isotopic composition of planktic and benthic foraminifers, lead to even question the contribution of Denmark Strait Overflow Water (DSOW) during these interglacials. The modern Atlantic meridional overturning, characterized by Labrador Sea Water formation, due to winter cooling and convection in this basin, and by intense DSOW formation, seems mostly exclusive to the present interglacial.
PP44B-06 INVITED
North Atlantic Overflows and Outflows
Past ocean circulation changes may be expressed in water mass tracers and dynamical proxies. We have examined a combination of the 231Pa/230Th and sediment grain size indicators with nutrient proxies to reconstruct the evolution of the North Atlantic meridional overturning circulation since the last ice age. Deep-sea core locations in the western basin monitor the export of northern-sourced waters, especially along the deep western boundary. Cores from a range of water depths allow reconstruction of both intermediate and deep waters. During the last glacial maximum, northern source waters shoaled relative to the modern circulation and may not have overflowed from the Nordic seas, but they appear to have been well ventilated and part of a vigorous overturning cell. Intermediate depth sediments south of Iceland display a similar grain size spectrum as the modern, and sedimentary 231Pa/230Th at shallower sites was generally reduced, in contrast to higher ratios at deep sites. During the deglaciation, large-amplitude variability characterized the climate as well as the meridional overturning circulation at all depths. The most prominent excursion to high 231Pa/230Th indicating reduced export occurred at deep sites along the western boundary. The circulation rebounded at all depths at the onset of the Bolling Allerod, and declined again during the Younger Dryas. Only smaller oscillations occurred once the modern configuration of overflows from the Greenland and Norwegian Seas was established at the start of the Holocene. Comparisons between the respective proxies reveal the interplay between the mean flow and bottom currents sourced in the north and the south.
PP44B-07
Millennial-scale deep ocean ventilation and sea-surface variability during the last four glacial cycles: a new assessment for the Northern Hemisphere ice-sheet growth
During Expedition 306 of the Integrated Ocean Drilling Program (IODP), site U1313 (41°N, 32.57°W, 3425 m wd) was drilled at the former DSDP site 607 to recover longer records of abrupt climate change. Here we report for the first time continuous millennial-scale, with a mean temporal resolution of ~200-500 yr, variability records of ice-rafted detritus (IRD), North Atlantic Deep Water (NADW) formation, and sea-surface properties during the last 460 ka from the subtropical North Atlantic Ocean. X-ray fluorescence (XRF) data show many-fold increase in Fe and Ti concentrations during the glacial compared to the warmer periods. We attribute such increases in Fe and Ti concentrations mainly to sediment remobilization from the continental shelves around the circum-Atlantic Ocean. IRD data demonstrate an increase in magnitude during the last glacial cycle and Marine Isotope Stage (MIS) 12 in comparison to the previous three glacial cycles. Most of the high IRD peaks are comprised of detrital carbonate grains, suggesting that they were mainly derived from the Laurentide ice- sheet. Enriched δ13C values in \it Cibicidoides wuellerstorfi were measured during MIS 3, 5.1, 7.1, and 11.3 whereas the lightest values were observed in MIS 4, 6.4, 8, 9.2, 10 and 12. Moreover, the δ13CCw data show progressive enrichment from MIS 12 to the Holocene suggesting waning influence of southern source waters relative to NADW at the site. The millennial-scale δ13CCw variability is not always tied to IRD-events. The abundance of \it U. peregrina is higher in MISs 3, 4, 5d, 6, 8, 10 and 12 whereas the \it C. wuellerstorfi is higher during the interglacial with some exceptions. Mg/Ca-derived sea-surface temperatures (SSTs) in \it Globigerina bulloides are not-in-phase with the June 21st insolation at 65°N. This relationship of Mg/Ca-SST with solar insolation is consistent with the foraminiferal transfer function-based SST.
PP44B-08 INVITED
Millennial-scale climate changes: linkages between low and high latitudes
Millennial-scale climate changes in the North Atlantic realm are thought to be associated with a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), a cooling of the North Atlantic Ocean and a warming of the South Atlantic Ocean (the "bipolar seesaw" hypothesis). A closer look at multi-proxy data from marine sediments off the coasts of tropical and subtropical South America and Africa reveals a more complex picture. During the Heinrich 1 and Younger Dryas events, the Sahel region suffered from draughts; precipitation decreased in northern South America (e.g.\ in the Cariaco Basin), while it probably increased in parts of Brazil. Furthermore, in the western South Atlantic Ocean, we found a large and abrupt warming only after the Heinrich 1 event, during the transition to the Bølling-Allerød period. We interpret our data with the help of an Earth-system model of intermediate complexity that includes a marine biogeochemical component as well as land-surface and vegetation components. With decreasing strength of the AMOC, evaporation decreases in the North Atlantic Ocean and increases in the South Atlantic Ocean, precipitation decreases over northwestern South America as well as central Africa, and the "plant-functional types"/biomes show a small southward shift in western Africa. At the same time, the east-west slope of the South Atlantic thermocline flattens, consistent with relatively cold central and intermediate waters in the western South Atlantic Ocean. Our aim is to elucidate the processes that cause abrupt climate change, through the analysis of linkages between low and high latitudes in terms of changes in the hydrological cycle and atmospheric dust transport.