PP21C-1434
Seismic and bathymetric evidence for four different episodes of iceberg scouring on the New Jersey outer shelf: possible correlation to Heinrich events
Reanalysis of densely-surveyed chirp seismic data on the outer New Jersey shelf reveal two sets of buried incisions. The incisions are linear to slightly sinuous, oriented ~NE-SW, and exhibit cross-cutting relationships; both sets are interpreted here as paleo-iceberg scours. One set cuts into the R horizon, a regional unconformity that formed ~45-40 ka, and subsequently buried by the outer shelf wedge (OSW). The second forms an erose boundary between a layered, lower unit of the OSW and an upper, transparent unit. The erose boundary scours are deeper and have steeper slopes than the R horizon scours. The latter often exhibit bermed edges, whereas the former do not. These differences in cross-sectional morphology are likely determined by the material properties of the sediments being scoured: sand at the R horizon, and clay at the erose boundary. The orientation of both sets of scours are consistent with iceberg scours in the seafloor of the outer New Jersey shelf seafloor analyzed previously. We argue that Heinrich events, periods of massive iceberg flux into the North Atlantic, represent the most likely times for transporting icebergs to these latitudes. This would imply that southward transport of icebergs in the Western Atlantic was a common characteristic of Heinrich events. Based on stratigraphic relationships, sea level curves and limited radiocarbon age constraints, we tentatively attribute the R horizon scours to Heinrich event H4 (~~37 ka), the erose boundary scours to H3 (~30 ka), and the seafloor scours to H2 (~23 ka) and H1 (~17 ka). Facies changes immediately following the R horizon and erose boundary scours may be related to Heinrich event-forced climate changes.
PP21C-1435
Rapid transmission of Climate Signals from the North Atlantic to the far Eastern Eurasian continent during the Last Deglacial period
Last ice age climate is characterized by millennial scale fluctuations between warm and cold states. Coupled glacial-oceanic interactions as well as atmospheric variations are thought as an amplifier and a transmitter of the rapid climate changes in the hemisphere and/or the entire globe. Here we present geochemical fingerprints of palaeoclimate recorded in deep sea sediments from Japan Sea located far-east of the Eurasian continent along with the modeling results obtained using fully coupled Atmosphere Ocean General Circulation Model (AOGCM) to seek the time scale, magnitude and sequences of climate events transmitting in the northern hemisphere from the North Atlantic. Deep sea sediment cores were taken from the Japan Sea and conducted various geochemical analyses. More than 20 radiocarbon measurements for the last deglacial section were made and consistent results of our age model and occurrence horizons of the wide spread tephra layers confirmed the reliability of the reservoir age estimations. Alkenone based sea surface temperature (SST) estimate indicates more than 5 degree C cooling during the latest Pleistocene and 2 temperature minima were marked at the times of both Heinrich event (HE) 1 and the Younger Dryas (YD) cold events. Onset of the B/A warming in our record seems to be synchronous with the North Atlantic data. Terrestrial biomarkers (C29 and C31 n-alkane) were also measured to monitor atmospheric changes and it reached more than 4 folds enhancements of dust influx during the last glacial period. Distinct peaks of terrestrial biomarkers before the major increase in the n- alkanes attributed to intrusion of the warm current into the basin after the Holocene were seen and they are again coincided with HE and YD. Correlated behavior of n-Alkane fluxes to those two major cold episodes that are independent from rapid sea-level rises (ie. 19ka mwp and Mwp1a) suggested the linkage between the Greenland and the Japan Sea climate via the atmosphere. The experiment designed specifically for reconstructing climate changes due to the freshwater forcing in the North Atlantic region was conducted using AOGCM (MIROC3.2). Almost instantaneous cooling trend is seen in the North Atlantic region after putting freshwater into the ocean between the 50-70 degree N. The initiation of cooler climate was found also in the North Western Pacific regions without any major delays. Consistent climate reconstructions from geochemical observation and AOGCM suggests that the major role played by the atmospheric systems due to changing in planetary wave field to transmit rapid climate signals from the North Atlantic to other part in the northern hemisphere.
PP21C-1436
Climatic Consequences of Barents Ice Sheet Collapse During the Last Glaciation: Comparison With Other Release Locations
An intermediate complexity climate model is used to simulate Henirich event-scale freshwater surges and fluxes of icebergs from the collapse of the Barents Ice Sheet during the last glaciation. The impact on convection and deep-water formation in the North Atlantic is compared to similar simulations from three other possible sources of icebergs: Hudson Strait, Gulf of St Lawrence and Norwegian Channel Ice Stream. We show that freshwater forcings and iceberg surges equivalent to 0.1 Sv from all release locations had a significant impact on the meridional overturning circulation, yet differences exist due to the time taken for icebergs to reach the North Atlantic, with the long trajectory of the European Icebergs resulted in a more gradual and delayed influx of meltwater, and a delayed return to previous conditions following the cessation of the influx. Whereas realistic iceberg surges from the Barents ice Sheet and NCIS do not halt the North Atlantic Meridional Overturning, it is halted for fluxes of 0.3 Sv from the Hudson Strait and Gulf of St Lawrence.
PP21C-1437
Geochemical and Sedimentological Proxies for the Extent of the Greenland Ice Sheet During the Penultimate Interglacial
The greatest uncertainty for future sea level rise is the contribution from the Greenland Ice Sheet (GIS) due to limited understanding of GIS sensitivity to a warming climate. To better understand GIS climate sensitivity, we present new geochemical (Pb-Sr-Nd isotopes) and sedimentological data (% silt) from sediment core MD99-2227 (58°12.64'N, 48°22.38'W, 3460 m of water depth) located on the Eirik Drift. Prior work indicates that this site records the sedimentological record of GIS deglaciation. With our new data, we provide constraints on the aerial extent of the southern GIS during the last (TI, 21-7 ka) and penultimate (TII, 135-117 ka) deglaciations and subsequent interglacials. The % silt record suggests a significantly longer period of terrestrial sediment input during TII than TI (18 kyr vs. 5 kyr) in agreement with prior Ti, Fe and magnetic records. During TII, the radiogenic isotope records indicate that the earlier part of this sediment input (135-125 ka) has an εNd signature consistent with an Archean province (εNd ~- 20) that is likely derived from the southern Greenland Archean block (~64 °N), whereas sediment from 125-117 ka have εNd values of -15. The Sr-Nd-Pb isotope signature of sediment from 125- 117 ka are consistent with a sediment source of Proterozoic or younger affinity. This period of terrestrial sediment flux corresponds with the +6 m sea level high stand suggesting that the greatest GIS retreat occurred between 125-117 ka. If this younger sediment was derived from the Proterozoic Central Nagssugtoqidian Orogenic Belt (~67.5 °N), then the sediment data are consistent with ice sheet models that suggest a 3.5-4.5 m sea level contribution from the GIS to the +6 m sea level high stand. This sustained freshwater flux may have reduced the surface density of the Labrador Sea, explaining the lack of Labrador Sea deepwater formation during this interglacial. Further Pb-Sr-Nd-isotope analyses of the silt fraction of suspended load sediment from Greenland streams will provide new end-member data that will aid in deciphering the provenance of the sediment to MD99-2227, helping untangle the relative proportions of Archean and Proterozoic sediment and thus the aerial extent of the GIS during TII.
PP21C-1438
Consistent Observational and Numerical Modeling Support for Ice Sheet Forcing of DOI event 8
The Binge/Purge (B/P) hypothesis of abrupt climate change (MacAyeal, 1993a,b, Paleoceanography) suggests that an instability and periodic collapse of the Laurentide ice sheet is responsible for the observed abrupt warmings of Greenland (Dansgaard-Oeschger Interstadials or DOIs) through the influence of ice sheet meltwater on ocean circulation and its poleward heat transport. If the B/P hypothesis is correct, then one should be able to use the Greenland paleotemperature data and a climate model to infer changes in global sea level associated with the DOIs. The B/P hypothesis could then be tested by comparing the inferred sea level changes with independent reconstructions based on marine records (corals and sediments). We use stochastic inversion of a seasonal resolving climate model (Schmittner and Stocker, 1999, J. Climate), composed of a zonally averaged ocean model (Wright and Stocker, 1992, J. Geophys. Res.) coupled to an energy and moisture balance model of the atmosphere, to infer the changes in ice sheet meltwater required to explain the Greenland paleotemperature record from 39 ka to 34 ka which include DOIs 8, 7, and 6. We found a compelling consistency between modeled and reconstructed sea level changes for DOI 8 (a particularly long interstadial following Heinrich Event 4), which tends to support the B/P hypothesis. On the other hand, the large estimated uncertainties in the inversion solution and sea level reconstructions do not permit a firm test of the B/P hypothesis for DOIs 7 and 6.
PP21C-1439
Glacial North Atlantic Millennial Scale Ice-rafting Events and Their Influence on Overflow Speed. Did Sea-ice Play a Role as a Source of IRD and Freshwater?
Ice-rafting events in the North Atlantic have been linked to millennial scale climate changes during the last glacial period through their influence on the Atlantic meridional overturning circulation. The detailed, unmixed grain-size record from core DS97-2P from the Reykjanes Ridge (58°56'N, 30°24'W, 1685 m water depth) displays multiple events of enhanced ice-rafted detritus (IRD) input each followed by a decrease in the grain-size of current-sorted sediments, suggesting a direct link between freshwater forcing (ice-rafting) and overflow speed (grain-size of current-sorted sediments). Experimental tests with the particle sizer quantitatively corroborate the grain-size record. Here we focus on part of the record spanning ~30-45 kyr BP to investigate the processes playing at the sea surface during these ice-rafting events. During IRD events the CaCO3-content started to increase and peaked just after the main IRD input. The CaCO3 signal cannot be attributed to dilution by IRD, thus reflecting primary production directly. The anti-correlation with the low resolution relative abundance of cold-water dwelling planktonic foraminifer N. pachyderma s. suggests that the CaCO3-record predominantly reflects temperature variability at or near the sea surface. This is supported by the concurrent shift to lighter δ18O values of N. pachyderma s. and few Mg/Ca paleothemperature reconstructions. Sea surface temperature thus started to increase during most ice-rafting events and peaked afterwards. We hypothesize that this warming is related to the invigoration of the warm North Atlantic Drift. During or just prior Heinrich events (HEs) 3-5 the CaCO3 and δ18O curve deviate, suggesting the input of meltwater. Strikingly, the remainder of the IR events does not show any sign of isotopically light meltwater input. As sea-ice is isotopically indistinguishable from seawater, its melting will not be visible in the δ18O signal. The record thus suggests IRD delivery to the site by sea-ice. Although all IR events affected deep current vigour similarly, processes at or near the sea surface differed markedly between HEs and non-HEs: sea-ice sourced freshwater input during non-HEs with additional glacier-derived freshwater input associated with HEs.
PP21C-1440
Oxygen isotopes of detrital carbonate in Heinrich Layers: Implications for reconstructing surface water δ18O
We measured oxygen isotopes of individual detrital carbonate grains in Heinrich layers at three North Atlantic sites, including Core HU90-013-028 from the Labrador slope, IODP Site U1302/03 at Orphan Knoll, and IODP Site U1308 (re-occupation of DSDP Site 609) in the eastern North Atlantic. Oxygen isotopic values of individual detrital carbonate grains from all Heinrich events at all sites average -5.55 ±1.46 permil (1 σ, n=166). The δ18O of bulk carbonate at Site U1308 records the proportion of detrital to biogenic carbonate and bulk δ18O decreases to -5 permil during Heinrich (H) events 1, 2, 4 and 5 relative to a background value of +1 to 2 permil for biogenic carbonate. Bulk δ18O also decreases during H3 and H6 but only attains values of -1 permil. Bulk carbonate δ18O is a sensitive proxy for detecting detrital carbonate and may be useful for identifying Heinrich layers in cores within and outside the North Atlantic ice-rafted detritus belt that lack conspicuous evidence of Heinrich layers in coarse-grained lithic proxies. Because the δ18O of detrital and bulk carbonate contained in Heinrich layers have very low values, any fine-grained detrital carbonate not removed from the inner test chambers of foraminfera will lower δ18O, perhaps leading to an overestimation of the influence of low-δ18O meltwater during Heinrich events. We assessed this potential problem at IODP Site U1308 by rigorously cleaning specimens of Neogloboquadrina pachyderma (sinistral) by breaking each specimen open to reveal internal chambers, immersing tests in an ultrasonic bath, and repicking clean test fragments prior to isotopic analysis. Broken tests clearly reveal the presence of some fine-grained detrital carbonate filling internal chambers. Comparison of δ18O results from Site U1308 with those obtained by Bond et al. (1992) at Site 609 indicate that for most Heinrich layers, the magnitude of the δ18O decrease is reduced, but not eliminated, when thoroughly cleaned specimens of N. pachyderma are analyzed. Benthic foraminifera may be similarly susceptible to contamination by detrital carbonate. Radiocarbon dates of foraminifera from Heinrich layers may be similarly affected by the presence of detrital carbonate with infinite radiocarbon age. Accurate assessment of meltwater influence during Heinrich events is important for providing boundary conditions for numerical models ("hosing experiments") that seek to evaluate the sensitivity of deepwater circulation to freshwater forcing. We recommend rigorous cleaning and screening procedures for foraminifera used for isotope and radiocarbon analysis in North Atlantic Heinrich layers to eliminate contamination by fine-grained detrital carbonate.
PP21C-1441
Geochemical constraints on North American freshwater discharge to the Northwest Atlantic during the Bølling-Allerød Warm Period
During the last deglaciation, the eastward routing freshwater from retreat of the Laurentide Ice Sheet along with periods of enhanced meltwater discharge presumably caused reductions Atlantic meridional overturning (AMOC) strength. While the timing of freshwater discharge is relatively well constrained during Heinrich Event 1 (H1) (~17.5-16 ka) and the Younger Dryas cold event (~12.9-11.5 ka), the history of LIS discharge to the North Atlantic between these two events remains poorly constrained. Here we present new deglacial (~17 to 13 ka) records of sea surface temperature (SST) (planktonic Mg/Ca) and sea surface salinity (SSS) (planktonic SST-ice volume-corrected δ18O of seawater (δ18Osw) and δ13C) from the Laurentian Fan. The SST record shows similarities to other North Atlantic temperature records with warming following Heinrich Event 1 (H1) and at the onset of the Bølling, with several SST oscillations superimposed on gradual cooling through the Allerød. The SSS dependent δ18Osw record decreases at ~16.5-16 (H1), 14.2-14.0 and 13.6-13.4 ka, with similar decreases observed in the δ13C record. These decreases indicate the arrival of water depleted in 18O and 13C, presumably of terrestrial origin, suggesting increased terrestrially derived freshwater discharge from northeastern North America. The timing of inferred increases in terrestrial freshwater discharge at ~14.1 and 13.5 ka are concurrent with the Older Dryas cold event and a cold oscillation during the earlier part of the Allerød, southward shifts in the Intertropical Convergence Zone and reductions in Southeast Asian Monsoon intensity, as well as two increases in atmospheric Δδ14C. This suggests that these relatively brief freshwater discharge events reduced AMOC strength with attendant affects on climate.
PP21C-1442
Meltwater and Abrupt Climate Change in the Gulf of Mexico During the Last Glacial Termination
During the Last Glacial Termination from 18,000-8,000 cal. yrs B.P., meltwater routing of the Laurentide Ice Sheet (LIS) may have been linked to abrupt climatic events, such as the Younger Dryas. Previous studies show episodic meltwater input from the LIS, via the Mississippi River to the Gulf of Mexico (GOM) several thousand years before the onset of the Younger Dryas until approximately 13,000 cal yrs B.P., when meltwater routing may have switched to a more northern spillway, causing an abrupt change in thermohaline circulation (THC). The exact timing and magnitude of this meltwater input to the GOM is poorly constrained due to the lack of high-resolution data. Also unknown are the detailed relationships between GOM sea surface temperature, sea surface salinity and ice volume, relative to Northern and Southern Hemisphere climate from Greenland and Antarctica ice core records. High sedimentation rates (~40 cm/kyr) from laminated, anoxic Orca Basin core MD02-2550 provide the necessary resolution to assess GOM paleoceanography. Paired Mg/Ca and δ18O values from planktonic Foraminifera species Globigerinoides ruber (pink and white varieties) provide the relative timing of meltwater input and temperature change in the GOM with nearly decadal resolution. δ18Ocalcite results show multiple cool and/or high salinity periods with isotopic excursions of at least 2‰ that coincide with abrupt climatic events in Greenland ice core records, including the Oldest Dryas from 16,200-15,000 cal. yrs B.P. and the Intra-Allerod Cold Period at 13,860-13,560 cal. yrs B.P. Meltwater input to the GOM is seen for several thousand years before the onset of the Younger Dryas with white G. ruber δ18Ocalcite values as low as -4‰. Thirty-three AMS radiocarbon dates and high-resolution δ18O results provide excellent temporal constraints on deglacial climate events, including an abrupt (<200 yrs) cessation of meltwater in the GOM centered at 10,970± 40 radiocarbon yrs B.P., with a δ18O increase of 3.5‰. The δ18Ocalcite of white G. ruber shows a clearly defined Younger Dryas interval (δ18Ocalcite ~ -0.5‰) after the Cessation Event from 13,000-11,700 cal yrs B.P. Pink G. ruber displays a similar cessation of meltwater, yet exhibits a much small isotopic range (4‰ vs. 6‰ for pink and white varieties, respectively) without a fully defined Younger Dryas period. The radiocarbon age of the Cessation Event in the GOM (10,970± 40) coincides with large changes in Δ14C, a proxy for THC strength, as well as the radiocarbon onset of the Younger Dryas in European lake sediments. These results suggest that the onset of the Younger Dryas and other abrupt climate events during the Last Glacial Termination may be linked to a decrease in THC due to a change in meltwater routing of the LIS.
PP21C-1443
Deglacial Diatom Assemblages And Surface Ocean Properties Overlying The Bermuda Rise
Diatom assemblages document the surface hydrographic changes over the Bermuda Rise between 19.2 and 14.5 ka BP. Subtropical warm water diatom species and Chaetoceros resting spores dominate the assemblages, as would be expected from more productive regions in the North Atlantic. Beginning 16.9 ka BP and lasting until 14.6 ka BP, brackish diatoms are common and sometimes accompanied by fresh water diatoms. In general, the contribution of brackish diatoms is coupled with the total diatom abundance. Much of this same interval is known to contain rare grains of Ice Rafted Debris (IRD). The coupling between diatom productivity, indicators of low salinity surface water and IRD suggests that successive discharge of icebergs over the Bermuda Rise might have stimulated the productivity over the site. As the site is distant from the iceberg sources, it is hypothesized that the migration of some icebergs to the subtropics might have been associated with cold-core rings, creating an isolated environment involving melt water enriched in nutrients and supporting the diatom productivity in an otherwise oligotrophic setting. Besides, the major contrast of those isolated environments with the temperatures and salinities with the Sargasso Sea waters might have resulted in turbulent mixing, which is a necessary oceanographic condition to diatom productivity. The relatively high diatom productivity coupled with lower salinity water (culminating around 15.5 ka BP) and its persistence beyond the last significant ice rafting suggests continued injection of low salinity water from the subpolar gyre by cold core rings after the end of Heinrich Event 1. Finally, opal productivity might have increased 231Pa scavenging during H1 and biased the 231Pa/230Th ratio in Bermuda Rise sediments.
PP21C-1444
Laurentide ice sheet discharge linked to North Atlantic deep water reduction 10,200 years ago
The stability of the thermohaline circulation is sensitive to freshwater forcing, but such forcing is rare in the Holocene, except the 8,200 years BP cooling event. Here we report an abrupt freshwater forcing event at ~10,200 years BP from three Labrador Sea and northwest Atlantic sediment cores HU97048-16 (59.42°N, 60.14°W; 1,085 mwd), MD99-2226 (57.04°N, 50.38°W; 3,580 mwd) and HU98039-05 (38.45°N, 49.54°W; 5,232 mwd). The sediment unit in the 10,200 years BP event is characterized by the yellowish to greyish brown high detrital carbonate beds in contrast to the northwestern Labrador Sea Heinrich layers which are identified by the distinctive nepheloid-flow layers deposits. Oxygen isotope data in the planktonic foraminifera Neogloboquadrina pachyderma (sinistral) and high bulk carbonate concentration suggest a fresh water sediment plume was transported southward along the Labrador margin, mixing with the North Atlantic Current. This freshwater forcing event was coeval with the lighter carbon isotope in the epibenthic foraminifera Cibicidoides wuellerstorfi in the deep Labrador basin. Carbonate-rich sediments and shallow-water benthic foraminifera suggest that some sediment was transported hyperpycnally by outburst floods through the Northwest Atlantic Mid-Ocean Channel to distal sites. This evidence for a major freshwater forcing event through Hudson Strait indicates a meltwater trigger for a North Atlantic Deep Water decrease during the 10,200 years BP event. Expression of this freshwater forcing event was found in ice-cores, speleothems and other North Atlantic marine sediments records.
PP21C-1445
An abrupt and prominent climatic reversal at 9.2 ka in the northeastern North America
Continental climate during the early Holocene (from 10 to 7 ka) is characterized by multiple abrupt climatic reversals such as the well-known 8.2 ka event that has been observed worldwide and attributed to the terminal collapse of the Laurentide Ice Sheet (LIS) in the North American continent. However, many episodes of meltwater releases occurred prior to the final collapse of LIS, their impact on the continental climate is much less understood. We present in this paper decadal-scale hydrogen isotopic records of aquatic and terrestrial plant biomarkers from Blood Pond, Massachusetts during the early Holocene. Our isotopic records infer a cooling of 3~4 degree between 9.3 and 9.1 ka against the millennial scale climate background, mainly induced by changes in precipitation seasonality. In comparison, the 8.2 ka event displays smaller amplitude of temperature cooling of 1~2 degree at our southern New England site. We interpret our observed climatic reversal at ~ 9.2 ka as representing increased proportion of winter precipitation in conjunction with a drier and cooler summer, triggered by slowdown in thermohaline circulation as a result of freshwater release from the proglacial lakes. We attribute the difference in climate response at 8.2 ka and 9.2 ka events to the configuration of LIS, with 9.2 ka LIS having a much stronger blocking effect on the moisture from the Gulf of Mexico during the summer. Our data suggest that the seasonality of the precipitation at the southern New England was highly sensitive to meltwater releases, especially prior to the final collapse of the LIS.
PP21C-1446
Changes in Stratigraphy Across the Lake Ojibway Basin, Implications for Lake Drainage Record
Changes in Stratigraphy Across the Lake Ojibway Basin, Implications for Lake Drainage Record Lake Ojibway's formation, sedimentation and drainage history was influenced by the retreat of the Laurentide ice sheet. Eleven Lake cores were taken from a geographic area ranging from the Cochran area in central Ontario 240km south east into western Québec to establish a varve chronology to help assess the timing of its final drainage as it is implicated in the 8200 yr event. Differences in stratigraphy have been found across the basin. In three sites interior to the Cochran re-advance limit (Paulen, 2001) the stratigraphy is: a till unit, at the bottom, grades into an ice proximal varve unit which thins upward and gives way to a pellet unit of fine grained silts and clays above which a finely laminated sediment becomes massive. Outside the Cochran re- advance limit the sequence is: an ice proximal varve unit containing contorted sections above which disturbed sediments become finely laminated or massive until a contact with darker organic "modern" sediments. A sudden drainage event across the entire basin should leave a distinct signature. The new cores east of the Cochran re-advance act as a test and allow the generation of two new hypotheses: (1) the finely laminated to massive units common in both sets of cores below the darker organics is the drainage stratigraphy or (2) the finely laminated to massive units are the post drainage signature representing the settling out after drainage, making the contact between the varved glacial sediments and the finely laminated sediments the drainage signature. This data may also place the Cochran re-advance closer to the drainage event because the stratigraphy between regions is similar above. It is possible that the Cochran could have been the triggering mechanism for the drainage event. If true, it could have ramifications for the size of the basin and geometry of the ice sheet, putting the ice margin further south and making the basin smaller until final collapse.
PP21C-1447
The Glacial Lake Ojibway Varve Stratigraphy and Implications for Final Drainage
Drainage of Lake Ojibway via the Hudson Straits to the North Atlantic Ocean is the leading hypothesized trigger for the 8,250 cal BP abrupt climatic cooling event, but there remain several outstanding questions: 1) when exactly did the lake drain (there are few direct dates and almost none are at 8,250 cal BP), 2) how did drainage occur (was it one or multiple episodes and how long did drainage require), and 3) what was the position of the ice margin during drainage (the ice margin position determines the volume of freshwater released to the Atlantic Ocean.) Sediment records from the Lake Ojibway basin should provide answers to these questions; Lake Ojibway sediments are typically varved and can often be correlated spatially and temporally. Recently recovered lacustrine sediment records from sites farther north than previous studies yield few additional Lake Ojibway varves. A reanalysis of varve thickness records suggests that final drainage was as late as 8,300 cal BP. This stratigraphy relies in part on a yet untested correlation to glacial varves from the Lake Superior basin and associated terrestrial radiocarbon dates. The Lake Ojibway sediment records recovered so far provide no evidence for multiple drainages, but substantial reworking of Lake Ojibway sediments following drainage, and an early Holocene climate that was significantly drier than modern, complicates the post drainage stratigraphy. The short lengths of the varve records recovered north of Cochrane, Ontario suggest that the ice margin at final drainage may have been farther south than commonly depicted. A more southerly ice margin position dramatically decreases the freshwater flood discharge below estimates used in most published attempts to model impact of this event on ocean circulation.
PP21C-1448
New constraints on the hydraulics of the final Lake Agassiz drainage event
Despite the increased attention received in recent years by the '8.2 kyr event', relatively little direct field- evidence has emerged of the timing and hydraulics of the flood event that accompanied the demise of the Laurentide ice-sheet, and that is presumed to have triggered a cessation of the North Atlantic overturning. Here we present new sedimentological and AMS radiocarbon evidence from uplifted flood deposits in the James Bay Lowlands. The flood deposits overlie a set of remnant Lake Agassiz blue-grey varves (which in turn overlie late-glacial Kipling Till). They are overlain by grey marine clay and intercalated silt layers that represent the offlap deposits of the Tyrell Sea. The deposits themselves consist of crudely stratified clay- and silt pebble 'breccias', with variable inclusion of dispersed sand- to cobble sized lithic clasts (drop-stones). The flood sequence culminates in a striking couplet of blue- and (overlying) red- 'pebble breccias', whose contrasting colouration is due to the presence or absence of red clay (as amorphous or rounded rip-up clasts and matrix) and the relative abundance of quartz silt/sand versus dark arenite clasts. Notably, the red clay breccia also includes allochthonous marine benthic foraminifera, probably sourced from pre-Tyrell Sea sediments. The sedimentary character and composition of the flood deposits suggests that the final drainage of Lake Agassiz occurred via rapid yet pulsed flow of sediment-laden water (perhaps controlled by tidal oscillations). This included periods of two-way (possibly estuarine) exchange with the opening Tyrell Sea, allowing marine water and sediment to flow into the James Bay lowlands prior to the establishment of open marine conditions. If the 'red breccia' flood deposits identified here are correlative or genetically associated with the red-bed deposits observed in the Hudson Strait (e.g. Kerwin, 1996; Barber et al., 1999), then a conundrum arises as to the source (or sources) of the red-coloured sediments. One immediate question that arises is: what type of flow could have transported material from the currently accepted Hudson Strait 'red bed' source (the Dubawnt Formation) from northern Manitoba to the James Bay lowlands? Future isotopic fingerprinting may yet resolve, or dispel, this conundrum.
PP21C-1449
Tracking and Dating The Drainage of Lake Agassiz: Re-Examining Sedimentary, Micropaleontological and Palynological Evidence Along the Eastern Canadian Margin and Linking it With the 8.2 ka Event
Large volumes of meltwater and detrital sediments were released during the drainage of postglacial Lake Agassiz in the early Holocene. The release of meltwater from Lake Agassiz could be responsible for the 8.2 ka event recorded in the Greenland Ice Sheet and tentative links with the Preboreal Oscillation have been made. Tentative links between the release of meltwater from Lake Agassiz and the 8.2 ka cooling recorded in the Greenland Ice Sheet, and possibly the Preboreal Oscillation, have been made. The path of the meltwater flow has been the object of debates, some studies showing clear evidence for the drainage of large volumes of meltwater through Hudson Strait, while others fail to show evidence for reduced salinity in sedimentary records from that region. The timing of the meltwater drainage also appears critical when attempting to link it with cooling events and slowing of thermohaline circulation in the North Atlantic. A dozen sedimentological, micropaleontological and palynological records from the Labrador, Northeast Newfoundland and Scotian Shelves were re-examined. Detrital carbonate (DC) beds suggest that plumes of suspended DC sediment were carried through Hudson Strait into the Labrador Current, and as far south as Grand Bank. Two periods of energetic meltwater flow are believed to originate from the Noble Inlet ice advance across the Strait (8.9-8.6 14C ka), and from outburst drainage of glacial lakes Agassiz and Ojibway (7.7-76 14C ka). We also re-evaluated the correction factor for radiocarbon dates from the Labrador Sea to 730 years, based on the age of pre-bomb shells and the increased duration in seasonal sea ice cover in the early Holocene. Onshore-offshore correlations of palynological records help to further validate our correction factors and we can demonstrate that the 8.2 ka cooling can indeed be correlated with meltwater drainage from Lake Agassiz. We have evidence for lower sea surface salinity in the detrital carbonate layers from the various records. Benthic foraminifera show that significant bottom water-mass changes accompanied initial DC deposition, indicating short-lived periods of enhanced flux of the outer Labrador Current that laterally entrained slope fauna into some shelf basins. Dinoflagellate cysts assemblages show increased proportions of Spiniferites elongatus and Impagidinium pallidum in the DC interval, indicating cooler sea-surface conditions. Dinocyst transfer functions allow quantification of the reductions in surface salinity and temperature, and increases in annual ice duration, supporting the interpretation that the Outer Labrador Current was significantly enhanced and cooled by glacial lake outburst flooding. With flux reduction, salinity gradually returned to normal ranges and shelf faunas reappeared.
PP21C-1450
Connecting the Final Drainage of Lake Agassiz to the 8.2 ka Event by Means of a High- Resolution Sea-Level Record From the Mississippi Delta
The catastrophic drainage of proglacial lakes Agassiz and Ojibway (LAO) in the early Holocene is widely believed to have triggered the 8.2 ka cold event. The timing of the LAO drainage, however, remains poorly constrained, which impedes our understanding of the abruptness of this climate change in response to a rapid freshwater forcing. Dating of the LAO drainage in the Hudson Bay area (Barber et al., 1999) yielded an age of 8.47 ± 0.30 ka, thus leaving large uncertainties about its connection with the 8.2 ka event. Here we refine the chronology of the final LAO drainage by constructing a high-resolution sea-level record from the Mississippi Delta with the basal peat approach. Two separate basal peat beds are interpreted to bracket an abrupt sea-level rise around 8.2 ka. The stratigraphically deeper level (Level A) contains a 2 cm basal peat layer that is abruptly overlain by Rangia cuneata– bearing lagoonal muds, representing the onset of an abrupt sea-level rise. The stratigraphically shallower level (Level B) contains a 6 cm basal peat layer that is covered by organic-rich muds, representing the end of the abrupt sea-level rise. Radiocarbon dating of different fractions of macrofossils from each peat layer yields consistent ages. The peat layer from Level A and the base of the peat layer from Level B yield ages of 8257 Cal. yr BP (median age with 2 sigma range of 8198-8270) and 8258 Cal. yr BP (median age with 2 sigma range of 8198-8270), respectively. The indistinguishable ages from these two peat layers, together with the diagnostic stratigraphic features, confirms that sea-level rise was likely instantaneous. This new chronology of the final LAO drainage is similar to the timings of the onset of the 8.2 ka event determined from high-resolution studies of Greenland ice cores and an Omani stalagmite, suggesting an instantaneous ocean-atmospheric system response to freshwater forcing, consistent with model predictions.
PP21C-1451
Model Simulations of the 8.2 ka Event
Since the Atlantic meridional overturning circulation (MOC) plays an important role in determining the climate of the North Atlantic and beyond, it is essential to test the ability of coupled models to predict changes in the MOC and any resulting climate impacts. The 8.2 ka event has been previously targeted for such a test (i.e., Alley, 2003; Schmidt and LeGrande, 2005). The hypothesized cause of the 8.2 ka event is drainage of freshwater from Lake Agassiz into Hudson Bay, which appears to have slowed the MOC and affected climate in various parts of the Northern Hemisphere. The climate system before the 8.2 ka event was generally similar to that of today with a few notable exceptions, including, lower greenhouse gas concentrations, increased seasonality of insolation due to orbital forcing, a remnant of the Laurentide ice sheet near Hudson Bay, and enhanced ice melt runoff down the St. Lawrence River. Using the NCAR CCSM 3.0 model, we have run experiments to determine how adding these different boundary conditions influences the background climate state prior to the drainage of Lake Agassiz into the Hudson Bay. We find these boundary conditions have significant effects on the ocean properties in the North Atlantic. We will discuss the reasons for these changes and their implications for the effects of freshwater forcing at 8.2 ka.
PP21C-1452
Timing and Characterization of the 8.2 ka Event With Atmospheric Gases (O2, CH4, N2, Ar)
Atmospheric gas records from ice cores can complement traditional single-location paleo records, because the atmosphere acts as a nearly-instantaneous integrator of innumerable gas fluxes over a wide (though ill- defined) spatial area. The 8.2 ka event left a clear signature in atmospheric methane and the O-18 of O2 (δ18Oatm), thought to be sensitive to low-latitude terrestrial rainfall (Severinghaus et al., this meeting). The local temperature at Greenland summit (GISP2) is also recorded in the 15N/14N of N2 and 40Ar/36Ar, via thermal fractionation of these gases. Taken together, the suite of gases affords a comparison of the precise relative timing of the event in geographically distant regions. The abrupt cooling in Greenland (3.3±1.1°C from 15N) that marked the onset of the 8.2 ka event was synchronous with a 15% drop in methane sources to within ±4 years (1σ). The total duration of the event was ~150 years in 15N, with the most intense cold period lasting ~60 years. Photosynthesis is ubiquitous in high-rainfall areas, making O2 a more broadly representative indicator than methane, which is produced in more restricted settings (high- productivity anoxic wetlands). The δ18Oatm record is smoothed by the 1000-year turnover time of O2 in the atmosphere, making timing indistinct, but supports the hypothesis of a widespread low- latitude (Asian and N. African monsoon) terrestrial rainfall change associated with the 8.2 ka event. This finding underscores the potential risk of future climate change to rainfall-based agricultural societies in these regions.
PP21C-1453
Identification and Dating of a key Late Pleistocene Stratigraphic Unit in the St. Lawrence Estuary and Gulf (Eastern Canada): Implications for the 400-m Thick Quaternary Sequence at the Former Margin of the Laurentide Ice Sheet
A recently acquired 8-m long sediment core along with high-resolution seismic-reflection and sub-bottom profiler sections allowed the identification, characterization and dating of a widespread seismic unit extending from the head of the Laurentian Channel (Lower St. Lawrence Estuary) to Honguedo Strait (Gulf of St. Lawrence), Eastern Canada. This seismic unit (termed unit 2) is characterized by a series of parallel high- amplitude reflections with thicknesses ranging from 68 m near the head of the Laurentian Channel to <5 m in Honguedo Strait. This seismic unit is generally observed below a very thick unit of postglacial sediments that can reach >250 m in the St. Lawrence Estuary, leaving it very difficult to be reached by conventional coring operations. Here, we reveal how we were able to trace and core this seismic unit in an area where it lays closer to the seafloor near the southern wall of the Laurentian Channel in the Lower St. Lawrence Estuary. This seismic unit consists of two sedimentary facies: sandy mud including ice rafted debris (IRD) underlying faintly laminated to homogenous and plastic silty clays. Based on the sedimentary facies, we interpret the upper clays as ice-distal glaciomarine sediments and the lower sandier sediments as ice- proximal glaciomarine sediments. Seismic unit 2 is highly disturbed by iceberg scouring in the Gulf of St. Lawrence where it is found at shallower depths, indicating that it was deposited during deglaciation. The available AMS 14C dates obtained in the ice-proximal glaciomarine sediments indicate that the lower part of seismic unit 2 was deposited during local re-advances of the Laurentide Ice Sheet margin in the Goldthwait Sea that began at or before the Younger Dryas cold event (11 100 to 10 000 yr BP) and that seismic unit 2 can be used as a chronostratigraphic marker throughout the St. Lawrence Estuary and northwestern Gulf of St. Lawrence. Moreover, because the seismic character of the sediments below seismic unit 2 is not typical of ice-contact or sub-glacial sediments, they represent either a very thick sequence (with very high sediment accumulation rates) of slightly older deglacial sediments or pre-Wisconsinan sediments (i.e., Marine Isotopic Stages >4). Nonetheless, without further evidence, we cannot rule out either one of these alternatives, highlighting the potential of the St. Lawrence Estuary for the recovery of a very detailed sequence of possibly at least the last interglacial/glacial cycle.
PP21C-1454
A Lower Threshold for North Atlantic ice Rafting in the Late Pliocene Than the Late Pleistocene in Response to Low-slung Slippery ice Sheets?
Suborbital variability in late Pleistocene records of IRD and SST in the North Atlantic Ocean appears most extreme during times of enlarged ice-sheets with a well-constrained benthic oxygen isotope-defined 'ice- volume threshold' (δ18OT) for the '100 ka (inter)glacial' world. Information on North Atlantic climate instability for the earlier Pleistocene and late Pliocene is more fragmentary and/or of much lower temporal resolution but the data available suggest similar behaviour with δ18OT remaining more or less constant over the past 2000 or 3000 ka. Here we report new high resolution records of stable isotope change and ice-rafting in the North Atlantic at IODP Site U1308 (a reoccupation of DSDP Site 609) during two glacials key to intensification of NHG (MIS G4, ~2640 ka and 100, ~2520 ka). We find a pattern of suborbital ice-rafting events showing clear evidence of threshold behaviour. However, contrary to previous reports, we find that δ18OT for the late Pliocene is up to 0.45 permil VPDB lower than for the late Pleistocene. Using the Eemian as a deglaciated baseline for the past 3000 ka we evaluate different potential explanations for this finding. We conclude that the observed offset in δ18OT is attributable to the existence of lower-slung Pliocene ice-sheets that flowed more readily than their late Pleistocene counterparts, likely associated with a smaller contemporaneous continental ice budget and/or less isotopically depleted ice.
PP21C-1455
Rapid Centennial Scale Reductions in North Atlantic Deep Water During the Peak of the Last Interglacial Period Recorded in Sediments From the Eirik Drift
One key uncertainty in future climate projections involves changes in the ocean meridional overturning circulation (MOC) and its response to possible increases in freshwater fluxes in a warmer future. The peak of the previous interglacial period (MIS 5e) has a number of elements in common with our projected future—much of the North Atlantic was warmer, the Greenland ice sheet was smaller, and sea level was higher than at present. Thus, a detailed characterization of North Atlantic Deep Water (NADW) properties and circulation during this period is instrumental for constraining the potential sensitivity of NADW to possible future warming and freshening of the North Atlantic. Here we present new high-resolution records of near surface and bottom water properties spanning MIS 5e from a core site on the Eirik Drift south of Greenland (Core MD03-2664 cored during the IMAGES P.I.C.A.S.S.O cruise of the R/V Marion Dufresne of the French Polar Institute). The site lies at 3440m, just below the main axis of the sediment laden Western Boundary Undercurrent (WBUC) and is optimally situated for recording changes in newly formed NADW—the deep, southward flowing branch of the MOC. We use δ13C and δ18O of benthonic foraminifera (C. wuellerstorfi) to reconstruct deep water chemical and physical properties, while near surface water properties are reconstructed using planktonic δ18O records (N. pachyderma sinistral). Due to the extreme accumulation rates at this location our records provide a sub-centennially resolved (~20 yr sample spacing) depiction of deep water and surface climate evolution spanning peak MIS 5e. Our preliminary results show gradually increasing benthonic δ13C values through MIS 5e, not reaching modern NADW values until the second half of the interglacial. Superimposed on this long term trend are distinct large-amplitude (>1 permil) reductions in benthic δ13C. These δ13C decreases are affected rapidly and last no more than a few of centuries before recovering rapidly to background values. The first two, and the most distinct, of these low carbon isotope excursions occur early in MIS 5e and are initiated during the two lowest intervals in our planktonic oxygen isotopic records. Taken together our results suggest that the influence of NADW was briefly but strongly curtailed at our site during the interval of MIS 5e when surface waters south of Greenland were their warmest/freshest.