Ice-rafted Detritus Research Articles

A reconstruction of sea surface warming in the northern North Atlantic during MIS 3 ice-rafting events

Marine isotope stage 3 (29–59 kyr BP) is characterised by rapid shifts from cold stadial to warm interstadial periods, which may be linked to changes in the vigour of the Atlantic Meridional Overturning Circulation due to variable freshwater input by melting ice. Here we present two northern North Atlantic multi-proxy records of sea surface conditions that indicate warm (near) sea surface conditions during such ice-rafting events. We infer near surface temperature from planktonic foraminiferal counts, Mg/Ca and oxygen isotopes of left-coiling Neogloboquadrina pachyderma and from calcite content. Temperatures increased during ice-rafting and rose rapidly to interstadial values after ice-rafting ceased. This pattern is clearest during Heinrich Event 4, but also present throughout the other millennial scale ice-rafting events. It indicates that stadials in the Greenland ice-cores are concurrent with a (near) surface warming in the North Atlantic, which was probably restricted to the summer, as winter temperatures must have remained low for sediment-laden ice to reach the site. As similar warming during ice-rafting events is seen regionally in the northern North Atlantic it cannot be explained by a rerouting of the North Atlantic Current. Rather, we attribute it to a shoaling of a warm subsurface water mass that was formed as a result of decreased ventilation of the upper waters and a continued northward subsurface flow of warm water. Planktonic carbon isotopes support this suggestion showing coincident decreased ventilation during deposition of ice-rafted detritus (IRD). The absence of a clear meltwater spike in the δ 18O records during IRD input suggests that besides glacial freshwater, sea ice may have been responsible for the ventilation decrease and associated near surface heat built up. The proposed scenario is in agreement with modelling studies that require the release of heat trapped below the surface to restart the overturning circulation.

Paleoceanographic evolution of North Pacific surface water off Japan during the past 150,000 years

Hydrographic variability in the Mixed Water Region of the Northwest Pacific Ocean at latitudes 35°–40°N, between the Kuroshio Extension and Oyashio Front, causes complex upwelling, leading to large primary productivity and thus great fishery resources. We reconstructed the periodicity of the variability in North Pacific Intermediate Water upwelling and surface ocean hydrography based on the high-resolution analysis of diatom assemblages in seven cores, representing the last 150,000 years. We derived annual sea surface temperatures (SSTs) through a diatom-based proxy ( Td ′). The Td′-derived annual SSTs (°C) are controlled by orbital forcing, and show a reversed saw-tooth in southern cores, in contrast to a normal saw-tooth pattern in the northern cores. Oceanic diatom abundances along the northern margin of the Mixed Water Region are twice times as high as beneath the axis of the Kuroshio Extension, and fluctuated in a revised saw-tooth pattern with higher overall abundances interglacials. After the last deglaciation, annual SSTs declined markedly during Heinrich and Bond events in the northern North Atlantic, when ice-rafted detritus transported by icebergs was abundant. Wavelet analyses of the record of oceanic diatom abundances show significant variability at 2.0-kyr, 2 to 5.6-kyr and 3.2 to 9.6-kyr periods. Wavelet analyses of the annual SST records show significant periodicity at 1.4 to 2.6-kyr, 3.3 to 4.0-kyr, 7.2 to 12.8-kyr cycles.

Late slowdown of the Atlantic Meridional Overturning Circulation during the Last Glacial Inception: New constraints from sedimentary ( 231Pa/ 230Th)

Our study gives new constraints on the response of Atlantic Meridional Overturning Circulation (AMOC) export to various forcings during the Last Glacial Inception. The decay corrected excess sedimentary ( 231Pa/ 230Th) activity ratio (hereafter referred to as (Pa/Th)) has been measured over that period in two deep cores from the Western (SU90-11, 44°04′N, 40°01′W, 3645 m) and Eastern (MD01-2446, 39°03′N, 12°37′W, 3547 m) basins of the North Atlantic. Both records display significant changes despite the relatively short half-life of 231Pa (∼ 32 kyr) compared to the period we investigate. The (Pa/Th) variability does not correlate to changes in local opal flux normalized to 230Th. Moreover, the (Pa/Th) profiles display a high degree of coherency with indirect proxies of AMOC activity such as the benthic foraminifera δ 13C and the mid-latitude summer Sea Surface Temperature in nearby reference cores. These additional pieces of evidence support our interpretation of the (Pa/Th) as reflecting AMOC export. The (Pa/Th) repeatedly underwent rapid changes during the Last Glacial Inception associated with the extension of ice rafted detritus in the North Atlantic, highlighting the control of ice-sheet dynamics through freshwater forcing on AMOC export. AMOC export remains large during periods of ice-sheet growth and its decreases lag the Northern Hemisphere summer insolation forcing. AMOC modulation appears driven by ice-sheet dynamics, itself driven by the seasonal insolation gradient between low and high Northern Hemisphere latitudes and the associated intensity of the meridional oceanic and atmospheric circulation.

Sedimentology and depositional history of Holocene sandy contourites on the lower slope of the Faroe–Shetland Channel, northwest of the UK

Thermohaline ocean currents with near-bed velocities of a few tens of cm s − 1 have the ability to transport and sort sand, which is often deposited in thin sheets referred to as ‘contourite’ sand sheets. We examine a small sandy contourite sheet located in 800–1000 m water depth on the west Shetland slope in the Faroe–Shetland Channel. We describe the sedimentology of the contourite, and the relationship between sediment transport processes and contourite distribution through the last deglaciation and Holocene, based on a set of 17 short cores. Four sedimentological units ranging from very poorly sorted mud at the base (unit 1) to moderately sorted, slightly muddy, fine sand at the top (unit 4) are recognised. Ice-rafted detritus (IRD) occurs in the lower three units, but is absent in unit 4. The upper two units, 3 and 4, make up the contourite sand sheet. Analysis of the sand fraction of units 3 and 4 shows that the mean sand grain size is remarkably constant at around 3 ϕ throughout, except in some unit 3 samples with unusually high IRD content, where the mean can be coarser. IRD content correlates strongly with mean grain size, sorting and skewness throughout unit 3 even when the IRD content is small, showing that IRD input is one of the key processes controlling the sedimentology of unit 3. We conclude that the muddy sand contourites form by incomplete winnowing of the underlying glaciomarine sediments. The mud component is transported with the sand from the same source, either as small particles attached to sand grains or as sand-sized mud aggregates. Thickness maps of units 3 and 4 show that they have different depocentres, with unit 4 displaced northwest (downslope) relative to unit 3. Sedimentological trends in unit 3 are inconsistent and show no consistent transport direction. In contrast, clear across slope trends are seen in unit 4, with weak alongslope trends from the centre to both ends of the elongate contourite body. This suggests that the upper part of the contourite, unit 4, is derived from the downslope reworking of unit 3 sediment. There is no evidence for significant alongslope transport in either unit 3 or unit 4. The patchy occurrence of sandy contourites along the west Shetland slope is interpreted to result from localised export of sand from further upslope. The usefulness of the contourite sand in palaeoceanographic terms is limited. No meaningful dates can be derived from the sediments because of reworking and bioturbation. However, the presence of IRD within unit 3 shows that the Holocene circulation pattern, with strong bottom water flow through the Faroe–Shetland Channel, was established while there was still significant floating ice in the area.

Sediment sources of northern Québec and Labrador glacial deposits and the northeastern sector of the Laurentide Ice Sheet during ice-rafting events of the last glacial cycle

Provenance studies of anomalously high-flux layers of ice-rafted detritus (IRD) in North Atlantic sediments of the last glacial cycle show evidence for massive iceberg discharges coming from the Hudson Strait region of the Laurentide Ice Sheet (LIS). Although these so-called Heinrich events (H events) are commonly thought to be associated with abrupt drawdown of the LIS interior, uncertainties remain regarding the sector(s) of this multi-domed ice sheet that conveyed ice through Hudson Strait. In Northern Québec and Labrador (NQL), large-scale patterns of glacial lineations indicate massive ice flows towards Ungava Bay and Hudson Strait that could reflect the participation of the Labrador–Québec ice dome in H events. Here we evaluate this hypothesis by constraining the source of NQL glacial deposits, which provide an estimate of the provenance characteristics of IRD originating from this sector. Specifically, we use 40Ar/ 39Ar ages of detrital hornblende grains in 25 till samples distributed along a latitudinal transect (lat. 58°) extending east and west of Ungava Bay. The data show that tills located west and southwest of the Ungava Bay region are largely dominated by hornblende grains with Archean ages (>2.6 Ga), while tills located east of Ungava Bay are characterized by grains with early Paleoproterozoic ages (2.0–1.8 Ga), although most samples contain a few Archean-age grains. IRD derived from the NQL region should thus be characterized by a large proportion of Archean-age detrital grains, which contrasts significantly with the predominant Paleoproterozoic 40Ar/ 39Ar ages (1.8–1.6 Ga) typically reported for the dominant age population of hornblende grains in H layers. Comparisons with IRD through the last glacial cycle from a western North Atlantic core off Newfoundland do not show evidence for any prominent ice-rafted event with the provenance characteristics of NQL glacial deposits, thereby suggesting that significant ice-calving event(s) from the Labrador–Québec sector may have been limited throughout that interval. Although these results tend to point towards a relative stability of this ice dome during H events, our study also indicates that further provenance work is required on IRD proximal to the Hudson Strait mouth in order to constrain with a greater confidence the sector(s) of the LIS that fed ice into Hudson Strait during H events. Alternatively, these results and other paleogeographic considerations tend to support models suggesting that part of the Ungava Bay glacial lineations could be associated with a Late-Glacial ice flow across Hudson Strait.

Growth, dynamics and deglaciation of the last British–Irish ice sheet: the deep-sea ice-rafted detritus record

The evolution and dynamics of the last British–Irish Ice Sheet (BIIS) have hitherto largely been reconstructed from onshore and shallow marine glacial geological and geomorphological data. This reconstruction has been problematic because these sequences and data are spatially and temporally incomplete and fragmentary. In order to enhance BIIS reconstruction, we present a compilation of new and previously published ice-rafted detritus (IRD) flux and concentration data from high-resolution sediment cores recovered from the NE Atlantic deep-sea continental slope adjacent to the last BIIS. These cores are situated adjacent to the full latitudinal extent of the last BIIS and cover Marine Isotope Stages (MIS) 2 and 3. Age models are based on radiocarbon dating and graphical tuning of abundances of the polar planktonic foraminifera Neogloboquadrina pachyderma sinistral (% Nps) to the Greenland GISP2 ice core record. Multiple IRD fingerprinting techniques indicate that, at the selected locations, most IRD are sourced from adjacent BIIS ice streams except in the centre of Heinrich (H) layers in which IRD shows a prominent Laurentide Ice Sheet provenance. IRD flux data are interpreted with reference to a conceptual model explaining the relations between flux, North Atlantic hydrography and ice dynamics. Both positive and rapid negative mass balance can cause increases, and prominent peaks, in IRD flux. First-order interpretation of the IRD record indicates the timing of the presence of the BIIS with an actively calving marine margin. The records show a coherent latitudinal, but partly phased, signal during MIS 3 and 2. Published data indicate that the last BIIS initiated during the MIS 5/4 cooling transition; renewed growth just before H5 (46 ka) was succeeded by very strong millennial-scale variability apparently corresponding with Dansgaard–Oeschger (DO) cycles closely coupled to millennial-scale climate variability in the North Atlantic region involving latitudinal migration of the North Atlantic Polar Front. This indicates that the previously defined “precursor events” are not uniquely associated with H events but are part of the millennial-scale variability. Major growth of the ice sheet occurred after 29 ka with the Barra Ice Stream attaining a shelf-edge position and generating turbiditic flows on the Barra–Donegal Fan at ∼27 ka. The ice sheet reached its maximum extent at H2 (24 ka), earlier than interpreted in previous studies. Rapid retreat, initially characterised by peak IRD flux, during Greenland Interstadial 2 (23 ka) was followed by readvance between 22 and 16 ka. Readvance during H1 was only characterised by BIIS ice streams draining central dome(s) of the ice sheet, and was followed by rapid deglaciation and ice exhaustion. The evidence for a calving margin and IRD supply from the BIIS during Greenland Stadial 1 (Younger Dryas event) is equivocal. The timing of the initiation, maximum extent, deglacial and readvance phases of the BIIS interpreted from the IRD flux record is strongly supported by recent independent data from both the Irish Sea and North Sea sectors of the ice sheet.

Late quaternary ice-rafted detritus events in the Chukchi Basin, western Arctic Ocean

The late Quaternary ice rafted detritus (IRD) events in the Chukchi Basin, western Arctic Ocean are indications of the provenance of the coarser detritus and ice export events, and also document the evolutionary histories of Beaufort Gyre and the North American Ice Sheet (NAIS). The sediment of core M03 from the Chukchi Basin was selected to study the regional response to the ice export events and the NAIS variability. The stratigraphic framework of M03 was established by a combination of lithological features and downcore color change cycles, AMS14C dating with foraminifera abundance and IRD events. The core was also compared with the adjacent core NWR 5 from the Northwind Ridge area. The core extends back to Marine Isotope Stage (MIS) 7. A sedimentary hiatus of 10–20 ka might occur between 16 to 20 cm core depth. Seven IRD events are distinguished from the studied core and are presented during the early MIS 1, MIS 3, MIS 5 and late MIS 7. These IRD are transported by sea ice and icebergs, which were exported to the Beaufort Sea from the M’Clure Strait Ice Stream, Canadian Arctic Archipelago, and brought to the Chukchi Basin by the Beaufort Gyre.

Provenance of Quaternary glacial and glacimarine sediments along the southeast Greenland margin

The Nd and Sr isotopic compositions of Quaternary glacial and glacimarine siliciclastic sediments deposited along the margin of southeast Greenland were determined to assess the roles of the Greenland, Iceland, and more distal ice sheets in delivering detritus to this portion of the northern North Atlantic. The isotopic compositions of detritus generated by portions of the southern Greenland Ice Sheet were defined through measurements of till and trough mouth fan sediments. Massive diamicts from the Scoresby Sund trough mouth fan show a restricted range of ε Nd (− 11.8 to − 16.6) and 87Sr/ 86Sr (0.7192–0.7246) consistent with their derivation from mixtures of sediments derived from Paleoproterozoic and/or Caledonian basement and Tertiary Greenland basalts. Further south at Kangerlussuaq, till isotopic compositions covary with the underlying basement type, with low ε Nd values in the inner fiord (− 18.1) reflecting the erosion of the local Precambrian gneisses, but with higher ε Nd values (− 2.3 to 2.5) found where the trough crosses East Greenland Tertiary basalts. Fine-grained (< 63 µm) sediments deposited along the southeast Greenland margin also show regular spatial isotopic variations. Ambient sediments and ice-rafted detritus in the southern Irminger Basin trend towards low ε Nd values (to ~ − 28) and 87Sr/ 86Sr ratios (~ 0.711 to ~ 0.715) and are likely derived from proximal Archean gneisses of SE Greenland. Further north in the northern Irminger and Blosseville Basins, sediments trend toward much higher ε Nd (> − 4) and low 87Sr/ 86Sr (< 0.709) reflecting a component derived from the local Iceland volcanic rocks and/or the East Greenland Tertiary basalts. In all three regions, the locally-derived detritus is intermixed with sediment with an intermediate ε Nd value (~ − 10) and 87Sr/ 86Sr (~ 0.718) that was likely delivered by icebergs emanating from the Eurasian Ice Sheets and not from eastern Greenland. Deposition of glacial sediments from both proximal and distal (Eurasian) sources occurred adjacent to SE Greenland throughout the past 50 Ka, with periodic increases in IRD deposition at various times including those of Heinrich events 1, 2 and 4. These results suggest that at least the southern portions of the Greenland Ice Sheet experienced periodic instabilities during the Last Glacial period.

Core-based reconstruction of paleoenvironmental conditions in the southern Drake Passage (West Antarctica) over the last 150 ka

A deep-sea sediment core (GC98-06) from the southernmost Drake Passage, West Antarctica, shows late Quaternary depositional environments distinctly different from sedimentary drifts commonly found along the southwestern Pacific margin of the Drake Passage. The chronology of the core has been inferred using geochemical tracers of paleoproductivity and diatom biostratigraphy, and represents the paleoceanographic conditions in a continental rise setting during the last 150,000 years. Three dominant sediment types associated with distinct sedimentary processes have been identified using textural/compositional analyses: (1) hemipelagic mud (interglacial sediments) deposited from pelagic settling of bioclasts, meltwater plumes, and ice-rafted detritus; (2) terrigenous mud (glacial sediments) delivered by turbid meltwater plumes; and (3) massive muds marking the boundaries from interglacial to glacial periods. The succession of the sedimentary facies in core GC98-06 is interpreted to reflect temporal changes in environmental conditions prevailing on the continental rise of the southern Drake Passage in the course of successive climatic stages over the last 150 ka: from the bottom upward, these are glacial, interglacial, glaciation, glacial, and interglacial episodes. Variability in sediment flux and diatom abundance seem to have been related to changes in glacial advance, sea-ice extent, and specific sedimentary environments, collectively influenced by mid- to late Quaternary climatic changes.

Sea Ice Formation Rates Recorded in Planktonic Foraminiferal Oxygen and Carbon Isotopes in the Western Arctic Ocean during the Late Quaternary

An integrated study was carried out on stable isotopes of planktonic foraminifera Neogloboquadrina pachyderma(sin.)(Nps),planktonic foraminiferal abundance and ice-rafted detritus(IRD) in the western Arctic Ocean to investigate sea ice formation during the late Quaternary.Our results show that light excursions in Nps δ18O and δ13C happened seven times,likely indicating the production and sinking of isotopically light brines caused by enhanced rate of the sea ice formation.Because concomitant decrease occurred in planktonic foraminiferal abundance and IRD,these light Nps δ18O and δ13C values unlikely reflect inputs of freshwater and warm Atlantic waters.They more likely suggest reduction of Atlantic waters into the Arctic Ocean and substantial decrease of Chukchi Sea shelf waters.On the contrary,heavy δ18O values would indicate reduction of freshwater and Pacific waters into the Arctic Ocean and heavy δ13C values be caused by well ventilated surface and halocline waters transported from the shelf into the Arctic Ocean.

Seeking a Holocene drift ice proxy: non‐clay mineral variations from the SW to N‐central Iceland shelf: trends, regime shifts, and periodicities

AbstractQuantitative X‐ray diffraction analysis of the &lt;2 mm sediment fraction was carried out on 1257 samples (from the seafloor and 16 cores) from the Iceland shelf west of 18° W. All but one core (B997‐347PC) were from transects along troughs on the NW to N‐central shelf, an area that in modern and historic times has been affected by drift ice. The paper focuses on the non‐clay mineralogy of the sediments (excluding calcite and volcanic glass). Quartz and potassium feldspars occupy similar positions in an R‐mode principal component analysis, and oligoclase feldspar tracks quartz; these minerals are used as a proxy for ice‐rafted detritus (IRD). Accordingly, the sum of these largely foreign minerals (Q&amp;K) (to Icelandic bedrock) is used as a proxy for drift ice. A stacked, equi‐spaced 100 a record is developed which shows both low‐frequency trends and higher‐frequency events. The detrended stacked record compares well with the flux of quartz (mg cm−2 a−1) at MD99‐2269 off N Iceland. The multi‐taper method indicated that there are three significant frequencies at the 95% confidence level with periods of ca. 2500, 445 and 304 a. Regime shift analysis pinpoints intervals when there was a statistically significant shift in the average Q&amp;K weight %, and identifies four IRD‐rich events separated by intervals with lower inputs. There is some association between peaks of IRD input, less dense surface waters (from δ18O data on planktonic foraminifera) and intervals of moraine building. Copyright © 2009 John Wiley &amp; Sons, Ltd.

The role of currents and sea ice in both slowly deposited central Arctic and rapidly deposited Chukchi–Alaskan margin sediments

A study of three long cores from the outer shelf and continental slope north of Alaska in the Arctic Ocean indicate that localized drift deposits occur here with sedimentation rates of more than 1.5 m/kyr during the Holocene. Currents in this area average about 5–20 cm/s but can reach 100 cm/s and these velocities transport the sediment found in these cores primarily as intermittent suspended load. These high accumulation sediments form levee-like deposits associated with margins of canyons cutting across the shelf and slope. Unlike most textural investigations of Arctic sediment that focus on the coarser ice-rafted detritus (IRD), this paper focuses on the > 95% of the sediment, which is finer than 45 μm. The mean size of this fraction varies between 6 and 15 μm in Holocene sediments from the Chukchi–Alaskan shelf and slope with the higher values closer to shore. Analysis of detailed size distributions of these Holocene deposits are compared to 34 sediment samples collected from sea ice across the Arctic Ocean and to Holocene sediment from central Arctic Ocean cores and indicate that similar textural parameters occur in all of these sediments. Principal components of these size distributions indicate that sea ice is an important link between the shelves and the central Arctic. Factor scores indicate nearly identical components in the clay and fine silt size fractions but very different components in the coarse silt for sea ice sediment and central Arctic ridge sediments compared to shelf and continental slope deposits. Sea ice must contribute to sedimentation in both of these Arctic regions, but bottom currents dominate in the slope region, forming drift deposits.

Onset of “Hudson Strait” Heinrich events in the eastern North Atlantic at the end of the middle Pleistocene transition (∼640 ka)?

Heinrich events are well documented for the last glaciation, but little is known about their occurrence in older glacial periods of the Pleistocene. Here we report scanning XRF and bulk carbonate δ18O results from Integrated Ocean Drilling Program Site U1308 (reoccupation of Deep Sea Drilling Project Site 609) that are used to develop proxy records of ice‐rafted detritus (IRD) for the last ∼1.4 Ma. Ca/Sr is used as an indicator of IRD layers that are rich in detrital carbonate (i.e., Heinrich layers), whereas Si/Sr reflects layers that are poor in biogenic carbonate and relatively rich in detrital silicate minerals. A pronounced change occurred in the composition and frequency of IRD at ∼640 ka during marine isotope stage (MIS) 16, coinciding with the end of the middle Pleistocene transition. At this time, “Hudson Strait” Heinrich layers suddenly appeared in the sedimentary record of Site U1308, and the dominant period of the Si/Sr proxy shifted from 41 ka prior to 640 ka to 100 ka afterward. The onset of Heinrich layers during MIS 16 represents either the initiation of surging of the Laurentide Ice Sheet (LIS) off Hudson Strait or the first time icebergs produced by this process survived the transport to Site U1308. We speculate that ice volume (i.e., thickness) and duration surpassed a critical threshold during MIS 16 and activated the dynamical processes responsible for LIS instability in the region of Hudson Strait. We also observe a strong coupling between IRD proxies and benthic δ13C variation at Site U1308 throughout the Pleistocene, supporting a link between iceberg discharge and weakening of thermohaline circulation in the North Atlantic.

Oxygen and carbon isotopes of detrital carbonate in North Atlantic Heinrich Events

We report oxygen and carbon isotope results of detrital carbonate grains from Heinrich layers at three sites in the North Atlantic located along a transect from the Labrador Sea to the eastern North Atlantic. Oxygen isotopic values of individual detrital carbonate grains from six Heinrich layers at all sites average − 5.6‰ ± 1.5‰ (1σ; n = 166), reflecting values of dolomitic limestone derived from source areas in northeastern Canada. The δ 18O of bulk carbonate at Integrated Ocean Drilling Program (IODP) Site U1308 (re-occupation of Deep Sea Drilling Project (DSDP) Site 609) in the eastern North Atlantic records the proportion of detrital to biogenic carbonate and δ 18O decreases to − 5‰ during Heinrich (H) events 1, 2, 4 and 5 relative to a background value of ∼ 1 to 2‰ for biogenic carbonate. Bulk δ 18O also decreases during H3 and H6 but only attains values of − 1‰, indicating either a greater proportion of biogenic-to-detrital carbonate or a different source. Because the δ 18O of detrital carbonate is ∼ 9‰ lower than foraminifer carbonate, any fine-grained detrital carbonate not removed from the inner test chambers will lower foraminifer δ 18O. We conclude bulk carbonate δ 18O is a sensitive proxy for detrital carbonate and may be useful for identifying Heinrich layers in cores within and near the margins of the North Atlantic ice-rafted detritus (IRD) belt.

Ice-rafted detritus events in the Arctic during the last glacial interval, and the timing of the Innuitian and Laurentide ice sheet calving events

Ice-rafted detritus events in the Arctic during the last glacial interval, and the timing of the Innuitian and Laurentide ice sheet calving events

Peat biostratigraphy and Holocene climate in the northwestern mountain periphery of Lake Baikal

We report new radiocarbon ages, pollen stratigraphy, and botanical composition of an organic sediment sequence from mountains along the northwestern side of Lake Baikal. The calendar age of the peat is ca. 9.5 kyr BP. The pollen spectra between 9.5 and 8.5 kyr ago show vegetation consisting mostly of open larch and spruce-larch forests with minor birch. The interval from 7.5 to 6.8 kyr BP is marked by broad fir spread confirmed also by the peat botanical composition. The climate inferred for the period of fir dominance combines mild winters, cool summers, thick snow cover, and abundant moisture, apparently due to more intense North Atlantic air transport. Fir percentages decrease dramatically after 6.8 kyr BP, when pines begin to spread in the Baikal watershed. The peat record bears imprint of North Atlantic cooling events identified as intervals of ice-rafted detritus deposition and low surface water temperatures. The new data allow a better insight into the regional climate and landscapes, and the reliable age model provides regional and inter-regional correlations of global change events.

Chronological constraints on Cordilleran Ice Sheet glaciomarine sedimentation from core MD02-2496 off Vancouver Island (western Canada)

Chronology based on 46 radiocarbon dates has been generated for MD02-2496, western Vancouver Island, Canada. Subsequent correlation between glaciomarine sedimentary sequences within the core and the terrestrial stratigraphy of the Cordilleran Ice Sheet for the last 50 ka provide new insight into Cordilleran Ice Sheet processes. Glaciomarine sedimentation associated with the most recent advance (Fraser Glaciation) of the Cordilleran Ice Sheet began at 30.5 cal kyr BP (∼25 14C kyr BP). Extremely high sedimentation rates indicate the ice sheet became proximal to the site at ∼19.5 cal kyr BP (16 14C kyr BP). Deposition of ice rafted detritus (IRD) began at ∼17 cal kyr BP (14 14C kyr BP), and abruptly terminated at ∼16.2 cal kyr BP (13.5 14C kyr BP) indicating rapid regional ice sheet retreat. A minor IRD event followed at ∼14.7 cal kyr BP (12.5 14C kyr BP). Glaciomarine sediment deposition abruptly ended at 10.2 cal kyr BP (∼8.8 14C kyr BP) with the resumption of biogenic sedimentation. The Olympia Interstade is represented by an interval of hemipelagic sedimentation between ∼43 and 31.5 ka. Prior to ∼45 ka sediment deposition is controlled by glaciomarine processes with evidence for a significant IRD event at ∼47 cal kyr BP (>41±1.06 14C kyr BP).

Late Quaternary stratigraphy and sedimentology of Orphan Basin: Implications for meltwater dispersal in the southern Labrador Sea

Orphan Basin is a deep-water basin on the continental margin off Newfoundland, which throughout the late Quaternary received proglacial sediment from local ice that crossed the continental shelf. Sediment from more distant sources was transported southward in the Labrador Current as proglacial plumes and in icebergs. Five sedimentary facies related to glacial processes are distinguished in cores recovered from Orphan Basin: hemipelagic sediment, nepheloid-layer deposits (layered mud), beds rich in ice-rafted detritus (IRD), sand and mud turbidites, and glaciogenic debris-flow deposits. IRD-rich beds correspond to periods of intensified iceberg calving, and layered mud, turbidites, and glaciogenic debris-flow deposits with glacial meltwater discharge. In the Late Wisconsinan, eight periods of meltwater discharge and iceberg calving from the Newfoundland ice sheet are interpreted from the sediment facies in Orphan Basin. These discharges coincide with the terminations of the colder periods of the D–O cycles recorded in Greenland ice cores. The oldest minor meltwater event (27.5–28.5 cal ka) corresponds to the first Late Wisconsinan ice advance across the Grand Banks and NE Newfoundland Shelf. The following three meltwater discharges (23–23.5, 23.8–24.5, and 25–27 cal ka) deposited sand turbidites and glaciogenic debris-flow deposits seaward of Trinity Trough, which was occupied by an ice stream at this time, and mud turbidites in the southern part of the basin derived from a mid-shelf ice margin on the Grand Banks. Four periods of meltwater discharge occurred during the deglaciation and are centered at 15, 18.5, 19.75, and 20.75 cal ka. The youngest is correlated to Heinrich event 1. In the literature, the 18.5 and 20.75 cal ka events have been recorded in multiple glacial settings in the North Atlantic, and therefore, are interpreted as large-scale events of meltwater discharge and iceberg calving, but in Orphan Basin the 19.75 cal ka event is also of similar scale.

Ice-rafted detritus events in the Arctic during the last glacial interval, and the timing of the Innuitian and Laurentide ice sheet calving events

Ice-rafted detritus (IRD) layers in the Arctic Ocean not only indicate the source of this detrital sediment, but give insights into the ice drift and ice sheet history. Detrital sand-sized Fe oxide mineral grains that are matched to precise sources using the microprobe chemical fingerprint of each grain, along with elevated coarse IRD abundance and radiocarbon ages, are used to define IRD peaks from the Innuitian and Arctic portions of the Laurentide ice sheets. Because grains from these two areas can be entrained by sea ice from the shelves just offshore of the calving areas, peaks in these grains must be correlated to coarse IRD to identify iceberg calving events, and to distinguish them from sea-ice rafting. The sequence of IRD peaks deposited by icebergs from these two ice sheets indicate that both ice sheets calved bergs at accelerated numbers, six or seven times, from 11 to 36 Kya. The relatively short times between most of these IRD events suggest that the ice sheets did not completely collapse with each IRD event, except the last event. Although there is some indication that one ice sheet may have begun calving bergs before the other, the resolution of the Arctic cores does not allow definitive determination of this. This emphasizes the need for higher resolution cores from the central Arctic, as well as from near the terminus of large Pleistocene ice sheets. Sea-ice rafting occurs throughout the last glacial stage, even during some glacial IRD events, as indicated by Fe grains from non-glacial sources.

Glacial-Marine Sedimentation, Canadian Polar Margin, North of Axel Heiberg Island

Sediment cores, taken at depths of 140 to 300 m across the northwestern shelf of Axel Heiberg Island (82° N), record the deposition of sediments under perennial sea ice. Five sedimentary fades are recognized: (A) soft pebbly-sandy-mud with dropstone structures; (B) bioturbated silty muds; (C) wispy-laminated silty clay/clay; (D) laminated sands/silts and mud; (E) firm pebbly-sandy-mud with chaotic pebble fabrics. Other sediments include terrestrial bedrock of Paleogene Eureka Sound Group, and a younger Tertiary deposit, possibly the Beaufort Formation. Ages range from 1530 ± 60 BP (Fades A) to 9950 ± 80 BP (Fades D). Sedimentation rates vary as follows: - 0.8 cm ka-1, Fades B; 4 cm ka"\ Fades A; 90 cm ka-1, Fades C; 134 cm ka~', Fades D. The sedimentation history, as interpreted from the sedimentology, palynology and foraminiferal results, suggests intervals of more continuous ice cover, with a reduced influx of coarse ice-rafted detritus, alternating with more open water conditions, and high sediment input from meltwater and/or floating icebergs. Only marine sediments overlie Neogene bedrock in the cores. The absence of diamictons at the core sites suggests that grounded ice perhaps never occupied this part of the Axel Heiberg Island shelf. The interpreted history of sedimentation generally corresponds to the land-based record from Ellesmere Island, but differs significantly from marine-based studies in more southern latitudes.