Ice-rafted Detritus Events Research Articles

Sea-Surface Characteristics of the Newfoundland Basin of the Northwest Atlantic Ocean during the Last 145,000 Years: A Study Based on the Sedimentological and Paleontological Proxies

Dramatic changes occur in the sea-surface characteristics (i.e., temperature and salinity) and freshwater input due to the interaction of cold and fresh Labrador Current and warm and salty North Atlantic Current (NAC) on the southeast Grand Banks. As a result, the biological productivity and seasonal stratification of the upper water masses are intensified. Such changes must have been more dramatic during the glacial times due to the penetration of the Polar and Arctic fronts and southward migration of the Gulf Stream/NAC. However, the extent to which such changes impacted the sea-surface characteristics in the Newfoundland Basin is poorly known. We report changes in the sea-surface characteristics using a piston core (Hu9007-08) collected from the Milne seamount during the last 145,000 years. Heinrich layers H1, H2, H4, and H5 and H11 within the MIS3 and at the penultimate deglaciation were identified by the ice-rafted detritus (IRD) and Neogloboquadrina pachyderma peaks and lighter oxygen isotopes. Rapid turnover by the foraminiferal species with distinct depth habitats and ecological niches in the mixed-layer and thermocline suggests an interplay between the polar and subpolar water masses during the Heinrich and non-Heinrich periods. Only two North Atlantic-wide cooling events, C24 and C21, in which the latter event linked to the minor IRD event during the marine isotope stage (MIS) 5 in Hu90-08, compared to the eight events in the eastern subpolar gyre (e.g., ODP site 984). Millennial-scale N. pachyderma variability in the western subpolar gyre appears to be absent in the eastern subpolar gyre during the MIS3 suggesting the occasional presence of salty and warm water by the NAC inflow, implying a different climate state between the western and eastern subpolar gyre. Although T. quinqueloba data are fragmentary, there are differences between the western and eastern subpolar gyre in addition to the differences within the western subpolar gyre during MIS5 that might imply a variable influence by the subpolar water. This finding suggests that the influence by the NAC outweighs the impact of cold and fresh polar water in the northern northwest Atlantic during the MIS5.

40 Ar/ 39 Ar hornblende provenance clues about Heinrich event 3 (H3)

Abstract Iceberg discharges into the North Atlantic are important sources of fresh water, and the sediments they deposit can provide constraints on which sectors of different ice sheets were contributing icebergs. 40 Ar/ 39 Ar ages of sand-sized hornblende grains provide useful constraints on IRD (ice-rafted detritus) source areas. Heinrich events are intervals of anomalously high percentages of IRD in marine sediment cores of the North Atlantic IRD belt. In contrast to the others, Heinrich event 3 (H3) records a significantly lower flux of IRD. This study compares 40 Ar/ 39 Ar hornblende age distributions from the interval around and including H3 in giant gravity core EW9303-GGC31 from Orphan Knoll, in the southern part of the Labrador Sea, with piston core V28-82 in the eastern part of the North Atlantic IRD belt. Collectively, these results confirm that H3 represents a Hudson Strait IRD event, but that it was smaller than during H1, H2, H4 and H5, and therefore comprises only a small fraction of the detritus at the eastern North Atlantic location of V28-82. These results support a previously published interpretation of across-strait ice flow during H3 at Hudson Strait. Supplementary material: Appendix 1 is 40 Ar/ 39 Ar data from core EW9303-GGC31; Appendix 2 is grain counts across H3 from core V28-82; Appendix 3 is 40 Ar/ 39 Ar data from core V28-82; these are available at http://www.geolsoc.org.uk/SUP18631 .

Paleoceanographic records in the Chukchi Basin, western Arctic Ocean during the late Quaternary

The late Quaternary paleoceanographic changes in the western Arctic Ocean are revealed by quantitative studies of foraminiferal abundance, ice-rafted detritus (IRD) and its mineralogical and petrological compositions, planktonic Neogloboquadrina pachyderma (sin.) (Nps)-δ 18O and -δ 13C, biogenic and non-biogenic components in Core M03 token from the Chukchi Basin during the Second Chinese National Arctic Expedition cruise. Seven IRD events appeared at MIS 7, 5, 3 and 1. These IRD were carried in massive icebergs, which were exported to the Beaufort Sea through the M’Clure Strait Ice Stream, Canadian Arctic Archipelago, and then transported into the Chukchi Basin by the Beaufort Gyre. Low IRD deposition occurred during the glacial times when more extended ice cover and weakened Beaufort Gyre, while the open water condition and the intensified Beaufort Gyre during interglacial periods favored the IRD deposition. Therefore, the IRD events not only indicate the provenance of coarser detritus and ice export events, but also reflect the evolutionary histories of the Beaufort Gyre and North American ice sheet. Seven light Nps-δ 18O and -δ 13C excursions could respond to enhanced rates of sea ice formation resulting in the production and sinking of isotopically light brines, but was irrelevant to the warm Atlantic water and freshwater inputs. Whereas, the heavy Nps-δ18O and -δ 13C values separately reflect the lessened Arctic freshwater and Pacific water, and well-ventilated surface water from the continental shelf and halocline water. Variations of CaCO3 content and planktonic foraminiferal abundance during the interglacial and glacial periods can demonstrate the incremental or diminishing input of the Atlantic water, while the total organic carbon (TOC) and opal contents increased and decreased during the glacial and interglacial periods, respectively, which could be related to the TOC degradation, opal dissolution and redox conditions of interface between the bottom water and sediments.

Ice-rafting from the British–Irish ice sheet since the earliest Pleistocene (2.6 million years ago): implications for long-term mid-latitudinal ice-sheet growth in the North Atlantic region

The Plio-Pleistocene intensification of Northern Hemisphere continental ice-sheet development is known to have profoundly affected the global climate system. Evidence for early continental glaciation is preserved in sediments throughout the North Atlantic Ocean, where ice-rafted detritus (IRD) layers attest to the calving of sediment-loaded icebergs from circum-Atlantic ice sheets. So far, Early-Pleistocene IRD deposition has been attributed to the presence of high-latitudinal ice sheets, whereas the existence and extent of ice accumulation in more temperate, mid-latitudinal regions remains enigmatic.Here we present results from the multiproxy provenance analysis of a unique, Pleistocene-Holocene IRD sequence from the Irish NE Atlantic continental margin. There, the Challenger coral carbonate mound (IODP Expedition 307 site U1317) preserved an Early-Pleistocene record of 16 distinctive IRD events, deposited between ca 2.6 and 1.7 Ma. Strong and complex IRD signals are also identified during the mid-Pleistocene climate transition (ca 1.2 to 0.65 Ma) and throughout the Middle-Late Pleistocene interval. Radiogenic isotope source-fingerprinting, in combination with coarse lithic component analysis, indicates a dominant sediment source in the nearby British–Irish Isles, even for the oldest, Early-Pleistocene IRD deposits. Hence, our findings demonstrate, for the first time, repeated and substantial (i.e. marine-terminating) ice accumulation on the British–Irish Isles since the beginning of the Pleistocene. Contemporaneous expansion of both high- and mid-latitudinal ice sheets in the North Atlantic region is therefore implied at the onset of the Pleistocene. Moreover, it suggests the recurrent establishment of (climatically) favourable conditions for ice sheet inception, growth and instability in mid-latitudinal regions, even in the earliest stages of Northern Hemisphere glacial expansion and in an obliquity-driven climate system.

Reconstructing North Atlantic deglacial surface hydrography and its link to the Atlantic overturning circulation

Paired Mg/Ca–δ 18O measurements on multiple species of planktic foraminifera are combined with published benthic isotope records from south of Iceland in order to assess the role North Atlantic freshwater input played in determining the evolution of hydrography and climate during the last deglaciation. We demonstrate that Globigerina bulloides and Globorotalia inflata are restricted to intervals when warm Atlantic waters reached the area south of Iceland, and therefore Mg/Ca–δ 18O data from these species monitor changes in the temperature and seawater δ 18O signature of the northward inflow of Atlantic water to the area. In contrast, Neogloboquadrina pachyderma (sinistral) calcifies within local subpolar/polar waters and new Mg/Ca–δ 18O analyses on this species document changes in this water mass. We observe two major surface ocean events during Heinrich Stadial 1 (∼ 17–14.7 ka): an early freshening of the Atlantic Inflow (∼ 17–16 ka), and a later interval (16–14.7 ka) of local surface freshening, sea-ice formation and brine rejection that was associated with a further reduction in deep ocean ventilation. Centennial-scale cold intervals during the Bølling–Allerød (BA, 14.7–12.9 ka) were likely triggered by the rerouting of North American continental run-off during ice-sheet retreat. However, the relative effects of these freshwater events on deep ventilation and climate south of Iceland appear to have been modulated by the background climate deterioration. Two freshwater events occurred during the Younger Dryas cold interval (YD, 12.9–11.7 ka), both accompanied by a reduction in deep ventilation south of Iceland: an early YD freshening of the Atlantic Inflow and local subpolar/polar waters, and a late YD ice-rafted detritus event that was possibly related to brine formation south of Iceland. Based on our reconstructions, the strengthening of the Atlantic Meridional Overturning Circulation at the onset of BA and Holocene may have been promoted by the subsurface warming of subpolar/polar water, brine formation that drew warm saline Atlantic water northwards, and the high background salinity of the Atlantic Inflow.

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.

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

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).

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.

Quartz content and the quartz-to-plagioclase ratio determined by X-ray diffraction: a proxy for ice rafting in the northern North Atlantic?

Many paleoceanographic reconstructions of the glacial North Atlantic include estimates of iceberg discharge, which are based on the variable abundance of ice-rafted detritus (IRD) in deep-sea sediments. IRD abundance is most often determined by the mechanical separation and painstaking counting of terrigenous particles larger than a specified threshold grain size, typically 150 μm. Here we present a new proxy for IRD based on X-ray diffraction (XRD) analysis of bulk sediments. This approach complements results obtained from standard techniques while offering several distinct advantages. In addition to the rapid production of objective data, XRD measurements on bulk sediments are sensitive to a broader and more characteristic grain size range than counts of individual coarse lithic fragments. The technique is demonstrated in a study of 12 sediment cores from the North Atlantic. Bulk quartz content and the quartz-to-plagioclase ratio exhibit peak-to-peak correspondence to manual counting results, which verifies the identification of large IRD influxes. The XRD data also reveal variations between the manually identified peaks, suggesting increased sensitivity to low-level, distal, or sea-ice sources of IRD. A saw-tooth pattern emerges in many IRD events, which supports a link between ice rafting and atmospheric temperature changes over Greenland, and providing further evidence of the influence of climate on iceberg discharges.

Were glacial iceberg surges in the North Atlantic triggered by climatic warming?

High-resolution physical, mineralogical, sedimentological and micropalaeontological studies were carried out on North Atlantic cores from the Reykjanes Ridge at 59°N and from the region southwest of the Faeroe Islands. All core sites are situated along the pathway of Iceland–Scotland Overflow Water (ISOW) and the various parameters measured display similar features. Previously identified carbonate oscillations [Keigwin and Jones (1994) J. Geophys. Res., 99, 12397–12410] in the time span back to the Marine Isotope Stage 5–4 transition and Late Glacial lithic events [Bond and Lotti (1995) Science, 267, 1005–1010], such as the Heinrich ice-rafting events, are all represented in the core records. Long-term trends and higher-frequency changes in ISOW intensity were reconstructed on the basis of various independent proxy records. The long-term trends in circulation match theoretical orbitally forced insolation changes. Our observed links between ice-rafted detritus (IRD) input, variations in sea surface temperature (SST) and circulation at greater depth point to the need to re-examine the origin of IRD events. We suggest that these events may have been triggered by enhanced, partly sub-surface, heat transport to the north. Enhanced northward heat transport may have caused bottom melting of floating outlet glaciers and ice shelves, leading to increased iceberg discharge and ice sheet destabilization. This discharge resulted in lower SST’s and a lower temperature over Greenland. Thus, as shown by our records, this scenario implies a temporary de-coupling of surface processes and circulation at greater depth. A key feature is the occurrence of a saw-tooth pattern in the marine data, which is similar to the Greenland ice core records. Moreover, the ‘warming’ theory of IRD events would explain the observed ‘out-of-phase’ relationship between the Greenland and Antarctic ice core records and also the rapid establishment of higher temperatures over Greenland immediately after the cold phases (stadials) of the Dansgaard–Oeschger cycles.

Ocean circulation and iceberg discharge in the glacial North Atlantic: Inferences from unmixing of sediment size distributions

Variability in iceberg discharge and deep-ocean circulation in the North Atlantic during the last glacial period is inferred from the grain-size distribution and trace elemental composition of terrigenous sediments in a deep-sea core taken on Reykjanes Ridge, south of Iceland. End-member modeling of the grain-size distributions is used to unmix the signals of varying bottom-current speed and iceberg discharge. The size distribution within the silt fraction appears to be influenced by both factors. Based on Th-Sc-La relationship, we established that during the ice-rafted detritus events, continental material of likely Greenlandic origin increased to 87%, and that bottom-current–derived material contains to 40% mid-oceanic ridge fines, probably of Icelandic origin. Our results have important implications for the use of silt grain size as an indicator for paleocurrent speed in the glacial North Atlantic. We show that reconstructions of variations in bottom-current speed based on the raw grain-size data are opposite to inferences from the unmixed record. The latter indicates that deep-water convection decreased during periods of enhanced iceberg discharge, which is in general agreement with paleoceanographic reconstructions of the North Atlantic.

The origin of Heinrich layers: evidence from H2 for European precursor events

Recent well-dated isotopic (Sr–Nd) fingerprinting of Heinrich layer ice-rafted detritus (IRD) on the European margin indicates supply from European ice sheets as precursors to Laurentide Ice Sheet (LIS) supply [F.E. Grousset et al., Geology 28 (2000) 123–126, H. Snoeckx et al., Mar. Geol. 158 (1999) 197–208]. These precursor events lead LIS input by up to 1.5 ka [F.E. Grousset et al., Geology 28 (2000) 123–126] and have been interpreted to indicate LIS collapse during Heinrich events stimulated by events originating on the European side of the Atlantic [F.E. Grousset et al., Geology 28 (2000) 123–126]. Such phasing of IRD supply from different sources within Heinrich layers therefore has implications for the origin and mechanics of Heinrich events. We present evidence here that the IRD comprising Heinrich layer 2 (H2; ∼20–21 14C ka BP) on the European continental margin contains detrital Campanian Upper Chalk deriving from bedrock sources eroded on the Celtic shelf by the British Ice Sheet (BIS) in addition to lithic material sourced from the LIS. High-resolution radiocarbon chronology indicates chalk grain deposition as discrete pulses both before and coincident with supply of LIS-sourced detritus. The specificity of the chalk fingerprint to the BIS enables a 700–1000 yr lag between the BIS and LIS events to be identified. This phasing indicates a more rapid response of the outlet lobes draining the smaller BIS than those draining the LIS and implicates external climatic forcing of Heinrich events. It is unlikely that this precursor event represents IRD event 18, the recently identified 1–2 ka IRD cycle event which immediately precedes H2, because the lag between precursor and main event is here less than 1.5 ka and because such pervasive periodicity is not apparent in European continental margin IRD records. The later synchroneity between the BIS and LIS input in H2 identifies glacio–eustatic sea-level rise associated with LIS discharges as a possible feedback mechanism causing destabilisation of ice streams elsewhere during Heinrich events.

Millennial-scale instability of the antarctic ice sheet during the last glaciation

Records of ice-rafted detritus (IRD) concentration in deep-sea cores from the southeast Atlantic Ocean reveal millennial-scale pulses of IRD delivery between 20,000 and 74,000 years ago. Prominent IRD layers correlate across the Polar Frontal Zone, suggesting episodes of Antarctic Ice Sheet instability. Carbon isotopes (delta(13)C) of benthic foraminifers, a proxy of deepwater circulation, reveal that South Atlantic IRD events coincided with strong increases in North Atlantic Deep Water (NADW) production and inferred warming (interstadials) in the high-latitude North Atlantic. Sea level rise or increased NADW production associated with strong interstadials may have resulted in destabilization of grounded ice shelves and possible surging in the Weddell Sea region of West Antarctica.

North Atlantic Ice Sheet Fluctuations 10,000–70,000 Yr Ago as Inferred from Deposits on the Reykjanes Ridge, Southeast of Greenland

Marine records from the Reykjanes Ridge indicate ice sheet variations and abrupt climate changes. One of these records, ice-rafted detritus (IRD), serves as a proxy for iceberg discharges that probably indicates ice sheet fluctuations. The IRD records suggest that iceberg discharge 68,000–10,000 yr B.P. happened more frequently than the 7000- to 10,000-yr spacing of the Heinrich events. An IRD peak 67,000 to 63,000 yr B.P. further suggests that the Middle Weichselian glaciation started about 12,000 yr earlier in the North Atlantic than in the Norwegian Sea. Several later IRD events, in contrast, correlate with Norwegian Sea IRD-rich layers and imply coeval ice sheet advances in the North Atlantic and the Norwegian Sea. Coccoliths in a core from the Reykjanes Ridge show distinct peaks in species that record occasional inflow of warm surface water during the last glaciation, as previously reported from the eastern Labrador Sea. High abundances of coccoliths, together with a decrease ofNeogloboquadrina pachydermasin. and relatively low δ18O values, imply enhanced advection of the North Atlantic Current 69,000–67,000 yr B.P., 56,000–54,000 yr B.P., 35,000–33,000 yr B.P., and 26,000–23,000 yr B.P. This advection provided a regional moisture source for extension of ice sheets onto the shelf. In contrast, most of the IRD events are characterized by cold polar surface water masses indicating rapid variations in ocean surface conditions.

Radiocarbon chronology of depositional regimes in the western Arctic Ocean

The foraminiferal abundance and percentage of coarse ice-rafted detritus (IRD) define glacial, deglacial, and interglacial depositional regimes in AMS radiocarbon-dated box cores from the western Arctic Ocean. Sediment deposition rates are generally less than 0.5 cm/ka for glacial regimes, greater than this for deglacial regimes and greater than 1–2 cm/ka for interglacial regimes. These differences in deposition rates might account for the much lower average sedimentation rates for the last 780 ka of 1–3 mm/ka in cores from the central Arctic Ocean if glacial regimes dominated this interval. Foraminiferal abundances are less than 500/g during glacial maxima and mostly higher than 2000/g during deglacial and interglacial regimes. Slightly higher coarse IRD percentages occur in deglacial intervals (> 5–10% up to 30%) compared with interglacial intervals (mostly ≤5%), which characterize the last 10–12 ka in the western Arctic Ocean. Glacial regimes occurred from about 40 to 11 ka except for a brief interglacial or deglacial interval around 24–28 ka in the central Arctic Ocean. The coarsest deglacial events occurred prior to 45 ka. The Late Wisconsin deglaciation sediments (approximately 9–16 ka) are difficult to detect in the central Arctic Ocean sediments because they are only slightly coarser than the Holocene and, in some box cores, less coarse than the Holocene. A previously unrecognized coarse IRD event occurred near the core tops (0–3 cm) between 1500 and 3500 radiocarbon years ago in the western Arctic Ocean. Only sediment older than 40 ka is coarser than this recent IRD event, which might correspond to a Neoglaciation recognized in the North Atlantic and elsewhere.

Ice rafting history from the Spitsbergen ice cap over the last 200 kyr

Three piston cores taken from the mid to upper continental slope of the Spitsbergen margin have been used to elucidate the calving history of the Spitsbergen ice mass for the last 200 kyr. The abundance of clasts larger than 2 mm was used as a measure of ice rafted detritus (IRD) in the cores, these clasts were counted from X-ray photographs. The fraction coarser than 63 μm was also measured and compared to the clast counts to assess the validity of this parameter as a measure of IRD content. The correlation between clast counts and the fraction >63 μm was poor, and it was concluded that the fraction >63 μm was a poor measure of IRD in the Spitsbergen cores. In cores close to a major continental landmass, or from an area of steep submarine relief the fraction >63 μm is likely to be affected by factors other than ice rafting alone. The IRD peaks identified in this study were thought to be produced by the following; Advance of the Spitsbergen ice cap during periods of increased moisture supply during interstadial periods (mid stage 5 and stage 3); Disintegration of a large ice mass on the continental shelf during deglaciations (stages 6/5e, 5b/5a and 2/1 boundaries); Summer melting of a large ice mass during glacial periods (stage 6). A good correlation was found between interpreted ice advances from IRD input to the Spitsbergen margin and postulated ice advances from terrestrial evidence. The IRD peaks found from the Spitsbergen margin appear to be produced by a different mechanism from that proposed for the production of the Heinrich layers of the North Atlantic. These are thought to be produced by the collapse of the Laurentide ice sheet once it reaches a threshold size. Correlation of Heinrich layers produced from a mid-latitude ice sheet with the IRD events of the Norwegian-Greenland Sea is not possible due to their different mechanism of formation. In fact, there is possibly an antiphase relationship as Heinrich layers are formed mainly during glacial periods, whereas IRD peaks in the Greenland Sea are produced during periods of rapid ice advance when moisture supply is high (when relatively warm Atlantic waters enter the Norwegian-Greenland Sea during interstadial periods), as well as during melting events.