Iceberg Discharge Research Articles

Increased productivity in the subantarctic ocean during Heinrich events

Massive iceberg discharges from the Northern Hemisphere ice sheets, 'Heinrich events', coincided with the coldest periods of the last ice age. There is widespread evidence for Heinrich events and their profound impact on the climate and circulation of the North Atlantic Ocean, but their influence beyond that region remains uncertain. Here we use a combination of molecular fingerprints of algal productivity and radioisotope tracers of sedimentation to document eight periods of increased productivity in the subpolar Southern Ocean during the past 70,000 years that occurred within 1,000-2,000 years of a Northern Hemisphere Heinrich event. We discuss possible causes for such a link, including increased supply of iron from upwelling and increased stratification during the growing season, which imply an alteration of the global ocean circulation during Heinrich events. The mechanisms linking North Atlantic iceberg discharges with subantarctic productivity remain unclear at this point. We suggest that understanding how the Southern Ocean was altered during these extreme climate perturbations is critical to understanding the role of the ocean in climate change.

Greenland ice sheet surface mass-balance variability: 1991–2003

AbstractThe Polar MM5 mesoscale atmospheric model was run for 13 years (1991–2003) over Greenland at 24 km horizontal resolution (Box and others, 2004). The model physics were driven by satellite, station and weather-balloon observational data assimilation, i.e. European Centre for Medium-Range Weather Forecasts (ECMWF) operational analysis. The analysis in this study focuses on the response of the surface mass balance to its primary controls: temperature and precipitation. The results indicate coherent spatial patterns of variability and statistically significant links with temperature and precipitation and the North Atlantic Oscillation. Precipitation trends have the same spatial pattern and sign as temperature, suggesting an association of precipitation and temperature variability. Increasing temperatures contribute to an increasing ablation trend and expansion of the ablation zone despite increasing accumulation trends. The Pinatubo (Philippines) volcanic cooling in the early 1990s enhances this apparent warming trend. Only in the northeast does precipitation appear to dominate the surface mass balance, where both temperature and precipitation have decreased. There is little evidence for a total ice-sheet surface mass-balance trend, although the meltwater runoff has a positive trend and, combined with iceberg discharge and basal melting estimates, suggests the ice sheet as a whole is in a state of net mass loss over this period.

A continuous record of temperature evolution over a sequence of Dansgaard‐Oeschger events during Marine Isotopic Stage 4 (76 to 62 kyr BP)

Our knowledge of the temperature evolution over Greenland during Dansgaard‐Oeschger events (DO) is currently qualitatively described through the water isotopic profile. Using two independent paleothermometry methods, one based on air isotopic measurements and the other on the combined measurements of water isotopes (δD and δ18O), we show a complete and quantitative reconstruction of temperature at the NorthGRIP site over the period 76 to 62 kyr BP (DO 18, 19 and 20). We confirm that the associated warmings are larger than those conventionally depicted by the water isotopes (11°C, 16°C and 11°C for DO 18, 19 and 20). Secondly, we demonstrate that the relationship between temperature and δ18O varies rapidly during the last glacial period, even over a DO. Finally, our temperature reconstruction over DO 19 agrees well with that predicted from simple climate models linking the DO to iceberg discharges.

Evolution of iceberg melting, biological productivity, and the record of Icelandic volcanism in the Irminger basin since 630 ka

Planktic δ 18O and δ 13C records and point count records of biogenic, volcanic, and nonvolcanic terrigenous [ice-rafted debris (IRD)] sediment components from Hole 919A in the Irminger basin, northern North Atlantic provide a comprehensive dataset from which a paleoceanographic reconstruction for the last 630 kyr has been developed. The paleoceanographic evolution of the Irminger basin during this time contains both long-term patterns and significant developmental steps. One long-term pattern observed is the persistent deposition of hematite-stained ice-rafted debris. This record suggests that the modern and late Pleistocene discharges of icebergs from northern redbed regions to the Irminger Sea lie in the low end of the range observed over the last 630 kyr. In addition, Arctic front fluctuations appear to have been the main controlling factor on the long-term accumulation patterns of IRD and planktic biogenic groups. The Hole 919A sediment record also contains a long-term association between felsic volcanic ash abundances and light δ 18O excursions in both interglacial and glacial stages, which suggests a causal link between deglaciations and explosive Icelandic eruptions. A significant developmental step in the paleoceanographic reconstruction based on benthic evidence was for diminished supply of Denmark Strait Overflow Water (DSOW) beginning at ∼380 ka, possibly initiated by the influx of meltwater from broad-scale iceberg discharges along the east Greenland coast. There is also planktic evidence of a two-step cooling of sea surface conditions in the Irminger basin, first at ∼338–309 ka and later at ∼211–190 ka, after which both glacials and interglacials were colder as the Arctic front migrated southeast of Site 919. In addition to offering these findings, this reconstruction provides a longer-term geologic context for the interpretation of more recent paleoceanographic events and patterns of deposition from this region.

Greenland ice sheet surface mass balance 1991–2000: Application of Polar MM5 mesoscale model and in situ data

The Polar Pennsylvania State University–National Center for Atmospheric Research Fifth‐Generation Mesoscale Model (Polar MM5) regional climate model was run over the North Atlantic region for 1991–2000. We analyze 24‐km output over the Greenland ice sheet to evaluate spatial and temporal variability of the surface mass balance and its subcomponents. The model output is compared with 3 years of automatic weather station (AWS) data from 17 sites to identify biases. Using the in situ data, we derive simple corrections for biases in melt energy and in water vapor fluxes from the surface and from blowing snow. The simulated accumulation rate is in agreement with AWS and snow pit observations. Estimates for runoff and the surface mass balance distribution over the ice sheet are produced using modeled melt volume and a meltwater retention scheme. From the decade investigated, the magnitude of interannual variability in surface mass balance components is tentatively established. The largest variability is concentrated along the ice sheet margin, where both accumulation and ablation rates are largest. The simulated interannual fluctuations suggest a large absolute variability, ±187 km3 yr−1 for total ice sheet surface mass balance. Variability in simulated equilibrium line altitude is suggestive of a dominance of thermal variability in the south with increasing importance of accumulation variability with increasing latitude. Empirical functions for the sensitivity of surface mass balance to temperature and precipitation anomalies are presented. The precise locations and regions of maximum and minimum surface energy and mass fluxes are suggested. Using an estimate for iceberg discharge and bottom melting, the total ice sheet mass balance is estimated be −78 km3 yr−1, producing 2.2 mm of eustatic sea level rise over the 1991–2000 decade and contributing 15% to the observed (1.5 mm yr−1) global sea level rise. The more negative mass balance is attributed to including blowing‐snow sublimation loss and to regional warming in the 1990s.

Eemian to early Würmian climate dynamics: history and pattern of changes in Central Europe

High-resolution pollen records from the northern alpine foreland are used to evaluate the patterns of climate change during the period between the late Eemian Interglacial (marine oxygen isotope stage OIS 5e) about 115 ky ago and the end of the St. Germain II (OIS 5a, Dansgaard–Oeschger interstadial D/O IS21) about 75 ky ago. For the quantitative climate interpretation of the fossil pollen assemblages, we compare two different methods, a modern analogue technique and a mutual climatic range approach. The results not only reveal major climatic changes at the transition to stadials and interstadials, but also indicate short-term oscillations which are followed by an almost full return to the preceding climate conditions, and components of gradual trends resembling saw-tooth structures. Based on the pollen data, the climate reconstructions suggest that the optimum in St. Germain II (OIS 5a, D/O IS21) in winter temperatures was about 0.8 to 1.4 °C and in summer temperatures about 0.7 to 1.2 °C warmer than the optimum in St. Germain I (OIS 5c, D/O IS22 to IS24). The severe and short-time Montaigu cold event (iceberg discharge event C23) clearly divided St. Germain I into two thermal periods. Thus, St. Germain Ia (D/O IS24) preceding the Montaigu event is slightly more favourable than the following St. Germain Ic (D/O IS23). Since several climate oscillations occurred during these interstadials, they show only limited stability. With respect to the stadial periods, the results reveal a coherent pattern for all records. Winter temperatures during Melisey II (OIS 5b, C21) are about 1 to 4 °C lower than those during Melisey I (OIS 5d, C24). As regards short-term climate changes during Melisey I and Melisey II, consistent amelioration episodes may be distinguished during these intervals.

Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes

The Atlantic meridional overturning circulation is widely believed to affect climate. Changes in ocean circulation have been inferred from records of the deep water chemical composition derived from sedimentary nutrient proxies, but their impact on climate is difficult to assess because such reconstructions provide insufficient constraints on the rate of overturning. Here we report measurements of 231Pa/230Th, a kinematic proxy for the meridional overturning circulation, in a sediment core from the subtropical North Atlantic Ocean. We find that the meridional overturning was nearly, or completely, eliminated during the coldest deglacial interval in the North Atlantic region, beginning with the catastrophic iceberg discharge Heinrich event H1, 17,500 yr ago, and declined sharply but briefly into the Younger Dryas cold event, about 12,700 yr ago. Following these cold events, the 231Pa/230Th record indicates that rapid accelerations of the meridional overturning circulation were concurrent with the two strongest regional warming events during deglaciation. These results confirm the significance of variations in the rate of the Atlantic meridional overturning circulation for abrupt climate changes.

Catastrophic ice shelf breakup as the source of Heinrich event icebergs

Heinrich layers of the glacial North Atlantic record abrupt widespread iceberg rafting of detrital carbonate and other lithic material at the extreme‐cold culminations of Bond climate cycles. Both internal (glaciologic) and external (climate) forcings have been proposed. Here we suggest an explanation for the iceberg release that encompasses external climate forcing on the basis of a new glaciological process recently witnessed along the Antarctic Peninsula: rapid disintegrations of fringing ice shelves induced by climate‐controlled meltwater infilling of surface crevasses. We postulate that peripheral ice shelves, formed along the eastern Canadian seaboard during extreme cold conditions, would be vulnerable to sudden climate‐driven disintegration during any climate amelioration. Ice shelf disintegration then would be the source of Heinrich event icebergs.

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.

Postglacial marine environmental changes in Maxwell Bay, King George Island, West Antarctica

Sediment textural properties and total organic carbon (TOC) contents of three sediment cores from Maxwell Bay, King George Island, West Antarctica, record changes in Holocene glaciomarine sedimentary environments. The lower sedimentary unit is mostly composed of TOC-poor diamictons, indicating advanced coastal glacier margins and rapid iceberg discharge in proximal glaciomarine settings with limited productivity and meltwater supply. Fine-grained, TOC-rich sediments in the upper lithologic unit suggest more open water and warm conditions, leading to enhanced biological productivity due to increased nutrient-rich meltwater supply into the bay. The relationship between TOC and total sulfur (TS) indicates that the additional sulfur within the sediment has not originated from in situ pyrite formation under the reducing condition, but rather may be attributed to the detrital supply of sand-sized pyrite from the hydrothermal-origin, quartz-pyrite rocks widely distributed in King George Island. The evolution of bottom-water hydrography after deglaciation was recorded in the benthic foraminiferal stable-isotopic composition, corroborated by the TOC and lithologic changes. The O18O values indicate that bottom-water in Maxwell Bay was probably mixed gradually with intruding 18O-rich seawater from Bransfield Strait. In addition, the O13C values reflect a spatial variability in the carbon isotope distribution in Maxwell Bay, depending on marine productivity as well as terrestrial carbon fluxes by meltwater discharge. The distinct lithologic transition, dated to approximately 8000 yr BP (uncorrected) and characterized by textural and geochemical contrasts, highlights the postglacial environmental change by a major coastal glacier retreat in Maxwell Bay.

Late Devensian marine deposits (Errol Clay Formation) at the Gallowflat Claypit, eastern Scotland: new evidence for the timing of ice recession in the Tay Estuary

Synopsis The Late Devensian arctic marine deposits (Errol Clay Formation) at the Gallowflat Claypit between Perth and Dundee are correlated with the type succession at nearby Inchcoonans, but include an expanded basal glaciomarine unit with rhythms interpreted as symmict varves. Laminated silt–clay couplets are thought to be tidal. Conditions were fully marine by 13.8 ka bp (based on new radiocarbon dates and a 400 a reservoir age). Following recession of the Tay–Forth glacier from its advanced Late Devensian position at the Wee Bankie Terminal Moraine, it is inferred that deglaciation of the middle Tay estuary some 100 km to the west took place between 14.5 and 14 ka bp . The possibility that there was a readvance of ice in the Tay and Forth estuaries during or close to the Heinrich iceberg discharge event H1 is briefly discussed.

Saw‐tooth pattern of North Atlantic current speed during Dansgaard‐Oeschger cycles revealed by the magnetic grain size of Reykjanes Ridge sediments at 59°N

Detailed mineral (rock) magnetic measurements, including high‐resolution magnetic hysteresis loops, were carried out on three sediment cores recovered from the Reykjanes Ridge in the North Atlantic. Supported by physical grain‐size analyses, titanomagnetite grain‐size variations form a proxy of the speed of near‐bottom currents and reveal a cyclic “saw‐tooth” pattern between Dansgaard‐Oeschger cycles 8 and 5. The magnetic grain‐size data suggest that these Dansgaard‐Oeschger cycles were characterized by gradual intensification of the near‐bottom current speed, while other paleoceanographic proxies indicate escalating iceberg discharge and declining sea surface temperatures. These observations are contrary to the frequently assumed and strictly applied positive relationship between high‐latitude warmth and North Atlantic Deep Water formation. Within the confines of temporal control, however, it would appear that peaks in iceberg discharge and slow near‐bottom current speeds were synchronous. The magnetic hysteresis data also demonstrate that magnetic concentration parameters, such as magnetic susceptibility, do not necessarily provide direct evidence of paleocirculation.

Postglacial marine environmental changes in Maxwell Bay, King George Island, West Antarctica

Sediment textural properties and total organic carbon (TOC) contents of three sediment cores from Maxwell Bay, King George Island, West Antarctica, record changes in Holocene glaciomarine sedimentary environments. The lower sedimentary unit is mostly composed of TOC-poor diamictons, indicating advanced coastal glacier margins and rapid iceberg discharge in proximal glaciomarine settings with limited productivity and meltwater supply. Fine-grained, TOC-rich sediments in the upper lithologic unit suggest more open water and warm conditions, leading to enhanced biological productivity due to increased nutrient-rich meltwater supply into the bay. The relationship between TOC and total sulfur (TS) indicates that the additional sulfur within the sediment has not originated from in situ pyrite formation under the reducing condition, but rather may be attributed to the detrital supply of sand-sized pyrite from the hydrothermal-origin, quartz-pyrite rocks widely distributed in King George Island. The evolution of bottom-water hydrography after deglaciation was recorded in the benthic foraminiferal stable-isotopic composition, corroborated by the TOC and lithologic changes. The O18O values indicate that bottom-water in Maxwell Bay was probably mixed gradually with intruding 18O-rich seawater from Bransfield Strait. In addition, the O13C values reflect a spatial variability in the carbon isotope distribution in Maxwell Bay, depending on marine productivity as well as terrestrial carbon fluxes by meltwater discharge. The distinct lithologic transition, dated to approximately 8000 yr BP (uncorrected) and characterized by textural and geochemical contrasts, highlights the postglacial environmental change by a major coastal glacier retreat in Maxwell Bay.

Land–sea correlations for the last glaciation inferred from a pollen and dinocyst record from the portuguese margin

Pollen and dinoflagellate cyst assemblages from Core SU 81-18 recovered off Portugal (37°46′N, 10°11′W; 3135-m water depth) have been used to document the short-term environmental changes that occurred in southwest Europe since 25,000 yr B.P. The relationship between the oceanic and continental environments has been further examined by the use of other marine proxies (coarse sedimentary fraction, foraminifera) and by comparison with proximal land pollen records. Heinrich 2 (H2) and Heinrich 1 (H1) events were the most extreme parts of the highly variable last glacial period, with the maximum extension of dry steppe on land and the occurrence of cool and dilute waters at the core site. Our study shows that H1 and H2 are divided in two distinct phases: one with Neogloboquadrina pachyderma left coiling associated with the maximum input of ice rafted debris, reflecting the in situ release of icebergs and the occurrence of cool and dilute seawater at the core site; the other with dinoflagellate cysts of subpolar affinity, Bitectatodinium tepikiense, reflecting a seasonal control marked by warm summer SST and cold winter SST.

Multidecadal ocean variability and NW European ice sheet surges during the last deglaciation

A multiproxy paleoceanographic record from the Atlantic margin off the British Isles reveals in unprecedented detail discharges of icebergs and meltwater in response to sea surface temperature increases across the last deglaciation. We observe the earliest signal of deglaciation as a moderate elevation of sea surface temperatures that commenced with a weakly developed thermocline and the presence of highly ventilated intermediate waters in the Rockall Trough. This warming pulse triggered a series of multidecadal ice‐rafted debris peaks that culminated with a major meltwater discharge at 17,500 years before present related to ice sheet disintegration across the NW European region. The impact of meltwater caused a progressive reduction in deep water ventilation and a sea surface cooling phase that preceded the collapse of the Laurentide Ice Sheet during Heinrich event 1 by 500–1000 years. A similar sequence of rapid ocean‐ice sheet interaction across the European continental margin is identified during the Bølling‐Allerød to Younger Dryas transition. The strategic location of our sediment core suggests a sensitive and rapid response of ice sheets in NW Europe to transient increases in thermohaline heat transport.

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.

Simulation of ocean‐ice sheet interactions during the last deglaciation

The last deglaciation, between 21 thousand years BP (kyr BP) and 8 kyr BP, was punctuated by a series of climatic oscillations, both in the Northern and Southern Hemispheres, often attributed to changes in ocean circulation. These oscillations are explored with an Earth system model of intermediate complexity, including a synchronous coupling between atmosphere, ocean, and ice sheet dynamics. We focused more particularly on the impact of Heinrich Event 1 (HE1) on the deglaciation process. Representation for HE1 was achieved by a parameterization of iceberg discharge included in the ice sheet model, based on paleoceanographic evidence. The model simulates the following sequence in response to the HE1: (1) The thermohaline circulation is drastically reduced as soon as HE1 starts, causing a sharp cooling in the North Atlantic and a smooth warming in the Southern Hemisphere. (2) Circulation restarts abruptly at the end of HE1, producing an intense warming in both hemispheres (up to 10°C warming in the Greenland‐North Atlantic area, with a rapidly decreasing amplitude as one moves southward). (3) Along with this warming, the melting of the North American and Eurasian ice sheets is enhanced, producing a small freshwater discharge to the North Atlantic, but there is no significant impact on Atlantic salinity and thermohaline circulation. (4) A few decades after the post‐Heinrich warm event, both the southern Atlantic and Antarctica entered a prolonged cold period caused by the reactivation of the thermohaline circulation. (5) Finally, about 1500 years later, depending on sensitivity experiments, meltwater flowing into the North Atlantic through the Hudson Strait and the Barents Sea causes a gradual vertical stratification of the northern North Atlantic. The presented simulations are compared to paleoarchives by taking into account the limits inherent to the model structure. One of the conclusions is that there is probably no direct link between the meltwater pulse 1A observed in sea level data and the onset of the Younger Dryas.

Late Quaternary ice-rafting events in the SW Pacific Ocean, off eastern New Zealand

New records of ice-rafted debris (IRD) in eight cores from Campbell Plateau reveal periodic incursions of icebergs over the last 200 kyr. Observations of modern iceberg trajectories and provenance studies show the icebergs originated mainly in Antarctica and traveled east and north to the plateau margins via the Antarctic Circumpolar Current. There, local currents and winds dispersed icebergs across the plateau and adjacent regions as far north as Chatham Rise at 43°S. The shallow rise crest, along-rise circulation and strong thermal gradient associated with the nearby Subtropical Front inhibited further northward transport. Although IRD concentrations are very low (≤26 grains/g), downcore profiles and a δ 18O chronology reveal distinct peaks during (i) the transition from marine isotope stages (MIS) 7 to 6, (ii) late interglacial MIS 5 and (iii) glacial MIS 2. In addition, smaller peaks occur in MIS 4 and 3. This generalised pattern is also evident off Antarctica, indicating that periodic destabilisation of ice shelves was the main driving force behind the Campbell Plateau IRD events. However, there are significant differences between Antarctic and New Zealand IRD records that probably reflect palaeoceanographic influences on iceberg dispersal. The MIS 5 IRD event is better represented off Antarctica, confirming that last interglacial changes in sea level and temperatures encouraged iceberg discharge and proximal melting. Conversely, the MIS 2 event is relatively better shown on Campbell Plateau, implying that last glacial temperatures inhibited proximal iceberg melting and, together with a more vigorous oceanic circulation, allowed more icebergs to reach the distant plateau. A tentative correlation with records from the Southeast Atlantic and Southwest Indian oceans suggests that the IRD events were felt over much of the Southern Ocean for at least the last 70 kyr.

Climatic and oceanographic changes in the Northeast Atlantic reflected by magnetic properties of sediments deposited on the Portuguese Margin during the last 340 ka

Rock magnetic parameters measured along two giant piston cores MD95-2040 (40°34′N, 9°51′W) and MD95-2042 (37°47′N, 10°09′W) collected off the Portuguese Margin, related to other proxy-climatic data, have been used to reconstruct magnetic mineralogical changes of, in relation to environmental and climatic conditions over the North Atlantic, Western Europe and Northwest Africa during the last three climatic cycles (since isotope stage 10). Thin discrete layers containing coarse grains of titano-magnetite are associated with events of iceberg discharge during Heinrich events 1–6 [Heinrich, Quat. Res. 29 (1988) 142] that have equivalent events in isotope stages 5–8. Concentrations of fine-grained (Ti-) magnetite and hematite/goethite, varying in phase opposition, are directly linked with alternations of cold and warm climatic periods. Spectral analyses of the rock magnetic signals reveal Milankovitch periods at 100 and 41 ka, confirming the relationship with long-term climatic changes at high latitudes. The nature (Ti-magnetite) and size range of the finest ferrimagnetic fraction as well as its variation, suggest a control by deep currents carrying a colloidal/clayey fraction from remote sources (Iceland, Faeroes, mid-Atlantic Ridge). Variation of hematite/goethite contents is linked with transport by rivers and winds from the neighbouring continent. A tight correlation with the D–O cycles in Greenland ice records confirms that North Atlantic oceanic regimes and continental wind regimes were strongly influenced by millennial scale climatic changes throughout the last 350 ka.

Simulations of Heinrich Events in a coupled ocean‐atmosphere‐sea ice model

Between 20 and 80 kyr ago several cooling cycles occurred in the Northern Hemisphere which culminated in a discharge of icebergs into the North Atlantic. These so called “Heinrich Events” (HEs) were followed by an abrupt shift to a warmer climate. Here we use a coupled ocean‐atmosphere‐sea ice model to study the response of the climate system under glacial conditions to a hypothetical HE. The HE is simulated by meltwater discharges and combined changes in land albedo and ice sheet topography mimicking a break‐up of a considerable part of the Laurentide ice sheet. Despite very different initial strengths of the overturning circulation in the Northern Hemisphere, the model response to the HEs is qualitatively similar. A warming of ocean and atmosphere temperatures due to the topography/albedo changes is simulated after the iceberg discharge. North Atlantic Deep Water (NADW) formation drops and reestablishes due to the meltwater event.