Iceberg Discharge Research Articles

Estimates of iceberg submarine melting from high-resolution digital elevation models: application to Sermilik Fjord, East Greenland

AbstractObserved increases in iceberg discharge from Greenland’s marine-terminating glaciers over the past two decades have altered the freshwater flux from glacial fjords into surrounding ocean basins. Although variations in freshwater flux due to ice-sheet discharge change have been investigated on a broad scale, the distribution of the freshwater flux due to melting of calved glacier ice (i.e. icebergs) has not been examined. Logistical challenges to collecting in situ data in glacial fjords have so far prevented a detailed examination of freshwater fluxes arising from melting beneath the waterline (i.e. submarine melting). Here we demonstrate that submarine melting of icebergs can be quantified using repeat digital elevation models derived from very high-resolution stereo satellite images. Analysis of volume changes for icebergs in Sermilik Fjord, East Greenland, yield area-averaged submarine melt rates of ~0.39 m d–1. These rates are in relatively good agreement with simulated winter melt rates along the submerged portion of the Helheim Glacier terminus, providing independent validation of the applied technique. Further, the volume flux of fresh water from iceberg melting scales with surface and submerged iceberg areas, which suggests that iceberg meltwater may be an important freshwater component in fjords with high iceberg concentrations and/or expansive ice melange.

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 .

Iceberg discharges of the last glacial period driven by oceanic circulation changes

Proxy data reveal the existence of episodes of increased deposition of ice-rafted detritus in the North Atlantic Ocean during the last glacial period interpreted as massive iceberg discharges from the Laurentide Ice Sheet. Although these have long been attributed to self-sustained ice sheet oscillations, growing evidence of the crucial role that the ocean plays both for past and future behavior of the cryosphere suggests a climatic control of these ice surges. Here, we present simulations of the last glacial period carried out with a hybrid ice sheet-ice shelf model forced by an oceanic warming index derived from proxy data that accounts for the impact of past ocean circulation changes on ocean temperatures. The model generates a time series of iceberg discharge that closely agrees with ice-rafted debris records over the past 80 ka, indicating that oceanic circulation variations were responsible for the enigmatic ice purges of the last ice age.

Establishment of modern circulation pattern at c. 6000 cal a BP in Disko Bugt, central West Greenland: opening of the Vaigat Strait

ABSTRACTVariations in the Holocene circulation of the West Greenland Current (WGC) in the Disko Bugt region have been reconstructed from a suite of sediment cores. Palaeoceanographic proxies include magnetic susceptibility (MS) and X‐ray fluorescence titanium counts, which document a major shift in circulation at c. 6000 cal a BP. Before this date, sediments in southern Disko Bugt were characterized by high terrigeneous and basaltic input, suggesting widespread influence of meltwater plumes. Our data show that the WGC re‐circulated in the southern Disko Bugt area because a potential northern pathway, the narrow Vaigat Strait, was blocked by icebergs that calved from marine outlet glaciers in eastern Disko Bugt. Sediments in southern Disko Bugt deposited after c. 6000 cal a BP have significantly lower terrestrial and basaltic sediment input, which coincides with minimum Holocene ice sheet extent. The reduced meltwater and iceberg discharge to the embayment caused the Vaigat Strait to become free of blocking icebergs and terrigenous input was partly diverted to the outer shelf. Thus, the modern circulation pattern of the WGC was established in the Disko Bugt region through the opening of the Vaigat Strait c. 6000 cal a BP.

Links between tropical rainfall and North Atlantic climate during the last glacial period

During the last glacial period, the North Atlantic regionexperienced pronounced, millennial-scale alternations between cold, stadial conditions and milder interstadial conditions—commonly referred to as Dansgaard–Oeschger oscillations—as well as periods of massive iceberg discharge known as Heinrich events1. Changes in Northern Hemisphere temperature, as recorded in Greenland2,3,4, are thought to have affected the location of the Atlantic intertropical convergence zone5,6 and the strength of the Indian summer monsoon7,8. Here we use high-resolution records of sediment colour—a measure of terrigenous versus biogenic content—from the Cariaco Basin off the coast of Venezuela and the Arabian Sea to assess teleconnections with the North Atlantic climate system during the last glacial period. The Cariaco record indicates that the intertropical convergence zone migrated seasonally over the site during mild stadial conditions, but was permanently displaced south of the basin during peak stadials and Heinrich events. In the Arabian Sea, we find evidence of a weak Indian summer monsoon during the stadial events. The tropical records show a more variable response to North Atlantic cooling than the Greenland temperature records. We therefore suggest that Greenland climate is especially sensitive to variations in the North Atlantic system—in particular sea-ice extent—whereas the intertropical convergence zone and Indian monsoon system respond primarily to variations in mean Northern Hemisphere temperature.

Massive iceberg discharges during Termination V: causes and consequences

A high resolution coccolithophore study was carried out in order to improve the understanding of the paleoceanographic evolution and changes in paleoproductivity occurring off the Iberian Margin (IM) between 445 and 360 ka, i.e. during late Marine Isotope Stage 12 to 11. Coccolithophore assemblages allowed reconstructing surface water changes characterized by millennial-scale oscillations (~ 1.5 kyr cycles) involving Portugal or Iberian Poleward Currents (PC and IPC) prevalence. Changes in paleoproductivity, possibly related to the upwelling of Eastern North Atlantic Central Waters (ENACW) – of sub-tropical (ENACWst) or sub-polar origin (ENACWsp) – were also recognized. This study also permitted detecting abrupt events (stadial/interstadial-type oscillations) and revealed that changes in paleoproductivity are related to opposite dynamics during glacial and interglacial stages, with the reversed setting being established during the deglaciation. Furthermore, a possible control of half and fourth precessional cycle components, on the occurrence of abrupt changes within the assemblages' structure, during the deglaciation, is proposed on the basis of wavelet analysis results applied to selected taxa.

The role of Southern Ocean winds, upwelling and CO2 in glacial abrupt climate change

The last glacial period (ca. 110-10 kyr before present, hereafter kyr BP) is characterized by substantial climate instability, manifested as climatic variability on millennial timescales. Two types of events dominate this variability: Dansgaard-Oeschger (DO) events, which involve decadal-scale warming by more than 10K, and Heinrich events, massive iceberg discharges from the Laurentide Ice Sheet at intervals of ca. 10 kyr during peak glacial conditions. Both DO and Heinrich events are associated with widespread centennial to millennial scale climatic changes, including a synchronous temperature response over the North Atlantic and an anti-phase temperature relationship over Antarctica and most of the Southern Ocean, as revealed by a wealth of deep sea sediments and terrestrial record. Recent studies indicate CO2 changes during deglaciation and, possibly, during glacial abrupt climate changes were preceded by significant increases of Southern Ocean upwelling caused by an enhancement and/or a shift of surface winds over that region. The proposed hypothesis is that periods of halted or reduced North Atlantic deep water (NADW) formation resulted in warming of the Southern Ocean through the bipolar see-saw effect leading to a reorganization of Southern Hemisphere (SH) surface winds, and thereby enhanced upwelling and atmospheric CO2 concentrations. Here, the role of SH surface wind and CO2 changes in the Atlantic meridional overturning circulation (MOC) is analyzed in a coupled climate model of intermediate complexity. We investigate whether changes in the former could eventually trigger an intensification of the Atlantic overturning circulation and a northward shift of NADW formation, which would allow to explain glacial abrupt climate changes as the result of an oscillation which involves the MOC, CO2 and the winds.

Strengthening of the Northeast Monsoon over the Flores Sea, Indonesia, at the time of Heinrich event 1

Paleoclimate evidence from South America and Asia has been interpreted to indicate that tropical rainfall migrated southward during the Northern Hemisphere cooling associated with Heinrich stadial 1 (HS1), an event of massive iceberg discharge to the North Atlantic ca. 18‐15 ka. Although arid conditions associated with such a shift are well documented in southern Asia, as far south as Borneo, debate still exists regarding the precipitation response in southern Indonesia and Australia during HS1. This study utilizes concentrations of the long-lived nuclide 232 Th as a proxy for detrital riverine input and 230 Th normalization to estimate the history of preserved fl uxes reaching the seafl oor in the Flores Sea, located between southern Sulawesi and the Lesser Sunda Islands, Indonesia. Because the only source of 232 Th to the ocean is continental minerals, this proxy is a robust indicator of continental weathering. The 230 Th normalized burial fl uxes of lithogenic and biogenic matter demonstrate that both detrital and biogenic fl uxes in the Flores Sea were higher during HS1 than any other period in the past 22 k.y. High detrital fl uxes indicate enhanced precipitation runoff from surrounding landmasses during a period of maximum southward shift of the Intertropical Convergence Zone. This study further constrains the northern limit of enhanced rainfall associated with a southward shift of Australian monsoon-related rainfall at the time of HS1 and highlights the value of 232 Th as a proxy of continental input to deep-sea sediment records.

The Pacific‐Atlantic seesaw and the Bering Strait

Paleo proxy data and previous modeling studies both indicate that the massive discharge of icebergs into the North Atlantic may have led to a (nearly) collapsed Atlantic meridional overturning circulation (AMOC), resulting in a seesaw‐like climate change between the North Pacific and North Atlantic, with a warming in the former and a cooling in the latter. Here by using a fully coupled climate model, we show that this Pacific‐Atlantic seesaw associated with changes of the AMOC can only occur when the Bering Strait is closed. As this strait is closed, the oceanic communication between the North Pacific and Atlantic is cut off. When AMOC collapses, the North Atlantic becomes cooler, but the North Pacific becomes warmer due to the buildup of the Pacific meridional overturning circulation which transports more warm and salty subtropical water into the North Pacific, leading to seesaw‐like climate changes in the two ocean basins.

Erratum: Determining the natural length of the current interglacial

terglacials can be used to draw analogies with the present, provided their duration is known. Here we propose that the minimum age of a glacial inception is constrained by the onset of bipolar-seesaw climate variability, which requires ice-sheets large enough to produce iceberg discharges that disrupt the ocean circulation. We identify the bipolar seesaw in ice-core and North Atlantic marine records by the appearance of a distinct phasing of interhemispheric climate and hydrographic changes and ice-rafted debris. The glacial inception during Marine Isotope sub-Stage 19c, a close analogue for the present interglacial, occurred near the summer insolation minimum, suggesting that the interglacial was not prolonged by subdued radiative forcing 7 . Assuming that ice growth mainly responds to insolation and CO2 forcing, this analogy suggests that the end of the current interglacial would occur within the next 1500 years, if atmospheric CO2 concentrations did not exceed 240 5 ppmv.

High-Latitude Forcing of the South American Summer Monsoon During the Last Glacial

The climate of the Last Glacial period (10,000 to 110,000 years ago) was characterized by rapid millennial-scale climate fluctuations termed Dansgaard/Oeschger (D/O) and Heinrich events. We present results from a speleothem-derived proxy of the South American summer monsoon (SASM) from 16,000 to 50,000 years ago that demonstrate the occurrence of D/O cycles and Heinrich events. This tropical Southern Hemisphere monsoon reconstruction illustrates an antiphase relationship to Northern Hemisphere monsoon intensity at the millennial scale. Our results also show an influence of Antarctic millennial-scale climate fluctuations on the SASM. This high-resolution, precisely dated, tropical precipitation record can be used to establish the timing of climate events in the high latitudes of the Northern and Southern Hemispheres.

南大洋における海洋フロントの南北シフト

The Southern Ocean plays an important role in the global climate system both at present and in the geologic past. To resolve the causes and processes of atmospheric CO2 change, it is important to understand the mechanisms and processes of sub-systems in the Antarctic Cryosphere such as change of biological productivity, sea-surface temperature, surface water frontal system, sea ice distribution, and the Antarctic Ice Sheet during the glacial-interglacial climate cycle. A large number of float observations made recently suggest that mid-depth Southern Ocean temperatures rose 0.17°C between the 1950s and 1980s. The Southern Ocean is warming faster than the global oceans, and this is concentrated within the Antarctic Circumpolar Current (ACC). Warming is consistent with a poleward shift of the ACC, probably driven by long-term poleward shifts in the winds of the region, as represented by the southern annular mode. Changes to the extent of Antarctic sea ice are difficult to quantify for the pre-satellite observation era. However, a substantially larger set of proxy records based on whaling positions indicates that a larger southward shift of the summer sea ice edge occurred between the mid-1950s and early 1970s. In the glacial to interglacial cycle, ice-rafted debris (IRD) is an important proxy for reconstructing past iceberg discharges and sea ice expansions. However, it is necessary to specify the origin of IRD in the Southern Ocean, because IRD deposition on the pelagic seafloor is controlled not only by the dynamics of the Antarctic ice sheet but also by surface water conditions such as sea-surface temperature and oceanic front migrations. For example, several layers rich in volcanic tephra were deposited in the eastern Atlantic sector of the Southern Ocean. Deposition of the tephra-rich IRD layers was controlled by changes in sea-surface temperature and sea ice conditions in the Polar Frontal Zone of the South Atlantic, rather than Antarctic ice sheet dynamics. Thus, IRD deposition is a signal of the expansion of sea ice in the South Atlantic. According to IRD records, it seems that sea ice expansion events occurred suddenly in the Atlantic sector of the Southern Ocean during the last glacial period.

Rapid response of Helheim Glacier in Greenland to climate variability over the past century

During the early 2000s the Greenland Ice Sheet experienced the largest ice-mass loss of the instrumental record 1 , largely as a result of the acceleration, thinning and retreat of large outlet glaciers in West and southeast Greenland 2‐5 . The quasi-simultaneous change in the glaciers suggests a common climate forcing. Increasing air 6 and ocean 7,8 temperatures have been indicated as potential triggers. Here, we present a record of calving activity of Helheim Glacier, East Greenland, that extends back to about AD 1890, based on an analysis of sedimentary deposits from Sermilik Fjord, where Helheim Glacier terminates. Specifically, we use the annual deposition of sand grains as a proxy for iceberg discharge. Our record reveals large fluctuations in calving rates, but the present high rate was reproduced only in the 1930s. A comparison with climate indices indicates that high calving activity coincides with a relatively strong influence of Atlantic water and a lower influence of polar water on the shelf off Greenland, as well as with warm summers and the negative phase of the North Atlantic Oscillation. Our analysis provides evidence that Helheim Glacier responds to short-term fluctuations of large-scale oceanic and atmospheric conditions, on timescales of 3‐10 years. The forcings behind the rapid increase in mass loss from the Greenland Ice Sheet in the early 2000s (ref. 1) are still debated. It is unclear whether the mass loss will continue in the near future and,ifso,atwhatrate.Theseuncertaintiesareaconsequenceofour limited understanding of mechanisms regulating ice-sheet variability and the response of fast-flowing outlet glaciers to climate variability. In southeast Greenland, Helheim Glacier, one of the regions largestglaciers,thinned,acceleratedandretreatedduringtheperiod 20032005

Characterizing the sediment provenance of East Antarctica's weak underbelly: The Aurora and Wilkes sub‐glacial basins

The Wilkes and Aurora basins are large, low‐lying sub‐glacial basins that may cause areas of weakness in the overlying East Antarctic ice sheet. Previous work based on ice‐rafted debris (IRD) provenance analyses found evidence for massive iceberg discharges from these areas during the late Miocene and Pliocene. Here we characterize the sediments shed from the inferred areas of weakness along this margin (94°E to 165°E) by measuring40Ar/39Ar ages of 292 individual detrital hornblende grains from eight marine sediment core locations off East Antarctica and Nd isotopic compositions of the bulk fine fraction from the same sediments. We further expand the toolbox for Antarctic IRD provenance analyses by exploring the application of 40Ar/39Ar ages of detrital biotites; biotite as an IRD tracer eliminates lithological biases imposed by only analyzing hornblendes and allows for characterization of samples with low IRD concentrations. Our data quadruples the number of detrital 40Ar/39Ar ages from this margin of East Antarctica and leads to the following conclusions: (1) Four main sectors between the Ross Sea and Prydz Bay, separated by ice drainage divides, are distinguishable based upon the combination of 40Ar/39Ar ages of detrital hornblende and biotite grains and the εNd of the bulk fine fraction; (2) 40Ar/39Ar biotite ages can be used as a robust provenance tracer for this part of East Antarctica; and (3) sediments shed from the coastal areas of the Aurora and Wilkes sub‐glacial basins can be clearly distinguished from one another based upon their isotopic fingerprints.

A Reason to Surge

Climate Science Heinrich events—massive, episodic iceberg discharges from the Laurentide Ice Sheet into the north Atlantic Ocean during the last glacial period—once were thought to be the consequence of internal ice sheet stability: the inability of the ice sheet to remain intact after having grown to a particularly large size. Support for this view has eroded over the past decade, however, and the suggestion that the events are instead the consequence of external causes, such as solar variability or the addition of glacial meltwater into the adjacent surface ocean, have gained favor. Marcott et al. add to that line of thought, with a combination of data from benthic foraminifera and results from a climate-model simulation, which suggest that basin-wide subsurface warming occurred at the same time as large reductions in Atlantic deep circulation, with subsurface temperature increasing by approximately 2° C during the 1000- to 2000-year interval before a Heinrich event. Such a temperature rise would erode the floating ice shelves that buttress terrestrial ice streams, and produce massive iceberg discharges. This finding could have important implications about how these ice sheets might behave in the future in our warming climate. Proc. Natl. Acad. Sci. U.S.A. 108 , 10.1073/pnas.1104772108 (2011).

Modeling the surface mass-balance response of the Laurentide Ice Sheet to Bølling warming and its contribution to Meltwater Pulse 1A

Meltwater Pulse (MWP) 1A occurred ~14.5–14ka and is the largest abrupt rise in sea level (10–20m of sea-level rise) of the last deglaciation. The timing of MWP-1A is coincident with or shortly follows the abrupt warming of the North Atlantic region into the Bølling warm period, which could have triggered a large Laurentide Ice Sheet (LIS) contribution to MWP-1A. Given that outside of the Arctic, LIS iceberg discharge probably did not increase during the Bølling, much of the LIS MWP-1A contribution likely occurred through surface ablation. Here we test the response of LIS surface mass-balance to Bølling warming by forcing a LIS energy–mass balance model with climate from an atmosphere–ocean general circulation model. Our modeling approach neglects changes in LIS mass from dynamics and iceberg calving, allowing us to isolate the surface mass balance response. Model results suggest that LIS surface ablation can explain much of the sea-level rise just prior to MWP-1A. LIS surface mass-balance becomes more negative in response to the Bølling warming, contributing an additional 2.9±1.0m of sea-level rise in 500yr in addition to the background contribution of 4.0±0.8m. The modeled LIS MWP-1A contribution is less than previous assumptions but agrees with geochemical runoff and LIS area-volume estimates. The fraction of MWP-1A attributable to other ice sheets, particularly Antarctica, depends on the total sea-level rise that occurred during this MWP.

Okhotsk Sea ice coverage and Kamchatka glaciation over the last 350 ka — Evidence from ice-rafted debris and planktonic δ 18O

High-resolution records of ice-rafted debris (IRD) and oxygen isotope records spanning an E–W-trending transect across the Okhotsk Sea unravel the marine and terrestrial cryogenic history of NE-Siberia over the last 350 kyr. IRD, predominantly dispersed basin-wide by sea-ice, shows lowest fluxes during interglacial periods implying a reduced and seasonal sea-ice coverage. Highest IRD accumulation rates are observed during glacial and deglacial periods with a more extended, but highly dynamic ice cover. Although being rather synchronous, IRD fluxes are on average higher in the western Okhotsk Sea than in the eastern part, pointing to a persistent but mobile, particle-supplying sea-ice cover even during full glacial conditions, presumably less dense in the eastern parts. MIS 6 is exceptional in this respect: Asynchronous fluxes of IRD, which vary spatially, reflect rapid paleoclimatic and paleo-glaciomarine changes. IRD accumulation rates were by factor 2–3 higher during MIS 6 as compared to the last glacial maximum, and the IRD depositional center shifted from the western Okhotsk Sea (early MIS 6) toward Kamchatka (late MIS 6), synchronous to a distinct change in the IRD mineral composition. Both, the characteristic composition of late MIS 6 IRD originating from the Sredinny Mountain Range of Kamchatka and their significantly enhanced accumulation rates refer to intensified iceberg dispersal across the eastern part of the Okhotsk Sea at ~ 138 ka, ~ 135 ka, ~ 129 ka, and ~ 128 ka BP. This scenario affords the presence of extended mountain glaciers protruding down to sea level on the western side of Kamchatka. Anomalously light planktonic stable oxygen isotopes during MIS 6.3, Termination II and MIS 5.5–5.4 suggest significant freshwater supply related to the westward drainage of Kamchatka glaciers. The intensified Kamchatka glaciation observed during late MIS 6 was repeated during MIS 3. Iceberg discharges into the eastern Okhotsk Sea are observed at ~ 60 ka, ~ 51 ka, ~ 42 ka, ~ 38 ka, ~ 36 ka, and ~ 31 ka, and may partly correspond to N-Atlantic Heinrich Events.

Vegetation history of the penultimate glacial period (Marine isotope stage 6) at Ioannina, north‐west Greece

AbstractThe vegetational history of the penultimate glacial period, Marine Isotope Stage (MIS) 6 (c. 185–135 ka), has remained relatively unexplored. Here we present a new record from the Ioannina basin, north‐west Greece, which constitutes the highest‐resolution terrestrial pollen record for this interval produced to date. It shows that the vegetation history of MIS 6 in this region can be divided into two parts: an early period (185–155 ka) with pronounced oscillations in tree population extent, and a later period (155–135 ka) with much smaller tree populations and subdued oscillations. This pattern is analogous to the MIS 3/MIS 2 division during the last glacial in the same sequence, although the early part of MIS 6 had larger Pinus populations and fewer temperate trees relative to the equivalent interval in MIS 3. This implies cooler and wetter conditions, which is somewhat counterintuitive given the high summer insolation during MIS 6e, but is in line with other palaeoclimatic evidence from the Mediterranean. Comparison with North Atlantic records suggests that despite the absence of pronounced iceberg discharges during MIS 6, North Atlantic millennial‐scale variability had a significant downstream impact on tree populations in north‐west Greece. Copyright © 2011 John Wiley & Sons, Ltd.

Early arrival of Southern Source Water in the deep North Atlantic prior to Heinrich event 2

The Atlantic Meridional Overturning Circulation (AMOC) plays an important role in the Northern Hemisphere climate system. Significant interest went into the question of how excessive freshwater input through melting of continental ice can affect its overturning vigor and, hence, heat supply, to higher northern latitudes. Such forcing can be tested by investigating its behavior during extreme iceberg discharge events into the open North Atlantic during the last glacial period, the so‐called Heinrich events (HE). Here we present neodymium (Nd) isotope compositions of past seawater, a sensitive chemical water mass tag, extracted from sediments of Ocean Drilling Program Site 1063 in the western North Atlantic (Bermuda Rise), covering the period surrounding HE 2, the Last Glacial Maximum, and the early deglaciation. These data are compared with a record of the kinematic circulation tracer (231Pa/230Th)xs extracted from the same sediment core. Both tracers indicate significant circulation changes preceding intense ice rafting during HE 2 by almost 2 kyr. Moreover, the Nd isotope record suggests the presence of deeply ventilating North Atlantic Deep Water early during Marine Isotope Stage 2 until it was replaced by Southern Source Water at ∼27 ka. The early switch to high (Pa/Th)xs and radiogenic ɛNd in relation to intensified ice rafting during HE 2 suggests that ice rafting into the open North Atlantic during major HE 2 was preceded by an early change of the AMOC. This opens the possibility that variations in AMOC contributed to or even triggered the ice sheet instability rather than merely responding to it.

Modelling abrupt glacial North Atlantic freshening: Rates of change and their implications for Heinrich events

The abrupt delivery of large amounts of freshwater to the North Atlantic in the form of water or icebergs has been thought to lead to significant climate change, including abrupt slowing of the Atlantic Ocean meridional overturning circulation. In this paper we examine intermediate complexity coupled modelling evidence to estimate the rates of change, and recovery, in oceanic climate that would be expected for such events occurring during glacial times from likely sources around the North Atlantic and Arctic periphery. We show that rates of climate change are slower for events with a European or Arctic origin. Palaeoceanographic data are presented to consider, through the model results, the origin and likely strength of major ice-rafting, or Heinrich, events during the last glacial period. We suggest that Heinrich events H1–H3 are likely to have had a significant contribution from an Arctic source as well as Hudson Strait, leading to the observed climate change. In the case of H1 and H2, we hypothesise that this secondary input is from a Laurentide Arctic source, but the dominant iceberg release for H3 is hypothesised to derive from the northern Fennoscandian Ice Sheet, rather than Hudson Strait. Earlier Heinrich events are suggested to be predominantly Hudson Strait in origin, with H6 having the lowest climate impact, and hence iceberg flux, but H4 having a climate signal of geographically variable length. We hypothesise that this is linked to a combination of climate-affecting events occurring around the globe at this time, and not just of Laurentide origin.