Matthias Moros

An extensive and dynamic ice sheet on the West Greenland shelf during the last glacial cycle

Considerable uncertainty surrounds the extent and timing of the advance and retreat of the Greenland Ice Sheet (GIS) on the continental shelf bordering Baffin Bay during the last glacial cycle. Here we use marine geophysical and geological data to show that fast-flowing ice sheet outlets, including the ancestral Jakobshavn Isbrae, expanded several hundred kilometers to the shelf edge during the last glaciation ca. 20 ka. Retreat of these outlets was asynchronous. Initial retreat from the shelf edge was underway by 14,880 calibrated (cal) yr B.P. in Uummannaq trough. Radiocarbon dates from the adjacent Disko trough and adjoining trough-mouth fan imply later deglaciation of Jakobshavn Isbrae, and, significantly, an extensive readvance and rapid retreat of this outlet during the Younger Dryas stadial (YD). This is notable because it is the first evidence of a major advance of the GIS during the YD on the West Greenland shelf, although the short duration suggests that it may have been out of phase with YD temperatures.

Climate and oceanographic variability in the SW Barents Sea during the Holocene

The Holocene section of the marine sediment core PSh-5159N, located in the SW Barents Sea, has been studied at high resolution with a multiproxy approach. A well-stratified water column occurred at the site 11—9.8 ka BP. The stratification was probably a result of a winter sea ice cover and/or fresh, warm surface waters during summer. Stratification and resultant reduction in air—sea interaction allowed for warmer bottom water temperatures. The general situation 11—9.8 ka BP could have been associated with an anomalous high-pressure system over the Nordic Seas and the Arctic Ocean. During the 11—10.5 ka BP interval the polar front was located close to the Barents Sea margin. The polar front moved towards the site from 10.5 ka BP, and from 9.8 to 7.5 ka BP it was probably located close to the site. At 7.5 ka BP the polar front retreated eastwards as the present-day oceanographic pattern established. The mid Holocene was in general characterized by rather stable conditions. In contrast, highly variable conditions are recorded throughout the late Holocene. Episodic expansions of the coastal water influenced zone are typical for the last 2.5 ka BP. Predominantly cold conditions and reduced southwesterly wind strength are suggested during these episodes. The Holocene temperature variability seems in general to be of larger amplitude than instrumentally recorded temperature changes in the SW Barents Sea.

Early Holocene temperature variability in the Nordic Seas: The role of oceanic heat advection versus changes in orbital forcing

Received 7 January 2011; revised 15 July 2011; accepted 21 July 2011; published 22 October 2011. [1] The separate roles of oceanic heat advection and orbital forcing on influencing early Holocene temperature variability in the eastern Nordic Seas is investigated. The effect of changing orbital forcing on the ocean temperatures is tested using the 1DICE model, and the 1DICE results are compared with new and previously published temperature reconstructions from a transect of five cores located underneath the pathway of Atlantic water, from the Faroe‐Shetland Channel in the south to the Barents Sea in the north. The stronger early Holocene summer insolation at high northern latitudes increased the summer mixed layer temperatures, however, ocean temperatures underneath the summer mixed layer did not increase significantly. The absolute maximum in summer mixed layer temperatures occurred between 9 and 6 ka BP, representing the Holocene Thermal Maximum in the eastern Nordic Seas. In contrast, maximum in northward oceanic heat transport through the Norwegian Atlantic Current occurred approximately 10 ka BP. The maximum in oceanic heat transport at 10 ka BP occurred due to a major reorganization of the Atlantic Ocean circulation, entailing strong and deep rejuvenation of the Atlantic Meridional Overturning Circulation, combined with changes in the North Atlantic gyre dynamic causing enhanced transport of heat and salt into the Nordic Seas.

A signal of persistent Atlantic multidecadal variability in Arctic sea ice

Satellite data suggest an Arctic sea ice-climate system in rapid transformation, yet its long-term natural modes of variability are poorly known. Here we integrate and synthesize a set of multicentury historical records of Atlantic Arctic sea ice, supplemented with high-resolution paleoproxy records, each reflecting primarily winter/spring sea ice conditions. We establish a signal of pervasive and persistent multidecadal (~60–90 year) fluctuations that is most pronounced in the Greenland Sea and weakens further away. Covariability between sea ice and Atlantic multidecadal variability as represented by the Atlantic Multidecadal Oscillation (AMO) index is evident during the instrumental record, including an abrupt change at the onset of the early twentieth century warming. Similar covariability through previous centuries is evident from comparison of the longest historical sea ice records and paleoproxy reconstructions of sea ice and the AMO. This observational evidence supports recent modeling studies that have suggested that Arctic sea ice is intrinsically linked to Atlantic multidecadal variability. This may have implications for understanding the recent negative trend in Arctic winter sea ice extent, although because the losses have been greater in summer, other processes and feedbacks are also important.

A 100 yr record of ocean temperature control on the stability of Jakobshavn Isbrae, West Greenland

An understanding of the interaction between ice sheet dynamics and forcing mechanisms, such as oceanic and atmospheric circulation, is important because of the potential contribution of these processes to constraining models that seek to predict future rates of sea-level change. Here we report new benthic foraminiferal data from Disko Bugt, West Greenland, showing a close correlation between subsurface ocean temperature changes and the ice margin position of the glacier Jakobshavn Isbrae over the past 100 yr. In particular, our faunal data show that warm ocean currents entered a bay, Disko Bugt, during the retreat phases of Jakobshavn Isbrae from A.D. 1920 to 1950 and since 1998. We also show a link between West Greenland ocean temperature and the Atlantic Multidecadal Oscillation, a key climate indicator in the North Atlantic Ocean. The close coupling between the oceans and the cryosphere identified here should be assessed in future projections of sea-level change.

Physical properties of Reykjanes Ridge sediments and their linkage to high-resolution Greenland Ice Sheet Project 2 ice core data

Five gravity cores taken from the Reykjanes Ridge have been used to establish a link between sediment physical properties and atmospheric records documented by δ18O variations in Greenland ice cores over the last 45,000 calendar years. Marine Gamma Ray Attenuation Porosity Evaluator density and magnetic susceptibility variations could be linked with the ice core Dansgaard-Oeschger and Bond cycles. This is supported by ice-rafted detritus (IRD), grain size, the quartz/feldspar ratio, and carbonate, isotopic, and foraminiferal records. The covariation of the sediment physical properties and δ18O in Greenland ice indicates a coupling of atmospheric temperature and paleocirculation variations. Gradual reduced bottom currents (Iceland-Scotland Overflow Water) and enhanced iceberg discharges have been reconstructed for cold atmospheric periods relative to interstadial times. In the study area the magnetic susceptibility signal is not related to the ice-rafted detritus input but most probably reflects the variations of the Iceland-Scotland Overflow Water intensity transporting titanomagnetite into the Reykjanes Ridge region.

A robust, multisite Holocene history of drift ice off northern Iceland: implications for North Atlantic climate

An important indicator of Holocene climate change is provided by evidence for variations in the extent of drift ice. A proxy for drift ice in Iceland waters is provided by the presence of quartz. Quantitative x-ray diffraction analysis of the < 2 mm sediment fraction was undertaken on 16 cores from around Iceland. The quartz weight (wt.)% estimates from each core were integrated into 250-yr intervals between −0.05 and 11.7 cal. ka BP. Median quartz wt.% varied between 0.2 and 3.4 and maximum values ranged between 2.8 and 11.8 wt.%. High values were attained in the early Holocene and minimum values were reached 6—7 cal. ka BP. Quartz wt.% then rose steadily during the late Holocene. Our data exhibit no correlation with counts on haematite-stained quartz (HSQ) grains from VM129-191 west of Ireland casting doubt on the ice-transport origin. A pilot study on the provenance of Fe oxide grains in two cores that cover the last 1.3 and 6.1 cal. ka BP indicated a large fraction of the grains between 1 and 6 cal. ka BP were from either Icelandic or presently unsampled sources. However, there was a dramatic increase in Canadian and Russian sources from the Arctic Ocean ~1 cal. ka BP. These data may indicate the beginning of an Arctic Oscillation-like climate mode.

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 Bolling-Allerod 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.

Holocene palaeoceanographic evolution off West Greenland

Benthic foraminiferal assemblages from a core southwest of Disko Bugt provide a Holocene perspective (last ~7 ka BP) on ice-sheet/ocean interactions between the West Greenland Current (WGC) and the West Greenland ice sheet. Changes in the fauna reveal significant variations in the water mass properties (temperature and salinity) of the WGC through time. From 7.3 to 6.2 ka BP, a relatively warm/strong WGC influences ice-sheet melt in Disko Bugt and causes enhanced meltwater production, resulting in low surface-water productivity. The most favourable oceanographic conditions occur from 5.5 to 3.5 ka BP, associated with ‘thermal optimum-like’ conditions, encompassing minimum ice sheet extent in the Disko Bugt area. These conditions are attributed to: (1) reduced meltwater influence as the ice sheet is land based and (2) enhanced contribution of warm/saline water masses from the Irminger Current to the WGC. The transition into the late Holocene (last ~3.5 ka BP) is characterized by a cooling of oceanographic conditions, caused by increased advection of cold/low-salinity water masses from the East Greenland Current. A longer-term late-Holocene cooling trend within the WGC is attributed to the onset of Neoglacial cooling within the North Atlantic region. Superimposed on this cooling trend, multicentennial-scale variability within the WGC matches reconstructions from a nearby coring site in Disko Bugt as follows: (1) cooling at ~2.5 ka BP, linked to the 2.7 ka BP ‘cooling event’; (2) a warm phase centred at 1.8 ka BP, associated with the ‘Roman Warm Period’; (3) slight warming between 1.4 and 0.9 ka BP, linked to the ‘Medieval Climate Anomaly’; (4) severe cooling of the WGC after 0.9 ka BP, culminating at 0.3 ka BP during the ‘Little Ice Age’. We show that multicentennial-scale palaeoceanography variability along the West Greenland margin is driven by ocean forcing, i.e. variations in the relative contribution of Atlantic (Irminger Current) and Polar (East Greenland Current) water masses to the WGC during the last ~7 ka BP, influencing ice sheet dynamics.

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 mum. 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