ABSTRACT: In search of Holocene high-resolution IRD signals from the margins of Labrador, East Greenland, and Iceland: is there a pervasive signal?

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Abstract

Changes in the concentration, extent, and duration of "drift ice" (sea ice and icebergs) to the sub-polar North Atlantic Ocean have a potentially large impact on the Climate System. This was well illustrated by the impact of the Great Salinity Anomaly of the late 1960's. Cores from sites well removed from present-day drift-ice limits have a pervasive 1.5 ky periodicity in the Holocene. However, work in the 1980's and 1990's on the East Greenland and Labrador/Baffin Is. margins showed a dissimilar history of drift-ice IRD. We expect a priori that the deposition of IRD is inherently noisy because of the variability of the sediment load , the duration and extent of drift ice across any 7-10 cm diameter portion of the seafloor, and in the melt rate of the ice. To better describe IRD variations in the more drift-ice proximal regions of East Greenland, North Iceland, and the Baffin/Labrador margins we quantify weight% variations in non-clay and clay minerals using X-ray diffraction on sediments < 2 mm in size, on a wide spatial array of both surface and high-resolution (20-100 yr per sample) down core samples, the latter largely derived from the 1999 IMAGES V cruise. On the Iceland margin, and East Greenland between 68-70°N, the mineralogy of glacial sediments is dominated by plagioclase feldspars and pyroxene with virtually no quartz. An initial focus is on the records from the Iceland margin because there are historical sources for drift-ice variations that go back to the time of the Settlement in ~870AD. The cores from the NW to N-central Iceland shelf shows an initial interval of high values of quartz (2- 5%), including the 8.2 cal ka BP event, followed by a prolonged interval of low (< 1%) values. However, starting between 4-5 cal ka BP the amount of quartz in the sediments increases toward the present day (2- 5%). The quartz may well derive, in part, from the Eurasian shelf because the abundant driftwood on the north Icelandic coast has been traced to that region and is carried there via the East Greenland and East Iceland currents. On the East Greenland shelf the variations in clasts > 2 mm parallel the quartz trends from Iceland but the actual quartz variations show no clear Neoglacial signal. At Orphan Knoll the variations in quartz and dolomite weight% in the < 2mm sediment have similar trends---the dolomite is probably derived from NW Greenland--which includes high values during the deglaciation of Arctic Canada and Greenland, followed by lower but persistent inputs (2-7 weight%) over the last 6 cal ka BP. Despite clear terrestrial evidence for glacial regrowth in Canada and Greenland there is no dramatic Neoglacial signal over the last few thousand years at this marine site. Despite the major differences in presumed IRD trends the weight% of calcite over the last 10 cal ka BP is remarkably similar along the three margins, with a major exception being a strong decline in calcite over the last 2 cal ka BP at Icelandic and East Greenland sites, whereas the Labrador Sea record shows no such decline. Analysis of the time-series shows a variety of multi-decadal to multi-century periodicities, depending on the sampling resolution, but there is no clear millennial signal at these sites. Thus interpretation of the Holocene history of drift-ice in the North Atlantic basin is not without ambiguities in terms of appropriate proxies and patterns of variability.

Bibliography of Canadian Geomorphology