A high resolution record of ice-rafting from the Northeast Pacific
Date
1996
Authors
Hewitt, Antony Trevor
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Abstract
Ice-rafted debris (IRD) in marine sediments can provide considerable information on a number of aspects of the paleoenvironment, including the timing of continental ice advance and retreat, and also paleoceanography. In this study, five deep sea cores from the northeast Pacific were investigated, consisting of four from the Gulf of Alaska, and one from off northern Vancouver Island, British Columbia. The supply of IRD was evaluated in each of the cores based on the mass accumulation rate of coarse lithic material, and on Gamma Ray Attenuation Porosity Evaluator (GRAPE) density in one of the cores from the Gulf of Alaska. The provenance (source) and shape of IRD grains were also evaluated under a microscope. Together, the results provide a high-resolution, 800,000 year account of ice-rafting in the Northeast Pacific.
It was found that the Chugach and St. Elias mountains of Southern Alaska were the dominant source of IRD in the Gulf of Alaska cores, but at times there was an increased proportion of debris from southeastern Alaska and/or northwestern British Columbia (Alexander Archipelago - Queen Charlotte Sound). Such times are proposed to represent cold ocean conditions when a greater number icebergs calved from eastern sources survived melting to reach the more northerly core sites. Northwestern British Columbia (Queen Charlotte Sound) was the chief source of IRD in the core off northern Vancouver Island. Periods of increased IRD flux in the Gulf of Alaska are concurrent with intervals of widespread continental ice advance in western North America during the last 40,000 years. This work suggests that many additional, older ice advance-retreat cycles occurred during the late Quaternary, and since the chronology extends beyond the range of radiocarbon dating it provides a detailed history of these events through the late Pleistocene. The single IRD event in the core off Northern Vancouver Island occurs out of phase with those recorded in the Gulf of Alaska cores, and in fact occurs during the initial stages of deglaciation of the shelf in that area. This may reflect a northward shift of the subarctic ocean gyre boundary, such that the surface ocean circulation favored iceberg transport over this core site during deglaciation, but not during full glacial conditions.
Fluctuations in IRD supply are concurrent between the four cores from the Gulf of Alaska, but in some cases the magnitude of IRD accumulation rates are quite different, in some cases. The close proximity of these core sites probably precludes paleoclimatic/oceanographic factors in producing this variability. Instead, it may be a result of random debris dumps off the top of icebergs. Correlation between the IRD record and 8180 record in Greenland ice (GRIP) reveals that IRD flux decreased markedly during especially long interstadials recorded in the GRIP core. These are the same interstadials that follow the North Atlantic Heinrich events. Correlation with the marine 8180 isotope record (SPECMAP) shows that the 100,000 year cycle in ice volume, that dominates the SPECMAP record, is in phase with a 100,000 year cycle in IRD flux. The correlations with GRIP and SPECMAP suggest fluctuations in Laurentide ice volume and consequent forcing of the climate system are important factors controlling the timing of ice advance/retreat in northwestern North America on both 100,000 year time scales, and on suborbital time scales.
IRD was also deposited during several global interglacial intervals, including isotope stage 5e, but not during the Holocene. The shorter, higher frequency Dansgaard-Oeschger interstadials (those that occur between interstadials that follow the Heinrich events) have no correlative interstadials recorded in North Pacific IRD accumulation. There is also no IRD event associated with Younger Dryas time. These events likely have origins different from the climatic effects of ice volume oscillations.