Sources of Sediment Found in Sea Ice From the Western Arctic Ocean, New Insights Into Processes of Entrainment and Drift Patterns

The geochemical fingerprint of entrained Fe oxide mineral grains in Arctic Ocean sea ice is used to determine precise sources of this sediment. This approach substantiates the importance of the Laptev Sea as a source of sea ice and even the presence of Russian ice in the Beaufort Sea off Alaska. Fe oxide grains from the Laptev Sea were found in floes in the Beaufort Sea, Chukchi Borderland, and central Arctic Ocean, demonstrating the importance of sea ice in distributing itself throughout the Arctic Ocean and the potential of transporting sediment from Russian rivers to North American shelves. Banks Island shelf is the most important source of sediment sampled from ice floes in the Beaufort Sea, northern Chukchi Sea, and Chukchi Borderland area. Although most of the entrained sediment fits the criteria for suspension freezing in shallow water, the presence of winter polynyas with offshore winds and not the size of shallow areas appears to be the critical factor for sea ice entrainment. Seven of the 18 ice floes sampled contained Fe oxide grains from more than one source area. The two most common sources that are found in the same ice floes are Banks Island and the Laptev Sea. Multiple sources in ice floes suggest that either mingling of fragmented ice floes occurs or that a source area containing grains from both these sources has yet to be located. The addition of fine, sand-sized, windblown sediment is not thought to be significant

Arctic Perennial Ice Cover Over the Last 14 Million Years

Knowledge of the long-term history of the perennial ice is an important issue that has eluded study because the Cenozoic core material needed has been unavailable until the recent Arctic Coring Expedition (ACEX). Detrital Fe oxide mineral grains analyzed by microprobe from the last 14 Ma (164 m) of the ACEX composite core on the Lomonosov Ridge were matched to circum-Arctic sources with the same mineral and 12-element composition. These precise source determinations and estimates of drift rates were used to determine that these sand grains could not be rafted to the ACEX core site in less than a year. Thus the perennial ice cover has existed since 14 Ma except for the unlikely rapid return to seasonal ice between the average sampling interval of about 0.17 Ma. Both North America and Russia contributed significant Fe grains to the ACEX core during the last 14 Ma

Trace-Elements in Ilmenite - A Way to Discriminate Provenance or Age in Coastal Sands

Trace elements and Ti percentage in ilmenite grains magnetically separated from modern and late Pleistocene coastal sands of southeastern Virginia and northwestern North Carolina were used to distinguish different deposits. Multivariate analysis of ilmenite composition (Ti, Mn, Mg, Cr, V, Ni, and Cu) from coastal deposits and potential source rivers enabled the identification of dominant source rivers. Using the trace element content of one mineral instead of heavy-mineral suites eliminated most of the hydraulic sorting, selective weathering, and intrastratal solution problems that often obscure heavy-mineral provenance determinations. Most ilmenite grains lacked exsolution or twinning, which are common to ilmenite; however, there were no significant optical differences between river and coastal deposits, and thus weathering effects were considered to be negligible in provenance determinations based on ilmenite composition. Owing to the dynamic mixing of beach sands during deposition, they contained more homogeneous ilmenite trace-element values than did river or bay sands. Late Pleistocene and modern beach deposits were compositionally similar, but different from associated bay sands. Bay sands were more similar to different source river deposits than were beach sands. Despite a similar primary or distal provenance, subtle differences in the mixture of proximal sources were revealed between the ilmenite composition of samples from a modern arid a late Pleistocene beach deposit. Besides aiding in provenance determination, ilmenite trace-element content thus might be used for distinguishing beach deposits of different ages and for subsurface correlation of discontinuous segments from a barrier-island chain or other similarly well mixed sand deposit

Ice-Rafted Detritus Events in the Arctic During the Last Glacial Interval, and the Timing of the Innuitian and Laurentide Ice Sheet Calving Events

Ice-rafted detritus (IRD) layers in the Arctic Ocean not only indicate the source of this detrital sediment, but give insights into the ice drift and ice sheet history. Detrital sand-sized FE oxide mineral grains that are matched to precise sources using the microprobe chemical fingerprint of each grain, along with elevated coarse IRD abundance and radiocarbon ages, are used to define IRD peaks from the Innuitian and Arctic portions of the Laurentide ice sheets. Because grains from these two areas can be entrained by sea ice from the shelves just offshore of the calving areas, peaks in these grains must be correlated to coarse IRD to identify iceberg calving events, and to distinguish them from sea-ice rafting. The sequence of IRD peaks deposited by icebergs from these two ice sheets indicate that both ice sheets calved bergs at accelerated numbers, six or seven times, from 11 to 36 Kya. The relatively short times between most of these IRD events suggest that the ice sheets did not completely collapse with each IRD Event, except the last event. Although there is some indication that one ice sheet may have begun calving bergs before the other, the resolution of the Arctic cores does not allow definitive determination of this. This emphasizes the need for higher resolution cores from the central Arctic, as well as from near the terminus of large Pleistocene ice sheets. Sea-ice rafting occurs throughout the last glacial stage, even during some glacial IRD events, as indicated by FE grains from non-glacial sources

A Holocene Record of Changing Arctic Ocean Ice Drift Analogous to the Effects of the Arctic Oscillation

The Arctic Oscillation (AO) controls the configuration of the Transpolar Drift (TPD). If thicker ice from the Beaufort Gyre were exported, the volume of fresh water/sea ice in the Nordic seas would significantly increase, decreasing the formation of North Atlantic deep water. This would cool large parts of the Northern Hemisphere and affect global climate. Therefore, in order to understand how the global climate system functions, it is imperative to know how the TPD changed over the last millennium or more. The provenance of grains in a sediment core located near the confluence of the TPD and the Beaufort Gyre provides a direct proxy for changes in seaice drift owing to these circulation systems. The core has more than 200 cm of Holocene sediment, with intervals dominated by grains from the Russian shelves alternating with intervals of abundant grains from North American sources. Grains matched to Russian shelves indicate that the TPD was shifted toward North America, similar to what occurs during a more positive phase of the AO. This condition alternated with intervals where few grains matched to Russian sources, presumably because the TPD was restricted to the Russian half of the Arctic, far from the core site. During the last 1300 years, increased influx of Russian grains occurred approximately every 50-150 years. This fluctuation might represent the long-term oscillation of the AO, which modulates the same TPD shift today

Variability of Sea Ice Cover in the Chukchi Sea (Western Arctic Ocean) During the Holocene

Dinocysts from cores collected in the Chukchi Sea from the shelf edge to the lower slope were used to reconstruct changes in sea surface conditions and sea ice cover using modern analogue techniques. Holocene sequences have been recovered in a down-slope core (B15: 2135 m, 75°44\u27N, sedimentation rate of ~1cm kyr-1) and in a shelf core (P1: 201 m, 73°41\u27N, sedimentation rate of ~22 cm kyr-1). The shelf record spanning about 8000 years suggests high-frequency centennial oscillations of sea surface conditions and a significant reduction of the sea ice at circa 6000 and 2500 calendar (cal) years B.P. The condensed offshore record (B15) reveals an early postglacial optimum with minimum sea ice cover prior to 12,000 cal years B.P., which corresponds to a terrestrial climate optimum in Bering Sea area. Dinocyst data indicate extensive sea ice cover (\u3e10 months yr-1) from 12,000 to 6000 cal years B.P. followed by a general trend of decreasing sea ice and increasing sea surface salinity conditions, superimposed on large-amplitude millennial-scale oscillations. In contrast, δ18O data in mesopelagic foraminifers (Neogloboquadrina pachyderma) and benthic foraminifers (Cibicides wuellerstorfi) reveal maximum subsurface temperature and thus maximum inflow of the North Atlantic water around 8000 cal years B.P., followed by a trend toward cooling of the subsurface to bottom water masses. Sea-surface to subsurface conditions estimated from dinocysts and δ18O data in foraminifers thus suggest a decoupling between the surface water layer and the intermediate North Atlantic water mass with the existence of a sharp halocline and a reverse thermocline, especially before 6000 years B.P. The overall data and sea ice reconstructions from core B15 are consistent with strong sea ice convergence in the western Arctic during the early Holocene as suggested on the basis of climate model experiments including sea ice dynamics, matching a higher inflow rate of North Atlantic Water

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 xray diffraction analysis of the \u3c 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

Clay Mineral Cycles Identified by Diffuse Spectral Reflectance in Quaternary Sediments From the Northwind Ridge: Implications for Glacial-Interglacial Sedimentation Patterns in the Arctic Ocean

A Quaternary record of fine-grained sediment composition is used to investigate Arctic Ocean climate variability on glacial-interglacial time scales. Diffuse spectral reflectance data from sediment core P1-92AR-P25 from the Northwind Ridge, north of Alaska, demonstrates cyclic variations in mineralogy. Varimax-rotated R-mode factor analysis of down-core data revealed three major mineralogical assemblages, which were then compared with the content of manganese, a proxy for basin ventilation, and thus glacial-interglacial cycles. Results indicate that factor 1, a smectite + chlorite clay assemblage, was delivered to the core site during interglacials, either by fluvial discharge or sea-ice drift from Siberian rivers or inflow from the Bering Sea. Factor 2, an illite + goethite assemblage, is related to glacial periods, and was probably transported from the Laurentide Ice Sheet by icebergs or meltwater. Factor 3, glauconite, might have been sourced from the North Slope region of Alaska during deglacial intervals, or from dolomites associated with Laurentide iceberg-discharge pulses. The observed variations in sediment source and transport mechanisms arise from glacial-interglacial changes in sea level, the size of the terrestrial ice sheets surrounding the Arctic Ocean, the extent of sea-ice cover and altered atmospheric circulation. The reconstructed glacial-interglacial circulation patterns from the Late Quaternary show some similarity with modern circulation changes presumably related to the monthly- to decadally-fluctuating Arctic Oscillation. However, because the Arctic Oscillation operates on much shorter time scales, further research is necessary to better understand the driving mechanism for the changes observed over glacial-interglacial cycles, and the potential role of ocean-atmospheric interaction

Arctic Ice Export Events and Their Potential Impact on Global Climate During the Late Pleistocene

Ice sheets in the North American Arctic and, to a lesser extent, those in northern Eurasia calved large quantities of icebergs that drifted through Fram Strait into the Greenland Sea several times during the late Pleistocene. These icebergs deposited Fe oxide grains (45-250 mum) and coarse lithic clasts \u3e250 mum matched to specific circum-Arctic sources. Four massive Arctic iceberg export events are identified from the Laurentide and the Innuitian ice sheets, between 14 and 34 ka (calendar years) in a sediment core from Fram Strait. These relatively short duration (\u3c1-4 kyr) events contain 3-5 times the background levels of Fe oxide grains. They began suddenly, as indicated by a steep rise in the number of grains matched to an ice sheet source, suggesting rapid purges of ice through Fram Strait, due perhaps to collapse of ice sheets. The larger events from the northwestern Laurentide ice sheet are preceded by events from the Innuitian ice sheet. Despite the chronological uncertainties, the Arctic export events appear to occur prior to Heinrich events