Jens F. Bischof

Contrasting glacial/interglacial regimes in the western Arctic Ocean as exemplified by a sedimentary record from the Mendeleev Ridge

Distinct cyclicity in lithology and microfaunal distribution in sediment cores from the Mendeleev Ridge in the western Arctic Ocean (water depths ca. 1.5 km) reflects contrasting glacial/interglacial sedimentary patterns. We conclude that during major glaciations extremely thick pack ice or ice shelves covered the western Arctic Ocean and its circulation was restricted in comparison with interglacial,modern-type conditions. Glacier collapse events are marked in sediment cores by increased contents of ice-rafted debris,notably by spikes of detrital carbonates and iron oxide grains from the Canadian Arctic Archipelago. Composition of foraminiferal calcite N 18 O and N 13 C also shows strong cyclicity indicating changes in freshwater balance and/or ventilation rates of the Arctic Ocean. Light stable isotopic spikes characterize deglacial events such as the last deglaciation at ca. 12 14 C kyr BP. The prolonged period with low N 18 O and N 13 C values and elevated contents of iron oxide grains from the Canadian Archipelago in the lower part of the Mendeleev Ridge record is interpreted to signify the pooling of freshwater in the Amerasia Basin,possibly in relation to an extended glaciation in arctic North America. Unique benthic foraminiferal events provide a means for an independent stratigraphic correlation of sedimentary records from the Mendeleev Ridge and other mid-depth locations throughout the Arctic Ocean such as the Northwind and Lomonosov Ridges. This correlation demonstrates the disparity of existing age models and underscores the need to establish a definitive chronostratigraphy for Arctic Ocean sediments. ; 2003 Elsevier B.V. All rights reserved.

Radiocarbon chronology of depositional regimes in the western Arctic Ocean

Abstract The foraminiferal abundance and percentage of coarse ice-rafted detritus (IRD) define glacial, deglacial, and interglacial depositional regimes in AMS radiocarbon-dated box cores from the western Arctic Ocean. Sediment deposition rates are generally less than 0.5 cm/ka for glacial regimes, greater than this for deglacial regimes and greater than 1–2 cm/ka for interglacial regimes. These differences in deposition rates might account for the much lower average sedimentation rates for the last 780 ka of 1–3 mm/ka in cores from the central Arctic Ocean if glacial regimes dominated this interval. Foraminiferal abundances are less than 500/g during glacial maxima and mostly higher than 2000/g during deglacial and interglacial regimes. Slightly higher coarse IRD percentages occur in deglacial intervals (> 5–10% up to 30%) compared with interglacial intervals (mostly ≤5%), which characterize the last 10–12 ka in the western Arctic Ocean. Glacial regimes occurred from about 40 to 11 ka except for a brief interglacial or deglacial interval around 24–28 ka in the central Arctic Ocean. The coarsest deglacial events occurred prior to 45 ka. The Late Wisconsin deglaciation sediments (approximately 9–16 ka) are difficult to detect in the central Arctic Ocean sediments because they are only slightly coarser than the Holocene and, in some box cores, less coarse than the Holocene. A previously unrecognized coarse IRD event occurred near the core tops (0–3 cm) between 1500 and 3500 radiocarbon years ago in the western Arctic Ocean. Only sediment older than 40 ka is coarser than this recent IRD event, which might correspond to a Neoglaciation recognized in the North Atlantic and elsewhere.

Origin of ice-rafted debris: Pleistocene paleoceanography in the western Arctic Ocean

The composition of Pleistocene ice-rafted debris (IRD) >250 gm was analyzed quantitatively by grain counting in five sediment cores t¾om the western central Arctic Ocean and compared with the composition of till clasts from NW Canada in order to determine the dropstone origin and to reconstruct the Pleistocene ice drillways and surface currents. The IRD composition alternates repeatedly between carbonate- and quartz-dominated assemblages, along with metamorphic and igneous rocks, clastic rocks, and some chert. The highest quartz content is found on the Alpha Ridge, while carbonate percentages are highest on the Northwind Ridge (NWR) and the Chukchi Cap. The source for the carbonates is the area around Banks and Victoria Islands and parts of northern Canada. Quartz most likely originated from the central Queen Elizabeth Islands. IRD on the southeastern Alpha Ridge is dominated by mafic crystalline rocks from northern Ellesmere Island and northern Greenland. At least six major glacial intervals are identified within the last 1 million years, during which icebergs drifted toward the west in the Beaufort Sea, straight northward in the central Arctic Ocean, and northeastward on the SE Alpha Ridge.

A Statistical Approach to Source Determination of Lithic and Fe Oxide Grains: An Example from the Alpha Ridge, Arctic Ocean

ABSTRACT Discriminant function analysis (DFA) of microprobe data on 12 elements in nine Fe oxide mineral types was used to match each Fe oxide grain from an Arctic Ocean core to similarly analyzed grains in probable source areas for ice-rafted detritus. This approach to provenance allows us to determine the proportion of multiple sources with a high degree of statistical probability. Counts of microscopically identified dropstones (> 250 µm) from centimeter-thick samples of the core and from source-area samples provide a direct link to the sediment source. Multiple types of DFA (e.g., direct vs. stepwise) on the dropstone data provide slightly different sources for many samples from the core. This is due to multiple sources for each core sample and provides a more accurate picture of ources than one DFA procedure alone. Most of the lithic grains were derived from the northwestern Queen Elizabeth Islands centered around Ellef Ringnes Island and from the vicinity of Victoria and Banks Islands. Whereas the microprobe data from individual Fe oxide grains led to these same source areas, they always showed that several sources contributed to each centimeter-thick core sample. Significant input from both the dominant source areas to the same intervals in the core indicates that the ice sheets or ice caps covering these different areas coexisted during several pre-Wisconsin glaciations.

The decay of the Barents ice sheet as documented in nordic seas ice-rafted debris

Abstract A large, marine-based ice sheet that covered the Barents Sea and Svalbard during the height of the Weichselian Glaciation began to disintegrate at 15 ka. The decay produced enormous quantities of icebergs that drifted parallel to the Norwegian continental margin southward and northwestward into the Fram Strait. This drifttrack is reflected by a trace of ice-rafted clastic sedimentary rocks from the bottom of the Barents Sea in deep sea sediments of the Norwegian Sea and the Fram Strait. The petrographic composition of ice-rafted lithic particles (dropstones) > 0.5 mm in late Quaternary (20 ka-Recent) deep-sea sediments of the Norwegian Greenland Sea was analyzed in order to determine the source areas of the dropstones. The dropstone provenance was deciphered from the regional distribution of some precisely defined lithotypes. During the maximum glaciation between 20 and 15 ka, the Norwegian Sea received crystalline rocks primarily from western Norway, but also from Svalbard and the northwestern British Isles. The Greenland Sea was supplied with clay-, silt- and sandstones from central and northern East Greenland and rocks from circum Arctic areas. At 15 ka, a dramatic shift in provenance from crystalline rocks to clastic sedimentary rocks is recorded in the dropstone composition of the northern and eastern Norwegian Sea. This impulse was the ultimate result of the deterioration of the Barents ice sheet. The dispersal area of clastic rock fragments reaches from the Barents shelf between 72° and 76°N, and 15° and 20°E to the Voring Plateau (eastern Norwegian Sea) at 67°N and 5°E. The main direction of iceberg drift was to the south and southeast. After an initial period of only 500 years duration and extremely high dropstone deposition rates, the influx from the Barents shelf ceased and gradually changed into the modern surface current system.

Nordic Seas surface ice drift reconstructions: evidence from ice rafted coal fragments during oxygen isotope stage 6

Abstract Sixteen long sediment cores from the eastern Arctic Ocean, the Fram Strait and the Norwegian-Greenland Sea, documenting 200 000 years of sedimentation, were studied for their qualitative dropstone composition (>500 µm-fraction). In sediments from oxygen isotope stages 1–5, coal particles are usually subordinate components of the coarse fraction. In contrast to younger deposits, coal content in oxygen isotope stage 6 (186-128 ka) varies between 20% and 65 0n the eastern Arctic Ocean and the Fram Strait and between 5% and 20 0n the Norwegian Sea. Southward decreasing coal content and similarities in maturity and petrography of the coals indicate that the coal was transported by iceberg or sea ice rafting more than 1000 km to the south. It is suggested that during intervals of oxygen-isotope stage 6 drifting ice carried abundant coal fragments from the eastern Arctic Ocean southward through the Fram Strait into the eastern Norwegian Sea. Thus, surface circulation was then opposite to that of today.

Palynomorphs of ice rafted clastic sedimentary rocks in Late Quaternary glacial marine sediments of the Norwegian Sea as provenance indicators

Abstract Palynomorph assemblages of ice rafted pebbles of clastic sedimentary rocks in surface sediments from 0 to 41 cm depth of the Norweigan Sea are dominated by Triassic and Lower Cretaceous taxa along with occasional Jurassic, Lower Paleozoic, and very rare Carboniferous-Permian taxa. Tertiary species were not found, and Upper Cretaceous species were only found in the western Norweigan Sea. The ice-rafted rock fragments originated from the Late Pleistocene ice sheets adjacent to the Norwegian Sea. The determined ages of the ice-rafted clastic sedimentary rocks indicate the Barents shelf as the main source, because the Barents shelf has exposures of clastic sedimentary rocks whose ages and lithologies match the material found in the majority of the analyzed samples. In addition some material originated from the West Norwegian shelf and some from NE Greenland. Our results confirm that an ice sheet extending from SW Svalbard onto parts of the northwestern Barents shelf existed during the Late Weichselian. The northern dropstone source areas suggest that currents in the Norwegian Sea from 15 to 9 ka before present were moving in the opposite direction of modern currents