Dennis A. Darby

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.

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.

Icebreaker expedition collects key Arctic seafloor and ice data

The recently completed Healy-Oden Trans-Arctic Expedition 2005 (HOTRAX′05) retrieved 29 piston cores averaging nearly 12 meters in length from a complete transect across the central Arctic Ocean (Figure 1). These cores provide a critically-needed sample cache for both a pan-Arctic stratigraphy and a long-awaited paleoclimate record that it is hoped will greatly improve the understanding of how deepwater is exchanged between Arctic basins, how the climate system in the Arctic works over longer time intervals, and how the Arctic system interacts with global systems. The coring was done from the U.S. Coast Guard Cutter (USCGC) Healy, while oceanographic measurements were made from the Swedish icebreaker Oden. In addition to coring and oceanography, HOTRAX mapped the seafloor with multibeam bathymetry and collected chirp sonar profiles that not only mapped the strata to a sub-bottom depth of 50–100 meters, but also provided detailed information on the geologic context of the core sites.

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.

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.

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.

Evidence for radionuclide transport by sea ice

Ice and ice-borne sediments were collected across the Arctic Basin during the Arctic Ocean Section, 1994 (AOS-94), a recent US/Canada trans-Arctic expedition. Sediments were analysed for 137Cs, clay mineralogy and carbon. Concentrations of 137Cs ranged from 5 to 73 Bq kg−1 in the ice-borne sediments. Concentrations of ice samples without sediment were all less than 1 Bq m−3. The sediment sample with the highest 137Cs concentration (73 Bq kg−1) was collected in the Beaufort Sea. This concentration was significantly higher than in bottom sediments collected in the same area, indicating an ice transport mechanism from an area with correspondingly higher concentrations. Recent results from the application of ice transport models and sediment analyses indicate that it is very likely that sediments are transported by ice, from the Siberian shelf areas to the Beaufort Sea.

New record shows pronounced changes in Arctic Ocean circulation and climate

Does the Arctic Ocean surface circulation north of Alaska oscillate to and fro like a slow washing machine on millennial timescales? New evidence from the sediment record over the last 10,000 years suggests that it does and that in the recent past, the western Arctic Ocean was much warmer than it is today. Similar Holocene climatic fluctuations are seen in many records worldwide, yet their origin remains enigmatic. Modeling and observational studies suggest that the Arctic may play an important role in these climate fluctuations through changes in surface albedo, modifications of oceanic thermohaline circulation, and changes in biogeochemical cycling of nutrients and radiatively important gases [PARCS, 1999].

Variation in ilmenite element composition within and among drainage basins; implications for provenance

ABSTRACT The elemental composition (Ti, Fe, Mn, Mg, V, Zn, Cr, Ni, and Cu) of detrital ilmenite grains from three large drainage basins in Virginia was found to be relatively unchanged over hundreds of kilometers distance downstream from the Blue Ridge Mountains to the Coastal Plain. The igneous Blue Ridge Complex apparently contributes a sufficient quantity of ilmenite in each river system such that the input of distinctly different ilmenite compositions from tributaries does not change the overall ilmenite composition in the main river. This is not the case for nonopaque heavy minerals, which change in abundance and type downstream due to tributary input. Despite a similar primary source, each major river has a distinctly different ilmenite composition which can be discriminated at better th n a 0.95 confidence level. This enhances the ability to determine the source river for sands in the depocenter. Initial results on ilmenite from small-drainage-basin tributaries dominated by one or two source-rock types known to contain ilmenite show that compositional differences exist for granitic, mafic igneous, metamorphic, and sedimentary sources.