Geology topics

D. A. Darby

Publications and source records attributed to D. A. Darby.

3 recordsLinked to original sources

USGS2009

Late Quaternary stratigraphy and sedimentation patterns in the western Arctic Ocean

Sediment cores from the western Arctic Ocean obtained on the 2005 HOTRAX and some earlier expeditions have been analyzed to develop a stratigraphic correlation from the Alaskan Chukchi margin to the Northwind and Mendeleev-Alpha ridges. The correlation was primarily based on terrigenous sediment composition that is not affected by diagenetic processes as strongly as the biogenic component, and paleomagnetic inclination records. Chronostratigraphic control was provided by 14 C dating and amino-acid racemization ages, as well as correlation to earlier established Arctic Ocean stratigraphies. Distribution of sedimentary units across the western Arctic indicates that sedimentation rates decrease from tens of centimeters per kyr on the Alaskan margin to a few centimeters on the southern ends of Northwind and Mendeleev ridges and just a few millimeters on the ridges in the interior of the Amerasia basin. This sedimentation pattern suggests that Late Quaternary sediment transport and deposition, except for turbidites at the basin bottom, were generally controlled by ice concentration (and thus melt-out rate) and transportation distance from sources, with local variances related to subsurface currents. In the long term, most sediment was probably delivered to the core sites by icebergs during glacial periods, with a significant contribution from sea ice. During glacial maxima very fine-grained sediment was deposited with sedimentation rates greatly reduced away from the margins to a hiatus of several kyr duration as shown for the Last Glacial Maximum. This sedimentary environment was possibly related to a very solid ice cover and reduced melt-out over a large part of the western Arctic Ocean.

Global and Planetary Change↗

USGS2004

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 ?? 18O and ??13C 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 14C kyr BP. The prolonged period with low ??18O and ??13C 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.

Palaeogeography, Palaeoclimatology, Palaeoecology↗

USGS1998

Comparing sea-ice sediment load with Beaufort Sea shelf deposits: Is entrainment selective?

Modern dispersal of sea-ice-rafted debris (IRD) is important for the Arctic Ocean sediment budget from sources to sinks. Sediment entrainment occurs mainly through the action of small ice crystals (frazil) attaching to sedimentary particles in shallow water, a mechanism that could be selective. The principal source for entrainment of IRD by suspension freezing into the Beaufort Gyre, western Arctic Ocean, is the adjacent shallow (<30 m) shelf, here called the source surface. The texture, clay-mineral composition, coarse sand (<250 micrometers) lithology, and carbon and carbonate content of IRD in the Beaufort Gyre were compared to sediments from the probable source surface, in order to determine whether preferential entrainment occurs with any of these sediment parameters. IRD is generally much finer grained than the source surface, showing that silt- and clay-size particles are preferentially entrained by frazil ice, although anchor ice can locally incorporate very high percentages of sand and coarser clasts. The coarsest IRD is also the most poorly sorted. The clay mineralogy of the <2 micrometer IRD fraction is very similar to that of the source surface, indicating no selective entrainment within the clay fraction. The lithology of sand in IRD also matches that of the source surface, although the number of coarse grains is too small (<100) in most samples for a statistically meaningful count. The average organic-carbon content of IRD is three times higher than that of the source surface, but we attribute this to summer algal growth on ice floes rather than to selective entrainment. A relatively low carbonate content in IRD may be because much of the carbonate in the source is of silt size while about 50% of the IRD measured is clay size. The low carbonate content may also reflect solution under acidic summer conditions on sea ice. Selective export of silt- and clay-size particles by ice rafting from the shallow shelf with time should lead to the formation of a slightly coarser lag, even though some of the dirty ice drops its sediment load in the entrainment area. Further mineralogical and lithological analysis on IRD promises to become a useful tool for the study of sediment dispersal paths by drift ice in the Arctic today and in the past, and also for the study of sources of anthropogenic pollutants found in sea ice.

Beaufort Sea shelf↗