Search for: All records

This is the repository for an R Shiny App that allows users to create sea ice records for the Bering and Chukchi seas from diatom data. Users will upload a data set of diatom percentages and the app will return predictions of spring (March, April, May, June_ sea ice concentrations based on a generalized additive model of surface sediment assemblages in the Bering and Chukchi Seas. Please direct questions to Beth Caissie bcaissie@usgs.gov. 

Polar bear (Ursus maritimus) is the apex predator of the Arctic, largely dependent on sea-ice. The expected disappearance of the ice cover of the Arctic seas by the mid 21st century is predicted to cause a dramatic decrease in the global range and population size of the species. To place this scenario against the backdrop of past distribution changes and their causes, we use a fossil dataset to investigate the polar bear’s past distribution dynamics during the Late Glacial and the Holocene. Fossil results indicate that during the last deglaciation, polar bears were present at the southwestern margin of the Scandinavian Ice Sheet, surviving until the earliest Holocene. There are no Arctic polar bear findings from 8,000-6,000 years ago (8-6 ka), the Holocene’s warmest period. However, fossils that date from 8-9 ka and 5-6 ka suggest that the species likely survived this period in cold refugia located near the East Siberian Sea, northern Greenland and the Canadian Archipelago. Polar bear range expansion is documented by an increase in fossils during the last 4,000 years in tandem with cooling climate and expanding Arctic sea ice. The results document changes in polar bear’s distribution in response to Late Glacial and Holocene Arctic temperature and sea ice trends. 

Founding populations of the first Americans likely occupied parts of Beringia during the Last Glacial Maximum (LGM). The timing, pathways, and modes of their southward transit remain unknown, but blockage of the interior route by North American ice sheets between ~26 and 14 cal kyr BP (ka) favors a coastal route during this period. Using models and paleoceanographic data from the North Pacific, we identify climatically favorable intervals when humans could have plausibly traversed the Cordilleran coastal corridor during the terminal Pleistocene. Model simulations suggest that northward coastal currents strengthened during the LGM and at times of enhanced freshwater input, making southward transit by boat more difficult. Repeated Cordilleran glacial-calving events would have further challenged coastal transit on land and at sea. Following these events, ice-free coastal areas opened and seasonal sea ice was present along the Alaskan margin until at least 15 ka. Given evidence for humans south of the ice sheets by 16 ka and possibly earlier, we posit that early people may have taken advantage of winter sea ice that connected islands and coastal refugia. Marine ice-edge habitats offer a rich food supply and traversing coastal sea ice could have mitigated the difficulty of traveling southward in watercraft or on land over glaciers. We identify 24.5 to 22 ka and 16.4 to 14.8 ka as environmentally favorable time periods for coastal migration, when climate conditions provided both winter sea ice and ice-free summer conditions that facilitated year-round marine resource diversity and multiple modes of mobility along the North Pacific coast. 

This dataset contains grain size records from three Integrated Ocean Drilling Program core sites (U1345, U1343, and U1339) in the Bering Sea. These records are used to determine the effectiveness of different grain size parameters as proxies for sediment transport, current strength, and primary productivity in the Bering Sea during a past warm interval (Marine Isotope Stage 11, 424-374 thousand years ago (ka)). Grain size is measured using a laser diffraction particle size analyzer (Malvern Mastersizer 3000), and is reported for bulk sediments, and for the terrigenous fraction only. The raw dataset provided by the Malvern software includes the volume % of grains in 109 bin sizes, as well as the 10th (Dx10), 50th (Dx50) and 90th (Dx90) percentiles. We also provide the volume distribution of grains in the following size fractions: clay (less than 2 micrometers (μm)); silt (2-63 μm); sand (63-2000 μm); gravel (greater than 2000 μm); ice-rafted debris (greater than 150 μm; greater than 250 μm), and sortable silt (10-63 μm). Additional grain size parameters, including mean size, sorting and skewness, are calculated in GRADISTAT.