Stable isotope profiles of Last Interglacial (Pelukian Transgression) mollusks and paleoclimate implications in the Bering Strait Region

Abstract

The marine paleoclimate during the last interglaciation in the Bering Strait region is inferred from the stable oxygen and carbon isotope profiles of fossil bivalve mollusks. Specimens were collected from the marine highstand deposits of the Pelukian Transgression from the Arctic Alaskan Coastal Plain, the Nome Coastal Plain, St. Lawrence Island, and the Val'katlen Suite on the Chukotka Peninsula. As a calibration, modern mollusks were obtained from the major water masses in the Bering–Chukchi seas system and from coastal environments near the fossil localities. The δ 18 O composition of the modern mollusks accurately reflects the seawater δ 18 O differences among the water masses and the ambient temperature range at each site. Seasonal paleotemperature ranges are estimated from the δ 18 O profiles of the fossils by comparison with those of the modern specimens. A simple model is developed to correct the paleotemperature ranges for the influence of differences in seawater δ 18 O between water masses and seasonal variations caused by freshwater discharge. Paleotemperatures calculated for the Skull Cliff site, Alaskan Arctic Coastal Plain, are close to but slightly cooler than those of modern specimens from Peard Bay and Icy Cape. The isotopic composition of a Pelukian specimen collected near Wainwright shows the obvious influence of locally reduced salinities; with a correction for lower seawater δ 18 O , this profile yields a summer temperature of 13°C, which is 3°C warmer than that from the modern Icy Cape specimen. The isotope records of two specimens from the Nome Coastal Plain suggest salinities that were 3.5 psu lower than the modern eastern Bering Shelf; the paleotemperature range of 0–15°C is consistent with modern conditions in Norton Sound. The character of the isotope profiles of the Pelukian specimens from St. Lawrence Island is very different than that of modern specimens near the island, but matches the δ 18 O values of shells from deeper sites in Anadyr Water. This suggests that the fossil mollusks were living in Anadyr Strait and were transported by a glacial advance from the Chukotka Peninsula. The last interglacial specimens from the Val'katlen Suite have isotope profiles with large seasonal δ 18 O ranges and the clear influence of a reduced-salinity coastal water mass. Calculated paleotemperatures from the Val'katlen specimens range from 0 to 13°C. Differences in the δ 13 C of shell carbonate between sites appears to reflect the regional distribution of the δ 13 C of sedimentary organic matter (SOM), which is a mixture between terrestrial organic matter (mean δ 13 C of −24‰) and marine phytoplankton (−21‰). Partitioning of δ 13 C between shallow and deep waters by phytoplankton productivity and remineralization may also affect the regional distribution. Differences in shell δ 13 C between taxa may relate to feeding mode and the type and isotopic composition of ingested particles. The filter feeders Mya and Astarte typically have δ 13 C values 1–2‰ more positive than the deposit feeders Macoma and Serripes; this effect is attributed to a δ 13 C difference between marine phytoplankton and SOM with a component of terrestrial organic carbon.