Marine paleoclimatology and paleoecology of a Pleistocene invertebrate fauna from Amchitka Island, Aleutian Islands, Alaska

Abstract

Sea cliff exposures at the head of South Bight on Amchitka Island contain an invertebrate fauna consisting of 24 bivale, 14 gastropod, 4 barnacle, 2 brachiopod, 1 amphineuran, 1 echinoid, 1 annelid and 2 planktonic foraminifer species and various unstudied bryozoans, benthonic foraminifers and ostracodes, as well as bone fragments of marine mammals. With the exception of 4 species of Chlamys, all taxa appear to be living along the Alaskan coastline today. Paleotemperature analyses based on the average size of adult Hiatella arctica, the median of midpoints technique, the extralimital species technique, a biogeographic method, species distribution and coiling ratios of planktonic foraminifers, and oxygen stable isotope ratios give varied values. The extralimital species, biogeographic and oxygen isotope techniques are judged to give the most reliable evidence of paleotemperature at South Bight. These data suggest that mean February sea surface temperature was about 3.9° C and that mean August temperatures were somewhere between 10.0° C and 11.7° C. Present-day mean February temperature at South Bight is 3.9° C and mean August temperature is 10.0° C. The fauna lived in shallow (0–23 m?) water in a sublittoral environment on the open coast. Remains of a few rocky-shore littoral organisms were displaced downslope a short distance where they accumulated with remains of epifaunal, infaunal and neritopelagic elements of the inner sublittoral zone and open ocean planktonic foraminifers. The fauna is most closely comparable to that of the present-day Aleutian Province, and to a lesser degree comparable to that of the Arctic. Astarte bennettii, A. borealis, Diplodonta aleutica, and Liomesus nux are unreported in North Pacific strata older than those of the Einahnuhtan transgression, and Chlamys hanaishiensis amchitkana, C. islandica powersi and C. coatsi middletonensis are unknown from strata younger than those of the Einahnuhtan. However, the almost entirely modern aspect of the fauna, coupled with the absence of Astarte mortonensis-like variants of the Astarte borealis lineage, and radiometric dates on bone and shell material of 130,000 ± 15,000 years, appear to indicate a Kotzebuan age.