ABSTRACT: Amphimelissa setosa is at present a species endemic to the Arctic province: the Arctic Ocean proper and its marginal seas. In the North Pacific Ocean and the Okhotsk Sea, its first occurrence was at 1.03 Ma and it became locally extinct at 72 ka. Its first recorded occurrence in the North Atlantic is at IODP Site U1314 at 414 ka in MIS 11, which is much older than previously documented by Matul and Abelmann (2005). This earlier first occurrence suggests that its migration from the North Pacific took place during MIS 11, with a higher sea-level stand than at present. The stratigraphic distribution of A. setosa shows seven peak occurrences: at 383.3 ka in MIS 11 (11.9%), 347.1 ka (64.6%) in MIS 10, at 319.2 ka (12.1%) in MIS 9, 291.1 ka (16.4%) near base in MIS 8, at 74.4 ka (8.1%) inMIS 5a, at 53.7 ka (18.2%) in MIS 3, and at 21.1 ka (48.2%) in MIS 2, the two major peaks corresponding with glacial maxima. The prominent abundance peaks at MIS 10 and MIS 2 clearly reflect the southward penetration of the Arctic front. On the other hand, A. setosa is very rare or absent in the upper part of MIS 8 and during most of MIS 5.We suggest that during the upper part of MIS 8, A. setosa is not observed because of considerably lower biogenic silica. Its near absence during MIS 6 may be the result of relatively warmer oceanographic conditions in this glacial period without a significant southward migration of the polar front. Finally, morphometric analyses of A. setosa at the abundance peaks in MIS 10 and MIS 2 indicate that the specimens occurring in MIS 10 are somewhat, but significantly larger than those occurring in MIS 2 with also a slight difference in shape.
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