Late Cenozoic Interactions between the Arctic and Pacific Oceans Inferred from Sublittoral Molluscan Faunas - a Review

Abstract

The analysis of original and published data on the distribution of modern and fossil molluscs in the Late Cenozoic deposits of the North Pacific and Eastern Arctic allowed to trace variations in the composition of fossil as- semblages with time and to reconstruct faunistic cxchanges between the two oceans. The first exchange between the Pacific and Atlantic molluscs via the Arctic Ocean probably occurred during the Late Miocene. Recent evidence came from southwestern Alaska showing the strait had opened by ar least the Late Miocene or earliest Pliocene (4.8-5.5. Ma). Detailed analysis of the Pliocene formations of Kamchatka gives evidence for immigrations of species of the genera Elliptica (4 Ma), Tridonta, Rictocvma, Nicania and Cyrtodaria (3.5 Ma) frorn the Arctic into the North Pacific. The most abundant migration 01' Pacific species into the Arctic and North Atlantic occurred during the Late Pliocene. Transgressive marine deposits of this agc containing molluscs of Pacific origin have been found in many localities along the Arctic coast frorn northern Chukotka to the American coast, Greenland. Iceland and even the Pechora Sea. Age detenninations of North Arnerican and Canadian shells range from 2.7 to 2.14 Ma, thus suggesting the Bering Strait was opened during this period. Abun- dance of boreal Pacific molluscs throughout the Arctic shelf, together with other paleofaunistic and paleofloristic data, gives evidence for the existence of sea- sonally ice-free coastal areas. However, no arctic species, in a biogeographical sense, have been found in the late Pliocene Beringian and Ust-Limimtevayam assemblages of the North Pacific. First traces of the arctic cold water species Ponlandia arctica in the North Pacific wcrc found in the Eopleistocene depos- its of Chukotka (Pinakul beds), Kamchatka (Lower Olkhovaya and Tusatuvayam beds), and Alaska (An vili an anel Middletonian beds). All these mainly boreal assemblages displaya unique coexistence 01' arctic cold water and lower boreal warm water species. Presently such a combination is not observed anywhere. The maximum immigration of arctic molluscs into the North Pacific occurred during the Middle Pleistocene, when boreal-arctic and arctic molluscs of atlantic origin (Yoldiellafraterna, Y. intermedia, Y. lenticula, Batltyarca glaeialis) ap- peared in this area for the first time, Latc Pleistocene regression isolated faunas of the two oceans for a long time and strongly influenced the modern dis- tribution of sublittoral molluscs: recent assemblages of the Pacific are warmer than Pleistocene ones (due to the absence of arctic species), and the modern high Arctic assernblages are the coldest among Pleistocene ones