Louie Marincovich

Evidence for an early opening of the Bering Strait

The first opening of the Bering Strait was an important palaeogeographical and biogeographical event for marine and terrestrial biotas in Asia and North America, and an oceanographic event of global importance. This event, however, has never been precisely dated, so it has not been accurately incorporated into models of global biogeography and oceanography. A recent find of the bivalve mollusc Astarte in southern Alaska is a clear signal that the strait had opened by at least the Late Miocene or earliest Pliocene epochs, because this genus otherwise dwelled only in the Arctic and North Atlantic oceans at that time,. Here we show that marine diatoms with Astarte are correlative with subzone b of the Neodenticula kamtschatica zone of the North Pacific diatom biochronology, yielding an age range of 4.8–5.5u2009Myr (refs 7, 8). Stratigraphic information suggests that this may be a minimum age range for the straits first opening, which evidently occurred between 4.8 and 7.3–7.4u2009Myr ago. Our results contrast with past studies that suggested an age of 3.1–4.1u2009Myr for the initial opening of the Bering Strait.

A refined age for the earliest opening of Bering Strait

Abstract Biostratigraphically and chronostratigraphically important diatoms from the Milky River Formation, Alaska Peninsula, southwestern Alaska, imply an age range of 5.4–5.5 Ma for the oldest North Pacific Cenozoic occurrence of the marine bivalve mollusk Astarte, which migrated from the Arctic Ocean into the North Pacific when Bering Strait first flooded. The data presented here are a refinement of the age range of 4.8–5.5 Ma reported earlier and imply that Bering Strait first opened very near the end of the Miocene at 5.32 Ma.

Late Paleocene Arctic coastal climate inferred from molluscan stable and radiogenic isotope ratios

Abstract Characterizing polar climates during past warm intervals is important for understanding ‘greenhouse’ climate dynamics because high-latitude surface temperatures and precipitation patterns are extremely sensitive to global climatic conditions. Model-data comparisons of high-latitude climates during past warm intervals (Cretaceous–Eocene, Pliocene) are currently at odds. Specifically, simulations of past warm climates produce polar regions characterized by sub-freezing temperatures and significant seasonality, whereas limited fossil proxy data indicate higher mean annual temperatures and low seasonality (i.e. an equable climate). We have constructed a data set to infer northern hemisphere polar marine temperatures during the late Paleocene. Seasonal and mean annual temperature and the oxygen isotopic composition of precipitation are reconstructed for an Arctic coastal setting during the Thanetian (57–58xa0Ma) using fossil shell stable isotope profiles. We estimate that coastal water temperatures varied between 10 and 15°C during the seasons of growth, presumably spring, summer and fall. These findings support paleontological evidence, implying Northern Hemisphere polar climates were seasonally warm during the late Paleocene. In addition, estuarine fossil oxygen isotope profiles show periodic excursions to low values (as low as −19‰ VPDB), which indicate seasonal pulses of isotopically-depleted freshwater.

New evidence for the age of Bering Strait

Abstract The earliest known opening of Bering Strait in signaled by the presence in southern Alaskan Neogene strata of the marine bivalve mollusk Astarte , which had dwelled throughout the Cenozoic in the Arctic and North Atlantic oceans. Astarte occurs with age-diagnostic marine diatoms in the middle and upper Miocene Bear Lake Formation of the Alaska Peninsula, southwestern Alaska. The diverse diatom flora of more than 100 taxa contains species characteristic of Subzone b of the Neodenticula kamtchatica Zone of the North Pacific diatom biochronology, which has an age range of 4.8–5.5xa0Ma. This inferred age for an early opening of Bering Strait predates the generally cited ages of 3.1–4.1xa0Ma. Stratigraphic data suggest that the first opening of the strait may have occurred even earlier.

Central American paleogeography controlled Pliocene Arctic Ocean molluscan migrations

The Pliocene interchange of North Pacific and North Atlantic marine faunas via the Arctic Ocean was long thought to have been a single episode of faunal exchange between the northern oceans that took place as soon as the Bering Strait first opened. New evidence implies that there were two northern migration events instead of one, and that the second phase of migration was much later than the first. The migration of Atlantic-Arctic mollusks into the Bering Sea and North Pacific when the Bering Strait first opened at 4.8–5.5 Ma constituted the initial phase of this interchange, but the abrupt appearance of North Pacific mollusks in the North Atlantic at 3.6 Ma postdated the first opening of the Bering Strait by 1.2–1.9 m.y. This second phase of trans-Arctic migration was also coeval with shoaling of the Central American seaway between North and South America. This late Pliocene trans-Arctic migration of North Pacific mollusks is evidence for the reversal of marine flow to northward through the Bering Strait, which was one consequence of the reorganization of Northern Hemisphere ocean circulation caused by substantial closure of the Central American seaway. This inferred causal link between the histories of the Beringian and Panamanian ocean gateways is in agreement with ocean circulation models.