The Pliocene - Pleistocene evolution of surface water masses in the Bering Sea is not well
understood, and the aim of this study is to establish a Plio-Pleistocene planktonic
foraminiferal biostratigraphy for the Bering Sea and investigate changes of the surface water
circulation. The Bering Sea is a marginal sea of the North Pacific connected to the
Arctic Ocean through the Bering Strait providing the only connection and exchange
of waters between the Pacific and Atlantic Oceans in the Northern Hemisphere.
IODP Site U1340 and Site U1343 in the Bering Sea have been investigated with
regard to planktonic foraminifers and fragmentation. The base of Site U1340 dates
back to the Early Pliocene and the base of Site U1343 to the Early Pleistocene.
Site U1340 is situated at Bowers Ridge, southern Bering Sea, under the axis of
the Alaskan Stream transporting warm water into the Bering Sea. Site U1343, is
situated near to the gateway to the Arctic Ocean in the northern Bering Sea. At both
sites there are none or very few planktonic foraminifers during the Pliocene and
early Pleistocene. After 1.3-1.4 Ma the planktonic foraminifers are continuously
present for most of the samples examined. Three stratigraphic events have been
identified in this study: 1) the first occurrence (FO) of Neogloboquadrina inglei is
observed at 1.4 - 1.5 Ma, 2) the change in the coiling ratio of Neogloboquadrina
pachyderma from right to left at 1.2 Ma, and 3) the last occurrence (LO) of N. inglei at
0.7 Ma. The oldest event may be affected by poor preservation of foraminifers in
older sediments. However, the ages of the latter two events seem to agree with
the dating of the same events at the Californian margin observed by Kucera and
Kennett (2000) implying that these events are robust regional events for the entire
northern Pacific. Multivariate analyses of the quantitative planktonic foraminifer
data show three main faunal assemblages. The oldest assemblage from 1.3 - 1.4
Ma to 1.2 Ma is dominated by the subpolar N pachyderma s.l. (dex) together with
Globigerina bulloides. Other species in this fauna are N. inglei, N. pachyderma s.l. (sin),
Globigerina umbilicata and Turborotalita quinqueloba. After 1.2 Ma the faunal
assemblage is dominated by the polar N. pachyderma s.l. (sin), but the remaining
species are the same as before. At 0.7 Ma N. inglei disappears, whilst the remaining
fauna assemblage stays the same, with N. pachyderma s.l. (sin) still dominating,
reflecting cool Pleistocene conditions. Prior to 1.4 – 1.3 Ma there are very few or no
planktonic foraminifers. Low shell fragmentation and lower TOC suggest that the lack of
planktonic foraminifers in these sediments cannot be explained either by a shoaling
of the carbonate compensation depth or by enhanced supralysoclinal dissolution.
Instead, we argue that the appearance of abundant foraminifers in the sediment
reflects the same evolutionary adaption of N. pachyderma (sin) to cold conditions
to have occurred after 1.1-1.0 Ma as in the North Atlantic (Huber et al., 2000). |