Benthic foraminifera indicate Glacial North Pacific Intermediate Water and reduced primary productivity over Bowers Ridge, Bering Sea, since the Mid-Brunhes Transition

Abstract

Abstract. The Mid-Brunhes Transition (MBT) saw an increase in the amplitude of glacial cycles expressed in ice core and deep ocean records from about 400 ka, but its influence on high-latitude climates is not fully understood. The Arctic Ocean is thought to have warmed and exhibited reduced sea ice, but little is known of sea ice marginal locations such as the Bering Sea. The Bering Sea is the link between the Arctic and Pacific Ocean and is an area of high productivity and CO2 ventilation; it hosts a pronounced oxygen minimum zone (OMZ) and is thought to be the location of Glacial North Pacific Intermediate Water (GNPIW) formation in the Pleistocene. To understand palaeoceanographic change in the region, we analysed benthic foraminiferal faunas from Bowers Ridge (Site U1342, 800 m of water depth) over the past 600 kyr, as they are uniquely well preserved and sensitive to changes in deep and surface ocean conditions. We identified and imaged 71 taxa and provide a full taxonomy. Foraminiferal preservation is markedly higher during glacials, indicating the presence of less corrosive GNPIW. The most abundant species are Bulimina exilis, Takayanagia delicata, Alabaminella weddellensis, Gyroidina sp. 2, Cassidulina laevigata, Islandiella norcrossi, and Uvigerina bifurcata, consistent with broadly high net primary production throughout the last 600 kyr. Correspondence analysis shows that the most significant Assemblage 1 comprises B. exilis, T. delicata, Bolivina spissa, and Brizalina, which occur sporadically within intervals of laminated, biogenic-rich sediment, mostly during glacials and also some deglacials, and are interpreted as indicating very high productivity. Other assemblages contain the phytodetritivore species A. weddellensis, I. norcrossi, and C. laevigata, indicative of seasonal phytoplankton blooms. Before the MBT, more numerous intervals of the very high-productivity Assemblage 1 and A. weddellensis occur, which we suggest reflect a time of more sea-ice-related seasonal stratification and ice edge blooms. Our inference of a decrease in sea ice meltwater stratification influence in the central Bering Sea after the MBT is consistent with records showing that the Arctic and Pacific Ocean warmed during glacials and suggests that high-latitude productivity and sea ice changes were an important feature of this climate event.

Coupled Impacts of Atmospheric Circulation and Sea‐Ice on Late Pleistocene Terrigenous Sediment Dynamics in the Subarctic Pacific Ocean

“…With the AL moved to the east and anomalously higher pressure affecting northeastern Siberia, stronger northerly winds prevail over the Bering Sea (Luchin et al., 2002; Rodionov et al., 2007) (Figure 5e). Such winds would have led to an incease in sea‐ice extent over the Bering Shelf and across the Aleutian Basin (Figure 5b), causing intense brine rejection and the associated intermediate or deep‐water ventilation in the northwestern Bering Sea (Kender et al., 2019). In parallel, a strengthening and southward shift of the Subarctic Front could restrict the transport of warmer and wetter air masses from the tropical Pacific (Gorbarenko et al., 2005).…”

Late quaternary sea-ice and sedimentary redox conditions in the eastern Bering Sea – Implications for ventilation of the mid-depth North Pacific and an Atlantic-Pacific seesaw mechanism

“…Benthic foraminiferal assemblages in the mid-depth Bering Sea is dominated by calcareous infaunal species, controlled primarily by the supply of organic matter to the sediments (Kender et al, 2019;Kender and Kaminski, 2017;Setoyama and Kaminski, 2015). The dominant species are typically tolerant of high-productivity-lowoxygen conditions, related to the pronounced mid-depth OMZ and export of organic carbon to the sediments, particularly within the Green Belt along the eastern Bering Sea slope (Expedition 323 Scientists, 2010;Kender et al, 2019;Khusid et al, 2006;Setoyama and Kaminski, 2015).…”

“…However, at Site U1343 the foraminiferal abundance is generally low (Expedition 323 Scientists, 2010) with 0-309 specimens in the 150-250 µm fraction per sample for the studied interval. We consider all samples with >50 specimens for environmental inferences, which has previously been shown to yield reliable diversity at the Bowers Ridge in the southern Bering Sea (Kender et al, 2019).…”

“…CA uses reciprocal averaging to compare the species within a given dataset (Greenacre, 1983), with CA scores characterizing similar faunal traits (e.g. Hammer and Harper, 2006;Kender et al, 2019). The CA axes describe the variance in the dataset, corresponding to the ecological parameter predominantly controlling the abundance of species within the assemblage.…”

“…While the last glacial interval was dominated by Assemblage 3, predominately composed of phytodetritivore species (Kender et al 2019), the BA and the early Holocene are dominated by Assemblage 1 (Figure 6). The latter is composed of species adapted to low oxygen conditions and very high export of organic carbon to the seafloor (Kender et al, 2019;Piña-Ochoa et al, 2010;Schumacher et al, 2007;Sen Gupta and Machain-Castillo, 1993), dominated by B. exilis (Figure 6).…”