High organic carbon deposition in the northern margin of the Aleutian Basin (Bering Sea) before the last deglaciation

Abstract

High-resolution geochemical, isotope and elemental data from core PC23A in the northern margin of the Aleutian Basin (Bering Sea) were used to reconstruct distinct paleoceanographic features of the last deglaciation (pre-Boreal[PB], Bølling-Allerød[BA], Younger Dryas[YD]). The PB and BA intervals are characterized by increased siliceous (diatom) and calcareous (coccolithophores and foraminifers) productivity represented by high biogenic opal and CaCO 3 contents, respectively. The enhanced productivity can plausibly be attributed to an elevated sea-surface nutrient supply from increased melt-water input and enhanced Alaskan Stream injection under warm, restricted sea-ice conditions. High C org /N ratios and low δ 13 C values of sediment organic matter during the PB and BA intervals reflect the contribution of terrestrial organic matters. The PB and BA intervals were also identified by laminated sediment layers of core PC23A, characterized by high Mo/Al and Cd/Al ratios, indicating that the bottom water condition remained anoxic. High δ 15 N values during the same period were attributed mainly to the increased nutrient utilization and subsequent denitrification of seawater nitrate. Part of high δ 15 N values may also be due to incorporation of inorganic nitrogen in the clay minerals. It is worthy of note that high total organic carbon (TOC) deposition occurred approximately 3,000 years before onset of the last deglaciation. Simultaneous high C org /N ratios and low δ 13 C values clearly suggest that the high TOC content should be related to terrestrial organic carbon input. Low δ 15 N values during the high TOC interval also confirm the contribution of terrigenous organic matter. Although abundant calcareous phytoplankton production under cold, nutrient-poor conditions represented by Ba ex data was reported for high TOC deposition preceding the last deglaciation in an earlier study of the Okhotsk Sea, the main reason for the enhanced TOC deposition in the Bering Sea is an increased terrigenous input from the submerged continental shelves (Beringia) with a sea-level rise; this is further supported by Al enrichment of bulk sediments during the high TOC deposition.

Millennial-scale variability of marine productivity and terrigenous matter supply in the western Bering Sea over the past 180 kyr

“…In contrast to TOC and CaCO 3 , we observed only minor increases in opal during the B/A, but a subsequent gradual increase into the Holocene. These results from the western Bering Sea are comparable to opal records from the northern slope (Itaki et al, 2009;Khim et al, 2010;Kim et al, 2011). Despite showing a similar temporal evolution, opal records from Bowers Ridge (Okada et al, 2005;Okazaki et al, 2005;Brunelle et al, 2007Brunelle et al, , 2010, Umnak Plateau (Okada et al, 2005;Okazaki et al, 2005), and the southern Okhotsk Sea (Gorbarenko, 1996;Gorbarenko et al, 2002a, b;Brunelle et al, 2010) have 1.5 to 3-times higher values.…”

“…Umnak Plateau: KH99-3-UMK-3A (Okada et al, 2005;Okazaki et al, 2005;Tanaka and Takahashi, 2005), RC14-121 (Cook et al, 2005). Northern slope: MR06-04-PC23A and -PC24A (Itaki et al, 2009;Khim et al, 2010;Kim et al, 2011;Rella et al, 2012). Dashed black line indicates average maximum sea-ice extent (after Niebauer et al, 1999;Zhang et al, 2010).…”

“…(3) After Eppley and Peterson (1979). Okazaki et al, 2005;Brunelle et al, 2007Brunelle et al, , 2010Itaki et al, 2009;Khim et al, 2010;Kim et al, 2011). At ODP Site 882 in the N Pacific interglacial maxima in CaCO 3 are accompanied by maxima in Ba bio .…”

“…Springer et al, 1996;Niebauer et al, 1999), it might have had a different influence on past oceanatmosphere CO 2 exchange. Paleoceanographic reconstructions in the Bering Sea also revealed reduced export production during the last glacial period, which increased during Termination I and remained high in the Holocene (Gorbarenko, 1996;Cook et al, 2005;Gorbarenko et al, 2005;Okada et al, 2005;Okazaki et al, 2005;Brunelle et al, 2007Brunelle et al, , 2010Itaki et al, 2009;Khim et al, 2010;Kim et al, 2011). This variability was explained by a complex interplay of changes in sea surface temperatures (SST), sea-ice coverage, inflow of Pacific surface waters, and upper-ocean stratification (e.g.…”

“…Recent progress has been made in detecting millennialscale climate variability in Bering Sea sediments (Cook et al, 2005;Gorbarenko et al, 2005Gorbarenko et al, , 2010Okazaki et al, 2005;Brunelle et al, 2010;Khim et al, 2010;Kim et al, 2011;Max et al, 2012;Rella et al, 2012). Most of these studies are restricted to the last ∼ 70 kyr or focus on deglacial changes in the northern, southern, and southeastern Bering Sea.…”