Relationship between δ15N values of bulk sediments and total organic carbon concentration in response to orbital-scale biogenic opal production in the Bering slope area over the last 600 kyrs

Abstract

Biogenic opal production in the surface water of the Bering Sea shows clear orbital-scale variations such as interglacial highs and glacial lows. In this study, we examined the variation patterns in δ15N of bulk sediment (δ15Nbulk) reflecting nitrate utilization, and total organic carbon (TOC) concentration in terms of organic carbon (OC) production, in response to the orbital-scale biogenic opal production during the last 600 kyrs at Site U1343 (IODP Exp. 323) in the Bering slope area. At Site U1343, δ15Nbulk value and TOC concentration do not show the distinct glacial-interglacial change. Moreover, contrary to the weak correlation between TOC and biogenic opal concentrations, TOC concentration is positively correlated with δ15Nbulk value, indicating the strong control of OC production on nitrate utilization. It should be noted that glacial terminations are characterized by high δ15Nbulk value, high TOC and low biogenic opal concentration whereas the following full interglacials are characterized by low δ15Nbulk value, low TOC and high biogenic opal concentration during the last 600 kyrs. In other words, glacial terminations are characterized by complete nitrate utilization associated with high OC production under low biogenic opal production, whereas the following full interglacial periods are characterized by decreased nitrate utilization associated with low OC production under high biogenic opal production. Such different glacial-interglacial variation patterns of OC production and nitrate utilization in response to biogenic opal production can best be explained by variable Si/C (or Si/N) uptake ratio by diatoms possibly owing to Fe availability. The glacial Fe input increase at Site U1343 is thought to be ascribed to more dust input or large discharge from the glacial Yukon River. These findings imply that Fe transport and contribution may play a significant role in governing the nutrient cycles in the Bering slope region of the Northern Hemisphere.