Nitrogen isotope and chlorin paleoproductivity records from the Northern South China Sea: remote vs. local forcing of millennial- and orbital-scale variability

Abstract

Variations in nitrogen isotopic composition (δ 15N) and total chlorin accumulation rate (AR) are employed as proxies to reconstruct oceanic nitrate inventory, the balance between denitrification and N fixation, and paleoproductivity in a rapidly accumulating sediment drift deposit beneath the Western Pacific Warm Pool for the last 145 Kyr. Subsurface and deep waters of the northern South China Sea (SCS) are sourced from the shallow Kuroshio Current (KC) and Pacific Intermediate Water, respectively. Their relative importance in determining sedimentary δ 15N and paleoproductivity have been altered by changes in equatorial circulation, summer- and winter-monsoon intensity and relative sea level. The location and basin configuration of the marginal SCS renders it especially sensitive to such changes. Assuming complete annual nitrate utilization, low δ 15N values during glacial stages are interpreted as a reflection of reduced remote denitrification in the Eastern Tropical North Pacific (ETNP) source waters, while much of marine isotope stage (MIS) 3 and the last interglacial were characterized by high denitrification. However, intervals of anomalously low δ 15N values are interpreted as reflecting the contribution from regional N fixation in West Pacific surface waters, transmitted to the site by the shallow KC. Unusually, the Holocene is characterized by declining δ 15N values and an inverse correlation with organic matter content since ca. 8.2 Ka. Millennial-scale variations during MIS 3 indicate higher frequency variations in both ETNP denitrification and local N fixation, which may be coherent with a hemispheric response to Dansgaard–Oeschger events recorded at high latitudes. For much of the last 145 Kyr, paleoproductivity was decoupled from δ 15N, and instead seems to reflect the extent of the global nitrate inventory stimulated by elevated dust fertilization, and regional mixed-layer deepening associated with the relative intensity of the SE Asian winter monsoon. Despite evaluation of possible conflicting influences on the record of each proxy, we interpret our data as clear evidence of glacial/interglacial changes in marine nutrient inventory across the whole of the North Pacific, with a corresponding biogeochemical response and important implications for global CO 2 drawdown via an invigorated biological pump. The relative importance of local, regional and global contributions to our records appears to be strongly modulated by relative sea level, controlling trans- and extra-basinal circulation in the SCS.