Nitrogen Isotope and Chlorin Paleoproductivity Records From the Northern South China Sea: Remote versus Local Forcing of Millennial- and Orbital-Scale Variability

Variations in nitrogen isotopic composition and total chlorin accumulation rate are employed as proxies to reconstruct oceanic nitrate inventory, the balance between denitrification and N fixation, and paleoproductivity in a sediment drift deposit beneath the Western Pacific Warm Pool for the last 145 Kyr. We have analyzed very rapidly accumulating sediments (mean 50cm per Kyr) from the northern SCS recovered during ODP Leg 184 (Site 1144) c. 400 Km SE of Hong Kong. Sub-surface and deep waters of the northern South China Sea (SCS) are sourced from the shallow Kuroshio Current 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 inter-glacial were characterized by high denitrification. However, intervals of anomalously low δ ¹⁵N values are interpreted as reflecting the contribution from regional N fixation in W. Pacific surface waters, transmitted to the site by the shallow Kuroshio Current. Unusually, the Holocene is characterized by declining δ ¹⁵N values and an inverse correlation with OM content since c. 8.5 Ka. Millennial-scale variations during MIS3 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 careful evaluation of possible conflicting influences on the record of each proxy, we interpret our data as clear evidence of glacial/inter-glacial changes in marine nutrient inventory across the whole of the North Pacific, with a corresponding biogeochemical response and important implications for global CO₂ drawdown via an invigorated biological pump. The relative importance of local, regional and global contributions to our records appear to be strongly modulated by relative sea level, controlling trans- and extra-basinal circulation in the SCS.