Paleoproductivity variations and implications in the subarctic northwestern Pacific since MIS 7: Geochemical evidence

Abstract

Knowledge of changes in marine primary productivity and export production in high-nutrient, low-chlorophyll regions of the subarctic NW Pacific Ocean is vital for understanding glacial-interglacial pCO2 variations induced by the biological pump mechanism. We conducted a multi-proxy of geochemical and biological productivity study of sedimentary core LV63-4-2, from the subarctic NW Pacific Ocean, spanning the last ~230 kyr (since ~MIS 7). The measurements included total organic carbon (TOC), opal, biogenic barium, and CaCO3 content as indicators of biological productivity; and the stable carbon isotope composition of TOC and trace elemental abundances to indicate sources of organic matter and redox conditions. The results reveal cyclic changes in paleoproductivity, with higher export production corresponding to periods of low Earth orbital obliquity and high winter insolation. Opal accumulation in core LV63-4-2 was higher during MIS 7, MIS 5d, and MIS 4, which differs from records from the Bering Sea, Okhotsk Sea, and the region to the south of the site LV63-4-2, but it is consistent with records from the Gulf of Alaska. We suggest that export production in the region is controlled by the combined effects of regional vertical convection associated with the formation of North Pacific Intermediate Water (NPIW), and the local iron supply sourced from the Gulf of Alaska. High export production is promoted by the coupling of weakened NPIW formation and an increased iron supply caused by enhanced upwelling in the Gulf of Alaska due to a strengthened Aleutian Low during warmer winters. By contrast, stronger NPIW formation dominated reduced export production during MIS 6 and MIS 2, despite an increased iron supply. Our results highlight the heterogeneity of export production in the subarctic North Pacific and the significant role of the local environment on productivity.