Reliability assessment and paleo-oceanographic signals of geochemical and isotopic indicators in the North Pacific Subtropical Gyre

Abstract

As the largest marine ecosystem on our planet, the North Pacific Subtropical Gyre (NPSG) plays an important role in global environmental and climate change. However, little is known about its environmental changes in an orbital scale. In this study, a constant flux model with excess 230Th was used to establish a time frame for our sediment core. Several geochemical and isotopic indicators were evaluated for their reliability in reconstructing paleoenvironment, including grain size distribution (GSD), Ti, Cu, Zn, Ba, total organic carbon (TOC), total nitrogen (TN), TOC/TN ratio, organic carbon and nitrogen isotopic composition (δ13C, δ15N). In addition, appropriate indicators were used to reveal changes in atmospheric dust deposition and biological productivity during the glacial and interglacial periods in the NPSG. Our results show that in such a low sedimentary environment, TOC, TN, TOC/TN ratio, δ13C, δ15N, Cu and Zn has been altered with the degradation of organic matter in the early diagenesis process. In contrast, GSD and Ti-derived eolian dust flux (Feolian) are suitable indicators for atmospheric dust deposition, and the mass accumulation rate of biogenic silica (MARBSi) and the deposition flux of excess Ba after correction for diffusion (Fexess Ba) are suitable for the reconstruction of biological productivity. The productivity during the glacial periods of Marine Isotope Stage (MIS) 2–4 and 6 is higher than the interglacial periods of MIS 1, 5, which corresponds well to the coarser dust particles and higher eolian dust supply. Therefore, we propose that different with former works in the subtropical Pacific, the fluctuations in productivity in the NPSG is mainly controlled by atmospheric dust deposition on a millennial scale.