Paleoproductivity and paleoredox conditions during late Pleistocene accumulation of laminated diatom mats in the tropical West Pacific

摘要

Paleoproductivity and paleoredox conditions were reconstructed in a sediment core in the Parece Vela Basin of the eastern Philippine Sea. The core consists of three units, from youngest to oldest: (1) laminated diatom mats (LDM) formed by Ethmodiscus rex during the Last Glacial Maximum (~18-28kyr B.P.), (2) diatomaceous clay (DC), and (3) pelagic clay (PC). Elevated levels of export productivity during LDM deposition are indicated by high values for excess Ba, opal content, and TOC/Ti ratios. Estimated rates of organic carbon degradation (ca. 98%), opal mass accumulation (average 1322gm~(-2)yr~(-1)), and corrected organic carbon flux (average 248gm~(-2)yr~(-1)) are comparable to high-productivity regions of the modern ocean. The LDM is also characterized by moderate enrichment of redox-sensitive elements such as U, Mo, Cd, and Zn, highly ~(34)S-depleted pyrite sulfur isotopic compositions (indicating bacterial sulfate reduction in a sulfate-unlimited system), and C-S-Fe systematics reflecting limitation of pyrite formation by organic matter rather than reactive Fe availability. These features suggest mainly suboxic conditions in bottom waters but development of sulfidic-anoxic conditions at or close to the sediment-water interface. Association of intensified anoxia with productivity maxima indicates that export production was a more important control on bottom water redox conditions than lateral ventilation. The DC and PC accumulated under oxic to suboxic conditions. Our observations suggest that redox environments during deposition of laminated marine sediments are more complicated and varied than previously thought, and, thus, the use of sediment lamination as an indicator of anoxic bottom water conditions must be approached cautiously.