Rare earth element geochemistry of laminated diatom mats from tropical West Pacific: Evidence for more reducing bottomwaters and higher primary productivity during the Last Glacial Maximum

Rare earth elements (REE) along with selected major and trace elements were measured in laminated Ethmodiscus rex diatom mats and associated sediments (diatomaceous clay and pelagic clay) in two sediment cores collected from the Parece Vela Basin of the eastern Philippine Sea. REE distributions are characterized by strong enrichment of middle REE (MREE) and heavy REE (HREE) over light REE (LREE) and slight enrichment of MREE over HREE. This distribution pattern suggests that REE compositions of the laminated E. rex diatom mats were not the product of E. rex blooms, an inference also supported by the fact that Ti-normalized REE concentrations are not clearly different from those in the diatomaceous clay and pelagic clay. Comparison of REE patterns with the vertical distribution of authigenic phases in the sediment indicates that LREE were dominantly associated with detrital materials, whereas MREE and HREE were selectively enriched by Mn oxyhydroxides and authigenic phosphates. In the pelagic clay, a weak negative Ce anomaly cannot be linked to oxic depositional conditions but instead is attributed to the retention of a negative Ce anomaly associated with authigenic phosphates. In the laminated E. rex diatom mats, a weak positive Ce anomaly is consistent with depositional sulfidic anoxic conditions and is interpreted to represent enhanced scavenging of reduced aqueous Ce3+ by barite in a strongly reducing environment. Moreover, variably negative to positive Ce anomalies in the diatomaceous clay may reflect the concurrent operation of the two processes controlling Ce accumulation in the laminated E. rex diatom mats and pelagic clay. The study cores show a ubiquitous positive Eu anomaly owing to the presence of volcanic materials. The smaller Eu anomaly in the laminated E. rex diatom mats and diatomaceous clay relative to that in the pelagic clay may be attributed to the release of reduced Eu2+ from mineral phases to the sediment pore water under strongly reducing conditions. Previous studies have shown that the advection of Lower Circumpolar Water (LCPW) strengthened during the Last Glacial Maximum (LGM), which should have resulted in better ventilation of the deep eastern Philippine Sea. Evidence presented here for more reducing bottomwater conditions during the laminated E. rex diatom mats formation implicates elevated primary productivity and enhanced organic sinking fluxes as primary controls on redox changes in the eastern Philippine Sea during the LGM. Enhanced primary productivity and more reducing bottomwaters during the laminated E. rex diatom mats deposition resulted in a larger respired carbon pool in the deep eastern Philippine Sea, which may have been part of a larger-scale modulation of atmospheric CO2 variation during Pleistocene glacial-interglacial cycles by deep-ocean carbon storage. (C) 2011 Elsevier B.V. All rights reserved.