Seasonal cycle of suspended barite in the mediterranean sea

Biogenic barium (Ba-xs) was measured in suspended particles at the DYFAMED site in the northwestern Mediterranean Sea, on a monthly basis between February and June 2003. The barium content of barite (BaSO4) micro-crystals was investigated using Scanning Electron Microscopy (SEM). Suspended particles were collected by filtration of small volumes of sea-water (similar to 10 L), as well as large volumes up to 2400 L in March and in May. The Ba-xs profiles obtained from small-volume filtration display the typical mesopelagic maximurn reported by earlier studies at similar to 200 m depth, with concentrations tip to 595 pmol L-1. In addition, suspended Ba-xs was found almost exclusively in the form of micro-crystalline barite, except in February. The Ba-xs profiles obtained from large-volume filtration are consistent with the small-volume filtration findings, but reveal a significant Ba-xs peak of 1698 pmol L-1 in the surface waters in May. Seasonal sampling at the DYFAMED site shows a net increase in barite concentration during phytoplanktonic blooms, confirming the involvement of biological systems in barite formation, as well as the potential role of barite as a primary productivity tracer. In addition, the coincidence between the mesopelagic barite maximum and the oxygen minimum layer suggests that barite is primarily found at depths of intense remineralization, in agreement with the hypothesis that barite forms within microenvironments of decaying organic matter. (C) 2008 Elsevier Ltd. All rights reserved.