Seasonal cycle of suspended barite in the mediterranean sea

Biogenic barium (Baxs) 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 seawater (∼10 L), as well as large volumes up to 2400 L in March and in May. The Baxs profiles obtained from small-volume filtration display the typical mesopelagic maximum reported by earlier studies at ∼200 m depth, with concentrations up to 595 pmol L−1. In addition, suspended Baxs was found almost exclusively in the form of micro-crystalline barite, except in February. The Baxs profiles obtained from large-volume filtration are consistent with the small-volume filtration findings, but reveal a significant Baxs 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.