A lamina-scale, SEM-based study of a late Quaternary diatom-ooze sapropel from the Mediterranean Ridge, Site 971

Abstract

A scanning electron microscope-based study of a laminated, late Quaternary sapropel from the moat of the Napoli mud volcano provides a new insight into the seasonal-scale sequence of fluxes involved in sapropel deposition. The sapropel is essentially an organic-rich diatom ooze in which the opal has been preserved in a topographic depression whose bottom waters were, at least intermittently, cut off from the silica-undersaturated waters characteristic of the Mediterranean. Back-scattered electron imagery of polished, resin-impregnated thin sections of this bed demonstrate an alternation of laminae containing diatoms from the family Rhizosoleniaceae, hereafter referred to as rhizosolenid diatoms, with laminae containing mixed assemblages including typical diatom bloom species. The occurrence of the rhizosolenid laminae are evidence for seasonal-scale mass sedimentation of diatoms in the form of diatom mats. These diatom mats may represent the deep cholorophyll maximum inferred to have existed during periods of sapropel formation documented by foraminifer and nannofossil studies. The mat-sedimentation model for the deposition of sapropels is consistent with both (1) the presence of stratified conditions in which nutrients trapped at depth could be exploited only by the vertically migrating mats; and (2) the evidence for high export production that occured by massive sedimentation of mats following the intermittent breakdown of stratification.