A possible capture of molybdenum during early diagenesis of dysoxic sediments

Abstract

Molybdenum is a trace metal commonly used as a marker of paleoredox conditions of ancient depositional settings. Pyrite is an important molybdenum (Mo) host for enriched sedimentary rocks. In this paper we study the relationship between Mo and pyrite in the Bancs Jumeaux Formation, a Jurassic succession in northern France consisting of limestone and pyrite-rich marls. This formation is relatively enriched in Mo compared to other redox-sensitive trace metals. Our approach is grounded on bulk rock chemical analysis and delineation of two contrasting types of pyrite that can be extracted from the rocks: polyframboids and nonframboidal concretionary masses. The morphological characteristics of both morphotypes were studied using scanning electronic microscopy. The polyframboids are richer in Mo than the concretions but are not markedly enriched in other trace metals. This discrepancy in geochemical composition could result from pyrite precipitation at different times during early diagenesis. Our results tend to indicate that the polyframboids would have formed very early in reducing “microniches”, within dominantly dysoxic sediment. This early pyrite precipitation occurred at shallow depth below the sediment-water interface close to the abundant Mo source in overlying oxic seawater (molybdate ions), and would have fostered Mo-capture by the polyframboids in relatively large amounts. The concretions would have formed later during early diagenesis (within the sulfidic zone) under conditions of more limited Mo availability.

Small-scaled lateral variations of an organic-rich formation in a ramp-type depositional environment (the Late Jurassic of the Boulonnais, France): impact of the clastic supply

“…The trace elements referred to as proxies to paleoproductivity, i.e., Ni and Cu (see discussion in Böning et al, 2015;Tribovillard et al, 2008), do not show any enrichment, thus suggesting that productivity was not high (we will not mention barium that cannot be used as a paleoproxy in the case of such relatively shallow environments). This conclusion is consistent with the one derived independently from organic parameters in the study by Tribovillard et al (2001).…”

Basinal restriction, black shales, Re‐Os dating, and the Early Toarcian (Jurassic) oceanic anoxic event

“…[7] Molybdenum is removed from euxinic waters by reduction to the thiomolybdate complexes (MoO x S 4-x 2À ) in the presence of free H 2 S and subsequent sequestration in either sedimenting organic matter, sedimenting pyrite formed in the water column, or both [Helz et al, 1996;Erickson and Helz, 2000;Vorlicek and Helz, 2002;Tribovillard et al, 2004aTribovillard et al, , 2006Tribovillard et al, , 2008. Because of this removal, the renewal time i.e., degree of restriction, of the subpycnoclinal water mass in modern, euxinic, marine basins, can be estimated from the slope of the regression line between Mo and TOC (rsMo/TOC hereinafter, to distinguish it from raw Mo/TOC ratios) in a crossplot of TOC against Mo in sediments [ Algeo and Lyons, 2006].…”

Rapid reorganization in ocean biogeochemistry off Peru towards the end of the Little Ice Age

“…Equatorial Subsurface Waters (ESSW), which are the main source of upwelling in the ETSP, are enriched in dissolved Cd Takesue et al, 2004). Cadmium is delivered to sediments in association with organic matter and becomes fixed in reducing sediments (Tribovillard et al, 2006;Böning et al, 2004). Cd in sediments can then be considered a proxy of paleoupwelling and paleoproductivity (Dean et al, 2006).…”