Controls on trace metal authigenic enrichment in reducing sediments: Insights from modern oxygen-deficient settings

Abstract

ABSTRACT. Any effort to reconstruct Earth history using variations in authigenic enrichments of redox-sensitive and biogeochemically important trace metals must rest on a fundamental understanding of their modern oceanic and sedimentary geochemistry. Further, unravelling the multiple controls on sedimentary enrichments requires a multi-element approach. Of the range of metals studied, most is known about the behavior of Fe, Mn, and Mo. In this study, we compare the authigenic enrichment patterns of these elements with a group whose behavior is not as well defined (Cd, Cu, Zn, and Ni) in three oxygen-poor settings: the Black Sea, the Cariaco Basin (Venezuela), and the Peru Margin. These three settings span a range of biogeochemical environments, allowing us to isolate the different controls on sedimentary enrichment. Our approach, relying on the covariation of elemental enrichment factors [EF, defined for element X as: EF X ‫؍‬ (X/Al) sample /(X/Al) lithogenic ], has previously been applied to Mo and U to elucidate paleoenvironmental information on, for example, benthic redox conditions, the particulate shuttle, and the evolution of water mass chemistry. We find two key controls on trace metal enrichment. First, the concentration of an element in the lithogenic background sediment (used in calculating EF X ) controls the magnitude of potential enrichment. Maximum enrichment factors of 376 and 800 are calculated for Mo (ϳ1 ppm in detrital sediments) and Cd (ϳ0.3 ppm), respectively, compared to values not greater than 17 in any setting for the other five metals (ϳ45 ppm to ϳ4.5 wt.% in detrital sediments). Second, there is a relationship between the aqueous concentration of the element in overlying seawater and its degree of enrichment in the sediment. We further identify four important processes for delivery of trace metals to the sediment. These are: (1) cellular uptake (especially important for Zn and Cd), (2) interaction/co-precipitation with sulfide (Mo, Cu, and Cd), (3) passive scavenging via the traditional particulate shuttle (Mo, Ni, and Cu), and (4) an association with the benthic Fe redox shuttle (Mn, Ni). Finally, we summarize the oceanic mass balance of Cd and Mo and place the first constraints on the contribution of reducing sediments to the oceanic mass balance of Cu, Zn, and Ni. We show that reducing sediments are the ultimate repository for up to half the total output flux of these elements from the oceanic dissolved pool.

Empirical links between trace metal cycling and marine microbial ecology during a large perturbation to Earth's carbon cycle

“…We distinguish between sulfidic sediments and a sulfidic water column and define reducing environments as those in which sulfide accumulation is restricted to pore fluids. These conditions often occur beneath low-oxygen bottom-waters similar to modern oxygen minimum zone environments (Little et al, 2015). In the modern oxic ocean, V is characterized by a shorter residence time [∼50 to 100 kyr, Algeo, 2004;Morford and Emerson, 1999] and a smaller dissolved marine reservoir [35 to 45 nM, Morford and Emerson, 1999, and references therein], regulated primarily through its removal as vanadate oxyanions (HVO 2− 4 and HVO − 4 ) associated with Mn-and Fe-oxides and by efficient burial in low oxygen settings (as reviewed by Tribovillard et al, 2006).…”

Selenium Isotopes as a Biogeochemical Proxy in Deep Time

“…The three sinks quoted above are also the most important sinks of molybdenum from the ocean (Anbar 2004). An updated mass balance by Little et al (2015) suggests that of the total molybdenum output (1.34-1.9 • 10 8 mol/yr), 6-8% (mean 7%) are removed in restricted euxinic basins, 13-77% (mean 45%) are removed in anoxic or suboxic parts of continental margins, and 18-77% (mean 48%) are removed in oxic settings, i.e. by adsorption to manganese oxides.…”

Paleoenvironmental evolution of the southern Neuquèn basin (Argentina) during the Tithonian-Berriasian (Vaca Muerta and Picún Leufú Formations): a multi-proxy approach

“…Briefly, dissolved chemical species present in seawater can easily bind onto the Fe and Mn oxy-hydroxides forming particles in the oxidizing part of the water column, thus becoming incorporated in a reversible way. During settling, Fe and Mn oxy-hydroxides can scavenge significant amounts of Mo (Crusius et al, 1996;Little et al, 2015). The oxy-hydroxides can in turn bind settling particles or flocs of OM, or be adsorbed onto clay minerals.…”

“…The restriction of deepwater circulation can influence authigenic U-Mo enrichment of marine sediments. All other variables being equal, restricted circulation lowers the degree of trace element enrichment in sediments owing to a decreased input of aqueous trace metal species supplied to the basinal water mass from the global ocean Little et al, 2015). However, water-mass restriction is often correlated with other environmental variables such as reduced redox conditions (especially within the suboxic-anoxic redox range), which may enhance trace metal uptake and offset the effects of increased restriction .…”