An evaluation of sedimentary molybdenum and iron as proxies for pore fluid paleoredox conditions

Abstract

Iron speciation and trace metal proxies are commonly applied together in efforts to identify 21 anoxic settings marked by the presence of free sulfide (euxinia) or dissolved iron (ferruginous) in the 22 water column. Here, we use a literature compilation from modern localities to provide a new empirical 23 evaluation of coupled Fe speciation and Mo concentrations as a proxy for pore water sulfide accumulation 24 at non-euxinic localities. We also present new Fe speciation, Mo concentration, and S isotope data from 25 the Friends of Anoxic Mud (FOAM) site in Long Island Sound, which is marked by pore water sulfide 26 accumulation of up to 3 mM beneath oxygen-containing bottom waters. For the operationally defined Fe 27 speciation scheme, 'highly reactive' Fe (Fe HR) is the sum of pyritized Fe (Fe py) and Fe dominantly present 28 in oxide phases that is available to react with pore water sulfide to form pyrite. Observations from FOAM 29 and elsewhere confirm that Fe py /Fe HR from non-euxinic sites is a generally reliable indicator of pore fluid 30 redox, particularly the presence of pore water sulfide. Molybdenum (Mo) concentration data for anoxic 31 continental margin sediments underlying oxic waters but with sulfidic pore fluids typically show 32 authigenic Mo enrichments (2-25 ppm) that are elevated relative to the upper crust (1-2 ppm). However, 33 compilations of Mo concentrations comparing sediments with and without sulfidic pore fluids underlying oxic and low oxygen (non-euxinic) water columns expose non-unique ranges for each, exposing false 35 positives and false negatives. False positives are most frequently found in sediments from low oxygen 36 water columns (for example, Peru Margin), where Mo concentration ranges can also overlap with values 37 commonly found in modern euxinic settings. FOAM represents an example of a false negative, where, 38 despite elevated pore water sulfide concentrations and evidence for active Fe and Mn redox cycling in 39 FOAM sediments, sedimentary Mo concentrations show a homogenous vertical profile across 50 cm 40 depth at 1-2 ppm. A diagenetic model for Mo provides evidence that muted authigenic enrichments are 41 derived from elevated sedimentation rates. Consideration of a range of additional parameters, most 42 prominently pore water Mo concentration, can replicate the ranges of most sedimentary Mo 43 concentrations observed in modern non-euxinic settings. Together, the modern Mo and Fe data 44 compilations and diagenetic model provide a framework for identifying paleo-pore water sulfide 45 accumulation in ancient settings and linked processes regulating seawater Mo and sulfate concentrations 46 and delivery to sediments. Among other utilities, identifying ancient accumulation of sulfide in pore 47 waters, particularly beneath oxic bottom waters, constrains the likelihood that those settings could have 48 hosted organisms and ecosystems with thiotrophy at their foundations. 49 INTRODUCTION 50 Iron speciation and molybdenum concentrations have been well calibrated in mod...

Scenarios of Deoxygenation of the Eastern Tropical North Pacific During the Past Millennium as a Window Into the Future of Oxygen Minimum Zones

“…Molybdenum enrichments can occur diagenetically (within the sediments), in the water column, or in some cases near the sediment-water interface, which was reported for OMZs (Zheng et al, 2000). While widely used as an indicator of past anoxic and euxinic (anoxic and sulfidic) conditions, several complications have been reported for this proxy (Morford and Emerson, 1999;Chappaz et al, 2014), including localized Mo enriched in turbidites despite fully oxygenated water column conditions (McKay and Pedersen, 2014) and uptake exclusively within the sediments when significant amounts of sulfide are present solely in pore waters (Zheng et al, 2000;Scholz et al, 2017;Hardisty et al, 2018)-although these enrichments tend to fall below those typical of euxinic settings (e.g., Scott and Lyons, 2012).…”

“…To assess the contribution of authigenic U through settling, we contrast U and Mo sedimentary records published in the literature with ours from the ETNP (Ivanochko and Pedersen, 2004;McManus et al, 2006). Typically, Mo is used as an indicator for water euxinia (Calvert and Pedersen, 1993;Algeo and Lyons, 2006;Algeo and Tribovillard, 2009;Lyons et al, 2009;Algeo and Rowe, 2012;Scott and Lyons, 2012), but most of the enrichments in modern OMZs are likely tied to the presence of sulfide only in pore waters (Scholz et al, 2017;Hardisty et al, 2018). We are not using V because, like U, there could also be important contribution of the redox sensitive element through settling particles, associated to reducing microenvironments ( Table 5; Choumiline, 2011).…”

“…Nevertheless, Mo is commonly elevated in ETNP sediment cores, reaching maxima several times higher than the average crustal value of 1.4 mg/kg (Wedepohl, 1995) (Figure 7; Table 5). These values are likely linked to the presence of sulfide only in the pore waters, leading to diagenetic enrichments (Zheng et al, 2000;Scholz et al, 2017;Hardisty et al, 2018). Because of this diagenetic enrichment, Mo is a less sensitive indicator of very low and transient sulfide in the water column.…”

Redox Chemistry and Molybdenum Burial in a Mesoproterozoic Lake

“…Constraining the geochemistry of Proterozoic lake environments can help to characterize the conditions wherein multicellular life may have evolved. Molybdenum (Mo), a redox‐sensitive trace element, has been intensively used as a paleo redox proxy, mostly to identify periods of past anoxia (Chappaz et al, ; Hardisty et al, ). In presence of sulfide (∑S(–II) = H 2 S + HS – + S 2– ), Mo can be scavenged in sediment via two pathways: (1) through association with Fe‐S minerals (e.g., Chappaz et al, ; Helz & Vorlicek, ) and (2) through interactions with organic matter (e.g., Ardakani et al, ; Tessin et al, ; Wagner et al, ).…”

The impact of declining oxygen conditions on pyrite accumulation in shelf sediments (Baltic Sea)

“…Moreover, sulfate reduction rate within the surface sediment layer varies seasonally (Aller 1980). Metabolic rate and hydrogen sulfide production decreases during the winter months when temperature and the input of organic matter to the bottom is low (Hardisty et al 2018).…”