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Title A model for the oceanic mass balance of rhenium and implications for the extent of Proterozoic ocean anoxia
Permalink https://escholarship.org/uc/item/1gh1h920
Journal GEOCHIMICA ET COSMOCHIMICA ACTA, 227
ISSN 0016-7037
Authors Sheen, Alex I Kendall, Brian Reinhard, Christopher T et al.
Publication Date 2018-04-15
DOI 10.1016/j.gca.2018.01.036
Peer reviewed
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Geochimica et Cosmochimica Acta 227 (2018) 75–95 www.elsevier.com/locate/gca
A model for the oceanic mass balance of rhenium and implications for the extent of Proterozoic ocean anoxia
Alex I. Sheen a,b,⇑, Brian Kendall a, Christopher T. Reinhard c, Robert A. Creaser b, Timothy W. Lyons d, Andrey Bekker d, Simon W. Poulton e, Ariel D. Anbar f,g a Department of Earth and Environmental Sciences, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada b Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada c School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, USA d Department of Earth Sciences, University of California, Riverside, CA 92521, USA e School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK f School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA g School of Molecular Sciences, Arizona State University, Tempe, AZ 85287, USA
Received 13 July 2017; accepted in revised form 30 January 2018; available online 8 February 2018
Abstract
Emerging geochemical evidence suggests that the atmosphere-ocean system underwent a significant decrease in O2 content following the Great Oxidation Event (GOE), leading to a mid-Proterozoic ocean (ca. 2.0–0.8 Ga) with oxygenated surface waters and predominantly anoxic deep waters. The extent of mid-Proterozoic seafloor anoxia has been recently estimated using mass-balance models based on molybdenum (Mo), uranium (U), and chromium (Cr) enrichments in organic-rich mudrocks (ORM). Here, we use a temporal compilation of concentrations for the redox-sensitive trace metal rhenium (Re) in ORM to provide an independent constraint on the global extent of mid-Proterozoic ocean anoxia and as a tool for more generally exploring how the marine geochemical cycle of Re has changed through time. The compilation reveals that mid-Proterozoic ORM are dominated by low Re concentrations that overall are only mildly higher than those of Archean ORM and significantly lower than many ORM deposited during the ca. 2.22–2.06 Ga Lomagundi Event and during the Phanerozoic Eon. These temporal trends are consistent with a decrease in the oceanic Re inventory in response to an expan- sion of anoxia after an interval of increased oxygenation during the Lomagundi Event. Mass-balance modeling of the marine Re geochemical cycle indicates that the mid-Proterozoic ORM with low Re enrichments are consistent with extensive seafloor anoxia. Beyond this agreement, these new data bring added value because Re, like the other metals, responds generally to low- oxygen conditions but has its own distinct sensitivity to the varying environmental controls. Thus, we can broaden our capac- ity to infer nuanced spatiotemporal patterns in ancient redox landscapes. For example, despite the still small number of data, some mid-Proterozoic ORM units have higher Re enrichments that may reflect a larger oceanic Re inventory during transient episodes of ocean oxygenation. An improved understanding of the modern oceanic Re cycle and a higher temporal resolution for the Re compilation will enable further tests of these hypotheses regarding changes in the surficial Re geochemical cycle in response to variations in atmosphere-ocean oxygenation. Nevertheless, the existing Re compilation and model results are in agreement with previous Cr, Mo, and U evidence for pervasively anoxic and ferruginous conditions in mid-Proterozoic oceans. Ó 2018 Elsevier Ltd. All rights reserved.
Keywords: Rhenium; Anoxia; Proterozoic; Organic-rich mudrocks; Ocean; Oxygen
⇑ Corresponding author at: Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada. E-mail address: [email protected] (A.I. Sheen). https://doi.org/10.1016/j.gca.2018.01.036 0016-7037/Ó 2018 Elsevier Ltd. All rights reserved. 76 A.I. Sheen et al. / Geochimica et Cosmochimica Acta 227 (2018) 75–95
1. INTRODUCTION 2017), such conditions imply poorly buffered surface O2 inventories and the potential for significant spatiotemporal The progressive oxygenation of the early Earth’s surface variability (Planavsky et al., 2014; Cole et al., 2016; had a profound impact on Earth’s biological and geochem- Reinhard et al., 2016; Daines et al., 2017; Hardisty et al., ical evolution (Lyons et al., 2014). A poorly oxygenated 2017). Indeed, there has been much recent interest in the atmosphere-ocean system in the Archean is indicated by possibility of dynamic spatiotemporal variations in several lines of evidence in the sedimentary record atmosphere-ocean redox conditions during this time (e.g., (Farquhar et al., 2000; Bekker et al., 2010; Sverjensky Sperling et al., 2014; Gilleaudeau and Kah, 2015; and Lee, 2010; Lyons et al., 2014), such as abundant Gilleaudeau et al., 2016; Mukherjee and Large, 2016; banded iron formations (BIF), common occurrence of Planavsky et al., 2016; Reinhard et al., 2016; Zhang et al., redox-sensitive detrital minerals, and preservation of sulfur 2016) against a background of lower atmospheric pO2 com- mass-independent fractionation (S-MIF). In addition, the pared with today (e.g., Cole et al., 2016). Hence, additional low concentrations of some redox-sensitive elements (e.g., data and proxies sensitive to global redox conditions are Mo, U) in sedimentary archives suggest low seawater con- needed to better understand the spatiotemporal evolution centrations of these elements because of their limited oxida- and the dominant redox state of the mid-Proterozoic tive mobilization from the Archean continental crust (Scott oceans. Such information is critical to a full understanding et al., 2008; Partin et al., 2013). The Great Oxidation Event of the relationship between Earth’s surface oxygenation (GOE) is marked by a permanent increase of atmospheric and the evolution of both eukaryotic organisms and com- O2 content to >0.001% present atmospheric level (PAL), plex metazoan life. The integrated use of diverse metals starting between 2.45 and 2.32 Ga (Pavlov and Kasting, spanning a broad range of redox sensitivity (e.g., 2002; Bekker et al., 2004; Bekker, 2014; Gumsley et al., Reinhard et al., 2013a), detrital backgrounds, uptake path- 2017). This transition was accompanied by the appearance ways, and crustal sources will strengthen our conclusions. of new mineral species containing redox-sensitive elements The concentrations of some redox-sensitive trace metals in their highest oxidation states, reduction in BIF deposi- in marine organic-rich mudrocks (ORM) can provide tion, disappearance of S-MIF, and an increase in seawater insight into global ocean redox conditions. Behaving largely Mo, U, and sulfate concentrations (Bekker et al., 2004, conservatively in oxygenated seawater, trace metals such as 2010, 2013; Schro¨der et al., 2008; Scott et al., 2008, 2014; Mo, Cr, Re, and U are removed to anoxic sediments at Sverjensky and Lee, 2010; Hazen et al., 2011; Planavsky higher rates compared with oxygenated sediments, with et al., 2012; Reuschel et al., 2012; Partin et al., 2013; removal rates dependent on the specific chemistry of the Reinhard et al., 2013a). The latter part of the GOE was water column and underlying sediment pore fluids (e.g., marked by a protracted episode of elevated organic carbon Morford and Emerson, 1999; Tribovillard et al., 2006). burial (Lomagundi Event) between ca. 2.22 and 2.06 Ga, Chromium, U, and Re become authigenically enriched in which suggests a long-lasting, but transient increase in sediments overlain by an anoxic water column (at both high atmosphere-ocean O2 contents to levels that may not have and low levels of dissolved H2S) and, to a lesser extent, in occurred again until the Neoproterozoic Oxidation Event anoxic sediments overlain by mildly oxygenated bottom (NOE) (Karhu and Holland, 1996; Scott et al., 2008, waters (Crusius et al., 1996; Morford and Emerson, 1999; 2014; Kump et al., 2011; Planavsky et al., 2011, 2012, Morford et al., 2005; Partin et al., 2013; Reinhard et al., 2014; Bekker and Holland, 2012; Partin et al., 2013). 2013a). By contrast, high authigenic Mo enrichments in Although dramatic swings in the extent of atmosphere- sediments require the accumulation of dissolved H2Sin ocean oxygenation are recognized in the early and late the water column. Anoxic sediments beneath mildly oxy- Proterozoic, surficial redox dynamics during the interven- genated bottom waters display mild Mo enrichments if dis- ing period were likely muted. Recent geochemical data solved H2S is present in sediment pore fluids at shallow (e.g., redox-sensitive trace metals and Fe speciation) suggest depths below the sediment-water interface (Crusius et al., stratified ocean redox conditions for the middle portion of 1996; Morford and Emerson, 1999; Morford et al., 2005). the Proterozoic (ca. 1.8–0.8 Ga; hereafter referred to as the Because these trace metals have a long seawater resi- mid-Proterozoic), wherein oxygenated surface waters were dence time relative to the average ocean turnover time underlain by euxinic (anoxic and sulfidic) waters in highly ( 1–2 kyr), their enrichment record in open-marine anoxic productive marginal settings, and ferrous iron (Fe2+) accu- sediments reflects, in principle, the global marine redox mulated in suboxic to anoxic deep waters in offshore set- state. Once an environment becomes authigenically active tings (ferruginous anoxia; Poulton et al., 2010; Planavsky for a particular redox-sensitive trace metal, the degree of et al., 2011; Reinhard et al., 2013a; Sperling et al., 2015). authigenic enrichment of that metal in sediments will be While Fe speciation data indicate ferruginous conditions broadly proportional to its dissolved seawater inventory at many times and localities in the mid-Proterozoic, the (Algeo and Lyons, 2006; Scott et al., 2008; Reinhard scarcity of preserved ancient seafloor, particularly of the et al., 2013a). On a global scale, that inventory will be con- deepest parts of the ocean, creates a challenge for constrain- trolled mainly by the collective state of marine redox condi- ing the marine redox landscape on a global scale. tions—once pervasive oxidative continental weathering and Although low O2 levels have been suggested for the mid- the associated riverine flux of dissolved anionic metal com- Proterozoic atmosphere–ocean system (<0.1–1.0% PAL; plexes are established (Scott et al., 2008; Partin et al., 2013; Canfield, 1998; Reinhard et al., 2013a; Planavsky et al., Reinhard et al., 2013a). In an ancient ocean that was more 2014; Liu et al., 2016; Tang et al., 2016; Hardisty et al., anoxic than today, a globally higher rate of metal burial in A.I. Sheen et al. / Geochimica et Cosmochimica Acta 227 (2018) 75–95 77 seafloor sediments should occur, leading to a decrease in Re in anoxic marine ORM may provide a novel and com- seawater metal concentrations and thus lower metal enrich- plementary perspective on global ocean redox conditions. ments in coeval ORM (Scott et al., 2008; Sahoo et al., 2012; To estimate first-order variations in the seawater con- Partin et al., 2013; Reinhard et al., 2013a). centration of Re through geologic time, with new insights Building from the approach of Scott et al. (2008), new into the evolution of marine redox conditions during the constraints on the extent of ocean anoxia were recently pre- mid-Proterozoic as the primary goal, we compiled Re con- sented for the mid-Proterozoic ocean using trace element centration data for ORM from the literature and report records. Through a mass-balance model combined with new Re concentration data from several Precambrian inter- Mo and Cr enrichments in ORM, Reinhard et al. (2013a) vals. We quantify the sources and sinks of the Re marine estimated the extent of mid-Proterozoic anoxia to be at cycle based on modern observations and integrate these least 30–40% of the modern seafloor area, with euxinic con- observations with the mass-balance approach developed ditions covering less than 1–10% of the modern seafloor. by Reinhard et al. (2013a). Using this mass-balance model, In fact, the Cr enrichment record is also consistent with vir- we discuss the sensitivity of the marine Re cycle with respect tually complete seafloor anoxia. A disadvantage of Cr, to the expansion and contraction of ocean anoxia during however, is its high detrital contribution to ORM this interval of Earth’s history and its implications for the (Reinhard et al., 2013a), which makes it an intrinsically less extent and variability of mid-Proterozoic ocean anoxia. precise tracer for the benthic redox landscape under condi- tions of pervasive marine anoxia. Based on the U record, 2. THE MODERN MARINE RE CYCLE Partin et al. (2013) postulated that anoxic conditions cov- ered over 50% of the modern seafloor area. However, this 2.1. Marine sources estimate assumes that the U burial flux in anoxic basins on continental margins applies to the global seafloor. Rhenium exists in seawater primarily as the soluble per- – Hence, the mass-balance model of Partin et al. (2013) is rhenate oxyanion ReO4, with a concentration of 40 pmol over-sensitive to anoxic conditions and consequently may kg–1 (Anbar et al., 1992; Colodner et al., 1993). In the mod- underestimate the true extent of ocean anoxia. ern ocean, most of this dissolved reservoir is derived from Rhenium (Re) behaves conservatively in oxygenated oxidative weathering of sulfide minerals, based on the good seawater and, unlike Mo, can be efficiently removed to correlation observed globally between Re and sulfate con- anoxic sediments at low dissolved H2S levels when the bot- centrations in rivers, and from weathering of ORM in the tom waters are weakly oxygenated or anoxic, thus making upper continental crust (Colodner et al., 1993; Miller it a more effective proxy for tracking general ocean anoxia et al., 2011; Dubin and Peucker-Ehrenbrink, 2015). Exclud- (i.e., combined euxinic and ferruginous anoxia) (Colodner ing anthropogenic inputs, the average Re concentration of et al., 1993; Crusius et al., 1996; Morford and Emerson, rivers has been calculated to be 11.2 pmol kg–1 based on 1999; Morford et al., 2005, 2012). Although Cr and U data from 38 rivers on five continents ( 37% of total water behave similarly in this regard (Partin et al., 2013; discharge; 25% of continental exorheic drainage area). This Reinhard et al., 2013a), the magnitude of authigenic Re estimate yields a riverine flux of 4.29 105 mol yr–1, which enrichment in anoxic marine sediments is significantly corresponds to a seawater residence time of 1.3 105 yr higher than the detrital background compared with Cr (Miller et al., 2011). and U, as reflected by higher Re enrichment factors in Seafloor hydrothermal vents have been considered as an organic-rich sediments expressed relative to average upper additional component of the marine Re cycle (Morford and crust (Table 1). These higher Re enrichments indicate that Emerson, 1999). It is postulated that Cl– complexation in Re is more sensitive to O2-deficient conditions than Cr high-temperature hydrothermal fluids may yield high Re and U and can potentially provide more quantitatively pre- concentrations (Xiong and Wood, 1999; 2002). However, cise information on deep-sea redox state. Because of these Re may be removed from such fluids by precipitation of sul- distinct geochemical properties, the enrichment record of fides under fluid-sulfide equilibrium (Miller et al., 2011).
Table 1 Comparison of Re, Cr and U enrichment levels in modern anoxic marine sediments. Element Upper crust concentrationa Average conc.a in modernb anoxic marine sediments Enrichment factorc Re 0.2–0.4 65.5 218 Cr 85 94.6 1.1 U 2.8 11.1 4.0 Re data: Esser and Turekian (1993); Peucker-Ehrenbrink and Jahn (2001); Dubin and Peucker-Ehrenbrink (2015); this study (see supplementary spreadsheet files, Tables S1–S2). Cr data: McLennan (2001), Reinhard et al. (2013a). U data: McLennan (2001), Partin et al. (2013). a Re in ppb; Cr and U in ppm. b Modern data defined as datasets with an age younger than 1 Ma. c Enrichment factor = anoxic sediment conc./upper crust conc. 78 A.I. Sheen et al. / Geochimica et Cosmochimica Acta 227 (2018) 75–95
Miller et al. (2011) presented the first published Re mea- with previous trace metal studies, we define oxic settings surements in high-temperature hydrothermal fluids in the as those with an O2 penetration depth >1 cm, at which Manus Basin, and estimated a flux of 1.2 103 mol yr–1 point Mn redox cycling begins (Morford and Emerson, from these data. If globally representative, this flux consti- 1999). Rhenium adsorption onto Fe–Mn oxides is minimal tutes 0.1% of the pre-anthropogenic riverine Re input and relative to other redox-sensitive trace metals such as Mo thus has a negligible influence on the modern oceanic Re (Koide et al., 1986; Colodner et al., 1993). As a reference mass balance. frame, and as discussed in Reinhard et al. (2013a), the Low-temperature hydrothermal fluids are a possible extent of modern seafloor covered by oxic sediments with minor source of Re to the modern ocean (as is the case an O2 penetration depth >1 cm is 84%. for Mo; Morford and Emerson, 1999), but no data exist The modern oxic Re sink is dominated by continental to quantify this flux. Reinhard et al. (2013a) calculated that margin sediments, where the sediment mass accumulation the modern low- and high-temperature hydrothermal fluxes rate is high. Previous Re budget reconstructions treat pela- of Mo and Cr are unlikely to be above 10% and 1% of gic sediments, which form the majority of modern-day oxic their riverine fluxes, respectively. Given the similar geo- seafloor, as a negligible sink. This assumption is supported chemical behaviour of Re and Mo, it is reasonable to by an average [Re]sed of 0.05 ppb in pelagic sediments of the assume that the modern hydrothermal flux of Re to the North Atlantic (Colodner, 1991), which translates to a bur- oceans is also small. ial flux of 1.2 10 5 ng cm–2 yr–1 (assuming an average lin- ear sedimentation rate). The pelagic burial rate in abyssal 2.2. Marine sinks regions of the seafloor, when combined with an average burial rate of 7.4 10 3 ng cm–2 yr–1 for Re in shelf sedi- 2.2.1. Overview ments deposited from well-oxygenated waters on continen- Sinks for Re removal from modern seawater comprise tal margins (Sea of Japan, Central Arctic Ocean, sediment burial in three redox settings: oxic, suboxic Northwestern US margin, and African margin; Table 2) (defined below), and anoxic. Because of the solubility of yields an area-weighted average oxic Re burial rate of 1.6 – 3 –2 –1 ReO4, Re accumulation is typically very slow in well- 10 ng cm yr (continental margin and deep-sea abys- 17 2 18 oxygenated marine settings where O2 penetrates more than sal regions cover an area of 7.4 10 cm and 2.7 10 1 cm below the sediment-water interface. By contrast, Re cm2, respectively; Sverdrup et al., 1942; Wollast, 2002). accumulation is significantly more efficient in anoxic marine Again, the oxic burial rate is dominated by continental settings. Rhenium removal from an anoxic water column is margin sediments and is unlikely to vary significantly with likely dominated by abiotic redox reactions rather than bio- new estimates for pelagic sediments. logical uptake. In anoxic conditions, Re is reduced from the soluble heptavalent (VII) to the insoluble tetravalent (IV) 2.2.3. Suboxic sink state (Colodner et al., 1993; Crusius et al., 1996; Morford Similar to Mo, Cr, and U, a significant difference exists and Emerson, 1999; Morford et al., 2005). Scavenging of in Re accumulation rates beneath anoxic water columns Re4+ by organic matter is suggested by the direct associa- and those under weakly oxygenated bottom waters tion of Re with organic matter in fine-grained sediments (Colodner et al., 1993; Crusius et al., 1996; Morford and and their lithified equivalents (ORM) (Selby and Creaser, Emerson, 1999; Morford et al., 2005). To be consistent with 2003; Georgiev et al., 2012). In some ORM, a good positive previous studies on trace element redox modeling, we define correlation is observed between Re concentration and total the suboxic setting as areas where the O2 penetration depth organic carbon (TOC) content (e.g., Rooney et al., 2010)as below the sediment-water interface is <1 cm (Partin et al., has been observed for Mo (Algeo and Lyons, 2006), but 2013; Reinhard et al., 2013a). This condition accounts for this relationship is not observed in all ORM (e.g., Cohen areas of the seafloor where dissolved O2 is present in bot- et al., 1999). Decoupling of Re and TOC contents in tom waters, but typically at levels low enough (<10–100 ORM may reflect temporal (i.e., stratigraphic) variations lM; higher values are also possible in shallow coastal loca- in local bottom water Re concentrations at the site of depo- tions; Morford and Emerson, 1999; Morford et al., 2005, sition. Co-precipitation of Re7+ with a Fe–Mo–S phase has 2007, 2012) so that Fe–Mn oxyhydroxides are not perma- been proposed for the removal of Re from sulfide-bearing nently buried, and that reduction of Re occurs in the anoxic waters (Helz and Dolor, 2012), which may also be respon- sediment pore-waters. The result is appreciable authigenic sible for a scatter of Re concentration versus TOC content accumulation of Re in sediments. Beneath modern suboxic in ORM. waters in continental margin environments, authigenic Re Rhenium burial rates characteristic of each of the three accumulation occurs without significant Mo accumulation settings are calculated based on observations in modern when the shallow pore water is anoxic but dissolved H2S marine environments (Table 2; Tables S1 and S2 in the sup- is low, thus demonstrating that Re removal rate does not plementary database). Below, we describe how representa- scale to H2S availability (Morford and Emerson, 1999; tive burial rates were calculated for each sink. Morford et al., 2005). Depending on the organic carbon flux and oxygen pen- 2.2.2. Oxic sink etration depth, modern suboxic Re burial rates range from Oxic settings are characterized by permanent burial of 0.2–0.3 ng cm–2 yr–1 on the northwestern U.S. margin to –2 –1 Mn oxides in sediments where O2 penetration depths below 1.5 ng cm yr in the Gulf of California. Our estimate the sediment-water interface are large. To be consistent for the suboxic Re burial rate combines these values with Table 2 Re burial rates in modern marine sediments. Basin mass accumulation rates are listed for anoxic sediments only. Oxygen penetration depth is given for suboxic and oxic sediments where available. Italicized names under the ‘‘Sampled Sediments” column are sub-basins within the same margin (in bold) and from which an arithmetic average is taken to represent the overall burial rate for that margin. Only non-italic values under the ‘‘Re Burial Rate” column are used in calculating weighted averages; this is done for the suboxic and oxic sinks. The modern anoxic sink burial rate is calculated from Cariaco Basin data (Supplementary information).
Sampled sediments Redox state O2 penetration Basin MAR Re burial rate Areal extent Weighing Representative sink References 2 1 2 1 2 depth (cm) (g cm yr ) (ng cm yr ) (cm ) factor Re burial rate, bi (ng cm 2 yr 1 ) OXIC Pelagic sediments Oxic 1.2 10 5a 2.7 1018b 78.4% 1.6 10 3 Colodner (1991) Oxic Continetal Margins 7.4 10 3 7.4 1017c 21.6%