Thallium-isotopic compositions of euxinic sediments as a proxy for global manganese-oxide burial

Abstract

Thallium (Tl) isotopes are a new and potentially powerful paleoredox proxy that may track bottom water oxygen conditions based on the global burial flux of manganese oxides. Thallium has a residence time of ~20 thousand years, which is longer than the ocean mixing time, and it has been inferred that modern oxic seawater is conservative with respect to both concentration and isotopes. Marine sources of Tl have nearly identical isotopic values. Therefore, the Tl sinks, adsorption onto manganese oxides and low temperature oceanic crust alteration (the dominant seawater output), are the primary controls of the seawater isotopic composition. For relatively short-term, ~million years, redox events it is reasonable to assume that the dominant mechanism that alters the Tl isotopic composition of seawater is associated with manganese oxide burial because large variability in low temperature ocean crust alteration is controlled by long-term, multi-million years, average ocean crust production rates. This study presents new Tl isotope data for an open ocean transect in the South Atlantic, and depth transects for two euxinic basins (anoxic and free sulfide in the water column), the Cariaco Basin and Black Sea. The Tl isotopic signature of open ocean seawater in the South Atlantic was found to be homogeneous with ε 205 Tl =-6.0 ± 0.3 (± 2 SD, n = 41) while oxic waters from Cariaco and the Black Sea are-5.6 and-2.2, respectively. Combined with existing data from the Pacific and Arctic Oceans, our Atlantic data establish the conservatism of Tl isotopes in the global ocean. In contrast, partially-and predominantly-restricted basins reveal Tl isotope differences that vary between open-ocean (-6) and continental material (-2) ε 205 Tl, scaling with the degree of restriction. Regardless of the differences between basins, Tl is quantitatively removed from their euxinic waters below the chemocline. The burial of Tl in euxinic sediments is estimated to be an order of magnitude less than each of the modern ocean outputs and imparts no isotopic fractionation. Thallium removal into pyrite appears to be associated with a small negative fractionation between-1 and-3 ε 205 Tl, which renders Tl-depleted waters below the chemocline enriched in isotopically-heavy Tl. Due to the quantitative removal of Tl from euxinic seawater, Tl isotope analyses of the authigenic fraction of underlying euxinic sediments from both the Black Sea and Cariaco Basin capture the Tl isotope value of the oxic portion of their respective water column with no net isotope fractionation. Since the Tl isotope composition of seawater is largely dictated by the relative fraction of Mn-oxide burial versus oceanic crust alteration, we contend that the Tl isotope composition of authigenic Tl in black shales, deposited under euxinic conditions but well-connected to the open ocean, can be utilized to reconstruct the Tl isotope composition of seawater, and thus to reconstruct the global history of Mn-oxide burial.

Thallium Mass Fraction and Stable Isotope Ratios of Sixteen Geological Reference Materials

“…, , 2006c, Rehkämper and Nielsen , Owens et al . ); anthropogenic mobilisation of Tl (Nriagu , Xiao et al . , , Peter and Viraraghavan , Kersten et al .…”

“…The utility and versatility of the system lie in the large isotope ratio differences and relatively high mass fractions (lg g -1 ) associated with low-temperature environments, in contrast with very low mass fractions and negligible isotope ratio differences in mantle-derived lavas (see review of Nielsen et al 2017. Applications include investigations of extraterrestrial materials (Nielsen et al 2006a, Andreasen et al 2009, 2012, Baker et al 2010b, Palk et al 2011; riverine and marine fluxes (Rehk€ amper et al 2002, 2006c, Owens et al 2017; anthropogenic mobilisation of Tl (Nriagu 1998, Xiao et al 2003, Peter and Viraraghavan 2005, Kersten et al 2014, Van ek et al 2018; ore exploration ; and use as a tracer of contributions to the source regions of igneous materials (Nielsen et al 2006b, 2014, Shu et al 2017, Blusztajn et al 2018.…”

Constraining oceanic oxygenation during the Shuram excursion in South China using thallium isotopes

“…Thallium (Tl) isotopes have shown rapid response to changes in global marine redox conditions during the onset of associated carbon isotope excursions (Ostrander, Owens, & Nielsen, ; Them et al, ). The modern marine residence time of Tl is ~18,500 years (Baker, Rehkämper, Hinkley, Nielsen, & Toutain, ; Nielsen, Rehkämper, & Prytulak, ; Rehkämper & Nielsen, ), significantly longer than the ocean mixing time of ~1,500 years, thereby yielding a globally homogenous Tl isotope composition (ε 205 Tl = 10,000 × ( 205/203 Tl sample ‐ 205/203 Tl SRM 997 )/ 205/203 Tl SRM 997 ) for oxic seawater of ε 205 Tl = −6.0 ± 0.3 (Owens, Nielsen, Horner, Ostrander, & Peterson, , and references therein) with concentrations of 65 ± 5 p.m. (Owens et al, ; Rehkämper & Nielsen, ). Sources of Tl to the ocean are dust aerosols, rivers, hydrothermal fluids, continental margin sediment pore fluids, and volcanic fumaroles, which are all characterized by relatively similar Tl isotope compositions of ε 205 Tl = −2 (Baker et al, ; Nielsen et al, , , , ).…”

“…In contrast, Tl deposition within altered oceanic crust imparts a relatively minor isotope fractionation from seawater (modern ε 205 Tl values of altered oceanic crusts are −10 to −6; Nielsen et al, ), constituting the majority of the remaining ~68% of the burial. There are additional sinks, but they are relatively minor and/or have limited known isotopic effect on seawater (Owens et al, ). Thus, the burial of Mn oxides is the most dominant flux that drives seawater Tl isotope variability, especially over shorter timescales (Owens, ; Owens et al, ).…”

“…There are additional sinks, but they are relatively minor and/or have limited known isotopic effect on seawater (Owens et al, ). Thus, the burial of Mn oxides is the most dominant flux that drives seawater Tl isotope variability, especially over shorter timescales (Owens, ; Owens et al, ). Manganese oxides are ubiquitous in oxic marine sediments deposited below an oxygenated water column but are absent under anoxic conditions including euxinic (anoxic and sulfidic water column) conditions at or above the sediment–water interface (Johnson, Webb, Ma, & Fischer, ; Rue, Smith, Cutter, & Bruland, ).…”