Thallium isotopes in early diagenetic pyrite – A paleoredox proxy?

Abstract

-This paper presents the first study of Tl isotopes in early diagenetic pyrite.Measurements from two sections deposited during the Toarcian Ocean Anoxic Event (T-OAE, ~183Ma) are compared with data from Late Neogene (<10Ma) pyrite samples from ODP legs 165 and 167 that were deposited in relatively oxic marine environments. The Tl isotope compositions of Late Neogene pyrites are all significantly heavier than seawater, which most likely indicates that Tl in diagenetic pyrite is partially sourced from ferromanganese oxyhydroxides that are known to display relatively heavy Tl isotope signatures. One of the T-OAE sections from Peniche in Portugal displays pyrite thallium isotope compositions indistinguishable from Late Neogene samples, whereas samples from Yorkshire in the UK are depleted in the heavy isotope of Tl. These lighter compositions are best explained by the lack of ferromanganese precipitation at the sediment-water interface due the sulphidic (euxinic) conditions thought to be prevalent in the Cleveland Basin where the Yorkshire section was deposited. The heavier signatures in the Peniche samples appear to result from an oxic water column that enabled precipitation of ferromanganese oxy-hydroxides at the sediment-water interface.The Tl isotope profile from Yorkshire is also compared with previously published molybdenum isotope ratios determined on the same sedimentary succession. There is a suggestion of an anti-correlation between these two isotope systems, which is consistent with the expected isotope shifts that occur in seawater when marine oxic (ferromanganese minerals) fluxes fluctuate.The results outlined here represent the first evidence that Tl isotopes in early diagenetic pyrite have potential to reveal variations in past ocean oxygenation on a local scale and potentially also for global oceans. However, much more information about Tl isotopes in 3 different marine environments, especially in anoxic/euxinic basins, is needed before Tl isotopes can be confidently utilized as a paleo-redox tracer.4

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

“…Until now, the phases bearing Tl in non‐euxinic reducing sediments have not been well understood. In euxinic basins, Tl is removed quantitatively through scavenging or incorporation into Fe sulfides precipitated from the water column below the chemocline (Nielsen et al, ; Owens et al, ). Although non‐euxinic reducing conditions can also support sulfide formation near the sediment–water interface, it is also possible that Tl incorporation in reducing sediments could occur through other processes.…”

“…Approximately 200 mg of powdered samples was digested in PTFE beakers using 10 ml concentrated HNO 3 at 135°C for 12 hr. The supernatant, which is considered to contain primarily authigenic Tl that may be bound to sulfides (Nielsen et al, ; Owens et al, ), was separated from the residual solids by centrifugation and complete evaporation. The samples were then redissolved in 3 ml concentrated HNO 3 and transferred into 90‐ml quartz glass vials.…”

“…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, , , , ). These sources are balanced by adsorption onto manganese (Mn) oxides and incorporation into oceanic crust during low‐temperature hydrothermal alteration (Nielsen et al, ).…”

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

“…The massive magmatic sulphide 14P was digested on a hot plate at 130°C for a minimum of 12 h in~5 ml HNO 3 (3 mol l -1 ), after the protocol of Nielsen et al (2011). Samples of 14P were then evaporated to near dryness at 155°C and redissolved in 12 ml HCl (1 mol l -1 ).…”

Mesozoic climates and oceans – a tribute to Hugh Jenkyns and Helmut Weissert

“…In this aspect of OAE research, concentrations and isotopes of redox‐sensitive elements, such as Cr, Fe, I, Mn, Mo, N, S, Tl, U and V, have proven particularly valuable in reconstructing changing redox conditions both locally and globally (e.g. Jenkyns et al ., , , ; Kuypers et al ., ; Pearce et al ., ; Jenkyns, ; Lu et al ., ; Montoya‐Pino et al ., ; Gill et al ., ; Nielsen et al ., ; Owens et al ., , ; Westermann et al ., ; Zhou et al ., ; Dickson et al ., , ; Gomes et al ., ; Holmden et al ., ). Through the integration of the different geochemical systems discussed here, it has been possible to develop a detailed understanding of the temporal (and, arguably, mechanistic) links between changes in the physical environment, seawater chemistry and biogeochemical cycles during OAEs (e.g.…”