Molybdenum isotope fractionations observed under anoxic experimental conditions

Abstract

Experiments were carried out to determine molybdenum isotope fractionation associated with adsorption to pyrite. Results show that the Mo isotope composition of the aqueous solution becomes progressively heavier as Mo is adsorbed, with a Mo isotope fractionation as large as 2.9‰. This fractionation is larger than observed for typical anoxic continental margin marine sediments (e.g., ~0.7‰), suggesting that Mo adsorption to pyrite is not the dominant process operating in these environments. However, our adsorption results do suggest the possibility that anoxic fractionation processes could impart an isotope signature within the geologic record similar to the isotope fractionation observed under oxygenated conditions. Additional experiments were conducted at high (~100 µM) dissolved Mo concentrations, but at varying pH and ∑H 2 S concentrations, and in each case these experiments promoted Mo-sulfide precipitation. Even though the experiments were conducted under differing conditions and produced different amounts of precipitate, the results suggest a constant fractionation of ~0.9‰ associated with this Mo removal process. This fractionation is more consistent with that inferred for anoxic continental margin marine sediments, suggesting that a process similar to Mo-sulfide precipitation, rather than an adsorption process, may be responsible for the Mo isotope compositions observed in these environments. The findings of this study suggest that, regardless of the geochemical mechanism employed, sediment Mo sequestration under anoxic conditions may impart a significant isotopic fractionation relative to parent seawater.

Anoxic development of sapropel S1 in the Nile Fan inferred from redox sensitive proxies, Fe speciation, Fe and Mo isotopes

“…Azrieli-Tal et al (2014) found light Mo isotope values (-0.9 to -0.5‰) in the lower part of sapropel S1 at ODP site 967D that coincided with the lowest Fe isotope values Siebert et al, 2003Siebert et al, , 2006. o isotope compositions (~1.6‰) of typical anoxic continental margin marine sediments (where sulphide may accumulate in porewaters) indicate a fractionation of -0.7‰ from seawater Mo Poulson-Brucker et al, 2009), which matches that ~-0.9‰ determined in Mo-sulphide co-precipitation experiments (Poulson-Brucker et al, 2012). Processes such as adsorption of isotopically light Mo to Fe (oxyhydr)oxides become significant in non-sulphidic waters (Goldberg et al, 2009;Goldberg et al, 2012).…”

“…In the sediments at or near the lower and upper sapropel boundaries (Fig. 5a), Mo adsorption to pyrite might become relevant (Poulson-Brucker et al, 2012 The main feature of the 9509 sapropel S1 is the asymmetric double minimum  98/95 Mo form with the intermediate peak corresponding to the 8.2 ka ventilation event (Fig. 5a).…”

Molybdenum‐isotope chemostratigraphy and paleoceanography of the Toarcian Oceanic Anoxic Event (Early Jurassic)

“…It would be difficult to explain the intersite differences in the sediment δ 98/95 Mo compositions of the Cleveland, West Netherlands, and South German Basins during the euxinic depositional conditions that prevailed during the T‐OAE by invoking the mixing into sediments of isotopically light oxic molybdenum phases, because they would be unstable in such conditions. Rather, the sediment Mo at each location must have been dominated by Mo sulphides [ Poulson‐Brucker et al , , ; Neubert et al , ; Nägler et al , ], with a negligible mixing contribution from Mo adsorbed to oxyhydroxides. Consequently, evolution of the local seawater δ 98/95 Mo in each basin is most likely to have been the principal cause of the differences in the sedimentary isotope records.…”

“…In oxic and suboxic environments, the lighter Mo isotopes are mainly removed by adsorption to Mn‐oxyhydroxide minerals, with compositions that are typically 3‰ lighter than seawater [ Siebert et al , ; Barling and Anbar , ; Goldberg et al , ; Wasylenki et al , ]. Conversely, in environments characterized by the presence of reduced sulphur (H 2 S) in seawater and sediment pore waters, Mo is removed from solution by the formation of Mo sulphides (thiomolybdates) [ Eriksson and Helz , ], with an isotopic composition ~0.90 to 0.50‰ less than local seawater (as observed in modern marine sediments [ Poulson et al , ; Poulson‐Brucker et al , , ; Nägler et al , ]). Any change in the balance of Mo removal fluxes between oxic and/or sulphidic sediments has a direct impact on the globally homogenous seawater isotopic composition, with a greater proportion of Mo removed into sulphidic sediments resulting in a lighter seawater Mo‐isotope composition.…”

Palaeoceanographic controls on spatial redox distribution over the Yangtze Platform during the Ediacaran–Cambrian transition

“…Molybdenum is present as molybdate (MoO 4 2− ; Broecker & Peng, ) in oxic sea water and is readily sequestered by Mn/Fe‐oxyhydroxides and subsequently released to pore waters under reducing conditions (Crusius et al ., ; Goldberg et al ., ). In the presence of free H 2 S (beyond a threshold concentration of ca 11 μM), molybdate can be converted to particle‐reactive thiomolybdate ions (MoO x S 4− x ; Helz et al ., ; Erickson & Helz, ) and subsequently scavenged by metal‐rich particles, sulphur‐rich organic molecules (Helz et al ., , ; Tribovillard et al ., ) and iron sulphide minerals (Vorlicek et al ., ; Poulson Brucker et al ., ). In the modern ocean, oxic–suboxic facies represent the most important sink for Mo but, despite only making up ca 0·3% of the sea floor, euxinic environments account for ca 30 to 50% of Mo removal from the water column (Bertine & Turekian, ; Morford & Emerson, ; Anbar, ; Algeo & Lyons, ), illustrating that Mo is mainly enriched in deposited sediments under sulphidic conditions (e.g.…”