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Arsenic-induced phosphate limitation under experimental Early Proterozoic oceanic conditions

Chi Fru, Ernest, Hemmingsson, Christoffer, Holm, Mikaela, Chiu, Beverly and Iñiguez, Enrique 2016. Arsenic-induced phosphate limitation under experimental Early Proterozoic oceanic conditions. Earth and Planetary Science Letters 434 , pp. 52-63. 10.1016/j.epsl.2015.11.009

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Abstract

Comparison of phosphorus concentrations associated with modern hydrothermal Fe(III)(oxyhydr)oxides and ancient Fe(III) oxide-rich iron formations, is used to estimate bioavailable Precambrian marine phosphorus (P) concentrations. This led to the proposition of a low dissolved P budget of ∼10–25% of present-day levels, before ∼1.9 billion years ago. Estimates incorporating ancient marine Si levels ≥0.67mM instead suggested global dissolved P levels greater than today. Here we unite current experimental models that have considered NaCl solutions containing elevated dissolved Fe(II), Si, Ca2+ and Mg2+ ions in the incorporation of P in Precambrian marine Fe(III)(oxyhydr)oxides, in addition to arsenic as a hydrothermal proxy. We show that the coprecipitation of dissolved P and Fe(III)(oxyhydr)oxides from arsenic-rich marine waters produces an average P distribution coefficient of ∼0.072 (±0.01)μM−1 . This is comparable to the ∼0.07μM−1 predicted for Fe(III)(oxyhydr)oxides in modern arsenic-rich, submarine hydrothermal settings, from which the lower Early Proterozoic dissolved marine P concentrations were predicted. As/P molar ratios below modern seawater ratios removed the negative feedback effect high Si impose on P scavenging by Fe(III)(oxyhydr)oxides. The binding of As(III) to Fe(III)(oxyhydr)oxides exhibits a lower competitive influence on P fixation. As(V) that likely became prominent in the surficially oxidized Early Proterozoic oceans induced dissolved P limitation because of preferential P sequestration at the expense of dissolved As(V) enrichment. The control of As on P scavenging by the precipitating Fe(III)(oxyhydr)oxides is strong regardless of common seawater cations (Mg2+ and Ca2+). The data suggest that the application of Si and Fe(III)(oxyhydr)oxides as an ancient seawater P proxy should consider chemical variability between depositional basins, taking into account the rather strong role hydrothermal arsenic has on the distribution of P in Fe(III)(oxyhydr)oxides. We propose that the generalized lower dissolved P budgets estimated from Early Proterozoic iron formations are consistent with oceans thought to be at least 3–4 times more hydrothermally active than at present.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Publisher: Elsevier
ISSN: 0012-821X
Date of Acceptance: 7 November 2015
Last Modified: 21 Nov 2017 22:08
URI: http://orca.cf.ac.uk/id/eprint/105872

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