Global importance of oxic molybdenum sinks prior to 2.6 Ga revealed by the Mo isotope composition of Precambrian carbonates

Marie Thoby,Wladyslaw Altermann,Kurt O Konhauser,Pieter T Visscher,Stefan V Lalonde,Philip W Fralick

doi:10.1130/g45706.1

Abstract

Abstract Sedimentary molybdenum (Mo) isotope compositions are a promising paleoredox indicator because the Mo isotope composition of seawater reflects the balance between anoxic and oxic sinks. Most available data are from shales; however, the Mo isotope composition of carbonates also reflects the composition of ancient seawater. Here, we provide an expanded data set of carbonate Mo isotope compositions, including the first data for carbonates older than 2.64 Ga, which we evaluate against a compilation of published data for carbonates, shales, and iron formations spanning geological time. Archean carbonate samples reveal maximum δ98Mo values that are generally above 1‰. These heavy values indicate that Mn(IV)-oxide or Fe(III)-oxide sinks were sufficiently important to influence the Mo isotope composition of seawater as far back as 2.93 Ga. Comparison of Mo isotope and rare earth element data, as well as residence time considerations, indicates that this metal-oxide influence was likely global. Available Mo isotope data for shales over the same time period generally show crustal values, which we attribute to negligible authigenic enrichment of Mo from seawater due to low ambient concentrations and a paucity of euxinic conditions. Our work demonstrates that the carbonate record provides important new insights into marine paleoredox conditions, especially when shale records are absent or unsuitable, and reinforces the emerging paradigm that oxic Mo sinks were important in the marine realm prior to 2.7 Ga.

Highlights

Molybdenum (Mo) stable isotopes have emerged as a powerful proxy for marine redox evolution that is based on the redox-sensitive nature of Mo sources and sinks
Modern Mo seawater sources are principally derived from the oxidative weathering of sulfide minerals in continental crust (~90%; mean δ98Mo value of 0.20‰; Siebert et al, 2003; Voegelin et al, 2012) variably modified during transport to values as high as 2.39‰ (e.g., Voegelin et al, 2012)
We turn to another sedimentary proxy capable of recording ancient Mo cycling: the molybdenum isotope composition of Mo hosted in carbonates (δ98Mocarb; Voegelin et al, 2009, 2010; Wen et al, 2011; Eroglu et al, 2015; Romaniello et al, 2016)

Summary

INTRODUCTION

Molybdenum (Mo) stable isotopes have emerged as a powerful proxy for marine redox evolution that is based on the redox-sensitive nature of Mo sources and sinks. Sediment isotope compositions approach that of seawater as Mo scavenging becomes near-quantitative (e.g., Neubert et al, 2008) This explains why black shales are used to track marine paleoredox using Mo isotopes. The lightest values are best explained by syndepositional adsorption of Mo to Mn(IV)-oxides, implying that sufficient O2 for Mn(II) oxidation, and the evolution of oxidative photosynthesis, had already occurred by 2.95 Ga. Here, we turn to another sedimentary proxy capable of recording ancient Mo cycling: the molybdenum isotope composition of Mo hosted in carbonates (δ98Mocarb; Voegelin et al, 2009, 2010; Wen et al, 2011; Eroglu et al, 2015; Romaniello et al, 2016).

RESULTS AND DISCUSSION

B South Africa

CONCLUSION