ASSESSING THE BIOAVAILABILITY OF MO IN ARCHEAN OCEANS

The bioavailability of Mo to early N₂-fixers is a matter of debate due to the limited constraints on Archean environmental conditions. Sedimentary enrichments of Mo from this time are ~100x less than in modern reducing sediments, suggesting that Mo in seawater could have been ~ 1 nM or less, as compared to 105 nM in modern seawater. Such low concentrations may have been limiting for Mo-based N₂-fixers, which appear to require [Mo] > 5 nM (Zerkle et al. 2006). However, recent N isotope analyses and genomic analyses indicate that Mo was likely used by the earliest N₂-fixing microbes (Stüeken et al. 2015; Schoepp-Cothenet et al. 2012), suggesting that seawater Mo concentrations were sufficiently high to be bioavailable. These findings highlight the need to better quantify Mo concentrations in Archean seawater.

To this end, we developed a simple steady-state mass balance model of the Archean ocean Mo budget that can be used to estimate the seawater concentration of Mo ([Mo]_sw) under different scenarios, subject to various constraints including Mo isotope measurements from the geologic record. The model balances Mo input from rivers against Mo burial in three marine sinks: (1) oxic sediments [i.e., those bearing Fe and/or Mn oxides]; (2) suboxic sediments; and (3) euxinic sediments. Because Mo mass balance is underconstrained, we use Monte Carlo analysis to explore distributions of [Mo]_SW as a function of variable sediment burial efficiencies, areas of deposition, and total burial fluxes of each type of sediment. We construct a distribution of potential [Mo]_SW and the environmental conditions that make them possible, consisting of all scenarios that satisfy constraints from the modern ocean and the geologic record.

We find that in an Archean ocean dominated by anoxic sediments, burial efficiency is high, and [Mo]_SW is kept < 5 nM. However, in scenarios in which oxide-containing sediments cover >50% of the deep ocean floor, Mo burial efficiency decreases—thereby increasing the residence time of Mo and allowing [Mo]_SW > 5 nM. Such scenarios may be consistent with extensive deposition of Fe oxide particles by photoferrotrophs, or widespread Fe photooxidation, even in anoxic Archean oceans. Further study of the feasibility of such scenarios is critical given the importance for Archean Mo cycling and N₂ fixation.