T7 Iron Formations, Ferruginous Sediments, and Redox through Time and Space

Sunday, 22 September 2019
8:00 AM - 12:00 PM
Room 229A, North Building

ASSESSING THE BIOAVAILABILITY OF MO IN ARCHEAN OCEANS

Paper number: T7-3
Program: Technical Sessions
Time: 8:30 AM – 8:45 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-341070

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.