Comment on bg-2021-253

doi:10.5194/bg-2021-253-rc1

Abstract

In the aqueous oligotrophic ecosystem of a post-mining lake (Lake Medard, Czechia), reductive Fe(II) dissolution outpaces sulfide generation from microbial sulfate reduction (MSR), and ferruginous conditions occur without quantitative sulfate depletion. An isotopically constrained estimate of the rates of sulfate reduction (SRR) suggests that despite a high genetic potential, this respiration pathway is limited by the rather low amounts of metabolizable organic carbon. This points to substrate competition exerted by iron and nitrogen respiring prokaryotes. Yet, the microbial succession across the nitrogenous and ferruginous zones of the bottom water column also indicates sustained genetic potential for chemolithotrophic sulfur oxidation. Therefore, our isotopic SRR estimates could be rather portraying high rates of anoxic sulfide oxidation to sulfate, probably accompanied by microbially induced disproportionation of S intermediates. Near and at the anoxic sediment-water interface, vigorous sulfur cycling can be fuelled by ferric and manganic particulate matter and redeposited siderite stocks. Sulfur oxidation and disproportionation then appear to prevent substantial stabilization of iron monosulfides as pyrite but can enable the interstitial precipitation of small proportions of equant microcrystalline gypsum. This latter mineral isotopically fingerprints sulfur oxidation proceeding at near equilibrium with the ambient anoxic waters, whilst authigenic pyrite-sulfur displays a 38 to 27 ‰ isotopic offset from ambient sulfate, suggestive of incomplete MSR and likely reflective also of an open sulfur cycling system. Pyrite-sulfur fractionation decreases with increased reducible reactive iron in the sediment. In the absence of ferruginous coastal zones today, the current water column redox stratification in the post-mining Lake Medard has scientific value for (i) testing emerging hypotheses on how a few interlinked biogeochemical cycles operated in nearshore paleoenvironments during redox transitional states; and (ii) to acquire insight on how similar early diagenetic redox proxy signals developed in sediments affected by analogue transitional states in ancient water columns.

Highlights

The biogeochemical reactions governing the distinctive redox structure of modern meromictic lakes have been studied, for the most part, in natural settings featuring high concentrations of dissolved iron and with common sulfate deficiency (Swanner et al, 2020)
In the upper anoxic sediments we conducted mineralogical analyses and a mineral-calibrated wet chemical speciation study of the reactive Fe and Mn pools. Using these 55 data, we developed a mechanistic model that assesses the potential regulatory roles of prokaryotes over the geochemical gradients detected in the water column, and their influence over iron (Fe) mineral transformations and biogeochemical cycling of sulfur (S), carbon (C), nitrogen (N) and manganese (Mn) across the redoxcline, and near the anoxic sedimentwater interface (SWI)
Salinity was estimated by using the measured conductivity values. It increases three-fold from the hypolimnion downwards (Fig. 2a). This could result from recharge of groundwater carrying high loads of dissolved salts or results from the lack of mixing of the legacy mine-impacted pit lake waters with those comprising the mixolimnion

Summary

Introduction

The biogeochemical reactions governing the distinctive redox structure of modern meromictic lakes have been studied, for the most part, in natural settings featuring high concentrations of dissolved iron and with common sulfate deficiency (Swanner et al, 2020). Ferruginous bottom water columns that contain elevated dissolved sulfate concentrations are not uncommon in shallow pit lakes (e.g., Denimal et al, 2005; Trettin et al, 2007), and have been reported from the post-mining Lake Medard in NW Czechia (Petrash et al, 2018; Fig. 1a) This newly formed, lacustrine system with a 60 m depth features oligotrophic and meromictic conditions, and given its recent water filling history it can be considered as a large-scale incubation 45 experiment reflecting an imbalanced transitional state between ferruginous and euxinic aquatic redox states (Scholz, 2018; van de Velde et al, 2021). Our research furthers understanding of the cryptic S cycle under ferruginous conditions 65 unaccompanied by a quantitative sulfate exhaustion, and for refining our proxy-based palaeoceanographic reconstructions of nearshore redox stratified water columns

Methods

Results

Conclusion