Bacterial and abiogenic carbonates formed in caves-no vital effect on clumped isotope compositions.

Attila Demény,László Rinyu,Nóra Enyedi,Péter Németh,Szabolcs Leél-Őssy,Judit Makk,Ivett Kovács,György Czuppon,Dóra Kesjár

doi:10.1371/journal.pone.0245621

Abstract

Speleothems (dominated by cave-hosted carbonate deposits) are valuable archives of paleoclimate conditions. As such, they are potential targets of clumped isotope analyses that may yield quantified data about past temperature variations. Clumped isotope analyses of stalagmites, however, seldom provide useful temperature values due to various isotope fractionation processes. This study focuses on the determination of the microbially induced vital effect, i.e., the isotope fractionation processes related to bacterial carbonate production. A cave site with biologically mediated amorphous calcium carbonate precitation was selected as a natural laboratory. Calcite deposits were farmed under a UV lamp to prevent bacterial activity, as well as under control conditions. Microbiological analyses and morphological investigations using scanning electron microscopy showed that the UV lamp treatment effectively reduced the number of bacterial cells, and that bacterial carbonate production strongly influenced the carbonate’s morphology. Stable oxygen isotope analyses of calcite and drip waters, as well as clumped isotope measurements revealed that, although most of the studied carbonates formed close to oxygen isotope equilibrium, clumped isotope Δ47 values varied widely from equilibrium to strongly fractionated data. Site-specific kinetic fractionations played a dominant role in the distribution of Δ47 values, whereas bacterial carbonate production did not result in a detectable clumped isotope effect.

Highlights

To test whether bacterial carbonate production affects the clumped isotope composition of the precipitating carbonate, the Baradla Cave was used as a natural laboratory, where carbonate samples were deposited and collected at one site illuminated by a UV lamp and at a control site that was not treated
Bacteria isolated from subsurface environments contain DNA repair mechanisms, which eliminate the damage caused by UV light [37,38]
Speleothems and farmed calcites were analyzed for stable oxygen isotope and clumped isotope compositions to determine if bacterial carbonate production had a detectable affect on Δ47 values

Summary

Introduction

To test whether bacterial carbonate production affects the clumped isotope composition of the precipitating carbonate, the Baradla Cave was used as a natural laboratory, where carbonate samples were deposited and collected at one site illuminated by a UV lamp and at a control site that was not treated. FTIR and micro-XRD analyses show that the 1–2 months old carbonate samples collected from the glass plates of the control and UV-illuminated samples are dominated by calcite.

Results

Conclusion