Plet, C., Grice, K., Pages, A., Ruebsam, W., Coolen, M. J. L. and Schwark, Lorenz (2016) Microbially-mediated fossil-bearing carbonate concretions and their significance for palaeoenvironmental reconstructions: A multi-proxy organic and inorganic geochemical appraisal. Chemical Geology, 426 . pp. 95-108. DOI 10.1016/j.chemgeo.2016.01.026.

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Abstract

Carbonate concretions are widespread within the geological record. However, the lack of recent known analogues creates a need for novel approaches to unravel the major microbial players involved in concretion formation and establish their suitability as palaeoenvironmental recorders. Here, we used a combination of geochemical and geological techniques to study two pyritiferous calcite concretions and compared the results with their host sediment (Toarcian "Posidonia Shale", 183 Ma, SW-Germany). The C-13-depleted nature of the concretion bodies, with average values of delta C-13(carb) (- 14.8), delta C-13(org) (- 32.4), and delta C-13(n-alkanes) (- 34.9), indicates that sulphate-reducing bacteria (SRB), played a major role in the concretion growth and preservation of the nucleus via the rapid decomposition of organic matter (OM). However, Rock-Eval analyses from both concretions revealed elevated hydrogen indices (HI) in the body and low HI values at the rim. These values suggest that most of the microbial activity did not occur in the concretion body but rather at the rim and at the surface of the nuclei, which generally supports the exceptional preservation of OM in carbonate concretions. Furthermore, enrichment in euhedral pyrite in the concretion rims suggests they were formed through increased activities of iron reducing (FeR) bacteria coupled to a decrease of SRB activity leading towards conditions more favourable to the direct precipitation of pyrite. Despite low delta C-13 values, the known lipid biomarkers such as acyclic extended isoprenoids or 3 beta-methyl-hopanes did not reveal evidence of an active methane cycling. The present study emphasises the crucial role of carbonate concretion in OM preservation and highlights their great potential as palaeoenvironmental recorders. (C) 2016 Elsevier B.V. All rights reserved.

Document Type:	Article
Keywords:	Toarcian, Palaeoenvironment, Sulphate reducing bacteria (SRB), Iron reducing bacteria (FeR), δ13C, Lipid biomarkers
Research affiliation:	Kiel University Kiel University > Kiel Marine Science OceanRep > The Future Ocean - Cluster of Excellence
Refereed:	Yes
Open Access Journal?:	No
Publisher:	Elsevier
Projects:	Future Ocean
Date Deposited:	14 Mar 2017 06:33
Last Modified:	23 Sep 2019 23:07
URI:	https://oceanrep.geomar.de/id/eprint/36264

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