Effects of organic matter on carbonate stable isotope ratios (δ¹³C, δ¹⁸O values) – implications for analyses of bulk sediments

Abstract

Bulk carbonate analyses from carbonate-OM mixtures are reliable only if the TIC/TOC values do not fall below certain thresholds. This has implications for carbonate isotope studies from bulk sediments for which the TIC/TOC ratios should be considered as an easy-to-determine measure for sample-quality assessment.

Carbon- and oxygen-stable isotopic signatures of methanogenesis, temperature, and water column stratification in Holocene siderite varves

“…Recent work also demonstrates that rapidly precipitated carbonates may be depleted relative to equilibrium δ 18 O values (Gabitov et al, 2012). Bulksediment samples can also be biased towards depleted δ 18 O values due to the presence of organic matter (Oehlerich et al, 2013). The latter mechanism may be the most likely explanation for the extremely depleted δ 18 O values of Group II samples in our case, although microbial influences cannot be fully discounted.…”

“…In a porewater environment, precipitation of siderite may be bio-induced when bacteria remineralize organic carbon, reduce ferric hydroxides, and create DIC with depleted δ 13 C values reflecting the initial composition of organic matter (Lovley, 1991;Fischer et al, 2009;Heimann et al, 2010). In OL this signal must be interpreted with caution, as biasing from the high organic matter content of Group II samples may also produce low isotope values (Oehlerich et al, 2013).…”

“…Potential contamination from organic matter in carbonate-isotope analysis would lead to more depleted mineral values (e.g. Oehlerich et al, 2013) and warmer inferred temperatures. Long term variability in lake water composition (e.g.…”

Lateglacial and Holocene climatic changes in south-eastern Patagonia inferred from carbonate isotope records of Laguna Potrok Aike (Argentina)

“…However, interpretation of these isotope data is an issue, because several factors can mask a palaeoclimatic signal, such as input of detrital carbonates from the catchment (Filippi et al, 1999;Thevenon et al, 2012), isotopic disequilibrium (Bluszcz et al, 2009) and changes in carbonate mineralogy and/or the mechanisms of carbonate precipitation (Fronval et al, 1995;Hu et al, 1997;Teranes et al, 1999;Leng and Marshall, 2004). Recent experimental data (Oehlerich et al, 2013a) demonstrates that amount and quality of admixed organic matter can bias carbonate isotope analyses performed on bulk sediment samples. Therefore, the potential influence of organic matter was considered in the presented record in the interpretation of the analytical results.…”

“…Another peculiarity of Laguna Potrok Aike is the occurrence of ikaite, a metastable calcium carbonate hexahydrate (CaCO 3 $ 6 H 2 O) (Oehlerich et al, 2013b). The ikaite crystals of Laguna Potrok Aike disintegrate at higher temperatures into mm-sized calcite crystals morphologically identical to the calcite crystals preserved in the sediment record (Oehlerich et al, 2013b).…”