Effects of detrital carbonate on stable oxygen and carbon isotope data from varved sediments of the interglacial Piànico palaeolake (Southern Alps, Italy)

Abstract

Naumann, R. 2010. Effects of detrital carbonate on stable oxygen and carbon isotope data from varved sediments of the interglacial Piànico palaeolake (Southern Alps, Italy).ABSTRACT: Contamination with detrital matter is a well-known bias in d 18 O records from lake carbonates but quantitative information of this effect is yet lacking. Therefore, we developed a new methodological approach combining isotope analyses with microfacies, X-ray diffraction and micro-X-ray fluorescence data and applied this in a case study for the Piànico interglacial lake record in order to provide a quantitative estimate of the effect of detrital carbonate on stable d 18 O and d 13 C data. The Piànico record contains a long series of distinct and well-preserved calcite varves and is correlated to Marine Isotope Stage 11. Intercalated in the varve sequence are detrital layers triggered by surface erosion events. These detrital layers are mainly composed of dolomite, thus reflecting the mineralogical signature of the catchment. Microfacies analyses of a 9350 varve year interval allows the identification of detrital layers down to sub-millimetre scale and a precise selection of three different types of samples for isotope analyses: (1) pure endogenic calcite varves (five varves per sample) without detrital contamination; (2) individual detrital layers; and (3) 'mixed' samples including five calcite varves and up to four thin detrital layers. Detrital samples show the isotopic signature of the catchment dolomite and are up to 5.7% enriched in d 18 O values with respect to endogenic calcite samples. In mixed samples, the degree of isotopic enrichment is directly related to the amount of detrital contamination; d 18 O of bulk carbonates is significantly biased when the detrital component amounts to more than 5% of the sample. It is also shown that samples containing detrital material have an influence on the calculation of the covariance between d 13 C and d 18 O. Covariance is high (r ¼ 0.76) when the correlation coefficient is calculated on the base of all samples, but absent (r ¼ À0.43) when samples containing detrital dolomite are excluded. It has been demonstrated that microfacies analysis is a quick tool to avoid or reduce detrital contamination in bulk carbonate samples during sample selection.

Multi-proxy evidence for early to mid-Holocene environmental and climatic changes in northeastern Poland

“…A bias of the δ 18 O calcite record through isotopically enriched detrital carbonates from the catchment (Leng & Marshall 2004) is excluded, as microfacies analysis proved the endogenic origin of the measured calcite (cf. Mangili et al . 2010).…”

“…A bias of the d 18 O calcite record through isotopically enriched detrital carbonates from the catchment (Leng & Marshall 2004) is excluded, as microfacies analysis proved the endogenic origin of the measured calcite (cf. Mangili et al 2010). Furthermore, effects of the epilimnetic water temperature during calcite precipitation on the d 18 O calcite record can be ruled out because of the parallel course of the d 18 O ostracods record, which is independent of seasonal changes in surface water temperature (von Grafenstein 2002).…”

Climatic implications of annual to decadal resolution stable isotope data from calcite varves of the Piànico interglacial lake record, Southern Alps

“…Consequently, 46 varves were omitted from the sampling, in order to avoid a bias of the stable isotope data through detrital carbonate (Mangili et al, 2010). Thirty-three of these varves occurred in the 352 varve interval at the beginning of the PMO, and the remaining 13 in the sampled interval at the end of the PMO.…”

Oxygen isotopic evidence for high‐magnitude, abrupt climatic events during the Lateglacial Interstadial in north‐west Europe: analysis of a lacustrine sequence from the site of Tirinie, Scottish Highlands

“…It is proposed that contamination by detrital carbonate is not an issue at Tirinie for three reasons. Firstly, several authors have suggested that isotopic datatsets derived from carbonate subsamples that are affected by varying degrees of detrital contamination typically show a strong degree of co-variance between d 18 O and d 13 C (Leng et al, 2010;Mangili et al, 2010), a pattern that is not seen in the Tirinie dataset (¼ r 2 ¼ 0.126, see Supplementary Information 1). Secondly, the process of sieving removed any carbonate material >63 mm, meaning that the analysed carbonate should be dominated by the fine-grained endogenic fraction.…”