Abstract
This study aimed to: (1) investigate intra-sample variability in the stable isotope composition (δ13C and δ18O values) measured in several individual mollusc shells; and (2) determine the optimum number of individual mollusc shells required to obtain stable isotope values representative of the mean climate signal for a sediment sample. Seven hundred and six gastropod shells were analysed, representing eight species. These eight species were most abundant and consistently present in the sediment sequence of the Paddenluch outcrop, Brandenburg, northeast Germany. Significant differences between minimum and maximum δ13C and δ18O values of the shells were recorded within all sediment samples and for all analysed species. Within the sediment samples, the greatest mean variability in both δ13C and δ18O values was found for Hippeutis complanatus, 5.84 and 5.23 ‰, respectively. The greatest intra-specific variability in C and O isotope values within individual sediment samples, however, was 10.2 and 6.8 ‰, respectively. Significant differences between minimum and maximum δ13C and δ18O values of individual mollusc shells within the same sediment sample are probably the result of population isotope variability, variations in species life span, thickness of the sediment sample, sediment accumulation rate, and environmental fluctuations during the time span when the sediment was deposited. Shells were randomly selected to determine the optimum number of shells necessary to obtain stable isotope values that are representative of the mean climate signal, from a 1-cm sample of sediment. For each layer of the Paddenluch sediment sequence, 15 or more shells (N ≥ 15) was the required sample size to provide representative isotope values and yield a reliable mean isotope value. Sample sizes with fewer shells displayed more variable mean δ13C and δ18O values. Stable isotope composition of the analyzed molluscs was species-specific, with differences in mean δ13C and δ18O values of 2.5 and 3 ‰, respectively. This illustrates the need to use mono-specific samples throughout a sediment sequence to generate reliable data.
Highlights
Carbon and oxygen stable isotope composition (d13C and d18O) of mollusc shells is used widely in palaeolimnological studies to reconstruct past environments
Carbon and oxygen stable isotope composition of the calcareous bulk sediment samples changed within the studied time frame, between -5.49 and -2.22 % and -9.56 and -7.57 %, respectively (Fig. 3)
From the mean ranges in d13C and d18O values (Fig. 6), we suggest that the differences between the minimum and maximum isotope values measured in 20 mono-specific shells per 1-cm sediment layer probably represent the full range of isotopic values, b Fig. 5 Mean isotope values, standard deviation (SD) and ranges of d13C and d18O values calculated for Anisus (D.) vortex and Gyraulus (A.) crista in the selected six layers on the basis of 2, 3, 5, 10 and 15 randomly selected shells from samples, which consisted of at least 20 shells because the differences in range values between N = 15 and N = 20 shells are minor compared to the range in smaller sample sizes (Figs. 5, 6)
Summary
Carbon and oxygen stable isotope composition (d13C and d18O) of mollusc shells is used widely in palaeolimnological studies to reconstruct past environments. Food or habitat preferences change during the animal’s life span and shell isotope values are not representative of the lake environment (Leng and Marshall 2004; Shanahan et al 2005). One approach is to measure the stable isotope composition of individual mollusc shells (Jones et al 2002; Escobar et al 2010) This approach allows researchers to determine changes in isotope composition throughout the full length of the sediment sequence and assess variability of shell d13C and d18O values within sediment samples. It provides information on both longterm climate changes and short-term climate variability. A common alternate approach involves use of homogenized shells from multiple freshwater molluscs, mostly gastropods, to average short-term (usually yearly, because most of the freshwater gastropods inhabiting temperate climate have a one to two-year life span, Fromming 1956; Taft et al 2012) changes in lake conditions during accumulation
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