Trace-element chemistry of Quarternary ostracods as a palaeosalinity indicator in marginal marine environments
The aim of this project is to evaluate the use of shell chemistry as a salinity indicator in marginal marine environments through the analysis of the elements Mg and Sr, although the significance of Mn and Fe concentrations are also assessed, within the ostracod valve and calculation of subsequent trace-element/Ca ratios. Two contrasting approaches were investigated; a modern environment where the salinity gradient is well established, and supported by previous work and field measurements (The Fleet, Dorset) and a Holocene sediment sequence where salinity was inferred from faunal palaeoecology of foraminifera and ostracods (Core SB1, Son Bou, Menorca). The species Cytherois fischeri, Leptocythere castanea, Leptocythere lacertosa, Loxoconcha rhomboidea and Xestoleberis nitida were analysed from the Fleet and generally show no relation to the host water chemistry and high variability at each study site, reflecting the tidal nature of the waterbody. However, L. lacertosa does show a weak negative trend in Mg/Ca across the transect, similar to the inverse relationships reported in a few previous studies (Teeter & Quick, 1990; Keyser et al., 1993 and Boomer, pers. comm.). Positive correlations between Sr/Ca ratios and nominal shell weight for all five species, suggests that part of the Sr content within the valve is simply related to the weight of the shell (i.e. the heavier the valve the more Sr within) and was not subsequently taken up in equilibrium with the host water. The species Cyprideis torosa and Loxoconcha elliptica were analysed from core SB1. The C. torosa Sr/Ca ratio correlated very strongly with the faunal and floral records, reflecting the environmental changes and salinity variations suggested along with the presence/absence of marine connections. The L. elliptica Sr/Ca record and Mg/Ca records for both species do not display such distinct correlations, although more subtle shifts involving increased variability do correlate with more unstable episodes and overall trends in the mean also correlate to hypothesised long term salinity trends. Positive correlation between L. elliptica Sr/Ca and nominal shell weight indicates that part of the Sr within the valve is simply related to the weight of the shell, thereby making the environmental evidence available from the L. elliptica chemistry record more limited than that of the C. torosa record. Therefore, despite the fact that the Fleet has a reduced tidal flow compared to a normal estuary, it would appear that the water chemistry is still too variable for the ostracod shell chemistry technique to be used as a reliable indicator of water chemistry, although the technique can be of limited use where corroborative evidence from other sources is necessary to support any conclusions.