Dinosterol δD values in stratified tropical lakes (Cameroon) are affected by eutrophication

Abstract In freshwater settings, dinosterol(4α,23,24-trimethyl-5α-cholest-22E-en-3β-ol) is produced primarily by dinoflagellates, which encompass various species including autotrophs, mixotrophsand heterotrophs. Due to its source specificity and occurrence in lake and marine sediments, its presence and hydrogen isotopic composition (δD) should be valuable proxies for paleohydrological reconstruction. However, because the purity required for hydrogen isotope measurements is difficult to achieve using standard wet chemical purification methods, their potential as a paleohydrological proxy is rarely exploited. In this study, we tested δD values of dinosterolin both particulate organic matter(POM) and sediments of stratified tropical freshwater lakes (from Cameroon) as a paleohydrological proxy, the lakes being characterized by variable degrees of eutrophication. In POM and sediment samples, the δD values of dinosterolcorrelated with lake water δD values, confirming a first order influence of source water δD values. However, we observed that sedimentary dinosterolwas D enriched from ca. 19 to 54‰ compared with POM dinosterol. The enrichment correlated with lake water column conditions, mainly the redox potential at the oxic–anoxic interface ( E h OAI). The observations suggest that paleohydrologic reconstruction from δD values of dinosterolin the sediments of stratified tropical lakes ought to be sensitive to the depositional environment, in addition to lake water δD values, with more positive dinosterolδD values potentially reflecting increasing lake eutrophication. Furthermore, in lake sediments, the concentration of partially reduced vs. non-reduced C 34 botryococcenes, stanols vs. stenols, and bacterial (diploptene, diplopterol and ββ-bishomohopanol) vs. planktonic/terrestrial lipids (cholesterol, campesteroland dinosterol) correlated with E h OAI. We suggest using such molecular proxies for lake redox conditions in combination with dinosterolδD values to evaluate the effect of lake trophic status on sedimentary dinosterolδD values, as a basis for accurately reconstructing tropical lake water δD values.