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Abstract

In freshwater settings, dinosterol (4a,23,24-trimethyl-5a-cholest-22E-en-3b-ol) is produced primarily by dinoflagellates, which encompass various species including autotrophs, mixotrophs and heterotrophs. Due to its source specificity and occurrence in lake and marine sediments, its presence and hydrogen isotopic composition (dD) 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 dD values of dinosterol in 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 dD values of dinosterol correlated with lake water dD values, confirming a first order influence of source water dD values. However, we observed that sedimentary dinosterol was 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 (Eh OAI). The observations suggest that paleohydrologic reconstruction from dD values of dinosterol in the sediments of stratified tropical lakes ought to be sensitive to the depositional environment, in addition to lake water dD values, with more positive dinosterol dD values potentially reflecting increasing lake eutrophication. Furthermore, in lake sediments, the concentration of partially reduced vs. non-reduced C34 botryococcenes, stanols vs. stenols, and bacterial (diploptene, diplopterol and bb-bishomohopanol) vs. planktonic/terrestrial lipids (cholesterol, campesterol and dinosterol) correlated with Eh OAI. We suggest using such molecular proxies for lake redox conditions in combination with dinosterol dD values to evaluate the effect of lake trophic status on sedimentary dinosterol dD values, as a basis for accurately reconstructing tropical lake water dD values.