Environmental significance of mid- to late Holocene sapropels in Old Man Lake,Coorong coastal plain,South Australia: An isotopic,biomarker and palaeoecological perspective

The Holocene successions of numerous shallow lakes located along the Coorong coastal plain in South Australia attest to the impact of rising sea level and changing climate on their depositional environment. Old Man Lake is one of the smallest perennial alkaline lakes in the region. Its succession comprises a basal lagoonal sand rich in humic organic matter (OM) overlain by a 3.7 m thick upward shoaling lacustrine mudstone. The latter features three discrete sapropel units deposited between 3270 and 4910 cal yr BP, a time of increasing aridity throughout southeastern Australia. A core taken from the lake’s eastern margin yielded sedimentological, mineralogical, geochronological and micropaleontological data. Coring at five other sites across the lake provided sections of the humic and sapropelic facies (n = 20) for total organic carbon and Rock–Eval analysis; isotopic characterization of their micritic carbonate (δ¹³C_carb, δ¹⁸O_carb) and co-existing OM (δ¹³C_org); and GC–MS and GC–irMS analysis of their free aliphatic hydrocarbons. For each ‘sapropel event’ high productivity of diatoms and green algae was the principal driver of the accumulation and preservation of OM in such high concentrations. The precursor algal blooms were likely triggered by the influx of fresh water following winter rainfall. The combination of kerogen hydrogen index and δ¹³C_carb–δ¹³C_org, previously employed to track secular changes in algal productivity and organic preservation, proved useful in identifying synchronous geographic differences in these processes across the lake. Highly branched isoprenoids (HBI: C_25:1 ≫ C_20:0) are prominent components of the aliphatic hydrocarbons in the sapropels, confirming the significant contribution of diatoms to their OM. The C isotopic signatures of the principal C_25:1 HBI isomer and the co-occurring C₂₃–C₃₁ odd carbon numbered n-alkanes further document the non-uniformity of biomass preservation within and between the three sapropel units. The evidence from this study suggests that seasonal algal blooms and meromixis, although not necessarily an anoxic hypoliminion, were required for sapropel formation in the Holocene lakes of the Coorong region. Higher resolution sampling, dating and comparative analysis (microfossil, biomarker and isotopic) of these sapropels is required to clarify their potential significance as palaeoclimate proxies.