An examination of the fidelity of n-alkanes as a palaeoclimate proxy from sediments of Palaeolake Tianyang, South China

摘要

This study examines n-alkane distributions from a 120 m long borehole extracted from Palaeolake Tianyang, South China, which covers the last five glacial cycles. We aim to determine the palaeoclimatic/environmental fidelity of n-alkane distributions in bulk organic matter (OM) from this tropical sedimentary archive. The Tianyang lithostratigraphy reveals a deep-lake sequence (120-42 m) dominated by diatomite, overlain by shallow-lake and marsh sediments in the upper 42 m. The Carbon Preference Index (CPI, C_(23)-C_(31)) suggests that there is weak odd/even predominance (OEP) in long chain n-alkane distributions (C_(27)-C_(31)) in OM from deep lake sediments, and a strong OEP of long chain n-alkane distributions in OM from shallow lake sediments. The opposite characteristics are evident in short chain n-alkane distributions (C_(16)-C_(20))-These CPI values suggest different sources of organic carbon within the deep (dominantly diatom synthesis) and shallow lake (dominantly high plant input), but cannot help differentiate glacial from interglacial sediments. The Terrigenous-to-Aquatic Ratio (TAR) shows high values throughout interglacial sediments (＞4) and low values in glacial sediments (＜4), corresponding to a higher productivity of high plants during interglacials, and vice versa. The proportion of aquatic plant n-alkanes (P_(aq)) also reveals a clear change between glacial (0.31-0.39) and interglacial periods (0.22-0.28) during shallow lake stage, indicating a reduction in terrestrial OM input during glacial periods relative to interglacial periods. There is a greater amount of Unresolved Complex Mixture (UCM) in glacial sediments than within interglacial sediments. We suggest that the above n-alkanes indexes are useful environmental/climatic indicators within Palaeolake Tianyang, despite the fact that some other indexes, like Average Chain Length (ACL), provide less definitive interpretations.