BIOGEOCHEMICAL RESPONSES TO CLIMATE TRANSITIONS OVER THE PAST 40 KYR RECORDED IN SEDIMENTS FROM LAKE IA M’HE, A MAAR LAKE IN THE CENTRAL HIGHLANDS OF VIETNAM

Ia M’He (14°11’N, 107°52’E) is a shallow maar lake in the Central Highlands of Vietnam with a low sediment accumulation rate. Its climate reflects dynamic interactions between Asian monsoonal circulation and seasonal movement of the Intertropical Convergence Zone (ITCZ), augmented regularly by El Niño events and intermittently by tropical storms of varying intensity. Sediments coring in the lake affords an opportunity to decipher and evaluate paleoclimatic records documenting the temporal history of climate of this region of southeast Asia. ¹⁴C dating confirmed that the sediments recovered extend back beyond the Last Glacial Maximum (LGM) enabling determination of biogeochemical profiles of temporal variations in algal productivity and terrestrial vegetation reflecting changes in the lake environment associated with the transition from glacial to interglacial climate. C_org contents are ~2 % in the lowest section of the core when δ¹³C values vary from -27 to -28 ‰, then increase progressively toward the LGM (~10 %) and in post glacial (~15 %) to the Holocene (~20 %) while δ¹³C values are typically ~-26 ‰. C/N ratios also reflect temporal changes in the dominant sources of sedimentary organic matter, increasing to a maximum value (>40) prior to the LGM, then decreasing to <15 by ~7 kyr b.p. when diatoms first appear. Diatoms become common by ~3 kyr b.p., coinciding with a pronounced negative shift in δ²H values for C₂₈ n-alkanoic acid indicative of changes in terrestrial vegetation, or precipitation, or both. Distributions of aliphatic hydrocarbons vary downcore with n-alkanes maximizing at C₂₇ in glacial samples and succeeded by a marked prominence of C₃₃ or C₃₅ homologs prior to the LGM – the observed predominance of n-C₃₅ in some intervals is unprecedented, likely reflecting a discrete biological source. The composition of hopanes through the core is consistent, with C₂₇ β-hopane dominant, suggesting uniform bacterial activity in the lake. Sediments from Lake Ia M’He, like other maar lakes, also contain botryococcene and structurally related alkenes, with C_34:2 acyclic and C_34:3 monocyclic components prominent. Their relative abundance varies, which likely reflects temporal shifts in the prevalence of different strains of Botryococcus braunii, or a response to changes in the lake environment or climate.