Compound-specific stable carbon and hydrogen isotope analyses of late-Holocene vegetation and precipitation change at Laguna Los Mangos, Costa Rica

New compound-specific isotope analyses of a sediment core from Laguna Los Mangos in southern Pacific Costa Rica improves understanding of late-Holocene precipitation change in a region with limited paleoprecipitation records that is vulnerable to future climate change. We established paleoprecipitation and paleovegetation records from compound-specific stable hydrogen and carbon isotopic compositions of terrestrially-derived n-al-kanes (delta H-2(alkane) and delta C-13(alkane)) to assess paleohydrologic variability and potential linkages to paleoecological change and human activity as revealed by prior analyses of the Los Mangos core. The delta H-2(alkane) values were corrected for isotopic fractionation using pollen counts from the same core. The Los Mangos record extends to 4200 cal yr BP and small increases in delta C-13 values of C-29, +0.6 parts per thousand, and C-31 alkanes, +0.3 parts per thousand, (delta(CC29,31)-C-13) indicate a slight increase in C-4 vegetation after initial introduction of maize agriculture to the watershed at ca. 3360 cal yr BP. This slight increase in C-4 vegetation is followed by the largest positive carbon isotope excursions in the record, as compared to record averages (delta C-13(C29) = +3.2 parts per thousand, delta C-13(C31) = +5.0 parts per thousand). Paleohydrologic variability likely influenced vegetation and human activity at Los Mangos. Lake desiccation during the late-Terminal Classic Drought (TCD) resulted in a sedimentary hiatus in the Los Mangos record from ca. 950 to 450 cal yr BP. Positive excursions in comparison to record averages occur for both delta H-2(C29) and delta H-2(C31) proxies (delta H-2(C29) = +25.3 to +13.4 parts per thousand and delta H-2(C31) = +6.5 parts per thousand) during the middle Little Ice Age (LIA) and indicate drier than average conditions, but there is no evidence of desiccation during this period. Thus, drought conditions during the LIA were apparently not as severe at Los Mangos as during the TCD, possibly because of differing forcing mechanisms for LIA climate that originated, or were more clearly expressed, in the Atlantic basin.