Sedimentary stable carbon isotope evidence of late Quaternary vegetation and climate change in highland Costa Rica

Continuous terrestrial records of paleoclimate and paleovegetation that extend to the late Pleistocene are rare for the circum-Caribbean uplands. In this study we analyzed the bulk and compound-specific carbon isotope composition of lake sediments spanning this period from Lago de las Morrenas 1 (LM1), a glacial lake in the highlands of southern Costa Rica, for evidence of climate and vegetation changes that may not have been apparent in previous analyses. The stable carbon isotope ratios of n-alkanes typically derived from terrestrial plants (δ¹³C_C27–C33) indicate an increased abundance of C₄ plant taxa during the late Pleistocene and earliest Holocene that may be related to decreased atmospheric carbon dioxide concentrations, increased aridity, or habitat availability. These n-alkane isotope ratios also provide evidence of more arid conditions during the early and late Holocene, and more mesic conditions during the middle Holocene, a pattern prevalent in other paleoclimate records from the region that is thought to be related to millennial-scale dynamics of the intertropical convergence zone (ITCZ). The sensitivity of the LM1 paleorecord to trade wind dynamics provides further support for the role of millennial-scale shifts in ITCZ dynamics in driving neotropical environmental change, and indicates that the effects of ITCZ migration were not limited to the lowlands.