Stable-isotopic and geochemical assessment of Andean terrestrial ecosystems during Pleistocene climate changes (Colombia)

A fundamental uncertainty in global change research is the response of the tropics to climate changes. The biogeochemical response of a tropical alpine system to Pleistocene climate forcing is assessed in this study on the basis of changes in organic carbon (C_org), total sulfur (S_tot), and the carbon isotopic composition of organic carbon (δ 13C_org) through a 120 m-long core of fine-grained lake deposits from the Bogota basin (Colombia). Biogeochemical parameters in these lacustrine strata exhibit strong cyclicity, with elevated contents of C _org and S_tot in interglacial intervals. Monosulfides are the dominant sulfur constituent in this sedimentary setting and are inferred to form as a result of enhanced weathering of iron in the watershed and strongly reducing benthic conditions in the lake. In contrast, organosulfur compounds dominate the sedimentary sulfur in glacial intervals when mildly reducing conditions promoted diagenetic incorporation of sulfur into organic matter. This weathering-dependant variability of monosulfides may serve as a useful proxy for shifting rainfall patterns from glacial to interglacial climates. Comparison between the monosulfide time-series and the goethite index developed for the equatorial Atlantic as a proxy for rainfall reveals a strong covariance, providing support for the suggestion of reduced rainfall during glacial times.; The δ13C_org record in the Bogota lake deposits show seven distinct positive shifts in glacial intervals, interpreted to correspond to expansions of C₄ grasses. Values of δ 13C_org in Upper Pleistocene paleosols indicate mixed C ₃/C₄ vegetation in the basin, confirming increased abundance of C₄ plants during the last glaciation. Two mechanisms could favor C₄-grass expansions in the Bogota basin: lowered pCO ₂ and/or decreased rainfall. Ice-core records provide evidence for significant lowering of pCO₂ values during glacial times. Evidence for decreased tropical rainfall is, however, inconclusive. Authigenic kaolinites serve to assess rainfall changes if these minerals form at isotopic equilibrium. This condition exists in Holocene soils of the Bogota basin as revealed by isotopic data of meteoric water and pedogenic kaolinites. High kaolinite δ18O and δD values of paleosols indicate a pronounced shift in atmospheric circulation during the last glacial interval, resulting in decreased precipitation rates in the basin.; The combined results of this study indicate a unified, but complex response of alpine tropical systems to radiative and pCO₂ forcing. Changes in the global heat budget during glacial intervals cause decreased zonality in the trade winds and/or a southward shift of the Intertropical Convergence Zone (ITCZ) to reduce rainfall in the Bogota basin. Lowered temperature, rainfall, and pCO₂ cause vegetation shifts in the tropical Andes, driving continental weathering and associated terrestrial and lacustrine productivity to lower rates.