Reconstruction of paleohydrology and paleohumidity from oxygen isotope records in the Bolivian Andes

Abstract

Cellulose-inferred lake water δ18O (δ18Olw) records from Lago Potosi (LP), a seasonally closed lake in a watershed that is not currently glaciated, and Lago Taypi Chaka Kkota (LTCK) [previously reported in Abbott et al., 2000. Quat. Sci. Rev. 19, 1801–1820], an overflowing lake in a glaciated watershed, provide the basis for late Pleistocene and Holocene paleoclimatic reconstruction in the Bolivian Andes. Deconvolution of the histories of changing evaporative isotopic enrichment from source water δ18O in the lake sediment records is constrained by comparison to the Sajama ice core oxygen isotope profile, whereas local hydrological influence is distinguished from the regional moisture balance history by the response of the different catchments to climate change. Overall, variations in the LP δ18Olw record appear to be dominantly controlled by evaporative 18O-enrichment, reflecting shifts in local effective moisture. This record is used to generate a preliminary quantitative reconstruction of summer relative humidity spanning the past 11 500 cal yr on the basis of an isotope-mass balance model. Results indicate that the late Pleistocene was moist with summer relative humidity values estimated at 10–20% greater than present. Increasing aridity developed in the early Holocene with maximum prolonged dryness spanning 7500–6000 cal yr BP at LP, an interval characterized by summer relative humidity values that may have been 20% lower than present. Highly variable but dominantly arid conditions persist in the mid- to late Holocene, with average summer relative humidity values estimated at 15% below present, which then increase to about 10–20% greater than present by 2000 cal yr BP. Slightly more arid conditions characterize the last millennium with summer relative humidity values ranging from 5–10% lower than present. Similar long-term variations are evident in the LTCK δ18Olw profile, except during the early Holocene when lake water evaporative 18O-enrichment in response to low relative humidity appears to have been offset by enhanced inflow from 18O-depleted snowmelt or groundwater from the large catchment. Although some temporal offset is evident, significant correspondence occurs between the isotope-inferred paleohumidity reconstruction and other paleohydrological proxies from the region. These results support the contention that millennial-scale variations in tropical moisture balance may be linked to changes in summer insolation.