Holocene environments from stable isotope stratigraphy of ostracods and authigenic carbonate in Chilean Altiplano Lakes

Sediment cores from three shallow (<10 m), saline lakes along a transect of the northern Chilean Altiplano (18°S-27°S) document changes in moisture balance for the mid- to late Holocene. Stable isotopic compositions of ostracods and authigenic carbonate were measured to test whether high altitude (>4000 m a.s.l.) paleoclimatic profiles show coherency within tropical-subtropical circulation patterns. Climatic signals however are partially masked by local environmental systems. Results suggest similar arid mid-Holocene to less arid late Holocene trends in the three lakes although each lake shows distinct response characteristics linked to differences in regional moisture balance. Isotopic signatures for each lake group in discrete δ¹⁸O vs. δ¹³C populations. High isotopic variability results from episodic flooding and brine evolution in the shallow lakes. Laguna Seca (18°11'S, 69°14'W, 4500 m a.s.l.) is characterized by a diverse ostracod assemblage of at least 8 species indicating highest moisture levels among the three sites. High δ¹³C values are the result of continuous evasion of CO₂ derived from volcanically charged groundwaters and springs plus the formation of travertine and photosynthetic activity in the lake. A 3‰ decrease in δ¹⁸O about -4.7 m suggests an abrupt shift to wetter conditions at the transition from mid- to late Holocene. Lagunas Miscanti (23°44'S, 67°46'W, 4140 m a.s.l.) and del Negro Francisco (27°28'S, 69°14'W, 4125 m a.s.l.) each contain only one ostracod species, Limnocythere sappaensis, attesting to higher salinities, although sediment facies and ostracod continuity and abundances imply a history of higher effective moisture at Laguna del Negro Francisco. Sediment cores from each lake document more arid conditions in the lower sections, with a sharp shift to wetter conditions in the upper sections. Highly variable isotope values at the transition from mid- to late Holocene suggest rapid, short-term climate shifts during this transition. This overall trend can be correlated from Laguna Seca, Laguna Miscanti and Laguna del Negro Francisco to Lago Titicaca implying climatic coherency along the Altiplano. Isotopic profiles show second order fluctuations and drought events that suggest short-term variability along the transect. In order to explain differences in the effective moisture among the three sites we postulate interactions of regional air masses. In the north, convective tropical precipitation dominates Laguna Seca, as for lakes in the Titicaca region. Effective moisture for Lagunas del Negro Francisco and Miscanti follows a south to north gradient generated by cold air masses from 'cut offs' of Pacific Westerlies colliding with moist tropical air masses. Coherent shifts in the past therefore imply changes in available tropical air-mass moisture.