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 degrees S-27 degrees S) document change s in moisture balance for the mid- to late Holocene. Stable isotopic compos itions 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 pa rtially 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 disc rete delta(18)O vs. delta(13)C populations. High isotopic variability resul ts from episodic flooding and brine evolution in the shallow lakes. Laguna Seca (18 degrees 11'S, 69 degrees 14'W, 4500 m a.s.l.) is characterized by a diverse ostracod assemblage of at least 8 species indicating highest mois ture levels among the three sites. High delta(13)C values are the result of continuous evasion of CO2 derived from volcanically charged groundwaters a nd springs plus the formation of travertine and photosynthetic activity in the lake. A 3 parts per thousand decrease in delta(18)O about -4.7 m sugges ts an abrupt shift to wetter conditions at the transition from mid- to late Holocene. Lagunas Miscanti (23 degrees 44'S, 67 degrees 46'W, 4140 m a.s.l .) and del Negro Francisco (27 degrees 28'S, 69 degrees 14'W, 4125 m a.s.l. ) each contain only one ostracod species, Limnocythere sappaensis, attestin g to higher salinities, although sediment facies and ostracod continuity an d abundances imply a history of higher effective moisture at Laguna del Neg ro Francisco. Sediment cores from each lake document more arid conditions i n the lower sections, with a sharp shift to wetter conditions in the upper sections. Highly variable isotope values at the transition from mid- to lat e 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 al ong the Altiplano. Isotopic profiles show second order fluctuations and dro ught events that suggest short-term variability along the transect. In orde r to explain differences in the effective moisture among the three sites we postulate interactions of regional air masses. in the north, convective tr opical precipitation dominates Laguna Seca, as for lakes in the Titicaca re gion. Effective moisture for Lagunas del Negro Francisco and Miscanti follo ws a south to north gradient generated by cold air masses from 'cut offs' o f Pacific Westerlies colliding with moist tropical air masses. Coherent shi fts in the past therefore imply changes in available tropical air-mass mois ture. (C) 1999 Elsevier Science B.V. All rights reserved.