Antje Schwalb

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

Abstract Sediment cores from three shallow ( 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.

Stable Isotopes and Sediments from Pickerel Lake, South Dakota, USA: A 12ky Record of Environmental Changes

Sedimentological parameters and stable O- and C-isotopic composition of marl and ostracode calcite selected from a 17.7-m-long core from the 8-m-deep center of Pickerel Lake, northeastern South Dakota, provide one of the longest (ca. 12ky) paleoenvironmental records from the northern Great Plains. The late Glacial to early Holocene climate in the northern Great Plains was characterized by changes from cold and wet to cold and dry, and back to cold and wet conditions. These climatic changes were controlled by fluctuations in the positions of the Laurentide ice sheet and the extent of glacial Lake Agassiz. We speculate that the cold and dry phase may correspond to the Younger Dryas event. A salinity maximum was reached between 10.3 and 9.5 ka, after which Pickerel Lake shifted from a system controlled by atmospheric changes to a system controlled by groundwater seepage that might have been initiated by the final withdrawal of Glacial Lake Agassiz. A prairie lake was established at approximately 8.7 ka, and lasted until about 2.2 ka. During this mid-Holocene prairie period, drier conditions than today prevailed, interrupted by periods of increased moisture at about 8, 4, and 2.2 ka. Prairie conditions were more likely dry and cool rather than dry and warm. The last 2.2 ka are characterized by higher climatic variability with 400-yr aridity cycles including the Medieval Warm Period and the Little Ice Age.Although the signal of changing atmospheric circulation is overprinted by fluctuations in the positions of the ice sheet and glacial Lake Agassiz during the late Glacial-Holocene transition, a combination of strong zonal circulation and strong monsoons induced by the presence of the ice sheet and high insolation may have provided mechanisms for increased precipitation. Zonal flow introducing dry Pacific air became more important during the prairie period but seems to have been interrupted by short periods of stronger meridional circulation with intrusions of moist air from the Gulf of Mexico. More frequent switching between periods of zonal and meridional circulation seem to be responsible for increased climatic variability during the last 2.2 ka.