Lacustrine Records of Holocene Climate Change From Southernmost South America

The westerly wind field is one of the most prominent atmospheric circulation features in the Southern Hemisphere, which has a major impact on the climate of southern South America and hydrographic conditions in the Southern Ocean. Recent studies have argued that latitudinal shifts in the westerly boundaries during the LGM played a role in the modulation of atmospheric CO2 through positive feedback mechanisms within the Southern Ocean. However, significant discrepancies exist among paleoclimate records and GCM simulations. Here we present lacustrine records of late Holocene climate variability from two sites in southern South America. The first, a 4.75 meter sediment core from Laguna Guanacos (51°S, 72°W) in Southern Patagonia, provides a continuous record of climatic change related to the westerly wind regime during the last ~11,000 14C yrs. The sediment cores obtained from this small, shallow closed-basin lake reveal high concentrations of organic mater and biogenic carbonate. Eleven AMS radiocarbon dates on organic and carbonate fractions indicate that the record spans the last ~13,000 calendar years and modern dates from core tops suggest little influence by old or dead carbon sources. Combined pollen, stable isotope and elemental data indicate that significant shifts in the forest-steppe ecotone have occurred during the late Holocene. Increases in C/N and Nothofagus/Graminae index values culminating between 100 and 550 14C yrs BP are indicative of forest expansion related to cooler and moister conditions during the Little Ice Age. A large >2‰ decrease in bulk organic δ13C and a concomitant decrease in Nothofagus and rise in Rumex during the last 100 years, marks extensive burning of the region and the replacement of a forested landscape by an anthropogenic-set steppe. A positive linear relationship between δ18O and δ13C on bi-valves and ostracodes indicates that the lake behaves as a closed system and will be sensitive to changes in moisture balance. Sediments from the second site, Lago Fagnano in Tierra del Fuego (54°S, 68°W), exhibit abundant mm- to cm-scale interbedded dark and light laminae. AMS radiocarbon dates on the pollen fraction indicate that the 2.4 meter long core spans the last 5300 years 14C yrs. Isotopic data from these two sites will be compared and discussed in the context of late Holocene westerly variability.