A multi-proxy record of climatic changes and Southern Westerlies dynamics in the subtropical Andes of South America since 12ka

Abstract

The subtropical Andes of South America are a key area to study the interaction of tropical and extra-tropical climatic systems. The climate is characterized by wet winters and dry summers related to its position at the northern edge of the Southern Westerlies Wind Belt (SWWB) which migrate seasonally so that during the austral winter they expand equatorward to about 30°S. The SWWB is modulated by the presence of the Southern Eastern Pacific Subtropical Anticyclone (SEPSA) which migrates in conjunction with the SWWB determining the strong precipitation seasonality of Subtropical Andes. Thus, the interplay of the SWWB modulated by the SEPSA at millennial-centennial scale superimposed to the interannual scale variability related to the Southern Oscillation is reflected in precipitation patterns over the Andes allowing to trace the winter activity of SWWB at millennial-to-interannual time scales. The present paper presents the dynamics of the northern edge of the SWWB since 12ka based on a multi-proxy record of Laguna El Cepo located at 30°S (2900masl). The pollen record is dominated of Poaceae with maximum values between 12-11ka, 9.6-8ka and since 5.8ka onwards which suggest moderate humid conditions peaking since 2.2ka given the increase of Portulacaceae and Apiaceae. Dry phases characterized by increases of Ephedra occurred between 11-9.6ka and particularly 8-5.8ka. Pollen accumulation rates recorded the driest conditions between 11-5.8ka. The charcoal record shows that fire occurrence increased from 7ka onwards with maximum values between 7-6ka, during the driest period for the whole record.The diatom record reflects cold temperature of the water given the high values of Eunotia spp at 12-10.5ka and 4.8-3.8ka alternating with warm conditions between 10-8.5 and 8-6ka pointed out by Chamaeopinnularia sp which together with Chamaeopinnularia hassiatica reveal the warmest conditions around 6.4ka. High percentages of Kobayasiella micropundata between 8.5-8ka and at 5.5ka indicate the occurrence of exogenous input into the lake. High values of Eutonia spp during the late Holocene reflect cooler conditions during the last 5ka, compared to the early-mid Holocene conditions.The chironomid record show high percentages of Cricotopus suggesting littoral environment dominance at 12ka. Cold conditions associated to discrete increases of Smitia between 8.5-8ka and 5ka. The presence of Chironomus between 8-4.8ka and its highest percentages at 7.3-6.4ka suggest warm conditions. Sedimentological proxies suggest a recurrence of storms around 8.5-8ka and 5.8-4ka and a decrease productivity of the lake between 7.7-5.4ka.The joint interpretation of different proxy records suggests cold and humid conditions during the end of Pleistocene while the basin was still becoming a lake. During the early and mid-Holocene up to 5.8ka, dry and warm conditions dominated, with increased occurrence of storms around 8.5-8ka. The last 5.8ka appeared to be relatively more humid and colder conditions than the early and mid-Holocene.