Abstract

Paleohydrological reconstructions from the Chilean Altiplano document abrupt moisture fluctuations during the last millennia. Although the end of the mid Holocene aridity and the onset of more humid conditions between 6 – 4 ka has been identified in numerous regional marine and terrestrial sites, the timing of late Holocene dry and humid phases shows large regional variability. Laguna Miscanti (23° 43\' S - 67° 46\' W, 4140 m asl, 10 m deep) and Laguna Miñiques (23° 46’S - 67° 47’W, 4100 m asl, 4 m deep) are topographically closed, but connected by surface outflow from Miscanti. Sedimentological and geochemical indicators from two new cores show large facies changes, i.e. higher carbonate and evaporite deposition during more arid periods and increased organic productivity (both algal and macrophyte) during more humid phases. As in most Andean lakes located in volcanic settings, large 14C reservoir effects occur complicating 14C dating, so the age models include 210Pb and U/Th dating. In spite of dating uncertainties, both lakes show similar patterns during the last millennium. A humid phase in Laguna Miscanti prior to ca 1200 CE is coherent with rodent middens and geomorphological features indicative of a major pluvial/recharge event at lower altitudes (Atacama Desert) during the Medieval Climate Anomaly (ca 800 - 1300 CE). The LIA (1300 – 1850 CE) is characterized by several arid/humid cycles and the last century by a productivity increase. The hydrological changes observed during the last millennium illustrate the complex dynamics of recent climate evolution over the high altitude Andean plateau. Discrepancies between the northern and southern Altiplano records The stratification of the surface mixed layer is controlled on seasonal time scales by solar radiation and the turbulent mixing processes of the upper ocean. On longer interannual to interdecadal timescales, large atmosphere-ocean coupled systems, such as El NiñoSouthern Oscillation and the Pacific Decadal Oscillation, control the low frequency variability of the California Current System. The objective of this work is to reconstruct the stratification of the surface ocean in the southernmost region of the California Current that will allow a better understanding of the natural variability beyond the instrumental record. The San Lazaro Basin (SLB) is a semi-enclosed basin located in the Gulf of Ulloa, off Baja California peninsula (25°10\'N, 112°24\'W). Its location, on the dynamic border between the California Current and the subtropical region, is highly sensitive to changes in the oceanography of both regions from interannual to centennial timescales. The restricted circulation in the basin and the high biological export production maintain suboxic conditions in the basin, which in conjunction with the high sedimentation rate (1 mm/yr), allows an excellent preservation of the laminated sediments on the sea-floor of this basin. To characterize the variability of the mixed layer, five cores were sampled continuously at 2-5 mm intervals. The δ 18O isotopic composition of 2 species of planktonic foraminifera with different niches in the water column (Globigerinoides ruber and Neogloboquadrina dutertrei) were used to generate a model of density stratification of the surface water column. The time series resulting from the stratification model was validated using instrumental records of the Pacific Decadal Oscillation (PDO) finding a significant correlation (r=0.6559, p=0.0000). The reconstruction of density stratification presents statistically significant periods from decadal to centennial timescales, which are further shared with other reconstructions of the eastern Pacific Ocean.