Significance of millennial-scale coastal upwelling and Rio Loa variability forAtacama paleoclimate during MIS 2

The Atacama Desert located in northern Chile is one of the driest places on earth. The factors determining recent hyperarid climate conditions and their interplay and variability on interannual and decadal time scales are generally understood. Evidence for wetter (yet arid) conditions in the Atacama’s past is mostly provided by ephemeral lacustrine and fluvial deposits. The main watercourse of the Atacama Desert is the Rio Loa sourced by rainfall in the Andean mountains. Information on changes in the terrestrial supply to the ocean is recorded in marine sediments off the Rio Loa mouth.

Sediment core SO-104-52KL has been collected on the upper continental slope (∼340 m water depth, 21°S) off the Rio Loa during cruise 104, Leg 3 by RV Sonne in 1995. The preliminary chronology of the core based on ¹⁴C datings constrains the top and base of the core to 16 and 42 ka, respectively, with a mean sedimentation rate of 30-40 cm kyr^-1. These very high average sedimentation rates allow reconstructions of the paleoceaonographic and paleoclimatic conditions during marine isotope stages (MIS) 3 and 2 on millennial to centennial time scales. The location of the core on the continental slope off the Rio Loa mouth allows for the parallel evaluation of the Humboldt Current System and Andean rainfall as moisture sources for the Atacama Desert.

Proxy data for upwelling properties are established from microfossil assemblages and lipid biomarkers. Preliminary results show that foraminifera are abundant and well-preserved in the upper 4 m of the core. While planktonic foraminifera are rare, benthic assemblages are rich. In total, 24 genera and 49 species of benthic foraminifera were distinguished. The three taxa Bolivina, Bulimina, and Suggrunda, which indicate hypoxic to dysoxic conditions at the seafloor, make up most of the individuals. The dominance of hypoxia tolerating taxa indicates strong upwelling conditions via the presence of a pronounced Oxygen Mimimum Zone impinging on the upper continental slope. These data are put into context with relatively warm sea surface temperatures of average 21 °C as derived from alkenone data. Together with XRF data and grain-size analyses, both applied to characterize the terrestrial input by the Rio Loa, the expected proxy data will provide new insights into the dynamics of land-ocean coupling between the Atacama Desert and the eastern Pacific Ocean.

This study is part of the CRC 1211 “Earth-Evolution at the dry limit” project, funded by the German Research Foundation (DFG).