Project D3

Stable isotopes: Development of a paleohumidity proxy from oxygen isotopes in Ca-sulfate hydration water

Research areas: Geochemistry, Climate research

Principal investigators: Prof. Dr. Michael Staubwasser

Project Info: Phase 2

Based on the PI’s group’s fundamental work in the first CRC‐phase on the systematics of triple O‐isotope fractionation during evaporation, the mass dependent ¹⁷O‐excess parameter measured in structurally bound water extracted from lacustrine gypsum sediments can be shown to allow for the principal quantification of past humidity. Our first analysis of the total water isotopes (¹H, ²H, ¹⁶O, ¹⁷O, ¹⁸O) in multiple selenite gypsum samples taken along individual stratigraphic units of Pliocene lacustrine deposits in the Atacama Desert suggest the potential to quantify the climate mean state of the past with absolute values for temperature, humidity, and a parametrized value for wind speed from evaporation trajectories in the above five isotope space.

In the second CRC‐phase, I propose to investigate the evaporation systematics of total water isotopes from gypsic and salic soils forming throughout the Central Depression and on alluvial fans typically progressing from the Coastal and Pre‐Andean Cordilleras. The objective is to apply total water isotope analysis to the entire gypsum depositional facies of pedogenic and lacustrine gypsum in order to achieve a quantitative reconstruction of climatic mean states since the desert’s formation in the Miocene. The first systematic application will be a comparison of the Miocene Batea formation, the Pleistocene Soledad formation, and the present state along the past and present basins along the Rio Loa. Total water isotope analysis will be complemented by elemental andmineralogic analysis of soil salts, and by analyzing mass‐independent fractionation in sulfate and nitrate oxygen. In order to cover the extremes of past Atacama climate variability, I also propose to explore the potential of climate mean state reconstruction from total water isotope analysis to the full range of lacustrine sediments known from the Atacama Desert, including halite and diatomite ‐ the latter a possible expression of past pluvial conditions. In halite, the analysis would be done on commonly present fluid inclusions. In diatomite, this would be done on Si‐bound oxygen after fluorination, and on Si‐bound hydroxyl groups extractable by heating.

Both types of analysis are routinely utilized in classic water isotope geochemistry by the community, so that the effort would focus on adaptation and modification of these techniques to ensure reproducible triple O‐isotope analysis.