We seek to apply a novel approach developed successfully during this CRC that combines quantitative stable water isotope paleo-hydrology and U-Th / U-Pb dating on the same samples of paleo-lacustrine gypsum. This approach eliminates systematic correlation and interpolation errors of commonly dated proxy records. U-Th dating allows to identify cluster ages of desert pluvials within the last three glacial‑interglacial Milankovitch cycles. U-Pb dating enables us to date desert pluvials older than 1 Ma. We demonstrate that fundamental hydrologic variables measurable today, such as relative humidity, and the isotopic composition of atmospheric vapour and of lake inflow – i.e. run-off and groundwater – are preserved in structurally bound gypsum water of geologic age. These variables can be quantified including statistical uncertainty intervals by fitting the Craig-Gordon isotope evaporation model to a series of paleo-water samples that experienced different degrees of evaporation within a terminal desert lake basin.   By completing the record for the last 15 Ma on paleo-lacustrine sequences from the Atacama Central Depression (1000 masl) and Altiplano (4000 masl), we seek to identify the roles of local vapour advection and distal high-altitude run-off as major moisture sources to the central Atacama and relate the hyperaridity record to the genetic adaptation record of endemic C4 plants, e.g. Tillandsia landbecki. By applying the same techniques to travertine with fluid inclusions, we intend to compare the Atacama and Namib deserts, whose landscape evolution differed markedly over the late Cenozoic. Specifically, we seek to identify the roles of mountain uplift (Atacama only) vs ocean temperature and circulation change (Atacama and Namib) on late Miocene hyperaridification trends. In addition, we provide extensive isotope analytical service to eleven other CRC projects.

Parts of the Atacama Desert reached hyperaridity during the mid to late Miocene. Some parts already experienced most likely already hyperarid conditions. In order to avoid misunderstanding of the reviewer and a potential contractions to other subprojects, the locality for which this assumption is made might be beneficial.

Central Depression added

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

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.

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, Dr. Daniel Herwartz

This study seeks to systematically investigate aqueous isotope exchange of ¹⁶O-¹⁷O-¹⁸O and H-D between crystal water of hydrated Ca-sulphates (gypsum, bassanite) and principal types of Atacama desert water (water vapor, fog water, salt lakes and plant water). The purpose of this study is to determine if humidity- a key environmental parameter in deserts - can be directly inferred from the isotopic composition of gypsum hydration water (GHW) and plant water. 

After determination of the respective fractionation factors we aim to conduct experiments on possible exchange dynamics between gypsum powder and fog water. Selected transects and profiles will be investigated to test the theoretical and experimental constraints. We hypothesize that the purely mass dependent variation in ¹⁷O-excess can be used as a paleo humidity proxy.

In our pilot study we discovered that not only SO₄ but also gypsum crystal water can carry a massindependent (MIF) ₁₇O signature. This water carries a low δD signature (-100‰) implying that some crystal water in the Atacama is of stratospheric origin. In this project we aim to quantify the proportion of this stratospheric water (and thus gypsum) within natural samples and to determine how resistant this 17O enriched crystal water is to isotopic exchange. Such MIF water is probably only preserved under hyper arid conditions and may serve as a tool to identify (paleo-) hyper arid conditions.

Finally we suggest to analyze plant water to test (1) if plants utilize GHW as a water source and (2) if humidity can be inferred from the ¹⁷O-excess in leaf and stem water.

Former Members:

• Prof. Dr. Michael Staubwasser (Principal Investigator)
• Dr. Daniel Herwartz (Principal Investigator)
• Jochen Scheld (Technician)

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