Project Info: Phase 2

Understanding the chronology of arid and hyper‐arid environments and intermittent wet periods is of pivotal importance to constrain the coupled evolution of the biosphere and Earth surface processes through time at Earth’s dry margin. Our primary objective is to develop and apply novel methodology for the direct dating of samples recording climate evolution and surface processes using the ²³⁸U‐²³⁰Th and U‐Pb systems. The ²³⁸U‐²³⁰Th system may provide robust chronometric information for samples <500 ka in age, while the U‐Pb system commonly covers ages >500 ka.

Lacustrine and brine pool gypsum deposits from the late Pleistocene Atacama Desert were successfully dated by ²³⁸U‐²³⁰Th and placed into environmental context. A ~300 ka old ²³⁸U‐²³⁰Th dated primary gypsum sample from the Soledad formation showing recrystallization to anhydrite at ~ 250 ka illustrates the potential to constrain time‐scales of secondary anhydrite formation. First U‐Pb results (Laser ICPMS) for gypcretes, calcretes, silcretes and lacustrine gypsum from the Atacama and Namib deserts demonstrate a great potential for dating pedogenic and lacustrine Miocene to Pleistocene deposits. In the second phase of CRC 1211, we propose to improve this toolbox and apply it to the full range of evaporites, pedogenic crusts, and travertines from the Atacama and the Namib deserts. With respect to ²³⁸U‐²³⁰Th, we will date young samples from the two deserts, both from surface transects and drill‐cores. To improve the method, we will develop (1) protocols for travertines to remove intrinsic open‐system components like finely dispersed organic inclusions and secondary minerals, such as Fe‐oxyhydroxides, by employing oxidative‐reductive leaching techniques; (2) protocols for sulfate and halite samples rich in silica detritus and secondary mineral coatings; and (3) near equilinedating protocols for samples with ages close to 500 ka in order to bridge the gap to the U‐Pb system. With respect to advancing laser U‐Pb dating of the entire range of sulfates, carbonates, and silcretes from the Atacama and Namib deserts, we will (1) develop protocols for gypsum, anhydrite, calcite and nitrate using MC‐ICPMS to obtain more precise ages and higher spatial resolution, (2) establish matrix‐ and age‐matched reference material for sulfates, calcretes and silcretes, (3) improve in‐house data processing software and (4) date Quaternary samples down to 500 ka complemented by ²³⁰Th and ²³⁴U solution analyses to correct for initial disequilibrium. Dating of travertine deposits and low U/Pb evaporites will be complemented by solution U‐Pb dating. Our working hypothesis is that carbonates and silica represent more humid conditions, whereas sulfates represent more arid conditions. We will collaborate with project C04 to unravel the hitherto unknown conditions of anhydrite formation common in all types of sulfate deposits of the Atacama Desert. In collaboration with project D03 we will develop a protocol for the ¹⁷O‐excess and classic water isotope analysis of fluid inclusions in travertine and calcrete, thereby introducing climate reconstruction on directly dated samples in a time series. Existing protocols and novel developments in U‐Th and U‐Pb dating techniques of D02 cover the entire time‐scale of interest in the CRC and will be made available as dating service for all CRC projects through partial financing of positions in project Z04.