Within the framework of the CRC1211 (projects C04, D02 and D03), the occurrence of calcium sulfate minerals, i.e., gypsum (CaSO₄•2H₂O) and anhydrite (CaSO₄), in the Atacama Desert was investigated with regard to their origin, timing, geological and environmental setting, hydrology and influence on landscape evolution. To achieve these goals, a number of proxies were developed and combined for the first time, including stable and radiogenic isotopes, major- and trace elements as well as mineralogical techniques. Despite much effort, a mineralogical mechanism by which the water-free calcium sulfate anhydrite can be formed or being transformed from gypsum could not be established experimentally. However, evidence by transmission electron microscopy and in-situ Raman spectroscopy collected on our own Atacama samples and found in recent literature suggest a non-classical, faster pathway for the nucleation of nanometre-sized calcium sulfatephases, in particular, anhydrite (Stawski et al., 2020). Furthermore, biological organisms (e.g. fungi, lichen, cyanobacteria) produce a variety of metabolic products in the soils of the Atacama (B04, Knief et al., 2020), which partially are suggested to aid in the extraction of crystal water from sulfates by dissolution of gypsum and reprecipitation of also nanometre-sized anhydrite crystals (Huang et al., 2020). Likewise, Sr isotope and trace element signatures and possibly genetic diversification of desert plants are controlled by the geochemistry of desert soils and their components.

The exact nature and connection of these processes and the impact on geochemical archives will be investigated from both a geochemical and biomineralogical perspective by comparative analysis and controlled lab experiments. For this purpose, samples of calcium sulfates containing biological lifeforms (microorganisms and plants) and/or organic substances shall be collected and analysed in collaboration with C09 and B04 in the Atacama and Namib desert to verify whether this behaviour is specific to the most hyperarid Atacama Desert or represents a more general phenomenon. Together with projects B01 and B07, we will also couple soil geochemistry with plant geochemistry to constrain sources mass balances of critical major and trace elements in plants.

Research Questions WP1:

How and to what extent is the gypsum-anhydrite transformation controlled by biological factors?

Gypsum desert & atmospheric deposition

Research areas: Geochemistry, Mineralogy, Crystallography and Geomorphology

Principal investigators: Prof. Dr. Sandro Jahn, Prof. Dr. Carsten Münker, Prof. Dr. Michael Staubwasser, Dr. Christoph Lenting, Dr. Benedikt Ritter

Gypsum and other CaSO₄ phases are the most common evaporites in the hyper‐arid Atacama Desert, along with major and minor salts, such as halites and nitrates. Halite and nitrate abundance follow a combination of large‐scale gradients in moisture availability (NO₃^2‐) combined with proximity to the ocean source (Cl^‐, SO₄^2‐, Na⁺). Sulfate distribution is the most complex, as it has marine and stratospheric sources, is less prone to flushing‐out, and is redistributed by aeolian deposition and moisture‐related surface activity over probably long time‐scales. The abundance ratio of primary gypsum and secondary anhydrite seemingly depends on a complex feedback of climatic, chemical, and geomorphologic factors not yet understood. Local field observations of gypsum and anhydrite distribution in the ubiquitous thick gypsic soil show abundant nodules, wedges and polygonal patterned ground, which confirm the importance of secondary redistribution. However, our experiments indicated that anhydrite cannot directly form from primary gypsum at arid conditions in the pure H₂O‐CaSO₄ system but may occur at hypersaline wet conditions that we anticipate in a hyper‐arid environment with episodic moisture supply and high abundance of halite and nitrates.

Based on the results from the first project phase, we therefore hypothesize

1. that gypsum phase transitions are the key in understanding the evolution of hyper‐arid environments like the Atacama Desert,
2. that climatic and geomorphologic feedbacks may drive or inhibit the transformation, and
3. that gypsum group minerals are a unique archive of palaeo‐environmental change through time.

The main objectives of this project in the second phase are therefore, to study the abundance of Ca‐sulfates with respect to

1. their formation kinetics, e.g. understanding phase transformations at variable temperature, humidity and salinity,
2. their chemical formation environments, e.g. principal sources of salts (marine, atmospheric, hydrothermal) and environmental fractionation of salt abundances at the surface, and
3. their dependence on geomorphologic surface processes.

We seek to explore the timing and rates of deposition and the interplay of reactions on the atomic and chemical level with geomorphological controlled water‐driven pedogenic and landscape processes. These three key topics will be addressed in four work packages. Three of those will apply a large suite of atomistic, mineralogical, elemental, isotopic, and sedimentological analytical methods. The fourth will provide existing screening methods, fieldwork coordination and interdisciplinary scientific exchange and synthesis.

Gypsum desert & atmospheric deposition

Research areas: Geochemistry, Mineralogy and Crystallography

Gypsum is the most dominant mineral in hyperarid Atacama soils. CaSO₄ ⋅2H2O (and other compounds CaSO₄ ⋅ xH₂O, x = 0...2), together with other water-soluble minerals such as nitrates, play an essential role concerning weathering of soil and rocks, water penetration and trapping in the soil and soil dynamics.

In the Atacama prominent features include landscape-draping by powdery gypsum blankets, gypsum crust formation and disintegration and polygon fissure formation, which we hypothesize to be key to the understanding of the evolution of landscape in the Atacama Desert, and likely also for Martian geomorphology.

The main objectives of this project are to explore the origin and deposition rates of CaSO₄ ⋅ xH₂O compounds and highly water-soluble minerals, e.g. nitrates; their pedogenic and geomorphologic distribution and preservation and processes affecting these minerals after deposition, i.e. transformation of phases CaSO₄ ⋅ xH₂O under variable conditions of temperature, relative humidity and chemical environs.

In this context the project will address the question of the present and long-term gypsum deposition rates and the relative contribution of marine, continental and atmospheric sulfates to gypsum deposition across the coastal and central regions of the Atacama from the South to the hyper-arid core in the North. Here, also the contents and spatial distributions (regionally across the coastal and central regions of the Atacama Desert) of highly soluble salts, i.e. minerals that are more soluble than gypsum, will be considered. Besides the regional spatial distribution of salts also their local distribution in the soil column, caused by either descending or ascending water in soils, which trace the present and/or past hydrological regimes will be regarded.

A central topic of the project will also be the investigation of transformation processes of phases CaSO₄ ·xH₂O, i.e. processes of dehydration of gypsum CaSO₄ ·2H₂O to subhydrates or soluble anhydrite and a rehydration, particularly concerning their occurrence and velocity under environmental conditions in in the Atacama, concerning the involved subhydrate phases and concerning the influence of further chemical components besides CaSO₄ ⋅ x H₂O (x = 0...2) in the soil on the kinetics of formation (crystallization) of the different phases of the system CaSO₄ – H₂O (and transformation between them). From these investigations more insight is expected into processes that are assumed to contribute to surficial gypsum crust formation and preservation in the Atacama, and, since CaSO₄ ⋅ xH₂O phase transitions imply in part substantial volume changes, into the geomorphic effects (e.g. haloturbation) of these phase changes. This includes in particular surface and near-surface features, such as nodules, wedges and the widespread soil-polygons, and their potential impact on surface modification in the Atacama will be studied.

Former members:

• Prof. Dr. Becker-Bohaty (Principal Investigator)
• Prof. Dr. Tobor Dunai (Principal Investigator)
• Prof. Dr. Sandro Jahn (Principal Investigator)
• Prof. Dr. Michael Staubwasser (Principal Investigator)
• Dr. Daniel Herwartz (Scientist)
• Dr. Clemens Prescher (Scientist)

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