Diversity and patterns of fracture-exposed alteration features in thick recharge-area regolith

Typically thick aeration zones of topographic groundwater recharge areas are hardly investigated parts of subsurface ecosystems and the subsurface water/matter cycles (Lehmann and Totsche 2020). In fractured bedrock settings, here, mineral surfaces and assemblages, exposed to major flow-paths, can largely differ from the bulk rock-forming compositions. Representing highly diverse and likely important reaction spaces for subsurface matter cycling and groundwater quality, yet, their compositional and morphological diversity, their provided habitat structure and endolithic dwellers, and their matter sources and dynamics are scarcely known. In drill core samples of Triassic limestone-mudstone alternations from the Hainich Critical Zone Exploratory (Collaborative Research Center AquaDiva), we characterized and classified alteration features across regolith down to the phreatic zone. Besides analysis of fracture/pore fillings and rock matrices by digital microscopy, SEM(-EDX), among others, we investigated possible controlling factors like lithofacies associations, depth, water saturation, groundwater flow patterns and oxicity. Generally, strong weathering features with up to 1 mm thick fillings and up to several centimeters thick zones of alteration in rock of the aeration zone contrast with minor features in the phreatic zone. In the limestones and mudstones, major classes of fracture surface coatings, are taken by secondary Fe-oxides and/or clay laminae. Our results highlight the typical presence of diverse and likely dynamic reaction spaces, providing highly diverse microbial habitats. We suggest to carefully consider and explore the diversity and dynamics of mineral fractures surfaces of the aeration zone, and their contributions to element cycling and groundwater quality evolution.

 

References:

Lehmann, R., Totsche, K. U. (2020). Multi-directional flow dynamics shape groundwater quality in sloping bedrock strata. Journal of Hydrology 580. https://doi.org/10.1016/j.jhydrol.2019.124291