Stress histories control rock-breakdown trajectories in arid environments

Rock and boulder surfaces are often exposed to weathering and /or rock-breakdown processes for extremely long time periods. This is especially true for arid environments on Earth and on planetary bodies such as Mars. One important, but largely unexplored, gap in knowledge is the influence of past stress histories on the operation of present rock-breakdown processes. Do rocks in the same area with different stress histories respond equally to newly imposed environmental conditions? This study investigates the influence of different physical and chemical stress histories on the response of basalt to salt weathering. We designed a fourstage approach of pre-treatment, field exposure, weathering simulation, and post-treatment: (1) physical, chemical, or no pre-treatment in the laboratory; (2) 3 yr exposure in either a hyper-arid sandy or salt-pan environment in the Namib desert (Namibia); (4) 60 cycles of a hot desert salt weathering simulation; and (4) desalination. Salt uptake and rock breakdown was assessed at each stage through comparison with baseline observations of mass, internal strength (Dynamic Young’s modulus) and surface morphology (three-dimensional microscopy). Clear differences in block responses were found. Physically pre-treated blocks (especially those left in the salt-pan environment) experienced the highest loss of strength overall, chemically pre-treated blocks showed the greatest mass loss in the sandy environment, and freshly cut blocks gained strength during exposure in the desert and maintained this during the experiment. These results imply that stress history matters for predicting breakdown rates, with humid, arid, and saline legacies influencing subsequent breakdown in distinctive ways. INTRODUCTION Rock-breakdown processes such as physical and chemical weathering are important agents of geomorphic change, producing erodible sediment and influencing slope instability. Rates of rock breakdown in arid environments are generally slow (e.g., ~1 mm k.y.−1; Ryb et al., 2014), although ‘hot spots’ of locally wet, salty conditions have much higher breakdown rates (e.g., ~10–150 mm k.y.−1; Viles and Goudie, 2007). In arid environments on Earth, salt weathering is an important rock-breakdown process (Goudie, 1993; Warke, 2007), as are thermal stresses from differential insolation (identified as a likely cause of boulder cracking by Eppes et al. [2010, 2015], and shown experimentally to cause deterioration in pre-stressed blocks by Viles et al. [2010]) and wind abrasion. Similarly, experimental, observational, and modeling studies show thermal cycling to be an important cause of rock breakdown on dry planetary bodies such as Mars (Viles et al., 2010; Eppes et al., 2015; Molaro et al., 2015), in addition to eolian abrasion (Bridges et al., 2014) and salt weathering (Jagoutz, 2006). The relative importance of these different rock-breakdown processes and their dynamics over space and time have not yet been clearly evaluated. The term ‘stress history’ has been used to describe how the legacy of past processes influences response to current weathering (Warke, 2007). For example, rocks exposed to long periods of chemical weathering in wetter phases may respond more quickly to eolian abrasion in subsequent drier periods than rocks without that history. Or, rocks that have experienced extensive thermal cycling in arid conditions may break down more rapidly than other rocks when exposed to salt weathering associated with wetter conditions (Warke, 2007). Such stress histories may partially explain spatial and temporal patterning in rockbreakdown rates and styles in arid environments, and help explain variability of landscape evolution in geomorphic settings such as desert pavements (Viles and Goudie, 2013) and alluvial fans (Eppes and McFadden, 2008) over decadal to millennial time scales. What is lacking is empirical evidence of how different stress histories affect subsequent weathering trajectories. This paper evaluates the influence of stress histories on a relatively resilient rock type (basalt) found widely on Mars and in many Earth deserts (e.g., northern Namibia and Saudi Arabia). Specifically, we assess how legacies from past environmental conditions (wetter, drier, or more saline) influence breakdown rates. We utilize a novel methodology (combining sequential laboratory and field experiments) to address the following questions: (1) how do past histories of chemical weathering (by acid) or physical weathering (by thermal cycling) influence subsequent rock breakdown in eolian and salt-rich environments, (2) how does exposure in eolian or salt-rich environments influence subsequent salt weathering, and (3) how can such influences on weathering trajectories best be quantified experimentally?