Martin A. Coombes

Cool barnacles: Do common biogenic structures enhance or retard rates of deterioration of intertidal rocks and concrete?

Sedentary and mobile organisms grow profusely on hard substrates within the coastal zone and contribute to the deterioration of coastal engineering structures and the geomorphic evolution of rocky shores by both enhancing and retarding weathering and erosion. There is a lack of quantitative evidence for the direction and magnitude of these effects. This study assesses the influence of globally-abundant intertidal organisms, barnacles, by measuring the response of limestone, granite and marine-grade concrete colonised with varying percentage covers of Chthamalus spp. under simulated, temperate intertidal conditions. Temperature regimes at 5 and 10mm below the surface of each material demonstrated a consistent and statistically significant negative relationship between barnacle abundance and indicators of thermal breakdown. With a 95% cover of barnacles, subsurface peak temperatures were reduced by 1.59°C for limestone, 5.54°C for concrete and 5.97°C for granite in comparison to no barnacle cover. The amplitudes of short-term (15-30min) thermal fluctuations conducive to breakdown via fatigue effects were also buffered by 0.70°C in limestone, 1.50°C in concrete and 1.63°C in granite. Furthermore, concentrations of potentially damaging salt ions were consistently lower under barnacles in limestone and concrete. These results indicate that barnacles do not enhance, but likely reduce rates of mechanical breakdown on rock and concrete by buffering near-surface thermal cycling and reducing salt ion ingress. In these ways, we highlight the potential role of barnacles as agents of bioprotection. These findings support growing international efforts to enhance the ecological value of hard coastal structures by facilitating their colonisation (where appropriate) through design interventions.

Population-level zoogeomorphology: the case of the Eurasian badger (Meles meles L.)

The zoogeomorphological impact of burrowing animals varies in time and space as a result of the particular life history traits of the organisms involved, the patchy distribution of habitat resources, and fluctuations in population size. Such ecological complexity presents a major challenge for biogeomorphologists wishing to upscale from individuals to populations. Using a unique ecological data set for Eurasian badgers (Meles meles L.) in Wytham Woods, Oxfordshire, UK, we show that direct zoogeomorphological impact (soil displacement during sett excavation) is constrained by fluctuations in overall population size. Modeled digging rates for individual badgers (0.19–4.51 m3 yr−1) varied depending on the ecological function of the sett they are associated with, and we estimate that the whole population has displaced 304–601 ± 72 m3 of soil during the construction of 64 setts. This represents an overall excavation rate of 6.7–19.4 m3 (6.0–17.5 t) yr−1 in sett areas or 1.42–4.12 g m−2 yr−1 when averaged over the whole 424 ha woodland. As well as direct soil displacement, badger digging exposes material that is initially susceptible to erosion by water relative to undisturbed and litter-covered soils. Over time, setts become stabilized, representing unique landforms that persist in the landscape for decades to centuries.

Chapter 5 The rock coast of the British Isles: weathering and biogenic processes

Abstract An abundance of moisture, salts and organic life make rock coasts a unique weathering environment. Here, mechanical and chemical processes act to break down rocks alongside the influence of waves, tides and geological factors. Organisms concurrently break down (bioweathering and bioerosion) and protect (bioprotection) coastal rocks in direct and indirect ways, enhancing or impeding other inorganic modes of decay. Some species also build physical structures (bioconstruction) that have geomorphological and ecological consequences. Studies of particular weathering processes are well represented in the British Isles, and demonstrate both the overriding controls of lithology and tidal position. The complexities arising from the interactive and combined influences of different processes are also evident. Biogenic processes are of greatest importance for the geomorphology of carbonate rock coasts and cohesive shores in Britain and Ireland, but weathering is largely secondary to waves in the evolution of harder rock coasts. The importance of typically fine-scale rock breakdown in facilitating larger-scale erosion is recognized, however, but warrants more attention, and the value of interdisciplinary and applied weathering research on rock coasts is stressed.

Thermal blanketing by ivy ( Hedera helix L.) can protect building stone from damaging frosts

The impact of plants growing on buildings remains controversial, especially for vulnerable historic walls and ruins requiring on-going conservation. English ivy (Hedera helix L.) can cause considerable damage where it is able to grow into deteriorating masonry, yet in some circumstances it may be protective. Here we focus on the potential of ivy to buffer damaging thermal cycles and frost events that can contribute to the deterioration of masonry materials. On limestone masonry test walls in central Southern England (Wytham near Oxford, UK), ivy foliage had a significant influence on stone-surface freezing regimes. Over two successive winters (2012/13 and 2013/14) the frequency of freezing events under ivy was reduced on average by 26%, their duration by 34% and their severity by 32%. A subsequent laboratory simulation showed that stone mass loss, surface softening, and textural development were all significantly reduced under an ‘ivy covered’ thermal regime. Cautious extrapolation indicates that ivy can reduce frost-driven granular-scale decay of limestone by the order of 30u2009gu2009m−2 yr−1, depending on the local freezing regime. Whilst the capacity of ivy to cause damage should not be underplayed, vertical greenery can aid heritage conservation efforts by mitigating specific environmental threats.

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?