Topographic signatures and a general transport law for deep-seated landslides in a landscape evolution model

A fundamental goal of studying earth surface processes is to disentangle the complex web of interactions among baselevel, tectonics, climate, and rock properties that generate characteristic landforms. Mechanistic geomorphic transport laws can quantitatively address this goal, but no widely accepted law for landslides exists. Here we propose a transport law for deep-seated landslides in weathered bedrock and demonstrate its utility using a two-dimensional numerical landscape evolution model informed by study areas in the Waipaoa catchment, New Zealand, and the Eel River catchment, California. We define a non-dimensional landslide number, which is the ratio of the horizontal landslide flux to the vertical tectonic flux, that characterizes three distinct landscape types. One is dominated by stochastic landsliding, whereby discrete landslide events episodically erode material at rates exceeding the long-term uplift rate. Another is characterized by steady landsliding, in which the landslide flux at any location remains constant through time and is greatest at the steepest locations in the catchment. The third is not significantly affected by landsliding. In both the “stochastic landsliding” and “steady landsliding” regimes, increases in the non-dimensional landslide number systematically reduce catchment relief and widen valley spacing, producing long, low angle hillslopes despite high uplift rates. The stochastic landsliding regime captures the frequent observation that deep-seated landslides produce large sediment fluxes from small areal extents while being active only a fraction of the time. We suggest that this model is adaptable to a wide range of geologic settings and is useful for interpreting climate-driven changes in landslide behavior

Supplementary Data Terrestrial Mollusk Location and Species Count

Supporting data in csv table format.Many species of terrestrial mollusks are small and difficult to find, with poorly known ranges and habitat preferences. Because desiccation is a primary cause of mortality for many terrestrial mollusks, incorporating wetness as a habitat variable may improve survey results for different species of terrestrial mollusks. We compared presence and abundance data from terrestrial mollusk surveys in Tillamook Resource Area to two measures of relative wetness: topographic wetness index (TWI) and geomorphic features (landslides, debris-flow channels, etc.). Hurdle Model regression revealed a positive correlation between increased TWI and likelihood of presence or abundance for five species, and a negative correlation for four species. Overall species diversity and total mollusk count were negatively correlated with increased TWI, but the effect size was small (p = 0.02, R2 = -0.03). Our Kruskal-Wallis Analysis of Variance of TWI between species was significant (p<0.001), indicating terrestrial mollusks occupy significantly different wetness regimes - but this relationship was driven entirely by the wetness specialization of Hemphillia glandulosa. Our chi square analysis of topographic features found significant preferences of different species for different topographic types, which correlated loosely but not precisely to the preferences indicated by TWI. These results show that altering current terrestrial mollusk survey protocol to include geomorphic features (which are simpler and less time-intensive than calculating TWI) would increase detection likelihood of certain species, including Hemphillia glandulosa, a species protected under the Survey and Manage guidelines of the Northwest Forest Plan

The Correlation of Topographically-Derived Relative Wetness with Terrestrial Mollusk Presence and Abundance

Many species of terrestrial mollusks are small and difficult to find, with poorly known ranges and habitat preferences. Because desiccation is a primary cause of mortality for many terrestrial mollusks, incorporating wetness as a habitat variable may improve survey results for different species. We compared presence and abundance data from terrestrial mollusk surveys in Tillamook Resource Area to two measures of relative wetness: topographic wetness index (TWI) and geomorphic features (landslides, debris-flow channels, etc.). Hurdle Model regression revealed a positive correlation between increased TWI and likelihood of presence or abundance for four species, and a negative correlation for six species. Overall species diversity and total mollusk count were negatively correlated with increased TWI, but the effect size was small (p = 0.02, R2= -0.03). Our Kruskal-Wallis Analysis of Variance of TWI between species was significant (p \u3c 0.001), indicating terrestrial mollusks occupy significantly different wetness regimes – but this relationship was driven entirely by the wetness specialization of Hemphillia glandulosa. Our chi-square analysis of topographic features found significant preferences of different species for different topographic types, which correlated loosely but not precisely to the preferences indicated by TWI. These results show that altering current terrestrial mollusk survey protocol to include geomorphic features, which are simpler and less time-intensive than calculating TWI, would increase detection likelihood of certain species, including Hemphillia glandulosa, a species protected under the Survey and Manage guidelines of the Northwest Forest Plan

Estuarine Dissolved Oxygen History Inferred from Sedimentary Trace Metal and Organic Matter Preservation

Environmental history recorded in estuarine sediment describes water quality regimes through the use of geochemical and biological proxies. We collected sediment cores from two locations in the Coos Estuary, Oregon, at South Slough and Haynes Inlet, spanning from ~ 1680 AD to the present. To reconstruct the historical water column oxygen in the estuary, we measured geochemical proxies including organic matter, magnetic susceptibility, and elemental composition, and we constructed sediment chronologies using the Pb210 profile and radiocarbon dates. Correlation of geochemical proxies and a detailed 15-year record of dissolved oxygen observations supports the inference of dissolved oxygen (DO) history from these sediment cores: a novel finding for small, seasonal Pacific Northwest estuaries. Geochemical evidence suggests that over the last 300 years, annually or semi-annually averaged dissolved oxygen stress has been increasing at South Slough, while remaining stable or even decreasing at Haynes inlet. This history was explained by changing climatic and land-use effects on erosion and organic matter as well as the role of shipping channel maintenance in providing a dissolved oxygen reservoir at Haynes Inlet relative to the more isolated South Slough

Shaping post-orogenic landscapes by climate and chemical weathering

The spacing of ridges and valleys reflects the competition between diffusive transport on hill slopes and advective transport in channels, although the underlying lithologic, tectonic, and climatic controls have not been untangled. Here, we measure geochemical and geomorphic properties of catchments in Kruger National Park, where granitic lithology and erosion rates are invariant, enabling us to evaluate how varying rainfall (MAP=470, 550, and 730mm) impacts ridge-valley spacing. Catchment-average erosion rates, based on 10Be concentrations in river sands, are low (3 to 6 m My-1) and vary minimally across the three sites. Our lidar-based, slope-area trends reveal that hill slopes in the dry site are gentle (3%) and short (area of hill-valley transition is 75m2). The terrain has low relief and is highly dissected by small channels. With increasing rainfall, hill slopes lengthen (transition area increases from 220 to 700m2) and increase in gradient (6 to 8%), resulting in fewer, higher relief catchments. The chemical depletion fraction of ridge-crest regoliths increases with rainfall from 0.3 to 0.7, indicating progressive increase in chemical relative to physical erosion. In the dry site, catenas are relatively undifferentiated, but intermediate and wet site catenas have leached sandy crests and upper side slopes complemented by clay accumulation zones further down slope. The texture of this granitic landscape appears to be set by climate-driven feedbacks among chemical weathering, regolith fabric differentiation and hydrological routing that enhances the vigor of diffusive transport relative to advective transport