Jane Kathrin Willenbring

Landscapes in humid climates often experience rapid chemical and mechanical weathering that can change groundwater fluxes, drainage pathways, and frictional strength via alterations in subsurface mineral composition and matrix porosity. These changes to hillslope critical zone structure and strength should control landslide susceptibility, size, and failure style. Here we test the impact of differential weathering on landslide failure processes by exploiting a natural experiment along the dioritic Utuado batholith in Puerto Rico's Cordillera Central, where a sharp weathering gradient follows an upstream‐propagating knickzone into an uplifted shoreline platform that was affected by Hurricane Maria in 2017. Through a combination of landslide mapping, field geophysical and geotechnical surveys, topographic analysis, calibrated stream profile inversion, 3D back‐analysis of hillslope strength, and debris‐flow inundation assessments, we show that differential weathering induces changes to critical zone structure and consequently landslide susceptibility and hazard. Our mapping reveals that the hurricane‐induced landslides preferentially cluster along less weathered hillslopes despite a marked increase in frictional strength. Jointly inverted seismic refraction and electrical resistivity profiles show that the frictionally weak, heavily weathered hillslopes are highly porous and dry, which we posit acts to stabilize slopes through limiting deeper hydrologic connectivity that can produce shallow water tables with destabilizing lateral seepage forces during storms. Debris‐flow inundation mapping and analysis reveals that major contributors to debris‐flow growth, channel steepness and landslide coalescence, scale linearly with hillslope friction angle. This leads to higher‐mobility debris flows along knickzone reaches, which disproportionately affects communities at the base of the plateau.

The lateral migration of a river meander is driven by erosion on the outer bank and deposition on the inner bank, both of which are affected by shear stress (and therefore channel slope) through complex morphodynamic feedbacks. To test the sensitivity of lateral migration to channel slope, we quantify slope change induced by glacial isostatic adjustment along the Red River (North Dakota, USA and Manitoba, Canada) and two of its tributaries over the past 8.5 ka. We demonstrate a statistically significant, positive relationship between normalized cutoff count, which we interpret as a proxy for channel lateral migration rate, and slope change. We interpret this relationship as the signature of slope change modulating the magnitude of shear stress on riverbanks, suggesting that slope changes that occur over thousands of years are recorded in river floodplain morphology.

Rocky coast cliff retreat presents a hazard to coastal communities and infrastructure that is potentially amplified under rising sea level conditions, among other factors. Unfortunately, constraints on retreat rates are typically limited to those derived from imagery and maps spanning the last ∼100 years. Here, we use a newly developed coupled model of shore platform profile evolution and cosmogenic radionuclide production that considers the influence of relative sea level (RSL) rise, weathering, material resistance, and wave height decay to model cliff retreat over millennial timescales and its potential drivers in Del Mar, California. We demonstrate the ability to use topographic and bathymetric measurements from a narrow shore platform along with a limited data set of nine cosmogenic ¹⁰Be concentrations extending ∼125 m from the cliff base to obtain modeled cliff retreat rates that steadily range from 5.0 to 12.5 cm yr⁻¹ over the last two millennia until 100 years before present. These rates are consistent with modern retreat rates of about 2–19 cm yr⁻¹ here. RSL rise in Southern California remained relatively constant during the late Holocene, potentially explaining the relatively stable modeled cliff retreat rate over this time. We also explore the relative influence of weathering, material resistance, and wave erosion efficacy and find that both weathering and wave‐driven erosion are necessary to replicate the measured data at this location, with the latter exerting a stronger control on model acceptance, suggesting that waves may provide a possible mechanism by which RSL rise may influence coastal cliff erosion in southern California.

Topography is commonly viewed as a passive backdrop on which vegetation grows. Yet, in certain circumstances, a bidirectional feedback may develop between the control of topography and the spatial distribution of vegetation and landform development, because vegetation modulates the erosion of the land surface. Therefore, if reinforcing feedbacks are established between erosion and land cover distribution over timescales relevant to landform development, then the interactions between vegetation and topography may create distinctive landforms, shaped by vegetation. We expose here a strong correlation between the spatial distribution of vegetation, erosion rates, and topography at a characteristic length scale of 10²-10³m (mesoscale topography) in the Luquillo Experimental forest (LEF) of Puerto Rico. We use high-resolution LiDAR topography to characterize landforms, satellite images to classify the vegetation into forest types, and in-situ produced cosmogenic ¹⁰Be in the quartz extracted from soils and stream sediments to document spatial variations in soil erosion. The data document a strong correlation between forest type and topographic position (hilltop vs. valleys), and a correlation between topographic position and ¹⁰Be-derived erosion rates over 10³−10⁴ years. Erosion is faster in valleys, which are mostly covered by monocot Palm Forest, and slower on surrounding hills mostly covered by the dicot Palo Colorado Forest. Transition from one forest type to the next occurs across a break-in-slope that separates shallowly convex hilltops from deeply concave valleys (coves). The break-in-slope is the consequence of a longer-lasting erosional imbalance whereby coves erode faster than hills over landscape-shaping timescales. Such a deepening of the coves is usually spurred by external drivers, but such drivers are here absent. This implies that cove erosion is driven by a process originating within the coves themselves. We propose that vegetation is the primary driver of this imbalance, soil erosion being faster under Palm forest than under Palo Colorado forest. Concentration of the Palm forest in the deepening coves is reinforced by the better adaptation of Palm trees to the erosive processes that take place in the coves, once these develop steep slopes. At the current rate of landscape development, we find that the imbalance started within the past 0.1–1.5 My. The initiation of the process could correspond to time of settlement of these mountain slopes by the Palm and Palo Colorado forests.

Datasets, regression data, and descriptions of minor corrections to the datasets, and a detailed description of the statistical methods.

Datasets, regression data, and descriptions of minor corrections to the datasets, and a detailed description of the statistical methods.

Measuring rates of valley head migration and determining the timing of canyon‐opening are insightful for the evolution of planetary surfaces. Spatial gradients of in situ‐produced cosmogenic nuclide concentrations along horizontal transects provide a framework for assessing the migration of valley networks and similar topographic features. We developed a new derivation for valley head retreat rates from the concentrations of in situ‐produced cosmogenic radionuclides in valley walls. The retreat rate is inversely proportional to the magnitude of the spatial concentration gradient and proportional to local nuclide production rates. By solving for a spatial gradient in concentration along a valley parallel transect, we created an expression for the explicit determination of valley head retreat, which we refer to herein as unzipping. We applied this expression to a seepage‐derived drainage network developing along the Apalachicola River, Florida, USA. Sample concentrations along a valley margin transect varied systematically from 2.9 × 10⁵ to 3.5 × 10⁵ atoms/g resulting in a gradient of 160 atoms/g/m, and from this value a valley head retreat rate of 0.025 m/y was found. The discrepancy between overall network age and current rates of valley head migration suggests intermittent network growth which is consistent with glacial‐interglacial precipitation variations during the Pleistocene. This method can be applied to a wide range of Earth‐surface environments. For the ¹⁰Be system, this method should be sensitive to unzipping rates bounded between 10⁻⁶ and 10⁰ m/y.

Measuring rates of valley head migration and determining the timing of canyon-opening are insightful for the evolution of planetary surfaces. Spatial gradients of in situ-produced cosmogenic nuclide concentrations along horizontal transects provide a framework for assessing the migration of valley networks and similar topographic features. We developed a new derivation for valley head retreat rates from the concentration of in situ produced cosmogenic radionuclides in valley walls. The retreat rate is inversely proportional to the magnitude of the spatial concentration gradient and proportional to local nuclide production rates. By solving for a spatial gradient in concentration along a valley parallel transect, we created an expression for the explicit determination of valley head retreat, which we refer to herein as unzipping. We applied this expression to a seepage-derived drainage network developing along the Apalachicola River, Florida, USA. Sample concentrations along a valley margin transect varied systematically from 2.9x105 atoms/g to 3.5x105 atoms/g resulting in a gradient of 160 atoms/g/m, and from this value a valley head retreat rate of 0.025 m/y was found. The discrepancy between overall network age and current rates of valley head migration suggests intermittent network growth which is consistent with glacial-interglacial precipitation variations during the Pleistocene. This method can be applied to a wide range of Earth-surface environments. For the 10Be system, this method should be sensitive to unzipping rates bounded between 10-6 m/y and 100 m/y.