Simon M. Mudd

Limits on the adaptability of coastal marshes to rising sea level

[1] Assumptions of a static landscape inspire predictions that about half of the world’s coastal wetlands will submerge during this century in response to sea‐level acceleration. In contrast, we use simulations from five numerical models to quantify the conditions under which ecogeomorphic feedbacks allow coastal wetlands to adapt to projected changes in sea level. In contrast to previous sea‐level assessments, we find that non‐linear feedbacks among inundation, plant growth, organic matter accretion, and sediment deposition, allow marshes to survive conservative projections of sea‐ level rise where suspended sediment concentrations are greater than ∼20 mg/L. Under scenarios of more rapid sea‐level rise (e.g., those that include ice sheet melting), marsheswill likelysubmerge neartheend ofthe 21stcentury. Our results emphasize that in areas of rapid geomorphic change, predicting the response of ecosystems to climate change requires consideration of the ability of biological processestomodifytheirphysicalenvironment.Citation: Kirwan, M. L., G. R. Guntenspergen, A. D’Alpaos, J. T. Morris, S. M. Mudd, and S. Temmerman (2010), Limits on the adaptability of coastal marshes to rising sea level, Geophys. Res. Lett., 37, L23401,

Numerical models of salt marsh evolution: ecological, geomorphic, and climatic factors

Salt marshes are delicate landforms at the boundary between the sea and land. These ecosystems support a diverse biota that modifies the erosive characteristics of the substrate and mediates sediment transport processes. Here we present a broad overview of recent numerical models that quantify the formation and evolution of salt marshes under different physical and ecological drivers. In particular, we focus on the coupling between geomorphological and ecological processes and on how these feedbacks are included in predictive models of landform evolution. We describe in detail models that simulate fluxes of water, organic matter, and sediments in salt marshes. The interplay between biological and morphological processes often produces a distinct scarp between salt marshes and tidal flats. Numerical models can capture the dynamics of this boundary and the progradation or regression of the marsh in time. Tidal channels are also key features of the marsh landscape, flooding and draining the marsh platform and providing a source of sediments and nutrients to the marsh ecosystem. In recent years, several numerical models have been developed to describe the morphogenesis and long-term dynamics of salt marsh channels. Finally, salt marshes are highly sensitive to the effects of long-term climatic change. We therefore discuss in detail how numerical models have been used to determine salt marsh survival under different scenarios of sea level rise.

How does vegetation affect sedimentation on tidal marshes? Investigating particle capture and hydrodynamic controls on biologically mediated sedimentation

[1] Plants are known to enhance sedimentation on intertidal marshes. It is unclear, however, if the dominant mechanism of enhanced sedimentation is direct organic sedimentation, particle capture by plant stems, or enhanced settling due to a reduction in turbulent kinetic energy within flows through the plant canopy. Here we combine several previously reported laboratory studies with an 18 year record of salt marsh macrophyte characteristics to quantify these mechanisms. In dense stands of Spartina alterniflora (with projected plant areas per unit volume of >10 m −1 ) and rapid flows (>0.4 m s −1 ), we find that the fraction of sedimentation from particle capture can instantaneously exceed 70%. In most marshes dominated by Spartina alterniflora, however, we find particle settling, rather than capture, will account for the majority of inorganic sedimentation. We examine a previously reported 2 mm yr −1 increase in accretion rate following a fertilization experiment in South Carolina. Prior studies at the site have ruled out organic sedimentation as the cause of this increased accretion. We apply our newly developed models of particle capture and effective settling velocity to the fertilized and control sites and find that virtually all (>99%) of the increase in accretion rates can be attributed to enhanced settling brought about by reduced turbulent kinetic energy in the fertilized canopy. Our newly developed models of biologically mediated sedimentation are broadly applicable and can be applied to marshes where data relating biomass to stem diameter and projected plant area are available.

A theoretical model coupling chemical weathering rates with denudation rates

Uplift of the Himalayas has been proposed to have locally accelerated chemical weathering, thus leading to enhanced CO2 sequestration and global cooling. This hypothesis assumes that rapid erosion exposes fresh, highly reactive minerals at Earths surface. Empirical studies quantifying the relationship between erosion and weathering have produced apparently conflicting results, where the nature of the relationship is dependent on the weathering regime of the sampled landscapes. We derive a quantitative model that defines this relationship across the range of weathering regimes, from supply-limited to kinetically limited conditions. The model matches trends in field data collected by others and reconciles apparently conflicting results. The model also demonstrates that, as erosion rates increase, potential increases in weathering rate from the exposure of fresher materials are offset by the decrease in the total volume of minerals exposed due to thinner regolith. We conclude that the relationship between weathering and erosion is one of diminishing returns, in which increases in erosion rate lead to progressively smaller increases in weathering rate; indeed, at the highest erosion rates, weathering rates may decline. The ability, therefore, of accelerated uplift and erosion to stimulate greater CO2 sequestration may be significant in landscapes eroding at rates of 10–102 t km−2 yr−1. However, where erosion rates are greater than 102 t km−2 yr−1, increases in denudation may not be matched by increases in chemical weathering. Finally, our results suggest that watersheds with regolith thicknesses of ~0.5 m will yield the greatest solute fluxes.

Response of salt-marsh carbon accumulation to climate change

About half of annual marine carbon burial takes place in shallow water ecosystems where geomorphic and ecological stability is driven by interactions between the flow of water, vegetation growth and sediment transport. Although the sensitivity of terrestrial and deep marine carbon pools to climate change has been studied for decades, there is little understanding of how coastal carbon accumulation rates will change and potentially feed back on climate. Here we develop a numerical model of salt marsh evolution, informed by recent measurements of productivity and decomposition, and demonstrate that competition between mineral sediment deposition and organic-matter accumulation determines the net impact of climate change on carbon accumulation in intertidal wetlands. We find that the direct impact of warming on soil carbon accumulation rates is more subtle than the impact of warming-driven sea level rise, although the impact of warming increases with increasing rates of sea level rise. Our simulations suggest that the net impact of climate change will be to increase carbon burial rates in the first half of the twenty-first century, but that carbon–climate feedbacks are likely to diminish over time.

The Gamburtsev mountains and the origin and early evolution of the Antarctic Ice Sheet

Ice-sheet development in Antarctica was a result of significant and rapid global climate change about 34 million years ago. Ice-sheet and climate modelling suggest reductions in atmospheric carbon dioxide (less than three times the pre-industrial level of 280 parts per million by volume) that, in conjunction with the development of the Antarctic Circumpolar Current, led to cooling and glaciation paced by changes in Earth’s orbit. Based on the present subglacial topography, numerical models point to ice-sheet genesis on mountain massifs of Antarctica, including the Gamburtsev mountains at Dome A, the centre of the present ice sheet. Our lack of knowledge of the present-day topography of the Gamburtsev mountains means, however, that the nature of early glaciation and subsequent development of a continental-sized ice sheet are uncertain. Here we present radar information about the base of the ice at Dome A, revealing classic Alpine topography with pre-existing river valleys overdeepened by valley glaciers formed when the mean summer surface temperature was around 3 °C. This landscape is likely to have developed during the initial phases of Antarctic glaciation. According to Antarctic climate history (estimated from offshore sediment records) the Gamburtsev mountains are probably older than 34 million years and were the main centre for ice-sheet growth. Moreover, the landscape has most probably been preserved beneath the present ice sheet for around 14 million years.

Rain splash of dry sand revealed by high‐speed imaging and sticky paper splash targets

[1] Rain splash transport of sediment on a sloping surface arises from a downslope drift of grains displaced ballistically by raindrop impacts. We use high-speed imaging of drop impacts on dry sand to describe the drop-to-grain momentum transfer as this varies with drop size and grain size and to clarify ingredients of downslope grain drift. The ‘‘splash’’ of many grains involves ejection of surface grains accelerated by grain-to-grain collisions ahead of the radially spreading front of a drop as it deforms into a saucer shape during impact. For a given sand size, splash distances are similar for different drop sizes, but the number of displaced grains increases with drop size in proportion to the momentum of the drop not infiltrated within the first millisecond of impact. We present a theoretical formulation for grain ejection which assumes that the proportion of ejected grains within any small azimuthal angular interval dq about the center of impact is proportional to the momentum density of the spreading drop within dq and that the momentum of ejected grains at angle q is, on average, proportional to the momentum of the spreading drop at q. This formulation, consistent with observed splash distances, suggests that downslope grain transport involves an asymmetry in both quantity and distance: more grains move downslope than upslope with increasing surface slope, and, on average, grains move farther downslope. This latter effect is primarily due to the radial variation in the surface-parallel momentum of the spreading drop. Surface-parallel transport increases approximately linearly with slope.

Using hilltop curvature to derive the spatial distribution of erosion rates

[1] Erosion rates dictate the morphology of landscapes, and therefore quantifying them is a critical part of many geomorphic studies. Methods to directly measure erosion rates are expensive and time consuming, whereas topographic analysis facilitates prediction of erosion rates rapidly and over large spatial extents. If hillslope sediment flux is nonlinearly dependent on slope then the curvature of hilltops will be linearly proportional to erosion rates. In this contribution we develop new techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates using high-resolution (1 m) digital elevation data. Published and new cosmogenic radionuclide analyses in the Feather River basin, California, suggest that erosion rates vary by over an order of magnitude (10 to 250 mm kyr � 1 ). Hilltop curvature increases with erosion rates, allowing calibration of the hillslope sediment transport coefficient, which controls the relationship between gradient and sediment flux. Having constraints on sediment transport efficiency allows estimation of erosion rates throughout the landscape by mapping the spatial distribution of hilltop curvature. Additionally, we show that hilltop curvature continues to increase with rising erosion rates after gradient-limited hillslopes have emerged. Hence hilltop curvature can potentially reflect higher erosion rates than can be predicted by hillslope gradient, providing soil production on hilltops can keep pace with erosion. Finally, hilltop curvature can be used to estimate erosion rates in landscapes undergoing a transient adjustment to changing boundary conditions if the response timescale of hillslopes is short relative to channels.

Using chemical tracers in hillslope soils to estimate the importance of chemical denudation under conditions of downslope sediment transport

[1] We present a model of hillslope soils that couples the evolution of topography, soil thickness, and the concentration of constituent soil phases, defined as unique components of the soil with collective mass equal to the total soil mass. The model includes both sediment transport and chemical denudation. A simplified two-phase model is developed; the two phases are a chemically immobile phase, which has far lower solubility than the bulk soil and is not removed through chemical weathering (for example, zircon grains), and a chemically mobile phase that may be removed from the system through chemical weathering. Chemical denudation rates in hillslope soils can be measured using the concentration of immobile elements, but the enrichment of these immobile elements is influenced by spatial variations in chemical denudation rates and spatial variations in the chemical composition of a soil’s parent material. These considerations cloud the use of elemental depletion factors and cosmogenic nuclide-based total denudation rates used to identify the relationship between physical erosion and chemical weathering if these techniques do not account for downslope sediment transport. On hillslopes where chemical denudation rates vary in space, estimates of chemical denudation using techniques that do not account for downslope sediment transport and spatial variations in chemical denudation rates may be adequate where the chemical denudation rate is a significant fraction of the total denudation rate but are inadequate in regions where chemical weathering rates are small compared to the total denudation rate. We also examine relationships between transient mechanical and chemical denudation rates. Soil particle residence times may affect chemical weathering rates, and the relationship between total landscape-lowering rates and soil particle residence times can thus be quantified.

Objective extraction of channel heads from high-resolution topographic data

Fluvial landscapes are dissected by channels, and at their upstream termini are channel heads. Accurate reconstruction of the fluvial domain is fundamental to understanding runoff generation, storm hydrology, sediment transport, biogeochemical cycling, and landscape evolution. Many methods have been proposed for predicting channel head locations using topographic data, yet none have been tested against a robust field data set of mapped channel heads across multiple landscapes. In this study, four methods of channel head prediction were tested against field data from four sites with high-resolution DEMs: slope-area scaling relationships; two techniques based on landscape tangential curvature; and a new method presented here, which identifies the change from channel to hillslope topography along a profile using a transformed longitudinal coordinate system. Our method requires only two user-defined parameters, determined via independent statistical analysis. Slope-area plots are traditionally used to identify the fluvial-hillslope transition, but we observe no clear relationship between this transition and field-mapped channel heads. Of the four methods assessed, one of the tangential curvature methods and our new method most accurately reproduce the measured channel heads in all four field sites (Feather River CA, Mid Bailey Run OH, Indian Creek OH, Piedmont VA), with mean errors of −11, −7, 5, and −24 m and 34, 3, 12, and −58 m, respectively. Negative values indicate channel heads located upslope of those mapped in the field. Importantly, these two independent methods produce mutually consistent estimates, providing two tests of channel head locations based on independent topographic signatures.