Brandon McElroy

The impact of humans on continental erosion and sedimentation

Rock uplift and erosional denudation of orogenic belts have long been the most impor- tant geologic processes that serve to shape continental surfaces, but the rate of geomor- phic change resulting from these natural phe- nomena has now been outstripped by human activities associated with agriculture, con- struction, and mining. Although humans are now the most important geomorphic agent on the planets surface, natural and anthro- pogenic processes serve to modify quite dif- ferent parts of Earths landscape. In order to better understand the impact of humans on continental erosion, we have examined both long-term and short-term data on rates of sediment transfer in response to glacio-fl u- vial and anthropogenic processes. Phanerozoic rates of subaerial denuda- tion inferred from preserved volumes of sedimentary rock require a mean conti- nental erosion rate on the order of 16 m per million years (m/m.y.), resulting in the accumulation of ~5 gigatons of sediment per year (Gt/yr). Erosion irregularly increased over the ~542 m.y. span of Phanerozoic time to a Pliocene value of 53 m/m.y. (16 Gt/yr). Current estimates of large river sediment loads are similar to this late Neogene value, and require net denudation of ice-free land surfaces at a rate of ~62 m/m.y. (~21 Gt/yr). Consideration of the variation in large river sediment loads and the geomorphology of respective river basin catchments suggests that natural erosion is primarily confi ned to drainage headwaters; ~83% of the global river sediment fl ux is derived from the high- est 10% of Earths surface. Subaerial erosion as a result of human activity, primarily through agricultural practices, has resulted in a sharp increase in net rates of continental denudation; although less well constrained than estimates based on surviving rock volumes or current river loads, available data suggest that present farmland denudation is proceeding at a rate of ~600 m/ m.y. (~75 Gt/yr), and is largely confi ned to the lower elevations of Earths land surface, primarily along passive continental margins; ~83% of cropland erosion occurs over the lower 65% of Earths surface. The conspicuous disparity between natu- ral sediment fl uxes suggested by data on rock volumes and river loads (~21 Gt/yr) and anthropogenic fl uxes inferred from measured and modeled cropland soil losses (75 Gt/yr) is readily resolved by data on thicknesses and ages of alluvial sediment that has been deposited immediately downslope from erod- ing croplands over the history of human agriculture. Accumulation of postsettlement alluvium on higher-order tributary channels and fl oodplains (mean rate ~12,600 m/m.y.) is the most important geomorphic process in terms of the erosion and deposition of sedi- ment that is currently shaping the landscape of Earth. It far exceeds even the impact of Pleistocene continental glaciers or the cur- rent impact of alpine erosion by glacial and/ or fl uvial processes. Conversely, available data suggest that since 1961, global cropland area has increased by ~11%, while the global population has approximately doubled. The net effect of both changes is that per capita cropland area has decreased by ~44% over this same time interval; ~1% per year. This is ~25 times the rate of soil area loss antici- pated from human denudation of cropland surfaces. In a context of per capita food pro- duction, soil loss through cropland erosion is largely insignifi cant when compared to the impact of population growth.

Interactions between turbidity currents and topography in aggrading sinuous submarine channels: A laboratory study

We present results from a laboratory experiment documenting the evolution of a sinuous channel form via sedimentation from 24 turbidity currents having constant initial conditions. The initial channel had a sinuosity of 1.32, a wavelength of 1.95, an amplitude of 0.39 m, and three bends. All currents had a densimetric Froude number of 0.53 and an initial height equal to the channel relief at the start of the experiment. Large superelevation of currents was observed at bend apexes. This superelevation was 85%–142% greater than the value predicted by a balance of centrifugal and pressure-gradient forces. An additional contribution to the superelevation was the runup of the current onto the outer banks of bends. This runup height is described by a balance between kinetic and potential energy. Runup resulted in deposition of coarse particles on levee crests that were indistinguishable from those deposited on the channel bottom. Deposit thickness and composition showed a strong cross-channel asymmetry. Thicker, coarser, steeper levees grew on the outer banks relative to the inner banks of bends. Zones of flow separation were observed downstream from bend apexes along inner banks and affected sedimentation patterns. Sedimentation from currents caused the channel to aggrade with almost no change in planform. However, channel relief decreased throughout the experiment because deposition on the channel bottom always exceeded deposition at levee crests. The first bend served as a filter for the properties of the channelized current, bringing discharge at the channel entrance into agreement with the channel cross-sectional area. Excess discharge exited the channel at this filtering bend and was lost to the overbank surface.

Earth is (mostly) flat: Apportionment of the flux of continental sediment over millennial time scales

We thank [Warrick et al. (2014)][1] for the Comment on our recent synthesis of 10Be-derived denudation rates ([Willenbring et al., 2013][2]), in which we suggested that gently sloping areas, representing ∼90% of the Earth’s land surface, have sufficiently high rates of denudation to produce a

Linking river-flood dynamics to hyperpycnal-plume deposits: Experiments, theory, and geological implications

Turbid river plumes entering ocean or lake water of lesser density (i.e., hyperpycnal plumes) can plunge to form turbidity currents providing an important link between terrestrial sediment sources and marine depositional sinks. A leading hypothesis suggests that hyperpycnal-plume deposits accurately record the rising and falling discharge of a flooding river (in terms of sediment-size grading, bedform sequence, and deposit thickness), which, if correct, has significant implications for unraveling river dynamics, reservoir potential, and Earth history from marine-event beds. Herein, we present one of the first experimental flume studies aimed at testing this hypothesis. Results indicate that depth-averaged hyperpycnal-plume velocities can be uncorrelated or even anti-correlated with river discharge at certain seabed locations because of translation of the plunge point resulting from temporal variations in discharge and sediment concentration through the duration of a river flood. An advection length scale of settling sediment is found to be an important control on hyperpycnal-plume deposits, where coarse sediment (sand) is most likely to record multiple flow accelerations and decelerations related to plunge-point translation even for a river flood with a single-peaked hydrograph. In contrast, fine sediment (mud) is relatively insensitive to local plunge-point dynamics and is most likely to preserve directly rising and falling river discharge. Finally, it was found that the necessary fluvial sediment concentration to form a plunging plume can be much larger than the concentration typically used assuming density equivalence because of deposition upstream of the plunge point.

Quantitative bounds on morphodynamics and implications for reading the sedimentary record

Sedimentary rocks are the archives of environmental conditions and ancient planetary surface processes that led to their formation. Reconstructions of Earths past surface behaviour from the physical sedimentary record remain controversial, however, in part because we lack a quantitative framework to deconvolve internal dynamics of sediment-transport systems from environmental signal preservation. Internal dynamics of landscapes--a consequence of the coupling between bed topography, sediment transport and flow dynamics (morphodynamics)--result in regular and quasiperiodic landforms that abound on the Earth and other planets. Here, using theory and a data compilation of morphodynamic landforms that span a wide range of terrestrial, marine and planetary depositional systems, we show that the advection length for settling sediment sets bounds on the scales over which internal landscape dynamics operate. These bounds provide a universal palaeohydraulic reconstruction tool on planetary surfaces and allow for quantitative identification of depositional systems that may preserve tectonic, climatic and anthropogenic signals.

Global geologic maps are tectonic speedometers—Rates of rock cycling from area-age frequencies

Relations among ages and present areas of exposure of volcanic, sedimentary, plutonic, and metamorphic rock units (lithosomes) record a complex interplay between depths and rates of formation, rates of subsequent tectonic subsidence and burial, and/or rates of uplift and erosion. Thus, they potentially serve as efficient deep-time geologic speedometers, providing quantitative insight into rates of material transfer among the principal rock reservoirs—processes central to the rock cycle. Areal extents of lithosomes exposed on all continents from two map sources (Geological Survey of Canada [GSC] and the Food and Agricultural Organization [FAO] of the United Nations Educational, Scientific, and Cultural Organization [UNESCO]) indicate that volcanic, sedimentary, plutonic, and metamorphic rocks occupy ~8%, 73%, 7%, and 12% of global exposures, respectively. Plots of area versus age of all mapped rock types display a power-law relation where ~6.5% of continental area is resurfaced with younger (~10% volcanic; 90% sedimentary) units every million years, and where areas of rock exposure decrease by ~0.86% for each 1% increase in outcrop age (r 2 = 0.90). Area-age relations for volcanic and sedimentary lithosomes are similar to the power-law distribution defined by all rock units (because ~81% of mapped area consists of these two lithologies) and reflect progressive decrease in amount of exposure with increasing age. Over the long term, continental surfaces are blanketed by new volcanic rocks and sediments at rates of ~1.5 and 12.1 × 10 6 km 2 /Ma, respectively. In contrast to power-law–distributed volcanic and sedimentary rocks that form at the Earth9s surface, age-frequency distributions for plutonic and metamorphic rocks exhibit lognormal relations, with modes at ca. 154 and 697 Ma, respectively. A dearth of younger exposures of plutonic and metamorphic rocks reflects the fact that these rock types form at depth, and some duration of tectonism is therefore required for their exposure. Increasing modal ages, from Quaternary for volcanic and sedimentary successions, to early Mesozoic for intrusive rocks, to Neoproterozoic for metamorphic rocks, demonstrate that greater amounts of geologic time are required for uplift to bring more deeply formed rocks to the Earth9s surface. The two different age-frequency distributions observed for these major rock types—a general power-law age distribution for volcanic and sedimentary rocks and a lognormal distribution for plutonic and metamorphic rock ages—reflect the interplay between depths of formation and mean rates of vertical tectonic displacement. Age-frequency distributions for each of the major rock types are closely replicated by a model that presumes that individual crustal elements behave as a large population of random walks in geologic time and crustal depth, and where the processes of surficial erosion associated with tectonic uplift serve to impose an absorbing boundary on this random-walk space. Comparisons between model-predicted age-frequencies and those apparent in global map data suggest that mean rates of crustal subsidence and uplift are approximately equal in magnitude, with mean rates of vertical tectonic diffusion of lithosomes from crustal depths of formation of about half a kilometer per million years. Rates of uplift and subsidence are strongly dependent on durations of tectonic dispersion (lithosome ages); however, mean rates on the order of hundreds of meters per million years suggested by map age-frequencies are the same as would be anticipated on the basis of hundreds of published rates of erosional uplift and exhumation determined by more conventional geochronometers. This agreement suggests that geologic maps serve as effective deep-time speedometers for the geologic rock cycle.

Empirical assessment of theory for bankfull characteristics of alluvial channels

We compiled a data set of 541 bankfull measurements of alluvial rivers (see supporting information) and used Bayesian linear regression to examine empirical and theoretical support for the hypothesis that alluvial channels adjust to a predictable condition of basal shear stress as a function of sediment transport mode. An empirical closure based on channel slope, bankfull channel depth, and median grain size is proposed and results in the scaling of bankfull Shields stress with the inverse square root of particle Reynolds number. The empirical relationship is sufficient for purposes of quantifying paleohydraulic conditions in ancient alluvial channels. However, it is not currently appropriate for application to alluvial channels on extraterrestrial bodies because it depends on constant-valued, Earth-based coefficients.

The fate of sediment, wood and organic carbon eroded during an extreme flood, Colorado Front Range, USA

Identifying and quantifying the dominant processes of erosion and tracking the fate of sediment, wood, and carbon eroded during floods is important for understanding channel response to floods, downstream sediment and carbon loading, and the influence of extreme events on landscapes and the terrestrial carbon cycle. We quantify sediment, wood, and organic carbon (OC) from source to local sink following an extreme flood in the tectonically quiescent, semi-arid Colorado (USA) Front Range. Erosion of >500,000 m3 or as much as ~115 yr of weathering products occurred through landsliding and channel erosion during September 2013 flooding. More than half of the eroded sediment was deposited at the inlet and delta of a water supply reservoir, resulting in the equivalent of 100 yr of reservoir sedimentation and 2% loss in water storage capacity. The flood discharged 28 Mg C/km2, producing an event OC flux equivalent to humid, tectonically active areas. Post-flood remobilization resulted in a further ~100 yr of reservoir sedimentation plus export of an additional 1.3 Mg C/km2 of wood, demonstrating the ongoing impact of the flood on reservoir capacity and carbon cycling. Pronounced channel widening during the flood created accommodation space for 40% of flood sediment and storage of wood and eroded carbon. We conclude that confined channels, normally dismissed as transport reaches, can store and export substantial amounts of flood constituents.

Quantifying the stratigraphic completeness of delta shoreline trajectories

Understanding the incomplete nature of the stratigraphic record is fundamental for interpreting stratigraphic sequences. Methods for quantifying stratigraphic completeness for one-dimensional stratigraphic columns, defined as the proportion of time intervals of some length that contain stratigraphy, are commonplace; however, quantitative assessments of completeness in higher dimensions are lacking. Here we present a metric for defining stratigraphic completeness of two-dimensional shoreline trajectories using topset-foreset rollover positions in dip-parallel sections and describe the preservation potential of a shoreline trajectory derived from the geometry of the delta surface profile and the kinematics of the geomorphic shoreline trajectory. Two end-member forward models are required to fully constrain the preservation potential of the shoreline dependent on whether or not a topset is eroded during base level fall. A laboratory fan-delta was constructed under nonsteady boundary conditions, and one-dimensional stratigraphic column and two-dimensional shoreline completeness curves were calculated. Results are consistent with the hypothesis derived from conservation of sediment mass that completeness over all timescales should increase given increasing dimensions of analysis. Stratigraphic trajectories and completeness curves determined from forward models using experimental geomorphic trajectories compare well to values from transects when subsampled to the equivalent stratigraphic resolution as observed in the actual preserved sequence. The concept of stratigraphic completeness applied to two-dimensional trajectory analysis and the end-member forward models presented here provide novel tools for a conceptual understanding of the nature of stratigraphic preservation at basin scales.

Climatic Control of Continental Physiography

The spatial distribution of elevations and slopes on continents is a primary expression of complex interactions between tectonic and climatic systems. Because rates of tectonism and climatically mediated erosion vary in both space and time, it is possible that heights of land and steepness of associated slopes change both across climatic regimes and during the tectonic evolution of any particular landmass. As a geomorphic record, spatial variations of continental physiography with latitude afford some insight into the relative importance of tectonic and climatic processes in controlling Earth surface elevations. Because modern digital elevation models afford abundant data on continent surfaces across the complete set of equator to pole climate ranges, we have undertaken an evaluation of the dependence of continental physiography on climate using the latitudinal gradient as a proxy for first‐order change in temperature and precipitation. Continental hypsometries can be largely explained by elevation distributed as an exponential function of the square root of area of land, and hypsometries with normalized maximum elevations and total areas exhibit no significant latitudinal variation. This suggests that spatial variation in Earth surface topography, as manifest by area‐elevation relations, is largely insensitive to latitudinal position and associated climate. Land surface slope, however, does covary with absolute latitude. Steeper slopes are apparent at higher latitudes and likely record the poleward influence of cryogenically mediated processes of continental denudation. The coincidence of increasing subaerial slopes with latitude and stasis in hypsometric distributions of elevations suggests that poleward regions are also more dissected than their equatorial counterparts.