Bruce H. Wilkinson

Spatial distribution of δ18O in meteoric precipitation

Proxy data reflecting the oxygen isotope composition of meteoric precipitation (δ18Oppt) are widely used in reconstructions of continental paleoclimate and paleohydrology. However, actual geographic variation in modern water compositions is difficult to estimate from often sparse data. A first step toward understanding the geologic pattern of change in δ18Oppt is to describe the modern distribution in terms of principal geographic parameters. To this end, we empirically model relationships between 18O in modern precipitation and latitude and altitude. We then identify geographic areas where large-scale vapor transport patterns give rise to significant deviations from model δ18Oppt compositions based on latitude and altitude. Model value and residual grids are combined to derive a high-resolution global map of δ18Oppt that can serve as a spatial reference against which proxy data for paleoprecipitation can be compared. Reiteration of the procedure outlined here, for paleo-δ18Oppt data, may illuminate past changes in the climatic and physiographic parameters controlling the distribution of δ18O regimes.

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.

Kinetic Control of Morphology, Composition, and Mineralogy of Abiotic Sedimentary Carbonates

ABSTRACT Conventional thinking has long held that the abiotic precipitation of calcium carbonate occurs with a causal relationship between fluid My/Ca ratios and crystal morphology, crystal composition, and carbonate mineralogy, resulting in the formation of meteoric, equant, low-magnesium calcite and marine, acicular, high-magnesium calcite and aragonite. Problematically, calcites with varying amounts of incorporated magnesium occur either as equant or acicular crystals, and aragonite may coexist with calcite in either environment. Commonly, however, a systematic relation exists between crystal morphology, composition, mineralogy, and rates of reactant supply to growing crystal surfaces. For example, equant rather than acicular crystals of calcite form in modern, deep and/or cold marine, meteoric-phreatic, and deep-burial settings where the degree of carbonate saturation and/or rates of fluid flow are low. In areas of higher saturation and/or fluid flow, such as in warm, shallow-marine and meteoric-vadose environments, acicular calcite may predominate. This relation is also seen in systems in which aragonite and calcite form in intimate association. Aragonite precipitation is favored when rates of reactant supply are high; calcite forms when rates are low. Such relations suggest that crystal morphology, composition, and mineralogy are controlled by the kinetics of surface nucleation and the amount of reactants, principally carbonate ions, at growth sites. Precipitating phases are the ones which can best accommodate such excess reactants; ambient Mg/Ca ratios only indirectly control the nature of inorganically precipitated carbonate phases.

Humans as geologic agents: A deep-time perspective

Humans move increasingly large amounts of rock and sediment during various construction activities, and mean rates of cropland soil loss may exceed rates of formation by up to an order of magnitude, but appreciating the actual importance of humans as agents of global erosion necessitates knowledge of prehistoric denudation rates imposed on land surfaces solely by natural processes. Amounts of weathering debris that compose continental and oceanic sedimentary rocks provide one such source of information and indicate that mean denudation over the past half-billion years of Earth history has lowered continental surfaces by a few tens of meters per million years. In comparison, construction and agricultural activities currently result in the transport of enough sediment and rock to lower all ice-free continental surfaces by a few hundred meters per million years. Humans are now an order of magnitude more important at moving sediment than the sum of all other natural processes operating on the surface of the planet. Relationships between temporal trends in land use and global population indicate that humans became the prime agents of erosion sometime during the latter part of the first millennium A.D.

Continental Drift and Phanerozoic Carbonate Accumulation in Shallow-Shelf and Deep-Marine Settings

Knowledge of past rates of transfer of rock‐forming materials among the principal geologic reservoirs is central to understanding causes and magnitudes of change in earth surface processes over Phanerozoic time. To determine typical rates of global sediment cycling, we compiled information on area, volume, and lithology of shallow‐water sediments by epoch for both terrigenous clastics and marine carbonates. Data on amounts of surviving continental terrigenous rock (as opposed to deep oceanic, and including “terrestrial,” “marginal marine,” and “marine” deposits) exhibit positive age/area trends wherein greatest areas and volumes of conglomerate, sandstone, and shale are represented by younger sequences. Global volumes of terrigenous‐clastic sediment yield a mean cycling rate of 0.00124/m.yr., similar to that determined for Eurasian (0.00127), North American (0.00058 [A. B. Ronov], 0.00352 [T. D. Cook and A. W. Bally]), African (0.0017), and South American (0.0021) clastic sequences. Surficial erosion results in the mean destruction of ∼0.124% of terrigenous rock volume per million years of reservoir age. In contrast, surviving epicratonic and shelf‐margin carbonate sequences yield negative cycling rates of about −0.164%/m.yr. Surviving areas and volumes increase with sequence age; that is, the amount of limestone and dolostone preserved in shallow‐water settings increases back in time to maximal areal extents in the Middle and Upper Cambrian. Mass/age data on terrigenous‐clastic successions indicate generally constant rates of crustal erosion over Phanerozoic time. Decrease in size of the shallow‐marine carbonate reservoir forward in time therefore suggests generally invariant rates of global limestone accumulation and a shift in sites of accumulation from shallow‐cratonic to deep‐oceanic settings over much of the past 540 m.yr. Causes of this eon‐scale depositional translocation of carbonate sediment from shallow‐ to deep‐marine settings cannot be satisfactorily linked to either changes in global sea level or the evolution of carbonate‐producing plankton because neither exhibits a pattern of unidirectional change in position or abundance since the Early Phanerozoic. However, tabulation of long‐term variation in areas of continental shelves from paleogeographic maps reveals a generally uniform decrease in low‐latitude (<30°) platform area with decreasing age over most of the Phanerozoic. Moreover, rate of change of low‐latitude shelf area (∼ \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Biomineralization, paleoceanography, and the evolution of calcareous marine organisms

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Periodicity of Mesoscale Phanerozoic Sedimentary Cycles and the Role of Milankovitch Orbital Modulation

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