https://cyberleninka.org/article/n/352063.pdf

87Sr/86Sr, δ13C and δ18O evolution of Phanerozoic seawater

Porphyry Cu systems host some of the most widely distributed mineralization types at convergent plate boundaries, including porphyry deposits centered on intrusions; skarn, carbonate-replacement, and sediment-hosted Au deposits in increasingly peripheral locations; and superjacent high- and intermediate-sulfidation epithermal deposits. The systems commonly define linear belts, some many hundreds of kilometers long, as well as occurring less commonly in apparent isolation. The systems are closely related to underlying composite plutons, at paleodepths of 5 to 15 km, which represent the supply chambers for the magmas and fluids that formed the vertically elongate (>3 km) stocks or dike swarms and associated mineralization. The plutons may erupt volcanic rocks, but generally prior to initiation of the systems. Commonly, several discrete stocks are emplaced in and above the pluton roof zones, resulting in either clusters or structurally controlled alignments of porphyry Cu systems. The rheology and composition of the host rocks may strongly influence the size, grade, and type of mineralization generated in porphyry Cu systems. Individual systems have life spans of ~100,000 to several million years, whereas deposit clusters or alignments as well as entire belts may remain active for 10 m.y. or longer.

The alteration and mineralization in porphyry Cu systems, occupying many cubic kilometers of rock, are zoned outward from the stocks or dike swarms, which typically comprise several generations of intermediate to felsic porphyry intrusions. Porphyry Cu ± Au ± Mo deposits are centered on the intrusions, whereas carbonate wall rocks commonly host proximal Cu-Au skarns, less common distal Zn-Pb and/or Au skarns, and, beyond the skarn front, carbonate-replacement Cu and/or Zn-Pb-Ag ± Au deposits, and/or sediment-hosted (distal-disseminated) Au deposits. Peripheral mineralization is less conspicuous in noncarbonate wall rocks but may include base metal- or Au-bearing veins and mantos. High-sulfidation epithermal deposits may occur in lithocaps above porphyry Cu deposits, where massive sulfide lodes tend to develop in deeper feeder structures and Au ± Ag-rich, disseminated deposits within the uppermost 500 m or so. Less commonly, intermediate-sulfidation epithermal mineralization, chiefly veins, may develop on the peripheries of the lithocaps. The alteration-mineralization in the porphyry Cu deposits is zoned upward from barren, early sodic-calcic through potentially ore-grade potassic, chlorite-sericite, and sericitic, to advanced argillic, the last of these constituting the lithocaps, which may attain >1 km in thickness if unaffected by significant erosion. Low sulfidation-state chalcopyrite ± bornite assemblages are characteristic of potassic zones, whereas higher sulfidation-state sulfides are generated progressively upward in concert with temperature decline and the concomitant greater degrees of hydrolytic alteration, culminating in pyrite ± enargite ± covellite in the shallow parts of the litho-caps. The porphyry Cu mineralization occurs in a distinctive sequence of quartz-bearing veinlets as well as in disseminated form in the altered rock between them. Magmatic-hydrothermal breccias may form during porphyry intrusion, with some of them containing high-grade mineralization because of their intrinsic permeability. In contrast, most phreatomagmatic breccias, constituting maar-diatreme systems, are poorly mineralized at both the porphyry Cu and lithocap levels, mainly because many of them formed late in the evolution of systems.

Porphyry Cu systems are initiated by injection of oxidized magma saturated with S- and metal-rich, aqueous fluids from cupolas on the tops of the subjacent parental plutons. The sequence of alteration-mineralization events charted above is principally a consequence of progressive rock and fluid cooling, from >700° to <250°C, caused by solidification of the underlying parental plutons and downward propagation of the lithostatic-hydrostatic transition. Once the plutonic magmas stagnate, the high-temperature, generally two-phase hyper-saline liquid and vapor responsible for the potassic alteration and contained mineralization at depth and early overlying advanced argillic alteration, respectively, gives way, at <350°C, to a single-phase, low- to moderate-salinity liquid that causes the sericite-chlorite and sericitic alteration and associated mineralization. This same liquid also causes mineralization of the peripheral parts of systems, including the overlying lithocaps. The progressive thermal decline of the systems combined with synmineral paleosurface degradation results in the characteristic overprinting (telescoping) and partial to total reconstitution of older by younger alteration-mineralization types. Meteoric water is not required for formation of this alteration-mineralization sequence although its late ingress is commonplace.

Many features of porphyry Cu systems at all scales need to be taken into account during planning and execution of base and precious metal exploration programs in magmatic arc settings. At the regional and district scales, the occurrence of many deposits in belts, within which clusters and alignments are prominent, is a powerful exploration concept once one or more systems are known. At the deposit scale, particularly in the porphyry Cu environment, early-formed features commonly, but by no means always, give rise to the best ore-bodies. Late-stage alteration overprints may cause partial depletion or complete removal of Cu and Au, but metal concentration may also result. Recognition of single ore deposit types, whether economic or not, in porphyry Cu systems may be directly employed in combination with alteration and metal zoning concepts to search for other related deposit types, although not all those permitted by the model are likely to be present in most systems. Erosion level is a cogent control on the deposit types that may be preserved and, by the same token, on those that may be anticipated at depth. The most distal deposit types at all levels of the systems tend to be visually the most subtle, which may result in their being missed due to overshadowing by more prominent alteration-mineralization.

Porphyry Copper Systems

Global cooling in the Cenozoic, which led to the growth of large continental ice sheets in both hemispheres, may have been caused by the uplift of the Tibetan plateau and the positive feedbacks initiated by this event. In particular, tectonically driven increases in chemical weathering may have resulted in a decrease of atmospheric C02 concentration over the past 40 Myr.

Tectonic forcing of late Cenozoic climate

Data drawn from a global compilation of studies quantitatively confirm the long-articulated contention that erosion rates from conventionally plowed agricultural fields average 1–2 orders of magnitude greater than rates of soil production, erosion under native vegetation, and long-term geological erosion. The general equivalence of the latter indicates that, considered globally, hillslope soil production and erosion evolve to balance geologic and climate forcing, whereas conventional plow-based agriculture increases erosion rates enough to prove unsustainable. In contrast to how net soil erosion rates in conventionally plowed fields (≈1 mm/yr) can erode through a typical hillslope soil profile over time scales comparable to the longevity of major civilizations, no-till agriculture produces erosion rates much closer to soil production rates and therefore could provide a foundation for sustainable agriculture.

https://www.pnas.org/content/pnas/104/33/13268.full.pdf

Soil erosion and agricultural sustainability

Humans have fundamentally altered global patterns of biodiversity and ecosystem processes. Surprisingly, existing systems for representing these global patterns, including biome classifications, either ignore humans altogether or simplify human influence into, at most, four categories. Here, we present the first characterization of terrestrial biomes based on global patterns of sustained, direct human interaction with ecosystems. Eighteen “anthropogenic biomes” were identified through empirical analysis of global population, land use, and land cover. More than 75% of Earth's ice-free land showed evidence of alteration as a result of human residence and land use, with less than a quarter remaining as wildlands, supporting just 11% of terrestrial net primary production. Anthropogenic biomes offer a new way forward by acknowledging human influence on global ecosystems and moving us toward models and investigations of the terrestrial biosphere that integrate human and ecological systems.

https://esajournals.onlinelibrary.wiley.com/doi/epdf/10.1890/070062

Putting people in the map: anthropogenic biomes of the world

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.

Spatial distribution of δ18O in meteoric precipitation

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 planet's surface, natural and anthro- pogenic processes serve to modify quite dif- ferent parts of Earth's 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 Earth's 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 Earth's land surface, primarily along passive continental margins; ~83% of cropland erosion occurs over the lower 65% of Earth's 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.

The impact of humans on continental erosion and sedimentation

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.

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

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.

Humans as geologic agents: A deep-time perspective

We report a quantitative analysis of regional differences in the the oxygen isotope composition of river water and precipitation across the USA because data are now available to undertake a more geographically and temporally extensive analysis than was formerly possible. Maps of modern, mean annual δ 18 O values for both precipitation (δ 18 O PPT ) and river water (δ 18 O RIV ) across the 48 contiguous states of the USA have been generated using latitude and elevation as the primary predictors of stable isotope composition while also incorporating regional and local deviations based on available isotopic data. The difference between these two maps was calculated to determine regions where δ 18 O RIV is significantly offset from local δ 18 O PPT . Additional maps depicting seasonal and extreme values for δ 18 O RIV and δ 18 O PPT were also constructed. This exercise confirms the presence of regions characterized by differences in δ 18 O RIV and δ 18 O PPT and specifically identifies the magnitude and regional extent of these offsets. In particular, the Great Plains has δ 18 O RIV values that are more positive than precipitation, while much of the western USA is characterized by significantly lower δ 18 O RIV values in comparison with local δ 18 O PPT . The most salient feature that emerged from this comparison is the 'catchment effect' for the rivers. Because river water is largely derived from precipitation that fell upstream of the sample locality (i.e. at higher elevations) δ 18 O RIV values are often lower than local δ 18 O PPT values, particularly in catchments with high-elevation gradients. Seasonal patterns in the isotopic data substantiate the generally accepted notion that amplitudes of δ 18 O variation are greatly dampened in river water relative to those of local precipitation.

/pdf/spatial-distribution-and-seasonal-variation-in-18o-16o-of-3npc34l1y9.pdf

Bruce H. Wilkinson

Papers

Spatial distribution of δ18O in meteoric precipitation

The impact of humans on continental erosion and sedimentation

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

Humans as geologic agents: A deep-time perspective

Spatial distribution and seasonal variation in 18O/16O of modern precipitation and river water across the conterminous USA