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The impact of forest conversion on soil weathering is studied in a subtropical humid setting in southern Brazil (Rio Grande do Sul). A geochemical tracer of mineral weathering processes, the Ge/Si ratio, was used at the pedon and catchment scales to compare a cropland and a forest catchment. Ge/Si measurements were performed on bedrock, bulk soil, soil pore water and stream waters during base flow and rain events. The Ge/Si ratio in bulk soils is interpreted as the result of a mixing between clay minerals and quartz. Based on the Ge/Si ratio in soil pore water, no change in mineral weathering has been induced by forest conversion at the pedon scale. In contrast, at the catchment scale, the Ge/Si ratio of stream waters indicates an increased contribution from mineral weathering after conversion of forest to cropland. The evolution of Ge/Si ratio in stream waters during rain events points to a change in the hydrological paths due to forest conversion. We argue that forest conversion to cropland led to increased water percolation in soil, allowing the weathering of deeper soil material and thus, a stronger contribution from mineral weathering to stream waters.

This study investigates the influence of water, carbon and energy fluxes on solute production and transport through the Jemez Critical Zone (CZ) and impacts on C-Q relationships over variable spatial and temporal scales. Chemical depletion-enrichment profiles of soils, combined with regolith thickness and groundwater data indicate the importance to stream hydrochemistry of incongruent dissolution of silicate minerals during deep bedrock weathering, which is primarily limited by water fluxes, in this highly fractured, young volcanic terrain. Under high flow conditions (e.g., spring snowmelt), wetting of soil and regolith surfaces and presence of organic acids promote mineral dissolution and provide a constant supply of base cations, Si, and DIC to soil water and groundwater. Mixing of waters from different hydrochemical reservoirs in the near stream environment during ‘wet' periods leads to the chemostatic behavior of DIC, base cations, and Si in stream flow. Metals transported by organic matter complexation (i.e., Ge, Al) and/or colloids (i.e., Al) during periods of soil saturation and lateral connectivity to the stream display a positive relationship with Q. Variable Si-Q relationships, under all but the highest flow conditions, can be explained by non-conservative transport and precipitation of clay minerals, which influences long- versus short-term Si weathering fluxes. By combining measurements of the CZ obtained across different spatial and temporal scales, we were able to constrain weathering processes in different hydrological reservoirs that may be flushed to the stream during hydrologic events, thereby informing C-Q relationships.

Plant uptake and biological cycling processes are commonly the largest flux of nutrients in terrestrial ecosystems. Hydrologic and other losses are offset by inputs from atmospheric deposition and weathering. This multi-tracer study investigates these effects using 87Sr/86Sr, Ca/Sr, and Ge/Si ratios from solid (soil profiles and bedrock), biological (leaves and sap waters), and water (pore-, ground-, stream-waters) samples to study Ca, Sr, Ge, and Si sources and cycling in a forest catchment underlying gray shale in the temperate climate of central Pennsylvania. Leaves and sap waters were found to have similar Ge/Si ratios

The application of multiple isotope proxies on the same location within a Critical Zone (CZ), which we term “CZ-tope”, elucidates the interactions of geochemical, geomorphological, hydrological and biological processes together with anthropogenic influences in the CZ across widely disparate timescales. We exemplify the CZ-tope approach by summarizing the emerging hypotheses developed from isotopic measurements at the Susquehanna Shale Hills CZ Observatory (SSHCZO), Pennsylvania (U.S.A). At SSHCZO, measurements of U-series isotopes and meteoric 10Be in regolith provide evidence that the catchment is approaching a steady state at the ridgetops where regolith production is balanced by erosive loss. Isotopic measurements of δ13C, 87Sr/86Sr, and δ34S in the regolith, bedrock and water, together with 3H, δ2H and δ18O in various water reservoirs (precipitation, soil water, stream water and groundwater) support the hypothesis that nested reaction fronts have developed in the subsurface over timescales of millennia. Combinations of U-series and meteoric 10Be in bedrock and regolith and measurements of soil water δ18O led to the hypothesis that freeze-thaw is the dominant soil creep mechanism controlling regolith fluxes and hillslopes. Utilizing the CZ-tope approach of measuring δ26Mg, δ56Fe and δ11B isotopes on identical samples, we also developed a working hypothesis that particle transport in the subsurface represents a significant weathering loss from the catchment. Likewise, the use of δ18O, Ge/Si and 87Sr/86Sr ratios in xylem source waters (precipitation, soil, stream- and ground-), along with 87Sr/86Sr ratios from soils, led to the hypothesis that O isotope fractionation occurs near clay surfaces. Finally, analyses of 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb and 137Cs in soil from hillslope profiles have also revealed the imprint of widespread human activity. CZ-tope — the interpretation of multiple elements' isotopic ratios quantified for identical samples in one landscape — thus paints an emerging picture of SSHCZO as a relatively fast-eroding but slow-weathering landscape in which: i) nutrients are tightly cycled by vegetation, ii) soils move downslope largely by freeze-thaw, iii) subsurface particle transport is an important flux for mass loss, iv) mobile and immobile reservoirs act to fractionate water and cations into trees and stream water, and v) the imprint of humans is manifested in the metal contents of the topsoil.

Concentrations of dissolved silicon in river waters reflect a complex interplay among chemical weathering of primary silicate minerals, formation and weathering of secondary clay minerals, hydrothermal input and biological cycling (formation and dissolution of opal phytoliths and growth of diatoms). We applied the Ge/Si ratio to assess the different sources of dissolved Si in rivers hailing from the eastern Tibetan Plateau — the Salween, Mekong, Chang Jiang (Yangtze), Hong (Red) and Huang He (Yellow) and from Mt. Baekdu — the Duman. Elevated riverine Ge/Si ratios were observed in arid regions with high geothermal activity in the Salween, Chang Jiang and Mt. Baekdu streams. In the Huang He and Hong River basins geothermal influence was not as pronounced, but weathering of sulfide- and coal-bearing minerals may be responsible for the high Ge/Si ratios. In rivers where inputs from hydrothermal and sulfide weathering are minimal, our data mostly fall in the weathering-limited regime of high riverine Si concentrations and low Ge/Si ratios.

An extensive, northward deepening blanket of Neoproterozoic and Cambrian sedimentary rocks once extended from the Himalayan margin far onto the Indian craton. Cambrian deposits of this “upper Lesser Himalayan” succession, which include deposits of the “outer” Lesser Himalaya tectonic unit, are enriched in radiogenic 187Os. They make up part of a proximal marine facies belt that extends onto the craton and along strike from India to Pakistan. By contrast, age-equivalent facies in the Tethyan Himalaya are more distal in nature. Neoproterozoic to Cambrian strata of the upper Lesser Himalayan succession are now missing in much of the Lesser Himalaya, with their erosion exposing older Precambrian Lesser Himalayan strata. We suggest that exhumation and weathering of the upper Lesser Himalaya and related strata caused dramatic changes in the 187Os/188Os and 87Sr/86Sr Neogene record of seawater starting at ∼ 16 Ma . First-order estimates for the volume of upper Himalayan strata, as well as the volume of all LH rock eroded since this time, and geochemical box modeling, support this idea. Exhumation at 16 Ma is a fundamental event in the evolution of the Himalayan orogeny and the geochemical evolution of the oceans, and will be a critical part of the construction of future models of Himalayan thrust belt evolution.

Germanium–silicon (Ge/Si) ratios were determined on quartz diorite bedrock, saprolite, soil, primary and secondary minerals, phytolith, soil and saprolite pore waters, and spring water and stream waters in an effort to understand Ge/Si fractionation during weathering of quartz diorite in the Rio Icacos watershed, Puerto Rico. The Ge/Si ratio of the bedrock is 2 μmol/mol, with individual primary mineral phases ranging between 0.5 and 7 μmol/mol. The ratios in the bulk saprolite are higher (∼3 μmol/mol) than values measured in the bedrock. The major saprolite secondary mineral, kaolinite, has Ge/Si ratios ranging between 4.8 and 6.1 μmol/mol. The high Ge/Si ratios in the saprolite are consistent with preferential incorporation of Ge during the precipitation of kaolinite. Bulk shallow soils have lower ratios (1.1–1.6 μmol/mol) primarily due to the residual accumulation of Ge-poor quartz. Ge/Si ratios measured on saprolite and soil pore waters reflect reactions that take place during mineral transformations at discrete depths. Spring water and baseflow stream waters have the lowest Ge/Si ratios (0.27–0.47 μmol/mol), reflecting deep initial weathering reactions resulting in the precipitation of Ge-enriched kaolinite at the saprolite–bedrock interface. Mass-balance calculations on saprolite require significant loss of Si and Al even within 1 m above the saprolite–bedrock interface. Higher pore water Ge/Si ratios (∼1.2 μmol/mol) are consistent with partial dissolution of this Ge-enriched kaolinite. Pore water Ge/Si ratios increase up through the saprolite and into the overlying soil, but never reach the high values predicted by mass balance, perhaps reflecting the influence of phytolith recycling in the shallow soil.

We investigated Ca and Sr cycling in a humid tropical forest by analyzing Ca/Sr ratios and 87Sr/86Sr ratios in soil minerals, soil exchangeable cations, soil porewater, and plant roots, wood and leaves, and calculating the relative contributions of Sr from atmospheric inputs and weathering of local bedrock. An unexpectedly large contribution of bedrock-derived Sr and presumably Ca is cycled through the vegetation, reflecting the important role of geological processes in controlling the cycling of base cation nutrients even in a system with intensely weathered soil. This is surprising because over 99% of the Ca and Sr that was originally in the bedrock is leached out of the soil and saprolite during early stages of weathering at this site, and because there are large atmospheric inputs to the site of both sea salt and Saharan dust. Substantial differences in Ca and Sr cycling are seen on small spatial scales between a ridgetop and an adjacent steep hillslope site. Measured Ca/Sr ratios reflect fractionation between these elements during biogeochemical cycling. Fractionation was particularly evident between wood and foliar tissue, but fractionation during soil exchange processes is also likely. In comparing the Ca/Sr ratios of plants, exchangeable cations, and bulk soils, we found that foliar Ca/Sr ratios were greater than exchangeable cation Ca/Sr ratios, which in turn were greater than soil Ca/Sr ratios, similar to patterns observed at other highly weathered tropical sites.

The Brahmaputra River of South Asia is the fourth largest river in the world in terms of annual discharge. The lower Brahmaputra River basin is susceptible to catastrophic flooding with major social, economic, and public health impacts. There is relatively little rainfall and snowpack information for the watershed, and the system is poorly understood hydrologically. Using a combination of available remotely sensed and gauge data, this study analyzes snow cover, rainfall, and monsoon period discharge for a 14-yr time period (1986–99). It is found that interannual rainfall variability is low and is a weak predictor of monsoon discharge volumes. Strong evidence is found, however, that maximum spring snow cover in the upper Brahmaputra basin is a good predictor of the monsoon flood volume. Despite the temporal and spatial limitations of the data, this study’s analysis demonstrates the potential for developing an empirical tool for predicting large flood events that may allow an annual early window for mitigating flood damages in the lower Brahmaputra basin, home to 300 million people.

Soils are mixtures of material derived from substrate weathering, plant decomposition, and solute and particulate deposition from the atmosphere. The relative contribution from each source varies widely among soil types and environments. Atmospheric deposition of marine and mineral aerosols can have a major impact on the geochemistry and biogeochemistry of the Critical Zone. Some of the best-studied examples are from ocean islands because of the strong geochemical contrast between bedrock and atmospheric sources, but for the most part continental areas are more severely impacted by atmospheric deposition. With dust flux greater than 10% of the global river sediment flux, deposition from the atmosphere plays an important role in the biogeochemistry of soils worldwide.

Refractory trace element concentrations in strongly weathered Hawaiian soils ranging in age from 20 to 4100 ka are highly elevated over parent-rock values due to extensive mass loss of more soluble major elements during pedogenesis. Nb and Ta exhibit virtually no mobility. Soil Nb/Ta ratios are within the range of fresh bedrock even when soil Nb concentrations are residually enriched by a factor of 10. In contrast, Al, Zr, and Hf are depleted relative to Nb in surface soil horizons but are enriched at depth, clearly indicating mobility of these elements. Variations in Th/Nb ratios in soil profiles indicate significant Th mobility within the soil column. However, mass-balance calculations require that accretion of Th-enriched Asian dust has resulted in a net increase in Th in some soils. Soils developed on a 150 ka rainfall gradient show that the mobility and loss of Zr increase with mean annual precipitation.

Modern river studies support the generalization that surface waters in regions undergoing extensive chemical weathering have elevated dissolved germanium/silicon (Ge/Si) ratios compared to regions with less extensive chemical weathering, thought to be the result of Ge fractionation in 2:1 clays. Temporal variations in Ge/Si observed in marine diatoms have thus been linked to past global trends in terrestrial weathering. However, this relationship has not been adequately ground-truthed by a terrestrial-based paleoclimate study utilizing Ge/Si ratios in lake diatoms. To this end, a sediment core was extracted from Dry Lake (el. 2763 m) in the headwaters of the Santa Ana River of southern California in July 1996. The Dry Lake drainage basin is comprised of biotite–muscovite gneiss and granite, with sparse pine forests and relatively high relief. The core had a basal AMS 14C age of 8,350±60 ybp. We successfully separated diatom samples large enough for cleaning, dissolution and chemical analysis without contamination by detrital materials (confirmed by trace metal analyses). Two dissolved opal samples were analyzed for Ge and Si concentrations along with modern water samples collected from waters within the drainage basin. Diatoms obtained from 8000-year-old sediments near the bottom of the core had an opal Ge/Si of 0.79×10−6 (mol/mol). A composite sample of diatoms deposited within the past 100 years yielded a significantly lower Ge/Si of 0.34×10−6. Analysis of Ge/Si was also performed from a variety of other materials in the Dry Lake basin, including stream and lake water, unweathered bedrock, soils, and mineral separates. Together with sedimentologic records from the lake sediments and Ge/Si recorded in other basin materials, it appears that the high Ge/Si values recorded in the 8000-year opal sample were the result of preferential early weathering of high Ge/Si biotite and muscovite minerals from the slopes. Thus, the detailed examination of Ge/Si cycling in this isolated basin indicates that factors other than clay mineral transformations may drive Ge/Si paleorecords in some settings.

We used isotopes of Sr to quantify weathering versus atmospheric sources of foliar Sr in 34 Hawaiian forests on young volcanic soils. The forests varied widely in climate, and in lava flow age and texture. Weathering supplied most of the Sr in most of the sites, but atmospheric deposition contributed 30-50% of foliar Sr in the wettest rainforests. A stepwise multiple regression using annual precipitation, distance from the ocean, and texture of the underlying lava explained 76% of the variation in Sr isotope ratios across the sites. Substrate age did not contribute significantly to variation in Sr isotope ratios in the range of ages evaluated here (11-3000 years), although atmospheric sources eventually dominate pools of biologically available Sr in Hawaiian rainforests in older substrates (≥150,000 years).

We report 87Sr86Sr measurements on a suite of well preserved sedimentary carbonates from Lower Cambrian strata of the Lena River region of Siberia. Stable isotopes and major and trace element chemistry have been used to identify potentially unaltered samples for Sr isotopic measurements. The Sr data define a smooth curve of paleoseawater 87Sr86Sr values from the Tommotian through to the early Middle Cambrian. During the Tommotian-Atdabanian interval, 87Sr86Sr rose rapidly from 0.7081 to 0.7085. The rate of change in Sr ratios decreased during the Botomian but rose to 0.7088 in the late Toyonian to early Middle Cambrian. The rate of 87Sr86Sr increase during the Tommotian-Atdabanian was ca. 0.0001/m.y., comparable to the late Miocene change in seawater Sr. We infer that an interval of enhanced erosion during the ‘Cambrian explosion’ was responsible for this increase. An important source for radiogenic Sr to the oceans may have been erosion of the Pan-African orogenic belt of southern Africa. The rapid change in paleoseawater Sr corresponds with an interval of highly variable marine δ13C values. Model results for the Sr and C isotopic records suggest that the quasi-periodicity in the δ13C record is not a consequence of direct erosional forcing. However, our inference of high erosion rates during the Tommotian-Atdabanian implies enhanced fluxes of nutrient elements such as P to the oceans. Phosphorite deposits and black shale deposition in coeval strata suggest that periods of high marine productivity and anoxia may be in part related to enhanced river dissolved fluxes. Our results thus provide some insight into environmental conditions during the ‘Cambrian explosion.’

We investigated rates of chemical weathering of volcanic and ophiolitic rocks on Luzon Island, the Philippines. Luzon has a tropical climate and is volcanically and tectonically very active, all factors that should enhance chemical weathering. Seventy-five rivers and streams (10 draining ophiolites, 65 draining volcanic bedrock) and two volcanic hot springs were sampled and analyzed for major elements, alkalinity and 87Sr/86Sr. Cationic fluxes from the volcanic basins are dominated by Ca2+ and Mg2+ and dissolved silica concentrations are high (500–1900 μM). Silica concentrations in streams draining ophiolites are lower (400–900 μM), and the cationic charge is mostly Mg2+. The areally weighted average CO2 export flux from our study area is 3.89 ± 0.21 × 106 mol/km2/yr, or 5.99 ± 0.64 × 106 mol/km2/yr from ophiolites and 3.58 ± 0.23 × 106 mol/km2/yr from volcanic areas (uncertainty given as ±1 standard error, s.e.). This is ∼6–10 times higher than the current best estimate of areally averaged global CO2 export by basalt chemical weathering and ∼2–3 times higher than the current best estimate of CO2 export by basalt chemical weathering in the tropics. Extrapolating our findings to all tropical arcs, we estimate that around one tenth of all atmospheric carbon exported via silicate weathering to the oceans annually is processed in these environments, which amount to ∼1% of the global exorheic drainage area. Chemical weathering of volcanic terranes in the tropics appears to make a disproportionately large impact on the long-term carbon cycle.

We present a new high-resolution 1-D intermediate-complexity box model (ICBM) of ocean biogeochemical processes for paleoceanographic applications. The model contains 79 reservoirs in three regions that should be generally applicable throughout much of Earth history: (1) a stratified gyre region, (2) a high-latitude convective region, and (3) an upwelling region analogous to those found associated with eastern boundary currents. Transport processes are modeled as exchange fluxes between boxes and by eddy diffusion terms. Significant improvement in the representation of middepth oxygen budgets was achieved by implementing nonlocal mixing between the high-latitude surface and gyre thermocline reservoirs. The biogeochemical submodel simulates coupled C, N, P, O, and S systematics with explicit representation of microbial populations, using a process-based approach. Primary production follows Redfield stoichiometry, while water column remineralization is depth- and redox couple–dependent. Settling particulate organic matter is incorporated into a benthic submodel that accounts for burial and remineralization. The C/P ratio of burial depends on bottom water oxygen. Denitrification takes place both by classical and anammox pathways. The ICBM was tested against modern oceanographic observations from the Global Ocean Data Analysis Project, Joint Global Ocean Flux Study, and other databases. Comparisons of model output with circulation tracers including θ, salinity, CFC-12, and radiocarbon permit a test of the physical exchange scheme. Vertical profiles of biogeochemically reactive components in each of the three regions are in good agreement with observations. Under modern conditions the upwelling zone displays a pronounced oxygen minimum zone and water column denitrification, while these are not present in the high-latitude or gyre regions. Model-generated global fluxes also compare well to independent estimates of primary production, burial, and phosphorous and nitrogen cycling. The ICBM appears to adequately simulate the long-term (kyr) evolution of several biogeochemical cycles and improves on previous box models in several important ways. In a companion paper, the model's performance under euxinic conditions is tested against modern Black Sea data. The simple and adaptable structure of the model should make it applicable to a wide range of paleoceanographic problems. The model source code is available in MATLABTM 7 m-files provided as auxiliary material.

[1] Numerous hot springs flow within the steeply incised gorges of the central Nepal Himalayan front. The spring fluids have total dissolved solids (TDS) up to 7000 mg/L and Na+, and K+ typically comprise >50% of the cationic charge, indicating that high-temperature silicate alteration is the dominant source of hot spring alkalinity. HCO3− is normally the dominant anion. Sr isotope ratios from the hydrothermal fluids are similar to the range of values found in the host rocks and imply significant fluid-rock interaction with local lithologies. To determine the impact of the hydrothermal solute load on the local and regional river chemistry, we use a chemical mass balance approach to quantify the hot spring discharge. The springs are ubiquitously enriched in germanium (Ge) with high but variable Ge/Si. Himalayan rivers upstream of the hot spring zones have Ge/Si systematics like other unpolluted rivers, but downstream they are highly anomalous, with Ge/Si from 2 to 20 μmol/mol. Ge and Si appear to behave conservatively during mixing of spring and river, and the large disparity between river and spring [Ge] and Ge/Si ratios makes germanium an effective tracer of hot spring input. We use the Ge/Si mass balance to estimate the spring flux to individual river systems. Our results show that the premonsoon spring flow over the entire Narayani basin is about 2 m3/s (with a factor of 2 uncertainty), or 0.5% of the total Narayani river discharge. We estimate that the springs provide 25 (±15)% of the silicate-derived alkalinity to the Narayani system during the low-flow season from October to May. Available monsoon season data indicate that the spring flux increases during the monsoon by a factor of 2–3, but this increased flow is diluted by the up to 10× increase in overall river flow. The annual river discharge-weighted mean spring flux is 3.0 ± 1.2 m3/s for the Narayani; hydrothermal alteration contributes ∼10% of the annual flux of silicate alkalinity to this large river system.

[1] Glacial to interglacial variation in the germanium to silicon ratio (Ge/Si) of biogenic opal in oceanic sediment cores has been proposed to record terrestrial silicate weathering intensity. However, data connecting Ge/Si ratios to weathering are sparse, especially for cold regions. To characterize Ge/Si of the dissolved loads of rivers draining modern glacial and periglacial regions, we measured Ge and major element chemistry of more than 30 streams in the Copper River Basin of south Alaska. We measured significantly higher Ge/Si in rivers with greater than 15% glacial cover in their basins than in rivers draining areas with less glacial cover, which we attribute to preferencial subglacial weathering of biotite. We also ran laboratory experiments to investigate the role of differential sorption of Ge and Si on hydrous iron oxides in moderating the Ge/Si ratio of rivers in recently deglaciated basins. These experiments indicate that Ge is preferentially adsorbed over Si by hydrous iron oxides over a wide range of pH values and support differential sorption as a potentially significant fractionation process. Our measured high Ge/Si in glaciated basins is in striking contrast to the low Ge/Si in the oceanic record during glacial periods. The glacial to interglacial changes in marine Ge/Si cannot simply reflect changes in glacier cover on land, as the mineralogy of silicates weathered and processes partitioning Ge in soils must also strongly influence the fluvial Ge and Si inputs to the ocean.

UPLIFT of the Tibetan plateau about 7–8 Myr ago1,2 may have been responsible3–5 for the apparent intensification of the Asian monsoon 6–10 around that time. Increases in the oceanic 87Sr/86Sr ratio during the Neogene period have been attributed11,12 to increased erosion from the Himalayan orogen. It has been suggested that the monsoonal intensification may have enhanced overall erosion rates in the region5,13. If this were the case, sediment accumulation rates would have increased in the surrounding basins at this time. Here we present a reanalysis of stratigraphic data from the Indo-Gangetic foreland and the Bengal fan, which demonstrates that both of these basins experienced a decline in sediment-accumulation rates 8 Myr ago. Thus it seems that monsoonal intensification was accompanied by a decrease in mechanical weathering. This decrease could be due to reduced tectonic activity, decreased Himalayan glaciation or slope stabilization from dense plant cover.

At the Earth's surface, a complex suite of chemical, biological, and physical processes combines to create the engine that transforms bedrock into soil (Figure 1). Earth's weathering engine provides nutrients to nourish ecosystems and human society mediates the transport of toxic components within the biosphere, creates water flow paths that carve and weaken bedrock, and contributes to the evolution of landscapes at all temporal and spatial scales. At the longest time scales, the weathering engine sequesters CO2, thereby influencing long-term climate change. Despite the importance of soil, our knowledge of the rate of soil formation is limited because the weathering zone forms a complex, ever-changing interface, and because scientific approaches and funding paradigms have not promoted integrated research agendas to investigate such complex interactions. No national initiative has promoted a systems approach to investigation of weathering science across the broad array of geology, soil science, ecology and hydrology. Such a program is certainly needed, and this article describes a platform on which to build the initiative to answer the following question: How does the Earth weathering engine break down rock to nourish ecosystems, carve errestrial landscapes, and control carbon dioxide in the global atmosphere?

87 Sr/ 86 Sr evidence from a soil chronosequence in the Hawaiian Islands demonstrates that the atmosphere supplies >85% of putatively rock-derived Sr in older sites. Initially, bedrock is the dominant source for Sr and other lithophile elements such as Ca, but high rates of weathering and leaching of the substrate by 20 ka lead to a shift to atmospheric sources. The loss of weathering inputs coincides with other physio-chemical changes in the soil and results in a steep decline of base cations in the soil pool. While these patterns imply the potential for limitation of biological productivity by low base cation supply, the atmosphere provides a supply of base cations in excess of nutritional needs, even after nearly all rock-derived base cations have been leached from the soil. This raises the possibility that P limitation in terrestrial ecosystems may develop at least as much because of low rates of atmospheric deposition of P (relative to Ca, K, and other rock-derived elements) as because of its chemical interaction in soil.

New high-resolution δ 13 C data through the Lower Cambrian of Siberia reveal multiple, positive excursions coincident with phases in the "explosion" of invertebrate phyla (Nemakit-Daldynian to middle Botomian stages). Comparison of the δ 18 O and trace element (Mg, Fe, Mu, Sr) chemistry indicate that six new δ 13 C cycles are primary rather than diagenetic features, with potential for global correlation. Positive δ 13 C excursions up to +3‰ indicate that fractional organic carbon burial rates were high but variable through the Cambrian explosion. Values for δ 13 C dropped sharply from around +2.2‰ to -1.6‰ in Botomian time, coincident with mass extinction of the archaeocyathan reef biota. The rapid fluctuation of the δ 13 C signal and the temporal coincidence of the pronounced negative shift with the extinction event hint that the δ 13 C record may record productivity variations during the Early Cambrian radiation.

The homeostatic balance of Earth's long-term carbon cycle and the equable state of Earth's climate are maintained by negative feedbacks between the levels of atmospheric CO 2 and the chemical weathering rate of silicate rocks. Though clearly demonstrated by well-controlled laboratory dissolution experiments, the temperature dependence of silicate weathering rates, hypothesized to play a central role in these weathering feedbacks, has been difficult to quantify clearly in natural settings at landscape scale. By compiling data from basaltic catchments worldwide and considering only inactive volcanic fields (IVFs), here we show that the rate of CO 2 consumption associated with the weathering of basaltic rocks is strongly correlated with mean annual temperature (MAT) as predicted by chemical kinetics. Relations between temperature and CO 2 consumption rate for active volcanic fields (AVFs) are complicated by other factors such as eruption age, hydrothermal activity, and hydrological complexities. On the basis of this updated data compilation we are not able to distinguish whether or not there is a significant runoff control on basalt weathering rates. Nonetheless, the simple temperature control as observed in this global dataset implies that basalt weathering could be an effective mechanism for Earth to modulate long-term carbon cycle perturbations.