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Trees, the most successful biological power plants on earth, build and plumb the critical zone (CZ) in ways that we do not yet understand. To encourage exploration of the character and implications of interactions between trees and soil in the CZ, we propose nine hypotheses that can be tested at diverse settings. The hypotheses are roughly divided into those about the architecture (building) and those about the water (plumbing) in the CZ, but the two functions are intertwined. Depending upon one's disciplinary background, many of the nine hypotheses listed below may appear obviously true or obviously false. (1) Tree roots can only physically penetrate and biogeochemically comminute the immobile substrate underlying mobile soil where that underlying substrate is fractured or pre-weathered. (2) In settings where the thickness of weathered material, H, is large, trees primarily shape the CZ through biogeochemical reactions within the rooting zone. (3) In forested uplands, the thickness of mobile soil, h, can evolve toward a steady state because of feedbacks related to root disruption and tree throw. (4) In settings where h ≪ H and the rates of uplift and erosion are low, the uptake of phosphorus into trees is buffered by the fine-grained fraction of the soil, and the ultimate source of this phosphorus is dust. (5) In settings of limited water availability, trees maintain the highest length density of functional roots at depths where water can be extracted over most of the growing season with the least amount of energy expenditure. (6) Trees grow the majority of their roots in the zone where the most growth-limiting resource is abundant, but they also grow roots at other depths to forage for other resources and to hydraulically redistribute those resources to depths where they can be taken up more efficiently. (7) Trees rely on matrix water in the unsaturated zone that at times may have an isotopic composition distinct from the gravity-drained water that transits from the hillslope to groundwater and streamflow. (8) Mycorrhizal fungi can use matrix water directly, but trees can only use this water by accessing it indirectly through the fungi. (9) Even trees growing well above the valley floor of a catchment can directly affect stream chemistry where changes in permeability near the rooting zone promote intermittent zones of water saturation and downslope flow of water to the stream. By testing these nine hypotheses, we will generate important new cross-disciplinary insights that advance CZ science.

Studies in the leeward Kohala field system on Hawai‘i Island have considered the processes and timing of agricultural development associated with sociopolitical transformations and the production of agricultural surpluses. Using extensive soil sampling, we explore the use of relatively mobile and immobile soil parameters within the agricultural landscape to interpret the development and maintenance of the system over time. Results show that in the context of the leeward Kohala field system, the immobile element niobium can be used to interpret the location of ancient ground surfaces and that phosphorus is stable enough to be used to understand anthropogenic influences. In contrast, total and exchangeable calcium are too mobile to indicate purely anthropogenic processes. Data suggest that there was more intensive depletion of soil phosphorus on agricultural alignments than in field areas between the alignments, potentially explained by traditional cropping methods associated with the system. Spatial analyses within the field system identifies an area with high naturally occurring soil nutrient levels.

PUBLICATIONS Journal of Geophysical Research: Earth Surface RESEARCH ARTICLE 10.1002/2016JF003864 Key Points: • Soil chemistry and weathering vary nonlinearly across a large rainfall gradient (400–4700 mm/yr) in NZ • Climate control evidenced in detailed soil chemistry, Fe and Al mobilities, and cation leaching • Moisture availability can act as a “switch” to enable rapid chemical weathering in young soils Supporting Information: • Supporting Information S1 • Data Set S1 Correspondence to: J. L. Dixon, jean.dixon@montana.edu Citation: Dixon, J. L., O. A. Chadwick, and P. M. Vitousek (2016), Climate-driven thresholds for chemical weathering in postglacial soils of New Zealand, J. Geophys. Res. Earth Surf., 121, 1619–1634, doi:10.1002/2016JF003864. Received 18 FEB 2016 Accepted 12 AUG 2016 Accepted article online 17 AUG 2016 Published online 16 SEP 2016 Climate-driven thresholds for chemical weathering in postglacial soils of New Zealand Jean L. Dixon 1,2 , Oliver A. Chadwick 2 , and Peter M. Vitousek 3 Department of Earth Sciences and the Institute on Ecosystems, Montana State University, Bozeman, Montana, USA, Department of Geography, University of California, Santa Barbara, California, USA, 3 Department of Biology, Stanford University, Stanford, California, USA Abstract Chemical weathering in soils dissolves and alters minerals, mobilizes metals, liberates nutrients to terrestrial and aquatic ecosystems, and may modulate Earth’s climate over geologic time scales. Climate-weathering relationships are often considered fundamental controls on the evolution of Earth’s surface and biogeochemical cycles. However, surprisingly little consensus has emerged on if and how climate controls chemical weathering, and models and data from published literature often give contrasting correlations and predictions for how weathering rates and climate variables such as temperature or moisture are related. Here we combine insights gained from the different approaches, methods, and theory of the soil science, biogeochemistry, and geomorphology communities to tackle the fundamental question of how rainfall influences soil chemical properties. We explore climate-driven variations in weathering and soil development in young, postglacial soils of New Zealand, measuring soil elemental geochemistry along a large precipitation gradient (400–4700 mm/yr) across the Waitaki basin on Te Waipounamu, the South Island. Our data show a strong climate imprint on chemical weathering in these young soils. This climate control is evidenced by rapid nonlinear changes along the gradient in total and exchangeable cations in soils and in the increased movement and redistribution of metals with rainfall. The nonlinear behavior provides insight into why climate-weathering relationships may be elusive in some landscapes. These weathering thresholds also have significant implications for how climate may influence landscape evolution and the release of rock-derived nutrients to ecosystems, as landscapes that transition to wetter climates across this threshold may weather and deplete rapidly. 1. Introduction 1.1. Climate’s Elusive Control on Chemical Weathering Soils lie at the interface of air, water, life, and rock, and the weathering dynamics that transform minerals and water in soils are shaped by diverse processes. Climate has long been recognized to be one of the major dri- vers of these weathering processes [Jenny, 1941]. Temperature controls the kinetics of chemical reactions, and water has a role in nearly every chemical weathering reaction that directly results in mass loss from a rock or mineral. Therefore, warmer and wetter conditions should lead to higher weathering rate and intensity in soils. However, a coherent understanding of how climate controls soil chemical weathering remains elusive. Several reasons emerge for the lack of consensus across studies. 1.2. Competing Variables ©2016. The Authors. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distri- bution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. DIXON ET AL. First, while the conceptual framework for climate control on weathering rates is validated by laboratory experiments quantifying dissolution rates under different temperature or water-flow conditions [White et al., 1999; White and Brantley, 2003], field-based studies reveal significant complexity among climate- weathering linkages [Brantley, 2003; Drever et al., 1994; White and Brantley, 2003], which may be explained by time-dependent factors such as changing mineral surface area, pore water concentrations, and secondary precipitates. Similarly, climate’s control on soil weathering can be modified by the complex influence of other competing variables such as lithology, erosion rates, and/or dust deposition [e.g., Ferrier et al., 2012; Riebe et al., 2004]. Furthermore, field-based weathering rates are often measured in locations where multiple variables (including climate variables such as temperature and water availability) exert competing controls on mineral weathering and the fate of released ions [Chadwick and Chorover, 2001]. These competing climatic controls may be deconvolved using careful sampling designs and accounting for multiple variables [e.g., Dixon et al., 2009a; Rasmussen et al., 2011; White and Blum, 1995]; however, derived relationships and models may be site specific or have limited applicability. CLIMATE-DRIVEN WEATHERING THRESHOLDS

Nutrient biolifting is an important pedogenic process in which plant roots obtain inorganic nutrients such as phosphorus (P) and calcium (Ca) from minerals at depth and concentrate those nutrients at the surface. Here we use soil chemistry and stable isotopes of the alkaline earth elements Ca, strontium (Sr) and barium (Ba) to test the hypothesis that biolifting of P has been an important pedogenic process across a soil climosequence developed on volcanic deposits at Kohala Mountain, Hawaii. The geochemical linkage between these elements is revealed as generally positive site-specific relationships in soil mass gains and losses, particularly for P, Ba and Ca, using the ratio of immobile elements titanium and niobium (Ti/Nb) to link individual soil samples to a restricted compositional range of the chemically and isotopically diverse volcanic parent materials. At sites where P is enriched in surface soils relative to abundances in deeper soils, the isotope compositions of exchangeable Ca, Sr and Ba in the shallowest soil horizons ( 10 cm depth) at those sites is consistently heavier than the volcanic parent materials. The isotope compositions of exchangeable Ca and Sr trend toward heavier compositions with depth more gradually, reflecting increasing leakiness from these soils in the order Ba

Organic matter–mineral associations stabilize much of the carbon (C) stored globally in soils. Metastable short-range-order (SRO) minerals such as allophane and ferrihydrite provide one mechanism for long-term stabilization of organic matter in young soil. However, in soils with few SRO minerals and a predominance of crystalline aluminosilicate or Fe (and Al) oxyhydroxide, C turnover should be governed by chemisorption with those minerals. Here, we correlate mineral composition from soils containing small amounts of SRO minerals with mean turnover time (TT) of C estimated from radiocarbon (14C) in bulk soil, free light fraction and mineral-associated organic matter. We varied the mineral amount and composition by sampling ancient soils formed on different lithologies in arid to subhumid climates in Kruger National Park (KNP), South Africa. Mineral contents in bulk soils were assessed using chemical extractions to quantify Fe oxyhydroxides and SRO minerals. Because of our interest in the role of silicate clay mineralogy, particularly smectite (2 : 1) and kaolinite (1 : 1), we separately quantified the mineralogy of the clay-sized fraction using X-ray diffraction (XRD) and measured 14C on the same fraction. Density separation demonstrated that mineral associated C accounted for 40–70 % of bulk soil organic C in A and B1 horizons for granite, nephelinite and arid-zone gabbro soils, and > 80 % in other soils. Organic matter strongly associated with the isolated clay-sized fraction represented only 9–47 % of the bulk soil C. The mean TT of C strongly associated with the clay-sized fraction increased with the amount of smectite (2 : 1 clays); in samples with > 40 % smectite it averaged 1020 ± 460 years. The C not strongly associated with clay-sized minerals, including a combination of low-density C, the C associated with minerals of sizes between 2 µm and 2 cm (including Fe oxyhydroxides as coatings), and C removed from clay-sized material by 2 % hydrogen peroxide had TTs averaging 190 ± 190 years in surface horizons. Summed over the bulk soil profile, we found that smectite content correlated with the mean TT of bulk soil C across varied lithologies. The SRO mineral content in KNP soils was generally very low, except for the soils developed on gabbros under more humid climate that also had very high Fe and C contents with a surprisingly short, mean C TTs. In younger landscapes, SRO minerals are metastable and sequester C for long timescales. We hypothesize that in the KNP, SRO minerals represent a transient stage of mineral evolution and therefore lock up C for a shorter time. Overall, we found crystalline Fe-oxyhydroxides (determined as the difference between Fe in dithionate citrate and oxalate extractions) to be the strongest predictor for soil C content, while the mean TT of soil C was best predicted from the amount of smectite, which was also related to more easily measured bulk properties such as cation exchange capacity or pH. Combined with previous research on C turnover times in 2 : 1 vs. 1 : 1 clays, our results hold promise for predicting C inventory and persistence based on intrinsic timescales of specific carbon–mineral interactions.

Plants are known to scavenge nutrients such as phosphorus (P) from rock subtrates and concentrate those nutrients at the surface in order to satisfy long-term nutritional requirements. Using the stable isotope systematics of calcium (Ca), strontium (Sr) and barium (Ba) as proxy tracers, here we test the hypothesis that soils along a Hawaiian climosequence have “biolifted” P and other nutrients such as Ca from depth and concentrated those nutrients at the surface. Relative to isotope compositions in the volcanic soil parent materials, exchangeable Ca, Sr and Ba in the shallowest soils at sites having evidence for P biolifting are light confirming the role for plant uptake of these elements while exchangeable Ca, Sr and Ba of deeper soils at those sites are similar or heavy suggesting a complementary pool. In this pedogenic system Ca and Sr are more susceptible to leaching than Ba, thus Ba isotopes provide the most resilient evidence for biolifting across the climate gradient and a useful proxy for P biodynamics.

Pedogenic thresholds, where multiple soil properties vary substantially and coherently in a narrow portion of a broad environmental gradient, are well-described on basaltic soils in Hawaii. One such threshold occurs along climate gradients where primary minerals virtually disappear, base saturation decreases sharply, and aluminum is mobilized within a narrow range of increasing rainfall. A recent study that evaluated thresholds along a climate gradient of non-basalt-derived soils on the South Island of New Zealand found that while base saturation declined steeply in a narrow range of rainfall on that gradient, the change was not coherent across soil properties; a substantial fraction of the Ca present in primary minerals (40–60 %) remained through the highest-rainfall sites ((Dixon et al. in J Geophys Res, doi: 10.1002/2016JF003864, 2016). We developed a simple model to explore potential mechanisms driving differences between basalt-derived and non-basalt soils. Incorporating a broader spectrum of mineral weathering rates (including some primary minerals that are highly recalcitrant to weathering) into simulated non-basalt than simulated basalt-derived soils (and accounting for the lower rates of evapotranspiration in New Zealand) was sufficient to simulate observed differences between these substrates. Further, we used the simple model to evaluate the consequences of rainfall variation in the short- (time step to time step) and long-term (a change in rainfall after 50,000 time steps). Results of these analyses demonstrated that year-to-year variation in rainfall could play an important role in controlling changes in the position of the pedogenic threshold during soil development.

Using field observations and geochemical and digital terrain analyses, we describe the structure and thickness of the regolith across a climosequence on the island of Hawai'i to gain insight into the relative roles of precipitation and the near-surface hydrologic structure in determining weathering patterns. In the wet portion of the climosequence, where the long-term water balance is positive, the regolith thickness reaches an observed maximum of ~40m and appears limited by the geomorphic base-level of the landscape. However, even within this thick regolith, distinct units of varying weathering intensity occur; the vertical ordering of which largely reflects differences in the initial permeability structure of the basalt flows rather than a systematic decrease in weathering intensity downwards from the ground surface. In the dry portion of the climosequence, where the long-term water balance is negative, the regolith thickness is confined to ~1m, is highly dependent on the inferred permeability structure of the basalt flows, and is independent of geomorphic base-level. Weathering intensity also varies according to permeability structure and decreases in this thin regolith with distance beneath the ground surface. The abrupt change in regolith depth and character that coincides with the transition from net-positive to net-negative long-term water balance implies that small changes in precipitation rates around a neutral water balance result in large changes in the distribution and depth of weathering. Together our observations indicate that the distribution and depth of weathering in basalts (and probably other lithologies) might be best understood by considering how precipitation interacts with the complicated near-surface permeability structure over regolith-forming timescales to weather rock in the vadose zone. © 2013 John Wiley & Sons, Ltd.

The spacing of hills and valleys reflects the competition between disturbance-driven (or diffusive) transport on hillslopes and concentrative (or advective) transport in valleys, although the underlying lithologic, tectonic, and climatic controls have not been untangled. Here, we measure geochemical and geomorphic properties of catchments in Kruger National Park, South Africa, where granitic lithology and erosion rates are invariant, enabling us to evaluate how varying mean annual precipitation(MAP = 470 mm, 550 mm, and 730 mm) impacts hill-valley spacing or landscape dissection. Catchment-averaged erosion rates, based on 10Be concentrations in river sands, are low (3-6 m/m.y.) and vary minimally across the three sites. Our lidar-derived slope-area analyses reveal that hillslopes in the dry site are gentle (3%) and short, such that the terrain is low relief and appears highly dissected. With increasing rainfall, hillslopes lengthen and increase in gradient (6%-8%), resulting in less-dissected, higher-relief catchments. The chemical depletion fraction of hilltop regoliths increases with rainfall, from 0.3 to 0.7, reflecting a climate-driven increase in chemical relative to physical erosion. Soil catenas also vary systematically with climate as we observe relatively uniform soil properties in the dry site that contrast with leached sandy crests and upper slopes coupled with downslope clay accumulation zones in the intermediate and wet sites. The geomorphic texture of this slow-eroding, granitic landscape appears to be set by climate-driven feedbacks among chemical weathering, regolith fabric differentiation, hydrological routing, and sediment transport that enhance the vigor of hillslope sediment transport relative to valley-forming processes for wetter climates. © 2013 Geological Society of America.

A catena is a series of distinct but co-evolving soils arrayed along a slope. On low-slope, slowly eroding catenas the redistribution of mass occurs predominantly as plasma, the dissolved and suspended constituents in soil water. We applied mass balance methods to track how redistribution via plasma contributed to physical and geochemical differentiation of nine slowly eroding (~ 5 mm ky − 1 ) granitic catenas. The catenas were arrayed in a 3 × 3 climate by relief matrix and located in Kruger National Park, South Africa. Most of the catenas contained at least one illuviated soil profile that had undergone more volumetric expansion and less mass loss, and these soils were located in the lower halves of the slopes. By comparison, the majority of slope positions were eluviated. Soils from the wetter climates (550 and 730 mm precipitation yr − 1 ) generally had undergone greater collapse and lost more mass, while soils in the drier climate (470 mm yr − 1 ) had undergone expansion and lost less mass. Effects of differences in catena relief were less clear. Within each climate zone, soil horizon mass loss and strain were correlated, as were losses of most major elements, illustrating the predominant influence of primary mineral weathering. Nevertheless, mass loss and volumetric collapse did not become extreme because of the skeleton of resistant primary mineral grains inherited from the granite. Colloidal clay redistribution, as traced by the ratio of Ti to Zr in soil, suggested clay losses via suspension from catena eluvial zones. Thus illuviation of colloidal clays into downslope soils may be crucial to catena development by restricting subsurface flow there. Our analysis provides quantitative support for the conceptual understanding of catenas in cratonic landscapes and provides an endmember reference point in understanding the development of slowly eroding soil landscapes.

Our understanding of climatic conditions, and therefore forcing factors, in North America during the past two glacial cycles is limited in part by the scarcity of long, well-dated, continuous paleoclimate records. Here, we present the first, to our knowledge, continuous, millennial-resolution paleoclimate proxy record derived from millimeter-thick pedogenic carbonate clast coatings (pedothems), which are widely distributed in semiarid to arid regions worldwide. Our new multiisotope pedothem record from the Wind River Basin in Wyoming confirms a previously hypothesized period of increased transport of Gulf of Mexico moisture northward into the continental interior from 70,000 to 55,000 years ago based on oxygen and carbon isotopes determined by ion microprobe and uranium isotopes and U-Th dating by laser ablation inductively coupled plasma mass spectrometry. This pronounced meridional moisture transport, which contrasts with the dominant zonal transport of Pacific moisture into the North American interior by westerly winds before and after 70,000–55,000 years ago, may have resulted from a persistent anticyclone developed above the North American ice sheet during Marine Isotope Stage 4. We conclude that pedothems, when analyzed using microanalytical techniques, can provide high-resolution paleoclimate records that may open new avenues into understanding past terrestrial climates in regions where paleoclimate records are not otherwise available. When pedothem paleoclimate records are combined with existing records they will add complimentary soil-based perspectives on paleoclimate conditions.

Chemical Geology, 445, ISSN:0009-2541, ISSN:1872-6836

Soil catenas integrate and amplify gravity transfer and differentiation processes of eluviation and illuviation in soil profiles. We quantified differences in these redistribution processes along granitic catenas across an arid to sub-humid climate gradient in Kruger National Park, South Africa. We measured soil properties in nine catenas sampled from three areas receiving annual rainfall of 470 mm (arid zone), 550 mm (semi-arid zone) and 730 mm (sub-humid zone). As rainfall increased, kaolinite replaced smectite as the dominant clay mineral in all landscape positions across the catenas. Toeslopes showed the strongest evidence of this transition with an excess of smectite in the arid catenas but complete prevalence of kaolinite in toeslopes of sub-humid catenas. The concentration and distribution of clay along the catenas were dependent on landscape position as well — soil profiles at and near the crests were clay depleted (as low as 1%) while those at the toeslopes had much more clay (up to 60%). Clay redistribution along catenas was sensitive to climate with the least amount of redistribution occurring in the dry sites and the most occurring in the wet sites. As a consequence, the sub-humid catenas had clay accumulation only in a small part of the toeslopes while the bulk of their length was represented by highly leached soils. In contrast, arid zone catenas showed little clay redistribution and semi-arid sites displayed the greatest within-catena clay redistribution and preservation. Clay movement and storage conditioned other soil properties such as CEC, base cation distribution, base saturation and pH.

Over the last 100 years, Owens Lake playa in eastern California has been one of the largest point sources of PM-10 dust in the United States. Here we evaluate the spatial impact of the salt-rich playa dust on the alluvial piedmont soils of Owens Valley with the expectation that those nearest the playa will have the highest concentrations of playa-derived salts. We sampled soils of similar age on alluvial fans derived from Sierra Nevada granites along the valley axis to determine soluble salt and specific ion concentrations. Salt concentrations are indeed highest in those soils most proximate to the playa, with higher levels of sodium and elevated pH. However, one site directly north of the playa has significantly higher salt concentrations as measured by electrical conductivity (EC) compared to all other soils sampled, an average pH of about 10 and a sodium adsorption ratio (SAR) greater than 15, suggesting a strong impact of local dust as a result of local topography and wind patterns. Although EC, pH, and SAR of other sampled soils fall into normal ranges for sandy granitic soils in a semi-arid climate, the soils closer to the playa show greater salt and sodium impact than those further away. The strong local impact of playa salts and the patterns of salt contribution away from the playa suggest a spatial effect of dust on the alluvial soils that will grow both in intensity of impact and in distance from the playa if dust fluxes remain at recent levels.

The efficacy of higher plants at mining Si from primary and secondary minerals in terrestrial ecosystems is now recognized as an important weathering mechanism. Grassland ecosystems are a particularly large reservoir of biogenic silica and are thus likely to be a key regulator of Si mobilization. Herein, we examine the effects of parent material (basaltic and granitic rocks) on the range and variability of biogenic silica pools in grass-dominated ecosystems along two precipitation gradients of Kruger National Park, South Africa. Four soil pedons and adjacent dominant plant species were characterized for biogenic silica content. Our results indicate that although soils derived from basalt had less total Si and dissolved Si than soils derived from granite, a greater proportion of the total Si was made up of biogenically derived silica. In general, plants and soils overlying basaltic versus granitic parent material stored greater quantities of biogenic silica and had longer turnover times of the biogenic silica pool in soils. Additionally, the relative abundance of biogenic silica was greater at the drier sites along the precipitation gradient regardless of parent material. These results suggest that the biogeochemical cycling of Si is strongly influenced by parent material and the hydrologic controls parent material imparts on soils. While soils derived from both basalt and granite are strongly regulated by biologic uptake, the former is a “tighter” system with less loss of Si than the latter which, although more dependent on biogenic silica dissolution, has greater losses of total Si. Lithologic discontinuities span beyond grasslands and are predicted to also influence biogenic silica cycling in other ecosystems.

Studies of weathering and pedogenesis have long used calculations based upon low solubility index elements to determine mass gains and losses in open systems. One of the questions currently unanswered in these settings is the degree to which mass is transferred in solution (solutes) versus suspension (colloids). Here we show that differential mobility of the low solubility, high field strength (HFS) elements Ti and Zr can trace colloidal redistribution, and we present a model for distinguishing between mass transfer in suspension and solution. The model is tested on a well-differentiated granitic catena located in Kruger National Park, South Africa. Ti and Zr ratios from parent material, soil and colloidal material are substituted into a mixing equation to quantify colloidal movement. The results show zones of both colloid removal and augmentation along the catena. Colloidal losses of 110 kg m −2 (− 5% relative to parent material) are calculated for one eluviated soil profile. A downslope illuviated profile has gained 169 kg m −2 (10%) colloidal material. Elemental losses by mobilization in true solution are ubiquitous across the catena, even in zones of colloidal accumulation, and range from 1418 kg m −2 (− 46%) for an eluviated profile to 195 kg m −2 (− 23%) at the bottom of the catena. Quantification of simultaneous mass transfers in solution and suspension provide greater specificity on processes within soils and across hillslopes. Additionally, because colloids include both HFS and other elements, the ability to quantify their redistribution has implications for standard calculations of soil mass balances using such index elements.

Gully erosion removes comparatively large volumes of soil from small areas. It is often difficult to quantify the loss of soil because the footprint of individual gullies is too small to be captured by most generally available digital elevation models (DEMs), such as the USGS National Elevation Dataset. Airborne LiDAR (Light Detection and Ranging) has the potential to provide the required data density, but an even newer class of ground-based sensors may provide better local resolution at lower cost. In this study, we compared digital elevation models produced by airborne and ground-based LiDAR systems with ground-based geomorphic and geodetic survey data to determine their utility in quantifying volumetric soil loss due to gully erosion in a heavily degraded watershed (7.55 × 10 − 2 km 2 ), on southwestern Santa Cruz Island in southern California. Volumetric estimates of the eroded sediment were produced by comparing the LiDAR-derived DEMs of the gully system to a modeled pre-erosion surface. Average point densities were significantly higher for the ground-based LiDAR system and provided more detailed information; however, its limited scanning footprint and side-looking orientation presented serious challenges in collecting continuous data from deeply incised gullies, making the airborne system preferable for this type of investigation and likely for most applications where heavy topographic shadowing is prevalent.

Silicon isotope ratios (δ30Si) of bulk mineral materials in soil integrate effects from both silicon sources and processing. Here we report δ30Si values from a climate gradient of Hawaiian soils developed on 170 ka basalt and relate them to patterns of soil chemistry and mineralogy. The results demonstrate informative relationships between the mass fraction of soil Si depletion and δ30Si. In upper ( 1 m deep) soil horizons generally correlate with the isotope fractionation predicted by a study of dissolved Si in basalt-watershed rivers and driven by preferential 28Si removal from the dissolved phase during precipitation. In contrast, after correcting for the influence of dust, secondary mineral Si depletion and δ30Si values in shallow (

Savannas are shaped by drought, herbivory, nutrient limitation, and fire. We assessed the interactions between two of these factors—nutrient limitation and fire—across a savanna landscape in Kruger National Park, South Africa, by quantifying increases in plant-available forms of soil phosphorus (P) following experimental or simulated burns. Plant-available pools of P were defined for granitic surface soils subjected to four experimental fire regimes (no fire, triennial fire in the wet season, triennial fire in the dry season, and annual fire in the dry season). To provide context for these results, we also fractionated surface soils from a granitic catena after simulating burning of these soils in a furnace. Burned soils showed comparable pyromineralization rates, with the experimental burn plot soils averaging 0.49 ± 0.04 g labile P m− 2 y− 1 and catena soils averaging 0.63 ± 0.12 g labile P m− 2 y− 1. Only soils from subplots burned triennially during the wet summer season with moderate fire intensities (∼ 1.1 MW m− 1) showed significant increases in labile P relative to control soils. Soils from other burned subplots with greater fire intensities showed smaller gains in labile P, suggesting pyromineralization rates may peak at intermediate fire intensities. We estimated ash contributed up to 33% of pyromineralized P. For catena soils, simulated burning led to significant increases in pyromineralized P for the relatively P-rich footslope soils and smaller increases for sandy crest and midslope soils. These pyromineralization P fluxes are of the same order of magnitude as plant P demand estimated using foliar P levels, and about one-half microbial mineralization rates. In P-limited ecosystems where chemical weathering rates are slow, moderate-intensity fires could play a critical biogeochemical role in the supply of labile P.

Strontium isotope ratios assist ecosystem scientists in constraining the sources of alkaline earth elements, but their interpretation can be difficult because of complexities in mineral weathering and in the geographical and environmental controls on elemental additions and losses. Hawaii is a “natural laboratory” where a number of important biogeochemical variables have either limited ranges or vary in systematic ways, providing a unique opportunity to understand the impact of time, climate, and atmospheric inputs on the evolution of base cation sources to ecosystems. There are three major sources of strontium (Sr) to these ecosystems, each with distinct isotopic compositions: basalt lava, Asian dust, and rainfall. We present Sr isotope and concentration data on both bulk soil digests and NH4Ac extracts from soil profiles covering a wide range of environments and substrate ages. Bulk soil material from dry climates and/or young substrate ages with > 80 µg g− 1 Sr retain basalt-like Sr isotopic signatures, whereas those with Sr concentrations 40% by mass. At elevated dust levels, lava-derived Sr is low and dust-derived Sr is the dominant control of 87Sr/86Sr in bulk soils; however, 87Sr/86Sr of NH4Ac-extractable Sr largely reflects atmospheric deposition of marine aerosol in these situations. Overall, whole-soil Sr isotope values are controlled by complex interactions between Sr provided by lava weathering but partially lost by leaching, and Sr provided by dust but held in more resistant minerals. The isotopic composition of NH4Ac-extractable Sr and of the biota is controlled by lava weathering and rainfall contribution of Sr with only minor contributions from radiogenic dust sources.

Early (>3 Gy) wetter climate conditions on Mars have been proposed, and it is thus likely that pedogenic processes have occurred there at some point in the past. Soil and rock chemistry of the Martian landing sites were evaluated to test the hypothesis that in situ aqueous alteration and downward movement of solutes have been among the processes that have transformed these portions of the Mars regolith. A geochemical mass balance shows that Martian soils at three landing sites have lost significant quantities of major rock-forming elements and have gained elements that are likely present as soluble ions. The loss of elements is interpreted to have occurred during an earlier stage(s) of weathering that may have been accompanied by the downward transport of weathering products, and the salts are interpreted to be emplaced later in a drier Mars history. Chemical differences exist among the sites, indicating regional differences in soil composition. Shallow soil profile excavations at Gusev crater are consistent with late stage downward migration of salts, implying the presence of small amounts of liquid water even in relatively recent Martian history. While the mechanisms for chemical weathering and salt additions on Mars remain unclear, the soil chemistry appears to record a decline in leaching efficiency. A deep sedimentary exposure at Endurance crater contains complex depth profiles of SO4, Cl, and Br, trends generally consistent with downward aqueous transport accompanied by drying. While no model for the origin of Martian soils can be fully constrained with the currently available data, a pedogenic origin is consistent with observed Martian geology and geochemistry, and provides a testable hypothesis that can be evaluated with present and future data from the Mars surface.

We used two hyperspectral sensors at two different scales to test their potential to estimate biophysical properties of grazed pastures in Rondonia in the Brazilian Amazon. Using a field spectrometer, ten remotely sensed measurements (i.e., two vegetation indices, four fractions of spectral mixture analysis, and four spectral absorption features) were generated for two grass species, Brachiaria brizantha and Brachiaria decumbens. These measures were compared to above ground biomass, live and senesced biomass, and grass canopy water content. The sample size was 69 samples for field grass biophysical data and grass canopy reflectance. Water absorption measures between 1100 and 1250 nm had the highest correlations with above ground biomass, live biomass and canopy water content, while ligno-cellulose absorption measures between 2045 and 2218 nm were the best for estimating senesced biomass. These results suggest possible improvements on estimating grass measures using spectral absorption features derived from hyperspectral sensors. However, relationships were highly influenced by grass species architecture. B. decumbens, a more homogeneous, low growing species, had higher correlations between remotely sensed measures and biomass than B. brizantha, a more heterogeneous, vertically oriented species. The potential of using the Earth Observing-1 Hyperion data for pasture characterization was assessed and validated using field spectrometer and CCD camera data. Hyperion-derived NPV fraction provided better estimates of grass surface fraction compared to fractions generated from convolved ETM+/Landsat 7 data and minimized the problem of spectral ambiguity between NPV and Soil. The results suggest possible improvement of the quality of land-cover maps compared to maps made using multispectral sensors for the Amazon region.

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.

A long-term budget of uranium calculated for a chronosequence of Hawaiian soils demonstrates the importance of mineral aerosol deposition as a geochemical pathway. A series of sequential extractions on the same soils shows a strong association between Fe-oxides and uranium, especially the amorphous and poorly-crystalline Fe-oxides present in intermediate-age soils (20 and 150 ka). Despite intense leaching conditions, soil U contents were found to increase between 20-fold and 60-fold with age along the chronosequence, from 9 to 17 μg cm − 2 at 0.3 ka sites to 344–543 μg cm − 2 at the 4100 ka site. An open-system transport function comparing U to an index “least mobile” element (Nb) was used to calculate U gains or losses from the soils, relative to the input from parent material weathering. Results show small losses or small gains (± 25%) of U occurred in soils 150 ka and younger, while gains of up to 531% U occurred in soils older than 150 ka, highlighting the importance of mineral aerosol (dust) deposition at the older sites. In the 4100 ka soil, the ratio of dust-derived U inputs to parent material-derived U inputs is approximately 6. Sequential extractions show that between 0.1% and 1.2% of the total U in the soils was extracted with ammonium acetate, indicating that very little U exists as exchangeable cations. A second extraction employing citrate–dithionate and hydroxylamine hydrochloride was performed to extract U associated with Fe-oxides. Average percent occurrences for U in the Fe-oxide extraction for each profile are, 7% at 0.3 ka, 53% at 20 ka, 40% at 150 ka, and 17% at 4100 ka, illustrating an increase with age to 20 ka followed by a subsequent decline. This parallels the trend observed in amorphous Fe-oxide content, suggesting that adsorption on amorphous Fe-oxides is the dominant control on U in the intermediate-age soils.

Humans colonized Hawaii about 1200 years ago and have progressively modified vegetation, particularly in mesic to drytropical forests. We use δ 13 C to evaluate the contribution of C 3 and C 4 plants to deep soil organic matter to reconstruct pre-humancontact vegetation patterns along a wet to dry climate transect on Kohala Mountain, Hawaii Island. Precontact vegetationassemblages fall into three distinct zones: a wet C 3 dominated closed canopy forest where annual rainfall is N2000 mm, a dry C 4 dominated grassland with annual rainfall b500 mm, and a broad transition zone between these communities characterized by eitherC 3 trees with higher water-use efficiency than the rainforest trees or C 3 trees with a small amount of C 4 grasses intermixed. Thelikelihood of C 4 grass understory decreases with increasing rainfall. We show that the total concentration of rock-derived nutrientsin the b2-mm soil fraction differs in each of these vegetation zones. Nutrient losses are driven by leaching at high rainfall and byplant cycling and wind erosion at low rainfall. By contrast, nutrients are best preserved in surface soils of the intermediate rainfallzone, where rainfall supports abundant plant growth but does not contribute large amounts of water in excess of evapotranspiration.Polynesian farmers exploited these naturally enriched soils as they intensified their upland agricultural systems during the last threecenturies before European contact.© 2006 Elsevier B.V. All rights reserved.

We determined the total mass loss and rate of chemical weathering from three minimally eroded, Hawaiian lava flows that are ∼10, 170, and 350 ka old. Using a backhoe, we sampled the entire weathering zone at 28 sites and measured the depletion or enrichment of each major element in each soil horizon relative to parent material. We were able to assess the influence of both climate and substrate age on chemical weathering because each flow crosses a precipitation gradient from ∼600 to ∼2500 mm yr −1 . Mass loss rates were highest for the 0–10 ka interval under the wettest climatic conditions (54 t km −2 yr −1 ), and decreased to near zero in the wet sites during the 10–170 and 170–350 ka intervals. Not surprisingly, weathering rates were lower in drier sites; ∼24 t km −2 yr −1 from 0–10 ka to b 2tk m −2 yr −1 thereafter. However the effects of precipitation were non-linear. There was a precipitation threshold below which mass loss was relatively small, and above which mass loss was substantial but insensitive to increased rainfall. Chemical weathering rates depend on tectonic uplift, erosion, climate, rock type or some combination thereof. By working on stable, uneroded surfaces of a single rock type across a well-constrained precipitation gradient, we were able to identify another potential driver: the rate of dust deposition. Although Hawaiʻi is one of the least dusty places in the northern hemisphere, dust inputs reached 82% of the total mass loss from the weathering zone at some sites, and averaged 30% on the 170 ka flow. This highlights the potential importance of dust as a component of observed weathering fluxes from catchments worldwide.

We measured iron isotopic composition of surface (10–20 cm) and subsurface (50–70 cm) basaltic soil horizons from the Island of Maui along a climate gradient (MCG) ranging from 2.2 to 4.2 m mean annual precipitation (MAP). All soil forming factors except climate were conserved. The MCG has a documented decrease in Fe with increasing rainfall that is highly correlated with decreasing mean annual Eh values. We found that increasing MAP from 2.8 to 4.2 m resulted in a surface plus subsurface average increase of 0.56‰ ± 0.09‰ δ56Fe with the subsurface consistently 0.33 ± 0.06‰ δ56Fe greater than the surface horizons. Based on loss of Fe relative to Nb, Rayleigh fractionation was observed with 103lnαlost–retained values of − 0.37 ± 0.03 and − 0.34 ± 0.04 for the surface and subsurface, respectively. Equivalent 103lnαlost–retained values for the surface and subsurface soils suggests Fe loss is driven by similar mechanisms throughout the soil profile. Our calculated fractionation factor is about 1/3 the magnitude of laboratory determined fractionation factors for Fe reduction, suggesting other processes (organic complexation, Fe re-precipitation) modulate the net Fe loss along the MCG. These results offer field-scale confirmation of laboratory experiments on model systems that show anoxic weathering reactions produce materials enriched in heavy Fe isotopes.

Primary productivity on old, weathered soils often is assumed to be limited by phosphorus (P), especially in the lowland tropics where climatic conditions promote the rapid depletion of rock-derived nutrients. This assumption is based on a static view of soils weathering in place with no renewal of the bedrock source. In reality, advection of material through the soil column introduces a spatially variable supply of rock-derived nutrients. This flux is dependent on the residence time of soil, which can range from a few hundred years in rapidly uplifting collisional mountain belts to tens of millions of years in tectonically quiescent tropical cratons. We modeled the effects of tectonic uplift, erosion, and soil depth on the advection of P through the soil column and P availability, calibrating rate of change in biologically available P over time with data from two basaltic chronosequences in Hawai’i and a series of greywacke terraces in New Zealand. Combining our model with the global distribution of tectonic uplift rates and soil depths, we identified tectonic settings that are likely to support P-depleted ecosystems—assuming that tectonic uplift and erosion are balanced (that is, landscape development has reached steady state). The model captures the occurrence of transient P limitation in rapidly uplifting young ecosystems where mineral weathering is outpaced by physical erosion—a likely occurrence where biological N fixation is important. However, we calculate that P depletion is unlikely in areas of moderate uplift, such as most of Central America and Southeast Asia, due to the continuous advection of P into the rooting zone. Finally, where soil advection is slow, such as the Amazon Basin, we expect widespread P depletion in the absence of exogenous nutrient inputs.

© 2014. Mass balance models have become standard tools for characterizing element gains and losses and volumetric change during weathering and soil development. However, they rely on the assumption of complete immobility for an index element such as Ti or Zr. Here we describe a dual-phase mass balance model that eliminates the need for an assumption of immobility and in the process quantifies the contribution of aqueous versus colloidal element transfer. In the model, the high field strength elements Ti and Zr are assumed to be mobile only as suspended solids (colloids) and can therefore be used to distinguish elemental redistribution via colloids from redistribution via dissolved aqueous solutes. Calculations are based upon element concentrations in soil, parent material, and colloids dispersed from soil in the laboratory. We illustrate the utility of this model using a catena in South Africa. Traditional mass balance models systematically distort elemental gains and losses and changes in soil volume in this catena due to significant redistribution of Zr-bearing colloids. Applying the dual-phase model accounts for this colloidal redistribution and we find that the process accounts for a substantial portion of the major element (e.g., Al, Fe and Si) loss from eluvial soil. In addition, we find that in illuvial soils along this catena, gains of colloidal material significantly offset aqueous elemental loss. In other settings, processes such as accumulation of exogenous dust can mimic the geochemical effects of colloid redistribution and we suggest strategies for distinguishing between the two. The movement of clays and colloidal material is a major process in weathering and pedogenesis; the mass balance model presented here is a tool for quantifying effects of that process over time scales of soil development.

Germanium is a trace element that behaves similarly to silicon and shows promise as a tracer of silicate weathering processes. Sequestration of Ge by clay minerals is thought to be the dominant mechanism responsible for Ge/Si fractionation during weathering. In some environments, Ge may be sequestered by iron-oxyhydroxides. We examine Ge behavior in soils along a well-characterized climate gradient on the Hawaiian island of Maui to clarify the relationship between Fe geochemistry and soil Ge / Si ratios. All of the sites are developed on basaltic lavas and tephras approximately 400 ky old, are highly Si-depleted, and have Ge / Si ratios higher than parent material. Sites experiencing less than ∼3300 mm rain/yr host oxidized, highly weathered basaltic soils, in which Fe is present as secondary Fe-oxyhydroxide minerals. In contrast, the wetter sites (> 3340 mm/yr) show extensive Fe reduction, mobilization, and loss. We show that Ge sequestration is independent of Fe redox behavior along the gradient, and thus precipitation of Fe-oxyhydroxides is not a major factor contributing to Ge / Si fractionation during weathering. Instead, elevated soil Ge / Si ratios reflect partitioning of Ge into secondary clay minerals, plus additional retention of Ge by refractory non-silicates.

Lithium isotopes are a potential tracer of silicate weathering but the relationship between lithium isotope compositions and weathering state still need to be established with precision. Here, we report Li concentrations and Li isotope compositions of soils developed along a 4 million year humid-environment chronosequence in the Hawaiian Islands. Li concentrations are variable with depth and age, ranging from 0.24 to 21.3 ppm, and significant Li depletions (up to 92%) relative to parent basalts are systematically enhanced towards the surface. Our calculations show that the relative contribution from atmospheric deposits to the Li soil budget remains small, with a maximum contribution from dust Li of 20% at the oldest site. This is explained by the capacity of the weathering products to retain, within the profiles, the Li coming from basalt alteration, and allows us to explore more specifically the role of alteration processes on soil Li isotope signatures. The delta Li-7 values display a large range between -2.5 parts per thousand and + 13.9 parts per thousand. The youngest soils (0.3 ka) display the same delta Li-7 value as fresh basalt, regardless of depth, despite similar to 30% Li loss by leaching, indicating that there is little Li isotope fractionation during the incipient stage of weathering. delta Li-7 values for the older soils (>= 20 ka) vary non-linearly as a function of time and can be explained by progressive mineral transformations starting with the synthesis of metastable short-range order (nano-crystalline) minerals and followed by their transformation into relatively inert secondary minerals. Results highlight significant Li isotope fractionation during secondary mineral formation and in particular during Li uptake by kaolinite. Finally, we suggest that the non-monotonous evolution of the regolith delta Li-7 value over the last 4 Ma is consistent with climatic variations, where congruent release of Li isotopes occurs during warmer periods. (C) 2014 Elsevier Ltd. All rights reserved.

Polynesians settled and farmed the leeward, relatively arid slopes of Haleakala Volcano beginning about ad 1400. Archaeological investigations at two sites revealed dense concentrations of conical impressions in a subsurface 20cm cinder layer that was previously undisturbed, interpreted as resulting from cultivation practices involving digging sticks. Ethnographic accounts of Hawaiian sweet potato and dryland taro cultivation techniques provide details on the use of such digging sticks. By puncturing this cinder layer, farmers created a loamy mixture of ash and cinder suited to root crop development. Archaeobotanical analyses suggest an intensive, short-fallow regime: macroscopic wood charcoal was absent (therefore no fallow sufficient for secondary re-growth of dryland forest) and evidence of disturbance-tolerant, weedy species is abundant. Geochemical analyses relying on stratigraphic relationships show that cultivation depleted nutrient levels in the digging-stick impressions, through harvest and leach...

We report lead isotopic ratios and concentrations from a basaltic soil chronosequence in Hawaii. Substrate ages at the sites range from 300 years to 4.1 million years. All soils show a net addition of Pb above the contributions made by basaltic weathering, indicating that atmospheric deposition of natural or pollutant sources is important across the sequence. Isotopic ratios from the chronoseqence soils indicate a mixture of basalt, mineral aerosol, and anthropogenic Pb sources derived from both long range and local inputs. Locally derived anthropogenic Pb dominates young soils. Intermediate age soils in remote locations have a strong mineral aerosol signature and likely contain some anthropogenic lead of Asian origin. Older soils have undergone extensive leaching of both basaltic and eolian lead. While the anthropogenic inputs in most sites are from less radiogenic local and Asian sources, we find evidence of “J-type” lead of North American origin in a remote soil on Kauai. The diversity of anthropogenic lead sources suggests that at least two different weather patterns dominate atmospheric Pb transport across the Hawaiian island chain.

The term “incision wave” was coined in the 1990s to describe wave-like patterns of stream incision propagating through numerically modeled bedrock-channel river systems. An incision wave consists of a crest where available stream power is greatest, and where vertical incision is most rapid. The crest is flanked by limbs where there is less available stream power and where vertical incision rates are slower. Farther away from the crest, available stream power is less and incision rates are slower than in the limbs. Modeling results suggest tectonically induced base-level fall triggers incision waves that propagate upstream through bedrock-stream networks. However, it has recently been postulated that the equation underpinning incision-wave models cannot realistically simulate river systems. This paper provides field-geological evidence supporting the hypothesis that tectonically induced base-level fall triggers incision waves that propagate through bedrock-stream networks for tens of kilometers. Mid and late Pleistocene alluvial morphostratigraphy in the Rio Andarax drainage basin, southeast Spain is interpreted to indicate that patterns of paleo stream power were controlled by an incision wave. In the conceptual model proposed in this paper, the signature of an incision wave in the Rio Andarax drainage network is recorded by the contrasting morphostratigraphy of oxygen isotope stage 8 (“OIS8”) time landforms. At the crest of the incision wave during OIS8, there is no record of a paleoalluvial level because available stream power exceeded critical stream power and streams were incising vertically. The crestal area is flanked by OIS8-time strath terraces, which record the position of the two limbs of the incision wave. Farther upstream, OIS8-time alluvial fills mark an area unaffected by the incision wave. During oxygen isotope Stage 2 (“OIS2”) and/or oxygen isotope Stage 1 (“OIS1”), sediment availability in the Rio Andarax catchment was low, and the pattern of relative stream power was threshold-like rather than wave-like. Differences in the manifestation of the incision wave during OIS8 vs. during OIS2 and/or OIS1 indicate that climate and sediment availability can control how incision waves propagate through fluvial systems.

Mineralogical and isotope results paired with field observations suggest that halloysite is the favored, albeit metastable, aluminosilicate end product of arid-zone basalt weathering on Kohala, Hawai'i, and that the formation of smectite has been inhibited by kinetic factors. Soils sampled on constructional lava flows having ages ranging from 4 to 350 ka provide a chronosequence that has had minimal physical disturbance, thus allowing us to interpret chemical and mineralogical changes as the result of a time-dependent process. The halloysite content of the soil increases with increasing soil age; its growth is at the expense of allophane, which, in turn, forms at the expense of primary minerals. These mineral relationships suggest that halloysite has been forming continuously throughout the lifetime of the soil. Smectite, the thermodynamically stable phyllosilicate phase predicted by soil solution composition, is not found in these arid soils. We determined that our soil system is controlled by kinetic factors, and that, therefore, thermodynamic predictions do not reflect reality. The main factors favoring kinetic control of halloysite formation are intense, but short wet periods followed by prolonged extremely dry seasons, and microenvironmental conditions leading to immediate uptake of released Al by the halloysite-precursor mineral allophane. The δ18O relationship between present-day soil water and halloysite, formed over the last 170 ka, was used as a tracer of past soil conditions. Results from a reconstruction of paleo-climate and -soil conditions, combined with δ18O data, observed mineral relationships along the 350 ka chronosequence, and field evidence for long-term aridity rule out the possibility that the arid side of Kohala was affected by prolonged periods of higher rainfall that could have produced more dilute soil waters and, therefore, altered mineralogical stabilities. We conclude that pedogenic halloysite has been continually forming from the early stage of soil formation, and that it has consistently formed under isotopic equilibrium conditions, indicating that halloysite δ18O compositions imply paleoclimatic conditions over the time of its formation that are similar to today's.

Rainfall and the amount of water available to leach ions from soil are among the most important features determining mineral weathering, secondary mineral synthesis and soil chemical properties. Along an arid to humid climosequence on Kohala Mountain, Hawaii, we sampled 16 soil profiles and found that weathering and soil properties change in a nonlinear fashion with increased rainfall. The lavas are influenced by a strong rain shadow with mean annual precipitation (MAP) averaging 160 mm near the coast and rising to >3000 mm near the summit. A temperature decline from 24 to 15 °C with increasing elevation is matched by lower potential evapotranspiration (ET). A water balance model (monthly precipitation minus monthly ET) defines three broad climate zones along the sampling transect: an arid zone with moisture deficit in every month, an intermediate zone with moisture deficit during low-rainfall summer months and moisture surplus during high-rainfall winter months, and a humid zone with moisture surplus during every month. The annualized water balance can be ratioed with the integrated porosity of the top meter of soil to provide a leaching index. The index reaches 1 (total filling of the pore space on an annual basis) at about 1400 mm MAP. Index values >1 imply intense leaching conditions because of pore water replacement. In these 170 ka soils, leaching losses of soluble base cations and Si are nearly complete at index values >1, whereas only 60% of Al has been lost. At index values 1400 mm, soil buffering capacity has been totally exhausted leading to low pH and low ECEC. The nonlinear decline in ECEC is irreversible under natural conditions; base cation depleted soils will remain so even if the climate shifts to drier conditions. In contrast, a climate shift to wetter conditions can drastically modify surface chemical properties existing in the drier soils as weathering depletes primary minerals, elements are lost to leaching, and surface chemistry is modified. There is a time-dimension implied in climate gradient studies; soils forming in recently rejuvenated landscapes contain more primary minerals and should experience loss of buffering capacity at higher rainfall. Loss of buffering capacity means that biological acidity will move more deeply into the vadose zone or into the aquatic system. The details of this transfer depend on present and past climate, and the age and erosional stability of landscapes.

Reliable and precise ages of Quaternary pedogenic carbonate can be obtained with230Th/U dating by thermal ionization mass spectrometry applied to carefully selected milligram-size samples. Datable carbonate can form within a few thousand years of surface stabilization allowing ages of Quaternary deposits and surfaces to be closely estimated. Pedogenic carbonate clast-rinds from gravels of glacio-fluvial terraces in the Wind River Basin have median concentrations of 14 ppm U and 0.07 ppm232Th, with median (230Th/232Th) = 270, making them well suited for230Th/U dating. Horizons as thin as 0.5 mm were sampled from polished slabs to reduce averaging of long (≥105yr), and sometimes visibly discontinuous, depositional histories. Dense, translucent samples with finite230Th/U ages preserve within-rind stratigraphic order in all cases. Ages for terraces WR4 (167,000 ± 6,400 yr) and WR2 (55,000 ± 8600 yr) indicate a mean incision rate of 0.26 ± 0.05 m per thousand years for the Wind River over the past glacial cycle, slower than inferred from cosmogenic-nuclide dating. Terrace WR3, which formed penecontemporaneously with the final maximum glacial advance of the penultimate Rocky Mountain (Bull Lake) glaciation, has an age of 150,000 ± 8300 yr indicating that it is broadly synchronous with the penultimateglobalice volume maximum.

We present a detailed study of germanium behavior in the soil weathering environment as an important step toward using the Ge/Si system as a tracer of silicate weathering processes in both modern and ancient environments. Intensely weathered soils developed on Hawaiian basalts have bulk soil Ge/Si ratios 2 to 10 times higher than fresh basalt (e.g., 10 to 25 μmol/mol vs. 2.5 μmol/mol). Soil Ge concentrations increase with Si, and decrease with Fe, suggesting that Ge sequestration is related to accumulation of secondary soil silicates, rather than retention in soil Fe oxy-hydroxides. Sequential extractions of these soils suggest that Ge/Si fractionation takes place by Ge sequestration during the initial precipitation of secondary soil aluminosilicates (principally allophane). Further Si loss and changes in mineralogy as these soils age result in little additional Ge/Si fractionation. Ge/Si ratios in granitic soils and saprolites are strongly influenced by relative weathering rates of primary minerals. Kaolinite has a Ge/Si ratio (5.9 μmol/mol) higher than the plagioclase from which it forms (3.1 μmol/mol), whereas accumulation of primary quartz (Ge/Si 0.5 μmol/mol) prevents granitic soils from attaining high Ge/Si ratios. Laboratory synthesis of allophane confirms that Ge is preferentially partitioned into the solid phase upon precipitation of secondary aluminosilicates from solution.

Pedogenic carbonate minerals have progressively developed on basalt in a Quaternary weathering chronosequence (5-350 ka) on the island of Hawaii. The sites were isolated from direct contact with seawater or the water table, and were minimally influenced by eolian input throughout their history. The carbonates consist chiefly of high-magnesium calcite (HMC) and well-ordered stoichiometric dolomite. Pedogenic development of such minerals is rare, and the conditions of their formation bear on interpretations of paleosols and the development of sedimentary magnesian carbonates. 87Sr/86Sr ratios of chronosequence carbonates range from 0.7038 to 0.7048. The data indicate that > 75% of the strontium was derived from the weathering of volcanic parent material, even in the oldest soils, where few primary minerals are available to provide Ca and Mg. This is in contrast to continental calcretes, which are commonly dominated by atmospheric inputs and are generally composed of low-Mg calcite (LMC). Stable-isotope data are consistent with carbonate precipitation from soil waters in an arid environment. The magnesium content of the carbonate increases with substrate age, and textural and mineralogical changes suggest direct precipitation of dolomite as well as dolomitization of early-formed LMC and HMC. Opal was detected in young (5-10 ka) samples, and gibbsite is present in older samples, indicating a shift from silica-dominated to aluminum-dominated soil solution. High concentrations of dissolved silica and aluminum during early weathering likely precluded formation of Mg-rich secondary clay minerals such as smectite or palygorskite. Weathering of basalt with a molar Mg/Ca ratio of 0.8-1.5, and the absence of secondary magnesian silicates, can account for elevated soil-water Mg/Ca ratios (0.7-1.0 in soils > 70 ka) that led to magnesian carbonate formation. Over time, the formation of a clay-rich soil profile prevented rapid evaporation of soil water, possibly facilitating crystallization of stoichiometric dolomite.

Sulfur, a nutrient required by terrestrial ecosystems, is likely to be regulated by atmospheric processes in well-drained, upland settings because of its low concentration in most bedrock and generally poor retention by inorganic reactions within soils. Environmental controls on sulfur sources in unpolluted ecosystems have seldom been investigated in detail, even though the possibility of sulfur limiting primary production is much greater where atmospheric deposition of anthropogenic sulfur is low. Here we measure sulfur isotopic compositions of soils, vegetation and bulk atmospheric deposition from the Hawaiian Islands for the purpose of tracing sources of ecosystem sulfur. Hawaiian lava has a mantle-derived sulfur isotopic composition (δ(34)S VCDT) of -0.8‰. Bulk deposition on the island of Maui had a δ(34)S VCDT that varied temporally, spanned a range from +8.2 to +19.7‰, and reflected isotopic mixing from three sources: sea-salt (+21.1‰), marine biogenic emissions (+15.6‰), and volcanic emissions from active vents on Kilauea Volcano (+0.8‰). A straightforward, weathering-driven transition in ecosystem sulfur sources could be interpreted in the shift from relatively low (0.0 to +2.7‰) to relatively high (+17.8 to +19.3‰) soil δ(34)S values along a 0.3 to 4100 ka soil age-gradient, and similar patterns in associated vegetation. However, sub-kilometer scale spatial variation in soil sulfur isotopic composition was found along soil transects assumed by age and mass balance to be dominated by atmospheric sulfur inputs. Soil sulfur isotopic compositions ranged from +8.1 to +20.3‰ and generally decreased with increasing elevation (0-2000 m), distance from the coast (0-12 km), and annual rainfall (180-5000 mm). Such trends reflect the spatial variation in marine versus volcanic inputs from atmospheric deposition. Broadly, these results illustrate how the sources and magnitude of atmospheric deposition can exert controls over ecosystem sulfur biogeochemistry across relatively small spatial scales.

Hawaiian soils contain a mixture of material derived from in situ weathering of parent material plus atmospheric inputs, including sea salt aerosols and Asian dust. We use soil mineralogy and radiogenic isotope geochemistry (Sr and Nd) to evaluate the impact of Asian dust on a chronosequence of Hawaiian soils. Dust becomes an important constituent of soils 20 ky and older. Near-surface (4% of the total Nb contained in this soil profile.

A climate transect across the Kohala Peninsula, Hawaii provides an ideal opportunity to study soil processes and evolution as a function of rainfall. The parent material is the ∼150 ka Hawi alkali basalt aa flow, and median annual precipitation (MAP) changes from ∼16 cm along the west coast to ∼450 cm in the rain forest near the crest of the peninsula. We measured labile (plant-available) base cation concentrations and 87Sr/86Sr ratios of labile strontium and silicate residue from soil profiles across the transect from 18 to 300 cm MAP. Depletion of labile cations and a shift in labile 87Sr/86Sr ratios toward rainwater values with increasing rainfall clearly show the transition from a mineral-supported to a rainwater-supported cation nutrient budget. In contrast, increases in soil silicate residue 87Sr/86Sr values with increasing MAP result primarily from input of exogenous eolian material (dust derived from Asian loess), with a greater dust fraction at the high MAP sites due to aerosol washout. Most of the soil silicate strontium in high-MAP sites is still derived from the original parent material, but the shallower portions of profiles can be dust-dominated. The variations in labile 87Sr/86Sr with rainfall allow us to calculate weathering rates as a function of MAP. The primary uncertainty is the degree to which Sr in rainwater actually interacts with the labile cation reservoir before being flushed from the system; mass balance calculations for the 150 ka evolution of the profile suggest that only on the order of 5 to 50% of rainwater strontium exchanges with the labile reservoir. Our models suggest that the present-day supply of strontium by weathering increases steadily with rainfall in the low-MAP (

The anomalously low affinity of yttrium (Y) for iron (Fe) (oxyhydr)oxides relative to lanthanides with similar ionic radius (e.g., Ho) has been demonstrated in experiments with isolated Fe minerals and in a variety of marine systems that contain high concentrations of solid phase Fe. However, it has not previously been demonstrated to occur during soil genesis, despite the common observation that many soils become enriched in Fe over time. We hypothesized that Y would become progressively depleted in soils relative to Ho with increased weathering. Since, trivalent Y has an anomalously low Misono softness relative to other trivalent ions included in the rare earth element and yttrium group (REY 3+ ), we also investigated whether soil REY fractionation reflects variation in Misono softness. To test this, we measured trends in total REY concentrations for Hawaiian soils derived from basaltic parent materials aged 0.3–4100 ky, and measured REYs released from the same samples during short-time (3 h) dissolution experiments conducted as part of a previous investigation linking dissolution with surface charge properties (Chorover et al., 2004). The chondrite-normalized Y/Ho ratios in the parent Hawaiian basalt ( Chond [Y/ Ho] = 0.998) and continental dust ( Chond [Y/Ho] = 0.994) inputs are remarkably similar, and thus we can interpret deviations from Chond [Y/Ho] � 1.0 to result from soil biogeochemical processes and not source mixing. Between 0.3 and 20 ky, the Chond[Y/Ho] ratio of the subsurface soils decreased from 0.96 ± 0.07(2r) to 0.71 ± 0.05, and then remained unchanged across the rest of the weathering sequence. In contrast, the Chond [Y/Ho] ratio of the surface soils decreased from 0.99 ± 0.07 to 0.76 ± 0.05 at 150 ky and then, most likely due to continued dust inputs, increased to 1.04 ± 0.07 in the oldest soils. Analysis of the short-time dissolution experiments revealed preferential release of Y relative to Ho (and also La relative Pr) at intermediate pH where aqueous REY concentrations are governed by proton competition for adsorption sites. Proton-competition-control over REY release is bounded at high pH by the onset of colloidal dispersion—represented by the point of minimum dissolution (p.m.d.) of Al—and at low pH by the soil’s point of zero net charge (p.z.n.c.) and/or when proton-promoted dissolution of REY-containing solids, including Fe-(oxyhydr)oxides, control REY release. Results of our dissolution experiments suggest that complexation of REYs by dissolved organic matter (DOM) does not drive Y–Ho fractionation during pedogenesis, but rather may suppress it. Synthesis of these field and laboratory experiments suggests the Y/Ho ratio decreases early in soil development (

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.