Scholarly Work - Earth Sciences
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Item Capturing the complexity of soil evolution: Heterogeneities in rock cover and chemical weathering in Montana's Rocky Mountains(Elsevier BV, 2022-05) Benjaram, Sarah S.; Dixon, Jean L.; Wilcox, Andrew C.We investigate the relationship between chemical weathering, persistence of soil cover, and topography in two neighboring mountain ranges in the northern Rockies of western Montana, USA. We augment existing tools for measuring chemical weathering with adjustments for both local and landscape-scale contributions from unweathered rock fragments, boulders, and bedrock exposure. Adjusted weathering intensities recognize that quantifying weathering in mountainous systems should account for rock exposure, rather than focusing solely on fine-grained soil mantles. Our study systems' distinct morphologies are shaped by their unique climate histories. The previously glaciated Bitterroot Mountains consist of steep hillslopes with abundant rock cover, while the neighboring unglaciated Sapphire Mountains display convex, soil-mantled hillslopes. Over 380 soil thickness measurements, 118 analyses of soil and rock geochemistry, and digital terrain analysis reveal that patchy soils in the bedrock-rich system are roughly half as thick as those in the continuously-soil-mantled landscape, and ~45% less weathered, despite wetter conditions that would be expected to enhance weathering. These disparities increase when accounting for coarse rock fragments in soils and bedrock cover across the study catchments. The near continuously soil-mantled Sapphire system experiences ~1.5 times greater weathering intensity at a catchment scale compared to the bedrock-rich Bitterroot system. Rock exposure across the mountainous study system increases with increasing slope gradient. However, we find no clear threshold at which soils decrease in abundance or weathering intensity, and soils are surprisingly resilient even at the steepest hillslopes (comprising ~60% of the landscape area at slopes >30°). Our new data quantify soil abundance and chemical weathering intensity at both local and landscape scales. This work highlights how measurements of soil and rock cover need to be incorporated into studies quantifying chemical weathering, as traditional approaches may significantly overestimate and mischaracterize weathering regimes in mountain environments.Item A Geospatial Approach for Identifying and Exploring Potential NaturalWater Storage Sites(2017-08) Holmes, Danika L.; McEvoy, Jamie; Dixon, Jean L.; Payne, ScottAcross the globe, climate change is projected to affect the quantity, quality, and timing of freshwater availability. In western North America, there has been a shift toward earlier spring runoff and more winter precipitation as rain. This raises questions about the need for increased water storage to mitigate both floods and droughts. Some water managers have identified natural storage structures as valuable tools for increasing resiliency to these climate change impacts. However, identifying adequate sites and quantifying the storage potential of natural structures is a key challenge. This study addresses the need for a method for identifying and estimating floodplain water storage capacity in a manner that can be used by water planners through the development of a model that uses open-source geospatial data. This model was used to identify and estimate the storage capacity of a 0.33 km2 floodplain segment in eastern Montana, USA. The result is a range of storage capacities under eight natural water storage conditions, ranging from 900 m3 for small floods to 321,300 m3 for large floods. Incorporating additional hydraulic inputs, stakeholder needs, and stakeholder perceptions of natural storage into this process can help address more complex questions about using natural storage structures as ecosystem-based climate change adaptation strategies.Item Quantifying nutrient uptake as driver of rock weathering in forest ecosystems by magnesium stable isotopes(2017-06) Uhlig, David; Schuessler, Jan A.; Bouchez, Julien; Dixon, Jean L.; von Blackenburg, FriedhelmPlants and soil microbiota play an active role in rock weathering and potentially couple weathering at depth with erosion at the soil surface. The nature of this coupling is still unresolved because we lacked means to quantify the passage of chemical elements from rock through higher plants. In a temperate forested landscape characterised by relatively fast (similar to 220 t km(-2) yr(-1)) denudation and a kinetically limited weathering regime of the Southern Sierra Critical Zone Observatory (SSCZO), California, we measured magnesium (Mg) stable isotopes that are sensitive indicators of Mg utilisation by biota. We find that Mg is highly bio-utilised: 50-100% of the Mg released by chemical weathering is taken up by forest trees. To estimate the tree uptake of other bio-utilised elements (K, Ca, P and Si) we compared the dissolved fluxes of these elements and Mg in rivers with their solubilisation fluxes from rock (rock dissolution flux minus secondary mineral formation flux). We find a deficit in the dissolved fluxes throughout, which we attribute to the nutrient uptake by forest trees. Therefore both the Mg isotopes and the flux comparison suggest that a substantial part of the major element weathering flux is consumed by the tree biomass. The enrichment of Mg-26 over Mg-24 in tree trunks relative to leaves suggests that tree trunks account for a substantial fraction of the net uptake of Mg. This isotopic and elemental compartment separation is prevented from obliteration (which would occur by Mg redissolution) by two potential effects. Either the mineral nutrients accumulate today in regrowing forest biomass after clear cutting, or they are exported in litter and coarse woody debris (CWD) such that they remain in \solid\" biomass. Over pre-forest-management weathering timescales, this removal flux might have been in operation in the form of natural erosion of CWD. Regardless of the removal mechanism, our approach provides entirely novel means towards the direct quantification of biogenic uptake following weathering. We find that Mg and other nutrients and the plant-beneficial element Si (\"bio-elements\") are taken up by trees at up to 6m depth, and surface recycling of all bio-elements but P is minimal. Thus, in the watersheds of the SSCZO, the coupling between erosion and weathering might be established by bio-elements that are taken up by trees, are not recycled and are missing in the dissolved river flux due to erosion as CWD and as leaf-derived bio-opal for Si. We suggest that the partitioning of a biogenic weathering flux into eroded plant debris might represent a significant global contribution to element export after weathering in eroding mountain catchments that are characterised by a continuous supply of fresh mineral nutrients.Item Climate driven thresholds for chemical weathering in post-glacial soils of New Zealand(2016-09) Dixon, Jean L.; Chadwick, Oliver A.; Vitousek, PeterChemical 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.Item Parent material and pedogenic thresholds: observations and a simple model(2016-10) Vitousek, Peter; Dixon, Jean L.; Chadwick, Oliver A.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.