Jean L. Dixon

Climate-driven processes of hillslope weathering

Climate controls erosion and weathering on soil-mantled landscapes through diverse processes that have remained diffi cult to disentangle due to their complex interactions. We quantify denudation, soil and saprolite weathering, and soil transport near the base and crest of the western slope of the Sierra Nevada to examine how large differences in climate affect these processes. Depth profi les of fallout radionuclides and fi eld observations show relative differences in erosion and weathering processes at these two climatically diverse sites, and our data suggest fundamentally different patterns of soil production and transport mechanisms: biotically driven soil transport at low elevation, and surface erosion driven by overland fl ow at high elevation. Soil production rates from cosmogenic 10 Be decrease from 31.3 to 13.6 m/Ma with increasing soil depth at low elevation, but show uncertain depth dependence at the high elevation site. Our data also show a positive correlation between physical erosion and saprolite weathering at both sites. Highly weathered saprolites are overlain by weakly weathered and rapidly eroding soils, while chemically less depleted saprolites are overlain by slowly eroding, more weathered soils. Our data are among the fi rst to quantify the critical role of saprolite weathering in the evolution of actively eroding upland landscapes, and our results provide quantitative constraints on how different climates can shape hillslopes by driving processes of erosion and weathering.

Climate-driven thresholds for chemical weathering in postglacial soils of New Zealand

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

Post-Miocene landscape rejuvenation at the eastern end of the Alps

In the debate on the causes of uplift and landscape evolution of the Alps, most studies focus on regions that were glaciated at some stage during the last 2 m.y. In these areas, it is difficult to separate glacial-driven versus tectonically driven rates of erosion. Here, we present 10 Be-derived erosion rates from unglaciated catchments in the Koralpe range at the eastern end of the Alps. This region features strong geomorphologic evidence for landscape transience with young valleys incised into a smooth relict landscape. Erosion rates average 49 ± 8 mm/k.y. for catchments located on the relict landscape and 137 ± 15 mm/k.y. for catchments in the incised landscape. From these data, we estimate the onset of incision at 4 ± 1 Ma, the surface uplift at 350 ± 90 m, and a total relative base-level fall of 540 ± 140 m. Our results are in close agreement with both the magnitude and the age of onset of uplift of the Styrian Basin and the northern Molasse Basin, as well as the incision rate of the Mur River into the Styrian karst. The inferred timing of the onset of uplift around 4 Ma relates to interpreted basin inversion in the Pannonian Basin. Since this uplift event appears to have involved both the Pannonian Basin and the entire eastern end of the Alpine mountain range, we suggest that it may have occurred in response to a deep-seated process in the lithosphere. As such, we argue for tectonic drivers for the post-Miocene uplift in the eastern Alps.

Climatically controlled delivery and retention of meteoric 10Be in soils

Meteoric 10Be (Bem) is a widely used tracer of soil processes in terrestrial ecosystems, but complexity surrounding its delivery and retention in soils is frequently oversimplified. We measured Bem in ten soil profiles sampled on 150 ka lava flows of Kohala, Hawaii. The sampled soils receive annual rainfall of 160–3000 mm, and exhibit strong gradients in chemical properties. Below ~1400 mm rainfall, Bem inventories in the upper 1–2 m of soils span ~37–270 × 10 9 atom cm–2 and increase linearly with rainfall, consistent with precipitationdriven depositional fluxes and the retention of Bem in clay-rich soil horizons. Bem-derived ages, based on flux estimates and measured inventories, are uniform and consistent with the known substrate age (150 ka) at these sites. However, nuclide inventories dramatically decrease for soils at >1400 mm rainfall, where water balance changes from negative to positive. This climate-driven threshold is associated with Bem mobility, high pore-water volumes, low base cation saturation, and high exchangeable Al. Though rainfall-mediated delivery of Bem to soil is relatively predictable, its mobility and retention are strongly constrained by water-mediated ion exchange properties. Our results indicate meteoric 10Be may not be a robust tracer of soil age in systems with high rainfall and weathering intensity. INTRODUCTION Meteoric 10Be (Bem) is a promising tracer of soil residence times and movement (Monaghan et al., 1983), though uncertainties in its delivery to and retention in soil have limited its widespread use. Its widely used twin, in situ 10Be, is produced by cosmic rays in the lattices of mineral grains, while the meteoric variety is produced in the upper atmosphere and deposited via both dry and wet processes (Field et al., 2006). Beryllium has a high partition coefficient in most environmental settings, binding strongly to soils and sediments at Earth’s surface, and has a long residence in natural materials (Pavich et al., 1986). Given assumptions about its production and delivery, Bem is used to calculate soil and surface ages, rates of erosion and weathering, and soil movement across a landscape (Graly et al., 2010; West et al., 2013; Willenbring and von Blanckenburg, 2010). One limitation to the use of Bem is uncertainty about its delivery from the atmosphere to land surface (Reusser et al., 2010; Ouimet et al., 2015). The majority of Bem is delivered as wet deposition, though dust contributions can be non-trivial (e.g., Brown et al., 1989; Ouimet et al., 2015). Atmospheric Bem scavenging is complex (Field et al., 2006; Monaghan et al., 1986; Willenbring and von Blanckenburg, 2010). Regardless, models of global Bem delivery generally show annual deposition rates are high in regions with high precipitation, and limited measurements of atmospheric Bem fluxes suggest strong variation with precipitation (Graly et al., 2011, Graham et al., 2003). A second limitation in the use of Bem concerns its behavior in terrestrial systems (Bacon et al., 2012). The assumption of Bem retentivity in soils is required to derive soil ages and erosion rates (Bacon et al., 2012; Maher and von Blanckenburg, 2016; Pavich et al., 1986). Mobility of Bem within soil profiles has been previously suggested in soil chronosequence studies (e.g., Monaghan et al., 1983; Pavich et al., 1984; Barg et al., 1997), and by comparison with the behavior of rock derived 9Be (Bacon et al., 2012). Laboratory experiments indicate organic C speciation and pH influence Be binding and mobility in soils and sediments (You et al., 1989; Boschi and Willenbring, 2016). However, there are few observations of the conditions governing 10Be mobility directly within soils. Here, we evaluate Bem delivery and retention along a well characterized soil climosequence on the leeward slope of Kohala, Hawaii. We quantify Bem in soils formed on constructional lava flows of the same age but under strongly differing annual rainfall. These data improve understanding of Bem systematics and retentivity in soil, and provide new insight into the limitations of this isotope system.