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