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Hillslopes have been intensively studied by both geomorphologists and soil scientists. Whereas geomorphologists have focused on the physical soil production and transport on hillslopes, soil scientists have been concerned with the topographic variation of soil geochemical properties. We combined these differing approaches and quantified soil chemical weathering rates along a grass covered hillslope in Coastal California. The hillslope is comprised of both erosional and depositional sections. In the upper eroding section, soil production is balanced by physical erosion and chemical weathering. The hillslope then transitions to a depositional slope where soil accumulates due to a historical reduction of channel incision...
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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...
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In most climates on Earth, biological processes control soil N. In the Atacama Desert of Chile, aridity severely limits biology, and soils accumulate atmospheric NO3 −. We examined this apparent transformation of the soil N cycle using a series of ancient Atacama Desert soils (>2 My) that vary in rainfall (21 to <2 mm yr−1). With decreasing rainfall, soil organic C decreases to 0.3 kg C m−2 and biological activity becomes minimal, while soil NO3 − and organic N increase to 4 kg N m−2 and 1.4 kg N m−2, respectively. Atmospheric NO3 − (Δ17O = 23.0‰) increases from 39% to 80% of total soil NO3 − as rainfall decreases. These soils capture the transition from a steady state, biologically mediated soil N cycle to a dominantly...
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We compiled new and published data on the natural abundance N isotope composition (δ15N values) of soil and plant organic matter from around the world. Across a broad range of climate and ecosystem types, we found that soil and plant δ15N values systematically decreased with increasing mean annual precipitation (MAP) and decreasing mean annual temperature (MAT). Because most undisturbed soils are near N steady state, the observations suggest that an increasing fraction of ecosystem N losses are 15N-depleted forms (NO3, N2O, etc.) with decreasing MAP and increasing MAT. Wetter and colder ecosystems appear to be more efficient in conserving and recycling mineral N. Globally, plant δ15N values are more negative than...
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We measured Ca stable isotope ratios (δ44/40Ca) in an ancient (2 My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5 kmol m−2) is present as sulfates (2.5 kmol m−2), and to a lesser extent carbonates (0.4 kmol m−2). In aqueous extracts of variably hydrated calcium sulfate minerals, δ44/40CaE values (vs. bulk Earth) increase with depth (1.4 m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ44/40Ca in the top six horizons resembles that of sulfate-δ44/40Ca, but with values 0.1–0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ44/40Ca values observed on Earth. Linear...
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