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Soil erosion in agricultural landscapes reduces crop yields, leads to loss of ecosystem services, and influences the global carbon cycle. Despite decades of soil erosion research, the magnitude of historical soil loss remains poorly quantified across large agricultural regions because preagricultural soil data are rare, and it is challenging to extrapolate local-scale erosion observations across time and space. Here we focus on the Corn Belt of the midwestern United States and use a remote-sensing method to map areas in agricultural fields that have no remaining organic carbon-rich A-horizon. We use satellite and LiDAR data to develop a relationship between A-horizon loss and topographic curvature and then use topographic data to scale-up soil loss predictions across 3.9 × 10⁵km²of the Corn Belt. Our results indicate that 35 ± 11% of the cultivated area has lost A-horizon soil and that prior estimates of soil degradation from soil survey-based methods have significantly underestimated A-horizon soil loss. Convex hilltops throughout the region are often completely denuded of A-horizon soil. The association between soil loss and convex topography indicates that tillage-induced erosion is an important driver of soil loss, yet tillage erosion is not simulated in models used to assess nationwide soil loss trends inmore »the United States. We estimate that A-horizon loss decreases crop yields by 6 ± 2%, causing $2.8 ± $0.9 billion in annual economic losses. Regionally, we estimate 1.4 ± 0.5 Pg of carbon have been removed from hillslopes by erosion of the A-horizon, much of which likely remains buried in depositional areas within the fields.

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Cosmogenic nuclide surface exposure dating and erosion rate measurements in basaltic landscapes rely primarily on measurement of 3He in olivine or pyroxene. However, geochemical investigations using 3He have been impossible in the substantial fraction of basalts that lack separable olivine or pyroxene crystals, or where such crystals were present, but have been chemically weathered. Fine-textured basalts often contain small grains of ilmenite, a weathering-resistant mineral that is a target for cosmogenic 3He production with good He retention and straightforward mineral separation, but with a poorly constrained production rate. Here we empirically calibrate the cosmogenic 3He production rate in ilmenite by measuring 3He concentrations in basalts with fine-grained (~20 lm cross-section) ilmenite and co-existing pyroxene or olivine from the Columbia River and Snake River Plain basalt provinces in the western United States. The concentration ratio of ilmenite to pyroxene and olivine is 0.78 ± 0.02, yielding an apparent cosmogenic 3He production rate of 93.6 ± 7.7 atom g-1 yr-1 that is 20–30% greater than expected from prior theoretical and empirical estimates for compositionally similar minerals. The production rate discrepancy arises from the high energy with which cosmic ray spallation reactions emit tritium and 3He and the associated long stopping distances thatmore »cause them to redistribute within a rock. Fine-grained phases with low cosmogenic 3He production rates, like ilmenite, will have anomalously high production rates owing to net implantation of 3He from the surrounding, higher 3He production rate, matrix. Semi-quantitative modeling indicates implantation of spallation 3He increases with decreasing ilmenite grain size, leading to production rates that exceed those in a large grain by ~10% when grain radii are <150 lm. The modeling predicts that for the ilmenite grain size in our samples, implantation causes production rates to be ~20% greater than expected for a large grain, and within uncertainty resolves the discrepancy between our calibrated production rate, theory, and rates from previous work. The redistribution effect is maximized when the host rock and crystals differ substantially in mean atomic number, as they do between whole-rock basalt and ilmenite.« less