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40Ar/39Ar detrital sanidine (DS) dating of river terraces provides new insights into the evolution and bedrock incision history of the San Juan River, a major tributary of the Colorado River, USA, at the million-year time scale. We dated terrace flights from the San Juan−Colorado River confluence to the San Juan Rocky Mountains. We report >5700 40Ar/ 39Ar dates on single DS grains from axial river facies within several meters above the straths of 30 individual terraces; these yielded ∼2.5% young (<2 Ma) grains that constrain maximum depositional ages (MDAs) and minimum incision rates. The most common young grains were from known caldera eruptions: 0.63 Ma grains derived from the Yellowstone Lava Creek B eruption, and 1.23 Ma and 1.62 Ma grains derived from two Jemez Mountains eruptions in New Mexico. Agreement of a DS-derived MDA age with a refined cosmogenic burial age from Bluff, Utah, indicates that the DS MDA closely approximates the true depositional age in some cases. In a given reach, terraces with ca. 0.6 Ma grains are commonly about half as high above the river as those with ca. 1.2 Ma grains, suggesting that the formation of the terrace flights likely tracks near-steady bedrock incision over the past 1.2 Ma. Longitudinal profile analysis of the San Juan River system shows variation in area-normalized along-stream gradients: a steeper (ksn = 150) reach near the confluence with the Colorado River, a shallower gradient (ksn = 70) in the central Colorado Plateau, and steeper (ksn = 150) channels in the upper Animas River basin. These reaches all show steady bedrock incision, but rates vary by >100 m/Ma, with 247 m/Ma at the San Juan−Colorado River confluence, 120−164 m/Ma across the core of the Colorado Plateau, and 263 m/Ma in the upper Animas River area of the San Juan Mountains. The combined dataset suggests that the San Juan River system is actively adjusting to base-level fall at the Colorado River confluence and to the uplift of the San Juan Mountains headwaters relative to the core of the Colorado Plateau. These fluvial adjustments are attributed to ongoing mantle-driven differential epeirogenic uplift that is shaping the San Juan River system as well as rivers and landscapes elsewhere in the western United States. 

Abstract Weathering of ultramafic rocks emplaced at low latitude during arc‐arc and arc‐continent collisions may provide an important sink for atmospheric CO₂over geologic timescales. Accurately modeling the effects of ultramafic rock weathering on Earth's carbon cycle and climate requires understanding mass fluxes from ultramafic landscapes. In this study, physical erosion and chemical weathering fluxes and weathering intensity are quantified in 15 watersheds across the Monte del Estado, a serpentinite massif in Puerto Rico, using measurements of in situ³⁶Cl in magnetite, stream solute fluxes, and sediment geochemistry. Despite high relief in the study watersheds, erosion fluxes are moderate (22–109 tons km⁻² yr⁻¹), chemical weathering fluxes are large (55–143 tons km⁻² yr⁻¹), and weathering intensities are among the highest yet reported for silicate‐rock weathering (up to 0.88). We use these data to parameterize power‐law relationships between weathering, erosion, and runoff. We interpret the relative importance of climate versus erosion in setting weathering fluxes and CO₂consumption from the best‐fit power‐law slopes. Weathering fluxes from tropical, montane serpentinite landscapes are found to be strongly controlled by runoff and weakly controlled by the supply of fresh rock to the weathering zone through physical erosion. The strong runoff dependence of weathering fluxes implies that, to the extent that precipitation rates are coupled to global temperature, ultramafic landscapes may be important participants in the negative silicate weathering feedback, increasing (decreasing) CO₂consumption in response to a warming (cooling) climate. Thus, serpentinite landscapes may help stabilize Earth's climate state through time. 

Abstract. Cosmogenic nuclide production rates depend on the excitation functions of the underlying nuclear reactions and the intensity and energy spectrum of the cosmic-ray flux. The cosmic-ray energy spectrum shifts towards lower average energies with decreasing altitude (increasing atmospheric depth), so production rate scaling will differ for production reactions that have different energy sensitivities. Here, we assess the possibility of the unique scaling of 36Cl production from Fe by modeling changes in the 36ClFe/36ClK and 36ClFe/10Beqtz production ratios with altitude. We evaluate model predictions against measured 36Cl concentrations in magnetite and K-feldspar and 10Be concentrations in quartz from granitic rocks exposed across an elevation transect (ca. 1700–4300 ma.s.l.) in western North America. The data are broadly consistent with model predictions. The null hypothesis that 36ClFe/10Beqtz and 36ClFe/36ClK production ratios are invariant with altitude can be rejected at the 90 % confidence level. Thus, reaction-specific scaling factors will likely yield more accurate results than non-reaction-specific scaling factors when scaling 36Cl production in Fe-rich rocks and minerals. 

Abstract The formation of magma‐poor continental rifts is an enigmatic process, as the weakening mechanism(s) for cratonic lithosphere remains uncertain in the absence of elevated lithospheric temperature. One view links weakening to melts hidden at depth, while another ascribes it to pre‐existing weaknesses. Long‐term extensional rates also influence lithospheric strength and rift evolution. We target the Linfen Basin (LB) in the magma‐poor Shanxi Rift System (SRS) in the North China Craton to understand these components. We apply cosmogenic²⁶Al/¹⁰Be burial dating on 14 core samples at different depths from three deep boreholes in the basin and obtain six valid burial ages ranging from 2.37^{+1.18/−1.21}to 5.86^+inf/−1.37 Ma. We further re‐interpret a seismic reflection profile and quantify the geometry and amount of extension by forward structural modeling with multiple constraints based on extensional fault‐bend folding theory. The timing of the basal sedimentation is estimated to be ∼6.1 and ∼4.2 Ma in the southern and northern portions, respectively, indicating diachronous, northward‐propagating rifting. The amount and mean rate of extension are ∼3.6 km and ∼0.9 km/Myr, respectively. The basin depths increasing northward indicates the clockwise rotation of the basin. We propose a basin‐scale non‐rigid transtensional bookshelf faulting model to explain the rotation patterns of the circum‐Ordos basins. We argue that the inherited structures weaken the cratonic lithosphere of the SRS, and the low extension rate contributes to its magma‐poor nature. We propose a lithospheric‐scale evolution model for the LB, invoking the inherited crustal weakness, low extension rate, and lower lithosphere counterflow.