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Plain-Language Summary: Volcanic landscapes begin with high permeability, but with time develop a weathered surface that reduces permeability and diverts increasing amounts of water to stream runoff. On the island of Hawai’i the young volcanoes have no permanent streams; stream incision becomes important the older surfaces (more than about 20,000 years). By treating the weathered surface as a porous-plastic medium we Wnd that weathering can induce compaction of the soil that reduces permeability. The reduction in inWltration and initiation of stream incision fundamentally changes the hydrologic and geomorphic evolution of the landscape. Weathering affects both the chemistry and material properties of the surface and strongly inNuences landscape development, in ways that can be predicted with reactive transport and mechanical modeling. Geochemical tracers can be used to identify and quantify weathering processes and constrain these models. 

Plain-Language Summary: The use of carbon isotopes to constrain the relative rates of carbonate and organic carbon cycling has a long history. Most workers have assumed that the inputs of C to the ocean atmosphere system have isotopic compositions close to that of mantle, but that leads to substantial lower estimate of organic carbon burial and potential oxygen generation than sediment inventory approaches have found. A re-evaluation of carbon input shows that oxidation of old organic carbon and methane result in a signibcantly lower value for the isotopic composition of inputs, implying larger rates of carbon burial that agree much more closely with the inventory approaches. The new results also show that the sedimentary reservoirs of carbonate and organic carbon are experiencing net growth over the last 35 million years and that should increase the oxidation state of the Earth surface environment. Consideration of carbon isotope cycling under low oxygen conditions that were characteristic of the Precambrian shows that the standard assumptions may be lead to substantial mass balance errors under such conditions 

Estimates of sedimentary organic carbon burial fluxes based on inventory and isotope mass balance methods have been divergent. A new calculation of the isotope mass balance using a revised assessment of the inputs to the ocean-atmosphere system resolves the apparent discrepancy. Inputs include weathering of carbonate and old kerogen, geogenic methane oxidation, and volcanic and metamorphic degassing. Volcanic and metamorphic degassing comprise ≈23% of the total C input. Inputs from isotopically lightOCpetroandCH_4-geodrive the mean δ¹³C of the input to =−8.0 ± 1.9‰, notably lower than the commonly assumed volcanic degassing value. The isotope mass balance model yields a modern burial flux =15.9 ± 6.6 Tmol y⁻¹. The impact of the mid-Miocene Climatic Optimum isotope anomaly is an integrated excess deposition ≈ 4.3 × 10⁶Tmol between 18 and 11 Ma, which is both longer and larger than estimates for the total degassing by the Columbia River Basalt eruptions, implying a complex carbon system response to large eruptive events. Monte Carlo evaluation finds that late Cenozoic net growth of the carbonate reservoir is very likely while net growth of theC_orgreservoir is less certain but more likely than not. At present, subduction does not appear to keep up with net sedimentation and the overall masses of sedimentary carbonate and organic carbon are likely increasing. Growth in the sedimentaryC_orgreservoir implies oxidation of the surface environment and likely increases in atmospheric pO₂. 

Concern over the impacts of anthropogenic greenhouse gas emissions has led to proposals to offset CO2 emissions by mechanisms that could increase carbon storage in soils. Estimates of the potential efficacy of various strategies vary, but an issue common to all of them is how to verify the magnitude and stability of the results of interventions. Soil sinks are “out of sight” which contrasts with strategies like reforestation. Below-ground carbon sinks, whether they be organic or inorganic, pose substantially more challenging issues for quantification and monitoring. ERW seeks to apply Ca, Mg-rich silicate rocks to enhance the consumption of CO2 by weathering reactions. Measurement of base cation losses from soils is used for quantifying CO2 uptake. Assessing CO2 uptake this way requires assessment of small differences between heterogeneous end members. Geochemical tracers can be used to estimate basalt input assuming that the endmembers are distinct. To compensate for open system behavior normalization to an “immobile” element is necessary. The limitation is typically the highly heterogenous nature of soils. Data from these settings often have high covariances. We reanalyzed published data on amended soils using Monte Carlo uncertainty analysis. We find that in a number of cases the Ca and Mg differences in pre- and post-amendment soils are not significantly different from zero (1 s.e.). High soil chemistry variances makes quantification of small differences difficult. Techniques for estimating relevant sample size and power for noisy data sets and modest effect sizes are well developed in other fields and can be appropriately applied to ERW problems. A simplified example using Lehr’s approximate rule for a two-sided test with s.d. for the pre- and post- data sets = 0.1 and the effect size (net change) = 0.03 yields a sample size n = 178 to obtain a sample power of 0.8 at the 95% CI, an optimistic estimate. Appropriate experimental design for ERW will require substantial sampling effort and rigorous a priori statistical assessment. Current studies so far cannot achieve this. The ocean ∆CO2/∆ALK ratio used to estimate CO2 uptake is important and unlikely to be as high as most studies have assumed. 

Concern over the impacts of anthropogenic greenhouse gas emissions has led to proposals to offset CO2 emissions by mechanisms that could increase carbon storage in soils. Estimates of the potential efficacy of various strategies vary, but an issue common to all of them is how to verify the magnitude and stability of the results of interventions. Soil sinks are “out of sight” which contrasts with strategies like reforestation. Below-ground carbon sinks, whether they be organic or inorganic, pose substantially more challenging issues for quantification and monitoring. ERW seeks to apply Ca, Mg-rich silicate rocks to enhance the consumption of CO2 by weathering reactions. Measurement of base cation losses from soils is used for quantifying CO2 uptake. Assessing CO2 uptake this way requires assessment of small differences between heterogeneous end members. Geochemical tracers can be used to estimate basalt input assuming that the endmembers are distinct. To compensate for open system behavior normalization to an “immobile” element is necessary. The limitation is typically the highly heterogenous nature of soils. Data from these settings often have high covariances. We reanalyzed published data on amended soils using Monte Carlo uncertainty analysis. We find that in a number of cases the Ca and Mg differences in pre- and post-amendment soils are not significantly different from zero (1 s.e.). High soil chemistry variances makes quantification of small differences difficult. Techniques for estimating relevant sample size and power for noisy data sets and modest effect sizes are well developed in other fields and can be appropriately applied to ERW problems. A simplified example using Lehr’s approximate rule for a two-sided test with s.d. for the pre- and post- data sets = 0.1 and the effect size (net change) = 0.03 yields a sample size n = 178 to obtain a sample power of 0.8 at the 95% CI, an optimistic estimate. Appropriate experimental design for ERW will require substantial sampling effort and rigorous a priori statistical assessment. Current studies so far cannot achieve this. The ocean ∆CO2/∆ALK ratio used to estimate CO2 uptake is important and unlikely to be as high as most studies have assumed. 

We investigated how runoff-to-groundwater partitioning changes as a function of substrate age and degree of regolith development in the Island of Hawai’i, by modeling watershed-scale hydrodynamic properties for a series of volcanic catchments of different substrate age developed under different climates. In the younger catchments, rainfall infiltrates directly into the groundwater system and surface runoff is minimal, consisting of ephemeral streams flowing on the scale of hours to days. The older catchments show increasing surface runoff, with deeper incision and perennial discharge. We hypothesize that watershed-scale hydrodynamic properties change as a function of their weathering history—the convolution of time and climate: as surfaces age and become increasingly weathered, hydraulic conductivity is reduced, leading to increased runoff-to-recharge ratios. To test this relationship, we calculated both saturated hydraulic conductivity (k) and aquifer thickness (D) using recession flow analysis. We show that the average k in the younger catchments can be between 3 to 6 orders of magnitude larger than in older catchments, whereas modeled D increases with age. Ephemeral streams with zero baseflow at daily timescales cannot be evaluated using the same method. Instead, we calculated the recession constant for two contiguous catchments developed on young ash or lava deposits of different ages. Increasing bedrock age results in slower recession response in these ephemeral streams, which is consistent with decreasing hydraulic conductivity. Our results highlight the role of the weathering history in determining the evolution of watershed-scale hydrologic properties in volcanic catchments.