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Large earthquakes can construct mountainous topography by inducing rock uplift but also erode mountains by causing landslides. Observations following the 2008 Wenchuan earthquake show that landslide volumes in some cases match seismically induced uplift, raising questions about how the actions of individual earthquakes accumulate to build topography. Here we model the two-dimensional surface displacement field generated over a full earthquake cycle accounting for coseismic deformation, postseismic relaxation, landslide erosion, and erosion-induced isostatic compensation. We explore the related volume balance across different seismotectonic and topographic conditions and revisit the Wenchuan case in this context. The ratio (Ω) between landslide erosion and uplift is most sensitive to parameters determining landslide volumes (particularly earthquake magnitude Mw, seismic energy source depth, and failure susceptibility, as well as the seismological factor responsible for triggering landslides), and is moderately sensitive to the effective elastic thickness of lithosphere, Te. For a specified magnitude, more erosive events (higher Ω) tend to occur at shallower depth, in thicker-Te lithosphere, and in steeper, more landslide-prone landscapes. For given landscape and seismotectonic conditions, the volumes of both landslides and uplift to first order positively scale with Mw and seismic moment Mo. However, higher Mw earthquakes generate lower landslide and uplift volumes per unit Mo, suggesting lower efficiency in the use of seismic energy to drive topographic change. With our model, we calculate the long-term average seismic volume balance for the eastern Tibetan region and find that the net topographic effect of earthquakes in this region tends to be constructive rather than erosive. Overall, destructive events are rare when considering processes over the full earthquake cycle, although they are more likely if only considering the coseismic volume budget (as was the case for the 2008 Wenchuan earthquake where landsliding substantially offset coseismic uplift). Irrespective of the net budget, our results suggest that the erosive power of earthquakes plays an important role in mountain belt evolution, including by influencing structures and spatial patterns of deformation, for example affecting the wavelength of topography.

The evolution of the global carbon and silicon cycles is thought to have contributed to the long-term stability of Earth's climate.sup.1-3. Many questions remain, however, regarding the feedback mechanisms at play, and there are limited quantitative constraints on the sources and sinks of these elements in Earth's surface environments.sup.4-12. Here we argue that the lithium-isotope record can be used to track the processes controlling the long-term carbon and silicon cycles. By analysing more than 600 shallow-water marine carbonate samples from more than 100 stratigraphic units, we construct a new carbonate-based lithium-isotope record spanning the past 3 billion years. The data suggest an increase in the carbonate lithium-isotope values over time, which we propose was driven by long-term changes in the lithium-isotopic conditions of sea water, rather than by changes in the sedimentary alterations of older samples. Using a mass-balance modelling approach, we propose that the observed trend in lithium-isotope values reflects a transition from Precambrian carbon and silicon cycles to those characteristic of the modern. We speculate that this transition was linked to a gradual shift to a biologically controlled marine silicon cycle and the evolutionary radiation of land plants.sup.13,14. Analysis of shallow-water marine carbonate samples from 101 stratigraphic units allows construction of a record of lithium isotopes from the past 3 billion years, tracking the evolution of the global carbon and silicon cycles., Author(s): Boriana Kalderon-Asael [sup.1] , Joachim A. R. Katchinoff [sup.1] , Noah J. Planavsky [sup.1] , Ashleigh v. S. Hood [sup.2] , Mathieu Dellinger [sup.3] , Eric J. Bellefroid [sup.1] [...]

Determining the sources of water provisioning streams, soils, and vegetation can provide important insights into the water that sustains critical ecosystem functions now and how those functions may be expected to respond given projected changes in the global hydrologic cycle. We developed multi-year time series of water isotope ratios (δ18 O and δ2 H) based on twice-monthly collections of precipitation, lysimeter, and tree branch xylem waters from a seasonally dry tropical montane cloud forest in the southeastern Andes mountains of Peru. We then used this information to determine indices of the seasonal origins, the young water fractions (Fyw), and the new water fractions (Fnew) of soil, stream, and tree water. There was no evidence for intra-annual variation in the seasonal origins of stream water and lysimeter water from 1 m depth, both of which were predominantly comprised of wet-season precipitation even during the dry seasons. However, branch xylem waters demonstrated an intra-annual shift in seasonal origin: xylem waters were comprised of wet-season precipitation during the wet season and dry-season precipitation during the dry season. The young water fractions of lysimeter (< 15 %) and stream (5 %) waters were lower than the young water fraction (37 %) in branch xylem waters. The new water fraction (an indicator of water ≤ 2 weeks old in this study) was estimated to be 12 % for branch xylem waters, while there was no significant evidence for new water in stream or lysimeter waters from 1 m depth. Our results indicate that the source of water for trees in this system varied seasonally, such that recent precipitation may be more immediately taken up by shallow tree roots. In comparison, the source of water for soils and streams did not vary seasonally, such that precipitation may mix and reside in soils and take longer to transit into the stream. Our insights into the seasonal origins and ages of water in soils, streams, and vegetation in this humid tropical montane cloud forest add to understanding of the mechanisms that govern the partitioning of water moving through different ecosystems. [ABSTRACT FROM AUTHOR]

Permafrost degradation is altering biogeochemical processes throughout the Arctic. Thaw‐induced changes in organic matter transformations and mineral weathering reactions are impacting fluxes of inorganic carbon (IC) and alkalinity (ALK) in Arctic rivers. However, the net impact of these changing fluxes on the concentration of carbon dioxide in the atmosphere (pCO2) is relatively unconstrained. Resolving this uncertainty is important as thaw‐driven changes in the fluxes of IC and ALK could produce feedbacks in the global carbon cycle. Enhanced production of sulfuric acid through sulfide oxidation is particularly poorly quantified despite its potential to remove ALK from the ocean‐atmosphere system and increase pCO2, producing a positive feedback leading to more warming and permafrost degradation. In this work, we quantified weathering in the Koyukuk River, a major tributary of the Yukon River draining discontinuous permafrost in central Alaska, based on water and sediment samples collected near the village of Huslia in summer 2018. Using measurements of major ion abundances and sulfate (SO42− ${{\text{SO}}_{4}}^{2-}$) sulfur (34S/32S) and oxygen (18O/16O) isotope ratios, we employed the MEANDIR inversion model to quantify the relative importance of a suite of weathering processes and their net impact on pCO2. Calculations found that approximately 80% of SO42− ${{\text{SO}}_{4}}^{2-}$ in mainstem samples derived from sulfide oxidation with the remainder from evaporite dissolution. Moreover, 34S/32S ratios, 13C/12C ratios of dissolved IC, and sulfur X‐ray absorption spectra of mainstem, secondary channel, and floodplain pore fluid and sediment samples revealed modest degrees of microbial sulfate reduction within the floodplain. Weathering fluxes of ALK and IC result in lower values of pCO2 over timescales shorter than carbonate compensation (∼104 yr) and, for mainstem samples, higher values of pCO2 over timescales longer than carbonate compensation but shorter than the residence time of marine SO42− ${{\text{SO}}_{4}}^{2-}$ (∼107 yr). Furthermore, the absolute concentrations of SO42− ${{\text{SO}}_{4}}^{2-}$ and Mg2+ in the Koyukuk River, as well as the ratios of SO42− ${{\text{SO}}_{4}}^{2-}$ and Mg2+ to other dissolved weathering products, have increased over the past 50 years. Through analogy to similar trends in the Yukon River, we interpret these changes as reflecting enhanced sulfide oxidation due to ongoing exposure of previously frozen sediment and changes in the contributions of shallow and deep flow paths to the active channel. Overall, these findings confirm that sulfide oxidation is a substantial outcome of permafrost degradation and that the sulfur cycle responds to permafrost thaw with a timescale‐dependent feedback on warming. Key Points: Sulfide oxidation sources approximately 80% of dissolved SO42− ${{\text{SO}}_{4}}^{2-}$ in the Koyukuk River, Alaska, and may act as a positive feedback on modern warmingInversion calculations reveal weathering is a CO2 sink on timescales of ∼5–10 kyr and a CO2 source on timescales of ∼10 kyr to ∼10 MyrIncreases in the absolute and relative abundance of SO42− ${{\text{SO}}_{4}}^{2-}$ in the Koyukuk River and Yukon River over the past decades suggests thaw‐induced sulfide oxidation [ABSTRACT FROM AUTHOR]

The role of topography in determining water transit times and pathways through catchments is unclear, especially in mountainous environments – yet these environments play central roles in global water, sediment, and biogeochemical fluxes. Since the vast majority of intensively monitored catchments are at northern latitudes, the interplay between water transit, topography, and other landscape and climatic characteristics is particularly underexplored in tropical environments. To address this gap, here we present the results of a multiyear hydrologic sampling campaign (twice-monthly and storm sampling) to quantify water transit in seven small catchments (<1.3 km 2 area) across the transition from the Andes mountains to the Amazon floodplain in southern Peru. We use the stable isotope composition of water (δ18 O) to calculate the fraction of streamflow comprised of recent precipitation ("young water fraction") for each of the seven small catchments. Flow-weighted young water fractions (Fyw) are 5 %–26 % in the high-elevation mountains, 22 %–52 % in the mid-elevation mountains, and 7 % in the foreland floodplain. Across these catchments, topography does not exert a clear control on water transit; instead, stream Fyw is apparently controlled by a combination of hydroclimate (precipitation regime) and bedrock permeability. Mid-elevation sites are posited to have the highest Fyw due to more frequent and intense rainfall; less permeable bedrock and poorly developed soils may also facilitate high Fyw at these sites. Lowland soils have low Fyw due to very low flow path gradients despite low permeability. The data presented here highlight the complexity of factors that determine water transit in tropical mountainous catchments, particularly highlighting the role of intense orographic precipitation at mountain fronts in driving rapid conveyance of water through catchments. These results have implications for the response of Earth's montane "water towers" to climate change and for water–rock reactions that control global biogeochemical cycles. [ABSTRACT FROM AUTHOR]

Sulphide oxidation coupled to carbonate dissolution can provide a transient source of carbon dioxide to Earth's atmosphere and so balance the Cenozoic increase in carbon dioxide consumption by silicate weathering, reconciling this increase with the need for mass balance in the long-term carbon cycle. Balancing the Cenozoic carbon budget It is thought that mountain uplift stimulated carbon dioxide consumption by silicate mineral weathering during the Cenozoic era, roughly spanning the past 66 million years, but there are no signs of a corresponding increase in volcanic carbon dioxide emissions that could balance the carbon budget. This paper suggests that some of the missing carbon dioxide could have been produced by coupled sulphide oxidation and carbonate dissolution, a process that may also have been accelerated in response to uplift. This hypothesis is consistent with isotopic records and may help explain the interactions between the long-term carbon cycle, tectonics, and Earth's climate. The observed stability of Earth's climate over millions of years is thought to depend on the rate of carbon dioxide (CO.sub.2) release from the solid Earth being balanced by the rate of CO.sub.2 consumption by silicate weathering.sup.1. During the Cenozoic era, spanning approximately the past 66 million years, the concurrent increases in the marine isotopic ratios of strontium, osmium and lithium.sup.2,3,4 suggest that extensive uplift of mountain ranges may have stimulated CO.sub.2 consumption by silicate weathering.sup.5, but reconstructions of sea-floor spreading.sup.6 do not indicate a corresponding increase in CO.sub.2 inputs from volcanic degassing. The resulting imbalance would have depleted the atmosphere of all CO.sub.2 within a few million years.sup.7. As a result, reconciling Cenozoic isotopic records with the need for mass balance in the long-term carbon cycle has been a major and unresolved challenge in geochemistry and Earth history. Here we show that enhanced sulphide oxidation coupled to carbonate dissolution can provide a transient source of CO.sub.2 to Earth's atmosphere that is relevant over geological timescales. Like drawdown by means of silicate weathering, this source is probably enhanced by tectonic uplift, and so may have contributed to the relative stability of the partial pressure of atmospheric CO.sub.2 during the Cenozoic. A variety of other hypotheses.sup.8,9,10 have been put forward to explain the 'Cenozoic isotope-weathering paradox', and the evolution of the carbon cycle probably depended on multiple processes. However, an important role for sulphide oxidation coupled to carbonate dissolution is consistent with records of radiogenic isotopes.sup.2,3, atmospheric CO.sub.2 partial pressure.sup.11,12 and the evolution of the Cenozoic sulphur cycle, and could be accounted for by geologically reasonable changes in the global dioxygen cycle, suggesting that this CO.sub.2 source should be considered a potentially important but as yet generally unrecognized component of the long-term carbon cycle., Author(s): Mark A. Torres [sup.1] , A. Joshua West [sup.1] , Gaojun Li [sup.2] Author Affiliations: (1) Department of Earth Sciences, University of Southern California, Los Angeles, USA (2) Department [...]