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Highlights • Plant wax was studied in soil pits under tropical forests at varied elevation. • Plant wax concentration and composition were characterized in litter and soil profiles. • Plant wax D/H invariant within the profiles. • Significant down-profile 13C-enrichment linked to Suess effect and diagenesis. • Below-ground plant wax stocks greatly exceed above-ground stocks. Abstract The waxy coating that protects the leaves and other soft tissues of plants includes n -alkane and n -alkanoic acid compounds that are commonly used as biomarkers to reconstruct past environment. Plant waxes have geological relevance given their persistence in soils and paleosols, as well as in lake and marine sediments, yet diagenesis may alter their molecular and isotopic signatures from synthesis to deposition. This study seeks to understand the fate of plant wax biomarkers in soils after leaf-fall as characterized by a series of tropical soil profiles. We investigate the changes in abundance, molecular distributions, and hydrogen (δ D) and carbon isotopic compositions (δ 13C) of plant waxes (n -alkanes and n -alkanoic acids) in six litter-to-soil profiles along a 2740 m elevation transect from the eastern flank of the Andes mountains down to the lowland Amazon floodplain in Peru. From litter to soil, we find acid/alkane ratios increase while absolute abundances decrease. In contrast, within each soil, acid/alkane ratios are roughly constant, and we find an equivalent exponential decline in concentration in both compound classes with depth, with molecular distributions indicating some new production. We observe a 4–6‰ 13C-enrichment from litter to deeper soils for both C 29 n -alkanes and C 30 n -alkanoic acids, of which the Suess effect accounts for ≤2‰. We infer that microbial degradation and production (or 'turnover') processes influence the δ 13C of plant waxes that survive in soils; in contrast, no systematic change in δ D values is observed. The plant wax signal in soils includes averaging of inputs and diagenetic effects, so this signature is particularly relevant for the interpretation of plant wax archives in paleosols and the plant waxes eroded from soils and exported to downstream sedimentary archives. We show that soils represent the major stock of plant wax under living ecosystems, suggesting that soils may be a quantitatively-important source of plant waxes available for fluvial erosion, with implications for studies of carbon cycling and paleoenvironmental reconstructions from downstream geological archives. [ABSTRACT FROM AUTHOR]

Little is known about the fractionation of Li isotopes during formation of biogenic carbonates, which form the most promising geological archives of past seawater composition. Here we investigated the Li isotope composition (δ 7 Li) and Li/Ca ratios of organisms that are abundant in the Phanerozoic record: mollusks (mostly bivalves), echinoderms, and brachiopods. The measured samples include (i) modern calcite and aragonite shells from various species and natural environments (13 mollusk samples, 5 brachiopods and 3 echinoderms), and (ii) shells from mollusks grown under controlled conditions at various temperatures. When possible, the mollusk shell ultrastructure was micro-sampled in order to assess intra-shell heterogeneity. In this paper, we systematically characterize the influence of mineralogy, temperature, and biological processes on the δ 7 Li and Li/Ca of these shells and compare with published data for other taxa (foraminifera and corals). Aragonitic mollusks have the lowest δ 7 Li, ranging from +16 to +22‰, echinoderms have constant δ 7 Li of about +24‰, brachiopods have δ 7 Li of +25 to +28‰, and finally calcitic mollusks have the largest range and highest δ 7 Li values, ranging from +25‰ to +40‰. Measured brachiopods have similar δ 7 Li compared to inorganic calcite precipitated from seawater (δ 7 Li of +27 to +29‰), indicating minimum influence of vital effects, as also observed for other isotope systems and making them a potentially viable proxy of past seawater composition. Calcitic mollusks, on the contrary, are not a good archive for seawater paleo–δ 7 Li because many samples have significantly higher δ 7 Li values than inorganic calcite and display large inter-species variability, which suggests large vital effects. In addition, we observe very large intra-shell variability, in particular for mixed calcite-aragonite shells (over 20‰ variability), but also in mono-mineralic shells (up to 12‰ variability). Aragonitic bivalves have less variable δ 7 Li (7‰ variability) compared to calcitic mollusks, but with significantly lower δ 7 Li compared to inorganic aragonite, indicating the existence of vital effects. Bivalves grown at various temperatures show that temperature has only a minor influence on fractionation of Li isotopes during shell precipitation. Interestingly, we observe a strong correlation ( R 2  = 0.83) between the Li/Mg ratio in bivalve Mytilus edulis and temperature, with potential implications for paleo-temperature reconstructions. Finally, we observe a negative correlation between the δ 7 Li and both the Li/Ca and Mg/Ca ratio of calcite mollusks, which we relate to biomineralization processes. To explain this correlation, we propose preferential removal of 6 Li from the calcification site of calcite mollusks by physiological processes corresponding to the regulation of the amount of Mg in the calcifying medium. We calculate that up to 80% of the initial Li within the calcification site is removed by this process, leading to high δ 7 Li and low Li/Ca in some calcite mollusk specimens. Collectively, these results suggest that Mg (and thus [Li]) is strongly biologically controlled within the calcifying medium of calcite mollusks. Overall, the results of this study show that brachiopods are likely to be suitable targets for future work on the determination of paleo-seawater Li isotope composition—an emerging proxy for past weathering and hydrothermal processes. [ABSTRACT FROM AUTHOR]

Several years have passed since the global phase-out of leaded petrol use. Nonetheless, emissions from anthropogenic activities remain the principal source of Pb to the oceans. The distribution of elemental Pb and its stable isotopes throughout the surface ocean provide information on the source and transport of these anthropogenic inputs. This study presents dissolved Pb concentrations and isotopic distributions from 110 surface water samples collected during the Malaspina 2010 Circumnavigation Expedition. Dissolved Pb concentrations ranged from 10 pM to 49 pM across the sampling stations covering all major ocean basins. The highest concentrations were found in the northeast Atlantic Ocean and the lowest in both the south Pacific and south Atlantic Oceans. Lead concentrations measured in the north Pacific Ocean, near Hawaii, were compared to previously published data from the same region. That comparison showed that Pb concentration has decreased ∼40% since 1975, although the rate of decrease has slowed in the past two decades. The overall decline in concentration probably has been induced by the cessation of leaded gasoline use in North America. The temporal evolution of stable Pb isotopes in this region shows a shift from dominant North American-like composition in 1979 towards a more Asian-like composition in later years. More widely, the distribution of Pb and Pb isotopes measured in the Malaspina sample set of global surface waters were compared with previously published ratios of aerosols and other atmosphere-derived Pb sources from the countries surrounding the different ocean basins. This comparison identified the potential Pb sources to each ocean basin, providing new insights into the transport and fate of Pb in the oceans. [ABSTRACT FROM AUTHOR]

Changes in δ 13 C carb and δ 13 C org from marine strata occur globally in association with the end-Triassic mass extinction and the emplacement of the Central Atlantic Magmatic Province (CAMP) during the break up of Pangea. As is typical in deep time, the timing and duration of these isotopic excursions has remained elusive, hampering attempts to link carbon cycle perturbations to specific processes. Here, we report δ 13 C carb and δ 13 C org from Late Triassic and Early Jurassic strata near Levanto, Peru, where intercalated dated ash beds permit temporal calibration of the carbon isotope record. Both δ 13 C carb and δ 13 C org exhibit a broad positive excursion through the latest Triassic into the earliest Jurassic. The first order positive excursion in δ 13 C org is interrupted by a negative shift noted in many sections around the world coincident with the extinction horizon. Our data indicate that the negative excursion lasts 85 ± 25 kyrs, longer than inferred by previous studies based on cyclostratigraphy. A 260 ± 80 kyr positive δ 13 C org shift follows, during which the first Jurassic ammonites appear. The overall excursion culminates in a return to pre-perturbation carbon isotopic values over the next 1090 ± 70 kyrs. Via chronologic, isotopic, and biostratigraphic correlation to other successions, we find that δ 13 C carb and δ 13 C org return to pre-perturbation values as CAMP volcanism ceases and in association with the recovery of pelagic and benthic biota. However, the initiation of the carbon isotope excursion at Levanto predates the well-dated CAMP sills from North America, indicating that CAMP may have started earlier than thought based on these exposures, or that the onset of carbon cycle perturbations was not related to CAMP. [ABSTRACT FROM AUTHOR]

Earthquakes cause widespread landslides that can increase erosional fluxes observed over years to decades. However, the impact of earthquakes on denudation over the longer timescales relevant to orogenic evolution remains elusive. Here we assess erosion associated with earthquake-triggered landslides in the Longmen Shan range at the eastern margin of the Tibetan Plateau. We use the M w 7.9 2008 Wenchuan and M w 6.6 2013 Lushan earthquakes to evaluate how seismicity contributes to the erosional budget from short timescales (annual to decadal, as recorded by sediment fluxes) to long timescales (kyr to Myr, from cosmogenic nuclides and low temperature thermochronology). Over this wide range of timescales, the highest rates of denudation in the Longmen Shan coincide spatially with the region of most intense landsliding during the Wenchuan earthquake. Across sixteen gauged river catchments, sediment flux-derived denudation rates following the Wenchuan earthquake are closely correlated with seismic ground motion and the associated volume of Wenchuan-triggered landslides ( r 2 > 0.6 ), and to a lesser extent with the frequency of high intensity runoff events ( r 2 = 0.36 ). To assess whether earthquake-induced landsliding can contribute importantly to denudation over longer timescales, we model the total volume of landslides triggered by earthquakes of various magnitudes over multiple earthquake cycles. We combine models that predict the volumes of landslides triggered by earthquakes, calibrated against the Wenchuan and Lushan events, with an earthquake magnitude–frequency distribution. The long-term, landslide-sustained “seismic erosion rate” is similar in magnitude to regional long-term denudation rates (∼0.5–1 mm yr −1 ). The similar magnitude and spatial coincidence suggest that earthquake-triggered landslides are a primary mechanism of long-term denudation in the frontal Longmen Shan. We propose that the location and intensity of seismogenic faulting can contribute to focused denudation along a high-relief plateau margin. [ABSTRACT FROM AUTHOR]

The interplay between the hydrologic processes that supply, store, and route water in catchment systems and the chemical weathering reactions that add and remove solutes acts as an important control on chemical weathering fluxes. In this study, we use paired measurements of solute chemistry and runoff in four nested catchments that span the transition from the Andes Mountains to the Amazonian foreland floodplain in Peru in order to investigate the links between hydrology and weathering processes and to determine how these links change across a geomorphic gradient. All of the sites show variation in elemental concentrations and ratios with runoff consistent with hydrologically driven changes in lithologic sources, the extent of secondary mineral precipitation, and, potentially, fluid flow paths. In the Andean sites, solute concentrations are relatively constant despite large changes in runoff. This is in direct contrast to the foreland floodplain site, where solute concentrations are diluted as runoff increases. In the Andes–Amazon, the concentration–runoff behavior is correlated with the mean catchment slope angle, which suggests that erosional processes, by modulating the timescales over which weathering reactions occur within the critical zone, can be an underlying control on solute production and therefore on chemical weathering. Due to the co-variation between the geomorphic and hydrologic controls on chemical weathering, weathering fluxes in Andean sites are more sensitive to seasonal changes in runoff than in the foreland floodplain site. [ABSTRACT FROM AUTHOR]

The Li isotopic record of seawater shows a dramatic increase of ~9‰ over the past ~60 million years. Here we use a model to explore what may have caused this change. We focus particularly on considering how changes in the "reverse weathering" sinks that remove Li from seawater can contribute to explain the observed increase. Our interpretation is based on dividing the oceanic sink, which preferentially removes light Li, into two components: (i) removal into marine authigenic clays in sediments at low temperatures, with associated high fractionation factors, and (ii) removal into altered oceanic basalt at higher temperatures and resulting lower fractionation factors. We suggest that increases in the flux of degraded continental material delivered to the oceans over the past 60 Ma could have increased removal of Li into sedimentary authigenic clays versus altered basalt. Because altered basalt is associated with a smaller isotopic fractionation, an increasing portion of the lower temperature (authigenic clay-associated) sink could contribute to the rise of the seawater Li isotope value. This effect would moderate the extent to which the isotopic value of continental inputs must have changed in order to explain the seawater record over the Cenozoic. Nonetheless, unless the magnitude of fractionation during removal differs significantly from current understanding, substantial change in the δ7Li of inputs from continental weathering must have occurred. Our modeling suggests that dissolved riverine fluxes in the early Eocene were characterized by δ7Li of ~0 to +13‰, with best estimates of 6.6-12.6‰; these values imply increases over the past 60 Myrs of between 10 and 24‰, and we view a ~13‰ increase as a likely scenario. These changes would have been accompanied by increases in both the dissolved Li flux from continental weathering and the removal flux from seawater into marine authigenic clays. Increases in δ7Li of continental input are consistent with a change in the global denudation regime as a result of increasing continental erosion rate through the Cenozoic. Changes in denudation may have meant increasing climate sensitivity of weathering over time but do not require globally supply-limited and thus entirely climate-insensitive weathering in the early Cenozoic. [ABSTRACT FROM AUTHOR]

Abstract: The concentration of 10Be in detrital quartz (10Beqtz) from river sediments is now widely used to quantify catchment-wide denudation rates but may also be sensitive to inputs from bedrock landslides that deliver sediment with low 10Beqtz. Major landslide-triggering events can provide large amounts of low-concentration material to rivers in mountain catchments, but changes in river sediment 10Beqtz due to such events have not yet been measured directly. Here we examine the impact of widespread landslides triggered by the 2008 Wenchuan earthquake on 10Beqtz in sediment samples from the Min Jiang river basin, in Sichuan, China. Landslide deposit material associated with the Wenchuan earthquake has consistently lower 10Beqtz than in river sediment prior to the earthquake. River sediment 10Beqtz decreased significantly following the earthquake downstream of areas of high coseismic landslide occurrence (i.e., with greater than ∼0.3% of the upstream catchment area affected by landslides), because of input of the 10Be-depleted landslide material, but showed no systematic changes where landslide occurrence was low. Changes in river sediment 10Beqtz concentration were largest in small first-order catchments but were still significant in large river basins with areas of . Spatial and temporal variability in river sediment 10Beqtz has important implications for inferring representative denudation rates in tectonically active, landslide-dominated environments, even in large basins. Although the dilution of 10Beqtz in river sediment by landslide inputs may complicate interpretation of denudation rates, it also may provide a possible opportunity to track the transport of landslide sediment. The associated uncertainties are large, but in the Wenchuan case, calculations based on 10Be mixing proportions suggest that river sediment fluxes in the 2–3 years following the earthquake increased by a similar order of magnitude in the 0.25–1 mm and the <0.25 mm size fractions, as determined from 10Beqtz mixing calculations and hydrological gauging, respectively. Such information could provide new insight into sediment transfer, with implications for secondary sediment-related hazards and for understanding the removal of mass from mountains. [Copyright &y& Elsevier]

Abstract Solutes derived from the dissolution of silicate minerals play a key role in Earth's climate via the carbon and other biogeochemical cycles. Silicon (Si) is a unique constituent of silicate minerals and a biologically important nutrient, so tracing its behavior in near-surface environments may provide important insights into weathering processes. However, Si released by weathering is variably incorporated into secondary mineral phases and biota, obscuring signals derived from primary weathering processes. Due to chemical similarities, Germanium (Ge) may help better understand the Si cycle and its relationship to chemical weathering. With this aim, we report new measurements of the concentration and isotopic composition of Ge for both the dissolved and particulate phases of a variety of global rivers. These measurements are combined with analyses of concentration and isotopic ratio of Si on the exact same sample set in order to make direct comparisons of the behavior of these two elements in natural river systems. With this dataset, we develop a new modeling framework describing the full elemental and isotopic systems of these solutes in rivers (i.e., Ge/Si, δ 74 Ge , and δ 30 Si). This multi-proxy approach allows us to ascertain the relative importance of biological versus mineral uptake in modulating the fluxes of these elements delivered to the modern ocean. Dissolved δ 74 Ge composition of rivers studied thus far range from 0.9 to 5.5‰ with a discharge-weighted global average of 2.6 ± 0.5‰. The Ge isotope composition of riverine suspended and bedload sediments is indistinguishable from silicate source rocks, which is consistent with mass balance expectations. The multi-proxy modeling suggests that, among the watersheds studied here, the isotopic fractionation of Si during secondary mineral phase precipitation (Δ 30 Si s e c) ranges from −2.7 to −0.2‰, which removes between 19–79% of the initial dissolved Si, while between 12–54% is incorporated by biota. For Ge, modeling indicates that 79–98% of the dissolved load is incorporated into secondary mineral phases with a Δ 74 Ge s e c ranging from −4.9 to −0.3‰. The fractionation induced by biological uptake is calculated to range from −2.6 to −1.3‰ for Δ 30 Si b i o and −0.7 ± 0.7‰ for Δ 74 Ge b i o. In addition to improving our understanding of the coupled Ge and Si cycles, our study provides a framework for using multiple isotopic tracers to elucidate the chemical behavior of solutes in natural waters. Highlights • Ge and Si isotope composition of water and sediments in various rivers is presented. • Dissolved δ 74 Ge range 0.9–5.5‰, river sediment δ 74 Ge range 0.5–0.7‰. • Dissolved δ 74 Ge is strongly fractionated due to low chemical Ge mobility. • Ge/Si-isotope multi-proxy provides quantitative constraints on secondary vs biological fractionation. [ABSTRACT FROM AUTHOR]

Mountainous areas affected by natural wildfires and highly urbanized settings both have been identified as sources of pollutants to receiving waters. In this study, the partitioning of metals between the colloidal (0.02–0.2 μm) and soluble (<0.02 μm) phases in storm-runoff was evaluated in three different environments: (i) a catchment affected by the 2012 Williams Fire (2–13 September 2012) in the San Gabriel Mountains, (ii) a neighboring control catchment in the San Gabriels, and (iii) three rivers draining the Los Angeles County area. Results from each region were compared to each other in order to gain information about potential sources of soluble (i.e., bioavailable) elements that are delivered to receiving waters. Results show that in runoff from recently burned areas, 58% and 24% of the total dissolved (<0.2 μm) Pb and Fe, respectively, was present in the soluble pool. In contrast, runoff from urban and natural unburned areas carried less than 17% and 8% of the total dissolved Pb and Fe, respectively, in the soluble pool. These results suggest that storm runoff from burned landscapes has the previously unrecognized potential to supply a greater proportion of trace elements in bioactive soluble form, compared to runoff from urban or unburned areas, potentially increasing the impact of wildfire-delivered metals on receiving waters. [ABSTRACT FROM AUTHOR]

The rate of chemical weathering has been observed to increase with the rate of physical erosion in published comparisons of many catchments, but the mechanisms that couple these processes are not well understood. We investigated this question by examining the chemical weathering and porosity profiles from catchments developed on marine shale located in Pennsylvania, USA (Susquehanna Shale Hills Critical Zone Observatory, SSHCZO); California, USA (Eel River Critical Zone Observatory, ERCZO); and Taiwan (Fushan Experimental Forest). The protolith compositions, protolith porosities, and the depths of regolith at these sites are roughly similar while the catchments are characterized by large differences in erosion rate (1–3 mm yr−1 in Fushan ≫ 0.2–0.4 mm yr−1 in ERCZO ≫ 0.01–0.025 mm yr−1 in SSHCZO). The natural experiment did not totally isolate erosion as a variable: mean annual precipitation varied along the erosion gradient (4.2 m yr−1 in Fushan > 1.9 m yr−1 in ERCZO > 1.1 m yr−1 in SSHCZO), so the fastest eroding site experiences nearly twice the mean annual temperature of the other two. Even though erosion rates varied by about 100×, the depth of pyrite and carbonate depletion (defined here as regolith thickness) is roughly the same, consistent with chemical weathering of those minerals keeping up with erosion at the three sites. These minerals were always observed to be the deepest to react, and they reacted until 100% depletion. In two of three of the catchments where borehole observations were available for ridges, these minerals weathered across narrow reaction fronts. On the other hand, for the rock-forming clay mineral chlorite, the depth interval of weathering was wide and the extent of depletion observed at the land surface decreased with increasing erosion/precipitation. Thus, chemical weathering of the clay did not keep pace with erosion rate. But perhaps the biggest difference among the shales is that in the fast-eroding sites, microfractures account for 30–60% of the total porosity while in the slow-eroding shale, dissolution could be directly related to secondary porosity. We argue that the microfractures increase the influx of oxygen at depth and decrease the size of diffusion-limited internal domains of matrix, accelerating weathering of pyrite and carbonate under high erosion-rate conditions. Thus, microfracturing is a process that can couple physical erosion and chemical weathering in shales. [ABSTRACT FROM AUTHOR]

Abstract Erosion of organic carbon from the terrestrial biosphere and sedimentary rocks plays an important role in the global carbon cycle across a range of timescales. Over geological timescales (>104 years), erosion and burial of particulate organic carbon (POC) from the terrestrial biosphere (POC biosphere) is an important CO 2 sink, while oxidation of organic carbon derived from sedimentary rocks (petrogenic, POC petro) releases CO 2 to the atmosphere. Over decadal to millennial timescales, the balance between POC biosphere production and degradation affects atmospheric CO 2 concentrations. To better constrain the controls on erosional carbon transfers, here we quantify POC biosphere and POC petro fluxes in a mountain range with relatively low runoff, the Longmen Shan, which drains the eastern margin of the Tibetan Plateau. We measure total organic carbon content ([OC total ]) and the carbon isotopic compositions (13C/12C expressed as δ13C; 14C/12C expressed as fraction modern or F mod) of organic matter in suspended sediments collected from six gauging stations on the Min Jiang, a tributary of the Yangtze River, from 2005 to 2012. We find that POC petro has a large range of δ13C, from −26.2‰ to −13.2‰. This POC petro mixes with POC biosphere to set the δ13C of POC in river sediments. Binary mixing models reveal the possibility of aged POC biosphere at two gauging stations which drain the high elevations of the eastern Tibetan Plateau, with modelled biospheric F mod values of 0.82 ± 0.09 and 0.84 ± 0.08. This is consistent with prior suggestions of aged biospheric carbon being eroded from the plateau. The annual POC petro yields range from 0.04 ± 0.02 tC km−2 yr−1 to 1.69 ± 0.56 tC km−2 yr−1 across the five study catchments, with basin average yield that appears to be linked to catchment average slope as a likely proxy for erosion rate. Here, the variability in the petrogenic organic carbon content of rocks masks the signal of the weathering and oxidation of this rock-derived organic carbon. The annual POC biosphere yields range from 0.21 ± 0.04 tC km−2 yr−1 to 3.33 ± 0.57 tC km−2 yr−1. These values are towards the lower end of those measured in mountain ranges around the world, which we suggest not only reflects the relatively low erosion rates of the Longman Shan, but also the low annual runoff (<1 m yr−1). Across this region, the river POC biosphere discharge is related to the intensity of runoff events. Our data suggest that a wetter (and/or stormier) climate could increase the erosional export of POC biosphere in this tectonically-active mountain range. Depending on the fate of POC biosphere downstream in larger river systems, this could act as carbon-cycle climate feedback over geological timescales. [ABSTRACT FROM AUTHOR]

Highlights • Leaf wax biomarkers were studied in soils and rivers from the Andes to the Amazon. • Elevation trend found in C and H isotopic compositions of plant wax in soils. • Plant wax in river-suspended sediments approximated uniform spatial integration. • Carbon isotopic composition of n -alkanes differentiated with depth in soil and river. • Petrogenic inputs of n -alkanes discerned by CPI and dual isotopic analyses. Abstract Tropical montane regions tend to have high rates of precipitation, biological production, erosion, and sediment export, which together move material off the landscape and toward sedimentary deposits downstream. Plant wax biomarkers can be used to investigate sourcing of organic matter and are often used as proxies to reconstruct past climate and environment in sedimentary deposits. To understand how plant waxes are sourced within a wet, tropical montane catchment, we measure the stable C and H isotope composition (δ 13C and δ D) of n -alkanes and n -alkanoic acids in soils along an elevation transect and from sediments within the Madre de Dios River network along the eastern flank of the Peruvian Andes, draining an area of 75,400 km2 and 6 km of elevation. Soils yield systematic trends in plant wax δ 13C (+1.75 and +1.31‰ km−1, for the C 29 n -alkanes and C 30 n -alkanoic acids respectively in the mineral horizon) and δ D values (−10 and −12‰ km−1, respectively) across a 3.5 km elevation transect, which approximates trends previously reported from canopy leaves, though we find offsets between δ 13C values in plants and soils. River suspended sediments generally follow soil isotopic gradients defined by catchment elevations (δ 13C: +1.03 and +0.99‰ km−1 and δ D: −10 to −7‰ km−1, for the C 29 n -alkanes and C 30 n -alkanoic acids respectively) in the wet season, with a lowering in the dry season that is less well-constrained. In a few river suspended sediments, petrogenic contributions and depth-sorting influence the n -alkane δ 13C signal. Our dual isotope, dual compound class and seasonal sampling approach reveals no Andean-dominance in plant wax export, and instead that the sourcing of plant waxes in this very wet, forested catchment approximates that expected for spatial integration of the upstream catchment, thus with a lowland dominance on areal basis, guiding paleoenvironmental reconstructions in tropical montane regions. The dual isotope approach provides a cross-check on the altitudinal signals and can resolve ambiguity such as might be associated with vegetation change or aridity in paleoclimate records. Further, the altitude effect encoded within plant waxes presents a novel dual-isotope biomarker approach to paleoaltimetry. [ABSTRACT FROM AUTHOR]

Abstract The ratio between the activity of dissolved 234U and 238U in river water, (234U/238U) Riv , shows promise for tracing weathering processes. It has been suggested that preferential 234U release generates a high (234U/238U) Riv under low weathering intensity while congruent weathering, as expected for high weathering intensity, would release uranium with a secular equilibrium (234U/238U) Riv of 1. However, weathering intensity depends on the combination of exposure age of weathering interfaces and dissolution rate, complicating the use of (234U/238U) Riv as a tool in weathering studies. This work attempts to resolve the weathering dynamics reflected by (234U/238U) Riv using catchment scale denudation rate (D) as a first order approximation for the age of weathering interfaces. A global dataset (n = 653), including 132 new measurements from Chinese catchments, indicates changing responses of (234U/238U) Riv to D. Higher values (>1.5) and larger variability of (234U/238U) Riv are observed in catchments both with high (>3 mm/yr) and low D (<0.1 mm/yr). In contrast, the lowest values (approaching secular equilibrium) and the least variable (234U/238U) Riv are associated with catchments of medium to high D (0.3–2 mm/yr). The observed pattern can be explained by a model that involves competing uranium release from active- and inactive-weathering interfaces in response to a shifting weathering regime, from kinetically-limited to supply-limited. (234U/238U) Riv decreases with decreasing D under a kinetically-limited weathering regime due to the increasing contribution of congruent weathering relative to the preferential dissolution of 234U from radioactively damaged sites as D decreases. In contrast, (234U/238U) Riv increases with decreasing D under a supply-limited weathering regime because the minerals that remain under these conditions continue to release 234U to solution by recoil, while contributing little to the dissolved 238U because of their slow dissolution rate. A much deeper depth of the weathering interface is suggested for catchments with high D due to higher weathering contribution from deep fractures relative to the poorly developed surface regolith. A young age of ∼10 kyrs is constrained by the model for complete dissolution of exposed weathering-active minerals, which implies a rapid shifting from kinetically-limited to supply-limited weathering regime and limited influence of weathering on the calculation of comminution age based on (234U/238U) in sediments. A site-specific dissolution rate that is consistent with those estimated from weathering profiles is also constrained, which suggests that the discrepancy between field and laboratory weathering rate may largely be controlled by the difference in chemical affinity associated with stagnant micro-pores, low permeability, and high rock/fluid ratios rather than the passivation of weathering interfaces. Highlights • We observed changing responses of riverine dissolved (234U/238U) value to changes in catchment denudation rates. • The observation reflects shifting weathering limitation regimes. • We proposed a model that captures most variability of the riverine (234U/238U). • Site-specific dissolution rates ∼3 orders of magnitude lower than laboratorial experiments are constrained. [ABSTRACT FROM AUTHOR]

Plant leaf waxes have been found to record the hydrogen isotopic composition of precipitation and are thus used to reconstruct past climate. To assess how faithfully they record hydrological signals, we characterize leaf wax hydrogen isotopic compositions in forest canopy trees across a highly biodiverse, 3 km elevation range on the eastern flank of the Andes. We sampled the dominant tree species and assessed their relative abundance in the tree community. For each tree we collected xylem and leaf samples for analysis of plant water and plant leaf wax hydrogen isotopic compositions. In total, 176 individuals were sampled across 32 species and 5 forest plots that span the gradient. We find both xylem water and leaf wax δ D values of individuals correlate ( R 2 = 0.8 and R 2 = 0.3 respectively) with the isotopic composition of precipitation (with an elevation gradient of −21‰ km −1 ). Minimal leaf water enrichment means that leaf waxes are straightforward recorders of the isotopic composition of precipitation in wet climates. For these tropical forests we find the average fractionation between source water and leaf wax for C 29 n -alkanes, −129 ± 2‰ (s.e.m., n = 136), to be indistinguishable from that of temperate moist forests. For C 28 n -alkanoic acids the average fractionation is −121 ± 3‰ (s.e.m., n = 102). Sampling guided by community assembly within forest plots shows that integrated plant leaf wax hydrogen isotopic compositions faithfully record the gradient of isotopes in precipitation with elevation ( R 2 = 0.97 for n -alkanes and 0.60 for n -alkanoic acids). This calibration study supports the use of leaf waxes as recorders of the isotopic composition of precipitation in lowland tropical rainforest, tropical montane cloud forests and their sedimentary archives. [ABSTRACT FROM AUTHOR]

Abstract: The Altai range (western Mongolia) accommodates NNE–SSW shortening across the northern India–Eurasia collision zone by dextral slip on faults trending NNW–SSE, and anticlockwise, vertical-axis rotations of fault-bounded blocks. However, fault slip-rates and the way in which faulting evolves over time are poorly understood, and form the motivation for this study. We focussed on the Har-Us-Nuur fault, a major transpressional fault bounding the eastern margin of the Altai. Three abandoned alluvial fan surfaces, each displaced right-laterally by the fault, were targeted for dating with cosmogenic 10Be and quartz optically stimulated luminescence (OSL). The first surface (A2) shows an exponential decrease in 10Be with increasing depth, with a significant inherited component. Modelling this profile yielded a minimum age of 74.1 ka. Material from the same sampling pit was dated at ~19 ka with OSL, but we consider this younger age to be incorrect, possibly due to feldspar contamination or abnormal quartz OSL characteristics. The A2 surface is displaced by 175 m, implying a (maximum) dextral slip-rate of 2.4±0.4 mm yr−1. A second fan surface (F1) was dated at ~6 ka with OSL and shows little variation in 10Be with depth, consistent with this young age. The inherited component is higher than for A2, indicating contrasting levels of inheritance for different periods of fan aggradation. A final surface (F2) shows scattered 10Be concentrations and lacks material suitable for OSL, so cannot be dated precisely. Using the total vertical displacement across the fault, we place the initiation of movement on the fault at ~2 Ma, significantly later than the late Oligocene to Miocene (28–5 Ma) onset of shortening in the Altai region. This suggests that deformation in the Altai has widened over time to incorporate new faults at the range margins (such as Har-Us-Nuur), possibly because older faults in the range interior have rotated about vertical axes into orientations that require work to be done against gravity. [Copyright &y& Elsevier]

• We document the precursor carbon isotope excursion at the Ferguson Hill site, USA. • Sulphidic sediments are documented preceding the end-Triassic mass extinction. • Episodic anoxic conditions restructured shallow marine benthic ecosystems. • Marine ecosystem restructuring likely related to the early phase of CAMP volcanism. During the Phanerozoic, major global upheavals in life history and the carbon cycle are predominantly linked to the emplacement of large igneous provinces, but the delineation of a cause and effect framework remains unclear. The end-Triassic mass extinction (ETE) is temporally associated with emplacement of the Central Atlantic magmatic province (CAMP). A better understanding of precursor events to the ETE is essential if the mechanisms for this mass extinction are to be fully delineated. Here, we present new high-resolution data integrating petrographic, biotic, mercury, and carbon isotope analyses of the pre-extinction interval at the Ferguson Hill locality, Nevada (USA). We document the "precursor" carbon isotope excursion along with low Hg concentrations and sulphidic sediments prior to the ETE. A combination of proxies reveals disruptions to shallow marine ecosystems and biogeochemical cycles prior to the main phase of CAMP volcanism. We propose that episodic anoxic conditions led to the restructuring of shallow marine benthic ecosystems towards overall lower diversity, including more low oxygen tolerant taxa preceding the ETE. The timing of the initial marine ecosystem restructuring in eastern Panthalassa could be related to the early phase of CAMP emplacement, and implies that an early intrusive event initiated the ecosystem changes. These restructured marine ecosystems reflect the deteriorating environmental conditions leading up to the ETE that ultimately resulted in the ETE. [ABSTRACT FROM AUTHOR]

Abstract: Large seasonal variations in the dissolved load of the headwater tributaries of the Marsyandi river (Nepal Himalaya) for major cations and 87Sr/86Sr ratios are interpreted to result from a greater dissolution of carbonate relative to silicate at high runoff. There is up to a 0.003 decrease in strontium isotope ratios and a factor of 3 reduction in the Si(OH)4/Ca ratio during the monsoon. These variations, in small rivers sampling uniform lithologies, result from a different response of carbonate and silicate mineral dissolution to climatic forcing. Similar trends are observed in compiled literature data, from both Indian and Nepalese Himalayan rivers. Carbonate weathering is more sensitive to monsoonal runoff because of its faster dissolution kinetics. Silicate weathering increases relative to carbonate during the dry season, and may be more predominant in groundwater with longer water–rock interaction times. Despite this kinetic effect, silicate weathering fluxes are dominated by the monsoon flux, when between 50% and 70% of total annual silicate weathering flux occurs. [Copyright &y& Elsevier]

Estimation of landslide erosion rates and hazard prediction require a firm understanding of the physical controls on landslide size. In this study we seek to understand how the characteristics of different landscapes and forcing events influence the distribution of landslide size at a regional scale. We explore the parameter space of a mechanically-based landslide model through a series of simulations using digital elevation data. Consistent with previous studies, we find that large slope failures are infrequent due to the scarcity of large, steep hillslopes in typical mountainous topography. On the other hand, we find that the occurrence of small landslides is limited by the cohesive strength of hillslope material, which overcomes the weaker driving forces on short slopes. We test our model results with an empirical investigation of frequency-size distributions for eight real co-seismic landslide inventories. Although empirical data are noisy, we find a positive correlation between landslide size and hillslope relief, while the effects of PGA on landslide size are less pronounced. We conclude that landslide size distributions reflect the available distribution of hillslope geometries in a given landscape, and that external forcing (e.g., seismic ground motion) determines which subset of the hillslope distribution fails during a particular event. For a given landscape and forcing event, there is a particular length-scale over which landsliding is statistically favored, leading to the concept of characteristic landslide size distributions. • Landslide size statistics can be approximated by the lognormal distribution. • Simulated landslide size correlates with cohesive strength and hillslope relief. • Average landslide size reflects the distribution of hillslope dimensions in a landscape. • Landslide distributions are characteristic of the landscape and forcing event. [ABSTRACT FROM AUTHOR]