High Chemical Weathering Rates Research Articles

Dissolved trace element concentrations and fluxes in the Irrawaddy, Salween, Sittaung and Kaladan Rivers

The Irrawaddy (Ayeyarwaddy) and Salween (Thanlwin) globally rank among the largest rivers for supplying dissolved and particulate material to the ocean. Along with the Sittaung and Kaladan rivers they have societal importance to Myanmar in terms water sources and food production. Despite their importance for global biogeochemical cycles and the ~50 million people who live in their catchments, the chemistry of these rivers is poorly known. This study presents a comprehensive survey of dissolved (<0.22 μm) trace element concentrations (Sr, Fe, Al, Ba, Mn, V, Rb, Cu, Zn, As, Li, Ni, Mo, Cr, U, Pb, Sb, Co, Cs, Tl and Cd) at 38 locations within these river catchments, spanning a period of 2 years. The results highlight the global importance of the Irrawaddy and Salween rivers for trace element global biogeochemical cycles; contributing between 1 and 17 % of global dissolved riverine fluxes to the land-ocean interface for the studied elements. Area normalized dissolved fluxes in these catchments are ~2 to 10 times higher than global average values for most elements, consistent with high rates of chemical weathering. In general, anthropogenic activities have yet to significantly perturb dissolved trace element fluxes in these river systems. The presented dataset should therefore serve as a useful ‘natural’ baseline, against which future perturbations driven by climate change and/or the development of Myanmar's mining industry could be assessed. Exceptions to this include As in the Sittaung River and Sb, Zn, Pb and As in the Salween River, which may already be significantly impacted by anthropogenic inputs. The former represents a water quality issue of concern for public health, and so constraining the exact sources of As in the Sittaung River should be considered a priority for future research.

Soil Formation and Mass Redistribution during the Holocene Using Meteoric 10Be, Soil Chemistry and Mineralogy

Soil development and erosion are important and opposing processes in the evolution of high-mountainous landscapes, though their dynamics are not fully understood. We compared soil development between a calcareous and a siliceous chronosequence in the central Swiss Alps at high altitudes, which both cover soil formation over the Holocene. We calculated element mass balances, long-term erosion rates based on meteoric 10Be and we determined the rates of soil formation. We also analyzed the shifts in the mineralogical composition, weathering indices, the particle size distribution, carbon stocks and oxalate extractable Fe, Al, and Mn. The siliceous soils had high chemical weathering rates at the early stage of soil formation that strongly decreased after a few millennia. The development of calcareous soil was characterized by high carbonate losses and a shift to finer soil texture. Soil erosion hampered the upbuilding of soil horizons in the early stages of soil development, which led to a delay in soil and vegetation development. This study shows how soil formation drivers change over time. In the early stages of soil development, the parent material predominantly drives soil formation while at later stages the vegetation becomes more dominant as it influences surface stability, hydrological pathways, and chemical weathering that determine water drainage and retention.

Geochemical and palynological records of rapid climatic change from the early to late Aptian: evidence from the Jiaolai Basin, eastern China

The Laiyang Group in the Jiaolai Basin, eastern China, contains Lower Cretaceous continental river and lake deposits that provide a complete record of the evolution of the paleoclimate in the Laiyang area from the early to late Aptian. This study used novel chemical weathering indices and palynological data for Lower Cretaceous mudstone in the Laiyang Group coupled with existing geochemical data, clay mineralogy and plant fossil record to reconstruct the geochemical weathering conditions and paleoclimatic evolution of the Jiaolai Basin over this period. Four distinctive episodes of paleoclimatic change were identified, with one linked to an extremely dry–hot climatic event correlated with the Oceanic Anoxia Event (OAE)1a. During Stage 1, strong chemical weathering was correlated with a relatively warm–humid climate during the early Aptian but the intensity of chemical weathering tended to decrease with an increasingly dry and hot climate. In contrast, Stage 2 was characterised by stable conditions with weak chemical weathering and an extremely dry, hot climate during the early and middle Aptian. The presence of organic-rich black shale and the associated distribution of organic carbon isotope values verified that the extremely dry–hot climate during this stage was related to OAE 1a. Stage 3 was a transitional period extending through the middle and late Aptian during which the climate became relatively warm and humid, triggering increases in chemical weathering. Finally, Stage 4 during the late Aptian was characterised by a return to high chemical weathering rates and a warmer climate. Notably, the paleoclimatic changes in the Aptian period indicated by the geochemical record of the Jiaolai Basin are synchronous with those indicated by marine records. The results of this study provide a reference for further study of this climatic event in terrestrial systems within the context of the greenhouse climate in the Cretaceous. KEY POINTS Geochemical and palynological records of rapid climatic change are preserved in lower to upper Aptian terrestrial sediments in eastern China. Chemical weathering in inland east Asia during OAE 1a first increased, then decreased and then increased again. Terrestrial vegetation responded to global climate change during the early Aptian period in eastern China.

Periglacial landforms in the high Drakensberg, Southern Africa: morphogenetic associations with rock weathering rinds and shrub growth patterns

ABSTRACT Here, we aim to establish possible morphogenetic associations between periglacial phenomena (sorted circles and turf-/stone-banked lobes) by examining (a) rock surface weathering rinds by thin section microscopy and SEM-EDX analysis, and (b) alpine shrub growth patterns of Helichrysum trilineatum supported by AMS 14C age determination. The study area is the Mafadi–Njesuthi summit zone, one of the highest alpine regions of southern Africa. Rock weathering results indicate that clasts in large sorted circle centres experience high chemical weathering rates on both exposed (upper) and non-exposed (lower) surfaces, whereas clasts in lobes and very large apparently ‘relict’ sorted circles have been inactive for a prolonged period of time, based on strongly contrasting weathering rates on both exposed and non-exposed clast surfaces. Large sorted circles originating from a previous (possibly Last Glacial Maximum or late-glacial) colder period are still marginally or episodically active, thus restricting plant succession on these landforms. Even where very large sorted circles and turf-/stone-banked lobes are inactive, their legacy continues, as expressed in today’s surface morphology, and still influence ground abiotic conditions through the generation of microrelief and microclimate which in turn have impacts on ecosystem patterns including the distribution of H. trilineatum. Such shrub growth patterns are influenced by microscale site morphology, associated abiotic controls, and ongoing seasonal cryogenic activity. Both active and inactive periglacial landforms on highest summits of the Drakensberg represent microscale environments with distinct fine-scale abiotic and geomorphic settings, which in turn are manifested through differences in plant ecology and rock surface weathering, respectively.

Temporary and net sinks of atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; due to chemical weathering in subtropical catchment with mixing carbonate and silicate lithology

Abstract. The study provided the major ion chemistry, chemical weathering rates and temporary and net CO2 sinks in the Bei Jiang, which was characterized as a hyperactive region with high chemical weathering rates, carbonate and silicate mixing lithology, and abundant sulfuric acid chemical weathering agent of acid deposition and acid mining drainage (AMD) origins. The total chemical weathering rate of 85.46 t km−2 a−1 was comparable to that of other rivers in the hyperactive zones between the latitudes 0 and 30∘. A carbonate weathering rate of 61.15 t km−2 a−1 contributed to about 70 % of the total. The lithology, runoff, and geomorphology had a significant influence on the chemical weathering rate. The proportion of carbonate outcrops had a significant positive correlation with the chemical weathering rate. Due to the interaction between dilution and compensation effect, a significant positive linear relationship was detected between runoff and total carbonate and silicate weathering rates. The geomorphology factors such as catchment area, average slope, and hypsometric integral value (HI) had nonlinear correlation with chemical weathering rate and showed significant scale effect, which revealed the complexity in chemical weathering processes. Dissolved inorganic carbon (DIC) apportionment showed that CCW (carbonate weathering by CO2) was the dominant origin of DIC (35 %–87 %). SCW (carbonate weathering by H2SO4) (3 %–15 %) and CSW (silicate weathering by CO2) (7 %–59 %) were non-negligible processes. The temporary CO2 sink was 823.41×103 mol km−2 a−1. Compared with the temporary sink, the net sink of CO2 for the Bei Jiang was approximately 23.18×103 mol km−2 a−1 of CO2 and was about 2.82 % of the “temporary” CO2 sink. Human activities (sulfur acid deposition and AMD) dramatically decreased the CO2 net sink, even making chemical weathering a CO2 source to the atmosphere.

High-frequency monitoring reveals how hydrochemistry and dissolved carbon respond to rainstorms at a karstic critical zone, Southwestern China

Hydrochemical behavior and dissolved carbon dynamics are highly-sensitive to hydrological variations in the monsoon-influenced karstic critical zone which has high chemical weathering rates and experiences strong anthropogenic impact. Continuous high-frequency monitoring in the spring outlet of a karstic catchment in Southwestern China revealed that most hydrochemical variables changed distinctively in response to hydrologic variations, influenced by mixing of different sources and miscellaneous biogeochemical processes. Na+, K+ and SO42− varied significantly with hydrology, showing weak chemostatic behavior controlled by dilution. The flushing effect and random behavior of NO3− and Cl− likely reflect agricultural inputs from high throughflow. Soil CO2 in infiltrated water supports carbonate weathering, enabling DIC (dissolved inorganic carbon) and weathering products (e.g., Ca2+ and Mg2+) to maintain chemostatic behavior. Biogenic DIC exhibited a stronger chemostatic response than carbonate sources and was the foremost control in DIC behavior. Carbon exchange between DIC and DOC (dissolved organic carbon) did not significantly influence DIC concentration and δ13C due to very low DOC concentration. More DOC was exported by flushing from increasing discharge. Hysteretic analysis indicated that the transport processes were controlled by proximal sources mixing and diverse mobilization in various periods responding to rainstorms. NO3− and Cl− presented different hysteresis behavior as sourced from agricultural activities. DOC increased on the hydrograph rising limb and was controlled by a transport-limited regime. However, the hysteresis behavior of most weathering products and DIC were regulated by a process-limited regime in the karstic critical zone. Overall, biogeochemical processes, hydrogeological properties, storm intensity/magnitude and the timing of storms (antecedent conditions) are main factors influencing the response of hydrochemical variables and dissolved carbon to storm events.

Dynamic responses of DOC and DIC transport to different flow regimes in a subtropical small mountainous river.

Transport of riverine dissolved carbon (including DOC and DIC) is a crucial process linking terrestrial and aquatic C reservoirs, but has rarely been examined in subtropical small mountainous rivers (SMRs). This study monitored DOC and DIC concentrations on a biweekly basis during non-event flow periods and at 3 h intervals during two typhoon events in three SMRs in southwestern Taiwan between January 2014 and August 2016. Two models, HBV (the Hydrologiska Byråns Vattenbalansavdelning model) and a three-endmember mixing model, were applied to determine the quantities of DOC and DIC transport from different flow paths. The results show that the annual DOC and DIC fluxes were 2.7–4.8 and 48.4–54.3 t C km−2 yr−1, respectively, which were approx. 2 and 20 times higher than the global mean of 1.4 and 2.6 t C km−2 yr−1, respectively. The DIC / DOC ratio was 14.08, which is much higher than the mean of large rivers worldwide (1.86), and indicates the high rates of chemical weathering in this region. The two typhoons contributed 12%–14% of the annual streamflow in only 3 days (about 1.0% of the annual time), whereas 15.0%–23.5% and 9.2%–12.6% of the annual DOC and DIC flux, respectively, suggested that typhoons play a more important role in DOC transport than DIC transport. The end-member mixing model suggested that DOC and DIC export was mainly from surface runoff and deep groundwater, respectively. The unique patterns seen in Taiwan SMRs characterized by high dissolved carbon flux, high DIC / DOC ratio, and large transport by intense storms should be taken into consideration when estimating global carbon budgets.

Sr isotopic characteristics in two small watersheds draining silicate and carbonate rocks: implication for studies on seawater Sr isotopic evolution

Abstract. We systematically investigated the Sr isotopic characteristics of a small silicate watershed, the Xishui River a tributary of the Yangtze River, and a small carbonate watershed, the Guijiang River a tributary of the Pearl River. The results show that the two rivers have uncommon Sr isotopic characteristics compared with most small watersheds. Specifically, the silicate watershed (Xishui River) has relatively high Sr concentrations (0.468 to 1.70 μmol L−1 in summer and 1.30 to 3.17 μmol L−1 in winter, respectively) and low 87Sr/86Sr ratios (0.708686 to 0.709148 in summer and 0.708515 to 0.709305 in winter). The carbonate watershed (Guijiang River) has low Sr concentrations (0.124 to 1.098 μmol L−1) and high 87Sr/86Sr ratios (0.710558 to 0.724605). As the 87Sr/86Sr ratios in the Xishui River are lower than those in seawater, the 87Sr/86Sr ratio of seawater will decrease after the river water is transported to the oceans. Previous studies have also shown that some basaltic watersheds with extremely high chemical weathering rates reduced the seawater Sr isotope ratios. In other words, river catchments with high silicate weathering rates do not certainly transport highly radiogenic Sr into oceans. Therefore, the use of the variations in the seawater 87Sr/86Sr ratio to indicate the continental silicate weathering intensity may be questionable. In the Guijiang River catchment, the 87Sr/86Sr ratios of carbonate rocks and other sources (rainwater, domestic and industrial waste water, and agricultural fertilizer) are lower than 0.71. In comparison, some non-carbonate components, such as sand rocks, mud rocks, and shales, have relatively high Sr isotopic compositions. Moreover, granites accounted for only 5% of the drainage area have extremely high 87Sr/86Sr ratios with an average of greater than 0.8. Therefore, a few silicate components in carbonate rocks obviously increase the Sr isotopic compositions of the river water.

Global chemical weathering and associated P-release — The role of lithology, temperature and soil properties

Because there remains a lack of knowledge about the spatially explicit distribution of chemical weathering rates at the global scale, a model that considers prominent first-order factors is compiled step by step and the implied spatial variability in weathering is explored. The goal is to fuel the discussion about the development of an “Earth System” weathering function. We use as a starting point an established model of the dependence of chemical weathering on lithology and runoff, calibrated for an island arc setting, which features very high chemical weathering rates and a strong dependence on lithology and runoff. The model is enhanced stepwise with further factors accounting for soil shielding and temperature, and the observed variation of fluxes is discussed in context of observed data from large rivers globally.Results suggest that the global soil shielding reduces chemical weathering (CW) fluxes by about 44%, compared to an Earth surface with no deeply weathered soils but relatively young rock surfaces (e.g. as in volcanic arc and other tectonically active areas). About 70% of the weathering fluxes globally derive from 10% of the land area, with Southeast Asia being a primary “hot spot” of chemical weathering. In contrast, only 50% of runoff is attributed to 10% of the land area; thus the global chemical weathering curve is to some extent disconnected from the global runoff curve due to the spatially heterogeneous climate as well as rock and soil properties. The analysis of carbonate dissolution reveals that about half of the flux is not delivered from labeled carbonate sedimentary rocks, but from trace carbonates in igneous rocks as well as from siliciclastic sediment areas containing matrix carbonate.In addition to total chemical weathering fluxes, the release of P, a nutrient that controls biological productivity at large spatial scales, is affected by the spatial correlation between runoff, lithology, temperature and soil properties. The areal abundance of deeply weathered soils in Earth's past may have influenced weathering fluxes and P-fuelled biological productivity significantly, specifically in the case of larger climate shifts when high runoff fields shift to areas with thinner soils or areas with more weatherable rocks and relatively increased P-content. This observation may be particularly important for spatially resolved Earth system models targeting geological time scales. The model is discussed against current process knowledge and geodata with focus on improving future global chemical weathering model attempts.Identified key processes and geodata demanding further research are a) the representation of flowpaths to distinguish surface runoff, interflow and baseflow contributions to CW-fluxes, b) freeze-thaw effects on chemical weathering, specifically for the northern latitudes, c) a more detailed analysis to identify to what extent the spatially heterogeneous distribution of Earth surface properties causes a decoupling of the Earth system rating functions between CW-fluxes and global runoff, as well as d) an improved understanding of where and to what extent trace or matrix carbonates in silicate-dominated rocks and sediments contribute to carbonate weathering. The latter demands e) an improved representation of carbonate content in lithological classes in the lithological representation of the Earth surface. Further improvement of the lithological database is needed for f) the age of rocks and g) the geochemistry of sediments with focus on unconsolidated sediments in the large basins. And clearly h) an improved global soil database is needed for future improvements with reliable soil depth, mineralogical composition as well as physical properties.

Recent human-induced trophic change in the large and deep perialpine Lake Lucerne (Switzerland) compared to historical geochemical variations

This study investigates the sedimentological and geochemical changes that occurred during the last 2200years in the meromictic Lake Lucerne (Switzerland), one of the largest freshwater lakes of Central Europe. The stable isotope composition (δ13C and δ18O values) of bulk carbonates is compared to changes in grain-size distribution (clay and silt fraction), natural trace element input (titanium and thorium concentrations), and organic material abundance (Corg, nitrogen and phosphorus) and composition (C/N ratios and hydrogen and oxygen indexes). A drop in carbonate accumulation and in the δ18O values of sediments between ca. AD 500 and 700 followed a large and consistent rise in chemical weathering, marked by increases in the silicate-clay fraction and in crustal element concentrations. During the following millennium, there was a long-term decreasing trend in the lithogenic trace element input and in the phosphorus loading, suggesting decreasing terrigeneous input from runoff water. The major sedimentological change over the studied period occurred after ca. AD 1800 with a significant increase in the erosion-driven silt-fraction and in the sedimentation rate. During the last century, human-induced increase in nutrient input to the lake highly enhanced the accumulation of organic matter in sediment. Changes in nutrients and oxygen conditions in the hypolimnion of Lake Lucerne during the eutrophication period (i.e., the last 40years) highly modified the geochemical fluxes compared to the relatively stable oligotrophic conditions that prevailed during the previous 2000years. Before the 19th century, climate driven meromixis had a limited impact on the organic matter flux to the sediments, but the accumulation of carbonate considerably decreased during periods of lower mechanical erosion rates and high chemical weathering rates.

Paleovalley morphology and fluvial system evolution of Eocene-Oligocene sediments ("auriferous gravels"), northern Sierra Nevada, California: Implications for climate, tectonics, and topography

Research Article| September 01, 2011 Paleovalley morphology and fluvial system evolution of Eocene–Oligocene sediments (“auriferous gravels”), northern Sierra Nevada, California: Implications for climate, tectonics, and topography Elizabeth J. Cassel; Elizabeth J. Cassel † 1Department of Earth and Environment, Franklin and Marshall College, Lancaster, Pennsylvania 17604, USA †E-mail: ejcassel@gmail.com Search for other works by this author on: GSW Google Scholar Stephan A. Graham Stephan A. Graham 2Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Elizabeth J. Cassel † 1Department of Earth and Environment, Franklin and Marshall College, Lancaster, Pennsylvania 17604, USA Stephan A. Graham 2Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA †E-mail: ejcassel@gmail.com Publisher: Geological Society of America Received: 21 Jun 2010 Revision Received: 03 Oct 2010 Accepted: 09 Nov 2010 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2011 Geological Society of America GSA Bulletin (2011) 123 (9-10): 1699–1719. https://doi.org/10.1130/B30356.1 Article history Received: 21 Jun 2010 Revision Received: 03 Oct 2010 Accepted: 09 Nov 2010 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Elizabeth J. Cassel, Stephan A. Graham; Paleovalley morphology and fluvial system evolution of Eocene–Oligocene sediments (“auriferous gravels”), northern Sierra Nevada, California: Implications for climate, tectonics, and topography. GSA Bulletin 2011;; 123 (9-10): 1699–1719. doi: https://doi.org/10.1130/B30356.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In the northern Sierra Nevada, California, Eocene–Oligocene fluvial sediments (the “auriferous gravels”) rest unconformably in well-defined basement-incised paleovalleys, providing a record of paleolandscape geomorphology and drainage system evolution. Paleovalley surface mapping shows that local topography and bedrock lithology controlled the location of broad, terraced paleovalley segments, which formed in the central and eastern metamorphic bedrock belts and subsequently became areas of significant fluvial deposition. High-gradient, high-energy deposits are present near the edges of these areas, suggesting that tributaries and outlets resided in steeper, narrow paleovalleys. Multiple strath terraces are common in broad paleovalleys, indicating that incision events occurred both before deposition and during paleovalley widening, followed by periods of aggradation. Broad paleovalley deposits contain predominantly braided stream facies deposited in an energetically variable system. Multiple upward-fining cycles, bounded by erosional surfaces, are composed of alternating coarse-grained channel-belt and fine-grained floodplain packages. An overall upward-fining succession suggests that the locus of fluvial deposition shifted and depositional energy waned over time as the system backfilled successive broad paleovalleys.Paleochannel slope estimates of 0.004–0.055 were obtained based on average grain sizes of coarse channel fills. Anomalously thick (up to 11 m), laterally continuous (1–3 km) floodplain units, containing kaolinite-rich clay and Oxisol intervals, record high fine-grained sediment supply, driven by high rates of chemical weathering in the warm Eocene climate. Based on sedimentary characteristics, combined with floral and detrital zircon data, braided stream deposition within the basin occurred from the late early Eocene through late Eocene, and aggradation primarily occurred in broad, lower-gradient reaches behind a major bedrock knickpoint, created in the late Mesozoic–early Cenozoic, possibly due to asymmetric batholith unroofing. The diachronous, localized nature of paleovalley incision and braided stream aggradation within the northern Sierra Nevada invalidates previous estimates of the timing and amount of range uplift based on paleochannel reconstructions and gradients. Both internal and external drivers influenced the evolution of the fluvial system through (1) the pattern of basin filling, as the locus of deposition migrated east to west, (2) the warm Eocene climate, which increased chemical weathering and contributed a high sediment supply, and (3) changes in base level due to eustasy and/or Laramide tectonism. This record of Laramide tectonics in the Sierra Nevada and Basin and Range is important for the timing of Sierra Nevada uplift, for tectonic reconstructions of western North America, and for studies of landscape evolution in tectonically active orogens. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Age of river basins in Guadeloupe impacting chemical weathering rates and land use

Abstract The Lesser Antilles have very high chemical weathering rates, with values that can reach 1290 t/km2/a. The tropical environment induces high precipitation rates, high temperature, dense vegetation, with sharp relief and thick soils. Because of volcanic activity, frequent pyroclastic flows produce very erodible and porous materials. In addition, agriculture induces important land use changes which replace existing native forest cover with banana and sugar cane plantations. Their surface can cover as much as 40% of the total area of a river basin. The aim of this study is to identify key parameters, either natural or anthropogenic, that control chemical weathering rates. Among the combined impact of all parameters (climate, runoff, slopes, vegetation etc.), basin age seems to be the control parameter: the younger the basin, the higher the weathering rate. A correlation between the chemical weathering rate and the basin age suggests that young volcanic rocks are more easily weathered than old ones: young fresh material is easily mobilized by erosion, while for older rocks with thick soil covers, chemical rates are much lower. A combined effect between the higher erodibility and a higher climate erosivity of the younger relief could be observed. Moreover, a correlation between banana plantations and the chemical weathering rates that can be explained by an increase of infiltration, due to stem flow processes is shown here. Banana plantations also have a correlation with the basin age, older basins being more favorable terrains for cultivation.

High chemical weathering rates in first-order granitic catchments induced by agricultural stress

Chemical erosion rates have been determined on two upland granitic catchments under agricultural pressure in Brittany, France. Intensive agriculture has been carried out for at least 30 years in this region. The influence of geochemical processes related to agriculture on the chemistry of streamwaters is determined through a geochemical mass balance. The elemental export fluxes from these two agricultural catchments are then compared with other catchments around the world. The volume and concentrations of the precipitation are taken into account, as well as the inputs of organic and chemical fertilizers, groundwaters and streamwaters, to estimate the relative influence on export fluxes, and then evaluate the elemental fluxes released by weathering. The relatively high Si flux of about 1.8 ± 0.9 kmol ha − 1 yr − 1 is directly attributed to the chemical weathering of soil and rock in the catchment system. However, the Si flux remains comparable to values found in both small and large-sized catchments under temperate and tropical conditions. On the other hand, extremely high fluxes of major cations (Ca, Na and Mg) are observed, ranging from 4.2 ± 2.6 to 8.0 ± 4.9 kmol ha − 1 yr − 1 , which can be attributed to chemical weathering. These fluxes remain dramatically higher than those found in granitic catchments worldwide. Despite an integrated agriculture, the soil acidification induced by fertilizer application leads mainly to a release of major cations from the system, by processes of soil ion-exchange leaching as well as weathering of soil and rock.

Evidence for widespread anoxia in the proglacial zone of an Arctic glacier

The major ion (SO 4 2−, HCO 3 −, NO 3 −) and stable isotope composition ( δ 18O–H 2O, δ 18O–SO 4 2−, δ 34S–SO 4 2− and DIC- δ 13C) of ground- and surface water samples collected from the proglacial zone of a retreating high Arctic glacier, (Finsterwalderbreen, Svalbard) indicate the widespread occurrence of microbially-mediated processes under anoxic conditions. The δ 18O–SO 4 2− of active layer groundwaters, lake and stream waters is strongly depleted in 18O compared to the δ 18O–H 2O, and stable isotope mass balance arguments indicate that 75–95% of SO 4 2− in these waters is produced by the oxidation of sulphide minerals using Fe(III) and NO 3 − under anoxic conditions. Consistent with this is the frequent loss or depletion of NO 3 − from ground- and surface waters. SO 4 2− concentrations and δ 18O–SO 4 2− and δ 18O–H 2O data suggest that much of the SO 4 2− is generated anoxically early in the melt season in 18O depleted snowmelt, and is immobilized as sulphate salts in near surface moraine due to evaporative concentration. These salts are remobilized later in the season by rainfall and glacial flood events and cause the δ 18O–SO 4 2− of proglacial meltwaters to be < δ 18O–H 2O. The latter become isotopically heavy through the melt season due to evaporation. The lack of any relationship between δ 34S–SO 4 2− and δ 18O–SO 4 2− and relatively heavy DIC- δ 13C values in proglacial waters suggests that sulphate reduction is not a significant process. Acidity produced during anoxic oxidation of sulphides appears to drive chemical weathering in the proglacial zone, enabling high chemical weathering rates to be maintained despite the absence of dissolved oxygen. Similar findings have been reported from subglacial environments, where anoxic sulphide oxidation is widely observed. The absence of evidence for sulphate reduction in the proglacial groundwaters, however, suggests that the degree of anoxia in deglaciated settings may not be widespread as in subglacial environments, where significant sulphate reduction has been observed.

Major ion chemistry of the Yarlung Tsangpo–Brahmaputra river: Chemical weathering, erosion, and CO 2 consumption in the southern Tibetan plateau and eastern syntaxis of the Himalaya

The Yarlung Tsangpo–Brahmaputra river drains a large portion of the Himalaya and southern Tibetan plateau, including the eastern Himalayan syntaxis, one of the most tectonically active regions on the globe. We measured the solute chemistry of 161 streams and major tributaries of the Tsangpo–Brahmaputra to examine the effect of tectonic, climatic, and geologic factors on chemical weathering rates. Specifically, we quantify chemical weathering fluxes and CO 2 consumption by silicate weathering in southern Tibet and the eastern syntaxis of the Himalaya, examine the major chemical weathering reactions in the tributaries of the Tsangpo–Brahmaputra, and determine the total weathering flux from carbonate and silicate weathering processes in this region. We show that high precipitation, rapid tectonic uplift, steep channel slopes, and high stream power generate high rates of chemical weathering in the eastern syntaxis. The total dissolved solids (TDS) flux from the this area is greater than 520 tons km −2 yr −1 and the silicate cation flux more than 34 tons km −2 yr −1. In total, chemical weathering in this area consumes 15.2 × 10 5 mol CO 2 km −2 yr −1, which is twice the Brahmaputra average. These data show that 15–20% of the total CO 2 consumption by silicate weathering in the Brahmaputra catchment is derived from only 4% of the total land area of the basin. Hot springs and evaporite weathering provide significant contributions to dissolved Na + and Cl − fluxes throughout southern Tibet, comprising more than 50% of all Na + in some stream systems. Carbonate weathering generates 80–90% of all dissolved Ca 2+ and Mg 2+ cations in much of the Yarlung Tsangpo catchment.

Groundwater hydrochemistry in the active layer of the proglacial zone, Finsterwalderbreen, Svalbard

Glacial bulk meltwaters and active-layer groundwaters were sampled from the proglacial zone of Finsterwalderbreen during a single melt season in 1999, in order to determine the geochemical processes that maintain high chemical weathering rates in the proglacial zone of this glacier. Results demonstrate that the principle means of solute acquisition is the weathering of highly reactive moraine and fluvial active-layer sediments by supra-permafrost groundwaters. Active-layer groundwater derives from the thaw of the proglacial snowpack, buried ice and glacial bulk meltwaters. Groundwater evolves by sulphide oxidation and carbonate dissolution. Evaporation- and freeze-concentration of groundwater in summer and winter, respectively produce Mg–Ca-sulphate salts on the proglacial surface. Re-dissolution of these salts in early summer produces groundwaters that are supersaturated with respect to calcite.There is a pronounced spatial pattern to the geochemical evolution of groundwater. Close to the main proglacial channel, active layer sediments are flushed diurnally by bulk meltwaters. Here, Mg–Ca-sulphate deposits become exhausted in the early season and geochemical evolution proceeds by a combination of sulphide oxidation and carbonate dissolution. At greater distances from the channel, the dissolution of Mg–Ca-sulphate salts is a major influence and dilution by the bulk meltwaters is relatively minor. The influence of sulphate salt dissolution decreases during the sampling season, as these salts are exhausted and waters become increasingly routed by subsurface flowpaths.

Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers

The main problem associated with the study of silicate weathering using river dissolved load is that the main control of solute chemistry is lithology and that all rivers are influenced by carbonate and evaporite weathering. In this paper, newly compiled data on the 60 largest rivers of the world are used to calculate the contribution of main lithologies, rain and atmosphere to river dissolved loads. Technically, an inverse method is used to solve a model containing of a series of mass budget equations relating river concentrations to chemical weathering products and atmospheric inputs. New estimates of global silicate weathering fluxes and associated CO2 consumption fluxes are given. The role of basalt weathering on oceanic islands and volcanic arcs is emphasized. For each large river, an attempt is made to calculate chemical weathering rates of silicates per unit area. Only relative chemical weathering rates can be calculated. The relationships between the chemical weathering rates of silicates and the possible controlling parameters are explored. A combined effect of runoff-temperature and physical denudation seems to explain the variability of modern silicate chemical weathering rates. The results of this study highlight the coupling between the physical and the chemical processes of silicate weathering. Only an active physical denudation of continental rocks seems to be able to maintain high chemical weathering rates and significant CO2 consumption rates.

Use of [formula omitted] to trace origin and cycling of inorganic carbon in the Rhône river system

Dissolved inorganic carbon (DIC) and particulate inorganic carbon (PIC) were sampled at 20 sites on the Rhône and its main tributaries from Switzerland to the Mediterranean, in March, September and July (flood event) 1996. Flux estimation and DIC and PIC δ13C values are used to trace watershed inputs and fluvial processes affecting the inorganic carbon budget. DIC concentration and δ13C values vary between ∼1 mM at −5‰ near the Rhône's source and ∼3 mM at −10‰ near its mouth, with tributaries draining lowland, carbonate-dominated areas at ∼3.8 mM and −11‰. There is an inverse relationship between river DIC concentration and DIC δ13C. The headwaters are characterized by high DIC δ13C values and low DIC, indicating a minor input of respired CO2. DIC should be mainly produced through the reaction of carbonate with atmospheric CO2 or organic/sulfuric acids. In lowland, carbonate-dominated areas, δ13C values of ∼−11‰ are consistent with production of DIC by weathering of limestone C (δ13C≈−1‰) with CO2 derived from the oxidation of organic matter (δ13C≈−21.5‰). The evolution of the Rhône's DIC concentration and δ13C appear to be largely determined by mixing between the Rhône and its main tributaries. The CO2 fluxes and DIC δ13C mass-balance, calculated for the different segments of the Rhône, point to a net, but very limited input of fluvial respired CO2 from the oxidation of organic C (<5%). The PIC represents a small but quite variable fraction of the inorganic carbon (<3%), except on Alpine catchments and during floods. In these cases, it can be up to 10% of the DIC concentration and export. The carbon and oxygen isotopic composition of PIC from the Rhône during a flood and from Alpine tributaries falls within the δ-range of limestones (δ13C≥−2‰; δ18O≥−6‰). Relatively light PIC isotopic composition indicate mixing of sedimentary rocks and carbonate precipitated in situ. Data for the Rhône system are typical of watersheds presenting high chemical weathering rates for HCO3, and Alpine/lowland conditions.

Variations in the strontium isotopic ratio of seawater during the Miocene: Stratigraphic and geochemical implications

A composite strontium isotopic seawater curve was constructed for the Miocene between 24 and 6 Ma by combining 87Sr/86Sr measurements of planktonic foraminifera from Deep Sea Drilling Project sites 289 and 588. Site 289, with its virtually continuous sedimentary record and high sedimentation rates (26 m/m.y.), was used for constructing the Oligocene to mid‐Miocene part of the record, which included the calibration of 63 biostratigraphic datums to the Sr seawater curve using the timescale of Cande and Kent (1992). Across the Oligocene/Miocene boundary, a brief plateau occurred in the Sr seawater curve (87Sr/86Sr values averaged 0.70824) which is coincident with a carbon isotopic maximum (CM‐O/M) from 24.3 to 22.6 Ma. During the early Miocene, the strontium isotopic curve was marked by a steep rise in 87Sr/86Sr that included a break in slope near 19 Ma. The rate of growth was about 60 ppm/m.y. between 22.5 and 19.0 Ma and increased to over 80 ppm/m.y. between 19.0 and 16 Ma. Beginning at ∼16 Ma (between carbon isotopic maxima CM3 and CM4 of Woodruff and Savin (1991)), the rate of 87Sr/86Sr growth slowed and 87Sr/86Sr values were near constant from 15 to 13 Ma. After 13 Ma, growth in 87Sr/86Sr resumed and continued until ∼9 Ma, when the rate of 87Sr/86Sr growth decreased to zero once again. The entire Miocene seawater curve can be described by a high‐order function, and the first derivative (d87Sr/86Sr/dt) of this function reveals two periods of increased slope. The greatest rate of 87Sr/86Sr change occurred during the early Miocene between ∼20 and 16 Ma, and a smaller, but distinct, period of increased slope also occurred during the late Miocene between ∼12 and 9 Ma. These periods of steepened slope coincide with major phases of uplift and denudation of the Himalayan‐Tibetan Plateau region, supporting previous interpretations that the primary control on seawater 87Sr/86Sr during the Miocene was related to the collision of India and Asia. The rapid increase in 87Sr/86Sr values during the early Miocene from 20 to 16 Ma imply high rates of chemical weathering and dissolved riverine fluxes to the oceans. In the absence of another source of CO2, these high rates of chemical weathering should have quickly resulted in a drawdown of atmospheric CO2 and climatic cooling through a reversed greenhouse effect. The paleoclimatic record, however, indicates a warming trend during the early Miocene, culminating in a climatic optimum between 17 and 14.5 Ma. We suggest that the high rates of chemical erosion and warm temperatures during the climatic optimum were caused by an increase in the contribution of volcanic CO2 from the eruption of the Columbia River Flood Basalts (CRFB) between 17 and 15 Ma. The decrease in the rate of CRFB eruptions at 15 Ma and the removal of atmospheric carbon dioxide by increased organic carbon burial in Monterey deposits eventually led to cooling and increased glaciation between ∼14.5 and 13 Ma. The CRFB hypothesis helps to explain the significant time lag between the onset of increased rates of organic carbon burial in the Monterey at 17.5 Ma (as marked by increased δ13C values) and the climatic cooling and glaciation during the middle Miocene (as marked by the increase in δ18O values), which did not begin until ∼14.5 Ma.

WEATHERING TRENDS IN FINE DEBRIS BENEATH A SNOW PATCH, NIWOT RIDGE, FRONT RANGE, COLORADO

Seven soil pits dug in a downhill sequence beneath Martinelli snow patch, Niwot Ridge, Front Range, Colorado, reveal that soils are coarse-grained with only limited evidence of silt translocation. Clay mineralogy within the pits exhibits a widespread and distinct change at a depth of ∼15–20 cm. The most common characteristic of this change is a distinct increase in vermiculite in the lower layer, usually accompanied by a decrease in kaolinite and/or smectite. This pattern is interpreted to reflect decreased weathering intensity with depth. Microenvironments beneath the snow patch which exhibit high chemical weathering rates on coarse surficial debris also appear to experience high rates of clay production relative to referenced dry alpine sites, with specific clay mineralogies varying in a fashion commonly found in humid temperate environments. Data generated in this study appear to provide site-specific support for the regionally-developed Synthetic Alpine Slope Model. Spatial variations in eolian additi...