Search

Analytical solutions to reactive-transport equations describing the evolution of Li concentrations and isotopic ratios are presented for one-dimensional flow paths where reaction stoichiometry is constant along the flow path. These solutions are considered appropriate for chemical weathering in rapidly eroding catchments. The solutions may be described by two dimensionless numbers; 1) a Damköhler number describing the product of reaction rate and fluid residence time, and 2) a net partition coefficient which describes the fraction of Li re-precipitated in secondary minerals as the product of a fluid-secondary mineral partition coefficient and the mass fraction of secondary mineral precipitates. In settings where water entering flow paths is dilute, Li concentrations will increase along the flow path until they reach a limiting value determined by the net partition coefficient. Simultaneously, 7Li/6Li isotopic ratios will increase to a limiting value of the source rock ratio minus the secondary mineral-fluid Li-isotopic fractionation factor. Waters with Li-isotopic ratios in excess of this limiting value must have evolved with a change of reaction stoichiometry and/or partition coefficient along the flow path such that at some point net removal of Li to secondary minerals exceeds that supplied by dissolution of primary minerals. The modelling shows that the multiple controls on chemical weathering rates (temperature, rainfall, erosion rate, hydrology) cannot be inferred from Li concentration and isotopic ratio data alone, which only provide two independent constraints. Caution should be exercised in interpretation of oceanic Li records in terms of potential climatic variables. The model is illustrated by a set of Li concentration and isotopic ratio measurements on river waters and bed sands in the Alaknanda river basin which forms the headwaters of the Ganges. This illustrates how values of the Damköhler number and net partition coefficient can be used to trace weath

This paper presents fundamental analysis of the injection and release of fluid into porous media or geological reservoirs saturated by a different fluid undergoing a background flow, and tests the predictions using analogue laboratory experiments. The study reveals new results important for an understanding of the transport of hazardous contaminants through aquifers and the long-term fate of carbon dioxide ( ) in geological sequestration. Using numerical and asymptotic analysis, we describe a variety of flow regimes that arise, and demonstrate an almost instantaneous control of injected fluid by the far field conditions in geological reservoirs. For a continuous input, the flow develops a horizontal interface between the injected and ambient fluids. The background flow thereby effectively caps the height of the injected fluid into a shallower region of vertical confinement. For a released parcel of fluid, gravitational spreading is found to become negligible after a short time. A dominant control of the interface by the background pressure gradient arises, and stems from the different velocities at which it drives the injected and ambient fluids individually. Similarity solutions describing these dynamics show that the parcel approaches a slender triangular profile that grows horizontally as , where is time, a rate faster than relaxation under gravity. Shock layers develop at the front or back of the parcel, depending on whether it is more or less viscous than the ambient fluid. New analytical results describing the long-term effects of residual trapping due to capillary retention are developed, which yield explicit predictions for the time and length scales on which a parcel of becomes retained. We end by applying our results to geological contexts, concluding that even slight background motion can have considerable implications for long-term transport through the subsurface.

This paper presents fundamental analysis of the injection and release of fluid into porous media or geological reservoirs saturated by a different fluid undergoing a background flow, and tests the predictions using analogue laboratory experiments. The study reveals new results important for an understanding of the transport of hazardous contaminants through aquifers and the long-term fate of carbon dioxide ( ) in geological sequestration. Using numerical and asymptotic analysis, we describe a variety of flow regimes that arise, and demonstrate an almost instantaneous control of injected fluid by the far field conditions in geological reservoirs. For a continuous input, the flow develops a horizontal interface between the injected and ambient fluids. The background flow thereby effectively caps the height of the injected fluid into a shallower region of vertical confinement. For a released parcel of fluid, gravitational spreading is found to become negligible after a short time. A dominant control of the interface by the background pressure gradient arises, and stems from the different velocities at which it drives the injected and ambient fluids individually. Similarity solutions describing these dynamics show that the parcel approaches a slender triangular profile that grows horizontally as , where is time, a rate faster than relaxation under gravity. Shock layers develop at the front or back of the parcel, depending on whether it is more or less viscous than the ambient fluid. New analytical results describing the long-term effects of residual trapping due to capillary retention are developed, which yield explicit predictions for the time and length scales on which a parcel of becomes retained. We end by applying our results to geological contexts, concluding that even slight background motion can have considerable implications for long-term transport through the subsurface.

Eclogite facies metamorphic rocks provide critical information pertaining to the timing of continental collision in zones of plate convergence. Despite being amongst Earth's best studied orogens, little is understood about the rates of Alpine metamorphism within the Eastern Alps. We present LA–MC–ICPMS and ID–TIMS U–Pb ages of metamorphic allanite from the Eclogite Zone, Tauern Window, which when coupled with rare earth element analysis and thermobarometric modelling, demonstrate that the European continental margin was subducted to between 8 and 13 kbar (30–45 km) by 34.2 ± 3.6 Ma. These data define: (i.) an upper limit on the timing of eclogite facies metamorphism at 26.2 ± 1.8 kbar (70–80 km) and 553 ± 12 °C, (ii.) plate velocity (1–6 cm·a−1) exhumation of the Eclogite Zone from mantle to mid-crustal depths, and (iii.) a maximum duration of 10 Ma (28–38 Ma) for juxtaposition of Alpine upper-plate and European basement units and subsequent conductive heating thought to have driven regional Barrovian (re)crystallisation at ca. 30 Ma. One-dimensional thermal modelling of tectonically thickened crust shows that conductive heating is too slow to account for Tauern Barrovian conditions (550 °C at 9–13 kbar) within the maximum 10 Ma interval between eclogite exhumation and the thermal peak (28–32 Ma). Given that the Tauern Window is a classic locality for understanding rates of conductive thermal relaxation in tectonically thickened crust, this work raises questions of fundamental importance concerning the length scales of the mechanisms responsible for heat transfer within orogenic crust.

Ostracod Mg/Sr/Ca and 87Sr/86Sr geochemistry from Tibetan lake sediments: Implications for early to mid-Pleistocene Indian monsoon and catchment weathering. Boreas, 10.1111/j.1502-3885.2010.00184.x. ISSN 0300-9483 Lacustrine sediment serves as a valuable

Eclogite facies metamorphic rocks provide critical information pertaining to the timing of continental collision in zones of plate convergence. Despite being amongst Earth's best studied orogens, little is understood about the rates of Alpine metamorphism within the Eastern Alps. We present LA–MC–ICPMS and ID–TIMS U–Pb ages of metamorphic allanite from the Eclogite Zone, Tauern Window, which when coupled with rare earth element analysis and thermobarometric modelling, demonstrate that the European continental margin was subducted to between 8 and 13 kbar (30–45 km) by 34.2 ± 3.6 Ma. These data define: (i.) an upper limit on the timing of eclogite facies metamorphism at 26.2 ± 1.8 kbar (70–80 km) and 553 ± 12 °C, (ii.) plate velocity (1–6 cm·a−1) exhumation of the Eclogite Zone from mantle to mid-crustal depths, and (iii.) a maximum duration of 10 Ma (28–38 Ma) for juxtaposition of Alpine upper-plate and European basement units and subsequent conductive heating thought to have driven regional Barrovian (re)crystallisation at ca. 30 Ma. One-dimensional thermal modelling of tectonically thickened crust shows that conductive heating is too slow to account for Tauern Barrovian conditions (550 °C at 9–13 kbar) within the maximum 10 Ma interval between eclogite exhumation and the thermal peak (28–32 Ma). Given that the Tauern Window is a classic locality for understanding rates of conductive thermal relaxation in tectonically thickened crust, this work raises questions of fundamental importance concerning the length scales of the mechanisms responsible for heat transfer within orogenic crust.

As one of the pioneering projects of the Chinese Environmental Scientific Drilling (CESD) Program, a 206.5 m long sediment core (CE) was retrieved from Co Ngoin (Co = lake) Basin in the central Tibetan Plateau. Limnic records of the abundance of ostracod

We review new and published analyses of river waters, bedloads and their constituent minerals from the Dhauli Ganga and Alaknanda, headwaters of the Ganges in Garhwal, and the Marsyandi in Nepal and their tributaries. These data are used to discriminate between the inputs of major cations and Sr from silicate and carbonate sources. Methods of estimating the proportion of the carbonate and silicate inputs to river waters using mixing arrays in Sr-Ca-Mg-Na-K 87Sr/86Sr space are shown to suffer from systematic correlations between the magnitude of the precipitation of secondary calcite and the fraction of the silicate component. This results in factor-of two overestimates of the fractions of silicate-derived Ca, Mg and Sr. To correct for this the magnitude of secondary calcite precipitated and relative fractions of silicate and carbonate-derived cations are instead calculated by modeling the displacement of water compositions from the compositions of the carbonate and silicate components of the bedload in subsets of Sr-Ca-Mg-Na-K 87Sr/86Sr space. The compositions of the carbonate and silicate end-members in the bedload are determined by sequential leaching. The results of this modeling are compared with modeling of the modal mineral inputs to waters where mineral compositions are derived from electronmicroprobe analyses of the minerals in the bedload. In the upper Marsyandi catchment, which drains low-grade Tethyan Sedimentary Series formations, a set of mainstem samples collected over a two-year period define tight correlations in Sr-Ca-Mg-Na-K- 87Sr/86Sr space. Modeling of the magnitude of secondary carbonate precipitation and fractions of silicate-derived Ca, Mg and Sr in Sr-Ca-Mg 87Sr/86Sr space gives selfconsistent results that are compatible with both the calculations of mineral modes and published Mg-isotopic compositions, if the ratio of chlorite to biotite weathering is hig

We review new and published analyses of river waters, bedloads and their constituent minerals from the Dhauli Ganga and Alaknanda, headwaters of the Ganges in Garhwal, and the Marsyandi in Nepal and their tributaries. These data are used to discriminate between the inputs of major cations and Sr from silicate and carbonate sources. Methods of estimating the proportion of the carbonate and silicate inputs to river waters using mixing arrays in Sr-Ca-Mg-Na-K 87Sr/86Sr space are shown to suffer from systematic correlations between the magnitude of the precipitation of secondary calcite and the fraction of the silicate component. This results in factor-of two overestimates of the fractions of silicate-derived Ca, Mg and Sr. To correct for this the magnitude of secondary calcite precipitated and relative fractions of silicate and carbonate-derived cations are instead calculated by modeling the displacement of water compositions from the compositions of the carbonate and silicate components of the bedload in subsets of Sr-Ca-Mg-Na-K 87Sr/86Sr space. The compositions of the carbonate and silicate end-members in the bedload are determined by sequential leaching. The results of this modeling are compared with modeling of the modal mineral inputs to waters where mineral compositions are derived from electronmicroprobe analyses of the minerals in the bedload. In the upper Marsyandi catchment, which drains low-grade Tethyan Sedimentary Series formations, a set of mainstem samples collected over a two-year period define tight correlations in Sr-Ca-Mg-Na-K- 87Sr/86Sr space. Modeling of the magnitude of secondary carbonate precipitation and fractions of silicate-derived Ca, Mg and Sr in Sr-Ca-Mg 87Sr/86Sr space gives selfconsistent results that are compatible with both the calculations of mineral modes and published Mg-isotopic compositions, if the ratio of chlorite to biotite weathering is hig