Refereed journal articles

1. Knight, A. C. G., Tipper, E. T., Bradbury, H. J., Turchyn, A. V., Andermann, C., Freymuth, H., Elliott, T., & Bridgestock, L. (2024). Experimental constraints on barium isotope fractionation during adsorption–desorption reactions: Implications for weathering and erosion tracer applications. Geochimica Et Cosmochimica Acta, 384, 194–212.
   PDF DOI BIB ABSTRACT

   @article{Knight2024GCA,
     title = {Experimental constraints on barium isotope fractionation during adsorption–desorption reactions: Implications for weathering and erosion tracer applications},
     journal = {Geochimica et Cosmochimica Acta},
     volume = {384},
     pages = {194-212},
     year = {2024},
     issn = {0016-7037},
     doi = {https://doi.org/10.1016/j.gca.2024.08.016},
     author = {Knight, Alasdair C.G. and Tipper, Edward T. and Bradbury, Harold J. and Turchyn, Alexandra V. and Andermann, Christoff and Freymuth, Heye and Elliott, Tim and Bridgestock, Luke},
     keywords = {Adsorption–desorption, Barium isotopes, Weathering, Erosion, Clay minerals, Iron oxyhydroxides},
     file = {Knight2024GCA.pdf}
   }
   

   Constraining the processes that fractionate barium isotopes is essential for utilising barium isotope ratios as environmental tracers. Barium concentration measurements from soils, rivers, and estuaries demonstrate that adsorption–desorption reactions significantly influence the distribution of fluid–mobile barium at the Earth’s surface, potentially driving isotopic fractionation. To quantify the direction and magnitude of isotopic fractionation resulting from these reactions, a riverine and an estuarine series of batch experiments were conducted using environmentally important adsorbent minerals and surface waters. Himalayan river sediment and water samples were used to validate the experimental results. Adsorption–desorption reactions were found to be rapid, relative to the average transit time of sediment and water in catchments, and largely reversible. The direction and magnitude of isotopic fractionation in the riverine experiment series were consistent with the riverine field samples (preferential adsorption of the lighter isotopes). The reaction rate, reversibility, and magnitude of isotopic fractionation were found to depend primarily on the mineral. Experiments performed with iron oxyhydroxides (goethite and ferrihydrite) resulted in a greater degree of fractionation compared to clay minerals (kaolinite and montmorillonite). Estuarine experiments, designed to simulate sediment passage through a salinity gradient, demonstrated a high degree of reversibility, with 77% to 94% of adsorbed barium desorbed upon the addition of seawater to freshwater-equilibrated clay minerals. The results of the estuarine experiments suggest that barium isotope ratios measured in marine paleo-archives (e.g., corals) will reflect both the adsorbed and dissolved freshwater barium inputs to the ocean. The combined findings of this study indicate that the chemical and isotopic behaviour of barium differs from more conventional group 1 and 2 metal isotope systems due to a significant proportion of barium released from bedrock dissolution partitioning to mineral surfaces, rapid reaction rates between fluid–mobile phases, and a high degree of reaction reversibility. Consequently, riverine barium isotope ratios are likely to provide unique insights into the complex array of terrestrial weathering and erosion processes that sustain life on Earth.

2. Knight, A. C. G., Stevenson, E. I., Bridgestock, L., Jotautas Baronas, J., Knapp, W. J., Adhikari, B. R., Andermann, C., & Tipper, E. T. (2024). The impact of adsorption–desorption reactions on the chemistry of Himalayan rivers and the quantification of silicate weathering rates. Earth and Planetary Science Letters, 641, 118814.
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   @article{Knight2024EPSL,
     title = {The impact of adsorption–desorption reactions on the chemistry of Himalayan rivers and the quantification of silicate weathering rates},
     journal = {Earth and Planetary Science Letters},
     volume = {641},
     pages = {118814},
     year = {2024},
     issn = {0012-821X},
     doi = {https://doi.org/10.1016/j.epsl.2024.118814},
     author = {Knight, Alasdair C.G. and Stevenson, Emily I. and Bridgestock, Luke and {Jotautas Baronas}, J. and Knapp, William J. and Adhikari, Basanta Raj and Andermann, Christoff and Tipper, Edward T.},
     keywords = {Adsorption-desorption reactions, Silicate weathering, River chemistry},
     file = {Knight2024EPSL.pdf}
   }
   

   Common environmental adsorbents (clay minerals, metal-oxides, metal-oxyhydroxides and organic matter) can significantly impact the chemistry of aqueous fluids via adsorption–desorption reactions. The dissolved chemistry of rivers have routinely been used to quantify silicate mineral dissolution rates, which is a key process for removing carbon dioxide (Image 1) from the atmosphere over geological timescales. The sensitivity of silicate weathering rates to climate is disproportionately weighted towards regions with high erosion rates. This study quantifies the impact of adsorption-desorption reactions on the chemistry of three large Himalayan rivers over a period of two years, utilising both the adsorbed and dissolved phases. The concentration of riverine adsorbed cations are found to vary principally as a function of the concentration and cation exchange capacity (CEC) of the suspended sediment. Over the study period, the adsorbed phase is responsible for transporting ∼70% of the mobile (adsorbed and dissolved) barium and ∼10% of the mobile calcium and strontium. The relative partitioning of cations between the adsorbed and dissolved phases follows a systematic order in both the monsoon and the dry-season (preferentially adsorbed: Ba > Sr & Ca > Mg & K > Na). Excess mobile sodium (Image 2=Na-Cl) to silicon (Si) riverine ratios are found to vary systematically during an annual hydrological cycle due to the mixing of low temperature and geothermal waters. The desorption of sodium from uplifted marine sediments is one key process that may increase the Na*/Si ratios. Accounting for the desorption of sodium reduces silicate weathering rate estimates by up to 83% in the catchments. This study highlights that surficial weathering processes alone are unable to explain the chemistry of the rivers studied due to the influence of hydrothermal reactions, which may play an important role in limiting the efficiency of silicate weathering and hence modulating atmospheric Image 1 concentrations over geological time.

3. Knapp, W. J., Stevenson, E. I., Renforth, P., Ascough, P. L., Knight, A. C. G., Bridgestock, L., Bickle, M. J., Lin, Y., Riley, A. L., Mayes, W. M., & others. (2023). Quantifying CO2 removal at enhanced weathering sites: a multiproxy approach. Environmental Science & Technology, 57(26), 9854–9864.
   PDF DOI BIB ABSTRACT

   @article{Knapp2023,
     title = {Quantifying CO2 removal at enhanced weathering sites: a multiproxy approach},
     author = {Knapp, William J and Stevenson, Emily I and Renforth, Phil and Ascough, Philippa L and Knight, Alasdair CG and Bridgestock, Luke and Bickle, Michael J and Lin, Yongjie and Riley, Alex L and Mayes, William M and others},
     journal = {Environmental science \& technology},
     doi = {10.1021/acs.est.3c03757},
     volume = {57},
     number = {26},
     pages = {9854--9864},
     year = {2023},
     publisher = {ACS Publications},
     file = {Knapp2023.pdf}
   }
   

   Enhanced weathering is a carbon dioxide (CO2) mitigation strategy that promises large scale atmospheric CO2 removal. The main challenge associated with enhanced weathering is monitoring, reporting, and verifying (MRV) the amount of carbon removed as a result of enhanced weathering reactions. Here, we study a CO2 mineralization site in Consett, Co. Durham, UK, where steel slags have been weathered in a landscaped deposit for over 40 years. We provide new radiocarbon, δ13C, 87Sr/86Sr, and major element data in waters, calcite precipitates, and soils to quantify the rate of carbon removal. We demonstrate that measuring the radiocarbon activity of CaCO3 deposited in waters draining the slag deposit provides a robust constraint on the carbon source being sequestered (80% from the atmosphere, 2σ = 8%) and use downstream alkalinity measurements to determine the proportion of carbon exported to the ocean. The main phases dissolving in the slag are hydroxide minerals (e.g., portlandite) with minor contributions (<3%) from silicate minerals. We propose a novel method for quantifying carbon removal rates at enhanced weathering sites, which is a function of the radiocarbon-apportioned sources of carbon being sequestered, and the proportion of carbon being exported from the catchment to the oceans.

4. Tipper, E. T., Stevenson, E. I., Alcock, V., Knight, A. C. G., Baronas, J. J., Hilton, R. G., Bickle, M. J., Larkin, C. S., Feng, L., Relph, K. E., & Hughes, G. (2021). Global silicate weathering flux overestimated because of sediment–water cation exchange. Proceedings of the National Academy of Sciences, 118(1), e2016430118.
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   @article{Tipper2020,
     author = {Tipper, Edward T. and Stevenson, Emily I. and Alcock, Victoria and Knight, Alasdair C. G. and Baronas, J. Jotautas and Hilton, Robert G. and Bickle, Mike J. and Larkin, Christina S. and Feng, Linshu and Relph, Katy E. and Hughes, Genevieve},
     title = {Global silicate weathering flux overestimated because of sediment–water cation exchange},
     journal = {Proceedings of the National Academy of Sciences},
     volume = {118},
     number = {1},
     pages = {e2016430118},
     year = {2021},
     doi = {10.1073/pnas.2016430118},
     eprint = {https://www.pnas.org/doi/pdf/10.1073/pnas.2016430118},
     file = {Tipper2020.pdf}
   }
   

   Large rivers transport water and sediment to floodplains and oceans, 
   supplying the nutrients that sustain life. They also transport carbon, removed from the atmosphere during mineral dissolution reactions, 
   which is thought to provide a key negative climate feedback on long timescales. We demonstrate that the (million-year) carbon flux associated with mineral dissolution 
   has been overestimated by up to 28% because of a reactive pool of elements transported with river-borne suspended sediment. This is most acute in regions of high erosion, 
   where silicate weathering is thought to be most intense. Rivers carry the dissolved and solid products of silicate mineral weathering, a process that removes CO2 from the 
   atmosphere and provides a key negative climate feedback over geological timescales. Here we show that, in some river systems, a reactive exchange pool on river suspended particulate matter, 
   bonded weakly to mineral surfaces, increases the mobile cation flux by 50%. The chemistry of both river waters and the exchange pool demonstrates exchange equilibrium, confirmed by Sr isotopes. 
   Global silicate weathering fluxes are calculated based on riverine dissolved sodium (Na+) from silicate minerals. The large exchange pool supplies Na+ of nonsilicate origin to the dissolved load, 
   especially in catchments with widespread marine sediments, or where rocks have equilibrated with saline basement fluids. We quantify this by comparing the riverine sediment exchange pool and 
   river water chemistry. In some basins, cation exchange could account for the majority of sodium in the river water, significantly reducing estimates of silicate weathering. At a global scale, 
   we demonstrate that silicate weathering fluxes are overestimated by 12 to 28%. This overestimation is greatest in regions of high erosion and high sediment loads where the negative climate 
   feedback has a maximum sensitivity to chemical weathering reactions. In the context of other recent findings that reduce the net CO2 consumption through chemical weathering, the magnitude of the 
   continental silicate weathering fluxes and its implications for solid Earth CO2 degassing fluxes need to be further investigated.