Liu, Chunyao; (2023) Exploring the alteration of river-derived particles in estuaries and its implication on elemental cycles: evidence from lithium isotopes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The estuary zone is the critical area that connects the continent to the oceans. Every year, global rivers transport billions of tonnes of continental weathering products, including both dissolved material and solid material (i.e., river-derived particles), to the ocean through estuaries. In principle, the river-derived particles can be further altered during sediment-water interaction in estuaries, such as by mineral dissolution and secondary mineral formation, similar to continental weathering processes. The alteration of river-derived particles in the estuary may influence element cycling in the ocean, the fate of alkalinity, and organic carbon burial. Although lithium (Li) isotopes are regarded as a good tracer for the carbon burial efficiency of silicate weathering, the influences of the alteration of river-derived particles in the estuary on global Li cycling have not been investigated widely. In this study, four series of interaction experiments were conducted to imitate natural estuarine processes and two natural estuaries, the Borgarfjörður estuary (Iceland) and the Amazon estuary were explored. To better distinguish the influence of different fractions (i.e., exchangeable, carbonate, oxide, clay, and residue) in the solid load, a sequential extraction method of silicate solid samples was developed. The element concentrations (e.g., Na, Ca, Mg, K, Fe, Al, and Mn) and Li isotope compositions were examined. The Li concentrations and Li isotope compositions in the dissolved load and the different fractions of the solid load indicated net uptake of Li from the dissolved load by the solids, mainly the clay fraction, with accompanying Li isotope fractionation, during the alteration of river-derived particles in estuaries. River-derived particles were estimated to uptake ~1–4 μg of Li per gram, with Li isotope fractionation between solids and seawater of −17.1 to −24.0‰. The estuarine Li uptake is likely to be a significant Li sink in the ocean with the high dissolved Li concentration in the estuarine zone likely to be the main reason for the Li uptake. Furthermore, the analysis of Li isotopes and element concentrations implied the formation of oxides and clays in estuaries, which, on the one hand, may hinder alkalinity sequestration in marine carbonates, but, on the other hand, may enhance organic carbon formation and burial.

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