The influence of river-derived particles on estuarine and marine elemental cycles: Evidence from lithium isotopes

To examine the alteration of river-derived sediments through a large estuary and the implications for elemental cycling and global climate, this study analyses lithium (Li) isotopes and elemental concentrations (e.g., Li, Na, Mg, K, Ca, Fe and Al) of both the dissolved load and different phases of the sediment load (i.e., exchangeable, carbonate, oxide, clay and residue) in the Amazon estuary. The results show that river-derived sediments remove Li from the dissolved load, largely due to cation retention in secondary clays. By modelling the Li mass-balance and isotope fractionation, we estimate that the river-derived sediments gain 3-4 mu g/g Li from the dissolved load in the Amazon estuary, with a Li isotope fractionation factor (alpha clay-solution) of approximately 0.975. Considering the whole Amazon estuary, the river-derived sediments remove around 3.6-4.8 x 109 g/yr of Li from the dissolved load. Specifically, around 1.0-1.7 x 108 g/yr of Li is removed from river water (-1.8-3.0% of the dissolved Li discharge flux of the Amazon River) and around 3.5-4.7 x 109 g/yr of Li is removed from seawater, which represents a significant sink from the ocean. This estuarine Li sink is likely to be related to continental erosion rates; thus, continental weathering and erosion regimes could influence not only riverine Li input, but could also directly affect the Li sink, leading to a dual control on the Li budget and isotope composition in the ocean.

Automated summary

This study analyzes lithium isotopes and elemental concentrations in the Amazon estuary and suggests that river-derived sediments play a significant role in removing lithium from the dissolved load and acting as a sink for the element in the ocean. Furthermore, the study indicates that continental weathering and erosion regimes can influence the dual control on the lithium budget and isotope composition in the ocean.