An experimental study of the interaction of basaltic riverine particulate material and seawater

Abstract

The riverine transport of elements from land to ocean is an integral flux for many element cycles and an important climate regulating process over geological timescales. This flux consists of both dissolved and particulate material. The world's rivers are estimated to transport between 16.6 and 30 Gt yr À1 of particulate material, considerably higher than the dissolved flux of $1 Gt yr À1 . Therefore, the dissolution of particulate material upon arrival in estuaries and coastal waters may be a significant flux for many elements. Here we assess the role of riverine particulate material dissolution in seawater with closed-system experiments using riverine bedload material and estuarine sediment from western Iceland mixed with open ocean seawater. Both particulate materials significantly changed the elemental concentrations of the surrounding water with substantial increases in Si concentrations indicative of silicate dissolution. Seawater in contact with bedload material shows considerable enrichment of Ca, Mg, Mn, and Ni, while Li and K concentrations decrease. Moreover, the 87 Sr/ 86 Sr of seawater decreases with time with little change in Sr concentrations, indicative of a significant two-way flux between the solid and fluid phases. Mass balance calculations indicate that 3% of the Sr contained in the original riverine bedload was released during 9 months of reaction. In contrast, the estuarine material has a negligible effect on seawater 87 Sr/ 86Sr and transition metal concentrations, suggesting that these reactions occur when particulate material first arrives into coastal waters. Solubility calculations performed using the PHREEQC computer code confirm that primary minerals are undersaturated, while secondary minerals such as kaolinite are oversaturated in the reacted fluids. These results demonstrate that riverine transported basaltic particulate material can significantly alter the composition of seawater, although the total concentrations of many major elements in seawater are regulated by the formation of secondary phases. This behavior has important implications for nutrient supply to coastal waters and the isotopic mass balance of several elements in the oceans.

Reactivity of neodymium carriers in deep sea sediments: Implications for boundary exchange and paleoceanography

“…4b), the volcanics contribute $12% to the bulk detrital sediment Nd budget. Therefore, if contamination of the leachates only occurs from the volcanic component, $8% dissolution of this phase is required, in good agreement with recent studies on the dissolution of volcanic materials (Jones et al, 2012). Based on the same approach, the small samples of experiment (i) on WIND 24B also require $8% dissolution of this volcanic phase, while comparable calculations for small samples from WIND 28B and WIND 32B are also consistent with $5-10% dissolution.…”

“…Few experimental studies have addressed these questions or investigated boundary exchange more generally. A boundary exchange process occurred in batch reactor experiments on various riverine sediments but not on the one estuarine sediment sample studied to date (Jones et al, 2012). Since estuarine sediments are likely to represent more closely the material that is transferred to the oceans, we do not yet have experimental evidence to support the observational evidence for boundary exchange in the deep ocean (Lacan and Jeandel, 2005).…”

Overview of the mechanisms that could explain the ‘Boundary Exchange’ at the land–ocean contact

“…Batch experiments or sequential leaching slightly illuminated the processes by confirming that a fraction of the lithogenic material can be dissolved, releasing some of the elements it contains and contributing to balance their isotopic composition [29][30][31][32]51]. These experiments also showed that secondary phases are rapidly formed, scavenging a large part of most of the released species.…”

“…However, because seawater is close to saturated or supersaturated with respect to many mineral phases, much of the chemical input into seawater from atmospheric or riverine transported particulate material dissolution should be readily precipitated as secondary mineral, consistent with the 'Boundary Exchange' hypothesis. This could affect the net element input flux, but not so clearly its isotopic composition and the impact of this composition on the local seawater [29][30][31][32]. Precise flux estimates will depend on a comprehensive understanding of processes governing particle-solution exchanges, opening a large field for further researches.…”

Impact of sediment–seawater cation exchange on Himalayan chemical weathering fluxes

“…When transferred to the oceans, the Ca 2+ adsorbed on sediment surfaces is partially exchanged for Na + , Mg 2+ and K + (Sayles and Mangelsdorf, 1977), which are more abundant in ocean waters. These exchange reactions represent an additional source of Ca to the oceans and a potential sink for Na, Mg and K. These estuarine exchange reactions cannot be directly compared to additional weathering fluxes from the chemical weathering of sediments in seawater that has recently been suggested (Jones et al, 2012) as the processes are different. Chemical weathering in the presence of seawater involves the formation of secondary minerals (Jones et al, 2012), which is not the case for exchange reactions.…”