Riverine particulate material dissolution in seawater and its implications for the global cycles of the elements

The total mass of material globally transported to the oceans as particulate material is as much as 30 times greater that transported to the oceans in dissolved form. The degree to which riverine transported particles impact ocean chemistry and influence global elemental cycles depends on the reactivity of this particulate material in seawater and the relative concentration of each element. The concentrations of elements in particulate material relative to those of seawater range from less than 1 for soluble elements such as Na, to more than 10⁷ for insoluble elements such as Al, Fe, and the Rare Earths. The reactivity of particulate material in seawater is difficult to assess directly because this fluid is saturated or supersaturated with respect to numerous elements. As such, element release from the particulate material is commonly matched by precipitation of secondary phases. The rates of element release can, however, be quantified by monitoring the isotopic evolution of seawater during its reaction with a variety of particulate material samples collected from rivers throughout the world. This research focuses on the behavior of Sr and Nd, representative of the most and least soluble elements transported to the oceans. Batch experiments demonstrate that between 0.15 and 27% of Sr is liberated from volcanic and continental particulates when interacted with seawater over 6 months. Similarly, from 1.5 to 8.5% of Nd is liberated from volcanic sediments over this timespan. This observed elemental release rate from riverine particulate material has important consequences for: (1) chemical and isotopic mass balances in the ocean; and (2) the application of the isotopic weathering proxies to the geological record.