Using radium isotope fingerprinting to quantify iron release and distribution from different Celtic Sea shelf sediment types

Continental shelf sediments represent a major source of essential trace elements to the world’s oceans, and are of notable importance in cycling of the micronutrient iron (Fe). However, considerable uncertainties remain concerning the release of such micronutrients, including the overall flux of Fe from sediments, seasonal variation in this flux, and in particular the influence of sediment type.

This study used naturally occurring isotopes of the radioactive element radium (Ra) to quantify sedimentary flux. Like Fe, Ra is released from sediments into pore waters. Each of the four isotopes decay at different known rates, facilitating the use of Ra as a tracer of source, diffusion into the overlying water column, and subsequent advection and mixing for multiple time and length scales.

We measured Ra and Fe concentrations across spring, summer and autumn in Celtic Sea shelf waters, at sites spanning a range of sediment lithologies from mud to sand. Short-lived Ra isotopes reveal significant horizontal gradients in waters overlying different sediment types, despite vigorous vertical mixing during winter.

In addition to the spatial distribution of Ra isotopes, we studied the coupled release of Fe and Ra from different sediments with ex situ incubations. Together with pore water measurements, this experimental approach can explain much of the observed broad-scale spatial variation. We further discuss seasonal variation in these inventories, and explore the implications for release of Fe (and other dissolved species) from continental shelf sediments on a larger scale.

Back to: Physical and Biogeochemical Processes and the Support of Shelf Sea Primary Productivity and Carbon Cycling IV Posters