Timescales of South Atlantic ridge flank hydrothermal exchange; UK-IODP Moratorium Award for Thomas Belgrano - Shipboard Scientist, Expedition 393.

About two thirds of the Earth's surface is made up oceanic crust produced by tectonic plate spreading and magmatism along the global network of mid-ocean ridges. Chemical interaction between this newly formed ocean crust and seawater has regulated ocean chemistry throughout geological history. In particular, minerals precipitated in the crust from this interaction provide a means for geologically, biologically, and societally important substances such as carbon dioxide, water, sulfur, and potassium to be sequestered for many millions of years before eventual redelivery to the mantle.

Despite this understanding, the rates and specifics of such uptakes by oceanic crust are not particularly well known, especially for 'middle aged' crust between 20-60 million years old. A well-known deficit between the amount of heat measured at the surface and that predicted by models based purely on conductive heat loss indicates that substantial seawater cooling of the crust occurs over this interval. Similarly, a measured gradual increase in the speed of seismic waves through oceanic crust over the same timeframe suggests mineral reactions continue over this middle aged period.

Defining what these sub-surface changes are and how they develop with time - smoothly or in pulses, quickly or slowly - is prerequisite to integrating this component of the Earth system into global models for biogeochemical cycling.

This study aims to determine the rate and the uniformity with which seawater-ocean crust exchange develops over the 20-60 million year stage of crustal maturation. The International Ocean Drilling Program (IODP) South Atlantic Transect Expeditions 390/393 were specifically planned to sample this age interval, with six drill holes into upper oceanic crust of 6-61 million years age. This will be achieved by dating the decay of natural, weakly radioactive elements in hydrothermal minerals.

By dating material from all of the holes, we will assess whether hydrothermal fluid flow persisted over the 20-60 million time interval. Comparison of ages from drill holes with different thicknesses of blanketing sediments and different amounts of nearby basement outcrops will further allow the influence of these factors on prolonging hydrothermal circulation to be assessed. Finally, the chronological reference points for features in the drill core such as mineral veins will allow other expedition scientists studying the origin and microbial colonisation of these features to place their results in a temporal context.