Sixty million years of seafloor alteration: spatial-temporal controls on volatile incorporation along a 1000 km transect of oceanic crust

Roughly 60% of the Earth's surface is covered by oceanic crust and submerged beneath the oceans. Oceanic crust is formed from magma at mid-ocean ridges as a result of plate tectonic spreading. As they cool, the igneous rocks of the oceanic crust are altered by chemical reactions with seawater and other fluids. At high temperatures near the spreading ridge these interactions result in the spectacular hydrothermal 'black smoker' vent systems observed by submersibles on the seafloor. Low temperature fluid-rock reactions away from the spreading ridges are likely to be longer-lived and may continue for tens of millions of years. The alteration of oceanic crust results in chemical exchange between the crust and the oceans, altering the composition of both. Volatile chemicals such as carbon, chlorine and water are rare in fresh lavas but become enriched in the crust through alteration. This removes them from the oceans and binds them into the oceanic crust over time. The eventual fate of most oceanic crust is to be subducted beneath other tectonic plates and sink into Earth's mantle, removing volatile elements from the Earth's surface for potentially long timescales.

The processes described above have significant implications for the natural cycles of many volatile elements. Carbon is particularly important due to its behaviour in the atmosphere. Trapping it in the oceanic crust and subducting that crust might be an important part of past global climate cycles. Similarly, removal of seawater chlorine in this way may have been responsible for reducing the saltiness of the oceans and helping to make them habitable for life, over Earth's history. In order to understand how alteration of the oceanic crust may have affected seawater and atmospheric chemistry in the past we need to understand what controls volatile geochemical behaviour in this setting. The age of crust, the spreading rate (the rate at which the oceanic plates are moving apart) and the amount of sediment covering the crust are all likely to affect alteration and can be estimated or predicted from plate tectonic reconstructions. However, working out spreading rate or how sediment thickness affects the trapping of volatiles in altered crust is complex. Because it is submerged by seawater, our only direct samples of the oceanic crust come from drilling or dredging the seafloor. The cores drilled to date are mostly from very old or very young crust and are biased to faster spreading rates and thicker sediment cover. To address these shortcomings, a new drilling expedition is being undertaken to drill a transect of 6 holes along a single ~1000 km segment of oceanic crust in the South Atlantic, allowing a unique sample set recording ~60 million years of alteration to be interrogated. The new drilling will also fill in key gaps in the existing collection of cores including oceanic crust characterised by slow spreading rates and thin sedimentary cover sequences.

From April to August 2022, an international team of scientists is due to sail twice across the Atlantic Ocean onboard the scientific drilling vessel JOIDES Resolution. As cores are recovered, they will be studied and curated onboard to produce a permanent and publicly available record of the material recovered. Back on shore, the cores will be sampled for a range of scientific research projects to be carried out at laboratories across the world. At the University of Manchester, we intend to measure the amounts of carbon, water, halogens and noble gases in a selection of the core samples. This will allow us to trace the sources of the alteration fluids, document how they are incorporated into the rocks over time and reinterpret the results of earlier studies to obtain a more complete picture of the importance of time to oceanic crustal alteration. Ultimately this will also enable us to make better predictions of how trapping of important volatiles such as carbon in the oceanic crust may have varied in the past.