UK-IODP Moratorium Award for Lewis Grant - Shipboard Scientist Expedition 390: The role of ridge flank dolomitization in the ocean Mg budget

Ocean crust is created at submarine volcanic mid-ocean ridges and spreads away over time to form the ocean basins that cover two thirds of Earth's surface. Seawater circulates through cracks in the cooling crust, removing heat and chemically interacting with the rocks. This 'hydrothermal circulation' influences both the composition of the oceans and Earth through time. As the crust ages, sediment accumulates above it, and seawater flows along the contact between these sediments and the underlying crust, making this an important site for seawater-rock chemical exchange.

Magnesium (Mg) is the second most abundant cation in seawater and is supplied to the oceans by rivers, before being removed during hydrothermal alteration of ocean crust and precipitation of Mg-rich carbonates (i.e., dolomite) in the sediments. These same processes also directly influence the global carbon budget, making Mg a useful proxy when reconstructing past climate change, which is key to understanding the future evolution of our planet.

In the geological 'rock record', dolomite was a major store for oceanic Mg, although the extent of dolomite formation has decreased significantly over the last 150 million years. However, dolomite has been found within the sediments immediately overlying the volcanic crust, where hydrothermal fluid-rock exchange is occurring. It is important to decipher the timing, temperature, and the source of Mg of this dolomite formation to determine how and why it forms, and whether it is an important Mg reservoir. Mg isotopes provide a geochemical fingerprint of the source of the Mg in different reservoirs in the Earth. However, the Mg isotopic composition of dolomites formed close to the sediment-basement interface has not yet been determined.

Scientific ocean drilling allows us to sample marine sediments and the oceanic crust buried deep below them. International Ocean Discovery Program (IODP) Expeditions 390 and 393 will drill a transect of six sites across the South Atlantic, recovering ocean crust formed at the Mid-Atlantic ridge between 7 and 61 million years ago and providing a unique opportunity to investigate dolomite formation along the contact between sediments and the ocean crust. I will use the recovered cores to reassess the Mg budget of the oceans over the last 60 million years and determine whether dolomite formation still exerts a significant control on modern seawater chemistry.