UK-IODP Moratorium Award for Rosalind Coggon - Co-chief scientist IODP Exp 390/393

Ocean chemistry reflects the shifting balance between fundamental processes such as mountain building, erosion and weathering, climate change, and biological evolution that have influenced Earth's surface throughout its history. Reliable records of past seawater chemistry therefore provide information crucial to understanding the future evolution of our planet. To investigate signatures of past climate or tectonics from these records we must quantify the other ocean inputs and outputs, including chemical exchanges with the underlying ocean crust. Ocean crust is produced along the submarine volcanic mid-ocean ridges repaving two-thirds of Earth's surface every 200 million years. However, it remains relatively unexplored, accessible only using submersibles or scientific ocean drilling. Such exploration revealed that seawater circulates through cracks in the cooling ocean crust, reacts with the rocks, transporting heat and chemicals to the oceans. At the ridges this occurs through spectacular 'hydrothermal' hot springs, but quantitatively more important reactions occur during lower-temperature fluid flow on the vast ridge flanks. During mid-ocean ridge volcanism CO2 gas from the mantle is released from the magma to the oceans and atmosphere. During 'hydrothermal circulation' calcium carbonate (CaCO3) forms from the fluid as it passes through the crust, storing CO2 in the rock. The formation and evolution of ocean crust at mid-ocean ridges therefore enables long-term C-cycling through the Earth system. Unfortunately, we do not know how long hydrothermal exchange persists in aging ocean crust, and hence how it affects ocean chemistry or the long-term C-cycle that influences climate, primarily because we lack drill core of ocean crust of different ages. However, we have shown that the hydrothermal carbonates record the chemistry of the fluids they form from and can be used to determine past ocean chemistry and conditions. 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. I will use rocks and minerals from these cores to develop new records of past ocean chemistry and quantify hydrothermal contributions to the oceans and the long-term global carbon cycle.