Ocean crustal veins record dynamic interplay between plate-cooling-induced cracking and ocean chemistry

• Ocean crustal veins reflect dynamic evolution in ocean chemistry and plate cooling. • Veins widths and strain increase with crustal age, while vein densities remain constant. • Elemental mapping reveals multiple precipitation and fracturing episodes. • Ocean plate cooling drives vein reactivation, sustaining fluid flow over tens of millions of years. • Excess vein material linked to higher CO 2 levels during crustal formation. As ocean crust traverses away from spreading ridges, low-temperature hydrothermal minerals fill cracks to form veins, transforming the physical and chemical properties of ocean crust whilst also modifying the composition of seawater. Vein width and frequency observations compiled from the International Ocean Discovery Program (IODP) South Atlantic Transect (∼31°S) and previous scientific ocean drilling holes show that vein width distributions progressively broaden and observed strain ( Σ m veins / m core ) increases with crustal age, whereas vein densities ( # veins / m core ) remain approximately constant. Elemental mapping and textural observations illuminate multiple precipitation and fracturing episodes that continue as the ocean crust ages. This challenges the existing notion that ocean crustal veins are passively filled; rather, they are dynamic features of ocean crust aging. These data, combined with thermal strain modelling, indicate a positive feedback mechanism where cooling of the ocean plate induces cracking and the reactivation of pre-existing veins, ultimately resulting in further cooling. Waning of this feedback provides a mechanism for the termination of the global average heat flow anomaly. Sites with total vein dilation greater than expected for their age correspond with crustal formation during periods of high atmospheric CO 2 . The amount of vein material thus reflects the changing balance between ocean plate cooling, ocean chemistry, and the age of the ocean crust. Our results demonstrate that ocean crust endures as an active geochemical reservoir for tens of millions of years after formation.