A geological carbon cycle sink hosted by ocean crust talus breccias

Calcium carbonate precipitation in ageing ocean crust sequesters carbon dioxide dissolved in seawater through seafloor weathering reactions, influencing atmospheric CO2 concentrations on million-year timescales. However, this crustal carbon sink, and the extent it balances CO2 degassing during crustal formation at mid-ocean ridges, remain poorly quantified due to limited sampling of the vast ridge flanks where CO2 uptake continues for millions of years. Here we quantify the carbon sink hosted within talus breccias that accumulated through mass wasting 61 million years ago during rift faulting at the slow spreading Mid-Atlantic Ridge, cored during International Ocean Discovery Program South Atlantic Transect Expedition 390. After 40 million years of carbonate cementation, these breccias contain ~7.5 wt% seawater-derived CO2, 2 to 40 times more than previously cored upper crust. Our estimates of talus breccia abundance based on fault geometries indicate that talus formed at slow-spreading ridges can accommodate a CO2 sink equivalent to a large proportion of the CO2 released during accretion of the underlying crust. The proportion of plate divergence accommodated by faulting, and hence talus abundance, increases nonlinearly with decreasing spreading rate. Consequently, past variations in spreading rate may have impacted the balance between ocean crust CO2 release and uptake in Earth’s carbon cycle.