Timing of carbon uptake during seafloor alteration: Insight from in situ U-Pb dating at DSDP sites 417A and 417D

Abstract

Formation of carbonate minerals during alteration of seafloor lavas serves as a significant global CO2 sink. Understanding the timing of carbonate formation is critical for evaluating the role of low-temperature seafloor alteration as a negative feedback on the global carbon cycle. However, whether carbonate mineral formation largely occurs soon after crustal accretion, or continues throughout the entire lifespan of the ocean crust, remains debated. In this study, we use in situ U-Pb dating techniques to investigate the formation ages of carbonate veins and vesicles in ∼120 Ma ocean crust at Deep Sea Drilling Project (DSDP) Sites 417A and 417D, located in the western Atlantic. Our results show that carbonate mineral U concentrations differ between these cores, which we interpret as reflecting a strong dependence of U uptake into calcite on the redox conditions of the aquifer during carbonate growth. The more oxidizing alteration conditions at Site 417A led to growth of carbonate minerals with much lower U concentrations than those formed at Site 417D, which was altered under more reducing conditions. Importantly, through a thorough evaluation of both published and our new carbonate U-Pb age data, we confirm that more than 90 % of carbonate mineral formation during seafloor alteration occurs within <20 m.y. after crustal accretion. Simple models based on this refined timescale of basalt alteration and carbonate mineral formation show that variation in bottom water temperature within the first 10 m.y. after crustal accretion can affect the final carbon content of the upper oceanic crust. Our study provides valuable insights for carbon cycle models and highlights the importance of seafloor alteration in regulating Earth's climate.