Consequences of a Global Slowdown in Seafloor Spreading for Sea Level and Mantle Heat Loss

Variations in global mean sea level over millions of years originate from changes in both the climate and solid Earth systems. Most previous studies of the solid Earth contribution to sea level focused on the past 100 Myr or longer with a temporal resolution of 10 Myr or coarser. Here, we consider how sea level was affected by a 35% global slowdown in ocean crust production that occurred during 15–6 Ma, as was recently identified from marine magnetic anomalies. We calculate how the seafloor area-age distribution evolves over time in response to the slowdown in crust production, exploring how assumptions about the initial condition and crust destruction rate affect the results. The slowdown decreases the proportion of young shallow seafloor relative to older deeper seafloor, which produces a sea-level fall of 24–30 m if the plate-cooling model is used to describe the age dependence of bathymetry and a slightly larger fall if the half-space cooling model is used. The slowdown also increases the prevalence of slow and ultraslow spreading centers, which, by inhibiting mantle melting, causes the ocean crust to thin by 0.5 km, with a concomitant additional 2-m sea-level fall. Although an easily interpretable record of sea-level variations from sequence stratigraphy does not exist for the past 15 Myr, data from coastal New Jersey and offshore Nova Scotia are broadly consistent with our predictions. If the slowdown affects volcanic degassing, atmospheric CO₂, and the climate, thermosteric and ice effects can produce additional sea-level fall of >60 m.