Sediment Characterization at the Equatorial Mid‐Atlantic Ridge From P‐to‐S Teleseismic Phase Conversions Recorded on the PI‐LAB Experiment

다국어 초록 (Multilingual Abstract)

Accurate marine sediment characteristics, for example, thickness and seismic velocity, are important for constraining sedimentation rates with implications for climate variations and for seismic imaging of deeper structures using ocean bottom seismic deployments. We analyze P‐to‐S seismic phase conversions from the sediment‐crust boundary recorded by the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) experiment to infer the sediment thickness across the Mid‐Atlantic Ridge covering 0‐ to 80‐Myr‐old seafloor. We find Pds‐P delay times of 0.04–0.37 s, or 5‐ to 82‐m thickness. Sediment thickness increases with age. Thickness agrees with global estimates for young (<15–20 Myr) seafloor but is thinner on older lithosphere. Our result may represent a lower limit on sediment thickness, given that several of our stations are on topographic highs. The sedimentation rate decrease observed from 5 to 1.2 mm/kyr at ∼10 Myr suggests a recent increase in productivity related to climate change, eolian dust fluxes, and/or biogenic marine activity.
Studies on marine sediment production rate provide historical information on the past climates and sedimentation processes. Unfortunately, such data from the seafloor is sporadic across the oceans, limited to localized drilled sites and seismic profiles. Here we analyze the sediment characteristics along the equatorial Mid‐Atlantic Ocean, starting from age 0 at the ridge to sediments 80 million years old. We use newly acquired seismic data from the recent state‐of‐the‐art Passive Imaging of the Lithosphere‐Asthenosphere Boundary experiment. Thirty‐nine seismometers were put at the ocean bottom down to about 5,000 m below sea level and left recording seismic data for one year between 2016 and 2017. Conversion of seismic phases from the boundary between the crust and the sediment were analyzed to determine the sediment seismic velocity and thickness below each station. It was found that the thickness increases with age at a slower rate than previously thought and that an abrupt increase in the sedimentation rate occurred in the last 10 million years, from 1.2 to 5 mm per thousand years. The recent increase in productivity may be due to factors such as changes in climate, changes in the eolian dust fluxes from Africa to the Atlantic, and/or changes in biogenic marine activity.


P‐to‐S delay times from the sediment‐crust boundary suggest 5‐82 m of sediment beneath PI‐LAB
The thickness generally agrees with global estimates but is less than expected on older lithosphere
Higher deposition rates at <10 Myr suggest higher productivity perhaps related to stronger monsoons