Constraining sediment subduction: A converted phase study of the Aleutians and Marianas

This paper presents a new method for constraining the thickness of the thin low-velocity layer observed at the upper surface of subducting slabs using the differential travel time between direct P and converted SP phases. To aid detection of these phases a three-component, frequency dependent, data-adaptive polarization filter was also developed. An inversion scheme was applied to the data from two areas previously characterised by different sediment subduction regimes: accretionary (Alaska) and non-accretionary (Marianas). In both areas, our results are consistent with the hypothesis that the entire oceanic crust and sedimentary column is subducted intact to depths of > 150 km. Assuming that our study areas are representative of all subduction zones we have recalculated the global CO2 cycle to include the increased volume of sediment subducted under this new regime. A brief box-model calculation suggests that continued pelagic sediment deposition and subduction will lead to a net sink of carbon into the mantle on geological timescales and that the amount of carbon that is deep subducted increases by 13%. A recalculation of GLOSS (Plank and Langmuir, 1998) with complete sediment subduction at all regions leads to an average 19% increase in the subduction flux of all components of the sedimentary column implying a greater flux of sedimentary elements into the lower mantle than previously estimated.