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

Abstract

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 > 150km. 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.