A stagnant subducted slab accelerates the deep Earth carbon cycle

Substantial amounts of carbon can be transported into the deeper mantle beyond sub-arc depths and are thus likely to participate in mantle processes over long time scales. However, the residence time of carbon in the deep mantle remains debated. The (Paleo-)Pacific plate has been stagnating in the mantle transition zone (MTZ) beneath northeast Asia since the late Mesozoic, dominating deep carbon cycling above the MTZ. We reconstruct the carbon subduction flux from the (Paleo-)Pacific plate during the late Mesozoic to early Cenozoic, revealing that carbon transported beyond the sub-arc depths range from 1.4 to 14.9 Mt/yr. Meanwhile, analysis of Sr−Nd−Pb−Zn isotope data for Cretaceous−Cenozoic intraplate basalts shows that their mantle sources contained 0.4−1.4 wt% of recycled carbon derived from the subducting slab. Over time, the carbon subduction flux and the amount of recycled carbon exhibit similar trends, including three corresponding abrupt decreases and a transition of the predominant carbon reservoir from altered oceanic crust to sediments. An average time lag of 17 m.y. between the two trends reflects a relatively short residence time for subducted carbon associated with the stagnant slab in northeast Asia. These results indicate that stagnant slabs can accelerate the deep carbon cycle and exert a strong effect on subduction dynamics.