ENHANCED CENOZOIC CHEMICAL-WEATHERING AND THE SUBDUCTION OF PELAGIC CARBONATE

THE observed trend of increasing oceanic Sr-87/Sr-86 ratios during the late Cenozoic led Raymo et al. 1 to propose that chemical weathering rates increased at this time as a result of enhanced weatherability of silicate rocks. They suggested that this was due in turn to continential uplift, primarily in the Himalayas and the Andes. Because weathering involves the reaction of silicates with atmospheric carbon dioxide, considerations of changes in weathering rates must take into account the need to balance the global carbon cycle. To maintain this balance on timescales greater than 10(6) yr, enhanced weathering requires an increased flux of CO2 into the atmosphere 2. Without such an increased flux, weathering rates could not increase, and one is then forced to search for other explanations for the observed Sr-87/Sr-86 trend, such as a changing riverine strontium isotope composition 3-6. Here I assume that the strontium isotope record does indeed reflect enhanced weathering in the late Cenozoic, but propose that its cause may have been an increased CO2 flux arising from the recycling of pelagic sedimentary carbon at subduction zones. Since the Jurassic, sedimentary carbonate accumulation has shifted from primarily cratonic to primarily pelagic environments 7-11; because sea-floor spreading transports pelagic carbonate to, and recycles it through, metamorphic environments in subduction zones, this shift to pelagic carbonate accumulation would provide the increased CO2 flux needed to increase weathering rates 12,13.