Description: Deep-sea carbonate represents Earth’s largest carbon sink and one of the least-known components of the long-term carbon cycle that is intimately linked to climate. By coupling the deep-sea carbonate sedimentation history to a global tectonic model, we quantify this component within the framework of a continuously evolving seafloor. A long-term increase in marine carbonate carbon flux since the mid-Cretaceous is dominated by a post-50 Ma doubling of carbonate accumulation to ∼310 Mt C/yr at present-day. This increase was caused largely by the immense growth in deep-sea carbonate carbon storage, post-dating the end of the Early Eocene Climate Optimum. We suggest that a combination of a retreat of epicontinental seas, underpinned by long-term deepening of the seafloor, the inception of major Himalayan river systems, and the weathering of the Deccan Traps drove enhanced delivery of Ca2+ and HCO3– into the oceans and atmospheric CO2 drawdown in the 15 m.y. prior to the onset of glaciation at ca. 35 Ma. Relatively stagnant mid-ocean ridge, rift- and subduction-related degassing during this period support our contention that continental silicate weathering, rather than a major decrease in CO2 degassing, may have triggered an increase in marine carbonate accumulation and long-term Eocene global cooling. Our results provide new constraints for global carbon cycle models, and may improve our understanding of carbonate subduction-related metamorphism, mineralization and isotopic signatures of degassing.

Description: Highlights • Compilation of rifting events in the Neoproterozoic • Analysis of continental arc, continental rift and connectedness of continental lithosphere for the last 1 Ga • Two stage supercontinent cycle may better explain changes in the connectedness of continental lithosphere • Extraversion and introversion models of successive supercontinents occur on different timescales Abstract The extent of continental rifts and subduction zones through deep geological time provides insights into the mechanisms behind supercontinent cycles and the long term evolution of the mantle. However, previous compilations have stopped short of mapping the locations of rifts and subduction zones continuously since the Neoproterozoic and within a self-consistent plate kinematic framework. Using recently published plate models with continuously closing boundaries for the Neoproterozoic and Phanerozoic, we estimate how rift and peri-continental subduction length vary from 1 Ga to present and test hypotheses pertaining to the supercontinent cycle and supercontinent breakup. We extract measures of continental perimeter-to-area ratio as a proxy for the existence of a supercontinent, where during times of supercontinent existence the perimeter-to-area ratio should be low, and during assembly and dispersal it should be high. The amalgamation of Gondwana is clearly represented by changes in the length of peri-continental subduction and the breakup of Rodinia and Pangea by changes in rift lengths. The assembly of Pangea is not clearly defined using plate boundary lengths, likely because its formation resulted from the collision of only two large continents. Instead the assembly of Gondwana (ca. 520 Ma) marks the most prominent change in arc length and perimeter-to-area ratio during the last billion years suggesting that Gondwana during the Early Palaeozoic could explicitly be considered part of a Phanerozoic supercontinent. Consequently, the traditional understanding of the supercontinent cycle, in terms of supercontinent existence for short periods of time before dispersal and re-accretion, may be inadequate to fully describe the cycle. Instead, either a two-stage supercontinent cycle could be a more appropriate concept, or alternatively the time period of 1 to 0 Ga has to be considered as being dominated by supercontinent existence, with brief periods of dispersal and amalgamation.

Description: Contourite drifts are anomalously high sediment accumulations that form due to reworking by bottom currents. Due to the lack of a comprehensive contourite database, the link between vigorous bottom water activity and drift occurrence has yet to be demonstrated on a global scale. Using an eddy-resolving ocean model and a new georeferenced database of 267 contourites, we show that the global distribution of modern contourite drifts strongly depends on the configuration of the world’s most powerful bottom currents, many of which are associated with global meridional overturning circulation. Bathymetric obstacles frequently modify flow direction and intensity, imposing additional finer-scale control on drift occurrence. Mean bottom current speed over contourite-covered areas is only slightly higher (2.2cm/s) than the rest of the global ocean (1.1cm/s), falling below proposed thresholds deemed necessary to re-suspend and redistribute sediments (10–15cm/s). However, currents fluctuate more frequently and intensely over areas with drifts, highlighting the role of intermittent, high-energy bottom current events in sediment erosion, transport, and subsequent drift accumulation. We identify eddies as a major driver of these bottom current fluctuations, and we find that simulated bottom eddy kinetic energy is over three times higher in contourite-covered areas in comparison to the rest o.f the ocean. Our work supports previous hypotheses which suggest that contourite deposition predominantly occurs due to repeated acute events as opposed to continuous reworking under average-intensity background flow conditions. This suggests that the contourite record should be interpreted in terms of a bottom current’s susceptibility to experiencing periodic, high-speed current events. Our results also highlight the potential role of upper ocean dynamics in contourite sedimentation through its direct influence on deep eddy circulation.