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.

Description: Global oceans are known to have alternated between aragonite and calcite seas. These oscillations reflect changes in the Mg/Ca ratios of seawater that control biomineralization and the composition of marine carbonates, and are thought to be caused mainly by the time dependence of crustal accretion at mid-ocean ridge crests and the associated high-temperature mid-ocean ridge fluid flux. Here we use global ocean basin reconstructions to demonstrate that the fluctuations in hydrothermal ocean inputs are instead caused by the gradual growth and destruction of mid-ocean ridges and their relatively cool flanks during long-term tectonic cycles, thus linking ocean chemistry to off-ridge low-temperature hydrothermal exchange. Early Jurassic aragonite seas were a consequence of supercontinent stability and a minimum in mid-ocean ridge length and global basalt alteration. The breakup of Pangea resulted in a gradual doubling in ridge length and a 50% increase in the ridge flank area, leading to an enhanced volume of basalt to be altered. The associated increase in the total global hydrothermal fluid flux by as much as 65%, peaking at 120 Ma, led to lowered seawater Mg/Ca ratios and marine hypercalcification from 140 to 35 Ma. A return to aragonite seas with preferential aragonite and high-Mg calcite precipitation was driven by pronounced continental dispersal, leading to progressive subduction of ridges and their flanks along the Pacific rim.

Description: The vigorous current systems in the Southern Ocean play a key role in regulating the Earth’s oceans and climate, with the record of long-term environmental change mostly contained in deep-sea sediments. However, the well-established occurrence of widespread regional disconformities in the abyssal plains of the Southern Ocean attests to extensive erosion of deep-sea sediments during the Quaternary. We show that a wide belt of rapid sedimentation rates (〉5.5 cm/k.y.) along the Southeast Indian Ridge (SEIR) is a global anomaly and occurs in a region of low surface productivity bounded by two major disconformity fields associated with the Kerguelen Plateau to the east and the Macquarie Ridge to the west. Our high-resolution numerical ocean circulation model shows that the disconformity fields occur in regions of intense bottom-current activity where current speeds reach 0.2 m/s and are favorable for generating intense nepheloid layers. These layers are transported toward and along the SEIR to regions where bottom-current velocities drop to 〈0.03 m/s and fine particles settle out of suspension, consistent with focusing factors significantly greater than 1. We suggest that the anomalous accumulation of sediment along an 8000-km-long segment of the SEIR represents a giant succession of contourite drifts that is a major extension of the much smaller contourite east of Kerguelen Plateau and has occurred since 3–5 Ma based on the age of the oldest crust underlying the deposit. These inferred contourite drifts provide exceptionally valuable drilling targets for high-resolution climatic investigations of the Southern Ocean.

Description: Atmospheric carbon dioxide (CO 2 ) data for the last 420 million years (My) show long-term fluctuations related to supercontinent cycles as well as shorter cycles at 26 to 32 My whose origin is unknown. Periodicities of 26 to 30 My occur in diverse geological phenomena including mass extinctions, flood basalt volcanism, ocean anoxic events, deposition of massive evaporites, sequence boundaries, and orogenic events and have previously been linked to an extraterrestrial mechanism. The vast oceanic crustal carbon reservoir is an alternative potential driving force of climate fluctuations at these time scales, with hydrothermal crustal carbon uptake occurring mostly in young crust with a strong dependence on ocean bottom water temperature. We combine a global plate model and oceanic paleo-age grids with estimates of paleo-ocean bottom water temperatures to track the evolution of the oceanic crustal carbon reservoir over the past 230 My. We show that seafloor spreading rates as well as the storage, subduction, and emission of oceanic crustal and mantle CO 2 fluctuate with a period of 26 My. A connection with seafloor spreading rates and equivalent cycles in subduction zone rollback suggests that these periodicities are driven by the dynamics of subduction zone migration. The oceanic crust-mantle carbon cycle is thus a previously overlooked mechanism that connects plate tectonic pulsing with fluctuations in atmospheric carbon and surface environments.