How typhoons trigger turbidity currents in submarine canyons.

Intense turbidity currents occur in the Malaylay Submarine Canyon off the northern coast of Mindoro Island in the Philippines. They start in very shallow waters at the shelf break and reach deeper waters where a gas pipeline is located. The pipeline was displaced by a turbidity current in 2006 and its rock berm damaged by another 10 years later. Here we propose that they are triggered near the mouth of the Malaylay and Baco rivers by direct sediment resuspension in the shallow shelf and transport to the canyon heads by typhoon-induced waves and currents. We show these rivers are unlikely to generate hyperpycnal flows and trigger turbidity currents by themselves. Characteristic signatures of turbidity currents, in the form of bed shear stress obtained by numerical simulations, match observed erosion/deposition and rock berm damage patterns recorded by repeat bathymetric surveys before and after typhoon Nock-ten in December 2016. Our analysis predicts a larger turbidity current triggered by typhoon Durian in 2006; and reveals the reason for the lack of any significant turbidity current associated with typhoon Melor in December 2015. Key factors to assess turbidity current initiation are typhoon proximity, strength, and synchronicity of typhoon induced waves and currents. Using data from a 66-year hindcast we estimate a ~8-year return period of typhoons with capacity to trigger large turbidity currents.

A strategy for the conservation of biodiversity on mid-ocean ridges from deep-sea mining.

Mineral exploitation has spread from land to shallow coastal waters and is now planned for the offshore, deep seabed. Large seafloor areas are being approved for exploration for seafloor mineral deposits, creating an urgent need for regional environmental management plans. Networks of areas where mining and mining impacts are prohibited are key elements of these plans. We adapt marine reserve design principles to the distinctive biophysical environment of mid-ocean ridges, offer a framework for design and evaluation of these networks to support conservation of benthic ecosystems on mid-ocean ridges, and introduce projected climate-induced changes in the deep sea to the evaluation of reserve design. We enumerate a suite of metrics to measure network performance against conservation targets and network design criteria promulgated by the Convention on Biological Diversity. We apply these metrics to network scenarios on the northern and equatorial Mid-Atlantic Ridge, where contractors are exploring for seafloor massive sulfide (SMS) deposits. A latitudinally distributed network of areas performs well at (i) capturing ecologically important areas and 30 to 50% of the spreading ridge areas, (ii) replicating representative areas, (iii) maintaining along-ridge population connectivity, and (iv) protecting areas potentially less affected by climate-related changes. Critically, the network design is adaptive, allowing for refinement based on new knowledge and the location of mining sites, provided that design principles and conservation targets are maintained. This framework can be applied along the global mid-ocean ridge system as a precautionary measure to protect biodiversity and ecosystem function from impacts of SMS mining.

Human activities on the deep seafloor in the North East Atlantic: an assessment of spatial extent.

BACKGROUND: Environmental impacts of human activities on the deep seafloor are of increasing concern. While activities within waters shallower than 200 m have been the focus of previous assessments of anthropogenic impacts, no study has quantified the extent of individual activities or determined the relative severity of each type of impact in the deep sea. METHODOLOGY: The OSPAR maritime area of the North East Atlantic was chosen for the study because it is considered to be one of the most heavily impacted by human activities. In addition, it was assumed data would be accessible and comprehensive. Using the available data we map and estimate the spatial extent of five major human activities in the North East Atlantic that impact the deep seafloor: submarine communication cables, marine scientific research, oil and gas industry, bottom trawling and the historical dumping of radioactive waste, munitions and chemical weapons. It was not possible to map military activities. The extent of each activity has been quantified for a single year, 2005. PRINCIPAL FINDINGS: Human activities on the deep seafloor of the OSPAR area of the North Atlantic are significant but their footprints vary. Some activities have an immediate impact after which seafloor communities could re-establish, while others can continue to make an impact for many years and the impact could extend far beyond the physical disturbance. The spatial extent of waste disposal, telecommunication cables, the hydrocarbon industry and marine research activities is relatively small. The extent of bottom trawling is very significant and, even on the lowest possible estimates, is an order of magnitude greater than the total extent of all the other activities. CONCLUSIONS/SIGNIFICANCE: To meet future ecosystem-based management and governance objectives for the deep sea significant improvements are required in data collection and availability as well as a greater awareness of the relative impact of each human activity.