To provide an observational basis for the Intergovernmental Panel on Climate Change projections of a slowing Atlantic meridional overturning circulation (MOC) in the 21st century, the Overturning in the Subpolar North Atlantic Program (OSNAP) observing system was launched in the summer of 2014. The first 21-month record reveals a highly variable overturning circulation responsible for the majority of the heat and freshwater transport across the OSNAP line. In a departure from the prevailing view that changes in deep water formation in the Labrador Sea dominate MOC variability, these results suggest that the conversion of warm, salty, shallow Atlantic waters into colder, fresher, deep waters that move southward in the Irminger and Iceland basins is largely responsible for overturning and its variability in the subpolar basin.

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for the AMOC deep limb. In order to assess the impact of the interior pathway relative to the DWBC pathway, this work seeks to quantify the AMOC deep limb pathways in ocean circulation models, compare the pathway signatures of these models to observations, and identify a mechanism driving the interior pathway. The partitioning of the AMOC deep limb into interior and DWBC pathways is observed in several ocean models. Furthermore, there is a good agreement between the structure of the export pathways in models and observations. Both Eulerian and Lagrangian techniques, in models and observations, are used to identify the DWBC and interior pathways and these two perspectives are shown to be compatible with one another.Finally, deep, eddy-driven, recirculation gyres are shown to be a mechanism driving the interior pathway and the existence of the interior pathway is consistent with the vorticity balance at depth. The interior pathway makes a significant contribution to the total transport of the deep limb of the AMOC. Since the interior pathway is much broader and slower than the DWBC pathway, the large-scale transport of climate signals, heat, and anthropogenic CO 2 associated with the AMOC are slower and mixed more broadly throughout the ocean than once thought. iv

Microplastics are widespread in the natural environment and present numerous ecological threats. While the ultimate fate of marine microplastics are not well known, it is hypothesized that the deep sea is the final sink for this anthropogenic contaminant. This study provides a quantification and characterisation of microplastic pollution ingested by benthic macroinvertebrates with different feeding modes (Ophiomusium lymani, Hymenaster pellucidus and Colus jeffreysianus) and in adjacent deep water > 2200 m, in the Rockall Trough, Northeast Atlantic Ocean. Despite the remote location, microplastic fibres were identified in deep-sea water at a concentration of 70.8 particles m, comparable to that in surface waters. Of the invertebrates examined (n = 66), 48% ingested microplastics with quantities enumerated comparable to coastal species. The number of ingested microplastics differed significantly between species and generalized linear modelling identified that the number of microplastics ingested for a given tissue mass was related to species and not organism feeding mode or the length or overall weight of the individual. Deep-sea microplastics were visually highly degraded with surface areas more than double that of pristine particles. The identification of synthetic polymers with densities greater and less than seawater along with comparable quantities to the upper ocean indicates processes of vertical re-distribution. This study presents the first snapshot of deep ocean microplastics and the quantification of microplastic pollution in the Rockall Trough. Additional sampling throughout the deep-sea is required to assess levels of microplastic pollution, vertical transportation and sequestration, which have the potential to impact the largest global ecosystem.

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