In order to better understand the sources, sinks and hydrodynamic/biogeochemical influences on particulate matter distribution and variability in Arctic basins, we combined data from two 2015 fall expeditions: one from Bering Strait (USCGC Healy) and the other from Barents Sea (R/V Polarstern) meeting at the North Pole. Sections of beam attenuation due to particles were overlain by salinity, temperature, and chlorophyll-a fluorescence (Chl-a), and with nitrate contours on Chl-a sections to compare with concentrations of particulate matter (PM) and particulate organic carbon (POC) from full water column filtered samples. Dense Pacific water moving swiftly through Bering Strait erodes and carries sediment-laden waters onto the Chukchi Shelf, much of it moving in and above Barrow Canyon or is entrained in eddies. This nutrient-rich Pacific water sinks below the low-salinity, nutrient-poor polar mixed layer, forming a thick lens of high salinity water known as Pacific halocline waters. The nutrient-poor mixed layer inhibits photosynthesis in surface waters of Canada and Makarov Basins, but subsurface Chl-a maxima are observed when nutrients are available. Surface-water POC biomass appears greater in Barents Sea than in Beaufort Sea because nutrient-rich Atlantic water entering Barents Sea is not isolated from surface waters by strong stratification. Surface water freezes, creating high-density water that cascades into 400 m basins in Barents Sea and into deep Nansen Basin, eroding sediment that forms patches of nepheloid layers in the shallow basins. Nepheloid layers in the deep basins are very weak, consistent with a lack of strong currents there.

Global maps of maximum bottom particle concentration, benthic nepheloid layer thickness, and integrated particle mass in benthic nepheloid layers (BNL) based on 2412 global profiles collected using the Lamont Thorndike nephelometer from 1964-1984 are compared with maps of those same properties compiled from 6,392 global profiles measured by transmissometers from 1979 to 2016. Outputs from both instruments were converted to particulate matter concentration (PM). We present here a visual global comparison of the location and intensity of BNLs measured with these two independent instruments over slightly overlapping decadal time periods and combine the data sets in order to expand the time scale of global in situ measurements of BNLs, and to gain insight about the factors creating/sustaining BNLs. The similarity between general locations of high and low particle concentration BNLs during the two time periods indicates that the driving forces of erosion and resuspension of bottom sediments are spatially persistent during recent decadal time spans, though in areas of strong BNLs, intensity is highly episodic. Topography and well-developed current systems play a role. These maps can be used to better understand deep ocean sediment dynamics, linkage with upper ocean dynamics, the potential for scavenging of adsorption-prone elements near the seafloor, and provide a comprehensive comparison of these data sets on a global scale. During both time periods, BNLs are weak or absent in most of the Pacific, Indian, and Atlantic basins away from continental margins. High surface eddy kinetic energy is associated with the Kuroshio Current east of Japan. Both data sets show weak BNLs south of the Kuroshio, but no transmissometer data have been collected beneath the Kuroshio itself. Sparse nephelometer data show moderate BNLs just north of the Kuroshio Extension, but with much lower concentrations than beneath the Gulf Stream. Strong BNLs are found in areas where eddy kinetic energy in overlying waters, mean kinetic energy near bottom, and energy dissipation within the bottom boundary layer are high. Areas of strongest BNLs include the Western North Atlantic, Argentine Basin, areas around South Africa tied to the Agulhas Current region, and somewhat random locations in the Antarctic Circumpolar Current of the Southern Ocean.