Global comparison of benthic nepheloid layers based on 52 years of nephelometer and transmissometer measurements
Introduction
Optical instruments measuring forward scattering (referred to here as nephelometers) have been used for many decades to estimate particle abundance and distribution in bodies of water (Jerlov, 1953, Thorndike, 1975, Biscaye and Eittreim, 1977). In the 1970s, transmissometers were developed to measure the attenuation of light across a known path length to estimate particle concentration (Bartz et al., 1978). Numerous papers have been written using data from these, and similar instruments. Two multi decades-long global data sets have been collected: first using the Lamont Thorndike nephelometer (1960–1984; Thorndike, 1975) and second, using the SeaTech and WetLabs transmissometers (1979–2016; Gardner et al., 2018a, Gardner et al., 2018b). During a 1979 expedition of R/V Knorr (KN74), both instruments were used simultaneously in profiling and bottom-moored configurations and compared well in predicting particle concentrations (Gardner et al., 1985). We now combine those global data sets to quantify and compare benthic nepheloid layer (BNL) characteristics and location in the ocean over the last 52 years.
It is well known that particles in the euphotic zone of the open ocean result mainly from primary production of phytoplankton. Phytoplankton and phytodetritus are grazed by zooplankton, pelletized or aggregated into marine snow and rapidly remineralized during sinking (Honjo et al., 2008), processes that quickly reduce the particle concentration with depth. Once particles reach the seafloor they are incorporated into the bottom sediments. Optical and filtration measurements have shown that although particle concentrations in the open ocean decrease to very low minimum values in the water column deeper than 100–200 m (5–12 µg l−1; Biscaye and Eittreim, 1974, Brewer et al., 1976, Gardner et al., 1985), particle concentration can increase near the seafloor, indicating either erosion and resuspension due to bottom currents (Biscaye and Eittreim, 1974), or inhibited settling through bottom boundary layer turbulent mixing. The geographic variability of particle concentrations near the seafloor is orders of magnitude greater than in the mid-water column, sometimes reaching 100's–1000's µg l−1 (Biscaye and Eittreim, 1974, Biscaye and Eittreim, 1977, Hill et al., 2011, Hayes et al., 2015b, Gardner et al., 2017, Gardner et al., 2018a). High-resolution vertical measurements through the entire water column collected primarily during CTD hydrocasts with attached optical sensors, allows detection of BNLs. Profiling floats and gliders equipped with optical sensors have increased temporal and spatial coverage (Johnson et al, 2009), however most floats or gliders presently profile to 2000 m or less. Gliders that will profile to 6000 m are being built. We have yet to see how close to the seafloor they can safely make measurements. Therefore, these two data sets are valuable in setting the baseline for understanding sediment-water interactions on a global scale.
Biscaye and Eittreim (1977) synthesized nephelometer data collected throughout the North and South Atlantic, showing areas of high concentrations in the Western North Atlantic and in the Argentine Basin. Their initial hypothesis was that the high concentrations were caused by sediment eroded and resuspended by deep boundary currents generated by cold, saline water sinking at the poles and moving equatorward. Others noted a spatial association between elevated particulate matter concentrations (PM) in nepheloid layers and eddy kinetic energy (EKE) (Hollister and McCave, 1984) or bottom trapped topographic Rossby waves (Grant et al., 1985). Nephelometer data were collected in all oceans, however, no global maps of BNL parameters were constructed from those data. More than thirty years ago the state of general understanding about nepheloid layers was reviewed by McCave (1986). Most of the transmissometer data presented and discussed in this paper were collected after that review, and the first global synthesis of the Lamont Thorndike nephelometer data is contained in this paper. The combined data sets provide a much clearer picture of global geographic distribution, intensity, and variability of the BNL. New physical oceanographic measurements and models have also improved our understanding of hydrodynamics in the ocean.
The purpose of this paper is to compare global maps of maximum bottom particle concentrations in the deep ocean, thickness of BNLs, and “excess mass” of particles integrated within BNLs compiled from the nephelometer data collected from 1964 to 1984 with the maps of the same variables compiled using transmissometer data we collected between the late 1970’s to the present (Gardner et al., 2018a). We compare these data with published distributions of EKE, benthic energy dissipation, mean near-bottom kinetic energy, and refer to newly published time-series measurements in BNLs to better understand the causes, likely location, and variability of strong and weak BNLs.
Section snippets
Methods and data
The Lamont Thorndike photographic nephelometer (Thorndike, 1975) was developed to provide quantitative turbidity profiles in the ocean. The nephelometer was mounted on a metal frame coupled with a deep-sea camera to photograph the seafloor down to >7000 m, so both instruments had to be rugged enough to withstand high pressures. The nephelometer had a shielded source of continuous white light. The near-forward scattering signal was recorded using a photographic camera with an open shutter whose
Results and discussion
The data collected during these two multi-decadal time periods are not identified or sorted for seasons or years. Based on long near-bottom time-series measurements one would not expect significant seasonal impact on sediment erosion/resuspension (Gardner et al., 2017). Measurements during the E/ED era (1964–1984) were made primarily during Lamont-Doherty Earth Observatory (LDEO) geophysical surveys with the ships stopping once or twice a day to take piston cores, bottom photos, and
Conclusions
The regions of “strong” benthic nepheloid layers are very similar during both the 1964 to 1984 period as well as from the 1980’s to present day. “Strong” BNLs and benthic storms are most intense and thickest beneath areas of high surface and bottom eddy kinetic energy, strongly suggesting a linkage with upper ocean dynamics. This connection is best explained by generation of cyclones/anticyclones beneath meanders/rings spun off by major surface boundary currents or from bottom-trapped
Acknowledgements
We thank Lawrence Sullivan and his team for their invaluable expertise in preparing and deploying the Lamont-Thorndike nephelometers, and producing quality E/ED data from the 52 cruises involved in this work for over > 20 years. We also extend thanks to Mary Parsons, Adele Hanley, Adrian Heredia plus other personnel aboard numerous ships, and many colleagues for help in collecting and processing these data. We thank Rebecca Gray Thomas and Kenna Nolen for digitizing the KN74 CTD/Transmissometer
Author contributions
WDG, MJR and PEB conceived and outlined the analysis and synthesis of the nephelometer parameters. WDG and MJR orchestrated transmissometer data collection, calibrations, and initial analysis. AVM further processed the transmissometer and nephelometer data with QA/QC assistance from WDG and MJR. AVM constructed the figures, interacting with WDG and MJR. WDG wrote the initial manuscript with further development and editing from AVM and MJR.
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