Nepheloid Research Articles

Bottom and Intermediate Nepheloid Layer Induced by Shoaling Internal Solitary Waves: Impacts of the Angle of the Wave Group Velocity Vector and Slope Gradients

AbstractThe widely recognized global phenomena of bottom nepheloid layer (BNL) and intermediate nepheloid layer (INL) are ubiquitous in the ocean. These phenomena are induced by shoaling internal solitary waves (ISWs), as observed in many studies. In this study, we analyzed the BNLs and INLs induced by shoaling ISWs and their detailed processes using flume experiments and field observations. ISWs suspended seabed sediment by the horizontal velocity in the vortex, and the near‐bottom vertical velocity lifted sediment into the water column to create a BNL, which detached from the slope and diffused along the isopycnals, forming more than one INL. Considering the results of previous researchers, we found that the numbers of BNLs and INLs were principally determined by the relationship between the angle of the ISW group velocity vector (α) relative to horizontal and the slope gradients (γ). In transmissive regions (γ/α < 1), one BNL and more than one INL were formed. In critical regions (γ/α~1), only one BNL and less than one INL were observed. In reflective regions (γ/α > 1), less than one BNL was formed and no INL. The BNL in the critical regions should be the thickest where the sediment resuspension was the greatest. The concentrations and thicknesses of BNLs and INLs were related to the energy and amplitude of ISW and the sediment condition. The results in the transmissive regions were proved by field observation. Our research will help to predict the number and magnitude of transport channels formed by shoaling ISWs from the ocean margin to the ocean interior.

Interaction of a deep-sea current with a blind submarine canyon (Mar del Plata Canyon, Argentina)

The Mar del Plata (MdP) Canyon at the Argentine continental margin is incorporated into a major contourite depositional system, built by the incursion of southern-sourced water masses affecting the seafloor at different waters depths. The new sedimentological, morphological and hydro acoustic data provide novel insights into contour and turbidity current interactions in mid-slope (blind) canyons, which do not have a connection to the shelf or an onshore river system. Such canyons are capable to record climate-related ocean stratification changes, current variability, and slope stability. Three sediment cores were obtained along the MdP Canyon thalweg covering the last 20,000 years and compiled with two cores from the adjacent Ewing Terrace. Turbidity-current activity within the MdP Canyon was limited to the time interval from Last Glacial Maximum (LGM) to the late deglacial. During the LGM and early deglacial, turbidites reached both the proximal sector and the distal northern flank of the canyon. During the late deglacial only the proximal sector was characterized by turbidite deposition. Similarities in mineralogy and grain-size data indicate that the material transported by the turbidity currents originated from the mid-slope Ewing Terrace. Glacial turbidity-current activity was most probably favored by increased sediment supply along the Ewing Terrace from a shallowed and/or enhanced glacial Antarctic Intermediate Water (AAIW) nepheloid layer. These sediments were trapped by the MdP Canyon, in particular at the head area. During the late deglacial, a displacement or limited AAIW nepheloid layer resulted in less sediment transfer along the Ewing Terrace and immediate accumulation in the MdP Canyon head restricting turbidite deposition to the proximal sector of the canyon. In general, contourite-turbidite interactions provide valuable information on variations in thermohaline circulation such as AAIW distribution and current strength.

Three-dimensional coupling between size-fractionated chlorophyll-a, POC and physical processes in the Taiwan Strait in summer

Coastal water masses are inherently diverse, often with multiple water sources of different origins. Knowledge of characteristic salinity and temperature fingerprints can be very useful to differentiate between component water masses. For example, summertime river plumes carry relatively warm and less saline water into coastal waters, along with nutrients, clastic and organic sediments, and terrestrial-sourced biogenic particles. In an upwelling regime, relatively colder water carries marine-sourced nutrients and particles in water masses from deeper depths offshore. These two regimes are subject to biological-physical coupling that affects their size-differentiated particulate chlorophyll-a and organic carbon contents, and we show here how both regimes can be identified in these terms in the Taiwan Strait in summer. A close examination shows that microphytoplankton is the major contributor to chlorophyll-a concentrations in both regimes at all depths, and picophytoplankton is a minor contributor in the subsurface cold-water regime. The contribution from nanophytoplankton is relatively insignificant. Shipboard hydrographic profiling and water sampling was conducted over 28 h at a fixed location off the mouth of the Minjiang River in the northern Taiwan Strait, where the river plume and upwelling regimes overlap. Water samples were filtered onboard using a nested filtration system to separate suspended particles into >153, 63–153, 10–63, and 0.7–10 μm size classes. Our findings show that in the surface nepheloid layer, the river plume accounted for the physical process affecting large (>10 μm) terrestrial-sourced non-phytoplankton POC (organic debris), with an average C/N ratio of 92.2. The Upwelling regime accounted for the physical process affecting marine-sourced POC <10 μm containing fluorescence. In the intermediate nepheloid layer, which coincided with the subsurface chlorophyll maximum layer, most large (>10 μm) POC that contained fluorescence were either upwelling- or offshore-sourced, with an average C/N ratio of 8.5. In the benthic nepheloid layer, terrestrial- and marine-sourced POC containing fluorescence of all sizes were associated with river plume and upwelling regimes. Large (>63 μm) non-phytoplankton organic particles were also present, with an average C/N ratio of 10.2. Both spatial and temporal datasets point to strong three-dimensional coupling between biogenic particles and physical processes. The river plume and upwelling regimes further differentiate size-fractionated chlorophyll-a and POC.

Experimental investigation of sediment dynamics in response to breaking high-frequency internal solitary wave packets over a steep slope

Abstract Previous studies have found that internal solitary waves (ISWs) can erode, resuspend and transport seabed sediments and form intermediate nepheloid layers (INLs) and bottom nepheloid layers (BNLs). In our recent flume experiments, the function of the BNLs and the fate of suspended sediment were studied by breaking high-frequency ISW packets over a steep slope. We found that a considerable amount of sediment was resuspended down the slope with the vortex, and it reached across the entire water column at almost the same time due to the vertical ejection (pump) of flow at the head of the bore by the breaking ISW. The maximum height of the suspended sediment reached the pycnocline. The suspended sediment may be transported slightly up to the top of the slope via the BNL. The downslope transporting distance was very short and the suspended sediment did not reach the foot of the slope via the BNL. The suspended sediment was mainly redeposited around the middle of the slope, and some fine-grained sediment suspended offshore was spread along the pycnocline via the INL to greater distances. The changes in slope elevation might be influenced by successive scouring, the redeposition of sediment, and the migration of sand waves. Our study was performed under low Iribarren number conditions, which can facilitate the development of new field experiments designed to obtain direct measurements and insights on the interactions between ISWs and seabed sediments.

The organic complexation of iron in the Ross sea

The Ross Sea Polynya (RSP) has the highest primary production of Antarctic waters. Iron (Fe) is one of the most important growth limiting factors in the Southern Ocean. Dissolved iron (DFe)-binding organic ligands play an important ecological role because they increase the residence time of the scarce Fe. Therefore, we studied the DFe-binding organic ligands in the vicinity of the Ross Sea during a cruise between 20 December 2013 and 5 January 2014. The DFe-binding organic ligands were measured using Competing Ligand Exchange Cathodic Stripping Voltammetry (CLE-CSV) with TAC as competing ligand. The DFe-binding organic ligand concentrations always exceeded the DFe concentrations except in the bottom nepheloid layer of the RSP. No relationship was found between depth and DFe-binding organic ligand concentrations in the RSP indicating that these ligands are resistant to degradation and are probably exported by high salinity shelf water into the circumpolar current. DFe-binding organic ligand concentrations were highest in the RSP and the Antarctic Circumpolar Current (ACC) west of the Ross Sea, in association with seasonal phytoplankton blooms, although no correlation was found with parameters reflecting phytoplankton abundance or species. Phytoplankton sources and sinks of DFe-binding organic ligands are likely related to the seasonal progression of the bloom. In 39% of the samples, two DFe-binding organic ligand groups were distinguished based on the difference in binding strength. The distinction was especially clear in the RSP and in the ACC west of the RSP (54 and 77% of the samples, respectively) where blooms occurred and much less in the low biomass waters of the ACC east of the RSP and ice covered eastern part of the Ross Sea (15 and 10% of the samples, respectively). In these waters, other environmental factors, like sea ice melt, probably explain the absence of distinct relationships between primary production and ligand characteristics.

Evidence of trawling-induced resuspension events in the generation of nepheloid layers in the Foix submarine canyon (NW Mediterranean)

Abstract The temporal evolution of water column turbidity was studied on a submarine canyon on the Barcelona continental margin. From April to June 2014, an instrumented mooring array equipped with an autonomous hydrographic profiler with a CTD and a turbidimeter was deployed in the Foix canyon axis at 870 m depth. The instruments were programmed to collect hydrographic profiles once per day to provide a view of the temporal evolution of water column characteristics from 200 to 800 m water depth. The results illustrate a well-defined water turbidity structure of particulate matter distributed in intermediate nepheloid layers (INLs) developed between 300 and 500 m water depth and above the canyon rims, and INLs and near-bottom nepheloid layers (BNLs) confined inside the canyon between 650 and 800 m water depth. Data from fishing vessels activity at the time of the deployment was obtained from Vessel Monitoring System (VMS). The presence and location of the fishing vessels in the study area suggested a relationship between trawling activity and the generation of such layers. Nepheloid layers were absent during the first part of the deployment, when there was no fishing activity within the Foix Canyon axis or at the adjacent continental slope. Later, with the beginning of trawling activity on the fishing grounds close to the canyon, both INLs and BNLs were observed in the profiling casts, suggesting a causative relationship with fishing activities. Additionally, the hydrodynamic conditions within the canyon also seem to favour particle retention and to increase water turbidity in thick BNLs when water circulation is directed up-canyon. Bottom trawling appears to act as a main sediment resuspension mechanism in the Barcelona continental slope regions, increasing suspended sediment concentration at specific water depths where fishing grounds are located. Suspended particles are then advected and propagate along and across-margin by ambient currents via nepheloid layers.

Distribution of dissolved and leachable particulate Pb in the water column along the GEOTRACES section GA10 in the South Atlantic

We measured concentrations of dissolved and leachable particulate lead in the water column along a 40°S zonal transect in the South Atlantic Ocean, as part of the international GEOTRACES section GA10. Dissolved Pb (DPb) concentrations were highest in surface waters (8–29 pmol L−1) and lowest near the sea floor (<5 pmol L−1). Estimates for mineral sourced DPb for the South Atlantic surface ocean yielded a concentration of 5 ± 1 pmol L−1, suggesting that about 59% of the 12 ± 2 pmol L−1 (n = 85) DPb present in today's South Atlantic surface ocean has come from anthropogenic sources. Available values for wet deposition of 14 ± 12 nmol Pb m−2 d−1 (Chance et al., 2015) allow the calculation of an average residence time of 35 ± 31 days for DPb in the South Atlantic surface ocean. Elevated concentrations of LpPb, reaching 28 pmol L−1, were restricted to turbid bottom waters on the South American shelf and in the Argentine Basin, suggesting that strong turbulent mixing and resuspension of sediments was the main source of this Pb fraction. A negative linear regression between LpPb and DPb in these turbid waters indicated that the main removal mechanism for DPb in bottom waters on the shallow South American shelf and in nepheloid layers in the abyssal planes of the Argentine basin was the scavenging of DPb onto the surfaces of resuspended sediment particles.Elevated DPb concentrations in surface waters near the South African (23 ± 3 pmol L−1 (n = 22)), and South American (18 ± 3 pmol L−1 (n = 14)), continental margins coincided with the position and extension of the Agulhas (AC) and Brazil Current (BC). Elevated DPb concentrations of 12 ± 1 pmol L−1 (n = 30) at depth in the Argentine Basin (the western basin of the 40°S transect) coincided with the spreading of North Atlantic Deep Water (NADW) in the depth range of 1750 to 3000 m. Lowest DPb concentrations of 5 ± 1 (n = 26) and 6 ± 1 pmol L−1 (n = 22) corresponded with Weddell Sea Deep Water and Antarctic Bottom Water, located below NADW in the Argentine and Cape Basins (the eastern basin of the 40°S transect).

Processes controlling the distribution of dissolved Al and Ga along the U.S. GEOTRACES East Pacific Zonal Transect (GP16)

Gallium (Ga) behaves similarly to aluminum (Al) in the ocean. However, there are key differences in their behaviors resulting from the small differences in their chemistry. Here we present the distributions of dissolved Ga and Al in the Eastern Tropical Pacific Ocean, obtained as part of the 2013 U.S. GEOTRACES GP16 Eastern Pacific Zonal Transect (EPZT) from Peru to Tahiti. Low surface water dissolved Ga and Al, as well as high and variable surface water Ga/Al ratios along the EPZT reflect low aeolian input and a longer Ga residence time than Al. A gradient of decreasing surface water Ga toward the Peru upwelling region but not seen in surface water Al can be explained by additional dissolved Al inputs from shelf sediments, the presence of less reactive Al species or advection of low-Ga water from elsewhere.Beyond the coastal upwelling zone, the absence of Ga and Al removal associated with the chlorophyll a maximum suggests minimal removal of Al and Ga by biological scavenging in the oligotrophic gyre waters in the EPZT. West of 105°W, an expanding shallow pool (<200 m) of Ga-enriched water suggests Ga accumulation within the gyre circulation. However, we did not observe dissolved Al enrichment in this Ga-enriched pool. West of 90°W, a dissolved Ga minimum is centered within the nutricline, right below the Ga-enriched pool, and is likely a result of advective input of low-Ga water.Anomalously high dissolved Al water in the upper 200 m at the middle of section (106–112°W) was not an artifact of contamination, though the reasons for this remain unknown. Slightly elevated dissolved Al was observed near the Peru margin and at mid-depth in the western part of the section. Surprisingly, this mid-depth dissolved Al enrichment in the west is more prominent than that on the Peru margin. This may result from greater biological Al removal along the margin and/or different dust sources with differing Al solubility. A Ga depletion was commonly observed in the intermediate waters, especially near the Peru margin. This is opposite from the observation of slightly increased Al in these waters and still remains to be explained.Generally, elevated deep water Ga and Al concentrations were observed in this section, indicating inputs from hydrothermal activity and resuspended sediments. However, ∼930 km east of the East Pacific Rise, decreased bottom water Al, even in the nepheloid layer, might be due to low Al dissolution from these resuspended hydrothermal particles and/or the advection of low-Al Pacific Deep Water from the north. West of 143°W, decreased Ga and increased dissolved Al near the bottom indicate Antarctic Bottom Water influence. In the hydrothermal plumes, dissolved Ga and Al behavior is largely regulated by dilution of the hydrothermal signal. Hydrothermal Al flux estimates show hydrothermal inputs of Al are likely to be of regional, not global importance while the hydrothermal Ga flux might be comparable to the aeolian Ga flux.

Inputs and processes affecting the distribution of particulate iron in the North Atlantic along the GEOVIDE (GEOTRACES GA01) section

Abstract. The aim of the GEOVIDE cruise (May–June 2014, R/V Pourquoi Pas?) was to provide a better understanding of trace metal biogeochemical cycles in the North Atlantic Ocean. As marine particles play a key role in the global biogeochemical cycle of trace elements in the ocean, we discuss the distribution of particulate iron (PFe), in relation to the distribution of particulate aluminium (PAl), manganese (PMn), and phosphorus (PP). Overall, 32 full vertical profiles were collected for trace metal analyses, representing more than 500 samples. This resolution provides a solid basis for assessing concentration distributions, elemental ratios, size fractionation, and adsorptive scavenging processes in key areas of the thermohaline overturning circulation. Total particulate iron concentrations ranged from as low as 9 pmol L−1 in surface waters of the Labrador Sea to 304 nmol L−1 near the Iberian margin, while median PFe concentrations of 1.15 nmol L−1 were measured over the sub-euphotic ocean interior. Within the Iberian Abyssal Plain, the ratio of PFe to PAl was identical to the continental crust molar ratio (0.21 mol mol−1), indicating the important influence of crustal particles in the water column. Overall, the lithogenic component explained more than 87% of PFe variance along the section. Within the Irminger and Labrador basins, the formation of biogenic particles led to an increase in the PFe∕PAl ratio (up to 0.64 mol mol−1) compared to the continental crust ratio. Continental margins induce high concentrations of particulate trace elements within the surrounding water masses (up to 10 nmol L−1 of PFe). For example, horizontal advection of PFe was visible more than 250 km away from the Iberian margin. Additionally, several benthic nepheloid layers were observed more than 200 m above the seafloor along the transect, especially in the Icelandic, Irminger, and Labrador basins, suspending particles with high PFe content of up to 89 nmol L−1.

REE distribution and Nd isotope composition of estuarine waters and bulk sediment leachates tracing lithogenic inputs in eastern Canada

The rare earth element (REE) concentrations and the neodymium (Nd) and strontium (Sr) isotope compositions of the detrital fraction and authigenic FeMn oxyhydroxide coatings of marine sediments may provide valuable information for better understanding the pathways of weathering inputs and estuarine and coastal exchange processes on different time scales. Here, we present the REE concentrations and 143Nd/144Nd (expressed in epsilon units, εNd) and 87Sr/86Sr ratios of detrital and authigenic (leached FeMn oxyhydroxides) fractions from sediment core-top samples and of estuarine water samples collected in the Estuary and Gulf of St. Lawrence (EGSL) and continental shelf off southeastern Canada. The REE distribution patterns, εNd values, and 87Sr/86Sr isotopic values from the detrital fraction allow for the discrimination of sediment from continental sources in the EGSL. Sediments in the Baie des Chaleurs and on the continental shelf, which have εNd values ranging from −14.3 to −16, 87Sr/86Sr values ranging from 0.72708 to 0.71475, and low La/Yb and Gd/Yb ratios, are mainly supplied by the early Paleozoic Appalachian Mountains. In contrast, sediments in the Laurentian and Esquiman channels (εNd = −18.7 to −21.8, 87Sr/86Sr = 0.72068 to 0.72607, and high La/Yb and Gd/Yb ratios) come from the Grenvillian metamorphic rocks in the Canadian Shield, and surface sediments on the southern Labrador Shelf (εNd = −28.7, 87Sr/86Sr = 0.73062, and high La/Yb and Gd/Yb ratios) mainly originate from the Hudson Strait and Baffin Bay. The εNd values obtained from estuarine water samples and bulk sediment leachates are unradiogenic, with values ranging between −18.9 and − 21.7 and between −16.1 and − 27.2, respectively. Based on these results and the dissolved REE concentrations, we speculate that salt-induced coagulation of colloidal matter, dissolution of lithogenic sediments from the adjacent continents (notably from the erosion of the Grenville Province on the North Shore), bottom scavenging within the nepheloid layer, and brine rejection during sea ice formation significantly influence the distribution of REEs and the authigenic εNd signal throughout the water column in the EGSL. Overall, our results both underscore the fact that caution must be exercised when interpreting authigenic εNd records due to bottom water-mass mixing in estuarine and coastal marine environments and highlight the potential of REE and NdSr isotope compositions in investigating changes in sediment sources and transport pathways in the EGSL.

Vertical marine snow distribution in the stratified, hypersaline, and anoxic Orca Basin (Gulf of Mexico)

We present a complete description of the depth distribution of marine snow in Orca Basin (Gulf of Mexico), from sea surface through the pycnocline to within 10 m of the seafloor. Orca Basin is an intriguing location for studying marine snow because of its unique geological and hydrographic setting: the deepest ~200 m of the basin are filled with anoxic hypersaline brine. A typical deep ocean profile of marine snow distribution was observed from the sea surface to the pycnocline, namely a surface maximum in total particle number and midwater minimum. However, instead of a nepheloid (particle-rich) layer positioned near the seabed, the nepheloid layer in the Orca Basin was positioned atop the brine. Within the brine, the total particle volume increased by a factor of 2–3 while the total particle number decreased, indicating accumulation and aggregation of material in the brine. From these observations we infer increased residence time and retention of material within the brine, which agrees well with laboratory results showing a 2.2–3.5-fold reduction in settling speed of laboratory-generated marine snow below the seawater-brine interface. Similarly, dissolved organic carbon concentration in the brine correlated positively with measured colored dissolved organic matter (r2 = 0.92, n = 15), with both variables following total particle volume inversely through the pycnocline. These data indicate the release of dissolved organic carbon concomitant with loss in total particle volume and increase in particle numbers at the brine-seawater interface, highlighting the importance of the Orca Basin as a carbon sink.

Variations in the abundance and distribution of aggregates in the Ross Sea, Antarctica

The vertical distribution and temporal changes in aggregate abundance and sizes were measured in the Ross Sea, Antarctica between 2002 and 2005 to acquire a more complete understanding of the mechanisms and rates of carbon export from the euphotic layer. Aggregate abundance was determined by photographic techniques, and water column parameters (temperature, salinity, fluorescence, transmissometry) were assessed from CTD profiles. During the first three years the numbers of aggregates increased seasonally, being much more abundant within the upper 200 m in late summer than in early summer from 50 to 100 m (12.5 L–1 in early summer vs. 42.9 L–1 in late summer). In Year 4 aggregate numbers were substantially greater than in other years, and average aggregate abundance was maximal in early rather than late summer (177 vs. 84.5 L–1), which we attributed to the maximum biomass and aggregate formation being reached earlier than in other years. The contribution of aggregate particulate organic carbon to the total particulate carbon pool was estimated to be 20%. Ghost colonies, collapsed colonies of the haptophyte Phaeocystis antarctica, were observed during late summer in Year 4, with maximum numbers in the upper 100 m of ca. 40 L–1. Aggregate abundance, particulate organic carbon and ghost colonies all decreased exponentially with depth, and the rate of ghost colony disappearance suggested that their contribution to sedimentary input was small at the time of sampling. Bottom nepheloid layers were commonly observed in late summer in both transmissometer and aggregate data. Late summer nepheloid layers had fluorescent material within them, suggesting that the particles were likely generated during the same growing season. Longer studies encompassing the entire production season would be useful in further elucidating the role of these aggregates in the carbon cycle of these regions.

Patterns of iron and siderophore distributions across the California Current System

AbstractCoastal upwelling of nutrients and metals along eastern boundary currents fuels some of the most biologically productive marine ecosystems. Although iron is a main driver of productivity in many of these regions, iron cycling and acquisition by microbes remain poorly constrained, in part due to the unknown composition of organic ligands that keep bioavailable iron in solution. In this study, we investigated organic ligand composition in discrete water samples collected across the highly productive California Coastal upwelling system. Siderophores were observed in distinct nutrient regimes at concentrations ranging from 1 pM to 18 pM. Near the shallow continental shelf, ferrioxamine B was observed in recently upwelled, high chlorophyll surface waters while synechobactins were identified within nepheloid layers at 60–90 m depth. In offshore waters characterized by intermediate chlorophyll, iron, and nitrate concentrations, we found amphibactins and an unknown siderophore with a molecular formula of C33H58O8N5Fe. Highest concentrations were measured in the photic zone, however, amphibactins were also found in waters as deep as 1500 m. The distribution of siderophores provides evidence for microbial iron deficiency across a range of nutrient regimes and indicates siderophore production and acquisition is an important strategy for biological iron uptake in iron limited coastal systems. Polydisperse humic ligands were also detected throughout the water column and were particularly abundant near the benthic boundary. Our results highlight the fine‐scale spatial heterogeneity of metal ligand composition in an upwelling environment and elucidate distinct sources that include biological production and the degradation of organic matter in suboxic waters.

Microbial communities in the nepheloid layers and hypoxic zones of the Canary Current upwelling system.

Eastern boundary upwelling systems (EBUSs) are among the most productive marine environments in the world. The Canary Current upwelling system off the coast of Mauritania and Morocco is the second most productive of the four EBUS, where nutrient‐rich waters fuel perennial phytoplankton blooms, evident by high chlorophyll a concentrations off Cape Blanc, Mauritania. High primary production leads to eutrophic waters in the surface layers, whereas sinking phytoplankton debris and horizontally dispersed particles form nepheloid layers (NLs) and hypoxic waters at depth. We used Catalyzed Reporter Deposition Fluorescence In Situ Hybridization (CARD‐FISH) in combination with fatty acid (measured as methyl ester; FAME) profiles to investigate the bacterial and archaeal community composition along transects from neritic to pelagic waters within the “giant Cape Blanc filament” in two consecutive years (2010 and 2011), and to evaluate the usage of FAME data for microbial community studies. We also report the first fatty acid profile of Pelagibacterales strain HTCC7211 which was used as a reference profile for the SAR11 clade. Unexpectedly, the reference profile contained low concentrations of long chain fatty acids 18:1 cis11, 18:1 cis11 11methyl, and 19:0 cyclo11–12 fatty acids, the main compounds in other Alphaproteobacteria. Members of the free‐living SAR11 clade were found at increased relative abundance in the hypoxic waters in both years. In contrast, the depth profiles of Gammaproteobacteria (including Alteromonas and Pseudoalteromonas), Bacteroidetes, Roseobacter, and Synechococcus showed high abundances of these groups in layers where particle abundance was high, suggesting that particle attachment or association is an important mechanisms of dispersal for these groups. Collectively, our results highlight the influence of NLs, horizontal particle transport, and low oxygen on the structure and dispersal of microbial communities in upwelling systems.

Global comparison of benthic nepheloid layers based on 52 years of nephelometer and transmissometer measurements

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 to 1984 are compared with maps of those same properties compiled from 6392 global profiles measured by transmissometers from 1979 to 2016. Outputs from both instruments were converted to particulate matter concentration (PM). The purposes of this paper are to compare global differences and similarities in the location and intensity of BNLs measured with these two independent instruments over slightly overlapping decadal time periods, to 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 will help to 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 (South Atlantic), areas around South Africa tied to the Agulhas Current region, and somewhat random locations in the Antarctic Circumpolar Current of the Southern Ocean.

A global scavenging and circulation ocean model of thorium-230 and protactinium-231 with improved particle dynamics (NEMO–ProThorP 0.1)

Abstract. In this paper we set forth a 3-D ocean model of the radioactive trace isotopes 230Th and 231Pa. The interest arises from the fact that these isotopes are extensively used for investigating particle transport in the ocean and reconstructing past ocean circulation. The tracers are reversibly scavenged by biogenic and lithogenic particles. Our simulations of 230Th and 231Pa are based on the NEMO–PISCES ocean biogeochemistry general circulation model, which includes biogenic particles, namely small and big particulate organic carbon, calcium carbonate and biogenic silica. Small and big lithogenic particles from dust deposition are included in our model as well. Their distributions generally compare well with the small and big lithogenic particle concentrations from recent observations from the GEOTRACES programme, except for boundary nepheloid layers for which, as of today, there are no non-trivial prognostic models available on a global scale. Our simulations reproduce 230Th and 231Pa dissolved concentrations: they compare well with recent GEOTRACES observations in many parts of the ocean. Particulate 230Th and 231Pa concentrations are significantly improved compared to previous studies, but they are still too low because of missing particles from nepheloid layers. Our simulation reproduces the main characteristics of the 231Pa∕230Th ratio observed in the sediments and supports a moderate affinity of 231Pa to biogenic silica as suggested by recent observations relative to 230Th. Future model development may further improve understanding, especially when this will include a more complete representation of all particles, including different size classes, manganese hydroxides and nepheloid layers. This can be done based on our model as its source code is readily available.

Carbon cycling in the Sargasso Sea water column: Insights from lipid biomarkers in suspended particles

This study compares lipid biomarker and bulk constituents (organic carbon and δ13C, nitrogen and carbonate) in particles (30–4400 m depth) collected at the Oceanic Flux Program site in the northern Sargasso Sea off Bermuda during three periods of contrasting oceanographic conditions: following the spring bloom (April 2015), during the low productivity period (November 2015), and two weeks after the passage of the Category 3 Hurricane Nicole (October 2016). Lipids biomarkers were used to evaluate the contributions of phytoplankton, zooplankton and bacterial sources to the particulate organic carbon (POC), diagenetic transformations of suspended POC throughout the water column and within the nepheloid layer, and seasonal and non-seasonal temporal variability, including the impact of an extreme weather event.Depth profiles of lipid concentration and molecular composition showed seasonal and non-seasonal variability in particle composition throughout the water column. Higher lipid concentrations in April versus November 2015 reflected differences in plankton productivity, while relative abundances in diagnostic phytosterols, C18 polyunsaturated fatty acids and alkenones reflected differences in phytoplankton community structure. A rapid decrease in lipid biomarker concentrations below the euphotic zone was accompanied by a marked transition in lipid composition, e.g. increases in the relative percentages of cholesterol, C16-C18 fatty alcohols, odd- and branched- chain fatty acids. This reveals the intense remineralization of algal-derived carbon and fresh inputs of organic materials from zooplankton and microbial production within the upper mesopelagic zone. Particulate lipids within the bathypelagic zone reveal a close connectivity of deep particulate carbon composition with upper ocean properties, while compositional heterogeneity with depth reflects continuous particle turnover and de novo particle production throughout the deep water column. Additionally, depth trends in the within-class composition of fatty alcohols and bacterial fatty acids were observed, suggesting depth zonation of zooplankton and microbial community structure.Hurricane Nicole (October 2016) strongly impacted upper ocean physics and nutrient supply, resulting in a transient phytoplankton bloom/flux event that increased concentrations of fresh particulate phytodetritus (e.g., C18 polyunsaturated fatty acids) throughout the mesopelagic and upper bathypelagic zones. Concurrent increases in zooplankton and bacterial lipid biomarkers (e.g., 18:1ω9, odd- and branched-chain fatty acids, cholesterol) indicated that this transient pulse of labile carbon stimulated mesopelagic zooplankton and microbial activity, resulting in an enrichment of labile materials in the suspended particle pool throughout the deep ocean. Our study demonstrates that extreme weather events can have a major impact on carbon flux and particle cycling in the deep ocean, with pronounced implications for the ocean carbon pump and deep ocean ecosystems.

Transport of organic-walled dinoflagellate cysts in nepheloid layers off Cape Blanc (N-W Africa)

Many aspects of to the vertical and lateral transport of organic matter particles produced in the upper ocean are still not clear. This study documents the production, vertical and lateral displacement of the particulate organic matter fraction formed by dinoflagellate cysts in nepheloid layers present off Cape Blanc in November 2015. The position and extent of cyst production in the upper water column has been assessed by collecting the export rain of cysts with floating sediment traps. This has been compared to cyst associations in different intermediate nepheloid layers in the water column along two onshore-offshore transects.We document that highest cyst export production took place at the rim of a newly formed upwelling eddy/filament. We observed lateral transport up to 130 km off the shelf break into the deeper ocean in the bottom nepheloid layers (BNL) and in intermediate nepheloid layer (INL). The depth of the INL gradually increased from 600 m closest to the shelf break to 1300 m at the most offshore station. Transport in the BNL was observed in the more onshore part between 750 and 2300 m water depth. Cyst associations in both layers were comparable, indicating a common source. The large abundance of living cysts in these layers suggest that this source had a recent origin rather than being the result of resuspension of older sediment. The cyst associations of these samples differed from those of a second INL deeper in the water column as well as from samples from the more offshore part of the BNL. This indicates that the particles in this deeper INL and the offshore part of the BNL had a different origin. This origin could not be identified therefore no conclusions were drawn with respect to the extent to which these particles have been transported vertically or laterally.The cyst associations in the surface sediments differed from those of the nepholoid layers and the upper water column and did not reflect the effects of lateral transport as registered from the water column. The surface sediments contained a higher proportion of cysts that are highly resistant to aerobic degradation than the samples from the water column. This indicates that the former were modified predominantly by species-specific post depositional degradation. Selective degradation altering the cyst associations was not observed for the nepheloid layers.

Sponge-dominated offshore benthic ecosystems across South China in the aftermath of the end-Ordovician mass extinction

Offshore benthic communities were sparse after the end-Ordovician mass extinction. The recent discovery of diverse, abundant sponges in south-eastern China (Anhui and Zhejiang provinces) that flourished during the Hirnantian (Late Ordovician) post-extinction interval raises questions over the extent and ecological significance of the sponge community at that time. This paper dramatically expands the scale of the Hirnantian sponge fauna within South China, with seven new occurrences across 2000 km. These occurrences cross the Ordovician–Silurian boundary interval in the marginal and submerged platform areas of the Yangtze Sea in Jiangsu, Hubei, Hunan, Chongqing and Sichuan provinces. In condensed sections (platform-margin facies), the occurrences are normally represented by extensive spiculite layers, but several sites yield entire, exceptionally preserved body fossils; at Ganggangshan (near Nanjing, Jiangsu Province), such preservation is abundant. The new occurrences are combined with palaeogeographic and tectonic reconstructions to produce a model for the distribution and preservation of the sponge fauna. We argue that the exceptional preservation was due largely to nepheloid layers that were generated during rapid post-glacial transgression that flooded the weathered land surface of the uplifted Cathaysia Block. Nepheloid-layer collapse led to rapid deposition of suspended sediment within the deeper parts of the intra-plate, restricted basin and on adjacent platform margins and slopes. The suspended sediment influx also introduced abundant nutrients to the water column, as indicated by a significant δ13C excursion during the sponge-bearing mudstone interval at Ganggangshan. The combined sponge assemblage was dominant over large areas of the Hirnantian offshore sea floor across South China. We predict that similar occurrences with exceptional preservation will also be found in other continental blocks: abundant articulated sponges may be encountered particularly in restricted basin successions containing rapidly deposited graptolitic mudstones near the Ordovician–Silurian boundary.

Vertical Fluxes of Suspended Sedimentary Matter in Arctic Sedimentogenesis of Intracontinental Seas

Based on long-term studies of a small sea in the Arctic Ocean, new regularities in the sedimentary process under the conditions of Arctic sedimentogenesis were established. The quantitative transition of particulate sedimentary materials into the concentrated forms (bottom sediments) in the White Sea follows the linear dependence with a local maximum in the deep nepheloid layer. Marginal filters (Severnaya Dvina River, etc.), which are areas of ultrarapid sedimentation, were distinguished. The long-term data on the concentration of suspension and fluxes of particulate sedimentary material clearly indicate stable annual nepheloid layers, i.e., the distribution of particulate forms of sedimentary material (suspension) in the water column proceeds by new regularities, which may be distinguished ever more clearly.