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Abstract:Marine snow was implicated in the transport of oil to the seafloor during the Deepwater Horizon oil spill, but the exact processes remain controversial. In this study, we investigated the concentrations and distributions of the 16 USEPA priority polycyclic aromatic hydrocarbons (PAHs) in marine snow aggregates collected during a mesocosm experiment. Seawater only, oil in a water accommodated fraction (WAF), and Corexit-enhanced WAF (DCEWAF) were incubated for 16 d. Both WAF and DCEWAF aggregates were enriched in heavy molecular weight PAHs but depleted in naphthalene. DCEWAF aggregates had 2.6 times more total 16 PAHs than the WAF (20.5 vs. 7.8 µg/g). Aggregates in the WAF and DCEWAF incorporated 4.4% and 19.3%, respectively of the total PAHs in the mesocosm tanks. Our results revealed that marine snow sorbed and scavenged heavy molecular weight PAHs in the water column and the application of Corexit enhanced the incorporation of PAHs into the sinking aggregates.Keywords: polycyclic aromatic hydrocarbons (PAHs); marine snow; oil spill; mesocosm; Gulf of Mexico; water accommodated fraction (WAF)
After the DWH explosion, oil accumulated at the sea surface and in subsurface plumes. Prior research has documented observations of large marine snow near surface slicks from the spill as well as flaky, oily material coating coral reefs near the spill site.
To learn more, Passow used roller table experiments to investigate conditions that induce marine snow formation. She also examined the effects of different types of oil (Louisiana light crude, Macondo oil and bucket-collected spill oil), photochemical weathering and the presence of phytoplankton and dispersant on marine snow formation.
Because marine snow is constantly falling, there is a thick layer of it on the seafloor. Just like sediments, digging a few layers into this is representative of a large expanse of time. This is extremely useful evolutionarily, as it makes it easy to track developments of microorganisms. Further, unlike many microorganisms which can evolve relatively quickly, these organisms operate on longer time scales because nutrients are so sparse.
Arne Diercks, Kai Ziervogel, Ryan Sibert, Samantha B. Joye, Vernon Asper, Joseph P. Montoya; Vertical marine snow distribution in the stratified, hypersaline, and anoxic Orca Basin (Gulf of Mexico). Elementa: Science of the Anthropocene 1 January 2019; 7 10. doi:
Sinking particles rich in organic matter (i.e., marine snow) are major drivers of the biological pump that removes organic carbon from the surface ocean, sequestering atmospheric CO2 in the deep ocean (Ducklow et al., 2001). The settling speed of marine snow in the ocean is influenced by water column stratification (Prairie et al., 2013, 2015), as particle velocity depends on the excess density of the particle relative to that of the surrounding water mass. When particles reach a zone of increased density, settling speed decreases proportionally to the loss in excess density (Condie and Bormans, 1997; Kindler et al., 2010; Prairie et al., 2013, 2015; Prairie and White, 2017). The integrity of a particle then depends on the amount of time that it remains suspended at this depth. Residence time within the transition zone strongly influences remineralization, breakage, reaggregation and partial dissolution of marine snow particles, as they are exposed to numerous physical, chemical, and biological alterations that can reshape parts or all of the individual particles. The deep Orca Basin pycnocline serves as a particle trap: Van Cappellen et al. (1998) observed that the highest concentration of particulate Mn in Orca Basin occurred at 2,200 m, in the middle of the chemocline/pycnocline that reflects a broad seawater-to-brine transition zone.
Limited information is available about the distribution of marine snow within Orca Basin, most of which is based on unpublished video data from manned submersible dives (S Joye, pers. observation). Here, we provide the first profile of marine snow concentration from the sea surface to within 10 m of the seafloor in the northern Orca Basin obtained using an in situ marine snow camera. We highlight a detailed set of our data that focus on the water column across the pycnocline from 2,150 m to 2,275 m. Below this depth, salinity is generally uniform. This section of the water column is characterized by a rapid increase in particle concentration and salinity, a decrease in temperature and oxygen availability, and the associated physical and chemical transformations of organic and inorganic materials.
Within the transition zone, characteristics of the marine snow particles changed rapidly, from numerous small ones, to very high concentrations of particulate matter causing complete loss of image data (blackout), to large aggregates below the depths of image blackouts. Both camera strobes were working, as can be detected in the image data; however, no individual particles were visible in the images and the images recorded were illuminated only very faintly near the edges where the strobes were located. The comet-shaped marine snow aggregates below these blacked-out images measured centimeters in length and a few millimeters in width (Figures 3, S4 and S5). Additional distinct layers of large numbers of small marine snow particles were observed in the transition zone. The 2.5-cm thick transition zone in the settling cylinder resembles natural conditions in Orca Basin with respect to salinity conditions and, on a different scale, the observed cloudy transition zone in the image data (Figures S4 and S5) between 2,140 m and 2,251 m.
Laboratory aggregate settling speeds, using roller-tank-generated marine snow aggregates and filtered Orca Basin brine and seawater from the deep Gulf of Mexico in graduated cylinder settling experiments. DOI:
In Orca Basin, marine snow retained at the pycnocline consisted of up to 60% organic matter (Trefry et al., 1984; Van Cappellen et al., 1998). Wong et al. (1985) and Sheu (1983) reported that the settling organic matter was trapped at the pycnocline long enough to undergo a detectable degradation that was reflected by a strong release of biogenic iodine at the density interface, a component that responds strongly to the excitation wavelength of the ECO-FLNTU used during our camera deployment (Figure 3). Wong et al. (1985) suggested that the rapid decrease in the observed peak in total iodine concentration, from 8.1 μM in the transition zone at 2,266 m to 3.8 μM below 2,300 m, may have been caused by (1) an advective core of water with high dissolved iodine, (2) a horizontal diffusive flux of iodide from surrounding sediments on the slopes, or (3) an in situ preferential remineralization of biogenic particles that had accumulated along the pycnocline.
The pycnocline constitutes a temporary barrier for marine snow, as presented in our laboratory settling experiments and in the marine snow abundance profile. In this transition zone, marine snow particles exchange pore water (Ploug et al., 2002; Prairie et al., 2013; Prairie and White, 2017) and settle at a reduced speed, which is proportional to the loss in excess density of the particles relative to the increased density of the brine solution. The results of our laboratory settling column experiments show that settling particles slow significantly upon entering the transition zone where salinity is rising steeply, similar to observations for midwater density stratifications presented by Prairie et al. (2013, 2015). Some of the material arriving from above will settle quickly through the pycnocline into the brine, while other material will be subject to fluid shear stress along this interface and become entrained in the nepheloid layer situated on top of the pycnocline, visible in our camera and transmissometer data. We suggest that marine snow particles in this transition zone above the salt brine experience similar changes as those in benthic nepheloid layers, which include aggregation, disaggregation, grazing, dissolution, microbial diffusion (Ploug et al., 2008) and finally sedimentation into the brine or, as under normal oceanographic settings, onto the seafloor.
The delayed settling of marine snow aggregates in the transition zone can have significant consequences for local carbon cycling, as these aggregates often represent hot spots of bacterial activity (Smith et al., 1992; Kiørboe et al., 2002; Ziervogel et al., 2010; Prairie et al., 2015). The partial dissolution of organic matter within aggregates, and with it the formation of DOC, was attributed by Smith et al. (1992) to hydrolytic ectoenzymes exuded by the particle-associated bacteria. Leakage of the resulting organic solutes from sinking aggregates was suggested by Kiørboe and Jackson (2001) to contribute to water column bacterial production. Prairie et al. (2017) showed that bacteria and ectoenzymes originally associated with sinking marine snow detach from these aggregates at transition zones, remaining active in the layer long after the aggregates have settled out of these zones.
The mean of measured DOC concentrations in the brine, 241.0 μM ± 65.2 μM (n = 10), results in a total amount of 2.47 × 109 moles of DOC stored in the brine, providing a potential source of (refractory) DOC for the abiotic formation of marine snow aggregates as described by (Hansell et al., 2009); in fact, comet-shaped marine snow aggregates several centimeters in length, not seen higher in the water column, were observed just below the transition zone. Some of the DOC comes in with the salt (Joye et al., 2010), but Shah et al. (2013) have also suggested microbial conversion (enzymatic hydrolysis) of POC to DOC, as well as physical disaggregation of POC into DOC, based on stable C isotope budgets evaluated across the chemocline.
The data presented in this paper present the first continuous profile of marine snow distribution in the Orca Basin. Recorded concentrations of marine snow highlight the physical settings of the permanent halocline, with the presence of a nepheloid layer of small particle sizes above this interface and large aggregates below in the brine, containing a higher volume of aggregated material due to reduced settling speeds in the brine, as shown in the presented laboratory data on settling speed. We suggest that physical, chemical, and biological alteration of material arriving from the surface and through lateral advection into the lower water column and the transition zone lead to increased CDOM and DOC concentrations in the transition zone and in the brine. The transition zone between normal seawater and the brine is a very dynamic part of the water column, characterized by processes similar to those in the transition of water to sediment at the seafloor, but with vertical export into brine instead of sediment. 2b1af7f3a8