Mesoscale eddies and fronts in the ocean greatly impact lateral transport and in turn the trajectories of sinking particles. Such influence was explored for April and October 2012 in the Gulf of Mexico using numerical simulations performed with a regional model at 1-km horizontal resolution. Results are compared qualitatively to field samples from two sediment traps located at GC600 (27°22.5 N, 90°30.7 W) and AT357 (27°31.5 N, 89°42.6 W), 81 km apart. In April the traps collected a comparable amount of material, while in October the flux at GC600 greatly exceeded that at AT357. Through inverse calculations, several thousand particle trajectories were reconstructed multiple times from the ocean surface to the depth of the traps (approximately 1,000 m) using a range of sinking velocities, 20–100 m d–1. Taken together, model results and trap data indicate that cross-shore transport of riverine input induced by mesoscale eddies, and convergence and divergence processes at the scale of a few kilometers, significantly impact the trajectory of sinking particles. The large majority of modeled particles reach the bottom faster than would be expected by their sinking speeds alone. This finding is associated with submesoscale-induced horizontal convergence in the mixed layer that aggregates particles preferentially in downwelling regions, accelerating their descent. Furthermore, this study confirms that the cone of influence of vertical fluxes is highly variable in both space and time in the presence of an energetic eddy field, especially for particles with sinking velocity of 50 m d–1 or less. It also demonstrates that the variability of vertical fluxes in the Gulf of Mexico is highly complex and can be understood only by considering the mesoscale circulation and seasonal cycle of primary productivity, which in turn are linked to riverine inputs, wind forcing and the seasonal cycle of the mixed-layer depth.
Gravitational settling of particles from the euphotic zone to the deep ocean plays an important role in controlling the ocean’s carbon cycle (Armstrong et al., 2001; Son et al., 2009; Alonso-González et al., 2010; Kiko et al., 2017). Particles sinking rapidly enough to reach significant depths before being degraded or solubilized also contribute to the biological pump (Buesseler et al., 2007; Passow and Carlson, 2012). These particles consist of various organic and inorganic materials such as plankton, detritus, bacteria and minerals (Alldredge and Silver, 1988; Simon et al., 2002; Passow et al., 2012). They are commonly defined as composite particles with size greater than 0.5 mm and include predominately large fecal pellets and marine snow, which is very sticky and may collect oil, pollutants and other substances from the water column (Passow et al., 2012).
Deep fluxes of sinking particles are measured using time-series sediment traps, and fluxes are often interpreted as originating from a Gaussian-shaped catchment area, a few tens of kilometers large, located above the trap (Asper, 1987; Waniek et al., 2000). As pointed out by Siegel et al. (1990), however, horizontal advection and mesoscale circulations need to be accounted in the interpretation of sediment trap data. Through a numerical analysis of Lagrangian particles with different sizes and sinking velocities, Siegel et al. (1990) evaluated the variability in the catchment area as a function of sinking velocity and presence/absence of mesoscale eddies, and found that the catchment area may increase by several tens to hundreds of kilometers if mesoscale eddies are present. Those results were confirmed by observational (Neuer et al., 1997; Guidi and Stemmann, 2007) and numerical works (Siegel and Deuser, 1997; Waniek et al., 2000) and suggest that under certain conditions horizontal advective motions can transport sinking particles laterally from afar. In other words, the particles found in the ocean mixed-layer above a deep trap may not be represented in the material collected by the trap.
Several studies have shown that in and near the mixed layer both the horizontal distribution and vertical mixing of biological tracers, such as biomass, nutrients, and particulate organic carbon (POC), depend largely on mesoscale geostrophic eddies (Martin and Richards, 2001; Martin et al., 2003; McGillicuddy et al., 2007; Lévy et al., 2009; Chelton et al., 2011; Lehahn et al., 2011). The residence time of particulate matter in the surface ocean may indeed be of the same order of magnitude as that of mesoscale motions (several days to weeks; Mahadevan, 2016), providing enough time for biological processes such as uptake of nutrients and transformation of sinking or suspended particles to occur (Lévy et al., 2012a).
Variability at even smaller scales may also play a role (Lévy et al., 2012b; Zhong and Bracco, 2013; Mahadevan, 2016; Zhong et al., 2017). Submesoscale circulations impact lateral mixing on spatial scales from 100 m to 10 km. They are broadly characterized by a Richardson (Ri) and Rossby (Ro) number of order 1 and by vertical velocities comparable to those found in frontal systems (10–100 m d–1; Thomas and Ferrari, 2008). Ri, defined as g’∇ρ/ρ(∇u)2, where g’ is reduced gravity, ρ is density and u is the flow velocity, quantifies the relative role of buoyancy and flow shear in the flow dynamics; Ro, defined as u/Lf, where L is the length scale of the circulation considered and f the Coriolis parameter, estimates the relative role of inertial and Coriolis forces in the flow dynamics. Submesoscale circulations can also influence plankton dynamics, being on the timescale of hours to days comparable to that of phytoplankton growth. For example, it has been shown that submesoscale eddies have been shown to contribute to the onset of plankton blooms by restratifying the mixed layer in early spring (Mahadevan et al., 2012), submesoscale filaments around the eddy perimeter subduct large amounts of POC during spring blooms at high latitudes (Omand et al., 2015), and submesoscale fronts can converge effectively and quickly any floating material into ‘lines’ a few meters in width and up to several kilometers in length (Zhong et al., 2012; Mahadevan, 2016). Submesoscale fronts are particularly abundant in the northern Gulf of Mexico (GoM) where surface density gradients are fueled by the energetic mesoscale field and also by the abundant riverine input (Zhong et al., 2012; Luo et al., 2016; Barkan et al., 2017a).
Our quantitative understanding of the role of mesoscale and submesoscale dynamics in regulating the transport pathways of sinking particles in the GoM remains incomplete. In this work we focus on the northern part of the basin where the inventory of particles that sink from the euphotic layer to the bottom is mostly controlled by the large riverine input from the Mississippi-Atchafalaya River system (Lohrenz et al., 1997; Dagg and Breed, 2003; Giering et al., 2018), and by the presence of over 2,000 natural hydrocarbon seeps (MacDonald et al., 2002). Most of the nutrients that fuel biogeochemical interactions near the ocean surface, and in turn the production of particulate organic matter through photosynthesis and decomposition processes (Lohrenz et al., 1997; Dagg and Breed, 2003), have riverine origin. The Mississippi-Atchafalaya River system contributes 90% of the total nitrogen and 87% of the total phosphorous load to the Gulf (Dunn, 1996; Cardona et al., 2016). A smaller, but not negligible, contribution to the basin’s nutrient budget is also given by oils and gases from natural seeps that, rising to the surface, transport nutrients from deep, nutrient-rich waters upwards into the euphotic layer through an ‘added mass’ bubble effect (D’souza et al., 2016). The transport and spreading of these nutrients within the euphotic layer is then facilitated by the mesoscale (10–250 km scale) circulations. In the GoM they consist of the Loop Current (LC), its detached mesoscale anticyclonic (rotating in the clockwise direction) Loop Eddies or Rings that are surrounded by strong vorticity filaments of opposite sign (Zhong and Bracco, 2013; Zhong et al., 2017), and in a large number of smaller cyclonic and anticyclonic eddies (Luo et al., 2016). Biggs and Müller-Karger (1994), for example, showed that cyclonic and anticyclonic eddies shed by the LC can transport nearshore water seaward at least 100–200 km, while Del Castillo et al. (2001) and Gilbes et al. (2002) provided evidence of intrusion events of the Mississippi River plume offshore using in situ measurements of riverine materials (i.e., detrital colored organic matter and chlorophyll a [Chl a]) and ocean color data. Numerical studies further support a role for mesoscale structures in the cross-shelf transport of river-discharged water. For example, Toner et al. (2003) applied a Lagrangian analysis to satellite-derived Chl-a plumes to show that the inter-eddy advection transports organic material from the shelf to the ocean interior, and Morey et al. (2003b) noticed a remarkably high-Chl-a, low-salinity tongue extending from De Soto Canyon toward the southeast along the eastern edge of the LC by analyzing 14 years of model simulation. Overall, these works argue for linkages between the LC, the mesoscale eddies and the riverine freshwater input that contributes to the spatial and temporal variability of the formation of particulate matters. They focus, however, on the upper ocean, and little is known about the interactions between mesoscale advection and vertical transport of particulate matter through the whole water column.
Here we have used integrations from a submesoscale-permitting three-dimensional regional ocean model to test the hypothesis that mesoscale and submesoscale variability can at times significantly impact the catchment area of sediment traps in the northern GoM. We considered specifically two traps deployed 81 km apart, each at a depth greater than 1000 m in the northern GoM and 120 m above the bottom, as part of the ECOGIG (ECOsystem impacts of Oil & Gas Inputs to the Gulf; www.ecogig.org) project. Through the analysis of Lagrangian backtracked trajectories, we explored the temporal and spatial variability of the sinking particles, providing an interpretation of the variability observed in the trap data over two collection periods, in April and in October 2012. We focused on these periods because the traps collected comparable amounts of POC in April yet very different amounts in October, leading us to hypothesize that mesoscale circulation contributed to the differences between the periods.
Our goal was to understand if and how much the physical circulation and in particular Loop Current eddies affect the catchment area of sinking particles. As we did not aim at reproducing the observations in the most realistic way, we have not used a data-assimilative model. We also integrated trajectories of passive Lagrangian sinking particles with vertical velocities between 20 m d–1 and 100 m d–1 without accounting for aggregation or dissolution, imposing constant size-related sinking velocities throughout the water column.
Materials and Methods
Sediment traps, Chl-a data and riverine discharge
The traps considered in this work are at the Green Canyon lease block GC600 (27°22.5’ N, 90°30.7’ W, 1380 m deep; trap depth 1260 m), the largest natural hydrocarbon seep in the northern GoM (Roberts et al., 2010), and at a nearby non-seep reference site in the Atwater Valley lease block AT357 (27°31.5’ N, 89°42.6’ W, 1160 m deep; trap depth 1040 m). The seepage activity at GC600 has been linked to a localized increase in primary productivity and surface Chl-a concentrations near the ocean surface (D’souza et al., 2016).
Both traps collected settling particles in 11–28-day intervals from April 2012 onward. In this work we concentrated on two collection periods, 20–30 April and 2–12 October 2012. A detailed analysis of the sediment trap data from spring of 2012 to spring of 2016 at GC600 and for summer of 2015 for AT357, with an attempt to attribute their variability to riverine outflow, seasonality of primary productivity and presence/absence of natural seeps can be found in Giering et al. (2018). While we focused on the time series of POC, most flux compounds measured were significantly correlated, and in particular the total mass flux with POC, and the particulate organic nitrogen (PON) with POC.
Additionally, daily Chl-a data for 20–30 March and 1–15 September 2012 at 4-km resolution were downloaded from NASA’s Goddard Space Flight Center (http://oceancolor.gsfc.nasa.gov/). The chosen periods represent the time at which the particles collected in the traps where likely near the ocean surface. Sea surface height (SSH) anomaly products used for model validation are calculated based on AVISO altimetry data and were obtained from the Gulf of Mexico Coastal Ocean Observing System (GCOOS, http://gcoos.org/). Finally, data on freshwater discharge from the Mississippi-Atchafalaya system were obtained from the USGS (https://waterdata.usgs.gov/nwis/rt) at daily frequency and averaged to match the trap collection periods.
Model configuration, validation and particle deployment
We used three-dimensional velocity fields generated by a regional ocean model, the Regional Ocean Modeling System (ROMS) (Marchesiello et al., 2003), to backtrack the trajectory of sinking particles from a depth of about 1000 m, at the grid points where the two ECOGIG sediment traps were located (Figure 1), to the ocean surface. Thus we can identify the specific source areas of the particles reaching the traps.
ROMS is a three-dimensional, free-surface, hydrostatic ocean model that we used to simulate the circulation in the GoM with a resolution of 1 km in the horizontal to partially resolve submesoscale dynamics and 50 stretched, terrain-following layers in the vertical. Biharmonic horizontal diffusion and the Mellor-Yamada vertical mixing scheme with level 2.5 closure (Mellor and Yamada, 1982) were applied as mixing parameterizations. The model bathymetry is derived from the 2-min Gridded Global Relief Data (ETOPO2) topography (Sandwell and Smith, 1997) interpolated to the model grid and modified to reduce horizontal pressure gradient errors using the Sikiric et al. (2009) method, with maximum slope factor (rx0) of 0.25 and maximum hydrostatic inconsistency number (rx1) of 15. The model domain extends over the whole GoM, covering the region between about 98°–82° W and 22°–32° N (Figure 1). It has open boundaries to the east, north and south sides that were nudged to the 6-hourly Hybrid Coordinate Ocean Model – Navy Coupled Ocean Data Assimilation (HYCOM-NCODA) Analysis system (GOMI0.04/expt_31.0, http://www7320.nrlssc.navy.mil/hycomGOM). Six-hourly wind stresses and heat fluxes as well as daily precipitation from the European Centre for Medium-Range Weather Forecast ERA-interim reanalysis were used to force the model (Poli et al., 2010). More details about this configuration can be found in Bracco et al. (2018).
For each trap collection period (20–30 April and 2–12 October 2012) we performed a 65 day-long integration covering the two months prior to the collection, each initialized from the HYCOM-NCODA hindcast. The first two weeks in each run were discarded as adjustment/spin-up time. The seasonality and impact of the freshwater fluxes from the Mississippi-Atchafalaya River system on the sea surface salinity (SSS) fields were retained in the initial conditions of each run with no further river outflow implementation in the fall case, while nudging to the surface salinity field from the HYCOM-NCODA hindcast was applied at the end of the spin-up in the spring case. In early spring the riverine outflow increased significantly between 18 February 2012, when our simulation was initialized, and early-to-mid March, when particles collected in the trap were assumed to be at the surface. The subsequent nudging to HYCOM-NCODA guarantees proper simulation of the generation of submesoscale circulations fueled by the lateral density gradients induced by the riverine discharge propagating into the Gulf (Luo et al., 2016). In fall of 2012, on the other hand, the influx of river water was small in August and early September, and no further nudging was required for a proper representation of the salinity distribution in the region of interest. The modeled SSS compares very well to the HYCOM-NCODA hindcast in the first three weeks of both simulations, when the particles were near the surface, but is underestimated by the end of them, mostly over the shelves. In the region of interest of this work, i.e., offshore the northern shelf of the GoM, the differences between the spatially averaged SSS anomalies in the hindcast and in our ROMS runs remain less than 8% by the end of the third week.
Neglecting the momentum flux associated with the river outflow makes this configuration unsuitable for exploring the origin of particles collected by traps close to the Mississippi River mouth and directly influenced by the river ‘jet’ (Barkan et al., 2017b), as in the case of data from OC26 (88.36° W, 28.70° N; Giering et al., 2018), but provides realistic surface salinities and salinity gradients for locations 100 km or more from the river mouths, as in the case of GC600 and AT357.
ROMS uses initial and boundary conditions from a hindcast that assimilates all available satellite and in-situ observations, but is run without any data assimilation. The mesoscale circulation can therefore deviate from that of HYCOM and from observations, and can do so significantly over the Eulerian time scale of the flow, which was estimated to be about 50 days in the GoM (Cardona and Bracco, 2016). Figure 2 presents a qualitative validation of the modeled mesoscale circulation over two days during which some of the particles collected at the trap locations in the relevant time window were backtracked by ROMS to the ocean surface. It compares SSH maps from the model with those from multiple altimetry satellites compiled on a 0.25° × 0.25° grid by the Colorado Center for Astrodynamics Research. Although the model slightly overestimates the magnitude of SSH in both seasons, the spatial pattern and in particular the location of major eddies and of the LC are well represented. The overestimation may be due to the stress formulation of the wind forcing that does not account for eddy-current feedbacks (Renault et al., 2016). We note, however, that small mismatches between modeled and observed circulation may translate into large differences in particle transport, and that our results are intended to shed light on processes that can impact the cone of influence of sediment traps in the Gulf, without hindcasting or forecasting aspiration.
For the objective of this work, validating the modeled vertical transport would also be important. Such validation, however, is complicated by the paucity of observations of vertical velocity, w, in the ocean. A 16 month-long record of w was collected in the GoM at one location, through a mooring equipped with two upward-looking 75 kHz acoustic Doppler current profilers (ADCPs), between 2003 and 2004 (Rivas et al., 2008). An investigation of w variability, in a nearly identical model configuration to that adopted here and with a comparison to the ADCP data, is contained in Zhong et al. (2013), indicating that ROMS captures the observed w intensity and variability in association with the LC eddies.
Finally, we verified that the differences in topographic slopes at the two trap locations are small, at least in the model, and that lateral velocities at the depth of the traps (120 m above the bottom) are comparable (Figures S1 and S2) during both of the seasons considered. These verifications ensure that the lateral flux of particles into the traps is not strongly influenced by bathymetric features.
In the following we describe the evolution of 3698 Lagrangian tracers backtracked multiple times from the trap locations to the ocean surface off-line using hourly averaged three-dimensional velocity data generated by ROMS. The error in the tracer trajectories introduced by off-line tracking is negligible when using hourly flow fields at the resolution considered (Bracco et al., 2016). The tracers were uniformly deployed at about 100 m above seafloor (about 1250 m at GC600 and 1050 m at AT357) and their trajectories were integrated using LTRANS v.2b (Schlag and North, 2012). Most of the organic aggregates collected in sediment traps are commonly regarded as marine snow (Siegel et al., 1990). The sinking velocity of marine snow varies widely, depending on size, porosity and component particles, ranging between a few meters to several hundred meters per day (Asper, 1987; Alonso-González et al., 2010; Passow et al., 2012). While sinking through the water column, the flux of marine snow declines with depth, due to losses to remineralization and grazing (De La Rocha and Passow, 2007; Buesseler et al., 2008). Slowly sinking aggregates (sinking speed < 10 m d–1) have a very high chance of not arriving at the bottom. Large particles with rapid sinking velocities (e.g., > 150 m d–1) have a short residence time in the water column and are not impacted or very moderately impacted by lateral advection. We therefore neglected both very slow- and very fast-sinking particles and explored the 3-dimensional trajectories of passive particles with sinking velocities of 20 m d–1, 30 m d–1, 50 m d–1 and 100 m d–1 which were taken to be constant through the water column. For each case, we considered an ensemble of six (for 20 m d–1) or seven integrations performed by releasing particles once a day over a week.
Results and Discussion
Trap data in April and October 2012
In this work we focused on the April and October sediment trap collections to investigate the hypothesis that the mesoscale circulation may explain similarities and differences between these two periods. Figure 3 presents the POC concentrations measured by the two traps in 2012. The concentrations were similar in the first two months, diverged slightly from mid-June to early July, were comparable again in the second half of July and August, and presented large differences in fall and through the end of the year, with the largest discrepancies recorded in early October.
POC concentrations generally co-vary with Chl a and, in the GoM, with the input of organic material in the basin through riverine discharge. Although directly impacted by nutrient inputs through the riverine system, Chl-a concentrations also depend on other physical conditions. In the northwestern portion of the Gulf, where the traps were located, space-averaged Chl-a anomalies have been observed to increase after episodic wind events, which are frequent in winter and have a climatological seasonal cycle that generally follows that of the mixed-layer depth. As a result, Chl-a concentrations decrease from a winter maximum, generally in February to early March when the mixed layer is deep, to a minimum that can vary significantly from year to year but is most commonly recorded in late August and September, when the mixed-layer is shallowest (Muller-Karger et al., 2015). River discharge, on the other hand, is usually greater in spring and reaches its minimum in fall, with large interannual changes (Cardona et al., 2016).
For the purpose of this work we compared the time series of freshwater discharge from the Mississippi-Atchafalaya system with the POC time series from April 2012 to August 2014 and during February 2015 at AT375 and GC600, respectively (trap records were interrupted for a few months after these dates; Giering et al., 2018), as shown in Figure S3. Despite the small distance between the traps, the influence of the discharge was different, with POC concentrations at AT375 co-varying with the outflow (maximum correlation with the discharge two months prior to collection; r = 0.61, statistically significant at the 99% level), and no correlation evident at GC600.
Mesoscale and submesoscale circulations in March/April and September/October 2012
The mixing and stirring processes induced by mesoscale and submesoscale activities play a central role in controlling the dispersion of biological and physical tracers in the ocean. The mesoscale circulation in the GoM is dominated by the presence of the LC and the LC eddies. The LC enters the GoM basin through the Yucatan Channel and leaves through the Straits of Florida (Weisberg et al., 2000; Alvera-Azcárate et al., 2009). It influences the south part of the domain directly and is usually confined to the east of 86°W (Vukovich, 2007). The northern boundary of the LC penetrates northward to about 26.5–27.5°N latitude based on previous observational and numerical studies of the LC intrusion (Vukovich, 1988, 2007). At irregular intervals the LC then retreats southward after the detachment of a large LC eddy. On average, eddy shedding takes place every 11 months, but the variance characterizing the shedding time is very large (Sturges and Leben, 2000). These eddies, with diameters of several hundred kilometers, move westward across the GoM at speeds of about 5 km day–1 (Elliott, 1982; Vukovich, 2007) and finally decay against the continental margin or merge with eddies existing in the western GoM. For this work, we note that AT357 is located at the western boundary of the Mississippi Slope (Figure 1). This topographic feature is sufficiently large and tall to interfere with the propagation of the LC and its detached eddies, which can be found more often in the proximity of GC600 but rarely atop AT375.
The GoM mesoscale circulation patterns differed appreciably between the two periods considered in this work. Going back to March, two anticyclonic features, a large LC eddy west of 90°W and the LC extension reaching as far as 27°N, occupied the basin (Figure 2), but no large mesoscale structure was found directly above the traps. In September, on the other hand, the central portion of the basin was dominated by a large anticyclonic LC eddy centered at about 25.5°N, 90°W that resulted from the shedding of the LC extension seen in March and extended as far as GC600, while the LC extended only to the southeastern portion of the Gulf.
The mesoscale variability is reflected in the salinity maps, with the LC water of tropical Atlantic origin carrying a fresher signature than the surrounding water. Significant differences between March and September 2012 can be seen also in the horizontal distributions of SSS. Low-salinity waters were mostly trapped in the coastal region in March, but spread more widely offshore in September. This difference is caused by the seasonal variation of wind stress in the Gulf (Morey et al., 2003). The freshwater discharged from the Mississippi-Atchafalaya River system moved toward the west in spring, along the broad, shallow Texas-Louisiana shelf, and then southward, remaining more effectively confined along the Mexico coastline. In summer and fall, on the other hand, the freshwater masses moved predominately eastward, encroaching more easily on the LC and the numerous eddies that populated the offshore waters. Those mesoscale circulations then carried them offshore through mixing and stirring, and in the year of this study towards the location of the traps.
At scales of a few kilometers, mesoscale eddies smaller than the LC ones, both cyclonic and anticyclonic, and vorticity filaments can be identified in both months offshore the shelf in the vorticity field shown in Figure 4. Relative vorticity, defined as ζ = ∂v/∂x – ∂u/∂y, where u is velocity along the x (longitudinal) direction, and v is velocity along the y (meridional) axis, quantifies the spinning (clockwise if negative or anticlockwise if positive) of the water parcels. In Figure 4, relative vorticity is plotted normalized by the Coriolis parameter f, providing a quantification of the local Rossby number (Ro = |ζ|/f). Ro of ~1 or greater is indicative of ageostrophic motions; i.e., of circulations for which the balance between the pressure gradient force and the Coriolis force is not attained, as commonly found in the case of submesoscale dynamics. In between mesoscale structures, a large number of submesoscale circulations such as vorticity filaments and small vortices, on a spatial scale of 1–10 km and with a lifespan of days, can be seen, particularly in September. Submesoscale processes extract available potential energy from the mixed layer (Molemaker et al., 2005; McWilliams, 2008) through processes such as frontogenesis and mixed layer instabilities (Thomas and Ferrari, 2008; Luo et al., 2016; Barkan et al., 2017a). Submesoscale features are stronger and more numerous when more available potential energy can be extracted. In general, for open ocean conditions, these characteristics imply a seasonal cycle of submesoscale activity that follows that of the mixed layer depth (Capet et al., 2008; Mensa et al., 2013, Qiu et al., 2014; Callies et al., 2015). In the GoM, however, the submesoscale seasonality is further modulated by the riverine outflow. The freshwater fluxes enhance the submesoscale circulations by increasing lateral buoyancy gradients, yet suppress them by decreasing the mixed-layer depth (Molemaker et al., 2005; Thomas and Ferrari, 2008; McWilliams et al., 2009). The end result for the region offshore the shallow shelves is a slightly weaker submesoscale activity in winter than it would be in the absence of riverine forcing, minimal submesoscale activity recorded in spring, but stronger submesoscale dynamics and especially intense frontogenetic processes in summer and early fall, when the riverine outflow has reached offshore following the maximum discharge in late spring. Those submesoscale circulations further contribute to the lateral mixing of freshwater and to the offshore transport of the nitrogen of riverine origin, which fertilizes the otherwise nitrogen-limited offshore waters (Cardona et al., 2016), and are characterized by elevated vertical velocities (Zhong and Bracco, 2013; Zhong et al., 2017), with the potential to accelerate particle sinking. In 2012 the mixed-layer depth was shallower than the climatological average in March by about 4 m, and deeper than the climatological average in September by about 5 m. As a consequence, submesoscale circulations were even more abundant and intense in fall than in spring.
Submesoscale/mesoscale variability and catchment area of sinking particles
After describing the circulations that affect transport and mixing in the northern GoM in different seasons, we explore in detail the connectivity between upper layer circulation and flux at the depth of the sediment traps using Lagrangian tracers with vertical velocities of 20, 30, 50 and 100 m d–1, released from GC600 and AT357. Figure 4 identifies the location of the particles backtracked to the surface for one of the ensemble members performed and three sinking speeds (30, 50 and 100 m d–1; the distribution of particles for 20 m d–1 is qualitatively similar to that for 30 m d–1), superimposed on the corresponding vorticity field averaged over the interval when the particles reached the surface. Results indicate that the horizontal extent of sinking particles varied from tens to hundreds of kilometers. In April, a newly formed anticyclonic eddy propagated to the west, with the LC dominating the southeastern Gulf (Figure 2). The backtracked particles sinking at GC600 and AT357 show comparable horizontal dispersion independently of the sinking velocity when at the surface, with an east-west distribution above the seep for those ending at GC600 and a more north-south pattern for those reaching AT357 (e.g., Figure 4b). In October, however, particles reaching GC600 originated from a widely spread area if their sinking speed was 50 m d–1 or less; most of them can be traced to the periphery of the large anticyclonic LC eddy, with others spread along the continental slope to the northeast of the site. Particles as far as ~490 km apart reach the trap site at about the same time. Previous studies have reported large vertical velocities in or near the cyclonic cells of vorticity that surround anticyclonic eddies (Martin and Richards, 2001; McGillicuddy et al., 2007; Zhou et al., 2013). Such velocities are particularly strong and effective in presence of ageostrophic motions; i.e., when |ζ|/f is of order 1 (Zhong et al., 2017) as in the case of the September LC.
Tracers sinking at AT357, on the other hand, were confined to a much smaller region when at the surface, limited to a maximal distance of ~240 km with low but positive vorticity, and were not directly impacted by the LC eddy (Figure 4b, 4d and 4f). Videos of the sinking particles, uploaded as Videos S1–S4, provide a three-dimensional view of the pathways taken by the particles in one ensemble member for most of the sinking speeds considered.
The impacts of riverine inputs on fluxes at depths
As typical for regions influenced by riverine inflows, the spatial and temporal variability of POC in the GoM is strongly affected by the discharge from Mississippi-Atchafalaya River system. As riverine waters are usually less abundant and more confined along the shelf in winter and early spring, they reach their maximum in April or May and spread more effectively offshore in summer and early fall (Morey et al., 2003a), aided by the winds and by the mesoscale eddies and the LC. Chl-a patterns averaged over the periods 20–30 March 2012 and 1–15 September 2012 (Figure 5) exemplify in the observations as well as in the model the role of cross-shore transport of nutrient-rich riverine water in the presence of a strong mesoscale field. In March and early April, the ‘background’ primary productivity (blue/light blue range in Figure 5) at both sites is on average two to three times higher than in September (see Figure 2 in Giering et al., 2018) following the February winter bloom that characterizes the GoM due to frequent wind-mixing events that bring nutrients into the euphotic layer (Muller-Karger et al., 2015). Additionally, particles that sink to the traps at AT357 and GC600 occupy at the surface regions that are comparable in terms of size and (relatively high) Chl-a concentrations.
In contrast, in the first two weeks of September, a southeastward low-salinity extension wraps around the LC eddy, as captured in both the Chl-a (Figure 5) and SSS maps (Figure 3). Abundant nutrients as well as organic matter carried by the intrusion of riverine water were likely to cover the area from which some of the particles that sank to GC600 may have originated. In the model runs, the remaining particles sinking into this trap came directly from the shelf, also a nutrient-rich area in the real ocean, or from other portions of the LC eddy circulation cell, which are characterized by strong vertical velocities. The coastal productive waters, the freshwater plume, or strongly ageostrophic vorticity, however, do not intersect the region where the particles that were collected at the model grid point of AT357 originated.
Figure 6 quantifies the riverine contributions for particles collected by the traps in April and October through the averaged probability density function (PDF) of the salinity at their surface positions in March and September for the seven ensemble members at sinking speeds of 30 m d–1 and 50 m d–1. The September–October histogram clearly displays a low-salinity tail (<34.5) only for the particles that end at GC600. In March–April PDFs are nearly identical for the two traps.
Lateral advection and sinking time
The cone of influence of sinking particles in the GoM amidst its mesoscale circulation was further evaluated through the PDFs of the lateral displacement of the point of origin of the particles at the surface with respect to the trap location (Figure S4). The cone of influence for fast-sinking particles (e.g., 100 m d–1) is observed to peak at about 70 km and increase to 200 km for particles sinking with speed around 20 m d–1. Some of the slowest particles, however, could be backtracked to regions as far as ~350 km. The impact of the LC eddies is especially evident at GC600 in September–October, where the distribution was centered around 300 km. Figure 7 conceptually shows these origins and pathways of simulated particles from the ocean surface to the depth of the traps in the presence of the LC and LC eddies in spring and fall 2012.
In terms of the time required for particles to reach the traps from the surface, the lower the sinking speed, the broader the distributions are, independently of the season (Figure 8). Slowly sinking particles can be stirred laterally more efficiently, especially in the upper 100–150 m of the water column, where mesoscale and submesoscale features are most intense, and can sample different circulation features. Noticeably in all cases particles have a large probability of reaching the bottom faster than would be expected by their sinking speed alone following a straight line. Submesoscale-induced horizontal convergence near the surface and in the mixed layer contributes to the aggregation of particles in regions of strong horizontal convergence (i.e., submesoscale circulations) that, on average, are characterized predominantly by downwelling velocities in the upper portion of the water column, accelerating their descent. Figure 9 quantifies this effect by displaying the distribution of the fluid vertical velocities sampled by the particles for all tracers with sinking speed of 30 m d–1 through their descent in the upper 200 m of the water column during the period of September–October. Downward (positive) velocities dominate the distributions independently of the site. Greater values are found for particles collected at GC600 that were concentrated along the convergent side of the circulation cell around the LC eddy (Zhong et al., 2012; 2017). The initial aggregation and sinking in one ensemble member for most cases is also clearly visualized in the videos uploaded as Videos S1–S4.
Summary and conclusions
The horizontal and vertical transport of sinking particles in the northern GoM was investigated using regional ocean model simulations at 1-km resolution. Our numerical experiments were performed to help interpret sediment trap data collected at two adjacent, offshore locations in the northern Gulf. POC fluxes at the two trap sites varied appreciably temporally and spatially, suggesting that the sources of POC were highly variable. One trap, at AT357, was slightly shallower and less prone to be directly impacted by LC eddies due to its proximity to the Mississippi Slope. The POC concentrations in this trap appear to be influenced at the first order by the riverine discharge, with a significant correlation between the time series. The other trap, at GC600, is more often near the path of an LC eddy and does not co-vary with the riverine discharge.
The backtracking analysis of about 28,000 trajectories of particles sinking at four different velocities was performed to investigate the source of deep fluxes at the two trap sites at the end of April and beginning of October 2012. Our results suggest that in the GoM the variability of particle fluxes at depth is controlled by mesoscale eddies and the convergence induced by submesoscale circulations, as well as the seasonal cycling of primary productivity, riverine forcing and winds. In the northern Gulf primary productivity is generally highest in February, substantial through spring and lowest in summer and early fall. Riverine forcing is usually greatest in April–May and lowest in late fall, but highly variable on interannual scales. The winds, jointly with the mesoscale structures, determine the cross-shore transport of freshwater away from the shelf, which is commonly maximal in summer and early fall, and reduced in winter and early spring. In March–April, when the LC and a shed eddy dominated the GoM circulation, our model results suggest that particulates collected near the bottom at GC600 and AT357 originated from a cyclonic area of about 360 km between the two anticyclonic structures. In October the particles collected at GC600 originated from the shelf and from the cyclonic circulation cell around the LC eddy boundary that had entrained water from the Atchafalaya River. Both areas were characterized by low salinity, indicative of riverine water and high nutrient and particulate concentrations, in agreement with the high POC fluxes observed. By contrast, the particles backtracked from AT357 originated more locally, in an area that lacked strong vertical velocities and river influence, and that was far away from the LC eddy.
Our modeling experiment revealed that the combination of processes induced by mesoscale and submesoscale circulations and cross-shore transport of riverine inputs can result in POC fluxes in offshore waters that are highly variable in both space and time as quantified through probability density functions. Mesoscale circulations control size and shape of the catchment area, as well as the riverine water content and therefore productivity (nutrient influx). Submesoscale circulations, on the other hand, influence the overall time required for the marine snow to reach the ocean bottom, especially when particle sinking velocities are 50 m d–1 or less by converging particles within the mixed layer in regions where downward velocities are predominant and accelerating their descent. If the cone of influence for rapidly sinking particles (with sinking velocities of 100 m d–1 or higher) is generally limited to an area of a few tens of kilometers nearby the trap locations (Guidi and Stemmann, 2007), the cone of influence for particles with sinking speeds of 50 m d–1 or less can extend to several hundred kilometers depending on the details of the mesoscale circulation, in agreement with previous studies (Siegel et al., 1990; Siegel and Deuser, 1997; Waniek et al., 2000).
Another potential source of POC in the GoM, not considered in this work, is provided by the large number of natural hydrocarbon seeps across the entire northern GoM (MacDonald et al., 2002). Yearly averaged concentrations of POC are indeed higher at GC600, a well-known location of natural seeps, than at AT357, possibly due to physical and biological processes in response to the released hydrocarbons (e.g., bubble-induced upwelling flow and utilization of carbon and nutrients by plankton (D’souza et al., 2016).
Data Accessibility Statements
Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at https://data.gulfresearchinitiative.org (DOI: https://doi.org/10.7266/N7H993KF, DOI: https://doi.org/10.7266/N7PN93PS, DOI: https://doi.org/10.7266/N7JW8BXM, DOI: https://doi.org/10.7266/N7KP80MP, DOI: https://doi.org/10.7266/N7NP22XW). This is ECOGIG contribution 507.
We thank two anonymous reviewers for their insightful comments that helped to improve the clarity of this paper.
The research was made possible by a grant from The Gulf of Mexico Research Initiative through the ECOGIG consortium.
The authors have no competing interests to declare.
Contributed to conception and design: GL, AB, UT
Contributed to numerical simulations: GL
Contributed to acquisition of field data: UT
Contributed to analysis and interpretation of data: GL, AB
Drafted and/or revised the article: GL, AB, UT
Approved the submitted version for publication: GL, AB, UT