Project Overview

Summary:

Research Highlights

Resuspension (Flume)

High resolution bathymetry from BOEM was used to model the bottom gravity driven flow along seafloor (Figure 1). With the help of these modelled data, we established site characteristics for coring locations, bathymetric cross sections, downslope and cross-slope profiles with statistical analyses of slope angle and direction. Preliminary data from flume analyses suggest that size specific particle resuspension varies at identical flow speeds. Different amounts of particles in individual size classes were eroded, indicating different material composition, and depositional and erosional environments on the seafloor.

First results were discussed during the 2020 GoMOSES and the 2020 Ocean Sciences Conference in San Diego. These results show a clear distinction in resuspension dynamics of different depositional regimes on the seafloor, with larger and easier to resuspend particles in lee depo centers and smaller particles being resuspended at higher current flow in streambeds, where less larger particles are present potentially due to removal by bottom currents. Sedimentary environment

Figure 1: Watershed analyses (colored lines) of sampling area used for site selection. Yellow line indicates cruise track and black dots indicate individual sampling locations.

likely plays a large role in the grains present in our erosional experiment. As represented in the graphs in Figure 2, we do recognize some differences in erosional characteristics between sites. A publication of these results is prepared to be submitted in the second quarter of 2020.

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Figure 2: Graphs of average particle counts and average total volume [cm ] vs. bed shear stress [Pa] for coring locations MC08 and MC19

 
MC08 (left panel) shows a small presence of particles around 0.075 [Pa] as well as an increase in volume around 0.15 [Pa] including the presence of the largest particle class. MC19 has erosion occur at lower bed shear stress (0.025 [Pa]) and shows a more gradual increase in eroded volume until it peaks at 0.275 [Pa]. >1.8 mm particles are more prevalent throughout the entire experiment for MC19 than for MC08, showing that either more large particles are present at MC19 or that these particles are more resistant to erosion at MC08. More analyses are planned with insights from other data acquired during this project.


Sedimentology and Geochronology (Pb and Th Isotopes)

There is a mixture of 210Pb profile shapes with exponential profiles indicating sites with relatively consistent sediment accumulation (Figure 2). Profiles that are not exponential have “plateaus” indicative of increased sediment accumulation, and likely mass emplacement (Figure 3). This is likely an indicator of mass sediment accumulation associated with down-slope transport, and is corroborated by sedimentologic indicators (sedimentary structures and elemental composition by XRF).

Sites showing consistent sediment accumulation rates are all located in the N, NW, and W side of the study area closer to the Mississippi River. All sites in the SE portion of the study area have indications of mass sediment accumulation likely due to down-slope transport.

Core scanning was performed on two gravity cores (1-2m in length) and three multicores for photography, color line scan, x-radiography, magnetic susceptibility, and XRF (elemental composition). Due to the high quality of the sediment records each analysis was performed at the highest resolution possible. XRF scans indicate variability in sediment composition throughout the record.

Sediment Texture and Composition analyses are performed after 210Pb and 234Th radioisotope analyses and are still in progress. Results show that sediments are input from three source areas by a variety of transport/depositional processes. Siliciclastic muds, found in cm-scale layers, likely represent input from the Mississippi River-dominated deposits to the NW (including the initial DwH impact zone). Sedimentary structures of these layers indicate low density turbidity currents and slumps/slides are likely the dominant mass transport processes. Siliciclastic mud layers are essentially devoid of sand-sized carbonate grains (e.g., planktonic foraminifera) supporting gravity flow input. Carbonate-rich sandy muds, also found in cm-scale layers alternate with siliciclastic muds. They are coarser grained and represent hemipelagic deposition consisting of a combination of gravity flow input from the adjacent west Florida carbonate platform, and pelagic input (planktonic foraminifera rich) from overlying surface waters. The alternating siliciclastic-rich muds and carbonate-rich sandy muds, are separated by thin, mm-scale, planktonic foraminifera ooze, reflecting relatively slow, pelagic deposition in between the much faster, but episodic, gravity flow deposition.

Oscillation between episodic gravity flow input of siliciclastic and carbonate sediments, punctuated by pelagic deposition of planktonic ooze, is supported by 210Pb profiles, mass accumulation rates, elemental composition, sedimentary structures, and the presence/absence of fractured benthic foraminifera (Patrick Schwing, collaborator). Gravity flow input/deposition of both siliciclastic-rich sediments from the NNW and carbonate-rich sediments from the N (eastern DeSoto Canyon region) includes the initial DwH seafloor impact zone (MOSSFA) as the potential source area.

All data will continue to be integrated to provide a more comprehensive assessment of the spatial and temporal variability in sediment accumulation, with a focus on indicators and records of down-slope sedimentation events.

Organic geochemistry analyses of sediment samples:

The general patterns in the organics seems to be matching the sedimentology and 14C patterns. The oldest OM, more depleted 14C data, are to the SE of the spill site, which could mean:

1. DWH residues can be seen better at this point using molecular tools, and  
2. Downslope redistribution of sediments enables the remineralization of organic material over and over with every resuspension event (most depleted 14C with a signature of microbial degradation of old sediments). It seems resuspension events are small (moving only about the 1-1.5 cm top layer).
3. Which goes along with the flume erosion/resuspension analyses that the initial resuspension only moves the very fluffy layer atop of the cores before going into a general steady state erosion of the material below the first 5- 10mm of the core.

A large variability was observed among the sites analyzed indicating the specific location of sites (e.g., seafloor morphology, depth, distance to DWH site) may be important for the accumulation of organic compounds on the seafloor. Most of the sites located on the north side of the study area (closer to the Mississippi River and DWH site) show oil-residues with biomarker source ratios pointing out the DWH spill as the primary source at specific sediment layers (consisting mostly of a 2-6 mm thick layer, except the core at DWH site with a 20 mm thick layer). Most sites in the SE portion of the study area have organic compound ratios (e.g., low molecular weight alkanes with even predominance) resulting from long-term microbial degradation of organic matter. Aerobic degradation may be the main weathering process, as is only observed on the surface of these cores.

Using 210Pb chronology, chemical profiles for each site were divided in three time periods (Figure 4) to separate the sediment pulse event observed in 2010-2013 as a result of the DWH spill from pre-spill (2006-2009) and post-spill periods (2014- 2018).

Figure 5: Inputs of hydrocarbons to sediments (shown as biomarkers concentrations – hopanes, steranes, triaromatic hydrocarbons) for three periods determined by 210Pb chronology.

In all periods, most of the sites located on the north side of the study area (closer to the Mississippi River and DWH site; Figure 4) show oil-residues at specific sediment layers (consisting mostly of a 2-6 mm thick layer, except the core at DWH site with a 20 mm thick layer). In contrast, most sites in the SE portion of the study area have organic compound ratios (e.g., low molecular weight alkanes with even predominance) resulting from long-term microbial degradation of organic matter. Aerobic degradation may be the main weathering process, as is only observed on the surface of these cores.

Comparison of the three time periods in Figure 5, indicate an increase over time in the deposition of hydrocarbons to deep-sea sediments (>1500 m) in the northern Gulf of Mexico.

 

Spatial and temporal variability of hydrocarbon sources to deep-sea sediments were observed in the study area (Figure 6). Predominant terrigenous sources are found in sites with low levels of biomarker inputs (200 ng/cm3) contain mostly petrogenic sources. Hydrocarbon source ratios indicated that sites with high biomarker inputs contain weathered oil-residues similar to DWH spilled oil (Figure 7).

Figure 7: Biplots of diagnostic ratios of hydrocarbon sources n sediments. Sites with oil-residues (denoted as red circles) are mostly located on the north side of the study area, while sites with larger natural sources (denoted as blue circles) are found in the SE portion of the study area.

The spatial and temporal trends observed support previous studies indicating a large scale deposition of oil-contaminated marine snow in the northern GoM as a results from the DWH spill (period 2010-2013). In addition, the inputs of oil-residues to in 2014-2018 with similar biomarker source ratios to DWH oil at sites located up to ~38 km SE of the DWH wellhead indicate downslope redistribution of oil-residues in the deep-sea area studied.


Bottom line:

We developed a spatial and temporal perspective of oil-residues deposited on the seafloor using sediment cores collected at >1500 m depth in the northern Gulf of Mexico. We found evidence of redistribution of organic matter moving DWH oil residues downslope in the last years (~38 km SE of the DWH wellhead). Results indicate a larger impacted area by the DWH spill, further away from previous records. The redistribution of oil-residues at depth indicate a possible petrogenic source to deep waters during resuspension events.

Figure 8.

Surface sediment varied along this trend form -150‰ to -300‰. Rogers et al., (2019) (given below) reported that within years, oil-contaminated sediments with 14C depleted values recovered towards less 14C depleted values as the oil was mineralized on a time scale of years. The locations where we observed more modern radiocarbon values suggest more recent input of sediment and higher sedimentation rates, and these locations are where Isabel Romero’s hydrocarbon tracer studies have indicated DWH residues at low concentrations. The values of δ13C in the multicores all indicated that deposition of Figure 8: Rapid change in ?13C and %C was observed in many of the multicores, indicating rapid mineralization in this layer and possible ease of resuspension marine carbon dominated the upper 50 cm of the seafloor. Values varied between -22.5 to -20‰. Percent carbon values varied from 2.5% at the surface to about 1% below 2 cm depth. An interesting pattern of decreasing carbon in the upper 2 cm, and increasing δ13C suggest rapid mineralization in this surface layer, possibly due to its occasional suspension (Figure 8). It will be interesting to correlate the change in this layer with the resuspension measurements. The %N, and 15N values followed similar trends.

Gravity cores, which captured the upper 2m of sediment revealed that the upper 50 cm of sediment are a Holocene sediment drape overlying older Pleistocene sediments with an age of 25,000-30,000 years.

The sedimentary organic carbon is noticeably not radiocarbon dead (Figure 9). It appears that at a minimum, a 1.5-meter-thick sequence of gravity flow deposited terrestrial material of Pleistocene age is widely distributed across the Eastern Gulf, below a less than one-meter-thick Holocene aged sediment drape. The estimate of 1.5-meter thickness is a minimum estimate, for our cores did not reach the bottom of the sequence, nor did they obtain radiocarbon –dead material > 50,000 years. The radiocarbon age of this Pleistocene layer did not decrease with depth, it was flat. The δ13C values of the gravity core samples transition from -20‰ near the surface to - 26‰ to -28‰ varying from 20-60 cm. Coincident with this transition are increases in the organic matter C/N ratio and changes in δ15N value all consistent with a thick layer of terrestrial material deposited in 1,800-3,300 m water depth on the seafloor during deglaciation. To our knowledge, only two other studies have reported only two other locations where this has been observed in the Gulf of Mexico. In addition, the Holocene drape is notably thinner where cores were obtained in channels on the seafloor, indicating erosional activity in those channels.

C-14 0.0 to 1.0 cm Average

Figure 9: Surface 14C values in ‰. More positive values, indicated by cooler colors indicate more recent sediment inputs. These locations are where DWH hydrocarbon deposition was indicated in tracer studies.

List of Publications:

Chanton, J. P., Jaggi, A., Radovi?, J. R., Rosenheim, B., Walker, B. D., et al. (2019). Mapping isotopic and dissolved organic matter baselines in waters and sediments of Gulf of Mexico. In S. A. Murawski, C. H. Ainsworth, S. A. Gilbert, D. J. Hollander, C. B. Paris, M. Schlüter, et al. (Eds.), Scenarios and Responses to Future Deep Oil Spills: Fighting the Next War. Springer International.

Diercks, A., Macelloni, L., D’Emidio, M. et al. (2019) High-resolution seismo- acoustic characterization of Green Canyon 600, a perennial hydrocarbon seep in Gulf of Mexico deep water. Mar Geophys Res 40, 357–370. https://doi.org/10.1007/s11001-018-9374-3

Diercks, A., Ziervogel, K., Sibert, R., Joye, S. B., Asper, V., & Montoya, J. P. (2019). Vertical marine snow distribution in the stratified, hypersaline, and anoxic Orca Basin (Gulf of Mexico). Elem Sci Anth, 7(1), 10.

Larson, R. A., Brooks, G. R., Schwing, P. T., Diercks, A. R., Holmes, C. W., Chanton, J., et al. (2019). Characterization of the sedimentation associated with the Deepwater Horizon blowout: depositional pulse, initial response, and stabilization. In S. A. Murawski, C. H. Ainsworth, S. A. Gilbert, D. J. Hollander, C. B. Paris, M. Schlüter, et al. (Eds.), Deep Oil Spills – Facts, Fate, and Effects. Springer International.

Perlin, N., Paris, C. B., Berenshtein, I., Vaz, A. C., Faillettaz, R., Aman, Z. M., et al. (2019). Far-Field Modeling of Deep-Sea Blowout: Sensitivity Studies of Initial Conditions, Biodegradation, Sedimentation and SSDI on surface slicks and oil plume Concentrations. In S. A. Murawski, C. H. Ainsworth, S. A. Gilbert, D. J. Hollander, C. B. Paris, M. Schlüter, et al. (Eds.), Deep Oil Spills – Facts, Fate, and Effects. Springer International.

Radovi?, J., Romero, I. C., Oldenburg, T. B. P., Larter, S., & Tunnell, J. W. (2019). 40 years of weathering of coastal oil residues in the Southern Gulf of Mexico. In S. A. Murawski, C. H. Ainsworth, S. A. Gilbert, D. J. Hollander, C. B. Paris, M. Schlüter, et al. (Eds.), Deep Oil Spills – Facts, Fate, and Effects. Springer International.

Rogers, K. L., Bosman, S. H., Lardie-Gaylord, M., McNichol, A., Rosenheim, B. E., Montoya, J. P., et al. (2019). Petrocarbon evolution: Ramped pyrolysis/oxidation and isotopic studies of contaminated oil sediments from the Deepwater Horizon oil spill in the Gulf of Mexico. PLoS ONE, 14(2), e0212433.

Romero, I. G., Chanton, J., Rosenheim, B., Radovi?, J. R., Schwing, P., Hollander, D., et al. (2019). Long-term preservation of oil spill events in sediments: the case for the Deepwater Horizon spill in the northern Gulf of Mexico. In S. A. Murawski, C. H. Ainsworth, S. A. Gilbert, D. J. Hollander, C. B. Paris, M. Schlüter, et al. (Eds.), Deep Oil Spills – Facts, Fate, and Effects. Springer International.

Bosman S.H., Schwing P.T., Larson R.A., Wildermann N.E., Brooks G.R. Romero I.C., et al. (2020). The southern Gulf of Mexico: A baseline radiocarbon isoscape of surface sediments and isotopic excursions at depth. PLoS ONE, 15(4), e0231678.

Brooks GR, Larson RA, Schwing PT, Diercks AR, Armenteros-Almanza M, Diaz-Asencio M, Martinez-Suarez A, Sánchez-Cabeza JA, Ruiz-Fernandez AC, Herguera Garcia JC, Perez-Bernal LH, Hollander DJ (2020) Gulf of Mexico (GoM) bottom sediments and depositional processes: A baseline for future oil spills (Chap. 5). In: Murawski SA, Ainsworth C, Gilbert S, Hollander D, Paris CB, Schlüter M, Wetzel D (eds) Scenarios and Responses to Future Deep Oil Spills – Fighting the Next War. Springer, Cham

Johansen, C., Macelloni, L., Natter, M., Silva, M., Woolsey, M., Woolsey, A., Diercks, A.R., Hill, J., Viso, R., Marty, E., Lobodin, V.V., Shedd, W., Joye, S., MacDonald, I.R.: Hydrocarbon migration pathway and methane budget for a Gulf of Mexico natural seep site: Green Canyon 600; Earth and Planetary Science Letters; Volume 545, 1 September 2020, 116411

 Proposal Abstract - RFP-VI PI Arne Diercks