Deepwater and ultra-deepwater oil drilling are expanding globally as technology improves, making it increasingly profitable to drill in water depths greater than 1500 m. On average, the oil reservoirs also increase in volume with water depth. In this context, it is extremely important to better prepare for deepwater blowouts. It has become increasingly apparent since the Deepwater Horizon oil spill in the Gulf of Mexico in 2010 that broad environmental baselines should be established in areas of deepwater drilling.
In the aftermath of the Deepwater Horizon oil spill, a new phenomenon called marine oil snow sedimentation and flocculent accumulation (MOSSFA) occurred. Marine oil snow formed as plankton and bacteria released sticky materials called exopolymeric substances as a stress response when they encountered oil and dispersants. This sticky material bound together plankton, bacteria, oil droplets, and tiny grains of sediment from the Mississippi River into long bands and quickly settled to the seafloor.
On the seafloor, marine oil snow provided bacterial communities with a food source, and the communities began to degrade the marine oil snow. This degradation, or respiration, caused a decrease in oxygen on the seafloor. By eating (respiring) the marine oil snow, the bacterial communities also changed the chemistry of the water just above the seafloor.
Considering the change in oxygen availability and the change in water chemistry, it is also important to assess any impacts on the organisms that live on the seafloor. To do that, our team examined benthic foraminifera.
Benthic foraminifera are single-celled organisms that secrete hard shells made out of calcium carbonate, just like the shells you might find on a beach. However, most foraminifera are the size of a pencil tip. Our team measured the carbon isotope ratios of the foraminifera shells to determine if the marine oil snow event caused any change in foraminifera shell carbon composition. There are two stable carbon isotopes (Carbon-12 and Carbon-13), and the team measured the ratio of Carbon-13 to Carbon-12 in the shells of the foraminifera. Most oil is composed of Carbon-12. Also, naturally, more Carbon-12 is produced as bacterial communities respire food sources on the seafloor. With increased bacterial respiration, the Carbon-13 to Carbon-12 ratio of the surrounding seawater decreases or becomes depleted. Foraminifera use the carbon in the surrounding seawater to produce their shells. So, if the water that they are living in is depleted in Carbon-13, then their shells will also become depleted in Carbon-13.
From 2010 to 2012, the stable carbon signature of benthic foraminifera on the surface of the seafloor was depleted beyond any natural variations recorded in the past from the areas that the research team sampled. The foraminifera shells recorded the marine oil snow event over two years. Interestingly, the records collected in 2014 show that the Deepwater Horizon layer was buried below the surface of the seafloor by natural sedimentation processes. The foraminifera in the Deepwater Horizon layer (approximately 1 cm below the surface) were depleted just like the records from 2010-2012.
However, the carbon composition of the foraminifera at the surface of the seafloor in 2014 was the same as prior to the Deepwater Horizon event. This means that the surface of the seafloor was returning to a more normal status as of 2014. This also means that benthic foraminifera have preserved evidence of the marine oil snow event in the seafloor sediment record. This implies that benthic foraminifera can be used to determine the spatial extent of marine oil snow deposition on the seafloor and potentially has implications for long-term preservation and burial of marine oil snow events. In the context of the global expansion of deepwater and ultra-deepwater drilling, benthic foraminifera would provide a valuable tool to assess impacts to the seafloor in the event of a future blowout and should be taken into account when constructing environmental baseline measurements.
These findings are described in the article entitled, Tracing the incorporation of carbon into benthic foraminiferal calcite following the Deepwater Horizon event, recently published in the journal Environmental Pollution.
This work was conducted by Patrick T. Schwing, Isabel C. Romero, David J. Hollander, Ethan A. Goddard, and Rebekka A. Larson from the University of South Florida, Jeffrey P. Chanton from Florida State University, and Gregg R. Brooks from Eckerd College.
Reference:
- Schwing, P.T., Chanton, J.P., Hollander, D.J., Goddard, E.A., Romero, I.C., Brooks, G.R., Larson, R.A. (2018). Tracing the Incorporation of Petroleum Carbon into Benthic Foraminiferal Calcite Following the Deepwater Horizon Event. Environmental Pollution 237:424-429. DOI: 10.1016/j.envpol.2018.02.066