• SciEnvy

“Let it snow” – Marine snow as a vehicle for oil pollutants | Jake Smallbone

The term marine snow was first used in the 1930s when the discovery of tiny sinking aggregates in the ocean water column created the illusion of snow. Marine snow consists of various organic and inorganic particles, including microalgae, archaea, bacteria, dead phyto/zooplankton, faecal matter and various other detritus. It is created when dissolved organic matter (DOM) in the form of extracellular polymeric substances aggregate or stick together to form particulate organic matter (POM). These aggregates then act as a “Sticky” matrix for other bits of matter to further aggregate (Figure 1).

Figure 1: Schematic of marine snow (MS) formation and sedimentation

Once marine snow has formed it will start to sink to the seafloor, transporting microbes and much needed nutrients to deep sea environments. It also supports the organic carbon pump, sequestering atmospheric CO2 across the deep-sea benthos. However, in the last decade it has come to our attention that marine snow may act as a “vehicle” for the transport of crude oil into deep-sea environments through the formation of “marine oil snow”. This idea came to light during the 2010 Deepwater Horizon oil spill in which large formations of marine oil snow were observed accumulating on the seafloor, causing the sedimentation of oil in the deep sea. This event was named a MOSSFA or a marine oil snow, sedimentation, flocculation and accumulation event. This major event was thought to be caused by the significant amount of chemical dispersant used to support the subsequent clean up, with many researchers suggesting the use of chemical dispersants enhanced the formation of marine oil snow. When chemical dispersants are used they help reduce the interfacial tension between oil and water, allowing the oil to disperse into the water column forming small droplets (Figure 2). These small droplets are more readily available for biodegradation via hydrocarbon degrading microbes, thus further supporting the clean-up efforts during an oil spill event. It is thought that dispersants may impact marine oil snow formation due to increased chances for oil to interact with marine snow, through increased dispersion. Whether or not the use of chemical dispersion enhances marine oil snow formation is still up for debate, with contradictory studies suggesting they either enhance or inhibit these formations. Many of these studies have been scrutinised due to the lack of comparison to in-situ conditions, which is where my own research comes in to play!

Figure 2: Schematic of Surfactant molecule and dispersant process during an oil spill event. A) Surfactant molecule (Hydrophilic head and Hydrophobic tail), B) As surfactant is added to the oil slick it becomes saturated and starts to sink forming aggregates of surfactant, C/D) Oil droplets form, becoming more readily available for biodegradation and marine oil snow formation.

My PhD project will be assessing the impact of chemical dispersants on marine oil snow formation and investigating how effective the use of chemical dispersant is in mitigating marine oil pollution. I’ll be addressing previous criticisms by conducting my experiments in natural insitu conditions to simulate a North Sea environment. I’ll be setting up microcosms under various treatments of oil and dispersant which will kept at insitu temperatures and placed on a roller table to simulate turbulent conditions. A high intensity lighting system will allow for a natural day:night cycle and varying light intensities, indicative of North Sea conditions. I hope to be able to perform this experiment under multiple relevant conditions to compare marine oil snow formation across regional and seasonal changes.

The majority of past research in this area has directly catered to conditions across the Gulf of Mexico, with a focus on the Deep Water Horizons incident. It is therefore imperative that further research is conducted over a multitude of conditions to better inform oil spill response organisations and protocol when considering chemical dispersant use as a remediation strategy. Identifying potential MOSSFA event sites and their associated conditions would allow for a quick assessment of a site and whether it is susceptible to marine oil snow formation. In the future, it is hoped that this research can be used to better inform first responders on whether dispersant used to mitigate coastal and pelagic damage outweighs the potential impacts that may occur if oil enters our fragile deep-sea environments.

Figure 3: Microcosm set up using 1000ml borosilicate glass bottles, placed on a roller table to simulate natural turbulence.