The Deepwater Horizon oil spill that took place in 2010 in the Gulf of Mexico is the largest marine oil spill in history. For months, the rig leaked oil and gas with almost 5 million barrels estimated to have spread into the ocean. Half of this oil was trapped at depth, while the other half rose to the surface, immortalized in photos, where it even reached the coast.

Estimates showed that some of the expected surface oil had disappeared before it could be cleaned, and researchers think that sunlight might have played a role in destroying some of it. Reporting in Science Advances, Danielle Haas Freeman and Collin Ward from the Woods Hole Oceanographic Institution think that the process of photo-dissolution could have cleared out as little as 3 percent and as much as 17 percent of the oil floating in the Gulf of Mexico.

"The big picture question is, where did all this oil go? Despite over a decade of research, questions remain about the environmental fate of the spilled oil," Hass Freeman and Ward told IFLScience.  "The most commonly discussed fates are biodegradation, where microbes eat the oil, evaporation, where the oil turns into a gas and goes into the air, and stranding, where oil washes up on coastlines. We hypothesized that there may be another important fate, called photo-dissolution, where the oil absorbs sunlight and is transformed into compounds that dissolve into seawater."

Photo-dissociation has not been properly looked into with regards to spill models until now, but light can break the molecules of oil into other organic compounds. By testing different wavelengths of light and different amounts of sunlight received, Haas Freeman and Ward found that UV light and low doses are the most efficient ways to get rid of the oil.

“We found that nearly 10% of the floating oil in the Gulf of Mexico after the Deepwater Horizon spill dissolved into seawater after sunlight exposure, an amount that rivals other oil environmental fates like biodegradation and stranding,” they explained.

"We also conducted a sensitivity analysis to determine which conditions are most important in controlling the rate of photo-dissolution at sea under hypothetical spill scenarios, which is useful because it allows us to identify other possible environmental conditions where this process might be relevant," they added. "For example, we found that in summer months, photo-dissolution rates in the Arctic could be equal to or are even greater than those in lower-latitude waters. This finding is important because cargo ship traffic in the Arctic is increasing as the extent of sea ice declines, and with this increase in traffic comes a heightened risk of oil spills.

This knowledge may help us better understand the evolution of oil spills but they both stress that a lot more work is needed to understand the role that photo-dissociation plays as well as investigations into any potential negative consequences.

“One implication of our findings is that photo-dissolution may reduce the load of material that responders must plan to burn, skim, and chemically disperse at sea, or physically remove from sensitive coastal ecosystems,” they told IFLScience.

“On the other hand, the potential impacts of the dissolved transformation products on marine ecosystems must also be considered by the oil spill science community. One potential negative consequence of this process is if the sunlight transformation products are persistent and toxic to aquatic animals. However, the persistence and toxicity of these products are largely unknown. We think the community should prioritize these gaps in knowledge moving forward.”

So, can this process actually be used to make clean-up efforts better? Incorporating this process into oil spill models could allow for more accurate estimates of the extent of oil spills and their location and refining estimates for other environmental processes, Haas Freeman and Ward explained.

This could lead to "more judicious allocation of resources during the spill response phase," especially when resources are limited and environmental agencies have to make calls on what and how to safeguard certain areas.