Iron just never ceases to amaze...

By Matt Quinan:

Terrestrially derived iron minerals may safeguard marine ecosystems from eutrophication and sulfidization, but the full extent of their ecosystem benefits has not been established.  Iron redox cycling can generate hydrocarbon-degrading hydroxyl radicals, with reactions probably occuring more frequently in sediments with elevated iron content such as those in the RiOMars (River-dominate Ocean Margins). To determine if iron in marine sediment affects the degradation of hydrocarbons, we augmented sediments with iron and Deepwater Horizon crude oil to simulate conditions similar to those of a post-spill seafloor environment. Significant differences in the redox environment and oil degradation result as a function of sediment composition. These results could have significant implications for environmental policy decision making.

In order to determine if redox conditions do indeed affect hydrocarbon degradation rates and pathways, artificial sediment cores were created with variable compositions of play-sand, iron-coated sand, Indian River Lagoon (IRL) mud, and clay. The cores were separated into three oil treatments: Control, Oil 1, and Oil 2. Five replicates of each core were assembled to provide five sacrificial timepoints. They were then incubated in natural IRL water using a flow through system constructed for this experiment in the Harbor Branch Sediment Incubation Laboratory.

A black layer consistent with FeS minerals and concentrated in microbes formed in the cores almost immediately after incubation started.  But, the concentration of crude oil and iron in the cores seems to control the extent of the black layer.  It is possible that the presence of iron slows the slows the establishment of sulfur chemistry in the core.





 
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Oil extraction and analysis (GC/MS) after 2 months of incubation, has shown that an increase in the sedimentary iron concentration leads to an increase in the n-alkane degradation rate.  Also, the shorter chain alkanes that are not present in the source crude oil have appeared in cores with higher oil concentrations after incubating.  It is possible that the longer chain alkanes are being transformed into shorter chain alkanes when iron is present in the sediment. 

The next phase of this project will include the incubation natural sediment from the northern Gulf of Mexico amended with crude oil. The culmination of this project will be the construction of a model capable of estimating the length of time required for natural river systems to degrade hydrocarbons under different discharge scenarios that affect mineral delivery.