Award: OCE-1044289

In 2009, I began a study to collect and analyse particles sinking to the seafloor in the northern Gulf of Mexico to understand what supplied the food required to sustain delicate deep-water coral communities at depth, below sunlight, there. One obvious source is sinking of waste organic matter from the sunlit upper ocean (shallower than 200m) where life is abundant. Alternately, nutrient supply might come from exotic chemically-fuelled ecosystems associated with natural hydrocarbon (oil and gas) seeps that are known to occur across the floor of the Gulf of Mexico, in and among the active oil fields. While our project was already underway, the Deep Water Horizon accident occurred and within days oil slicks were mapped north of the Macondo well-head that passed directly over my sampling site. My samples was being collected using a sediment trap which acts as a giant circular funnel with a large surface area at the top (about 3ft diameter) that narrows and empties out into a plastic bottle at its base. Anything solid falling into the top of the funnel gets collected into the sample bottle at the base where, every two weeks, a carousel rotates to line up a fresh sample bottle with the base of the funnel (meanwhile all the other sample bottles are covered so nothing gets in or out). That way, when we analyse the samples, we know how much material arrived at the seafloor in each 2-week period and we can analyse how things vary with time. In this project I extended the original planned sampling so that, instead of our time-series running out while the Macondo well-head was still uncapped, we could compare, on a like-for-like basis, how the amount and composition of material sinking to the seafloor changed, from six months before the DWH accident happened until well beyond when the well-head was finally capped, several months later. What I found, when the samples were analysed in the laboratory was that there was no immediate clear sign of oil input reaching the seafloor at the study site but that there were important results to pay attention to. During the period that the Macondo well head was discharging oil in an uncontrolled way from the seafloor and contributing to surface ocean oil slicks, the results show an increase in the total amount of material sinking back to the seabed at the northern Gulf of Mexico site (about 30 miles NE of Macondo) and, further, that the concentration of organic carbon (which might or might not be oil related) in this material increased with time, too. Two questions then arose that we could not answer from those results alone: 1) Were the increases in mass and carbon flux in 2010 different from what would be expected in the Gulf of Mexico anyway in Spring and Summer where, (just as happens on land after Winter) increased hours of daylight and warmer temperatures can lead to a sudden flourishing of life in the surface ocean, which can also lead to an increase in the fall of waste material to the seafloor? 2) Whether the results observed at the time of the DWH accident were typical or atypical, did they carry any distinctive geochemical signatures that might allow us to "fingerprint" them as being related to the oil spill? In continuing work we have made important progress. By completing similar studies in 2011 we showed that the total amount of material and the carbon content of that material sinking to the same site over the Spring/Summer cycle of 2011 was not significantly different from that of 2010. However, in one measurement that can be made in my laboratory it was found that, by comparing the ratio of carbon [C] to nitrogen [N] in the sinking material, the highest material flux samples with the highest carbon contents at the height of the DWH accident in 2010 appeared to be composed of significantly different material in terms of its C:N ratio. The values in Spring 2010 were what scientists would normally conclude means this was "old" carbon and, hence, these results could ...