MARINE SNOW FORMATION AND SEDIMENTATION IN THE AFTERMATH OF THE DEEPWATER HORIZON/MACONDO WELL OIL SPILL

About one month after the spill, in May, large, extremely mucous rich flocks were observed in the presence of oil in the GOM. These flocks differed both in size and in consistency from marine snow usually observed in the GOM or elsewhere. These mucus flocks were very stringy, veil-like with thick mucous threads creating a web like structure. We suggest that these flocks sank en masse one month later, in June at high sinking velocities, taking suspended plankton and particles with them. Rapid sedimentation of these mucous flocks would explain their absence in the water 1-2 months later, and the appearance of oil and large concentration of calcium carbonate shells on the sediment surface at this time. We will present results (i) from aggregation experiments investigating the formation mechanisms of such flocks, (ii) on in situ marine snow distribution and (iii) from sediment traps deployed in the GOM August and September 2010. The most striking result of the aggregation experiments conducted onboard ship is that no flocks similar to those observed in May formed in any of the experiments, although aggregates formed. This absence of mucous flocks is in stark contrast to the in situ and in vitro observations made 3 weeks after the accident, and suggests that only very specific conditions lead to the formation of the observed mucus flocks. Sedimentation rate was relatively low along a transect south of the spill site, and two to three times higher nearer to the site of the accident. Small and medium sized miscellaneous aggregates dominated flux and were responsible for high sedimentation rates at stations 16 to 27, near the site of the spill.