Marine Oil Snow Sedimentation and Flocculent Accumulation (MOSSFA) refers to the process of formation, sinking, and seafloor deposition of oil-contaminated marine snow and oil-mineral aggregates. MOSSFA was well documented in the northern Gulf of Mexico (GoM) in the aftermath of the Deepwater Horizon(DWH 2010) and likely occurred in the southern GoM during Ixtoc 1 (1979–1980). This chapter introduces Part IV: Oil Spill Records in Deep Sea Sediments and addresses a series of questions regarding MOSSFA in the sedimentary record:

• What were the characteristics of MOSSFA sedimentary inputs?

• What was the extent of MOSSFA on the seafloor?

• What postdepositional processes took place as a result of MOSSFA?

• Can MOSSFA be preserved in the sedimentary record?

MOSSFA sedimentary inputs were comprised of three main components (biogenic, lithogenic, and petrogenic), many of which were surface derived. MOSSFA resulted in a four- to ten fold increase in bulk sediment accumulation rates, a two- to three fold increase in oil-derived hydrocarbon concentrations, a two- to three-order of magnitude increase in Corexit 9500A dispersant concentration, and two- to three fold increases in surface-derived biotic material (e.g., planktic foraminifera, diatoms). Estimates of the total spatial extent of MOSSFA on the seafloor of the northern GoM range from 1030 to 35,425 km2, accounting for between 3.7% and 14.4% of the total petroleum released during DWH. Increased microbial respiration of organic carbon caused depleted surface sediment oxygen, intensifying reducing conditions up to 3 years following DWH. Multiple proxies provided evidence of multi-year preservation of both oil residue in the sediments associated with DWH, MOSSFA, and the sedimentary event in the geologic record. Despite confounding factors in the southern GoM including regional events (e.g., volcanoes, hurricanes) and complex hydrocarbon backgrounds (e.g., natural seeps, oil, and gas infrastructure), multiple sedimentary proxies have provided evidence of degraded Ixtoc 1 oil-residue input and the MOSSFA sedimentary event preserved in the geologic record greater than 35 years after Ixtoc 1. Federal and international policies can be benefitted by incorporating MOSSFA with regard to response strategies, weighing the ecological trade-off between oiled coastlines and offshore benthic environments.