Physical Controls on the Creation and Persistence of Natural
Marine-Seepage Slicks
Abstract
Physical processes involved in the ascent of naturally seeped oil from
the seafloor and its persistence as a slick are considered. Simplified,
physics-based models are developed, drawing in part from the extensive
literature concerned with anthropogenic releases of oil at sea. The
first model calculates the ascent of oil droplets or oil-coated gas
bubbles as they ascend to the sea surface from the seep source. The
second model calculates slick longevity as a function of the effect of
wind-driven breaking waves. Both models have simplified inputs and
algorithms making them suitable for Monte Carlo-type analysis. Using the
oil ascent model, we find that slicks from shallower seeps are offset
farther relative to their water depth than those from deeper sources.
The slick longevity model reveals four growth modes for seepage slicks:
persistent (low wind speeds), ephemeral (high wind speeds), reset (all
slicks are cleared from an area by high wind speeds), and aging (slick
growth after a reset). A year’s worth of modeled winds from the Gulf of
Mexico indicate average slick ages of ~ 12 hours. Taking
account of the expected oil release duration implied by slick
recurrences yields average slick longevities for high recurrence seeps
of ~6.5 hours and ~ 5 hours for low
recurrence seeps. Seep flux estimates that include the length of
individual slicks and the constraints of local currents and wind
implicitly take into account the impact of wind-speed history. Those
that assume a slick age should be re-evaluated in light of the current
findings.