LATE JURASSIC IMPACT FIRE OF HYDROCARBON SOURCE ROCKS IN THE BARENTS SEA

The Late Jurassic, organic-rich shales of the Barents Sea make up one of the main source rock for oil and gas in the Arctic. These sediments covered the target area of the Lat Jurassic Mjølnir impact. In this paper it is suggested that the extreme richness of organic matter and highly volatile components in the target rock resulted in a colossal and intense fires in the impact area, both in the air and on the seafloor. This theory is supported by numerical simulations and explains the large quantities of soot that have been found in samples associated with the Mjølnir impact.

The soot formation occurred just after impact (due to shock heating of target material and probably later due to ejecta heating in impact plume) and most likely had an initial distribution similar to that of other ejecta. The soot generation may be tied to both an early phase of pyrolyses and succeeding combustion phases, but both the size and distribution of the soot are consistent with a combustion origin.

At the time of impact the paleogeographic position of the impact site was hundreds of km from the closest forest, making wildfires on surrounding land not very probable. Spontaneous ignition of vegetation would also need higher temperatures. An extraterrestrial carbon source seems less likely, since the geochemical analysis indicates an iron-nickel rich impactor. We consequently find it most likely that the soot particles came from organic rich, partly volatile, dark clays of the sea bed. Presently we have no information to distinguish between soot particles of different origins, but the soot grains inspected so far have an appearance comparable to those of combustion origin from oils or hydrocarbons. The fires in the air and on the seafloor must have started immediately at impact and may have been on and off in periods, partly controlled by the wash and back wash into the crater.