Time of accumulation of hypersaline anoxic brine in Orca basin (Gulf of Mexico)

Abstract

The Orca basin is an intraslope basin of 400 sq km area at a depth of ∼2400 m, on the continental slope in the northern Gulf of Mexico, containing 200 m of highly saline anoxic brine. The sedimentary units in a core from this basin are black mud from 0 to 485 cm with three turbidite beds of gray mud totalling 70 cm and gray mud from 485 cm to the bottom of the core at 1079 cm. It is interpreted that the black mud was deposited in a highly anoxic saline environment, and gray mud deposition took place in an oxic environment. The lack of sulfate reduction and bioturbation in the black mud, the occurrence of a manganese oxide peak in the gray mud, and geochemical differences between black and gray mud support this interpretation. The major black—gray boundary at 485 cm which has been dated by 14C to be 7900 ± 170 yrs represents the time when brine began to accumulate in the Orca basin and the depositional environment became anoxic. The interstitial water salinity in the core decreases from 238 g/kg at the top to 112 g/kg at the bottom, suggesting that the Orca brine did not diffuse from the sediments underneath but entered the basin from the surrounding slopes. The salinity profile in the sediments calculated for a downward-diffusing brine is similar to the observed profile and supports the beginning of brine accumulation at 7900 yrs B.P. Possible exposures of a salt surface to the sea water to the southeast and north of the basin are interpreted from two multichannel seismic profiles.