Relationships between anoxia, glacial meltwater and microfossil preservation in the Orca Basin, Gulf of Mexico

Abstract

The isotopic and microfossil records from piston core EN32-PC6 from the Orca Basin, Gulf of Mexico, reveal that various factors have moderated the sediment composition of the basin during the last 29,000 yrs (29 ka B.P.). Coincident with the introduction of isotopically depleted glacial meltwater into the Gulf approximately 16 ka B.P., anoxic conditions commenced within the intraslope basin, as evidenced by sedimentary laminations and improved microfossil (pteropod, planktonic foraminifera and radiolaria) preservation. The increased delivery of terrigenous organic matter with the glacial meltwater may have been responsible for inducing the anoxia. Also at this time, the water of the Orca Basin was most likely somewhat stratified due to elevated salinities, and the increased supply of organic matter onto the continental slope from the continent would have depleted the oxygen supply of the Orca Basin by oxidative consumption. About 13 ka B.P., oxygenated conditions resumed and microfossil abundance/preservation decreased when meltwater input diminished. At 8.5 ka B.P., the Orca Basin again became anoxic due to the development of the Orca Basin brine.