Rainy periods and bottom water stagnation initiating brine accumulation and metal concentrations: 1. The Late Quaternary

Abstract

A working hypothesis is proposed to account for the present accumulation of brines in isolated pockets of the ocean floor and for the formation of the underlying organic and metal‐rich sediments. These are the Tyro and Bannock basins in the East Mediterranean, the Red Sea Deeps, and the Orca Basin in the northern Gulf of Mexico. Initiation of brine‐derived deposition in the Red Sea Deeps and Orca Basin occurred between 12,000 and 8000 years B.P. This time bracket also encompasses the formation of the latest East Mediterranean sapropel and the wettest global climate since the last glacial maximum. This wet period first appeared in the tropics around 12,000 years B.P, then in the subtropical and middle latitudes. During the same period, the 23,000 year precession cycle brought the summer insolation of the northern hemisphere to its peak at 11,000 years B.P. with retreating northern hemisphere ice sheets. The Red Sea Deeps and the Orca Basin became anoxic during this humid period, and metal‐rich sapropel deposition then began. In contrast, the Tyro and Bannock basins began accumulating a brine long before and persisted beyond this climatic stage. The hypothesis involves two propositions: (1) As in the Eastern Mediterranean Sea, marine anoxia was mainly the consequence of the large influx of continental runoff and local precipitation. Longer residence time of bottom waters, so‐called “stagnation,” in silled rimmed basins would have resulted from lower salinity at the sea surface in areas of deep water formation in the Eastern Mediterranean, the Red Sea, and the Gulf of Mexico and (2) Miocene or older evaporites underlie these basins or outcrop on their flanks. Leaching from these evaporites was an ongoing process before the quasi‐stagnation phase, but the initial leachate, much less saline than the present brines, was continuously flushed by bottom circulation. The climate‐induced quiescence of bottom waters in these basins enabled the leachate to accumulate. The feedback of stagnation by increased density progressively raised the salinity of entrapped bottom waters to the present brine concentration. The high density has resisted brine removal by bottom circulation until present time, long after cessation of the initiating wet period. The brines therefore are stagnant, fossil waters.