Red Sea geochemistry

The Red Sea drillings reveal a number of new facets of the hot-brine-metalliferous system and other geochemical aspects of the sea, its sediments, and its past history as follows:

1) Dark shales rich in organic material, and containing enhanced Mo and V concentrations, are characteristic of Plio-Pleistocene strata in the Red Sea. Values as high as 1500 ppm V and 500 ppm Mo were obtained in sediments containing up to 8 percent organic carbon.

2) Metalliferous sediments in the hot brine deep (Site 226) are similar in composition in both solids and interstitial water to previously analyzed sediments. However, one site (228) well south of the known hot-brine deeps shows zinc mineralization reaching 5 percent Zn in late Miocene shale-anhydrite breccias.

3) Pore fluid studies show that near-saturated (NaCl) brines having similar total salt concentration to the hot-brine fluids are associated with Miocene evaporites at Sites 225, 227, and 228. However, their chemical and isotopic composition precludes such fluids being part of the "hot brine plumbing system." Hydrogen and oxygen isotope studies demonstrate that fluids trapped between and among the evaporitic rocks have a strong meteoric water component, presumed to have entered the rocks during or shortly after formation in shallow evaporating pans. The composition of pore fluid at Site 227 suggests the presence of late-stage evaporite minerals of the tachyhydrite CaMg₂Cl₆ • 12H₂O series in the in situ rocks.

4) Diffusivity measurements show that the pre-Miocene strata permit dissolved salt or gas diffusion to the extent of from 1/2 to about 1/10 the rate in free solution. However, in anhydrites diffusivity is reduced more than 100-fold, and no diffusion could be detected through halite rock. The rates applied to interstitial salt gradients at Site 225 suggest that less than 1 meter of rock salt is removed per million years by diffusion processes. The diffusion of salt can already be detected a few meters below the sediment-water interface, and based on the interstitial water studies, one can affirm the presence of salt at depth at Sites 228, 230, and possibly 229, where rock salt was not encountered by the drill.

5) Isotopic measurements on leads show that both leads from Site 228 and the hot brine deep (Site 226) require input from igneous or volcanic sources (e.g., volcanic ash). Elsewhere, however, leads of sedimentary-pelagic origin are noted.

6) Isotopic and other evidence indicates that the long-distance transport of subterranean brines advocated by Craig (1969) is unlikely. Instead, it is proposed that the source of the hot brines is subevaporite clastic or other aquifers of early to middle Miocene age that have been disrupted by rifting. These discharge in the deeps by virtue of hydrodynamic continuity with heavy brines at higher positions on the nearby flanks of the Red Sea. In this case, the waters might be fossil (middle Miocene) Red Sea waters of relatively normal salinity that have acquired greater salt concentration by diffusion from overlying late Miocene evaporites. The model is consistent with the isolated nature of the brine deeps and suggests that flow might have been enhanced by increased hydraulic gradients during periods of lowered Red Sea levels.

7) Interstitial water evidence indicates that Pleistocene lowerings of sea level did not cause evaporative conditions leading to actual gypsum or other evaporite deposition in the deeper water zones, as has been postulated. This in turn suggests that sill depths were greater than have been assumed.