Porewater geochemical evidence for fluid flow in Miocene Peri-platform Sediments of the Marion Plateau, Leg 194 Ocean Drilling Program

One of the goals of studying the Marion Plateau was to use the sediments and pore fluids recovered to investigate fluid circulation in ancient carbonate platforms of the plateau. The extensive dolomitization found in both platforms is itself indirect evidence for past fluid circulation in the platform. But when and how fluids may have circulated and the nature of the fluids, whether they were normal seawater or hypo- or hyper-saline, are open questions. Although sampling of porewaters from within platform MP3 was simply not possible, samples taken from sediments above and below the adjacent peri-platform facies provide evidence that seawater continues to circulate through a these sediments even though they are overlain by ~200 m of hemi-pelagic deposits. Site 1198 drilled to basement 5 km southeast of the margin of Miocene platform MP3. For most of the porewater constituents, nearly symmetrical, arcuate porewater profiles are found in the upper 200 mbsf, the hemipelagic sediments of seismic Megasequence D. From essentially seawater values near the sediment surface, concentrations either increase or decrease in the interval from 0 to 100 mbsf. In the interval from 100 to 200 mbsf, the trends of the upper 100 mbsf reverse, and concentrations return to values close to those of normal seawater. This pattern is found for alkalinity, sulfate, ammonium, strontium, potassium, and magnesium. Pore water samples could not be recovered from most of the peri-platform sediments of seismic Megasequence C. However, at the base of this sequence the porewater concentrations of most dissolved species are also close to seawater values. Concentration changes typical of pelagic sediments are seen in the deepest sediments of seismic Megasequence B. The shapes of the porewater profiles in the upper 100 mbsf are not atypical for pelagic sediments. But the changes in concentration seen in the interval between 100 and 200 mbsf are quite unusual. They are most easily explained by relatively constant reaction rates, with diffusion acting upon both the upper and lower bounds of this sediment package. The near-seawater concentrations at the upper and lower bounds of seismic Megasequence C implies that the fluid within the peri-platform sediments is close to seawater in composition, and suggests active circulation of seawater through the sediments of seismic Megasequence C, between 200 and 350 mbsf. Neither the mechanism nor direction of fluid flow can be determined. Based on the seismic profiles of the Marion Plateau, sequence C is not exposed on the seafloor and thus has no direct connection to seawater. The unit is in contact with the MP2/MP3 platform, and a hydraulic connection between the platform and the peri-platform sediments of seismic Megasequence C seems likely. Thus one possible explanation for the observed evidence for seawater circulation in Megasequence C is that it is coupled with active circulation of seawater within the platform.