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Fluid Flow in Carbonate Platform and
Peri-Platform Sediments of the Marion Plateau

One of the main objectives of Leg 194 was to study fluid circulation in the Marion Plateau using the sediments and pore fluids recovered from the MP2 and MP3 platforms. The extensive dolomitization found in both platforms is itself indirect evidence for past fluid circulation. Dolomite formation on a large scale requires fluid flow to deliver the required magnesium to the precursor calcium carbonate sediments. But when and how fluids may have circulated and the nature of the fluids, whether they were normal seawater or hypo- or hypersaline, are open questions. It was also thought that the present-day pore waters might hold evidence of past or even continuing fluid movement. In that regard, the pore fluid sampling program on Leg 194 has yielded intriguing results. Although sampling of pore waters from within the southern MP2/MP3 platform was not possible, samples taken from sediments above and below the adjacent periplatform facies provide clear evidence that seawater continues to circulate through these sediments even though they are overlain by ~200 m of hemipelagic deposits. By inference, seawater is likely to be circulating through the MP2/MP3 platform at present. Similarly, the pore water samples from directly above and below the carbonate platform facies of MP2 at Site 1193 are also close to seawater in composition, suggesting seawater circulation. Site 1198 drilled through seismic Megasequences D, C, and B to basement 5 km northwest of the margin of platform MP3 (Fig. F20). In seismic Megasequence D, which is equivalent to lithologic Unit I, 24 samples were taken at ~10 m intervals to a depth of 196.6 mbsf, the base of lithologic Unit I. From 197 to 350 mbsf, poor recovery of the unconsolidated sediments of lithologic Subunits IIA and IIB (seismic Megasequence C and the upper ~60 m of Megasequence B) precluded pore water sampling. Sampling resumed in Megasequence B at 350 mbsf, with samples taken at ~10 m intervals to a depth of 505.0 mbsf, just above basement.

For most of the pore water constituents, nearly symmetrical, arcuate pore water profiles are found in the upper 200 mbsf. From essentially seawater values near the sediment surface, concentrations in the interval from 0 to 100 mbsf either increase or decrease depending on the ion measured. 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.

Two profiles, sulfate and strontium, are illustrated in Figure F20. These two constituents were chosen because they are affected by completely independent processes. The two profiles are shown plotted on top of the seismic section that crosses the platform margin and continues across the adjacent periplatform sediments now buried beneath the hemipelagic sediments of seismic Megasequence D. Note that the data points all lie on the line showing the location of Site 1198; the x-axis is concentration, not distance. Sulfate concentration initially decreases as a result of bacterial sulfate reduction. At ~100 mbsf, however, sulfate concentration increases, returning to ~29 mM at the transition from hemipelagic sediments of seismic Megasequence D to Megasequence C. A cessation of sulfate reduction in the lower part of Megasequence D is an unlikely cause for the increase in sulfate concentration because there is no decrease in the organic carbon content within this unit. At the base of seismic Megasequence C, the sulfate concentration is also ~29 mM, and thereafter decreases with depth.

The strontium concentration profile found in the upper ~100 mbsf is typical of pelagic or hemipelagic, carbonate-rich sediments. Strontium is added to the pore fluids by carbonate dissolution and recrystallization. Usually, this process continues for many hundreds of meters and high strontium concentrations (500 to 1000 mM) are a standard feature of sediment pore waters below 100-200 mbsf. At Site 1198, however, the concentration of strontium in the lower portion of seismic Megasequence D reverses the trend seen in the upper ~100 mbsf and is close to the seawater value at the contact with seismic Megasequence C. The strontium concentration is also low in the uppermost sample from seismic Megasequence B and then increases with depth to the bottom of the sampled interval. As for sulfate, there is no sedimentological reason for the reversal in trend. The carbonate content of seismic Megasequence D is relatively constant and in fact increases in Megasequence C.

The strontium profile is in many ways similar to sulfate, although the chemical reaction controlling strontium concentration is independent of that which affects sulfate. The pore water profiles of both constituents in seismic Megasequence D suggest diffusion-reaction control without fluid flow. The shape of the upper portion of the pore water profiles is typical for pore waters in many parts of the ocean, but the changes in concentration seen in the lower half of lithologic Unit I are unusual. They are most easily explained by relatively constant rates of reaction in the sediments, with diffusion acting upon both the upper and lower bounds of this sediment package. The fact that the concentrations at the lower boundary of Unit I are close to seawater values implies that the fluid that fixes the concentration at the lower boundary condition is also close to seawater in composition. The near-seawater concentration of sulfate and strontium (and other constituents) at the base of seismic Megasequence D strongly 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 from the available data. 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.

Figure F21 is a similar plot to Figure F20 but shows the concentrations of sulfate and strontium for Site 1193, drilled through the northern MP2 platform. The ~200-m-thick sequence of platform facies of MP2 are overlain by a thin cover of hemipelagic sediments of seismic Megasequence D and underlain by several hundred meters of hemipelagic sediments of seismic Megasequence B. Both sulfate and strontium concentration profiles of the upper 40 mbsf of Megasequence D faintly suggest the curved profiles found in Site 1198. More importantly, the pore fluids from the surface down to the base of the MP2 platform at 230 mbsf are close to seawater in composition. The similarity was initially interpreted to be the result of a near lack of chemical reaction in highly stabilized lithologies, low-magnesium calcite and dolomite, of the platform. Although this explanation remains a possibility, a second possibility also exists. Chemical reaction may be continuing in the platform, but the resulting changes to pore fluid chemistry could be swept away by active fluid circulation. Chemical reactions certainly continue below the platform facies, in Megasequence B, where concentrations of sulfate decrease and concentrations of strontium increase with depth. The evidence for fluid circulation at Site 1193 is perhaps not as strong as found at Site 1198. Fluid circulation is, however, a viable explanation for the observed pore water profiles.

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