Middle Valley

Although Hole 1036A is only 9 m away from an active vent, no major hydrothermal vein network was intercepted at depth and most sediments are not highly lithified by hydrothermal alteration. Only a few hydraulic breccias and some anhydrite veins are observed. The absence of a stockwork zone underlying the mound and the lack of extensive induration suggest that the Dead Dog hydrothermal mound is an immature feature, consistent with the occurrence of collapsed chimney material overlying very young sediments.

Biostratigraphy

Biostratigraphic zonation of the quaternary sequence at Middle Valley is limited to planktonic foraminifers, because other fossils such as diatoms, radiolarians, and calcareous nannofossils are poorly preserved in hydrothermal settings. Planktonic foraminifers are abundant to common to 9.94, 18.61, and about 26 mbsf in the core from Holes 1036A, B, and C, respectively. The depth where foraminifers disappear coincides roughly with the shallowest occurrence of weakly indurated sediments. Regional sedimentation rates constrained by the depth to the 125 ka horizon identified by foram assemblages range from 17 to 33 cm/1000 yr, which is surprisingly slow considering the young age of the basement and the total thickness of the sediment filling Middle Valley.

Organic Geochemistry

Interstitial water collected from Hole 1036C cores contains methane and hydrocarbon derived from high temperature alteration of organic matter. Hydrothermal petroleum is found in discrete horizons in sediments from Hole 1036A (12 -17 mbsf), Hole 1036B (24-35 mbsf), and Hole 1036C (32-35 mbsf). This petroleum has migrated only a short distance ahead of the heat front and resides at an in situ temperature of 100 -150°C. CO₂ increases near the oil horizon, but there is no correlation between methane and the oil horizon. Total organic carbon decreases with increasing depth in Hole 1036A, close to the vent, and increases with depth in Hole 1036C, at the base of the mound.

Inorganic Geochemistry

Normal seawater concentrations are present in pore fluids sampled from shallow depths, but below 20 mbsf there is 10-m-thick layer that shows high magnesium (twice the seawater value), low calcium, and low sodium concentrations. This implies high-temperature (> 200°C) alteration in the vicinity of the vent fluid conduit, probably involving downwelling seawater that has undergone reaction and is subsequently transported laterally through this section. Fluids with compositions similar to those sampled at Dead Dog vent are present below 40 mbsf in cores from Hole 1035B, whereas end member hydrothermal fluids were not detected in samples from Hole 1035C, probably because the depth of influence of the hydrothermal fluids rapidly deepens away from the vent. The evidence does not support the presence of a cap rock controlling hydrothermal fluid flow below Holes 1036A and 1036B.

Physical Properties

High gamma-ray and wet bulk densities correspond to the occurrence of nodules in cores from Holes 1036A, B and C. Both measures of bulk density indicate a general increase down section. This is correlated to a decrease in porosity from 70%, at the surface, to 50% or less, at 40 mbsf. Near the vent, porosity in cores from Hole 1036A drops abruptly to less than 30% at about 30 mbsf near the contact between lithologic Units I and II. The depth at which the magnetic susceptibility of sediments decreases to near zero (magnetic wipe out zone) increases with distance from the vent and may be related to hydrothermal alteration and shallowing of isotherms near the vent. Magnetic peaks in the deeper cores from Holes 1036A and B in the magnetic wipe out zone are due to local pyrrhotite precipitation. The natural remanent magnetization of cores from Site 1036 was measured for quasi-continuous sections. Hydrothermal activity significantly affects the paleomagnetic signal and overprints the record of the Brunhes Chron, with the level of affected sediment decreasing with proximity to the hydrothermal vent.

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