Esacanaba Trough

Central Hill Area


Most samples examined from Site 1038 contain very few foraminifers or are barren. Their alteration is clearly associated with thermal and hydrothermal effects. Hole 1038F provides confirmation that Holocene sedimentation is exceptionally fast, probably in excess of 290 cm/1000 yr.

Igneous Petrology

A basaltic layer from 1 to at least 5-m thick was intersected in three holes (Holes 1038G, 1038H, and 1038I) at depths between 142 and 162 mbsf. This unit could have been either a thin flow erupted on sediment or a thin sill. The basalt intersected at the base of Hole 1038I (403 mbsf) is thought to be a flow, based on the presence of fresh glass and a very narrow (2 mm) baked upper contact. There is, however, insufficient core penetration in this interval to ascertain if this is indeed a flow, in which case it most likely was erupted over sediment, or is a sill.

Inorganic Geochemistry

Pore fluids collected from Site 1038 show a wide range in chemical compositions. A hydrothermal component is obvious in all the holes and dominates pore fluid chemistry at shallow depths below hydrothermally active and inactive sulfide mounds. The Cl concentration ranges from 300 to 800 mM indicating the presence of hydrothermal fluid affected by phase separation. In most holes, the high salinity component dominates pore fluids. Low salinity pore fluid is particularly evident in sand-rich layers in Holes 1038A, H, and I. This suggests a preferential vapor loss and migration through sand layers after boiling. Pore fluid compositions also appear to be modified by anhydrite dissolution, Mg-metasomatism, and chlorite formation.

Hole 1038I allowed recovery of pore fluid from deeper in the section. The thermal gradient in the upper part of Hole 1038I is approximately 2°C/m. High temperature results in concentrations of Li and B below 300 mbsf that are far higher than those recorded in the reference hole. Furthermore, the concentration of Mg declines far more rapidly with depth than in the reference hole. There is a marked change in the composition of pore fluid in the sand layer of lithologic Unit 3. Ca, Cl, Mg, Sr, and Na/Cl ratio decrease while Li, B, K, and NH4 concentrations increase. This is consistent with lateral flow of a hydrothermal component with low Cl through the sand layer. This fluid acquired high concentrations of Li, B, K, and NH4 by reaction with sediment. Fluid with similar composition was sampled at Holes 1038A and 1038H, where the low Cl concentration is related to phase segregation following boiling. High salinity fluids were recovered from sediment beneath a thin basalt layer penetrated at 161 mbsf in Hole 1038I. These fluids could be conjugate brines to the low salinity fluids sampled in the overlying sediment intervals. Concentration of Cl, Ca, K, Na, Mg, and NH4 increase while H4SiO4, Li, and B show a small reduction. The relationship between the Na and Ca content with Cl fits on the mixing line, indicating a single source for the high and low-Cl fluids.

Organic Geochemistry

High methane concentration in gas was found in Holes 1038E, 1038H, 1038F, 1038G, and 1038I. Except for Hole 1038I, which was drilled away from the hydrothermal field, methane in the recovered cores has a thermogenic origin. The presence of benzene and toluene confirm high temperature cracking of organic matter. Bitumen fluorescence also indicates a high maturation temperature of 150°C to 250°C. The estimated temperature of organic maturation is higher than in Middle Valley. The full range of maturity is observed. Hydrothermal petroleum occurs in the shallow section of several holes. The regional maturation of organic matter has been rapid for all holes. In Hole 1038I, methane has a biogenic origin at shallow levels and a thermogenic origin deeper in the section. Hydrocarbon compositions reflect the terrigenous source of organic matter in the sediments deposited in Escanaba Trough.

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