166 Preliminary Report


The primary objective of Site 1004 is to obtain heat flow and interstitial water geochemistry measurements to be used in conjunction with data obtained from Sites 1003, 1005, 1006, and 1007 to ascertain possible fluid flow and diagenetic reactions in the carbonate margin. In addition, this site serves as a location for the study of high-resolution sea-level changes during the Pleistocene, as well as providing additional information for the broader sea-level objectives of Leg 166.

A Pliocene-Pleistocene section (200 m) was drilled at Site 1004, which is located on the slope of Great Bahama Bank. The strata consist of a series of mixed pelagic and bank-derived carbonates with a carbonate content of 92-97%. Adara and WSTP temperature measurements confirm the presence of a non-steady state temperature profile suggesting an inflow of water into the slope sediments. Pore-water geochemistry and gas analyses show an extremely active diagenetic zone between 110 and 200 mbsf with alkalinity values as high as 70 mM, depletions in Mg and Ca, and enrichments in Sr. This zone corresponds with high concentrations of methane and hydrogen sulfide. Hydrogen sulfide readily degassed from the cores between 50 and 135 mbsf as soon as they were brought to the surface.

One major lithologic unit was distinguished. Unit I (0-200 mbsf; latest Pliocene to Holocene) is equivalent to Unit I described in Site 1003. It consists of several 5-to-10-m-thick intervals of light-colored, unlithified to partially lithified peloidal wackestones to mudstones, which alternate with thin 0.5 to 1 m intervals of gray, partially lithified wackestones to packstones, grainstones, and floatstones. Subunit IA (0-81.1 mbsf) contains multiple oscillations between zones of coarse, blackened grains with lithoclasts and fine periplatform ooze. Subunit IB (81.1-200 mbsf) contains an overall coarsening-upward sequence of partially dolomitized mudstones and wackestones with some coarser grained intervals.

Sediments recovered from Hole 1004A provide a record for the Pleistocene through the uppermost Pliocene, although the abundance and preservation of both nannofossils and foraminifers vary throughout the recovered sequence. The sedimentation rate is high throughout the Pleistocene, and modern platform production and shedding began to influence this site much earlier than the more distal site, Site 1003. Throughout Subunit IA, a number of alternations in sediment composition, color, and mineralogy can be recognized that probably correspond to sea-level changes. Lowstands are characterized by higher concentration of low-Mg calcite and darker color compared to highstands which contain abundant aragonite and high-Mg calcite. Below the bottom of IA, these cycles can no longer be recognized because of poor core recovery and the influence of diagenesis. The main effect of diagenesis is the dissolution of aragonite and HMC and the formation of low-Mg calcite (LMC) and dolomite.

Based on pore-water geochemistry, two distinct geochemical zones were identified at Site 1004. An upper zone, extending from the seafloor to a depth of 40 mbsf, is characterized by an absence of significant changes in the interstitial pore-water geochemistry. Geothermal measurements also indicate a reduced temperature gradient in this interval, suggesting the influx of seawater. The upper zone gradually merges into a region with salinity between 45 and 50, below 110 mbsf. In this lower region, concentrations of both Ca and Mg are depressed and Sr is elevated. This region corresponds to high concentrations of CH4 (up to 20,000 ppm) and H2S (up to 21,000 ppm). Although the C1/C2 ratio was over 2000 in this region, there were still detectable concentrations of C3 and C4 gases.

Site 1005 Results

166 Table of Contents

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