POREWATER CHEMISTRY OF THE ICDP-USGS EYREVILLE COREHOLE, CHESAPEAKE BAY IMPACT STRUCTURE

Cores were collected during the ICDP-USGS drilling project in the Chesapeake Bay Impact structure 3 km north of Cheriton, Virginia in the fall of 2005 for the purpose of extracting pore waters for chemical analyses. A total of 141 ten-cm-long cores were collected between the depths of 95 and 1,760 m at intervals of 12-15 m. For each core an outer rind was cut off in order to remove contamination from the drilling fluids, and the core was then sealed in nitrogen in a jar and refrigerated to inhibit chemical alteration during storage. Contamination from drilling fluid was checked by the analysis of trace organic compounds that are standard drilling additives (e.g. soybean oil) (see Rostad and Sanford, this session). No evidence of drilling fluid contamination was found in any of the pore water samples that were analyzed. Most pore water was extracted from the cores using a high-speed centrifuge, although a few of the samples that were predominantly clay had to be squeezed under pressures of ~2,000 psi. The extractions yielded pore water volumes from <1 to >20 ml. After extraction, the samples were quickly analyzed for pH, specific conductance, and alkalinity, and then divided into aliquots and refrigerated until the individual chemical analyses were performed.

The pore waters were analyzed for major ions, ²H and ¹⁸O of water, total dissolved organic carbon (DOC), and ⁸⁷Sr/⁸⁶Sr ratio. The solids from each sample were also analyzed for Fe²⁺ and Fe³⁺, and total carbon and nitrogen. Chloride values range from 530-25,000 mg/L in the post-impact section and 20,000-38,000 mg/L in the impact section. Calcium and Mg values range from 60-8,000 and 16-1,700 mg/L, respectively. Sulfate and bicarbonate values range from 60-11,000 and 20-540 mg/L respectively. There is a consistent increase in the Ca/Mg ratio with depth from <1 in the post impact, to about 10 in the Exmore megablocks, to >300 in some deep waters, suggesting increasing thermal alteration of minerals with depth. Deuterium and ¹⁸O have values indicative of freshwater/seawater mixing, but little to no evidence of evaporation. Cl-Br ratios are mostly close to that of seawater. Additional analyses are currently being performed to estimate porosity, soluble salts, sulfur isotopes and cation-exchange capacities. Further interpretations are given in a separate presentation in this session.