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doi:10.2204/iodp.proc.327.106.2011

Pore water geochemistry

The goal of the pore water program at Site U1363 was to recover basal sediments for pore water extraction to document trends in fluid composition with distance from the exposed basaltic portion of Grizzly Bare outcrop. Pore waters were recovered from five holes at four locations. Some analyses (alkalinity and ion chromatography) were conducted immediately to guide drilling operations. Fifty-eight pore water samples were collected: 15 from Hole U1363G, 14 from Hole U1363F, 14 from Hole U1363B, and 15 from Holes U1363C/U1363D. Basement depths are 17, 35, 57, and 231 mbsf, respectively, at these locations, as shown in Table T2 and Figures F2 and F3.

Pore water chemical profiles from three of the holes (U1363B, U1363F, and U1363G) are similar (Tables T3, T4). Results from Hole U1363B are described in the rest of this section. The upper portion of Hole U1363B was cored with the APC. Because Cores 327-U1363B-3H through 6H were partial strokes and sandy conditions caused poor recovery and sediment flow-in, the XCB was used for Cores 7X through 10X. Material from Cores 1H through 8X, with the exception of Core 6H, was squeezed to extract pore waters. Pore water profiles are similar in many ways to those from ODP Site 1026 and IODP Site U1301. In the upper portion of the sediment column, biogenic processes release dissolved Mn and Fe near the sediment/water interface and consume sulfate, with a sulfate minimum of 20.9 mM at 20.95 mbsf. There is a corresponding increase in alkalinity (Fig. F4), phosphate, and ammonium and an initial decrease in Ca resulting from carbonate formation from biogenic reactions that generate alkalinity. There are similar trends for sulfate, Mn, and Fe near the sediment/basalt interface; however, phosphate and ammonium are more influenced by diffusion and reaction within the upper basaltic basement. The gradients of cations Ca, Mg, and K near the sediment/basalt interface are indicative of a formation fluid that is slightly altered relative to seawater. Minor and trace elements in seawater also show reaction within the sediment and gradients in the basal sediment section that project to a formation fluid that is slightly altered relative to seawater.

Pore water profiles from Holes U1363F and U1363G, where the sediment section is considerably thinner, generally follow trends observed in Hole U1363B. Surprisingly, the fluid composition near the sediment/basalt interface is similar at these three locations, even though the holes are at different distances from the outcrop and have different amounts of sediment cover above basement (Figs. F2, F3). Sulfate, chlorinity, Li, and Sr concentrations differ significantly among the holes.

Only the lower portion of the sediment section was cored in Holes U1363C and U1363D. Nevertheless, spatial trends in pore water composition near the sediment/basalt interface are consistent when one considers the large-scale flow system. The composition of pore fluid at the base of Hole U1363D is more altered relative to seawater than it is in the three shallower holes discussed above (Holes U1363B, U1363F, and U1363G) but is less altered than spring fluids sampled on Baby Bare outcrop (Wheat and Mottl, 2000). An example of data quality and trends near the sediment/basalt interface is shown using alkalinity data (Fig. F4). Alkalinity concentrations increase with depth to a maximum value and then decrease in proximity to the sediment/basalt interface.

In Hole U1363G, which was APC cored to basement, alkalinity increases from 3.31 mM/L at 1.45 mbsf to 4.61 mM/L at 7.95 mbsf and then decreases to ~2.9 mM/L at a curatorial depth of 24.6 mbsf, which is 7 m deeper than the sediment/basalt contact determined by tagging basement in an unnumbered adjacent hole. Thus, fluids from a curatorial depth greater than 17 mbsf in Hole U1363G likely originated from material that was sucked into the liner during recovery of the APC core. Hole U1363F, which was XCB cored through the sediment/basalt interface, has a similar alkalinity profile (Fig. F4). In this hole, alkalinity rises from 6.65 mM/L at 4.45 mbsf to 10.5 mM/L at 12.45 mbsf and then decreases to 1.81 mM/L near the sediment/basalt interface at 33 mbsf. The highest alkalinity measured during Expedition 327 is 13.9 mM/L at 16.45 mbsf in Hole U1363B; alkalinity at the sediment/basalt interface is ~2.1 mM/L. We lack pore fluid data from the upper sediment portion of Holes U1363C and U1363D, but deeper data indicate a linear trend in composition with depth approaching the sediment/basalt interface. The lowest alkalinity measured within these holes is 1.25 mM/L at 222.65 mbsf, near the sediment/basalt interface, which is consistent with a progressively altered basement fluid with increasing distance from Grizzly Bare outcrop. Third-party shore-based geochemistry data are included in Tables T3 and T4, and these data will be available in LIMS just like analyses that are typically run on the ship.

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