Continuous advanced hydraulic piston corer (APC) coring from the seafloor to 200 mbsf enabled high-quality sampling for geochemistry and microbiology throughout the drilled sediment column of Site 1228. Because of the overall predominance of sulfate reduction in the highly sulfidic sediment and the presence of sulfate throughout the sediment column, there were no distinct chemical interfaces to target in the sampling scheme for Site 1228. Concentrations of chloride range linearly from a typical seawater concentration at 0 mbsf to twice the seawater concentration at 200 mbsf. This linear profile demonstrates the long-term stability of brine diffusion and provides a reference for all other interstitial water constituents. Analyzed nonconservative species that are affected by microbial activity in the subsurface included sulfate, dissolved inorganic carbon (DIC), and ammonium. Interstitial water analyses at high depth resolution show unexpected details with implications for both the rates of long-term processes and for more recent changes.
Sulfate reduction in the upper 50 m of the sediment column is not sufficient to deplete sulfate at depth. The overall sulfate distribution shows a steep drop in concentrations over the first few meters below the sediment/water interface, a sigmoidal curve in dissolved sulfate concentrations over the following 10 m, a decrease to 2.5 mM at 38 mbsf, and then a continuous increase to 30 mM at 200 mbsf. The sigmoidal curve of the first 10 m indicates that the near-surface distribution of sulfate reduction and/or transport processes changed strongly in geologically recent time and that diffusion through the sediment column has not yet fully adjusted to a new steady state. The continuous increase in sulfate concentrations from 40 to 200 mbsf results from upward diffusion of the underlying sulfate-rich brine.
The depth profiles of DIC and ammonium closely match the described sulfate distribution. The overall DIC profile reveals a distinct DIC maximum of 19 mM at 2 mbsf, a decline to 15 mM, a rise to a second, broader maximum of 20 mM at 25 mbsf, and then a gradual downhole decrease to 4 mM. Ammonium similarly increases from ~2000 然 near the sediment surface to a local maximum of 2600 然 at 2 mbsf, declines slightly, and then increases gradually to 5000 然 downhole. Comparison to Site 680 biochronostratigraphic data (Shipboard Scientific Party, 1988b) suggests that the sediment that contains the DIC and ammonium maxima may have been deposited a few tens of thousands of years ago. These near-surface interstitial water anomalies indicate that steady-state diffusion of biologically active chemicals past the upper sediment column was disrupted by late Pleistocene environmental change and has not yet fully recovered. The exact nature of these changes will be analyzed when a more complete data set becomes available.
Concentrations of manganese and iron in the interstitial water are extremely low (<0.1 然) down to ~60-80 mbsf. Below this depth, they increase gradually to ~10 然 (manganese) and 50 然 (iron) at 200 mbsf. The source of these dissolved metals at depth may be either diffusion from below or in situ manganese or iron reduction in the lower sediment column.
In contrast to most other ocean-margin sites, including Site 1227, a sulfate/methane interface is absent from the sediment of Site 1228. Methane concentrations remain low throughout the 200-m sediment column, reaching a maximum of only 8 然. Yet, the distribution of methane clearly reflects the sulfate distribution, with a maximum coinciding with the sulfate minimum and a general inverse correlation between sulfate and methane concentrations throughout the sediment column. These results indicate that even at a concentration above 9% of its seawater level (minimum = 2.5 mM; seawater = 28.9 mM), sulfate regulates the ability of methane-oxidizing consortia to take up methane and maintain a low background concentration. In this respect, Site 1228 provides a unique opportunity to analyze the energetics of anaerobic oxidation of methane and to test current theories of the limiting parameters for this key microbial process.
Acetate and formate are important fermentation products as well as substrates for sulfate-reducing bacteria. Their concentrations in this organic-rich shelf sediment are tenfold higher than in deep-sea sediments of the tropical Pacific (Sites 1225 and 1226) but only about one-half their concentrations at Peru shelf Site 1227. The Site 1228 data show considerable scatter with depth. Acetate concentrations fall mostly in the range of 1-4 然 and formate concentrations in the range of 0.5-3 然. The higher concentrations of both fatty acids are present below 100 mbsf. These concentrations are regulated by uptake mechanisms that are not yet fully understood.
Interestingly, the depth of the distinct sulfate minimum at ~40 mbsf is present in an interval of strong lithologic and physical change. At this depth, the sediment shifts from a diatomaceous silt of predominantly hemipelagic origin to older quartz- and feldspar-bearing silt with a more abundant terrestrial component. At 43 mbsf, there is a distinct minimum in porosity and maxima in density, thermal conductivity, and magnetic susceptibility. It is intriguing to speculate that such a physical boundary may temporarily lock the position of biogeochemical zonations in the sediment column.
The temperature gradient in the Site 1228 sediment column was defined from two discrete temperature measurements taken with the Davis-Villinger Temperature Probe (DVTP). The results were combined with Leg 112 data to define a linear temperature gradient of 34蚓/km and a heat flow of 32 mW/m2. This heat flow estimate is lower than the 46 mW/m2 estimated for Site 680 by the Leg 112 Shipboard Scientific Party (1988b) and confines the previous broad estimate of 20-70 mW/m2 for this site (Yamano and Uyeda, 1990). The temperature increases down through the sediment column from an estimated annual mean of 12.5蚓 at the seafloor to an extrapolated 19.3蚓 at 200 mbsf. These temperatures are all within the low mesophilic range for prokaryotes.
Samples were taken for total counts, viable (MPN) counts, and isolations of prokaryotes from selected depths throughout the sediment column. Because of the short transfer time between Sites 1227, 1228, and 1229, the acridine orange direct counts (AODCs) of total prokaryotic cell numbers at Site 1228 will be conducted postcruise. A large number of MPN samples and isolation incubations target a broad physiological spectrum of heterotrophic and autotrophic prokaryotes that utilize diverse electron acceptors and donors in their energy metabolism. The selective influence of increased salinity and brine composition is also targeted in some incubations. The expected slow growth of deep subsurface prokaryotes will require long postcruise incubation of samples before definite results are obtained from these experiments. This is also the case for the many experiments on microbial processes measured by radiotracer techniques on samples taken from throughout the entire sediment column.
Because the absence of prokaryotic cell contamination from drilling and sampling operations is critical for the isolation of indigenous prokaryotes and measurement of their activities, a perfluorocarbon tracer (PFT) was continuously added to the drilling water. Tracer samples were taken on the catwalk or in the laboratory from all core sections and subsamples used for microbiology. It was demonstrated that PFT concentrations are typically higher at the periphery than at the sampled center of whole-round core segments. The microbiology subsamples had PFT concentrations below or near the detection limit. This limit corresponds to the potential introduction of 0.01 無 seawater/g sediment. Such seawater introduction could maximally introduce 5 cells/g, based on the mean cell density in seawater (5 x 10-8 cells/L). An additional contamination test uses fluorescent microbeads dispersed on impact at the head of the core barrel. At Site 1228, this test consistently indicates that contamination is unlikely. This method releases 2 x 1011 prokaryote-sized beads at the most sensitive position during drilling, and the tests on microbiological samples showed no beads or, at most, one bead in the >60 microscopic fields of view routinely scanned. The extensive contamination tests applied at this site thus confirm the high quality of microbiology samples that can be taken by careful techniques from APC cores without visible disturbance.