Long-term objectives and postcruise science for Leg 205 include documenting the chemistry of pore fluids and associated sediments at Site 1253 in order to (1) determine the distribution of key tracer elements and isotope ratios between pore fluids and the various sediment types, (2) quantify the fluxes of those key elements and isotopes into the subduction zone, (3) utilize these data as a starting point for establishing geochemical and material mass balances (including sediment underplating) through the subduction zone, and (4) better constrain the fluid flow system in the upper oceanic section inferred from Leg 170 results (Kimura, Silver, Blum, et al., 1997).
Shortly after coring, pore waters were squeezed from whole rounds (14-20 cm long) and analyzed for salinity, pH, alkalinity, Cl, SO4, Ca, Mg, K, Sr, Ba, Mn, Li, B, Si, and NH4 concentrations. Sodium was determined from the above data by charge balance calculations.
In conjunction with sediment lithology and bulk chemistry, pore water chemistry can often provide a very sensitive picture of in situ diagenetic reactions, the nature of diffusion of chemical species within the sediment column, and advective fluid flow. This is possible because the low concentrations of many elements in the pore fluids mean that changes too small to affect the sediment chemistry can produce significant changes in the fluid composition. This section first documents variations in pore water chemistry then discusses their implications for diagenesis. It concludes by examining the evidence from shipboard data for the character of the advective fluid with approximately seawater composition, derived from depth, presumably the upper oceanic basement.
Calcium and magnesium concentrations were determined by routine titrimetric methods, as well as by ICP-AES. Silica concentrations were determined both by ICP-AES and spectrophotometry. Comparison of the accuracy and precision of the analytical methods for determining Ca, Mg, and Si concentrations are reported in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. Sediment concentrations of Rb, Cs, Ni, and Ce could not be determined by the shipboard ICP-AES. Pore water Rb concentrations were below the ICP-AES detection limit in all samples. One pore water Rb sample, analyzed at 1:5 dilution, just reached the 50-ppm detection limit of the ICP-AES. The reasons for this high level are not understood. Subsequent samples were not analyzed for Rb.
Chloride concentrations were approximately equal to that of seawater, except for a slightly lower concentration (1.5% seawater dilution) encountered just above the gabbroic sill at 399 mbsf. The Cl concentration was slightly higher (0.6% above seawater concentration) just below the sill at 432 mbsf. Note that the Cl concentration profile above the sill is similar to the corresponding Cl concentration profile observed at Site 1039 (Fig. F70). The salinity is approximately that of seawater throughout the sediment cored at Site 1253 (Table T8).
Sodium concentrations and the Na/Cl ratios in Figure F70 are slightly higher than those in seawater and increase toward and below the sill, even though Na concentrations in the sediment are fairly constant with depth. The tephras identified just above and below the sill are an exception to this pattern because of their different original composition. Potassium concentrations decrease sharply immediately above the gabbro sill, which is accompanied by a very sharp decrease in Si concentrations. Note that K concentrations approach seawater value below the sill, forming a trend subparallel to that observed at Site 1039 but at greater depth (Fig. F70).
The SO4 profile at Site 1253 is almost identical to the corresponding profile observed at Site 1039 (Fig. F71). Sulfate concentrations are slightly less than that of seawater in the basal sediment section, in contrast to values <27 mM measured higher in the section (between 330 and 370 mbsf) during Leg 170 (Kimura, Silver, Blum, et al., 1997). The high SO4 concentrations approaching seawater value indicate that the pore waters are not completely reduced.
Alkalinity is slightly lower than seawater concentration within the sediments above the lower igneous unit and remains fairly constant with depth (2.00 to 2.06 mM). The NH4 concentrations above the gabbro sill decrease with depth, varying between 82.75 and 49 然. Below the sill, NH4 concentrations are lower and also decrease with depth from 58 然 at 432.1 mbsf to 47 然 at 437.0 mbsf (Fig. F71B).
Calcium, Mg, Mg/Ca, and Sr decrease with depth toward seawater values with similar gradients to those observed at Site 1039 (Fig. F72), although they are shifted to deeper levels. Immediately below the gabbro sill, Ca and Sr concentrations remain nearly constant rather than continuing to decrease. The Mg profile in pore waters taken from the basal sediments is similar to that observed at Site 1039 (Fig. F72B), although Mg concentrations are locally lower below the sill than above it. Barium concentrations decrease with depth above the sill; however, they are anomalously high considering the seawater-like sulfate concentrations within the sediment above and below the gabbro sill (Fig. F73A). Typically, at these sulfate concentrations, BaSO4 solubility would predict Ba concentrations in the 100- to 200-nM range; however, Ba concentrations are as high as 5 然. In addition, where pore water Ba concentrations are highest, sediment Ba concentrations are the lowest. The interstitial water (IW) sample immediately above the sill has a high Ba concentration (4.84 然, relative to 1.04 然 at shallower levels). Below the sill, Ba concentrations decrease with depth.
Silica concentrations in the pore waters are similar to those observed at Site 1039 and decrease with depth, despite the fact that diatoms are abundant in the basal sediment section (Fig. F73B; Table T9). There is a very sharp decrease in Si concentration in the sample above the sill (changing from 964 to 297 然 over 14 m), most probably reflecting the diagenesis of amorphous Si to opal-CT and/or quartz as indicated by the chert layers described in "Lithostratigraphy". The sample just below the gabbro sill also has a lower silica concentration but higher than just above the sill.
Iron and manganese concentrations are relatively low (Fig. F71C, F71D; Table T9). Concentrations generally increase with depth in the basal sediment section, most likely from dissolution of Fe-Mn oxyhydroxide hydrothermal particles, which slightly color these sediments. Iron concentrations, however, sharply decrease at the upper sill contact and between the sill and Subunit 4B. Iron concentrations reach a maximum dissolved value of 43.55 然 just below the sill, and Mn reaches a maximum value of 23.27 然 immediately above it.
The Li profile at Site 1253 (Fig. F73C; Table T9) shows increasing concentrations with depth. The gradient is similar to that from Site 1039 but shifted downward by 40 m, which may reflect the deeper level of the gabbro sill at this site. Below the sill, Li concentrations are comparable to those just above it. Although B was expected to behave similarly to Li, values appear to decrease slightly.
The lithologic variations observed in the cores (see "Lithostratigraphy") control the in situ reactions responsible for the gradients and offsets in the pore fluid profiles for many, although not all, elements. Several features in the pore water chemistry suggest a role for ash alteration and associated authigenic mineral formation, primarily zeolites, just above and below the sill. Higher Na and much lower K and Si are observed just above the sill. This is consistent with the uptake of these elements by the authigenic formation of zeolites and quartz, which are also observed lithologically. Chloride concentrations are slightly lower (1.5%) relative to seawater, which may reflect opal-A or clay dehydration reactions immediately above the sill. The implied liberation of Na, Ca, and Sr to the fluids suggests ash alteration and carbonate recrystallization. Just below the sill, the low Mg concentrations in the fluid and the constant Li gradient are consistent with the authigenic formation of more Mg-rich clay minerals associated with ash alteration. Also noted in the section is a systematic decrease in NH4, which may reflect decreasing organic matter content downsection.
The clear overall gradients with depth noted for Ca, Sr, SO4, Si, and Li are also striking. The gradients parallel those measured during and after Leg 170 (Kimura, Silver, Blum, et al., 1997) but are shifted deeper by ~40 m, thus maintaining the same depth relationship to the top of the sill. The gradients trend toward values typical of modern seawater, and element systematics are similar to those associated with the 87Sr/86Sr variations observed in Leg 170 samples and discussed in "Margin Hydrology" in the "Leg 205 Summary" chapter. These gradients supported in the basal sediment pore waters suggest communication with an advective flow system of approximately seawater composition at greater depth, perhaps in the upper basement. Such a flow regime would mine heat from the plate and thus may be responsible for the extremely low geothermal gradient at this site.