The main objectives for determining key geochemical parameters of the pore waters at Site 1255, ~1 km west of Site 1254 and closer to the deformation front, are the same as those for Site 1254 (see "Inorganic Geochemistry" in the "Site 1254" chapter), with particular emphasis on the determination of the key depth interval in which to place the long-term geochemical observatory. Limited core recovery, however, permitted only a few samples to be measured, and interpretations should be regarded as preliminary. A total of three 38- to 43-cm-long whole rounds were sampled at Site 1255 for pore fluid geochemistry. One whole round was taken per core from 134.2 to 152.2 mbsf, and pore water recovery ranged from 14 to 46 mL. Pore waters were analyzed for Ca, Mg, K, Na, B, Ba, Fe, Mn, Sr, Si, NH4, SO4, and alkalinity concentrations, as well as for pH and salinity by methods described in "Inorganic Geochemistry" in the "Explanatory Notes" chapter. Pore water geochemistry data is shown in Tables T7 and T8.
Lithium, Ca, K, Mg, and Na were analyzed in "real time" on the shipboard inductively coupled plasma-atomic emission spectrophotometer to facilitate identification of the zone of maximum fluid flow within the décollement, as was done at Site 1254. Potassium, Na, and Ca concentrations at Site 1043 (Leg 170) decrease abruptly at the top of the décollement, whereas Mg concentrations increase. These constituents show opposite gradients within the transition from the décollement to the underthrust sediments. Therefore, the objective of real-time chemical monitoring for these constituents was to quickly identify these concentration gradients to ensure that the underthrust sediments were not penetrated too deeply during coring, such that the long-term observatory will sample décollement fluid and not a mixture of décollement and underthrust fluids. Because of the limited amount of core recovered at Site 1255, the real-time chemical analyses were not as successful as those at Site 1254; however, alkalinity and SO4 could be determined fairly rapidly and were helpful in approximately determining the transition between the wedge sediments and underthrust sediments.
Salinity is below seawater concentration in all samples analyzed at Site 1255 (Table T7) and increases with depth from 32 (8.5% lower than seawater) at 134.2 mbsf to 34 (3% lower than seawater) at 152.2 mbsf in the underthrust sediments. The lithologic boundary between wedge sediments and underthrust sediments occurs at 144.08 mbsf (see "Lithostratigraphy"), and the lower boundary of the décollement occurs at the same depth (see "Structural Geology"). As at Sites 1254 and 1043, the low salinity signature of the fluid within the décollement zone is interpreted as the result of advection of a low-salinity deeply sourced fluid from greater depth.
Chloride concentrations increase with depth from 549 mM (~2% lower than seawater concentration) within the décollement at 134.2 mbsf to seawater concentration within the transition between the deformed wedge sediments and the sediments of the underthrust section (Table T7). The low-chloride signature within the décollement at Site 1255 is less pronounced than the low-salinity signature and is most likely the result of mixing with a low-chloride deeply sourced fluid as observed at Sites 1254 and 1043.
Within the décollement, the Na concentration was calculated assuming alkalinity is very low (Kimura, Silver, Blum, et al., 1997). The calculated value is ~6% lower than seawater concentration. Sodium concentration increases to approximately seawater value within the underthrust sediment section at 145.8 mbsf (Table T7). Potassium concentrations are above seawater concentration in all three samples and range between 11.18 and 13.42 mM. There was not a sharp decrease in K concentrations within the décollement as observed at Site 1043, which suggests that the samples from Site 1255 were not within the zone of the minimum concentration of 8.8 mM, observed at 142.45 mbsf at Site 1043.
Calcium concentrations are below seawater concentration and increase with depth from 4.66 mM (56% lower than seawater) at 134.2 mbsf to 8.45 mM (20% lower than seawater) at 152.2 mbsf. At Site 1254, Ca concentrations within the décollement were approximately twice seawater concentration suggesting migration of a deeply sourced fluid through conduits within the décollement and upper fault zone (see "Inorganic Geochemistry" in the "Site 1254" chapter). At Site 1255, pore waters from the limited amount of core recovered do not show evidence for the high-Ca deeply sourced fluid sampled at Site 1254 in the lower part of the wedge section. This difference between sites may simply reflect low resolution as a result of poor recovery, although mixing between pore fluids of the uppermost few meters of the underthrust hemipelagic sediment section and pore fluid from the lower wedge sediments is a possible explanation.
In the one sample recovered from the wedge sediments (134.2 mbsf), the Mg concentration is 38.08 mM (29% lower than seawater concentration). An increase in Mg concentrations to 46.05 mM at 145.8 mbsf reflects the transition between wedge sediments and the underthrust hemipelagic sediments. Also, the lower Mg concentration within the décollement provides evidence for infiltration of the deeply sourced fluid, sampled at Site 1254, into the décollement at Site 1255. Furthermore, the higher Mg concentrations in the décollement at Site 1255 compared to those at Site 1254 (16-18 mM) suggest mixing between pore fluids of the underthrust sediments with those of the lowermost wedge sediments, or they may be the result of low resolution resulting from poor core recovery.
Strontium concentrations are variable within the depths sampled at Site 1255 and range between 94.76 and 98.42 µM. (Table T8).
Sulfate concentrations are zero within the lower wedge sediments at Sites 1255 and 1043. Sulfate concentrations increase sharply between 134.2 and 145.8 mbsf from 0 to 8.01 mM, reflecting the transition to the underthrust sediment section. Correspondingly, methane concentrations are high throughout the wedge section and are at background levels below the décollement (see "Organic Geochemistry"). At Site 1255, the uppermost underthrust sediments have been isolated from the external supply of dissolved SO4 from seawater and the remaining internal supply of SO4 at the top of the section has been mostly but not completely depleted by bacteria to concentrations of 7.25 mM at 145.8 mbsf and 8.61 mM at 152.2 mbsf (25% to 30% of seawater concentration, respectively), reflecting the location of Site 1255 (closer to the deformation front than Site 1254) where SO4 in the underthrust sediments was completely exhausted. Alkalinity concentrations at Site 1255 are high and range from 18.422 mM (8 times seawater alkalinity) to 22.645 mM (~10 times seawater alkalinity), which contrasts with Site 1040, where the alkalinity values are low (~5 mM) in the lower wedge section and immediately above the décollement. These high concentrations most likely result from mixing with the high alkalinity pore fluids that are characteristic of the uppermost hemipelagic section at Site 1039.
Ammonium concentrations decrease with depth from 2943.1 µM at 134.2 mbsf to 643.1 µM at 152.2 mbsf, which may reflect decreasing sedimentary organic matter content with depth. Barium concentrations in pore fluids from the wedge sediments at Site 1255 are similar to those at Site 1254 (~9.3 µM Ba); however, Ba in the underthrust sediment pore fluids increases sharply to 160 µM at Site 1254, whereas Ba concentrations decrease abruptly to 3.2 µM at 152.2 mbsf at Site 1255. At Site 1254, SO4 depletion (i.e., SO4 = 0 mM) extends into the underthrust sediments, but at Site 1255, SO4 concentrations in the underthrust increase to 8.61 mM at 152.2 mbsf. Therefore, the decrease in Ba concentration within the underthrust sediment pore fluids at Site 1255 may be due to barite precipitation.
Iron concentrations are variable at Site 1255 and range between 3.34 and 8.34 µM. The highest Fe concentrations are present right below the boundary between the wedge sediments and underthrust sediments at a depth of 145.8 mbsf (see "Lithostratigraphy") and may represent bacterial reduction of Fe(OH)3 within the uppermost underthrust sediments. Manganese concentrations increase with depth and range between 2.97 µM at 134.2 mbsf and 26.08 µM at 152.2 mbsf. The increase in Mn concentrations reflects the transition between wedge sediments and underthrust sediments. Sulfate has not been totally depleted in the upper underthrust section as at Site 1254, and active reduction of MnO2 may be occurring, which may keep dissolved Mn concentrations relatively high.
Lithium concentrations decrease with depth from 32.89 µM (22% higher than seawater) at 134.2 mbsf to 26.69 µM (approximately seawater concentration) at 152.2 mbsf. The higher than seawater Li concentration observed within the wedge sediments is similar to that observed at the base of the décollement at Site 1254 and suggests infiltration of a deeply sourced fluid within the décollement at Site 1255. The decrease to approximately seawater concentration between 134.2 and 152.2 mbsf reflects the transition from wedge sediments to the hemipelagic sediments of the underthrust section.
Silica concentrations are relatively high at Site 1255 and increase with depth from 623.9 µM at 134.2 mbsf to 717.9 µM at 152.2 mbsf. The increase in Si concentrations with depth is due to the dissolution of diatoms and radiolarians that are abundant in the underthrust sediments but more rare in the décollement sediments. However, Si concentrations in the décollement at Site 1254 range from 51.4 to 61.4 µM, which is only 9% the concentration observed at Site 1255; Si concentrations within the underthrust sediments at Site 1254 range from 553 to 549 µM, which is 88% of the concentration observed in underthrust sediments at Site 1255. The discrepancy in Si concentrations between the two sites may reflect differing amounts of diatoms within the sediments (see "Lithostratigraphy," also see "Lithostratigraphy" in the "Site 1254" chapter) or may be due to mixing between pore fluids from the uppermost few meters of the hemipelagic sediments with pore fluid from the lower wedge sediments at Site 1255 but not at Site 1254.
Boron concentrations range from 352.9 µM (22% lower than seawater concentration) at 134.2 mbsf to 624.67 µM (39% higher than seawater concentration) at 152.2 mbsf, which reflects the transition from deformed wedge sediments to sediments of the underthrust section.
As we observed at Site 1254, the chemical compositions of the pore fluids at Site 1255 are distinctly different in the wedge and underthrust section, with a less-sharp transition at the base of the décollement zone at ~144 mbsf. Fluid flow is suggested by salinity, Cl, Na, and Mg concentration minima and a Li concentration maximum within the décollement sample at 134.2 mbsf. These concentration minima were observed at Site 1043 along with a Mg concentration maximum within the same interval. Across the décollement, the shift in concentrations of some chemical components is reversed from the gradients observed at Site 1254 (i.e., Ca and Mg). In general, the pore fluids within the upper fault zone and in the décollement at Sites 1254 and 1040 are characterized by having a significantly stronger signature of a deeply sourced fluid than the pore fluids from Sites 1255 and 1043 (see "Inorganic Geochemistry" in the "Site 1254" chapter). At Sites 1255 and 1043, the pore fluid chemistry in the wedge and the uppermost underthrust sediments can be attributed to sampling mixing between the lower wedge and uppermost hemipelagic pore fluids, therefore, partially obscuring the deeply sourced fluid signature observed at Site 1254.