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doi:10.2204/iodp.proc.301.106.2005

Biogeochemistry

Site U1301, with a depth to basement of ~265 mbsf, lies along a buried basement ridge paralleling the Endeavor segment of the Juan de Fuca Ridge axis. Two basement exposures lie along this ridge near Site U1301: Baby Bare outcrop, located ~6 km to the south-southwest, and Mama Bare outcrop, located ~8 km north-northeast (see "Geologic context"). At Baby Bare outcrop, hydrothermal springs have been sampled, revealing a highly altered and warm fluid that circulates within basaltic basement (e.g., Mottl et al., 1998; Wheat et al., 2002). The source for this altered fluid is seawater, which is inferred to enter basement 52 km to the south-southwest at Grizzly Bare outcrop, another basaltic formation located along the same basement ridge (Wheat et al., 2000, 2002; Fisher et al., 2003). This along-strike pattern of hydrothermal circulation through the crust provides a potential habitat for a diverse biosphere not only within basaltic basement but also in the deepest sediments within a transitional environment between diagenetic- and water-rock-dominated chemical gradients.

Compositions of hydrothermal fluids within basaltic basement along this ridge between Baby Bare outcrop and Mama Bare outcrop have been documented extensively by sampling fluids directly from springs, boreholes, and surficial pore waters in zones of focused upflow (e.g., Wheat and Mottl, 2000; Wheat et al., 2000, 2002, 2004). Estimates of the composition of basaltic formation fluids were attempted using basal pore water gradients during Leg 168; however, fluids could have been affected by sampling artifacts caused by changes in pressure, temperature, and redox state. Potential artifacts have been determined for the major ions in seawater (Wheat et al., 2004). Concentrations for many trace elements are compromised by exposing the sediments to oxygen during pore fluid extraction. The introduction of oxygen to the sample creates the potential for reduced iron to oxidize and precipitate, affecting a variety of elements that coprecipitate and adsorb onto iron oxides. The sediment sampling and pore water extraction during Expedition 301 differed from that during Leg 168 in two important ways. First, we used an APC system to collect the sediment cores rather than the XCB or RCB systems. Second, to minimize potential artifacts caused by oxidation, whole-round sediment samples were placed into the squeezer within a nitrogen atmosphere. We report chemical data from 38 pore water samples, 45 gas headspace samples, and 30 solid-phase samples that were analyzed during IODP Expedition 301.

Results

Pore water analyses

Due to operational and time constraints, the sediment column in Hole U1301C was not continuously cored. At the end of the expedition we had some extra time, so we returned to Site U1301 to fill the gaps in the section (Hole U1301D). The results presented in this section integrate the data from Holes U1301C and U1301D (Table T10; Figs. F47, F48, F49, F50). For comparison, we also present pore fluid compositions from Leg 168 Site 1026 and spring fluid compositions from Baby Bare outcrop (Shipboard Scientific Party, 1997; Mottl et. al, 2000; Wheat and Mottl, 2000). Before selecting whole-round samples for pore water extraction, great care was taken to ensure that the samples consisted mostly of a fine-grained matrix whenever possible. One sample (301-U1301C-5H-2, 0–40 cm) consisted of sediment that was mostly mud in the lower section and mostly sand in the upper section. Both sections were squeezed and denoted as mud and sand in the table, respectively. Chemical analyses of these two samples are nearly identical. Sample 301-U1301C-19H-1, 110–150 cm, was a slurry of drilling fluids and sediment and was clearly altered by drilling. Data from this sample are not shown in the figures.

The majority of pore water chemical depth profiles from Site U1301 are similar to those from Site 1026. As observed in numerous DSDP and ODP holes drilled to basaltic basement, there are distinct biogeochemical zones identified on the basis of steep geochemical gradients at the seawater/sediment and sediment/basement interfaces. The gradients are particularly well defined in the dissolved sulfate, manganese, and iron profiles (Fig. F50). For example, sulfate concentrations decline from a seawater value of 28 mmol/kg at the sediment surface to ~0.4 mmol/kg at ~57 mbsf. At Site U1301, sulfate concentrations remain near zero to a depth of ~125 mbsf and then increase again to ~16 mmol/kg at 263 mbsf. This downhole pattern of sulfate concentrations indicates active sulfate reduction at depths of ~50 and ~125 mbsf and diffusive sources from bottom seawater and the basaltic formation fluid, respectively, as is the case at Site 1026. Concentrations of dissolved barium are high in between these depths (Fig. F50). The downhole pattern of methane concentrations (Fig. F50) indicates that methane is created at depths of 80–110 mbsf and diffuses to the overlying and underlying zones of active sulfate reduction.

Alkalinity, chlorinity, and ammonium profiles are also nearly identical at Sites 1026 and U1301 and have end-member compositions that approach those of spring fluids from Baby Bare outcrop at the sediment/basement interface. However, there are several significant differences in the profiles from these two sites for the minor elements, most notably for dissolved iron. Data from Site U1301 show a maximum iron concentration of 133 µmol/kg, compared to 14.8 µmol/kg at Site 1026. This highlights the importance of squeezing the sediment within a nitrogen atmosphere. Other differences between chemical profiles from Sites U1301 and 1026 exist for Mn, B, Sr, and Li. For these elements, the upper portion of the profiles are identical but differences occur within the basal sediments. These differences are likely caused by differences in composition (see "Lithostratigraphy") and not sampling artifacts, as was the case for iron. Although these elements are highly reactive within the sediment section, they do not rapidly oxidize like Fe, nor do they coprecipitate with Fe.

The carbon content of the pore water increases in the uppermost 40 m of sediment, reaching a maximum at 47 mbsf with values of 43.5 mmol/kg dissolved total carbon (TC), 24.3 mmol/kg dissolved inorganic carbon (IC), and 19.1 mmol/kg dissolved organic carbon (OC) (Table T10; Fig. F47). From 179 mbsf to the bottom of the hole at 260 mbsf, dissolved carbon concentrations are very low, ranging 0.57–1.79 mmol/kg TC, 0.01–0.76 mmol/kg IC, and 0.4–1.4 mmol/kg OC. The concentration profiles of TC, IC, and OC show the same trend as the alkalinity profile, which increases from bottom seawater of 2.5 mmol/kg to a maximum of 31.6 mmol/kg at 31.3 mbsf and then decreases to concentrations of ~0.33 mmol/kg (Fig. F47).

Gas analyses

Results from headspace gas analyses are listed in Table T11 and illustrated in Figure F50. At Site U1301, the depth profile of methane varies inversely with sulfate and indicates the presence of two sulfate/methane interfaces. Methane concentrations are low in the upper part of the sediment but increase sharply in the depth interval between 60 and 70 mbsf and reach a maximum at 111 mbsf. At and below 132 mbsf, concentrations of methane are near 0 mmol/kg. Here, a second, deep sulfate/methane interface occurs. Higher molecular weight hydrocarbon gases were not detected in samples from Site U1301.

The highest methane concentrations occur within the interval where sulfate is nearly depleted. This relationship is consistent with a methane source from microbiological production. Typically, substantial concentrations of methane are not produced until the dissolved sulfate concentration drops below 1–2 mmol/kg, which is close to the threshold value of 0.5 mmol/kg sulfate generally thought necessary for the accumulation of methane (Whiticar et al., 1994). Methane that diffuses out from the methane production zone is oxidized at the sulfate/methane interface by a consortium of archaea and sulfate-reducing bacteria (Boetius et al., 2000). The disappearance of almost all of the methane at the depths of sulfate depletion indicates that most of this methane is likely consumed by anaerobic methane oxidation. Consequently, methane concentrations remain low in zones without active methanogenesis.

Sediments

At Site U1301, the solid phase of the cored sediments has relatively low organic carbon, nitrogen, and hydrogen contents (Table T12; Fig. F51). Organic carbon contents are highest close to the sediment/water interface (0.9 wt%) but decrease rapidly and fluctuate around 0.3 wt% throughout the sediment column. Total nitrogen averages ~0.04 wt% and has a depth trend similar to organic carbon. In general, calculated atomic C/N ratios vary around a mean value of 7.7, which indicates organic matter of marine origin. However, there are some deviations from this general trend. C/N values increase in discrete sediment layers and reach values of up to 19.6 and 37.3 at sediment depths of 34.6 and 56.7 mbsf, respectively. These higher C/N ratios indicate significant input of terrestrial organic matter, likely a result of turbidite deposition. Overall, these findings agree well with previous observations at Site 1026 (Shipboard Scientific Party, 1997).

Throughout most of the cored sediment, inorganic carbon contents are low, with corresponding CaCO₃ values ranging from 0.75 to 5.7 wt% (Table T12; Fig. F51). Nonetheless, we find distinct layers with highly elevated carbonate contents below the postulated lower zone of anaerobic methane oxidation. CaCO₃ values increase to 7.6 wt% at 184 mbsf and reach 1.5 wt% at the sediment/basement interface. Both carbonate peaks coincide with elevated carbonate levels found at Site 1026 below the lower zone of anaerobic methane oxidation and at the sediment/basement interface (Fig. F49).

Discussion

Calculated compositions of fluids within uppermost basement derived from pore water chemical gradients in basal sediments from Expedition 301, in the absence of external sources or sinks, should equal those measured in spring fluid from Baby Bare outcrop and borehole fluids from Site 1026 (Wheat et al., 2004). There is interest in the basal gradients for determining fluxes to and from the sediment for elements and chemical species that are important to microbial processes. Examples of chemical species that can be utilized by microbial life are sulfate, iron, manganese, silicate, and phosphate. Fluids flowing past Baby Bare outcrop continue to undergo reaction in the basement and are exposed to diffusive exchange with the overlying sediment before arriving at Site U1301. We can use the calculated diffusive gradient for a range of ions to model the average fluid flux through the upper basement. Deviations from the modeled flux should be caused by reactions, either inorganic or microbially mediated, thus providing estimates for microbial consumption rates within the deep biosphere.

The chemical compositions for the major ions in pore waters at the base of the sediment column in Hole U1301C agree with measurements made from borehole fluids from Site 1026 and spring fluids from Baby Bare outcrop. Concentrations of Na, K, Ca, and Mg in the deepest sample differ by <7 mmol/kg from those measured at Baby Bare springs. Sulfate and chlorinity concentrations in the deepest samples are even closer to those of Baby Bare springs—1.7 and 0.1 mmol/kg, respectively. One of the hopes for these data and interpretations is to obtain better estimates for the composition of minor and trace elements in the formation fluid. Initial interpretations for Si, ammonium, and Mn are consistent with a better approximation to the spring and borehole compositions compared with those from Site 1026. In contrast, other basal gradients of minor elements from Site U1301 (e.g., Sr and Li) point to different values.

Evidence of microbial activity at the sediment/basement interface is consistent with elevated concentrations of Fe and Mn near this interface. For example, in the deepest sample concentrations of dissolved iron increase to 9.8 µmol/kg and concentrations of dissolved Mn rise to values >150 µmol/kg. The observed gradients are much higher than those recorded from Site 1026, probably a result of differing sampling techniques. In contrast, sulfate gradients from both sites appear to be similar in profile and demonstrate the existence of a deep zone of anaerobic methane oxidation and sulfate reduction that is maintained by sulfate diffusion from sulfate-rich hydrothermal fluids from the sediment/basement interface.

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