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doi:10.2204/iodp.proc.308.106.2006

Geochemistry and microbiology

Inorganic geochemistry

Interstitial water geochemistry

Interstitial water chemistry data for Site U1322 are listed in Table T12 and shown in Figures F26, F27, F28, and F29. Alkalinity is generally constant with minimal variation in concentration from 3.75 to 6.25 mM (Table T12). From the seafloor to ~34 mbsf (i.e., seismic Reflector S10), alkalinity increases to a maximum of 5.85 mM and then decreases to a minimum of 3.75 mM at ~85 mbsf (i.e., seismic Reflector S20). Between 90 and 103 mbsf (i.e., seismic Reflector S30), alkalinity once again reaches a local maximum of 5.6 mM. Below seismic Reflector S30, alkalinity increases slightly at first and then remains constant until the bottom of the core (Fig. F26). The pH of the interstitial water varies between 6.46 and 7.65. The acidic nature of the interstitial water (pH <7) above 100 mbsf at this site is similar to pH at Site U1324, but large pH fluctuations with several mimima and maxima occur within this part of the core. From 100 mbsf to TD, pH values are higher and average 7.2 ± 0.1 (Fig. F26).

Salinity varies between 3.3 and 3.8 parts per hundred (pph) (Table T12). Above 34 mbsf (i.e., seismic Reflector S10), salinity is higher, centered at 3.7–3.8 pph. Between 34 and 84 mbsf salinity decreases from 3.8 to 3.3 pph, and then it remains relatively constant downhole to the bottom of the hole. Interstitial water chlorinity varies from 542 to 578 mM. The seafloor chloride concentration is similar to the standard seawater value (International Association of the Physical Sciences of the Ocean [IAPSO] seawater standard = 559 mM), increases to 578 mM within the upper 15 mbsf (Fig. F26), and decreases back to approximately seawater chlorinity values within 50 mbsf.

Dissolved sulfate concentrations approximate standard seawater concentrations (28.9 mM) above 34 mbsf (i.e., seismic Reflector S10). Below this depth, sulfate concentrations rapidly decrease to a minimum of 0 mM at 75 mbsf and remain constant to TD (Fig. F27). The sulfate/​methane interface (SMI) depth is 74 mbsf. Interstitial water ammonium concentrations are very high, ranging from 428 to 4432 µM, and a general downhole increase is observed to 130 mbsf (i.e., seismic Reflector S30). Below seismic Reflector S30, ammonium contents are generally high and constant (~4000 µM) to 190 mbsf and then decrease to 2274 mM at 207 mbsf. Below this depth, ammonium concentrations remain constant to the bottom of the hole. Dissolved phosphate concentrations are very low (<4.0 µM), in particular above 34 mbsf (<2.8 µM) (Table T12). Below 34 mbsf to TD, several local phosphate maxima are observed (Fig. F27). Interstitial water data also show elevated concentrations of dissolved silica (H₄SiO₄) from 259 to 532 nM (Fig. F27).

Na⁺ concentrations do not vary significantly downhole. Mg²⁺ displays minimal downhole variation, with slightly higher concentrations above seismic Reflector S10 than those elsewhere. K⁺ concentrations decrease and Ca²⁺ concentrations increase significantly from the seafloor to ~34 mbsf (i.e., seismic Reflector S10). Below this interval to seismic Reflector S30 (165 mbsf), concentrations of K⁺, Mg²⁺, and Ca²⁺ decrease slightly. Below seismic Reflector S30, concentrations of these elements remain relatively constant to the bottom of the hole (Fig. F28).

Downhole variations of Li⁺, B³⁺, and Sr²⁺ concentrations are similar and generally increase to a maximum at ~34 mbsf (i.e., seismic Reflector S10) and then decrease toward seismic Reflector S30, although small local maxima are observed both for B³⁺ and Sr²⁺ within this interval (Fig. F29). Li⁺, B³⁺, and Sr²⁺ concentrations remain constant below seismic Reflector S30 to TD. Concentrations of Ba²⁺ are generally low (<1.3 µM) in depths shallower than 56 mbsf. The maximum concentration of 36.4 µM, at 75 mbsf (above seismic Reflector S20), decreases to 7.9 µM at 130 mbsf (i.e., seismic Reflector S30). Below this depth, Ba²⁺ contents are constant at ~10 ± 2 µM (Fig. F29). Dissolved Mn²⁺ concentrations rapidly decrease from a maximum of 61.2 µM near the seafloor and are generally <2 µM below 121 mbsf (Fig. F29). The interstitial water contains high concentrations of dissolved Fe²⁺ at shallow depths, with two local maxima of 351 and 476 µM at 8.5 and 84 mbsf, respectively (Fig. F29). High concentrations of dissolved Fe²⁺ caused iron oxyhydroxide precipitation (lepidocrocite, FeO[OH], as deduced from XRD analyses of the precipitates from the pore waters immediately after squeezing in the laboratory). Several interstitial water samples from Holes U1322C and U1322D were also analyzed, and the data generally fit with chemical pore water profiles for Hole U1322B. In summary, large pore water chemical variations are observed at shallow depths, in particular around seismic Reflector S10.

Solid-phase chemistry

Initial results for total inorganic carbon (TIC), calcium carbonate (CaCO₃), total organic carbon (TOC), total nitrogen, molar ratio of organic carbon to total nitrogen (C/N), and total hydrogen analyses on sediment squeeze cakes are listed in Table T13 and presented in Figure F30.

TIC contents range from 0.62 to 2.82 wt% (average = 1.74 wt%, which corresponds to an average CaCO₃ concentration of 14.54 wt%). The foraminifer-nannnofossil-rich clays within the uppermost 2–3 m of Site U1322 account for the peak maximum of 23.46 wt% in calcium carbonate. Inorganic carbon concentrations increase throughout lithostratigraphic Unit I to a maximum concentration of 2.82 wt% at ~130 mbsf. This peak concentration coincides with seismic Reflector S30 and the abrupt change from the clay-rich formation of lithostratigraphic Unit I to clay and mud interbedded MTDs in lithostratigraphic Unit II.

TOC contents generally decrease with depth, with local peak maxima at 10, 150, and 200 mbsf. TOC concentrations range between 0.26 and 1.64 wt% (average = 0.83 wt%). The spike in organic carbon at 10 mbsf corresponds with organic-rich black bands observed within the red and green color-banded clays of lithostratigraphic Subunit IA (see Fig. F7). Increases in TOC and decreases in TIC within lithostratigraphic Unit II at 150 and 200 mbsf are coincident with MTDs 2-2 and 2-5 in lithostratigraphic Unit II (see Fig. F6).

Total nitrogen concentrations, as a whole, are lower than concentrations observed at Ursa Basin Site U1324. Total nitrogen concentrations range from 0.05 to 0.21 wt% (average = 0.093 wt%). Peak concentrations in the total nitrogen curve occur opposite to local decreases in TOC in both lithostratigraphic Units I and II. Average total hydrogen contents vary between 0.01 and 2.84 wt% (average = 0.52 wt%).

C/N values at Site U1322 were the highest molar ratios observed during Expedition 308. C/N ratios range between 1.71 and 25.28 (average = 12.73). This broad range in ratios suggests input of organic matter from hemipelagic to predominantly terrestrial sources. C/N ratios center around an average value of 14.69 within lithostratigraphic Unit I, with rapid decreases at 20.9 and 44.2 mbsf and a spike in the ratio at 8.5 mbsf. C/N ratios exhibit far greater variability within the MTDs interpreted in lithostratigraphic Unit II. C/N minima at 143.8, 178.8, and 218.5 mbsf suggest greater relative input of hemipelagic organic matter, and maxima at 154.3, 186.7, and 199.9 mbsf imply higher amounts of terrestrial material.

Solid-phase initial interpretations

Site U1322 has the highest average TOC content among all Expedition 308 sites. Interbasinal differences in organic matter concentrations indicate greater accumulation of TOC toward the center of Ursa Basin at Site U1322 relative to Site U1324 along the flank of the basin. Organic carbon concentrations and C/N ratios at Site U1322 peak within the uppermost 8.5 m in lithostratigraphic Unit I and again at 154.3 and 199.9 mbsf, coincident with MTDs 2-2 and 2-5 in lithostratigraphic Unit II (see Fig. F6), respectively. MTDs 2-1, 2-3, and 2-7 within lithostratigraphic Unit II (see Fig. F6) are also associated with decreasing TOC concentrations and lower C/N ratios. The origin of sedimentary organic matter may be better assessed with carbon and nitrogen isotopic analyses (Jasper and Gagosian, 1990).

Organic geochemistry

Hydrocarbon gas composition

The vertical distribution of headspace methane and ethane is shown in Figure F31. Methane concentrations in the headspace volumes range between 1 and 51,001 ppmv. Rapid increases in methane concentrations begin in the middle of lithostratigraphic Unit I, whereas there are only minor amounts of methane from the seafloor to 60 mbsf. Methane was the predominant hydrocarbon species determined at Site U1322. Only very minor amounts of ethane (<3.4 ppmv) and ethylene (<2.6 ppmv) were detected in headspace samples (Table T14). No higher hydrocarbons were detected at Site U1322. Concentrations of methane fluctuate with depth, and the calculated SMI occurs at 74 mbsf (Fig. F32). The highest concentrations of methane were in the range of 29,536–51,001 ppmv between 75 and 129 mbsf. According to the lithologic description, this gas-bearing interval consists of mud and clay, both confined within lithostratigraphic Unit I. These strata are also positively correlated with slight increases of diagenetic ethane.

The maximum concentrations of hydrocarbons in expansion void gas (EVG) in the headspace sampled in Section 308-U1322B-14H-2 were as follows:

• methane = 707,229 ppmv,
• ethane = 38 ppmv,
• ethylene = 1 ppmv, and
• butane = 3 ppmv.

The concentrations of methane in EVG were one order of magnitude higher than those of the maximum range in headspace samples. When interpreting C₁/C₂ ratios or C_n/C_n+1, we need to consider that minor amounts of C₂ through C₅ compounds can also be generated in situ during early (low temperature) diagenesis of organic matter (e.g., Kvenvolden and Barnard, 1983). The C₁/C₂ ratios at Site U1322 are generally high, suggesting biogenic origin of the methane, possibly derived from in situ microbial activities or migrated from elsewhere. Hence, the almost exclusive presence of methane suggests that the hydrocarbon gases found at Site U1322 are of biogenic, not thermogenic, origin. Stable carbon isotopic ratios (δ¹³C) at the molecular level will help verify the origin of CH₄ in headspace and EVG.

Microbiology

Biomass enumeration

Microbial cell biomass in Hole U1322B was estimated using whole-round core samples (Fig. F33). The sampling frequency was every 4–5 m between 2.9 and 21.0 mbsf, every 7–11 m between 21.0 and 129.2 mbsf, and every 14–20 m between 129.2 and 231.5 mbsf.

A maximum cell density of 4.0 × 10⁵ cells/mL was observed at 2.9 mbsf. Microbial abundances decreased with depth; at 74.5 mbsf, biomass was below the cell enumeration confidence limit of 1.0 × 10⁴ cells/mL (Table T15). The extremely low cellular biomass at Site U1322 is consistent with microbial abundance levels at Site U1324. The microbial biomass in Ursa Basin is an order of magnitude less than cellular densities observed at Sites U1319 and U1320 in Brazos-Trinity Basin IV.

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