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doi:10.2204/iodp.proc.342.105.2014

Geochemistry

The geochemistry program during operations at Site U1404 included

• Hydrocarbon analysis on headspace gas samples,

• Analysis of interstitial water constituents (i.e., major and minor elements in whole-round and Rhizon samples), and

• Elemental geochemistry of carbon and nitrogen on solid sediment samples.

Headspace gas samples

As part of the routine environmental protection and safety monitoring program, evolved gases from 35 headspace gas samples were analyzed at a frequency of one sample per core in Hole U1404A (Table T18), usually from the bottom half of each core (i.e., Sections 4, 5, or 6). Methane concentrations are 1.75–28.8 ppmv. In the upper 212 mbsf, methane concentration reaches a maximum of 4.24 ppmv. Below 212 mbsf (Core 342-U1404A-25H), methane and ethane concentrations increase to ~28.8 and ~1.79 ppmv, respectively.

Interstitial water geochemistry

Interstitial water samples were typically taken at a frequency of one per core (Tables T19, T20) immediately after sectioning on the catwalk, although some cores were too disturbed or recovery was too low to allow a whole-round sample to be taken for analysis. Manual titrations of alkalinity, pH, and chlorinity were routinely performed on all samples from Hole U1404A. Ion chromatography analysis of interstitial water splits included Ca²⁺, Cl^–, Mg²⁺, K⁺, Na⁺, and SO₄^2– measurements using standard calibration and drift correction techniques. Routine inductively coupled plasma–atomic emission spectroscopy sample measurements included Fe, Mn, and Sr (Fig. F28).

Alkalinity and pH measurements show gradual increasing and decreasing trends, respectively, in the upper 230 mbsf of Hole U1404A, perhaps reflecting the decomposition of organic matter by the Mn oxide reduction pathway. Salinity, chloride, and sodium show little downhole change until 230 mbsf. pH and alkalinity, along with salinity, chloride, and sodium, all show marked inflections around 230 mbsf, corresponding to the ooze–claystone transition near the lithostratigraphic Unit III/IV boundary (see “Lithostratigraphy”) and may reflect carbonate recrystallization. The lowermost 75 m of Hole U1404A is very biscuited and fractured; thus, the interstitial fluid profiles below ~225 mbsf were possibly also influenced by drilling.

Sulfate concentrations show a modest and steady decline with increasing depth through Hole U1404A, whereas manganese concentrations rapidly and steadily increase downhole. Iron concentrations are low and variable. These profiles suggest sedimentary diagenesis driven by organic matter degradation. For example, the typical sequence of electron acceptor use is manganese, followed by iron, then sulfate. The modest downhole decrease in interstitial sulfate together with the low iron and high manganese argues for a sedimentary redox sequence that does not progress beyond manganese oxidation. Interstitial fluid geochemistry is not driven to sulfate reduction because of low organic carbon contents (cf. Emerson and Hedges, 2003.) An unsampled deep zone of reduction leading to sulfate depletion possibly has resulted in downward diffusion of sulfate (e.g., Berner, 1980).

Calcium concentrations steadily increase with depth in Hole U1404A, whereas magnesium concentrations decline rapidly in the upper 100 m and then taper to a steady decline to the bottom of Hole U1404A. Strontium concentrations show a near-uniform profile downhole to the lithostratigraphic Unit III/IV boundary (230 mbsf) and then increase below the ooze–claystone transition). Anomalously low strontium concentrations occur between 50 and 70 mbsf. Whether these anomalies are related to variations in the interstitial fluid profiles or to issues occurring during the analysis of these samples is not known. Overall, however, the increase in strontium concentrations is similar to that seen at Site U1403, although the profile at Site U1404 is more linear. A concomitant change in Sr/Ca ratio does not occur below the Unit III/IV boundary, suggesting that carbonate recrystallization (cf. Baker et al., 1982) has a negligible effect on interstitial water calcium concentrations. It is more likely that either dolomite deposition, exchange reactions with the basaltic basement rocks (Gieskes and Lawrence, 1981), or secular variability in the calcium concentrations of seawater (Stanley and Hardie, 1998; Hardie, 1996) are responsible for the downhole increase in calcium concentrations and declining Sr/Ca ratio. Interstitial fluids show decreasing Mg/Ca ratios with depth, consistent with basaltic basement exchange, yet this change is largely driven by increased calcium concentrations rather than decreased magnesium concentrations. This argues for carbonate recrystallization as the dominant driver of interstitial fluid profiles. Potassium concentrations decrease from maximum values of 16 mM in Subunit IIb to minimum values of 6 mM at the bottom of Hole U1404A. Although there are some prominent inflections (e.g., 100 and 140 mbsf), these changes do not strictly correspond to lithostratigraphic unit boundaries) and may reflect continual sorption onto clay minerals (Arthur, 1979).

Collectively, the interstitial fluid chemical profiles may reflect a combination of secular variations in seawater chemistry as well as postdepositional modifications. The modifications include those associated with organic matter degradation, sorption/desorption of chemical components on the surfaces of clay minerals, and chemical exchange with minerals undergoing diagenetic reactions.

Testing for the presence of gas hydrates

Hole U1404C was drilled and sampled to test for the presence of gas hydrates, the occurrence of which was suggested by several peculiarities in Cores 342-U1404B-4H and 5H (e.g., the presence of effervescent sediment, expelled section caps indicating high pressure, bulging core liners, a drop in bulk density, and the presence of pyrite). After scanning the core on the catwalk with a thermal imaging device, headspace gas samples and 5 cm whole rounds for interstitial water samples were taken at a sampling interval of one per section (Table T20). After logging Cores 342-U1404C-2H through 4H on the Special Task Multisensor Logger (STMSL), Rhizon samples were taken at an interval of 50 cm through Hole U1404C. Methane concentrations (1.82–2.99 ppmv) were not above atmospheric levels, and no other hydrocarbon traces were detected.

In Hole U1404C, 28 interstitial water samples were taken from 5 cm whole rounds in Cores 342-U1404C-2H through 4H at a sampling frequency of one sample per section to test for the existence of methane hydrates. In addition to squeezing whole-round samples for interstitial water analysis, 54 Rhizon samples were taken at a resolution of 50 cm in Cores 342-U1404C-2H through 4H. Manual titrations for pH, alkalinity, and chlorinity were performed on Hole U1404C interstitial water whole-round samples (Fig. F29). Uniform profiles, particularly of chlorinity, show that pore fluid geochemistry was not influenced by a shallow deposit of methane hydrate. Based on the headspace analysis of gas compounds (see “Headspace gas samples”) and interstitial fluid data from whole-round samples, a decision was made to archive the high-resolution Rhizon samples from Hole U1404C without analyzing their chemical components onboard.

Sediment geochemistry

Sediment plugs (5 cm³) for downhole analysis of sediment elemental geochemistry were taken from Cores 342-U1404A-1H through 36X at an average resolution of one sample per section adjacent to the moisture and density sample (Table T21). Carbonate contents are generally low (<5 wt%) in the upper 200 mbsf, with prominent increases to ~10 wt% around 60, 75, 95, and 170 mbsf (Fig. F30). High-resolution sampling (20–40 cm spacing) was carried out on targeted intervals in Cores 342-U1404A-23H through 36X in order to resolve carbonate fluctuations in lithologically distinct portions of Holes U1404A and U1404B. Between 200 and 300 mbsf in Hole U1404A, carbonate contents oscillate between 0 and 57 wt% in multiple pulses of three high CaCO₃ values at 200–206, 208–213, and 239–245 mbsf. These carbonate peak values range from 17 to 57 wt%, with a few less pronounced events with peaks of as much as 14 wt% at 213–226 mbsf. The maximum carbonate value is 57 wt% at 202.75 mbsf, within the Eocene/Oligocene boundary transition (Fig. F31), and 265–279 mbsf, with two less distinct events between 213 and 226 mbsf.

The deepest carbonate pulse seen in Hole U1404A is centered on Chron 19n in the magnetostratigraphy of Hole U1404A (Fig. F32; see “Paleomagnetism”). The stratigraphic position of this pulse of high carbonate content is consistent with the timing of carbonate accumulation Event 3 identified by Pälike et al. (2012) in equatorial Pacific sediment (Lyle et al., 2002). Based on this interpretation, the low-carbonate interval immediately above Chron C19n likely corresponds to the MECO (e.g., Bohaty et al., 2009), marked by high temperatures and deep-sea carbonate dissolution. The interpretation of the low–carbonate content interval between 244 and 260 mbsf as corresponding to the MECO is supported by identification of the calcareous nannofossil D. bisectus (first abundant disappearance at 40.36 Ma) in Section 342-U1404A-28H-CC, the uppermost extent of an interval barren of calcareous nannofossils (see “Biostratigraphy”).

Total organic carbon (TOC) values in Hole U1404A increase from 0.1 to 0.4 wt% at the lithostratigraphic Subunit IIa/IIb boundary (see “Lithostratigraphy”) (Table T21). TOC remains at 0.4 wt% until ~150 mbsf, where TOC begins to decline, reaching 0.15 wt% at 200 mbsf. These TOC values persist until the bottom of Hole U1404A. High TOC values in Subunit IIb correspond to the interval of green and greenish gray carbonate-poor Oligocene to Miocene clay characterized by a low magnetic signature.

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