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doi:10.2204/iodp.proc.342.112.2014

Geochemistry

The geochemistry program during operations at Site U1411 included

• Analysis of interstitial gas compounds on headspace samples;

• Measurement of minor and trace element concentrations in interstitial water squeezed from whole-round samples from Hole U1411B; and

• Inorganic carbon, total carbon, and total nitrogen determinations of solid sediment samples from Hole U1411B.

Headspace gas samples

Headspace gas samples for routine safety monitoring were collected typically at a frequency of one sample per core in Hole U1411B (Table T15), generally in the bottom half of each core (i.e., Sections 4, 5, or 6). Methane increases very slightly downhole, with values between 2.11 and 4.12 ppmv. Higher molecular weight hydrocarbons were not detected in measurable amounts.

Interstitial water samples

Seventeen interstitial samples were squeezed from whole-round samples that were typically taken at a frequency of one per core in Hole U1411B (Table T16). Whole-round samples were collected immediately after the cores were sectioned on the catwalk. In some cases, disturbed cores or low recovery precluded whole-round sampling, as was the case with Cores 342-U1411B-1H, 12H, 19X through 22X, and 27X, which were either too disturbed to collect a whole-round sample or were not recovered.

Results

Salinity, pH, alkalinity, ammonium, manganese, iron, and sulfate

The interstitial fluid profiles of sulfate, alkalinity, and ammonium in Hole U1411B reflect typical changes associated with organic carbon cycling (Fig. F21). The downhole pH profile decreases from 7.9 to 7.2 over 8–85 mbsf. Below 85 mbsf, the downhole trend becomes more uniform to 234 mbsf, with pH decreasing and salinity profiles near uniform. Alkalinity decreases downhole 3.2 to 2.5 mM in the upper 170 m, with a further, more rapid decrease from 204 to 253 mbsf.

Manganese concentrations decrease rapidly with depth in lithostratigraphic Unit I and the uppermost part of Unit II (see “Lithostratigraphy”) to 47 mbsf and then remain within 3–5 μM to the bottom of the hole.

Sulfate concentrations are 28 mM near the top of the recovered sequence and decrease to ~20 mM at the base of the sequence.

Ammonium concentrations increase downhole from 8 to ~250 μM at the base of the sequence. Ammonium concentrations in Hole U1411B are among the highest measured during Expedition 342.

Calcium, magnesium, sodium, chloride, boron, and potassium

Calcium concentrations in Hole U1411B increase downhole from 8 to 21 mM throughout the sequence with no major superimposed inflections.

Magnesium concentrations are relatively uniform in the upper 95 m, with concentrations of 52 mM. From 114 to 253 mbsf, magnesium concentrations decrease from 52 to 40 mM. Below this depth, the downhole magnesium profile becomes near uniform at concentrations of 44 mM.

Potassium concentrations decrease downhole in the upper 180 m, with a pronounced minimum of 7.7 mM at 67 mbsf. Following the nadir of the excursion to minimum potassium concentrations, downhole potassium concentration decreases, with values falling from 11 to 6.7 mM.

Magnesium/calcium ratios show a steady decline from the top to the base of the sediment column.

Throughout the sequence, sodium and chloride show no discernible downhole trends. However, sodium and chloride concentrations are positively correlated throughout the sequence (R² = 0.76).

Interstitial water boron concentrations increase downhole to a broad maximum from 375 to 500 μM over the 0–100 mbsf interval and then show an overall decrease downhole to 300 μM near the bottom of the sequence (250 mbsf).

Discussion

Interstitial fluid constituents in Hole U1411B are consistent with consumption of organic matter under oxic to suboxic conditions. The typical succession of electron acceptor use during early diagenesis is manganese, followed by iron then sulfate (Berner, 1980). Elevated concentrations of manganese in the upper 30 m coupled with low iron concentrations within the upper 50 m of the sediment column indicate oxic to suboxic diagenesis driven by microbial reduction of manganese oxides. Iron oxide reduction increases below 50 mbsf, where manganese concentrations become uniform. High sulfate concentrations throughout the sequence indicate that interstitial fluid diagenesis does not proceed to sulfate reduction.

Overall, interstitial water profiles of potassium, calcium, and magnesium are consistent with those resulting from exchange through alteration of basaltic basement at depth (Gieskes and Lawrence, 1981). Potassium concentrations also indicate possible adsorption onto clay particles.

Laboratory experiments under controlled temperatures and pressures have shown that boron is leached from terrigenous sediments into fluids (e.g., James et al., 2003), and a study of Ocean Drilling Program Leg 186 interstitial water samples concluded that the removal of boron from clays and volcanic ash was responsible for boron enrichment in interstitial water (Deyhle and Kopf, 2002). Therefore, the broad downhole peak in concentrations at Site U1411 at 100 mbsf presumably indicates increased supply from the terrigenous sediment component in lithostratigraphic Unit II (see “Lithostratigraphy”).

Sediment samples

Sediment plugs (5 cm³) for downhole analysis of sediment elemental geochemistry were taken from Cores 342-U1411B-1H through 27X at an average resolution of one sample per section, adjacent to the moisture and density (MAD) samples (Table T17).

Results

Concentrations of inorganic carbon vary from 0.04 to 6.6 wt% in Holes U1411A–U1411C (Table T17; Fig. F22). These concentrations are equivalent to 0–60 wt% CaCO₃, assuming that all of the carbonate is calcite.

Carbonate concentrations are 5–50 wt% in lithostratigraphic Unit I and decrease to 0–20 wt% in Unit II, which is consistent with low carbonate levels observed in other Oligocene sequences recovered during Expedition 342. In the expanded lower Oligocene sequence, carbonate content increases to 60 wt%. Below the Eocene/Oligocene boundary in the lower part of Unit II, carbonate content decreases to 30 wt%. Thereafter, carbonate content fluctuates between 30 and 40 wt% through the base of the sequence.

Total organic carbon and total nitrogen content fluctuates between 0 and 0.6 wt% and 0 and 0.14 wt%, respectively.

Discussion

As with other sites at which the Eocene/Oligocene boundary was recovered (Sites U1404 and U1406), carbonate content shows the characteristic increase across the boundary followed by a decrease in carbonate within the lower Oligocene.

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