Previous   |    Next

doi:10.2204/iodp.proc.342.109.2014

Geochemistry

The shipboard geochemistry program during operations at Site U1408 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 U1408A; and

• Inorganic carbon, total carbon, and total nitrogen determinations of solid sediment samples from multiple holes.

Headspace gas samples

Headspace gas samples for routine safety monitoring were collected at a typical frequency of one sample per core in Hole U1408A (Table T18), usually in the bottom half of each core (i.e., Section 4, 5, or 6). Methane was the only hydrocarbon detected in measurable amounts. Methane concentrations increase from 1.9 to 5.9 ppmv in the upper 150 mbsf of Hole U1408A. After reaching peak concentrations at 150 mbsf, methane decreases to 2.1 ppmv between 150 and 260 mbsf.

Interstitial water samples

Twenty-six interstitial water samples were squeezed from whole-round samples taken at a typical frequency of one per core in Hole U1408A (Table T19). 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 in the case with Core 342-U1408A-22X, which was too disturbed for collection of a whole-round sample.

Results

The interstitial fluid profiles of sulfate, alkalinity, and ammonium in Hole U1408A reflect typical changes associated with organic carbon cycling (Fig. F27). The downhole pH and salinity profiles are near uniform with no substantial inflections. In contrast, alkalinity shows strong increases downhole, from <3.3 mM in the uppermost 50 mbsf to a maximum of ~5.5 mM at 150–180 mbsf and then declining to ~4.5 mM at the base of the recovered section. Chloride and sodium show very limited variations in interstitial water, with very slight overall decreasing trends downhole.

Manganese concentrations decrease rapidly with depth in lithostratigraphic Units I and II (see “Lithostratigraphy”) to 30 mbsf and then increase gradually to a maximum value of 10 μM at 135 mbsf. The downhole trend reverses below this depth and decreases to 2 μM at the bottom of the recovered sequence.

Iron concentrations show three distinct peaks in the recovered sequence. First, there is a broad peak from 50 to 90 mbsf with maximum concentrations of 30–40 μM. The second and third peaks occur at 135 and 190 mbsf with concentrations reaching 20 μM in both instances.

Sulfate concentrations are 30 mM near the top of the recovered sequence. The downhole profile shows gently decreasing sulfate concentrations with no major inflections or correlations to lithostratigraphic unit boundaries. Overall, the sulfate concentrations at Site U1408 are high (average = 26.9 mM), suggesting that the influence of organic matter respiration within the sediment column at Site U1408 is modest.

The downhole ammonium profile is more variable than that documented at previous Expedition 342 sites. It is possible, however, to discern two main maxima, one at 25 mbsf (40 μM) and another at 175 mbsf (78 μM).

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

Calcium concentrations in Hole U1408A show an increasing downhole trend in the upper 180 mbsf. Below this depth, the downhole calcium concentration profile becomes more or less uniform at ~16 mM.

Magnesium and potassium concentrations show decreasing downhole trends in the upper 180 mbsf. Below this depth, the downhole magnesium concentration profile becomes more or less uniform at 45 mM. Mg/Ca ratios show a remarkably gentle decline to 180 mbsf, where the ratios become uniform at ~2.8.

Sodium and chloride concentrations range from 460 to 470 mM and 520 to 560 mM, respectively, and are highly variable, with no discernible downhole trends that correspond to lithostratigraphic units.

Boron concentrations are ~400 μm in lithostratigraphic Units I and II and then increase to a broad maximum of 480 μm. Between 20 and 70 mbsf, boron concentrations show an overall downhole decrease to 310 μm at 205 mbsf.

Discussion

The downhole patterns of alkalinity, manganese, and iron suggest two zones of organic matter degradation within the recovered sequence. The typical succession of electron acceptor use during early diagenesis is manganese, followed by iron then sulfate (cf. Berner, 1980). Elevated concentrations of manganese coupled with iron concentrations of 0 μM within the uppermost 30 mbsf of the sediment column indicate ongoing oxic to suboxic diagenesis driven by microbial reduction of ferromanganese oxide. Below 30 mbsf, the reduction of iron oxide is evident from the rapid increase in iron concentrations. The broad peak in iron between 40 and 90 mbsf suggests a locus of modest organic matter consumption within this depth. Little change in sulfate concentrations and increasing alkalinity concentrations beginning at 50 mbsf corroborate this interpretation.

Increasing manganese concentrations below 30 mbsf point to the existence of a deeper manganese source. Further reduction of ferromanganese oxide occurs deeper in the sequence, likely around 120–130 mbsf, where interstitial water iron concentrations peak at 125 and 190 mbsf. This portion of the sequence would appear to be a reservoir of relatively high organic carbon contents (Fig. F28), and the interstitial fluid profiles reflect the diffusional signal following the degradation of this reservoir, but the interstitial water sulfate profile shows that this reservoir is not a zone of active sulfate reduction. It is also possible that these strata are richer in ferromanganese oxide.

Site U1408 downhole trends in potassium, calcium, magnesium, and Mg/Ca ratios reflect diffusion profiles resulting from calcium-magnesium exchange associated with alteration of volcanic basement (Gieskes and Lawrence, 1981), with potassium and magnesium decreasing and calcium slightly increasing with depth (Fig. F27). Inflections in calcium and alkalinity at ~180 mbsf indicate possible dissolution/reprecipitation of carbonate, which is high in the nannofossil chalk that comprises the basal lithostratigraphic Unit IV (see “Lithostratigraphy”).

Laboratory experiments under regulated temperatures and pressures have shown that boron is leached from terrigenous sediment into fluids (James et al., 2003), and a study of Ocean Drilling Program Leg 186 interstitial water samples concluded that boron removal from clays and volcanic ash accounted for boron enrichment in interstitial water (Deyhle and Kopf, 2002). Therefore, the broad downhole peak in interstitial water boron concentrations at 20–70 mbsf at Site U1408 presumably indicates increased supply from the terrigenous sediment components in lithostratigraphic Units II and III (see “Lithostratigraphy”).

Sediment samples

Sediment plugs (5 cm³) for downhole analysis of sediment elemental geochemistry were taken from Cores 342-U1408A-1H through 27X at an average resolution of one sample per section, adjacent to the moisture and density (MAD) samples (Table T20).

Results

Concentrations of inorganic carbon vary from 0.12 to 10.9 wt% in Hole U1408A (Table T19; Fig. F28). These concentrations are equivalent to 0.6 to 90.5 wt% CaCO₃, assuming that all of the carbonate is calcite. Total organic carbon (TOC) values are typically 0–0.5 wt%, excluding data from the 30.96–89.44 mbsf interval, which were run with a different calibration standard and thus show much higher (>1 wt%) and very likely incorrect values. Of the data plotted, TOC contents are slightly higher (0.4 wt%) in the 100–150 mbsf interval than in the underlying and overlying sediment. Total nitrogen values range from 0 to 0.15 wt%.

Carbonate concentrations are roughly 40 wt% in lithostratigraphic Unit I and decrease to 0–10 wt% in Unit II, which is consistent with low carbonate levels observed in other Oligocene age sequences recovered during Expedition 342. In the expanded middle Eocene sequence represented by Unit III, carbonate content values are 40–50 wt%, with a few peaks to 80–90 wt%. Carbonate increases to 90 wt% in Unit IV, which corresponds to early Eocene sediment.

Discussion

Moderately high carbonate contents throughout the recovered sequence are consistent with the relatively shallow paleodepth of the site throughout the Eocene. The most prominent change in carbonate content is a step increase associated with the downhole transition from dominantly nannofossil clay to nannofossil chalk at the lithostratigraphic Unit III/IV boundary (from ~50 to >80 wt% CaCO₃; ~225 mbsf). This step increase is typical of the pattern seen in lower to middle Eocene boundary sequences recovered at Expedition 342 sites and reflects a transition from pelagic chalk sedimentation to clay deposition in the initial stages of sediment drift development. An unconformity is present between middle Eocene sediment of Unit III and lower Eocene sediment of Unit IV, precluding further interpretation at this stage.

Top of page   |    Previous   |    Next