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doi:10.2204/iodp.proc.342.110.2014

Geochemistry

The geochemistry program during operations at Site U1409 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 U1409A;

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

• Characterization of organic matter by source-rock pyrolysis.

Headspace gas samples

Headspace gas samples for routine safety monitoring were collected typically at a frequency of one sample per core in Hole U1409A (Table T19), generally in the bottom half of each core (i.e., Sections 4, 5, or 6). Methane increases very slightly downhole, with values between 1.79 and 6.43 ppmv. Higher molecular weight hydrocarbons were not detected in measurable amounts.

Interstitial water samples

Eighteen interstitial water samples were squeezed from whole-round samples, which were typically taken at a frequency of one per core in Hole U1409A (Table T20). 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 Cores 342-U1409A-15H and 16H, which were too short to collect a whole-round sample.

Results

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

The pH profile decreases uniformly throughout the hole from 7.6 at the seafloor to 6.9 at 200 mbsf (Fig. F32). Alkalinity decreases in lithostratigraphic Units I and II (0–35 mbsf) from 3.7 to 2.9 mM and then increases uniformly downhole to 4.1 mM. Ammonium ranges from ~10 to ~75 µM, with an average downhole trend of ~20 µM in Unit I through Subunit IVa (0–124 mbsf) and an average trend of ~45 µM in Subunits IVb and IVc. Manganese shows a characteristic, lithology-correlated profile of relatively high (>18 µM) values in Unit II, low values (<18 µM) in Unit III and Subunit IVa, and again higher values in Subunits IVb and IVc. Iron is below detection limit except for two samples with values of <5 µM. Sulfate increases from 28 to ~30 mM in Unit II and then decreases uniformly to ~23 mM at the bottom of the hole, except for Subunit IVa, where sulfate shows increased values. Overall, the sulfate concentrations at Site U1409 are high (average = ~27 mM), suggesting that the influence of organic matter respiration within the sediment column at Site U1409 is modest.

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

Calcium concentrations at Site U1409 uniformly increase downhole from lithostratigraphic Unit I through Subunit IVa (0–124 mbsf), where a step increase from 15 to 18 mM occurs. Below this depth, through Subunits IVb and IVc, calcium concentrations increase further to ~20 mM at the bottom of the hole. Magnesium mirrors the calcium profile, with slightly decreasing values in Unit I through Subunit IVa, where a step decrease from ~51 to ~44 mM occurs. Below this depth, magnesium concentrations decrease further to 40 mM at the base of the hole. Magnesium/calcium ratios show a gentle decline from 4.5 near the top of the core to ~3.0 at the Subunit IVa/IVb boundary and then decrease further to ~2.0 at the base of the hole.

Potassium concentrations decrease downhole throughout the hole. Throughout the sequence, sodium and chloride concentrations covary. Sodium concentrations range from 480 to 505 mM. Chloride concentrations range from ~580 to ~620 mM, with an overall increasing downhole trend.

Interstitial water boron concentrations show a broad maximum of ~550 µM within Unit III at ~65 mbsf and significantly lower values of ~300–350 µM in Unit IV at depths greater than 130 mbsf.

Discussion

Interstitial water profiles display evidence of compartmentalization with pronounced abrupt downhole shifts in magnesium, manganese, and potassium at ~125–130 mbsf, suggesting that the unrecovered sequence of cherts acts as an aquiclude. Overall, interstitial water profiles of potassium, calcium, and magnesium are consistent with those resulting from exchange with and alteration of basaltic basement at depth (Gieskes and Lawrence, 1981). Potassium and magnesium concentrations decrease and calcium concentrations increase with depth (Fig. F32). Inflections at ~180 mbsf correspond to an increase in alkalinity and indicate possible dissolution/reprecipitation of carbonate, which is high in the nannofossil chalk that comprises the basal litho-stratigraphic Subunit IVc (see “Lithostratigraphy”).

The downhole patterns of alkalinity and manganese 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 and then sulfate (cf. Berner, 1980). Elevated concentrations of manganese in the upper 30 m coupled with iron concentrations of 0 µm within the upper 50 m of the sediment column indicate oxic to suboxic diagenesis driven by microbial reduction of manganese oxides. The manganese maximum is coincident with an interval comprised of disseminated manganese nodules as well as dispersed sulfides. Further reduction of manganese oxides occurs below the aquiclude between ~130 and 140 mbsf. This portion of the sequence was likely a reservoir of relatively high organic carbon contents, and the interstitial fluid profiles reflect the diffusional signal following the degradation of this reservoir. The degradation of this reservoir of organic matter below the chert aquiclude resulted in sulfate reduction, as evidenced by the concomitant decrease in sulfate concentrations.

Laboratory experiments under controlled temperatures and pressures have shown that boron is leached from terrigenous sediment 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 the interstitial water (Deyhle and Kopf, 2002). Therefore, the broad downhole maximum in concentrations at Site U1409 at 59 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-U1409A-1H through 24X at an average resolution of one sample per section, adjacent to the moisture and density samples (Table T21).

Additional sampling commenced at roughly 40 cm intervals in Cores 342-U1409B-18X and 342-U1409C-20X to resolve high-amplitude features in carbonate contents associated with lithologic changes associated with the PETM (see “Biostratigraphy” and “Lithostratigraphy”). Samples were also taken at ~20–40 cm spacing from Core 342-U1409C-6H over a series of decimeter-scale dark–light intervals in Sections 342-U1409C-6H-3 through 6H-5.

Results

Concentrations of inorganic carbon vary from 0.04 to 12.4 wt% in Holes U1409A–U1409C (Table T21; Fig. F33). These concentrations are equivalent to 0–93 wt% CaCO₃, assuming that all of the carbonate is calcite.

Carbonate concentrations are between 5 and 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 age sequences recovered during Expedition 342. In the expanded middle Eocene sequence represented by Unit III, the alternating clay-rich bedding and white nannofossil ooze layers have fluctuating carbonate values of ~30 to ~80 wt%, with a few peaks of ~85 wt% carbonate. The clay and ooze layers are further differentiated by physical properties including color reflectance (L*), magnetic susceptibility, and NGR. Carbonate increases to 90 wt% in Unit IV, which corresponds to early Eocene age sediment. A further decrease to 20 wt% at ~154 mbsf is associated with a ~45 cm thick silicified interval representing at least a portion of the PETM. Placement is based on the occurrence of nannofossil excursion taxa (nannofossil Zone NP9b; see “Biostratigraphy”). The downhole carbonate decrease is associated with a decrease in sediment lightness (L*) and increase in magnetic susceptibility (Fig. F34). In general, NGR and L* show five major peaks at ~38, 47, 70, 100, and 155 mbsf, all of which correlate with the major variations in calcium carbonate content.

Total organic carbon (TOC) values are typically 0.1–0.3 wt% throughout the sediment column. TOC values, together with total nitrogen values, appear to decrease downhole from lithostratigraphic Unit III to IV. Organic matter is thermally immature and relatively well preserved with low T_max values (380°–420°C). Organic matter is a mixture of Type II (algal and microbial) and Type III (land plant/detrital) kerogen. (Fig. F35).

Discussion

As with other sites drilled to date on the Southeast Newfoundland Ridge (Sites U1407–U1409), the most prominent change is a step increase in carbonate (from 50 to 90 wt% CaCO₃; ~100 mbsf) in sediment during nannofossil Zone NP14 (around the early/middle Eocene boundary). This step correlates with shifts in several proxies (e.g., color reflectance, magnetic susceptibility, NGR, TOC, and total nitrogen values) and marks a transition from pelagic chalk sedimentation to clay deposition in the initial stages of sediment drift development.

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