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doi:10.2204/iodp.proc.342.111.2014

Geochemistry

The geochemistry program during operations at Site U1410 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 U1410A;

• 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 U1410A (Table T18), 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 6.72 ppmv. Higher molecular weight hydrocarbons were not detected in measurable amounts.

Interstitial water samples

Twenty-three interstitial water samples were squeezed from whole-round samples, which were typically taken at a frequency of one per core in Hole U1410A (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 Cores 342-U1410A-10H and 22X, which were too disturbed.

Results

The pH profile decreases uniformly throughout the hole from ~7.5 at the seafloor to 6.8 at 254 mbsf (Fig. F24). The salinity profile is near uniform at 36–38. In contrast, alkalinity shows low values of ~3.5 mM at the core top to 5.6 mM at the base of lithostratigraphic Subunit IVb (254 mbsf).

Manganese concentrations display a complex profile, with values of ~4–8 μM in lithostratigraphic Units I and II (0–130 mbsf), a maximum of ~15 μM near the base of Unit III (190 mbsf), and values varying between 11 and 4 μM in underlying Subunits IVa and IVb (220–254 mbsf).

Sulfate concentrations decrease downhole from a maximum of 30 mM at the top of the recovered sequence (lithostratigraphic Unit I) to <22 mM at the base. Ammonium concentrations are highly variable but are lowest at the core top (40 μM) and highest at the base (155 μM) in Subunit IVb (254 mbsf). An intermediate maximum of 105 μM occurs in Unit I (15 mbsf) and in the middle of Unit III (120 μM at 130 mbsf).

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

Calcium concentrations in Hole U1410A show an increasing downhole trend, from lithostratigraphic Unit I through Subunit IVb (0–254 mbsf), in which a step increase from ~11 to ~24 mM occurs.

Magnesium concentrations mirror the calcium profile, which shows slightly decreasing values in Unit I through Subunit IVa where a step decrease from 54 to 39 mM occurs. Magnesium/calcium ratios show a smooth monotonic decline from ~4.9 at the core top to <2 at the base of the hole. Potassium concentrations show a decreasing downhole trend to a depth of 150 mbsf. Below this depth, the concentration profile is near uniform at ~11 mM.

Sodium concentrations range from 465 to 494 mM, with a weak overall increasing downhole trend. Chloride concentrations range from 535 to 580 mM, with no discernible downhole trend.

Interstitial water boron concentrations are low (~400 μM or less) at the core top and in lithostratigraphic Subunit VIa (220–240 mbsf) but rise to values between 450 and 580 μM in Unit II (30 mbsf) and Subunit IVa (210 mbsf). Otherwise, boron concentrations show a gentle decline downhole.

Discussion

The interstitial fluid profiles of sulfate, alkalinity, and ammonium in Hole U1410A reflect typical changes associated with organic carbon cycling. As with other sites drilled to date on Southeast Newfoundland Ridge (Sites U1407–U1409), interstitial water profiles display evidence of compartmentalization with pronounced abrupt downhole shifts in magnesium, manganese, and potassium at ~220–230 mbsf, suggesting that the unrecovered sequence of chert functions 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. F24). Inflections at ~240 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 lithostratigraphic Subunit IVb (see “Lithostratigraphy”).

Depth profiles of alkalinity and manganese concentrations are well correlated with each other. This covariation of alkalinity and manganese concentrations indicates modest organic matter degradation associated with manganese reduction. Alkalinity, manganese, and iron show multiple peaks through the recovered sequences, which implies the presence of complex diffusional processes and organic matter reservoirs and subsequent organic matter degradation in each peak. Peak concentrations of manganese (16 μM) and iron (80 μM) are attained in lithostratigraphic Unit III. Disseminated pyrite formation in Unit II is reflected in the low iron content (0–4 μM), indicating syndepositional removal under highly reducing conditions. Both redox-sensitive metals not only reflect the generally reducing character of the sediment but also the difference between the clay contents of Unit III and the rest of the remaining units (see “Lithostratigraphy”). Sulfate concentrations steadily decrease with depth suggesting diffusion of seawater sulfate rather than solely microbial sulfate reduction.

The boron profile shows low values (380 μM) in the top of the sedimentary section (0–30 mbsf) and then increases sharply to high values (500–600 μM) at 30–40 mbsf, from where values decrease downhole back to low levels (400 μM) at 240 mbsf and then increase one more time toward the bottom of the hole. The sharp downhole increase near the top of the hole occurs near the boundary between lithostratigraphic Units I and II (35 mbsf; see “Lithostratigraphy”), where physical properties profiles also show step changes. Unit I is an ~34 m thick succession of Pleistocene sediment with alternating reddish brown clay, gray to dark brown foraminiferal ooze, grayish brown foraminiferal sand, and occasional sand- to pebble-sized lithics. Unit II is an ~30 m thick succession of clay-rich sediment of early Miocene to Oligocene age. The boron increase at the bottom of the hole corresponds to Subunit IVb, consisting of (pinkish) white nannofossil chalk of Eocene age containing several chert beds.

Boron has been demonstrated in laboratory experiments and from field studies to be leached from terrigenous sediments into fluids (e.g., James et al., 2003; Deyhle and Kopf, 2002). The boron profile at Site U1410 suggests that the boron supply varied significantly in the different lithostratigraphic units.

Sediment samples

Sediment plugs (5 cm³) for downhole analysis of sediment elemental geochemistry were taken from Cores 342-U1410A-1H through 24X at an average resolution of one sample per section, adjacent to the moisture and density samples (Table T20).

Results

Concentrations of inorganic carbon vary from 0.04 to 12.4 wt% in Holes U1410A–U1410C (Table T20; Fig. F25). These concentrations are equivalent to 0.7–93 wt% CaCO₃, assuming that all of the carbonate is calcite.

Carbonate concentrations are 5–50 wt% in Pleistocene nannofossil ooze of lithostratigraphic Unit I and decrease to 0–20 wt% in upper Miocene to Oligocene clay of 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 (see “Lithostratigraphy”), the alternating clay-rich bedding and white nannofossil ooze layers have fluctuating carbonate values of 30–85 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 content increases to 90 wt% in Unit IV, which corresponds to early Eocene–age sediment.

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) kerogen.

Discussion

Based on shipboard age models, the low-carbonate interval (80–83 mbsf) immediately above Chron C19n corresponds to the MECO, which is widely regarded as the terminal Eocene hyperthermal event (e.g., Bohaty et al., 2009), marked by high temperatures and deep sea carbonate dissolution.

The striking intervals of rhythmic greenish gray nannofossil clay and white nannofossil ooze of middle Eocene age were also recovered at Southeast Newfoundland Ridge Sites U1407 (lithostratigraphic Unit III), U1408 (Unit III), and U1409 (Unit III). A similar succession, although diminished in its visual appearance, was also recovered in Oligocene–Miocene sediment at J-Anomaly Ridge sites (e.g., Site U1405).

As with other sites drilled to date on the Southeast Newfoundland Ridge (Sites U1407–U1410), the most prominent change in Hole U1410A is a step increase (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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