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doi:10.2204/iodp.proc.342.107.2014

Geochemistry

The geochemistry program during operations at Site U1406 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 U1406A; 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 frequency of one sample per core in Hole U1406A (Table T18; Fig. F25), usually in the bottom half of each core (i.e., Sections 4, 5, or 6). Methane was the only hydrocarbon detected in measurable amounts. Methane concentrations increase from 1.4 to 3 ppmv in the upper 140 mbsf of Hole U1406A. After reaching peak concentrations at 140 mbsf, methane decreases to 1.6 ppmv between 140 and 290 mbsf.

Interstitial water geochemistry

Thirty interstitial water samples were squeezed from whole-round samples taken at a typical frequency of one per core in Hole U1406A (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-U1406A-24X and 25X, which had no recovery. In other instances, systematic whole-round sampling was temporarily suspended because of the suspected presence of critical lithologies or chronostratigraphic boundaries in the core. For example, we did not sample Cores 342-U1406A-21H and 22H, which contain an expanded lowermost Oligocene sequence (see “Biostratigraphy”).

Results

As with Sites U1402–U1405, manual titrations of pH and alkalinity were carried out on Hole U1406A interstitial water samples. Chloride concentration measurements by manual titration were suspended during operations at Site U1406 because results from ion chromatograph measurements of chloride concentration in Hole U1405A were of equivalent precision and accuracy.

The interstitial fluid profiles of sulfate, alkalinity, ammonium, and methane in the upper ~170 m of Hole U1406A reflect typical changes associated with organic carbon cycling (Figs. F25, F26). Downhole patterns of pH and alkalinity concentrations in Hole U1406A can be divided into two trends. First, from the top of Hole U1406A to ~180 mbsf, the boundary between Oligocene–Miocene nannofossil ooze of lithostratigraphic Unit II and Eocene–Oligocene nannofossil chalk of Unit III (see “Lithostratigraphy”), pH decreases from 7.5 to 7.0 and alkalinity concentrations increase from 3 to 5.8 mM. Second, below 180 mbsf, pH and alkalinity trends reverse, with pH increasing to 7.3 and alkalinity concentrations decreasing to 4 mM. Salinity shows a uniform downhole profile with no substantial inflections.

Interstitial fluid constituents that are sensitive to redox changes resulting from organic matter consumption (e.g., manganese and iron) show significant inflections at 170 mbsf, broadly coincident with the lithostratigraphic Unit II/III boundary and the reversal in pH and alkalinity trends described above. Manganese concentrations increase from 1 to 15 μM at the top of the sediment column at 170 mbsf and then decrease to 0 μM at the base of the recovered sequence. Iron concentrations are low (<2 μM) from 0 to 150 mbsf and then rise rapidly to peak concentrations of 8 μM from 150 to 170 mbsf, corresponding to maximum manganese concentrations. Iron concentrations fall again to <2 μM at 180 mbsf and below.

The downhole patterns of pH, alkalinity, manganese, and iron suggest a diagenetic sequence driven by organic matter consumption that does not progress beyond iron oxide reduction. The typical sequence of electron acceptor use during early diagenesis is manganese followed by iron then sulfate (cf. Berner, 1980). Sulfate concentrations remain relatively high within the depth range where manganese and iron reach their maximum concentrations (~170 mbsf). However, manganese and iron concentrations measured in Hole U1406A are lower than those at Sites U1403–U1405, suggesting that less organic matter has been consumed at Site U1406. Ammonium concentrations, which peak at relatively low values (10 μM at 170 mbsf; Fig. F26) compared to Sites U1403–U1405, corroborate this interpretation.

Calcium concentrations in Hole U1406A show an increasing downhole trend in the upper 180 mbsf (lithostratigraphic Units I and II). Below the sampling gap between 180 and 200 mbsf, calcium concentrations increase downhole to a maximum between ~150 and 180 mbsf, corresponding to a decrease in alkalinity and the lithologic transition from ooze to chalk (see “Lithostratigraphy”). Calcium concentrations then slightly decrease below 240 mbsf, the depth that marks the lithostratigraphic Unit III/IV (nannofossil chalk to nannofossil chalk with foraminifers) boundary.

Magnesium concentrations gradually decrease in Hole U1406A within the upper 180 mbsf. Across the sampling gap (180 mbsf), magnesium concentrations drop rapidly from 51 to 46 mM. Below 210 mbsf, the magnesium concentration depth gradient reverses, with magnesium concentrations reaching 49 mM at the bottom of Hole U1406A. Mg/Ca ratios first decrease sharply with depth and then decrease more gradually until becoming roughly constant with depth from 140 to 220 mbsf. Below 220 mbsf, Mg/Ca ratios increase to the bottom of Hole U1406A.

Strontium concentrations increase from 80 to 130 μM in the upper 210 mbsf of Hole U1406A. Below 210 mbsf, the strontium concentration depth gradient reverses and strontium concentrations fall to 100 μM at the bottom of Hole 1406A. Sr/Ca ratios decrease from 7.25 to 6.5 in the upper 70 mbsf of Hole U1406A and then become roughly constant with depth until 180 mbsf, where values sharply increase to 7.5 across the sampling gap and then steadily decrease to 6.5 at the bottom of Hole U1406A.

Potassium concentrations range from 9 to 17 mM and are highly variable with no discernible downhole trends that correspond to lithostratigraphic units.

Discussion

Typically, calcium concentrations in deep-sea sediment interstitial fluids show pronounced systematic increases with increasing depth, whereas magnesium concentrations decrease. A marked discontinuity in the interstitial water chemistry profiles at Site U1406 occurs at the sampling gap between 180 and 200 mbsf. This disparity is associated with an inferred low-porosity chert horizon that appears to serve as an aquiclude (Fig. F26) with strong physical control on the transport of dissolved chemical constituents within interstitial water.

The prominent downhole gradients in calcium, magnesium, and strontium concentrations in Hole U1406A point to multiple processes controlling the interstitial fluid chemistry at Site U1406. First, comparison of calcium concentrations from Sites U1403–U1405 suggest that exchange reactions and alteration with the underlying basalt likely play a strong role. Gently sloping concentration gradients in the upper 180 mbsf are diagnostic of deep exchange reactions with basaltic basement and/or volcaniclastic sediment and subsequent diffusion of calcium and magnesium through the sedimentary sequence (Gieskes and Lawrence, 1981). During these reactions, calcium is released into interstitial fluid and magnesium is removed, which appears to be reflected in decreasing Mg/Ca ratios in the upper lithostratigraphic units of Hole U1406A. A potential problem with this interpretation for Site U1406 is that the low-porosity chert horizon at 180–200 mbsf should have acted as an impediment to this exchange. Additionally, the reversal in the trends of calcium, magnesium, Mg/Ca ratio, and several other parameters below the chert interval appears to preclude attributing the gradients in the uppermost 180 m of core to exchange with deep reservoirs such as basaltic basement. The reversal of magnesium below the chert suggests a deep source for magnesium, perhaps seawater or flow through reefal sediments below the drilling depth.

The gradient of calcium concentrations at Site U1403 is the steepest, whereas that of Site U1406 is the weakest among all J-Anomaly Ridge sites. Because water depth is greatest at Site U1403, the variation in these gradients appears to reflect carbonate dissolution controlled by the CCD. Indeed, carbonate content at Site U1403 is lower than at the shallower sites, whereas Site U1406 is characterized by the highest carbonate concentrations. At Site U1406, the decrease in calcium concentration below 200 mbsf corresponds to a decrease in alkalinity and an interval of well-cemented chalk in lithostratigraphic Unit IV, indicating precipitation of carbonate. Carbonate overgrowths on benthic foraminifers in Unit IV (see “Biostratigraphy”) are consistent with this interpretation.

Sediment geochemistry

Sediment plugs (5 cm³) for downhole analysis of sediment elemental geochemistry were taken from Cores 342-U1406A-1H through 34X at an average resolution of one sample per section, adjacent to the moisture and density (MAD) samples (Table T20). Additional samples were taken at roughly 40 cm intervals from Cores 342-U1406B-24X through 30X and 342-U1406C-22X and 24X to resolve high-amplitude changes in carbonate contents. Elemental determinations of total organic carbon and total nitrogen were not made on all samples from Holes U1406B and U1406C.

Results

Carbonate content in Site U1406 sediment ranges from 21 to 92 wt% (Fig. F27). In lithostratigraphic Unit I (composed of foraminiferal sand and nannofossil ooze), carbonate contents are >50 wt%. Carbonate contents of 40 wt% are typical in the nannofossil ooze of Unit II. Several fluctuations within Unit II where carbonate contents increase to 60 wt% in 5 to 10 m thick intervals are evident. At the base of Unit III, carbonate contents increase to 60 wt% and reach 80 wt% in a discrete interval at ~199 mbsf. In the nannofossil chalk of Unit III, carbonate contents increase to 92 wt% and remain above 70 wt% to the bottom of Hole U1406A.

Total organic carbon, calculated as the difference between total carbon and inorganic carbon, ranges from 0.01 to 0.5 wt%, with many samples falling below detection limit (Table T20). Total nitrogen values generally fall below 0.1 wt%, with slightly lower values at the bottom of Hole U1406A.

Discussion

The prominent increase in carbonate content at 199 mbsf is associated with the Eocene–Oligocene transition (see “Biostratigraphy” and “Paleomagnetism” for identification criteria). Carbonate concentrations reach their maximum (92 wt% at 250.45 mbsf) at Site U1406 in calcareous nannofossil Zone NP21 (Chron C13n) in middle to upper Eocene strata. This increase is provisionally interpreted to reflect CCD deepening and overshoot events (see “Background and objectives") but could also reflect the onset of sediment drift formation.

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