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doi:10.2204/iodp.proc.342.106.2014

Geochemistry

The geochemistry program at Site U1405 included

• Analysis of hydrocarbon gas compounds on headspace samples;

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

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

Headspace gas samples

Headspace gas samples for routine safety monitoring were collected at a frequency of one sample per core in Hole U1405A (Table T18), usually from the bottom half of each core (i.e., Sections 4, 5, or 6). Methane increases very slightly downhole, with values between 1.59 and 3.49 ppmv. Higher molecular weight hydrocarbons were not detected in measurable amounts.

Interstitial water samples

Twenty-eight interstitial water samples were collected at a typical frequency of one per core immediately after sectioning the cores on the catwalk. In some cases, cores were too disturbed or recovery was too low to allow a whole-round sample to be taken for analysis. Results of the chemical analyses are presented in Table T19 and Figure F31.

Overall, pH measurements show a gradually decreasing downhole trend at Site U1405, whereas alkalinity measurements show an overall increasing then decreasing trend. Alkalinity reaches maximum values at ~190–210 mbsf with a modest decline downhole thereafter, suggesting that the depth interval centered on ~200 mbsf represents the locus of peak organic matter consumption. This explanation is corroborated by the downhole profile of interstitial water ammonium concentrations, a common respiration product of organic matter consumption. Ammonium concentrations are 0 mM in the upper 20 mbsf of Hole U1405A (corresponding to the Pleistocene–Pliocene clay and nannofossil ooze in lithostratigraphic Subunits Ia and Ib; see “Lithostratigraphy”) and then increase to peak values around 350 mM at ~210–300 mbsf, near the middle of lithostratigraphic Subunit IIb, an interval composed of silty clay. The decrease in alkalinity at the bottom of the sequence corresponds to a marked transition from biogenic ooze to lithified clay near the middle of Subunit IIc in upper lower Oligocene biosiliceous clay.

Manganese concentrations show a marked increase from seawater values at 5 mbsf to peak values of ~120 µM (210–220 mbsf) and then decrease to 60 µM at the bottom of the hole. Dissolved iron concentrations are low through much of the core but increase to an isolated maximum of 100 µM at 213 mbsf before returning to low values at the base of the section. These profiles suggest sedimentary diagenesis driven by organic matter degradation. The typical sequence of electron acceptor use during early diagenesis is manganese, followed by iron, then sulfate. In Hole U1405A, although there is a modest decreasing trend in sulfate concentrations downhole, the low iron concentrations combined with high manganese concentrations from the top of the sediment column to the top of lithostratigraphic Subunit IIc (Fig. F31) indicates a sedimentary redox sequence that does not progress beyond manganese oxidation until within the lowermost parts of the subunit. This conclusion is corroborated by high sulfate concentrations and a near-linear profile within the locus of peak organic matter consumption (190–210 mbsf) inferred from the alkalinity, ammonium, manganese, and iron profiles. Thus, it is likely that interstitial fluid geochemistry is not driven to sulfate reduction by reactions within the sampled interval (cf. Emerson and Hedges, 2003). Instead, the near-linear, slightly decreasing trend in sulfate may be the result of downward diffusion of sulfate through the Cenozoic sedimentary sequence (e.g., Berner, 1980). The cause of this downward diffusion is unclear. It is possible that an unsampled deeper zone of reduction has superimposed a modest sulfate depletion profile on the entire sampled section.

Potassium interstitial water concentrations decrease from maximum values of 13–16 mM near the top of Hole U1405A in lithostratigraphic Unit I to a minimum value of 6 mM at the bottom of the hole. A prominent inflection to higher values occurs at 190 mbsf and may reflect enhanced sorption onto clay minerals (e.g., Arthur, 1979), particularly illite, which is found in greater abundance in Subunit IIc (see “Lithostratigraphy”).

The calcium and magnesium profiles in Hole U1405A mimic the overall form of those encountered in deep-sea pelagic settings. The interstitial water profile for calcium in Hole U1405A shows a steady, near-linear increasing trend from ~10 mM at the core top to 28 mM at the base. (Fig. F31). Magnesium concentrations decrease from the core top (53–54 to ~40 mM at ~205 mbsf). Mg/Ca ratios decline steeply from the core top value of ~5 to ~1.5 at depth, with a flattening profile. The Sr profile is markedly different from the classic situation. Instead of the typical increase to a maximum value within the upper 200 m of the sediment column and a concentration plateau to the base of the hole (Gieskes, 1981), the Sr profile at Site U1405 shows a linear increase from the sediment/water interface to the deepest sample analyzed (300 mbsf), suggesting a deep source for Sr. The increase in strontium concentrations is similar to that seen at Sites U1403 and U1404, suggesting a carbonate-rich neritic section at the base of the site below the drilled section.

Sr/Ca ratios remain nearly constant between 6 and 8, suggesting that carbonate recrystallization and dissolution reactions within the sediment column drilled at Site U1405 (cf. Baker et al., 1982) have a negligible effect on interstitial water calcium concentrations. The variations in Ca, Sr, and Mg concentrations most likely reflect some combination of exchange reactions with the basaltic basement rocks (Gieskes and Lawrence, 1981), formation of authigenic dolomite deposition (e.g., resulting in a reduction in magnesium and increase in calcium), and diffusion. In Figure F32, we plot calcium and magnesium concentrations and Ca/Mg ratio for Sites U1403, U1404, and U1405 with the distance to basement noted. Overall, the calcium, magnesium, and Mg/Ca ratio profiles in Site U1405 interstitial water follow closely with those observed at Site U1404 but differ from the significantly steeper trends observed at Site U1403. These differences may relate to the closer proximity of Site U1403 to basement, which permitted stronger exchange of calcium and magnesium in the Site U1403 interstitial water with the basaltic basement rocks

Collectively, the downhole profiles of interstitial water chemistry in Hole U1405A reflect a combination of diffusion and postdepositional modifications. The modifications include those associated with organic matter degradation, sorption/desorption of chemical components on the surfaces of clay minerals, and chemical exchange with minerals undergoing diagenetic reactions.

Sediment samples

Sediment plugs (5 cm³) for downhole analysis of sediment elemental geochemistry were taken from Cores 342-U1405A-1H to 33X at an average resolution of one sample per section, adjacent to the moisture and density samples (Table T20).

Results

Carbonate content ranges from 0 to 46 wt% (Fig. F33). Values decline gradually from a maximum of 42 wt% in the uppermost Pliocene–Pleistocene foraminiferal sand to <1 wt% in the uppermost part of lithostratigraphic Subunit Ib. In the upper portion of Subunit IIa, carbonate contents are generally between 0 and 10 wt% and increase to 10–20 wt% in the lower portion of Subunit IIa. Several distinct intervals can be identified in the carbonate record in Hole U1405A:

1. An interval of 0 wt% carbonate within Core 342-U1405A-10H (~80–90 mbsf),

2. Prominent peaks in carbonate content at 60–70 and 110–125 mbsf,

3. Stratigraphically thin intervals of high carbonate content in light-colored intervals around the lithostratigraphic Subunit IIa/IIb and IIb/IIc boundaries (30 and 46 wt%, respectively), and

4. Low carbonate content (<15 wt%) in the lower portion of Subunit IIc.

Elemental analysis of Site U1405 sediment reveals that most of the total carbon in Site U1405 sediment samples is present as calcium carbonate. Total carbon values range from ~0 to ~7 wt%, with the downhole pattern of total carbon largely resembling that of carbonate content (Fig. F33).

Most of the total organic carbon (TOC) values (i.e., the difference between total carbon and inorganic carbon) are <0.5 wt%, with a smattering of values exceeding 0.6 wt%. Downhole trends in TOC are very subtle, with generally low values (0.1–0.4 wt%) in the upper 150 mbsf, slightly higher values (0.2–0.5 wt%) from 150 to 250 mbsf, and low values (0.1–0.4 wt%) in the lowermost 60 m of Hole U1405A. Most of the total nitrogen values fall into the 0.1–0.2 wt% range, with some higher values (i.e., >0.2 wt%) in the upper 110 m of Hole U1405A (Table T20; Fig. F33).

Discussion

Based on the shipboard biostratigraphic- and paleomagnetic-derived age model for Site U1405 (see “Biostratigraphy”), the intervals of high calcium carbonate contents in Subunit IIa correspond with intervals of elevated carbonate content observed in the lowermost Miocene and upper Oligocene at Site U1404. These intervals of increased carbonate content in the uppermost Oligocene appear to be driven by calcareous nannofossil abundance, particularly the shelf taxon Braarudosphaera (see “Biostratigraphy”), which is absent in any other interval. Collectively, these conspicuous fluctuations of carbonate content possibly reflect the result of CCD fluctuations, carbonate production, and/or sea level. Regardless of the cause, the occurrence of multiple intervals with similar characteristics during the Oligocene–Miocene transition suggest repeated oceanographic changes over an extended stratigraphic interval (e.g., >70 m of sediment) at Site U1405.

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