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doi:10.2204/iodp.proc.342.108.2014

Geochemistry

The geochemistry program carried out routine shipboard analyses for Site U1407, including

• 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 U1407A;

• 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 standard safety monitoring were collected at a frequency of one sample per core in Hole U1407A (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 1.4 and 4.12 ppmv. Higher molecular weight hydrocarbons were not detected in measurable amounts.

Interstitial water samples

Twenty interstitial water samples were collected from Hole U1407A, covering a depth range from close to the sediment/seawater interface to ~270 mbsf. Samples were collected at a typical frequency of one per core (Table T19). In some cases, disturbed cores or low recovery prohibited collection of whole-round samples for interstitial water analysis, as was the case with Cores 342-U1407A-13H through 15H, 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 Core 342-U1407A-28X, which contains a black shale sequence deposited during OAE 2 (see “Biostratigraphy”; see also “Ocean Anoxic Event 2” in the “Expedition 342 summary” chapter [Norris et al., 2014a]). Results of the chemical analyses are presented in Table T19 and Figure F31. Depletion of shipboard supplies of the laboratory strontium standard prompted a mission to collect seawater for preparation as a replacement standard. We were ultimately unable to confirm the calibration range, therefore strontium values for Sites U1407–U1411 are not reported.

Interstitial water profiles show evidence of compartmentalization, with prominent abrupt downhole shifts in magnesium, manganese, and potassium at ~100–120 mbsf, suggesting that the unrecovered sequence of cherts acts as an aquiclude (Fig. F31) with strong physical control on the transport of dissolved chemical constituents within interstitial water.

The downhole decrease in interstitial water Mg concentration is gradual from the sediment/water interface to 110 mbsf, where concentrations reach a minimum of 51 mM just above the chert-rich interval. Below the chert-rich interval, the downhole profile for Mg shows a reversed gradient to the base of the sediment column suggestive of a source of Mg within the highly porous underlying reefal carbonate sediment of lithostratigraphic Unit VI (see “Lithostratigraphy”). Superimposed on these general trends, the transient increases at 60, 140, 190, and 270 mbsf observed in Hole U1407A (Fig. F31) indicate a local net magnesium source within the sediment column. Possible sources include weathering of igneous minerals (Gieskes and Lawrence, 1981) and the release of magnesium absorbed to sediment-particle surfaces by ion exchange with interstitial fluid ammonium (von Breymann and Suess, 1990). Support for the first interpretation is the presence of an ash layer at 140 mbsf (see “Lithostratigraphy”). Support for the latter interpretation is corroborated by broadly correlative increases in ammonium concentrations. The calcium concentration of interstitial fluid shows a relatively subtle depth increase to the top of the chert-rich interval. Below the chert, Ca shows a slightly reversed gradient (downhole decrease). This reversed gradient in calcium corresponds to a downhole decrease in alkalinity and increased lithification (~180 and ~270 mbsf, respectively; see “Lithostratigraphy”), suggesting that alkalinity and Ca are partially controlled by carbonate diagenesis. Within this stratigraphic interval, preservation of foraminifers varies from good to occasionally poor, including calcareous nannofossils at ~270 mbsf (see “Biostratigraphy”).

Overall, potassium interstitial water concentrations decrease downhole from maximum values of 12–13 mM above the chert-rich layer to minimum values of 10–12 mM below the chert-rich layer. Prominent superimposed inflections to higher values occur at 200 and 250 mbsf and may reflect enhanced sorption onto clay minerals (e.g., Arthur, 1979), particularly illite, which is found in greater abundance in lithostratigraphic Subunit Vb (see “Lithostratigraphy”).

In contrast to Sites U1403–U1406, changes in the chemical composition of interstitial water at Site U1407 do not show a strong influence of the respiration of organic carbon.

The interstitial water profiles of the redox-sensitive metals Mn and Fe exhibit low to below detection limit concentrations. Manganese concentrations are 0–4.9 µM above the chert layer, with a large, broad peak extending from 180 to 200 mbsf below the chert layer, close to the deposition of the organic matter–rich black shale sequences (up to 16.7 wt% organic matter) in lithostratitigraphic Unit IV. The peak is very modest compared to previous sites and is possibly a function of the age of the black shale. Under conditions in which reducible Mn oxides are not abundant, continuing anaerobic microbial activity would shift to reduction of sulfate, and lower sulfate concentrations and increased alkalinity would be expected (e.g., Gieskes, 1981). However, whereas alkalinity increases slightly downhole to the chert-rich layer, the sulfate interstitial water profile is characterized by relative stability, with values between 30 and 28 mM over the entire interval, suggesting the modest manganese peak may not be associated with anaerobic microbial activity. Instead, it is possible that the Mn enrichment is the result of inorganic sedimentary diagenesis. Shipboard-based X-ray diffraction measurements did not detect the presence of manganese carbonate phases (e.g., rhodochrosite or ankerite), which are common in organic matter–rich sediment. Interstitial water squeeze cakes were sampled for further shore-based chemical analysis.

Sediment samples

Sediment plugs (5 cm³) for downhole analysis of sediment elemental geochemistry were taken from Cores 342-U1407A-1H through 31X at an average resolution of one sample per section, adjacent to the moisture and density samples (Table T20). Additional samples were taken from Cores 342-U1407B-7H and 10H at 40 cm intervals to resolve high-amplitude features in carbonate contents captured in Hole U1407A.

Results

Concentrations of inorganic carbon vary from 0.05 to 11.06 wt% in sediment at Site U1407. These concentrations are equivalent to 0% to 93% CaCO₃, assuming that all of the carbonate is calcite. Total organic carbon (TOC) values are typically 0.1–0.5 wt% over the entire interval. The 0 wt% carbonate values at 10 mbsf are recorded in the Oligocene clay beneath the Pleistocene veneer at Site U1407. The most prominent change in carbonate content at this site is a downhole step increase associated with the downhole transition from middle Eocene to lower Eocene sediment (from 59 to 82.5 wt%; ~80 mbsf); this step correlates with shifts in several proxies (e.g., color reflectance, magnetic susceptibility, NGR, TOC, and total nitrogen values; Fig. F32). This decrease in carbonate is linked to a change in production, preservation, or dilution by other sedimentary components. Although this interval corresponds to an increase in TOC, it does not correspond to high productivity, as inferred by increased siliceous tests.

A sedimentary sequence of zeolitic claystone (black shale) and chalk representing the Cenomanian–Turonian OAE 2 was recovered at ~220 mbsf in Hole U1407A. Two comparable OAE 2 successions, but with different lithostratigraphic expressions, were recovered in Holes U1407B and U1407C. Homogeneous to laminated organic-rich black shale from Hole U1407A has pyrolysis-derived TOC content of 4–16.7 wt% and variable carbonate content (1.2–4.4 wt%), reflecting the calcite-rich laminae in the black shale. Organic matter is Type II kerogen, derived from algal and microbial primary production. Hydrogen indexes (~600–620 mg hydrocarbons per g organic carbon) and T_max values (<415°C) indicate that the organic matter is thermally immature and relatively well preserved (Fig. F33). In addition, improved organic matter preservation is implied by C/N ratios that increase as organic carbon concentrations increase, suggesting that nitrogen-rich components were more readily degraded than other organic matter components during sinking of organic matter through a strongly developed oxygen-minimum zone, elevating C/N ratios of surviving organic matter (cf. Twitchell et al., 2002).

The downhole TOC profile (Fig. F33) shows a sharp increase near the onset of black shale deposition followed by a trend of ever-decreasing amounts of TOC to near-zero levels toward the top of the bed. The TOC profile suggests that the environment of deposition in the black shale initially produced a high degree of preservation in the kerogen, but with time the degree of preservation decreased to the point at which little TOC was preserved and carbonate deposition once again became dominant (85 wt% carbonate at 230 mbsf; Figs. F13, F32).

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