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doi:10.2204/iodp.proc.318.104.2011

Geochemistry and microbiology

Organic geochemistry

Volatiles

Methane and gas analysis was performed typically once per core using the methods described in the “Geochemistry and microbiology” in the “Methods” chapter.

Methane concentrations varied by four orders of magnitude downhole from undetected to >10,000 ppmv (Fig. F22). Levels are low between 0 and 200 m, increasing to between 1000 and 6000 ppmv between 300 and 1000 mbsf, with two spikes at 600 and 690 mbsf. C₁/C₂ levels were high throughout the hole and indicated no threat to drilling operations.

Assay of organic biomarkers

Twenty-five samples from Site U1356, ranging in age from early Eocene to late Miocene, were taken for extraction of lipids. Details of the methods are given in the “Geochemistry and microbiology” in the “Methods” chapter. One advantage of the methodology employed is the production of six chemical fractions for separation and clean-up of the different biomarker groups of principal utility for paleoceanographic and environmental reconstructions. The six fractions comprised aliphatic hydrocarbons, hopanes, and unidentified complex mixtures of branched alkanes (N1); polyaromatic hydrocarbons (N2); ketones (including alkenones) and aldehydes (N3); n-alkanols, sterols, and diols (N4); and n-alkanoic acid methyl esters (FAMES). The presence and concentrations of the high-utility tetraethers could not be determined on board because the chemistry laboratory is not presently equipped with a liquid chromatography–atmospheric pressure chemical ionization–mass spectrometry (LC-APCI-MS) device. We expect the tetraethers to elute in the N4 fraction, which will be measured on shore by LC-APCI-MS. Moreover, to ensure that the tetraethers were fully recovered, the silica gel columns (see “Geochemistry and microbiology” in the “Methods” chapter) were flushed with methanol and an additional fraction containing the most polar compounds (and any tetraethers not eluting in N4) was collected (N5 fraction).

The N1, N3, N4, and FAMES fractions were analyzed by gas chromatography mass spectrometry (GC-MS), and the results from each fraction are summarized in Tables T10, T11, T12, and T13, respectively (the N2 and N5 fractions were archived). Examples of GC-MS total- and single-ion chromatograms (of the four analyzed fractions) from four samples are presented in Figures F23 (middle Miocene), F24 (late Oligocene), F25, and F26 (both early Eocene). The primary purpose of this section is to ascertain biomarker composition and concentrations to aid the focus and priorities of future shore-based organic geochemical work. In particular, we wish to determine which compounds are present in sufficient concentration and with sufficient resolution (i.e., clean isolated peaks) for subsequent compound-specific isotope analysis for δ¹³C and δD. Current analytical requirements are ~10 ng per GC peak (in duplicate) for δ¹³C and ~100 ng per GC peak (in triplicate) for δD analyses.

N1 fraction

Homologous series of higher molecular weight (HMW) n-alkanes (along with n-alkanoic acids and n-alkanols) are synthesized by terrestrial higher plants as constituents of the epicuticular waxes and are some of the earliest biomarkers studied by science (Eglinton and Calvin, 1967; Eglinton and Hamilton, 1967). Low molecular weight (LMW) n-alkanes are much less indicative, being ubiquitous in cellular material and the primary constituents of oil and petroleum. Concentrations of HMW n-alkanes are low in the Miocene and Oligocene samples but increase in the early Eocene samples (Table T10). Fresh, higher plant HMW n-alkanes are characterized by a higher carbon preference index (CPI; a measure of the relative predominance of the compounds with odd-numbered carbon chains), whereas diagenetic, microbial, or crude oil n-alkanes are characterized by lower CPI values. When sufficiently abundant to be measured, the Eocene HMW n-alkanes are characterized by relatively high CPI values (Table T10). Figure F25 illustrates an example of thermally immature, higher plant wax n-alkanes from the Eocene. In contrast, Figure F24 shows an example of lower CPI n-alkanes, indicative of diagenetic or microbial source alkanes or contamination by petroleum byproducts.

A characteristic of the Eocene samples, which yield abundant higher plant n-alkanes, is the coeval presence of abundant hopane compounds derived from soil bacteria (Rohmer et al., 1984). Traditionally, hopanes and their precursor hopanoids have been attributed to aerobic bacteria, but recent work has revealed an anaerobic origin for hopanoids as well (Härtner et al., 2005). Moreover, their utility in paleoclimate studies has been recently highlighted by work that reports a decrease in the carbon isotope values of the hopanes extracted from the Cobham lignite (United Kingdom) at the onset of the PETM interval (Pancost et al., 2007). The isotopic excursion suggests an increase in the methanotroph population, reflecting an increase in methane production potentially driven by changes to a warmer, wetter climate. The presence of hopanes in the Eocene samples, along with abundant plant waxes (n-alkanes and n-alkanoic acids) confirms significant inputs of thermally immature organic material to Site U1356 from proximal terrestrial environments during the Eocene.

N3 fraction

Alkenones are synthesized by a limited number of unicellular algae of the Haptophyta, which include the coccolithophorids and are often informally called haptophytes (Conte et al., 1994; Marlowe et al., 1984; Volkman et al., 1980). Alkenones have been studied extensively in the open ocean, where the temperature-dependent distribution of the C_37:2 and C_37:3 alkenones (as expressed in Uk₃₇′) has been confirmed by culture, surface sediment, and water column particulate organic matter studies (e.g., Conte et al., 2006; Müller et al., 1998; Prahl and Wakeham, 1987).

Furthermore, alkenone δ¹³C has been used as a proxy for the estimation of past pCO₂ levels (e.g., Pagani et al., 2002). At Site U1356, alkenones were found in the majority of samples from the Eocene (Table T11) and possibly one sample from the early Oligocene (the concentration is too weak for confident assignment). Superior sensitivity for the detection of alkenones was achieved by operating the GC-MS in single-ion monitoring mode. However, concentrations could only be reasonably estimated (by external standards) using scanning mode (Table T12). Only the C_37:2, C_38:2, and C_39:2 components were detected, suggesting either Eocene sea-surface temperatures during the alkenone production season at Site U1356 were ≥30°C and/or preferential degradation of the tri-unsaturated compounds. Concentrations of the alkenones suggest compound-specific δ¹³C measurements might be feasible on some early Eocene samples.

N4 fraction

High molecular weight alkanols are constituents of higher plant waxes, whereas LMW n-alkanols are indicative of nonspecific marine phytoplankton and zooplankton sources. Compared to the n-alkanes, and especially the n-alkanoic acids, the HMW n-alkanols were sparse at Site U1356; some were found in the Miocene and Eocene but in low concentrations. The LMW n-alkanols were detected in many samples and are noticeably more abundant in the Eocene.

Sterols are abundant components of lipid membrane in many organisms. The relative abundances of different sterols (Villinski et al., 2008) can be used to reconstruct changes in the productivity of different algal groups (e.g., dinosterol for dinoflagellates and brassicasterol for diatoms). Two samples from the Oligocene (Table T12) contained a suite of sterols, as well as the diatom-produced C₂₈ diol. However, in other samples the nonspecific cholesterol (if present) was the only measurable sterol and no other sterols or diols were detected.

FAMES fraction

The LMW components of the n-alkanoic acids (C₁₄–C₂₃), as with the n-alkanols (C₁₅–C₂₃), are indicative of nonspecific marine phytoplankton and zooplankton sources, especially the n-C₁₄, n-C₁₆, and n-C₁₈ homologs (Gagosian et al., 1983). The HMW n-alkanoic acids (C₂₄–C₃₄), along with the n-alkanes (C₂₅–C₃₅) and n-alkanols (C₂₄–C₃₀), are the major components of higher plant leaf waxes (Eglinton et al., 1962; Eglinton and Hamilton, 1967; Kolattukudy, 1976). At Site U1356, the HMW n-alkanoic acids occur as the most abundant higher plant biomarkers. Occurrence and concentrations are markedly higher in the Eocene samples than the Oligocene and Miocene (Table T13). Moreover, compared to the n-alkanes the n-alkanoic acids generally elute as well-resolved peaks, with a clean baseline signal and few co-eluting compounds. All these factors suggest the HMW n-alkanoic acids have great potential for both δ¹³C and D/H compound-specific isotope analysis at Site U1356.

Inorganic geochemistry

Ninety-seven sediment samples from Hole U1356A were taken for analyses of percent carbonate, carbon, nitrogen, and sulfur content, as well as major and trace element analyses. Sample selection was carried out in close collaboration with the sedimentology group to sample the main lithologies represented in the hole. Sampling density was approximately one sample per core (2.0–997.9 mbsf).

Calcium carbonate (CaCO₃; in weight percent) was determined on all samples by coulometric methods (see “Geochemistry and microbiology” in the “Methods” chapter). CaCO₃ content is very low (≤2 wt%) for most major lithologies (Fig. F27). Low CaCO₃ values of 2–5 wt% were found in claystones of litholostratigraphic Unit III, nannofossil-bearing claystones of Unit III, and silty claystones of Unit VIII (see “Lithostratigraphy”). Higher CaCO₃ contents of 5–30 wt% were found in clay-bearing nannofossil limestones (Unit III), carbonate-rich claystones (Unit V), claystones (Unit VI), and silty claystones (Unit VI). Minor lithologies showed a wide range in CaCO₃ content from <1 to 71 wt%, with the highest values associated with limestones (Unit IV) and carbonate-bearing siltstones (Unit VIII).

Carbon, nitrogen, and sulfur contents were measured on 33 selected samples. Most data obtained were below instrumental detection limits. Five samples show carbon contents >2 wt% (2.2–8.7 wt%). Considering the results in context with the coulometer data, only one sample yielded measurable amounts of total organic carbon (3.17 wt%; Sample 318-U1356A-96R-CC, 16–17 cm). This sample was also the only sample with significant amounts of sulfur (3.2 wt%).

Concentrations of a selection of elements (silicon, titanium, aluminum, iron, manganese, calcium, magnesium, sodium, potassium, phosphorus, strontium, barium, vanadium, scandium, and cobalt) were obtained for all 97 bulk sediment samples by inductively coupled plasma–atomic emission spectrophotometer (ICP-AES). Representative results are shown in Figure F28, and data are reported in Table T14. Three broad intervals can be distinguished:

1. An upper interval (0 to ~878 mbsf),

2. A transitional interval (~878 and ~920 mbsf), and

3. A lower interval (~920 to ~1000 mbsf).

In the upper interval, silicon dioxide values fluctuate mainly between 65 and 75 wt%. Aluminum oxide concentrations range from 10% to 15% and show a clear inverse correlation with silicon dioxide. Values for titanium, potassium, magnesium, and iron oxides are positively correlated with aluminum oxide contents. A different pattern is displayed by barium, which shows a cyclic signature from 0 to 878 mbsf, most likely associated with productivity changes. Absolute concentrations vary between 400 and 1000 ppm. Calcium oxide patterns are dominated by CaCO₃-rich lithologies but show an anticorrelation with SiO₂ on the finer scale.

In the transitional interval (~878 to ~920 mbsf), most elemental concentrations show abrupt changes (Fig. F28). Silicon dioxide varies between 58 and 73 wt% and is inversely correlated with Al₂O₃ values (12–23 wt%). A similar inverse relationship with SiO₂ holds for TiO₂. In general, the transitional interval is characterized by widely fluctuating elemental concentrations rather than smooth increases/decreases in elemental concentrations.

The lower interval (~920 to ~1000 mbsf) shows a narrower range of elemental concentrations, at a level different than that recorded above the transition. Silicon dioxide concentrations are generally lower, between 60 and 63 wt%, and Al₂O₃ concentrations (as well as Fe₂O₃ and TiO₂ concentrations) are higher (21–25 wt%). MgO and K₂O concentrations in this unit are decoupled from Al₂O₃ and instead correlate with the trend observed in SiO₂ (i.e., lower absolute values).

Outliers to the overall trends described above occur in carbonate-enriched sediments (lower SiO₂ and higher CaO contents) and sandy diamicts and sandstones (higher SiO₂ contents). For example, the most extreme data point in the transitional interval just below 900 mbsf (i.e., high SiO₂ and low Al₂O₃, TiO₂, MgO, and Fe₂O₃) is associated with a sandstone unit (Fig. F28).

Elemental records in the upper interval (0–878 mbsf) reflect the association with biogenic/diagenetic (SiO₂ and Ba) and terrigenous (Al₂O₃, TiO₂, MgO, Fe₂O₃, and K₂O) phases. The lower interval, in contrast, shows parallel patterns for SiO₂, K₂O, and MgO, indicative of a primary terrigenous signal. When calculating the chemical index of alteration (CIA = [Al₂O₃/(Al₂O₃ + CaO* + K₂O + Na₂O)] × 100, where CaO* represents the CaO fixed in silicate minerals) (Nesbitt and Young, 1982) (Fig. F29), values between 50 and 70 can be observed in the upper interval. The values are intermediate between fresh feldspar (33–50) and chemically weathered detritus (70–75) and are typical for environments affected by physical weathering (e.g., Passchier and Krissek, 2008). In detail, CIA values show a gradual increase from 0 to 878 mbsf. At the depths of unconformity WL-U5 (~880 mbsf), an abrupt increase in CIA values is observed. Higher values in the lower interval (>80) strongly indicate intense chemical weathering and manifest different environmental conditions in the source area of the sediments.

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