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doi:10.2204/iodp.proc.303306.112.2006

Geochemistry

Inorganic geochemistry

Interstitial water chemistry

A total of eight interstitial water samples were extracted from 5 cm whole-round sediment sections from Hole U1313A with a resolution of one sample per core for the first six cores and thereafter one sample per core for Cores 306-U1313A-9H and 12H, covering a depth of 106.2 mbsf. Interstitial water samples were processed for routine shipboard geochemical analyses (see “Geochemistry” in the “Site U1312–U1315 methods” chapter). The concentrations of dissolved elements from Hole U1313A are given in Table T30, and their downhole profiles are illustrated in Figure F25. In comparison to Hole U1312A, none of the sediments used for interstitial water sampling in the upper section of Hole U1313A were physically disturbed or influenced by flow-in (see “Stratigraphic correlation;” Table T28). A noticeable change in the parameters of most pore water profiles is observed below 77.6 mbsf depth.

Chlorinity, salinity, alkalinity, and pH

Chloride (Cl^–) concentrations in Hole U1313A increase from 568 mM at 3 mbsf to ~572 mM at 77.6 mbsf, except for a low value of 559 mM at 30.2 mbsf. Below 77.6 mbsf, chlorinity decreases to a depth of 106.2 mbsf, where a value of ~554 mM is measured (Fig. F25A).

Downhole salinity increases from 33 to 35 g/kg between 3 and 20.7 mbsf. It then decreases to 34 g/kg between 20.7 and 30.2 mbsf, remains uniform to 77.6 mbsf, and then further decreases to 33 g/kg to the bottom of the profile (Fig. F25B).

Alkalinity increases with depth from 3.52 to 6.07 mM between 3.0 and 106.2 mbsf (Fig. F25C). These values are similar to those reported from Hole U1308A (see the “Site U1308” chapter) but higher than those reported from Hole U1312A.

pH values in Hole U1313A range from 7.20 to 7.39, which is similar to the values in Hole U1312A, albeit more variability is observed between 11.2 and 47.8 mbsf (Fig. F25D).

Sodium, potassium, magnesium, and calcium

Sodium (Na⁺), potassium (K⁺), and magnesium (Mg²⁺) concentrations in the interstitial water of Hole U1313A range from ~418 to ~494.3 mM, 10.9 to 12.4 mM, and 46.1 to 51.8 mM, respectively. Their downhole profiles show trends that are roughly similar to each other except for an increase in Mg²⁺ (49.8 mM) at 20.7 mbsf (Fig. F25E). Ca²⁺ values range from 8.9 to 10.1 mM throughout the profile.

Iron, boron, barium, lithium, manganese, and strontium

Iron (Fe²⁺) concentrations in Hole U1313A could be divided into two parts: a sharp downhole decreasing trend from ~16 to 1.9 µM between 3 and 20.7 mbsf and a moderate downhole increasing trend between 20.7 and 106.5 mbsf (Fig. F25I). The lowest value of 1.9 µM is measured within the lithologic interval displaying darker and black streaks, suggesting the presence of diagenetic iron minerals.

Boron (B) concentrations, mostly as boric acid (H₃BO₃) in the interstitial water samples of Hole U1313A, are highly variable and range from ~448 to 509 µM (Fig. F25J). Barium (Ba²⁺) concentrations in Hole U1313A are relatively uniform, ranging from 3.12 to 3.20 µM, and are nearly three times higher than those measured in Hole U1312A.

The lithium (Li⁺) and strontium (Sr²⁺) profiles show opposite trends to each other. Lithium (Li⁺) concentrations exhibit a linear decrease with depth, whereas strontium (Sr²⁺) concentrations increase downcore (Fig. F25L, F25N). These trends can be explained by the utilization of Li⁺ during transformation of ash fragments into clay minerals and by the expulsion of Sr²⁺ from the carbonate constituents in the sediments during dissolution and reprecipitation into the pore water (Baker et al., 1982; De Carlo, 1992). Manganese (Mn²⁺) concentrations are also higher in the shallowest samples (~45 µM), but they decrease rapidly at a depth of 20.7 mbsf and remain low through the rest of the downhole profile (Fig. F25M).

Dissolved silica, sulfate, and ammonium

The dissolved silica (H₄SiO₄) profile can be divided into two parts: the upper portion shows a downhole increasing trend to 39.3 mbsf, whereas the lower portion exhibits a decreasing trend from 39.3 mbsf to the bottom of the profile (Fig. F25O). The highest dissolved silica concentration was measured at 39.3 mbsf and likely reflects the initial presence of biogenic silica in the sediments and its subsequent dissolution.

Sulfate (SO₄^2–) concentrations show a slightly decreasing downhole trend (Fig. F25P).

Ammonium (NH₄⁺) concentrations increase downcore and exhibit the highest values (615 µM) at 47.8 mbsf (Fig. F25P). It is usually suggested that the diagenesis of organic matter depletes the dissolved SO₄^2– by sulfate reduction reaction, which can be shown as follows:

Sulfate concentrations are near seawater values at the top of the section and do not decrease downcore, significantly suggesting that the sulfate reduction process is not complete (Fig. F25P). As can be seen in Figure F25, sulfate concentrations decrease while those of NH₄⁺ increase. Moreover, as stated above, alkalinity increases downhole, similar to the increases in downhole NH₄⁺ values, are consistent with the prevalent reductive process. This is concordant with the relatively low methane (C₁) in headspace samples (Table T31). Note that the available HS^– can react with Fe²⁺ to form iron sulfide minerals (e.g., FeS and FeS₂), attesting the presence of dark and black pyritic substances (see “Lithostratigraphy”).

Organic geochemistry

Volatile hydrocarbons

Headspace gas analysis was conducted as part of the standard protocol required for shipboard safety and pollution prevention monitoring. A total of 32 headspace samples from Hole U1313A, with a sample resolution of one sample per core, were analyzed (Table T31). Methane was the only hydrocarbon gas detected at the site. The concentrations of CH₄ in Hole U1313A are relatively constant and range from 1.5 to 2.4 ppmv (Fig. F26). The average methane concentration in Hole U1313A is 1.8 ppmv, slightly below the background level.

Sedimentary bulk geochemistry

Sediment samples for the analysis of solid-phase bulk inorganic carbon, total carbon, and total nitrogen (TN) were collected from the working halves from Hole U1313A at a resolution of two samples per core. In addition, splits of squeeze cakes from interstitial water samples intended for investigations of solvent-extractable organic matter (see below) were also used for bulk measurements. Data from the bulk geochemical analysis performed on a total number of 75 samples are shown in Table T32 (see “Geochemistry” in the “Site U1312–U1315 methods” chapter for analytical methods and the derivation of total organic carbon [TOC] values).

Downhole variations of calcium carbonate contents along with the lightness (L*) data from Hole U1313A are shown in Figure F27. For comparison, the calcium carbonate profile from DSDP Site 607 (Ruddiman, Kidd, Thomas, et al., 1987) is also plotted in Figure F27.

CaCO₃ contents in Hole U1313A range from 31.5 to 96.7 wt% (average = 80.5 wt%). The most obvious change in the carbonate distribution in Hole U1313A is found at ~120 mbsf. CaCO₃ contents are relatively constant at >90 wt% below this depth, whereas they are highly variable between 0 and ~120 mbsf. High-amplitude fluctuations in CaCO₃ contents from ~30 to 90 wt% are characteristic of the upper ~40 mbsf (average = 60 wt%). Between ~40 and 120 mbsf, a ~10% reduction in the amplitude and CaCO₃ values from ~40 to ~90 wt% are observed. A shift toward higher (69 wt%) average carbonate content compared to the top 40 mbsf is also noted.

Low-amplitude variability (~5%–10%) between 120 mbsf and the bottom of the profile is observed where the carbonate content is constantly high (average = 94 wt%). Color reflectance (L*) data display a similar general trend as the carbonate profile and further reveal that the oscillation pattern is in fact preserved at much higher frequencies at Site U1313 (e.g., L* versus global δ¹⁸O stack) (see also “Stratigraphic correlation”), as known from the CaCO₃ record at Site 607 (Ruddiman, Kidd, Thomas, et al., 1987) (Fig. F27).

TOC and TN contents range from 0 to 0.65 wt% and 0.05 to 0.23 wt%, respectively, in sediments of Hole U1313A (Fig. F28). Both TOC and TN average 0.1 wt%. Despite overall low mean TOC contents, the upper ~170 m generally show higher variability with higher-amplitude fluctuations (0.05–0.6 wt%) between ~20 and 70 mbsf and less variation from ~70 to 170 mbsf. Below 170 mbsf, the TOC contents remain uniformly low (<0.1 wt%).

TN concentrations are low overall and relatively uniform.Variations in amplitude are in the order of 0.05%. However, a decrease in TN from ~0.15 to 0.1 wt% between 0 and ~150 mbsf is notable, which is comparable to the TN profile at Hole U1312A. At Site U1313, TN versus depth also seems to roughly mimic the downhole clay content as obtained by the smear slide estimates (see “Description of units”). Because clay minerals tend to absorb ammonium ions (Müller, 1977) and most of the samples contain relatively low TOC (<0.2 wt%), TOC/TN ratios are not useful to distinguish between marine and terrestrial organic matter.

Extractable organic matter and sources

In Hole U1313A, 16 samples were used for investigation of the solvent-extractable matter (Table T33). Unfortunately, the gas chromatography/mass spectrometry system could not be used at this site. However, using a flame ionization detector (FID) instead of the mass spectrometer allowed us to obtain a visual overview of the extractable compound inventory as well as to identify its major constituents (see “Geochemistry” in the “Site U1312–U1315 methods” chapter). As an example, the compound distribution of Sample 306-U1313A-5H-4, 145–150 cm, is shown in Figure F29. Two compound classes constitute the majority of the organic matter fraction, namely (1) a series of n-alkanes in a carbon atom number range of ~C₂₀–C₃₃ with a notable preference to long-chain (>C₂₅) odd-numbered compounds and (2) homologs of C₃₇–C₃₉ unsaturated methyl and ethyl ketones (alkenones).

1. The n-alkanes derived from cuticular waxes of higher land plants typically range from C₂₃ to C₃₅ with a distinct dominance of odd carbon chain lengths and a maximum concentration at C₂₇, C₂₉, and C₃₁ (Eglinton et al., 1962; Eglinton and Hamilton, 1967; Rieley et al., 1991; Kunst and Samuels, 2003). The carbon preference index (CPI) (e.g., Bray and Evans, 1961) is an expression of this odd-numbered n-alkane predominance, and n-alkane mixtures from natural vegetation waxes have high CPI (>5) (e.g., Eglinton and Hamilton, 1963). Similarly, n-alkanes in marine sediments showing a pronounced preference of long-chain (>C₂₅) odd-numbered homologs (and high CPI) are interpreted to be of terrigenous origin. Thus, the identification of long-chain n-alkanes with high CPIs of 2.0–4.7 (Table T33) in the sediments of Hole U1313A suggests that these compounds were derived from terrigenous sources.

2. Alkenones are only biosynthesized by a few extant species of haptophyte algae, primarily E. huxleyi and Gephyrocapsa spp. (see review in Conte et al., 1994; Volkman, 2000). Therefore, the presence of these compounds in the sediments of Site U1313 indicates that certain portions of the organic matter are also derived from a marine source.

As a first proxy to estimate marine and terrigenous organic matter contributions in the sediments of Hole U1313A, the ratio of plant wax-derived n-alkanes and haptophyte-derived alkenones can be used. A similar approach has also been used to characterize the organic matter deposited in the North Atlantic during the last climatic cycle (Villanueva et al., 1997). Figure F30 shows the downcore variations in the proportions of terrigenous and marine organic matter calculated as a normalized percentage from the summed peak areas of C_27, C₂₉, and C₃₁ n-alkanes and C₃₇ alkenones, respectively. A wide range of variability is observed in the composition of the organic matter, although it is based on a limited number of samples. Marine-derived organic matter composes between ~20% and 80%, whereas the remaining portion is of a terrigenous source. It may be mentioned here, however, that this approach does not consider amounts of marine organic matter potentially derived from diatoms, which partly occur in significant abundances in sediments of Site U1313 (see “Biostratigraphy”).

Alkenone-derived sea-surface temperatures

Besides indicating marine-derived sedimentary organic matter contributions, alkenones have also been widely used in paleoceanography for assessing the past sea-surface temperature (SST) changes because there exists a strong relationship between the degree of alkenone unsaturation and growth temperature. This degree of unsaturation can be evaluated from the abundance of the dominant di- and triunsaturated C₃₇ alkenones, commonly referred to as the U^k′₃₇ index:

The initial U^k′₃₇ growth temperature calibration derived from a culture of E. huxleyi shows a simple but clear linear relationship between U^k′₃₇ and temperature in a range of 5°–25°C (Prahl and Wakeham, 1987; Prahl et al., 1988):

Notably, equation 3 is statistically identical to a regression between U^k′₃₇ indexes determined in core-top sediments between 60°N and 60°S latitude (n = 370) and ocean-atlas mean annual SSTs (Müller et al., 1998). Equation 3 was used to calculate alkenone-derived SSTs at Site U1313 (41°N).

In Hole U1313A, SST ranges from 12.7° to 22.1°C (Table T33). Pleistocene SSTs (n = 11) display a high overall variability between 12.7° and 18.7°C. Notably, the 6°C variability as well as the absolute temperatures calculated from alkenones at Site U1313 match almost perfectly with the difference between the modern SSTs (~16°–18°C) (Fig. F5) and the reduced LGM SSTs (~10°–12°C) obtained by foraminiferal transfer functions for the relevant area of the North Atlantic by Pflaumann et al. (2003). Pliocene SSTs are significantly higher (16.8°–21.6°C; n = 4) and a maximum SST of 22.1°C is obtained for one sample from the latest Miocene (306-U1313A-29H-6, 70–72 cm). As shipboard data already allowed the construction of an age model for Site U1313 (see “Stratigraphic correlation;” Table T29), Pliocene–Pleistocene alkenone SSTs are compared to the global climate record based on benthic δ¹⁸O data (Lisiecki and Raymo, 2005). Figure F31 shows that alkenone-derived SSTs from sediments of Hole U1313A are in excellent agreement with glacial–interglacial cycles in terms of the recorded amplitudes, the overall variability, and the Pliocene–Pleistocene climatic evolution. Although these results are based only on a very limited set of samples and the interpretation is still preliminary, the alkenone approach for estimating SSTs and their short- and long-term variability at Site U1313 in future research is very promising.

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