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doi:10.2204/iodp.proc.303306.103.2006

Geochemistry

Volatile hydrocarbons

Headspace gas analysis was performed as a part of the standard protocols required for shipboard safety and pollution prevention monitoring. A total of 20 headspace samples from Holes U1302A and U1303A were analyzed (10 from each site at a sampling resolution of 1 per core) (Table T31). In general, sediments from Sites U1302 and U1303 yield low interstitial gas concentrations. Methane (C₁) concentrations are slightly higher than the atmospheric background level, ranging between 2.1 and 3.8 ppmv, and show no notable downhole trend (Fig. F29). The average C₁ concentrations for Holes U1302A and U1303A are 2.7 and 3.3 ppmv, respectively. No hydrocarbons higher than C₁ were detected.

Sedimentary geochemistry

Sediment samples were collected for the analyses of solid-phase geochemistry (inorganic carbon and elemental C, N, and S) at a sampling resolution of approximately two per core from all the holes drilled at Sites U1302 and U1303. At each site, the samples collected from multiple holes are taken to constitute a single depth profile using the composite depth scale (see “Composite section”). Figures F30 and F31 show calcium carbonate (CaCO₃) concentrations, total organic carbon (TOC) contents, N elemental concentrations, and organic C/N atomic ratios for Sites U1302 and U1303, respectively. Results of the coulometric and elemental analyses are reported in Table T32.

Determinations of bulk CaCO₃ contents by coulometry were made for 59 samples from Site U1302 and 30 samples from Site U1303. In general, carbonate contents for the sampled strata range from 1.4 to 48.6 wt% at Site U1302 (Fig. F30) and from 3.7 to 46.8 wt% at Site U1303 (Fig. F31). At Site U1302, the carbonate record is characterized by three distinct maxima of ~46 wt% at 129.8, 64.0–60.3, and 22.1–20.6 mcd, whereas minima (<10 wt%) are recognized at 107.1–91.1, 69.4, 26.7, and 10.4 mcd (Fig. F30). The carbonate contents appear to decrease gradually upward from each of the maxima toward the minima, particularly between 129.8 and 107.1 mcd, ~64 and 26.7 mcd, and ~20.6 and 5.5 mcd. In contrast, upward increases in carbonate content from each of the minima to the maxima are rapid, as observed between 69.4 and 64.0 mcd, 26.7 and 20.6 mcd, and 5.5 and 1.3 mcd. Although obscured by lower sampling resolution, the carbonate record at Site U1303 (Fig. F31) exhibits upward trend and maximum and minimum peaks that are similar to those in the Site U1302 record.

The variable carbonate contents recognized at Sites U1302 and U1303 primarily reflect relative abundance of calcareous components (i.e., nannofossils, foraminifers, and detrital carbonate) and noncarbonate materials dominated by silty clay and siliceous microfossils (see “Lithostratigraphy”). The lithology of deep-sea sediments can be traced using various downcore physical property and color reflectance records (see “Physical properties” and “Lithostratigraphy,” both in the “Site U1302–U1308 methods” chapter). To supplement the lower resolution shipboard carbonate data from Sites U1303 and U1302, we compare them to a high-resolution color reflectance L* record from Site U1302. As illustrated in Figure F32, the L* variation at Site U1302 corresponds well to the Site U1302 and U1303 carbonate profiles visually in that the high L* values (i.e., high reflectance) are associated with high carbonate contents and the low L* values correspond to low carbonate contents. Such visual correspondences between the carbonate and L* records are also recognized with other high-resolution physical property data (not shown) and, therefore, our shipboard low-resolution carbonate data document the first-order lithologic variability reasonably well.

Sedimentary total C, N, and S concentrations were determined on 36 samples from Holes U1302A–U1302E and 31 samples from Holes U1303A and U1303B. TOC content for most intervals at Sites U1302 and U1303 is <0.7 wt% (average = ~0.5 wt%) (Figs. F30, F31). Total N content at Sites U1302 and U1303 ranges from 0.03 to 0.10 wt% (average = ~0.05 wt%) (Figs. F30, F31). No significant downcore trends are recognized for TOC and N records at Sites U1302 and U1303. C/N ratio is a useful indicator to interpret origin of organic matter (i.e., marine, degraded marine, or terrestrial) in the sediments (Emerson and Hedges, 1988; Meyers, 1997). At Sites U1302 and U1303, most of the C/N values are ~4–13 ratio (Figs. F30, F31). This suggests that the organic materials preserved in the upper ~140 m of the sediments at Sites U1302 and U1303 are marine in origin (i.e., C/N = ~4–10 ratio) (Meyers, 1997). We recognize a sporadic occurrence of high C/N values (~20 ratio) that appears to be associated with intervals of high carbonate content. These relatively high C/N values could represent oxidized and degraded marine organic matter. None of the analyzed samples contained measurable S.

Interstitial water chemistry

We collected a total of 16 interstitial water samples from whole-round sediment sections from Sites U1302 and U1303. Seven samples from Hole U1302A and nine samples from Holes U1303A and U1303B were processed for routine shipboard geochemical analyses. For Site U1303, the samples from the two holes are taken to constitute a single depth profile using the composite depth scale. Besides whole-round samples, interstitial water samples were collected from small plug (~10 cm³) sediment samples for the upper ~100 mbsf for shore-based studies. Results of interstitial water analyses for Sites U1302 and U1303 are reported in Table T33 and Figure F33. Interstitial water chemistry data from Sample 303-U1303A-10H-1, 145–150 cm (i.e., 92.58 mcd), is not considered for further discussion due to clear evidence of core disturbance (see“Site U1302 and U1303 visual core descriptions” in “Core descriptions”) and contamination by drill fluid in the sample evident in several interstitial water chemistry parameters shown in Figure F33.

Chloride, sodium, salinity, and pH

The Cl^– records from Site U1302 and U1303 interstitial water are similar in terms of the overall trends, although some offsets exist between them because of low sampling resolution at each site (Fig. F33). In the upper ~40 mcd of the sediments, Cl^– increases downhole from ~555 to ~567 mM, returning to the near-seawater values in the underlying sediments to ~110 mcd. Previous workers have attributed the chlorinity maximum of ~40–50 mbsf to a remnant of higher-salinity bottom water masses during the Last Glacial Maximum (LGM) preserved in the sediment pore spaces (e.g., McDuff, 1985; Adkins et al., 2002; Adkins and Schrag, 2003). The overall downhole trends of our shipboard pore fluid chlorinity data in the upper ~100 m of the sediment columns are similar to those reported from other deep-sea sections. This implies that conservative chemical proxies preserved in the interstitial water samples collected from Sites U1302 and U1303 may record properties of bottom water masses prevailed during the LGM.

Downhole profiles of pore fluid Na⁺ concentrations derived from charge balance calculation for Sites U1302 and U1303 exhibit trends that are roughly similar to those of Cl^– (Fig. F33). The Na⁺ values at the sites range from 468 to 486 mM. The pH profiles at Sites U1302 and U1303 do not exhibit any significant downhole trends, with values ranging from 7.4 to 7.7 (Fig. F33). Salinity (not shown) is generally higher at Site U1303 than Site U1302 in the upper ~60 mcd while exhibiting decreasing trend downhole at both sites.

Alkalinity, sulfate, ammonium, and dissolved silica

Alkalinity increases with depth from 4.5–10.4 mM in the upper ~30 mcd at Site U1303, followed by relatively constant values at ~10.5 mM downhole (Fig. F33). Alkalinity values at Site U1302 are generally lower than those at Site U1303 by ~1 mM overall and decrease slightly from 9.7 mM at 36.4 mcd to 8.7 mM at 110.7 mcd. Sulfate profiles from Sites U1302 and U1303 are in concert with one another in terms of their values and downhole trends (Fig. F33). In general, SO₄^2– concentrations decrease steadily downhole over the entire drilled sequences, ranging from 26.7 mM at ~2.3 mcd to ~5.8 mM at ~110.7 mcd. The downhole trends in NH₄⁺ show antithetic relationships to SO₄^2– (Fig. F33). NH₄⁺ concentrations at Site U1303 are slightly higher than those at Site U1302 and exhibit a downward increasing trend in the upper 70 m, ranging from 165 to 977 µM. Below ~70 mcd, NH₄⁺ decreases by ~150 µM at Site U1302. The downhole increases in alkalinity and NH₄⁺ and decrease in SO₄^2– at Sites U1302 and U1303 primarily reflect decomposition of organic materials driven by SO₄^2– reduction. The highest alkalinity values (i.e., ~10.6 mM) below ~40 mcd at Sites U1302 and U1303 are not as high as expected from the degree of SO₄^2– reduction (i.e., ~42 mM), implying that alkalinity produced through SO₄^2– reduction is being consumed in the sediments.

Dissolved silica concentrations at Sites U1302 and U1303 increase with depth from 579 µM at 2.3 mcd to 880 µM at ~79.7 mcd (Fig. F33). The elevated dissolved H₄SiO₄ at Sites U1302 and U1303 likely reflects the presence and dissolution of biogenic H₄SiO₄ in the sediments. The lowest dissolved H₄SiO₄ concentration of ~327 µm recognized at 110.7 mcd in Hole U1302A coincides with an interval of opal-free silty clay sediments (see “Lithostratigraphy” and “Biostratigraphy”).

Calcium, strontium, lithium, magnesium, and potassium

At Site U1303, pore fluid calcium (Ca²⁺) concentrations decrease downhole gradually from the near-seawater value of ~9.8 mM at ~2.3 mcd to ~4.6 mM at ~60.8 mcd (Fig. F33). A similar downhole trend is recognized at Site U1302, but values are offset and higher than those at Site U1303 by ~0.8 mM. The most depleted Ca²⁺ values are recognized at 60.8 and 79.7 mcd at Sites U1303 and U1302, respectively. The Sr²⁺ and Li⁺ trends resemble one another in terms of overall downhole trends (Fig. F33); the highest values close to the seawater average are recognized at the top of the interstitial water profiles, decrease gradually downward to ~40–80 mcd, and increase slightly toward the bottom of the recovered sections. The parallel downhole trends recognized in the Ca²⁺, Sr²⁺, and Li⁺ profiles suggest the uptake in a diagenetic phase. These trends and the consumption of alkalinity likely reflect precipitation of carbonate minerals. However, biogenic carbonate dissolution and recrystallization are not important diagenetic processes at the sites because Sr²⁺ does not increase with depth (e.g., Baker et al., 1982).

Mg²⁺ and K⁺ concentrations decrease steadily and consistently with depths at the two sites (Fig. F33). By the deepest sample at Site U1303 (i.e., at 68.9 mcd), Mg²⁺ decreases by ~13% to 44.8 mM and K⁺ decreases by ~17% to 10.1 mM from the near-seawater values at the topmost sample. These correlating profiles (r = 0.83, n = 15) indicate that Mg²⁺ and K⁺ ions are being consumed in a related process perhaps through reactions with silicate minerals (i.e., clay diagenesis) within or below the cored interval. Another possible explanation for the downhole decreasing trend in the Mg²⁺ concentrations is precipitation of authigenic dolomite. However, dolomite precipitation may not be a significant diagenetic process governing the Mg²⁺ profiles at the sites because the magnitude of downhole decrease in Mg²⁺ concentrations is small relative to Ca²⁺.

Manganese, iron, boron, and barium

The Mn²⁺ profile at Site U1303 is characterized by a maximum concentration (52.2 µM) at the shallowest sample and subsequent rapid decrease to 17.0 µM at 8.9 mcd (Fig. F33). Below ~10 mcd, Mn²⁺ concentrations decrease moderately downhole at Sites U1302 and U1303, reaching a minimum concentration of ~6 µM at ~60–80 mcd. The downhole Mn²⁺ profiles suggest that we may have sampled the lowermost part of the manganese reduction zone. Pore fluid Fe profiles at Sites U1302 and U1303 exhibit downhole decreasing trends, ranging from 18.3 µM in the shallowest sample to 1.4 µM at 68.9 mcd (Fig. F33). In particular, the Fe profile at Site U1302 shows relatively uniform and very low concentrations of ~2–3 µM from 36.4 mcd toward the bottom of the recovered sections. At these sites, available Fe is probably rapidly precipitated into iron sulfides, which are ubiquitous in the sediments (see “Lithostratigraphy”).

Boron concentrations, mostly as boric acid (H₃BO₃), in the interstitial water samples at Sites U1302 and U1303 (not shown), exhibit a wide variety of values ranging from 496 to 623 µM, while no significant downhole trends are recognized. Barium (Ba²⁺) concentrations (not shown) are very low (<1.0 µM) as expected within the sulfate reduction zone, as available Ba²⁺ is sequestered in barite.

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