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doi:10.2204/iodp.proc.344.208.2019

Results

Results from the pore water analyses are listed in Table T1, and data from carbonate extractions are listed in Table T2. We compare results from the two different leaching approaches to determine the method from which a minimum amount of clay is removed so as to minimize the influence of clays in the strontium isotopic composition attributed to the carbonate phase. The soft leaching method was shown to be the more favorable method in terms of both clay contamination and fraction of CaCO₃ leached. Aluminum leached was far greater for all samples using the hard leach method, whereas CaCO₃ leached was greater for all samples when the soft leach method was employed (Figure F2). In both cases, the amount of strontium leached remained relatively consistent. All subsequent carbonate elemental and Sr isotope data reported here for Hole U1380C samples were obtained using the soft leaching procedure.

The elemental (Sr/Ba, Ba/Ca, and Mn/Ca) and isotopic (⁸⁷Sr/⁸⁶Sr, δ¹³C, and δ¹⁸O) composition of carbonate samples is shown in Figure F3 (triangles); the downcore plots also include pore water data from Sites U1378 and U1380 (circles) for comparison; the oxygen and carbon isotope data are from this study, the cation data are from Harris et al. (2013c) and Torres et al. (2014), and dissolved strontium isotope data are from Ross et al. (2015).

Downhole ⁸⁷Sr/⁸⁶Sr values for the Hole U1380C carbonates range from 0.70797 to 0.70899; these values are lower than seawater and are consistent with pore water data. Several samples from 707 to 796 mbsf exhibit ⁸⁷Sr/⁸⁶Sr ratios in a narrow range, from 0.70854 to 0.70872, departing from the decreasing trend of the other samples in that depth range. The strontium concentration in the leached carbonate ranges from 0.52 to 4.46 µmol/g, corresponding to molar ratios with respect to calcium of 0.21–3.78 mmol/mol. Barium and manganese concentrations range from 0.06 to 0.69 µmol/g and from 2.7 to 77.8 µmol/g, respectively, which correspond to molar ratios to calcium that range from 0.04 to 0.84 mmol/mol and from 2.3 to 13.8 mmol/mol for barium and manganese, respectively. Although strontium concentrations generally show a slight decrease downcore, depth plots of Ba/Ca and Mn/Ca display no such trend.

DIC (Figure F3E) shows a strong depletion in ¹³C with depth, reaching a low value of –22.35‰ Peedee belemnite (PDB) at 12 mbsf. This minimum corresponds to the depth of the sulfate depletion and methane increase (not shown), indicative of ¹³C depletion driven by anaerobic oxidation of methane, as documented by Expedition 334 Scientists (2012c). Below 12 mbsf, DIC is progressively enriched in ¹³C, reaching a δ¹³C highest value of 10.88‰ PDB at 84 mbsf. All values above seawater from 20 to 350 mbsf likely reflect residual DIC from methanogenesis. Most carbonate samples have δ¹³C values in the range of –2.62‰ to 0.24‰ PDB, and only three samples (344-U1380C-41R-1, 50–52 cm; 42R-1, 30–31 cm; and 52R-2, 67–68, cm) have values exceeding 3.5‰ PDB, which sets them apart from the relatively consistent trend of the others. These three samples (open triangles) also exhibit slightly higher Mn/Ca ratios and a more radiogenic strontium signal but are unexceptional with respect to the other elemental and isotope data.

The δ¹⁸O composition of the carbonates shows a slight decreasing trend with depth, whereas pore water data show fairly constant values to 552 mbsf. The isotopic composition of pore water recovered from the squeezed sediment has oxygen values that range from –0.71‰ to 3.38‰ standard mean ocean water (SMOW) in δ¹⁸O (Figure F3F), with only a few samples outside that range. The δ¹⁸O versus δ¹³C plot shown in Figure F4 illustrates the deviations in the isotopic composition of the carbonate relative to seawater (dashed lines) and the clear enrichment in ¹³C in three of the samples.

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