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Analytical methods

The isotopic composition of DIC was measured in pore fluid subsamples that had been filtered through disposable 0.45 μm Gelman polysulfone filters, preserved with HgCl2, and flame sealed in glass vials immediately after collection.

Samples from Sites U1325, U1326, and U1329 were analyzed at Oregon State University using a GasBench II automated sampler interfaced to a gas source stable isotope mass spectrometer as described in Torres et al. (2005). The sample headspace is flushed automatically with helium to reduce residual air. Acid (100 μL of 43% H3PO4) is then injected with a gas-tight syringe and the samples are allowed to evolve DIC as CO2 gas into the headspace. After an equilibration period of ~12 h, the headspace gases are flushed with a helium stream, which passes through a sample loop of selected volume (50, 100, or 250 μL). The sample loop is charged with gas, and a known volume of sample is then transferred to a second helium stream that flows through a gas chromatography column (to separate CO2 from other gas compounds) and a porous membrane trap (to remove water). The dry sample stream is transferred to a Finnigan DELTAplusXL mass spectrometer, which integrates the relevant isotope masses (m/z = 44, 45, and 46) as the CO2 peak enters the source. The sample volumes used for these pore fluid samples (with alkalinity values ranging from seawater values to concentrations >100 meq/L) ranged from 30 to 50 μL. This low volume requirement allowed for duplicate measurements in selected samples to ascertain data precision. Based on multiple standard measurements, the overall precision of this technique is conservatively estimated to be better than ±0.15‰ (Torres et al., 2005). Standardization is provided by tank CO2 (referenced to an array of international standards), and analyses are monitored against a stock solution of reagent NaHCO3. The precision of the δ13C measurements based on replicate analyses of a NaHCO3 stock solution over a 7 week period is ±0.07‰.

Pore fluid samples from Sites U1327 and U1328 were analyzed for δ13CDIC at Scripps Institute of Oceanography. Samples (2–3 mL) were treated with phosphoric acid, and after headspace equilibration, the isotopic ratios were measured using a Thermo-Finnigan Delta XP Plus stable isotope ratio mass spectrometer. The average precision of the analyses, determined from multiple determinations of National Bureau of Standards (NBS) and in-house standards, is 1.8% (~0.1‰ of seawater).