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doi:10.2204/iodp.proc.344.208.2019

Analytical methods

Pore water analyses (carbon, oxygen, and hydrogen isotopes)

Pore water extraction was conducted on board the R/V JOIDES Resolution following the procedures described by Expedition 334 Scientists (2012b) and Harris et al. (2013b). The squeezed pore fluids were collected in precleaned, plastic syringes attached to the squeezing assembly and subsequently filtered through a 0.45 µm Gelman polysulfone disposable filter. Samples collected for oxygen and hydrogen analyses were sealed in glass ampoules. Samples for isotopic composition of dissolved inorganic carbon (DIC) were collected in 2 mL glass vials and preserved with 10 µL of saturated HgCl₂.

Pore water oxygen and hydrogen stable isotopic compositions were measured at the University of Washington (USA) on a Picarro Cavity Ring-Down Spectrometer water analyzer (Model L2130-i). The data are reported in standard delta notation relative to Vienna standard mean ocean water (VSMOW). The precision of the δ¹⁸O and δD measurements based on repeated analyses of VSMOW and several in-house standards is better than ±0.05‰ and ±0.2‰, respectively.

The stable isotopic composition (δ¹³C) of the DIC was measured at Oregon State University (USA) using a GasBench II automated sampler interfaced to a gas source stable isotope mass spectrometer as described by Torres et al. (2005). The precision of the δ¹³C measurements based on replicate analyses of a NaHCO₃ stock solution is better than ±0.1‰. Instrument calibration was accomplished by comparison to National Institute of Standards and Technology (NIST)-8544 limestone (also known as NBS-19) prepared using a Kiel-III online acid digestion device, which yields isotopic values of –2.19‰ ± 0.06‰ for δ¹⁸O and 1.94‰ ± 0.02‰ for δ¹³C (N = 25). These values compare well with certified values of –2.20‰ and 1.95‰ for δ¹⁸O and δ¹³C.

Solid-phase analyses

Carbonate samples from Hole U1380C were selected from the lithified sediments in Units II and III. The total CaCO₃ content was measured on board the JOIDES Resolution using a UIC 5011 CO₂ coulometer, as described by Expedition 334 Scientists (2012b) and Harris et al. (2013b).

Two leaching procedures were compared in their effectiveness at maximizing carbonate leached (represented by fraction carbonate leached) while minimizing clay contamination (represented by a molar ratio of aluminum to calcium): a 4.4 M acetic acid leach (henceforth referred to as the “hard” leach) and a buffered acetic acid leach (referred to as the “soft” leach).

For the hard leach approach (method of Joseph et al., 2012), 8 mL of 4.4 M acetic acid was added to 10 mg of powdered sediment in Teflon vials (Savillex) and refluxed for 12 h at 70°C. Samples were then sonicated and centrifuged, dried down, and redissolved in 3% nitric acid (v/w) for analysis. The soft leach procedure uses a modified version of the buffered acetic acid leaching method outlined by Du et al. (2016). About 0.2 g of powdered sample was added to preleached 15 mL Falcon tubes to which 6.5 mL ultrapure (resistivity >18 MΩ) water (Milli-Q water purification system; Millipore; USA) and 0.6 mL buffered acetic acid (0.5 M sodium acetate and 3.3 M acetic acid in ultrapure water buffered to pH 4) were added. The tubes were shaken on a rotary benchtop shaker for 3 h, centrifuged for 10 min at 4000 rpm, and the leachate was decanted and filtered using 0.45 µm filters into clean Teflon vials. These leached fractions were then dried down, refluxed for 1 h with 1 mL concentrated HNO₃, dried down a second time, and refluxed for 1 h with 2 mL of 6 M HNO₃.

Elemental analysis of carbonate samples

Major (Al, Ca, Fe, and Mg) and minor (Ba, Mn, and Sr) elements in carbonate were analyzed on an Ametek SPECTRO ARCOS inductively coupled plasma–optical emission spectrometer at the W.M. Keck Collaboratory for Plasma Spectrometry at Oregon State University. Element concentrations are reported as a molar ratio to calcium. Experimental uncertainties reported are 2σ errors based on the three replicate analyses performed by the instrument per sample tube. Detection limits were calculated by multiplying the standard deviation of the three blank measurements for each element by the corresponding t-value for a 99% confidence interval (df = 2; t = 6.965). This value was then regressed on the standard curve to give the detection limit in parts per million.

Isotopic analyses of carbonate samples

In preparation for strontium isotopic analysis, aliquots of the sediment leaches containing ~200 ng Sr were loaded onto in-house made microcolumns containing 50 µL of Sr spec 100–150 µm resin from Eichrom. The Sr fractions were subsequently diluted 50/50 with 3% HNO₃, and the isotopic analysis was performed using a Nu Plasma multicollector inductively coupled plasma–mass spectrometer at the W.M. Keck Collaboratory for Plasma Spectrometry. The ⁸⁷Sr/⁸⁶Sr ratios were doubly corrected: once by using the intraelemental correction for mass fractionation (⁸⁶Sr/⁸⁸Sr = 0.1194) and again by internally normalizing the ratios to NIST standard NBS-987, with repeated measurements yielding a ⁸⁷Sr/⁸⁶Sr ratio of 0.701245 ± 0.000027 (2σ_mean; N = 115). Instrument precision was determined through replicate analysis of an in-house standard with a measured ⁸⁷Sr/⁸⁶Sr ratio of 0.708183 ± 0.000028 (2σ_mean; N = 100). These standards were run every six samples and in replicate at the beginning and end of every batch run.

Carbonate samples were isotopically characterized at Oregon State University using traditional phosphoric acid digestion and isotope ratio mass spectrometry (IRMS). Carbonate powders were reacted with 100% phosphoric acid at 75°C; and isotopic composition was measured using the Kiel III device connected to a Thermo Fisher 252 gas source mass spectrometer. Isotopic analyses with this technique yield a precision better than ± 0.1‰ and 0.2‰ for δ¹³C and δ¹⁸O, respectively. Instrument calibration was accomplished by comparison to NBS-19.

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