Porewater pH samples were collected in 3-mL capacity gas-tight borosilicate glass syringes with triple O-ring seals. A 2 – 2.5 mL volume was drawn from each coil into a syringe. 1 mL of sample was added to a cuvette for AT measurements and the remaining 1 mL was used for pH measurements. To each sample, 4 μL of 2 mM mCP (R ≃ 1.2) was added using a 10 μL Eppendorf Research pipette fitted with thin Teflon tubing at the end of the pipette tip. The sample with added dye was capped and thoroughly mixed, then housed in a custom-made cell warmer and thermostatted to 20°C (±0.05 °C). During the sampling process, care was taken to ensure that samples were not exposed to the atmosphere and that there was no entrapment of air bubbles within the syringe.
For small volume total alkalinity (AT) measurements, 1 mL of sample was pipetted directly into a semi-micro cuvette (1-cm pathlength, Fisher Cat 14-955-127) with a syringe or an Eppendorf Research pipette. Samples were covered with a polyethylene cuvette stopper (Cat. S29264) and placed inside a semi-submerged cuvette tray thermostated to a set temperature (e.g., 20°C) in a large-capacity water bath (Lauda Ecoline RE 120 water bath).
DIC and d13C of DIC were analyzed with a Picarro Cavity Ring-Down Spectrometer (G2131-i) with Liaison autosampler; the detailed methodology is described in Subhas et al. (2015). These measurements were made on board the ship. Dickson seawater CRM was used as the standard for DIC; pre-weighed optical calcite powder was used as the standard for d13C. Exetainer vials were pre-acidified and pre-weighed in the lab prior to the cruise. After the vials were filled with 3-5 mL of porewater and analyzed on the cruise, the stored vials were weighed again in the lab to obtain the sample mass. Results using this methodology were corrected by normalizing to measured values of Dickson seawater CRM. Uncertainty (1σ) for replicate DIC and d13C measurements were ±23 µmol/kg and ±0.15‰ (VPDB), respectively. DIC uncertainty was higher than reported in our previous studies (e.g., Subhas et al., 2015) likely due to mass determination: ship-board analysis necessitated weighing after analysis, but there was uncertainty in precisely how much sample mass may be removed during the analysis. Additionally, lower sample volume (5 mLs, instead of 7-8 mLs in our previous studies) may have added error.
Porewater dissolved manganese [Mn] was analyzed at Caltech by ICP-MS (inductively coupled plasma - mass spectrometry) using an Agilent 8800 triple-quadrupole instrument. In situ porewater samples were stored in acid-cleaned HDPE bottles and acidified using 6M distilled HCl. After at least one week, 1 mL aliquots were diluted 10:1 in distilled 5% HNO3 for ICP-MS analysis. Concentrations of [Mn] were determined via calibration to multi-element commercial ICP-MS standards, matrix-matched to samples by addition to artificial seawater and diluted in 5% HNO3, identically to the samples. Blanks were assessed by analysis of both pure 5% nitric acid and artificial seawater with no metal standard added. ICP-MS measurements were made in MS/MS mode using He as a carrier gas in the collision cell. The detection limit was 0.03 μmol/kg [Mn].
Porewater calcium, strontium, and sodium were measured at Northwestern University using a Thermo iCap7600 ICP-OES (inductively coupled plasma - optical emission spectrometer). In order to capture signals from high- to low-abundance cations ([Na] and [Sr], respectively), samples were diluted 1:100, by mass, with trace metal clean 3% HNO3. Each sample was weighed, diluted, and analyzed in five replicates. Samples were run in randomized order to account for any instrumental drift not corrected through standard bracketing. Cation concentrations were based on counts per second using two wavelengths from each cation (Ca393.366, Ca396.847, Sr407.771, Sr421.552, Na588.995, and Na589.592 nm). The two wavelengths for each element produced virtually identical results but were averaged, nonetheless. Calibration standards were synthetic mixtures of 1000 ppm cation solutions (Mg, Sr, Na, Ca, and K) diluted in 5% HNO3 to match expected sample concentrations. Calibration standards were used to correct for instrumental drift within a single run by interpolating between standards run at the beginning and end of the run. Blank measurements were made on HNO3 and were included in the calibration curve to account for instrumental blank. IAPSO standard seawater was used to adjust for drift across multiple days. Between 3-8 IAPSO standards were measured every run; the IAPSO average in a single run was normalized to the IAPSO average for all runs. This normalization was applied to all porewater samples. In this manner, samples analyzed on different days were normalized to a single measured IAPSO average. [Ca] and [Sr] were normalized to [Na]; all stations had a CTD measured salinity of 34.6 ppt; salinity was converted to [Na] using a seawater Na:salinity ratio of 10.781 g/kg:35 ppt (Pilson, 1998). IAPSO seawater measurements had the following one standard error spreads: ± 0.026 mmol/kg Ca, ± 0.56 μmol/kg Sr, and 1.5 mmol/kg Na.
Porewater silica was measured at USC. A spectrophotometric molybdenum blue method, modified from Parsons et al. (1984), was used to determine the concentration of silicic acid (H4SiO4 or DSi) in the pore water samples. Filtered pore water samples collected on the cruise were stored at 4°C and transported back to USC for analysis. Standards were made via dilution of 1,000 ppm Si standard solution (Aqua Solutions) in DSi free artificial seawater. The precision of this method, determined by duplicate analyses, was 1.3% for samples with DSi concentrations > 20 µM (< 20 µM analytical precision = 13.1%).
Instruments:
The pH and total alkalinity (AT) of porewater samples were measured using a multiparameter CO2 instrument named mvMICA (minimal volume Multiparameter-Inorganic Carbon Analyzer). The mvMICA is a dual parameter system engineered to simultaneously measure AT with sample volumes as low as 0.5 mL and pH with 1 and 1.5 mL (Fleger et al., in prep). The mvMICA configuration consisted of pH and AT channels encased in a thermoregulating water bath. Optical fibers connected channels to a custom tungsten halogen light source and Ocean Optics USB4000 spectrophotometers. Spectrophotometers were housed in a small refrigerator regulated to 18°C. The spectrophotometers were connected to a portable computer with custom software that monitors and records absorbances and sample information. The encased water bath had a lid with openings for AT sample input, CO2 gas tubing, and two Luer lock ports for pH sample input and output.
The submerged pH channel was affixed to the bottom of the case. The pH optical cell consisted of a gas impermeable PEEK tubing (1/8 in OD, 1/16 in ID) with T-connectors on either side to control sample throughput and extend fiber-optical leads within the cell.
The pH and AT of porewater samples were measured using the mvMICA (see section 3.2). Small-volume spectrophotometric AT measurements were made by directly purging the sample with CO2 gas (Fleger et al., in prep) and monitoring the absorbances until equilibration was reached.
A semi-micro disposable cuvette has a small volume whereby 1 mL of sample fills the cell to more than 50% of the cell height. A mounted cuvette holder secured the AT cuvette inside the mvMICA. The holder was positioned so that the top of the cuvette (∼0.5 cm) was above the water bath level, but sample volume remained equilibrated to bath temperature. Optical fibers are positioned just above the bottom of the cuvette. The purging tube assembly includes a CO2 equilibration tube that fits into the cuvette cover and is secured to control the positioning of the purging tube without obstructing the light path. Before purging the sample, water-saturated 10% CO2 gas was pre-equilibrated to 20°C by passing through a two-meter-long copper coil immersed in a thermostated water bath.
DIC and d13C of DIC were analyzed with a Picarro Cavity Ring-Down Spectrometer (G2131-i) with Liaison autosampler.
Porewater dissolved manganese [Mn] was analyzed by ICP-MS (inductively coupled plasma - mass spectrometry) using an Agilent 8800 triple-quadrupole instrument.
Porewater calcium, strontium, and sodium were measured using a Thermo iCap7600 ICP-OES (inductively coupled plasma - optical emission spectrometer).
Porewater silica was measured on a spectrophotometer.
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