Porewater analyses

During the Equatorial Pacific expedition, NO3- concentrations were analyzed with a Metrohm 844 UV/VIS Compact IC equipped with a 150 × 4.0 mm Metrosep A SUPP 8 150 column. The column temperature was set to 30°C. The eluent was a 10% NaCl solution filtered through a 0.45 μm pore size filter. Approximately 0.8 ml of interstitial water was injected manually into a 250 μL sample loop. UV absorption was determined at 215 nm.

We analyzed porewater samples for dissolved Mn⁴⁺ and Fe³⁺ by inductively coupled plasma emission spectrometry (ICP-ES) at Boston University. We acidified the samples at sea with Fisher Optima HNO₃ and stored them in a refrigerator until analysis. The samples were diluted 10-fold in 2% triple distilled HNO₃, and calibrated against a seven point calibration curve of matrix matched, spiked International Association for the Physical Sciences of the Ocean standard seawater (IAPSO, S = 35, Ocean Scientific International Ltd.) that covered the full concentration range of the unknown samples. We measured Mn⁴⁺ at 257.609 and 259.373 nm using the ICP-ES monochromator and at 257.61 nm using the ICP-ES polychromator with the Jobin-Yvon Ultima C ICP-ES instrument. Using the ICP-ES monochromator, we measured Fe³⁺ at 238.203 and 259.939 nm, as well as at 259.94 nm using the ICP-ES polychromator. We corrected final concentrations for the 1% acid dilution due to on-board acidification. Analytical precision for both elements is ~3% of the measured value. Accuracy was confirmed by analysis of spiked IAPSO standards that were not included in the original calibration.

We quantified dissolved SO42- in porewater samples from Lake Van and Barents Sea samples using a SYKAM IC system (SYKAM, Fürstenfeldbrück, Germany) equipped with an LCA A14 column, a suppressor (SAMS™, SeQuant, Sweden) and a S3115 conductivity detector. The mobile phase was a 6.25 mmol L⁻¹ Na₂CO₃ with 0.1 vol% modifier (1g 4-hydroxy-benzonitrile in 50 mL methanol). We eluted the samples at isocratic conditions at 50°C with the eluent flow set to 1 mL min⁻¹, injection volume was 30 μL.

During the Equatorial Pacific expedition, we quantified SO42- concentrations with a Metrohm 861 Advanced Compact IC. The IC was comprised of an 853 CO₂ suppressor, a thermal conductivity detector, a 150 × 4.0 mm Metrosep A SUPP 5 150 column, and a 20 μL sample loop. A Metrohm 837 IC Eluent/Sample Degasser was coupled to the system. We set the column oven to 32°C. The eluent was 3.2 mmol L⁻¹ Na₂CO₃, and 1.0 mmol L⁻¹ NaHCO₃. Each sample was analyzed as a 1:50 dilution of interstitial water with 18 MOhm cm⁻¹ deionized water. If the samples were anoxic, we added 5 μL of 10% Zn-acetate per mL of analyte to precipitate ZnS.