Sediment and pore water collection:

Short sediment cores were collected using a Bowers & Connelly megacorer, a multiple coring device that can collect ~20-40 cm long sediment cores with undisturbed sediment surfaces. At two sites (stations 41 and 64) longer cores (up to ~2 m) were also collected with a Kasten corer.

Megacorer cores were either sectioned for solid phase analysis, profiled with polarographic microelectrodes to determine dissolved O2 concentrations, or sectioned in a cold van under N2 for pore water sample extraction. Samples for solid phase analyses were placed in pre-cleaned screw-capped vials and frozen for porosity determinations at the New England Oceanographic Laboratory (NEOL).

Kasten cores were brought into a large cold room on-board ship, laid on their side and one side of the core box removed to expose the sediment in the core. A plastic block was placed against the top of the core to prevent slumping of the sediment during processing, and pore waters were collected from these cores using Rhizon samplers inserted directly into the cores at measured intervals. After pore water sampling was complete, samples for solid phase analyses were removed from the cores with plastic spoons and again placed in pre-cleaned screw-capped vials for porosity determinations.

While it is possible to recover cores with intact sediment-water interfaces using a megacorer, loss of surface sediments is typical during Kasten coring, making it not possible to directly determine absolute depths below the sediment-water interface in a Kasten core. We therefore determined the absolute depths of pore water and solid phase sample intervals from Kasten cores by aligning Kasten core profiles of pore water alkalinity to megacore alkalinity profiles from the same site (Berelson et al., 2005; Komada et al., 2016).

Sediment porosity determination:

The water content of the frozen porosity samples was determined by weight difference, where each sample was thawed, and a portion weighed, dried at 60°C for 24 hours and then reweighed. The porosity was expressed in terms of ml of seawater per cubic centimeter of whole sediments, where the weight of dried sediment was reduced by the weight of sea salts within the pore water (assuming bottom water salinity), and the volume of dried sediments was calculated assuming a dry density of 2.65 g cm-3 (Christensen, 1989).