Dataset: Sediment porosity collected from R/V Oceanus cruise OC1906A and R/V Sikuliaq cruise SKQ202016S off the coast of California in 2019 and 2020

Sediment cores were collected using a gravity corer and a multicorer on both cruises. Following recovery, the gravity cores were secured horizontally on the ship's deck and sampled from the bottom of the core upwards. This was done by sequentially removing 10-centimeter (cm) sediment intervals by cutting the core liner using a pipe cutter. Freshly exposed sediment was placed in pre-weighed scintillation vials, and the vials were frozen on-board the ship.

Most multi cores were sectioned at intervals ranging from 1 to 5 cm over the 30-40 cm length of the core. Some cores were sampled using pre-drilled holes in multi-core tubes that were sealed with tape during deployment. Upon recovery of the cores, the holes were cut open to sample the sediment with open-faced syringes inserted horizontally into the holes in the core tube. In both cases, the collected sediment was again placed in pre-weighed scintillation vials, and the vials were frozen on-board the ship.

Frozen vials were returned to the shore-based lab at ODU where the samples were dried to constant weight, to obtain the wet-to-dry mass ratio of the sediment sample (±1% precision). A dry sediment density of 2.67 gr/cm3 and a pore water density of 1.024 gr/cm3 were used in the porosity calculations (cm3 pore water/cm3 total sediment). All values were corrected for the salt content of the pore water.

While it is possible to recover intact sediment-water interfaces using multi-corers, loss of surface sediments is typical during gravity coring, making it impossible to directly quantify absolute depths below the sediment-water interface in a gravity core. We therefore determined absolute depths of sediment sample intervals in gravity cores by aligning DIC,  sulfate, ammonium, and porosity gravity core profiles to multicore profiles from the same site (Berelson et al., 2005; Iversen and Jørgensen, 1985; Komada et al., 2016).