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. 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.

Dissolved oxygen analyses:

This was carried out by microprofiling sediment cores in a cold room using voltametric gold-amalgam (Au/Hg) microelectrodes, as described in detail elsewhere (Brendel and Luther, 1995; Luther et al., 1998; Luther et al., 2008). An Analytical Instrument Systems model DLK-100 voltammetric analyzer was used for all measurements, with profiling carried out using an AIS computer-controlled automated micromanipulator. Oxygen was determined by linear-sweep voltammetry (voltage range: -0.1 to -1.8 V, scan rate: 200 mV/s), and quantified based on the peak height at roughly -0.3 V. Electrodes were calibrated using a two-point calibration curve with O2 saturated bottom water and the electrode response in the deeper O2-free zone of the sediments (Cai and Sayles, 1996). Five scans were carried out at each depth, with the last three scans typically averaged for concentration determinations.