Sediment cores were retrieved from bioturbated, intertidal sediments at low tide with a 7.5 cm (inner diameter) clear Plexiglas liner by pushing it directly into the sediment with minimum pressure as not to artificially force the sediment horizons together. The end of the core (i.e., the deepest horizon) was plugged with a rubber stopper and the sediment core was placed on ice. Typical transport back to the laboratory for pore water extraction was 0.5 hours. Sediment temperature and bottom water salinity were recorded at the time of sampling with an alcohol thermometer and refractometer, respectively.

Once back to the laboratory, the cores were removed from ice and 5 cm Rhizons (0.16-0.19 um pore size) were inserted into pre-drilled 7 mm holes at 1 cm depth intervals to a depth of 10 cm. Pore waters were extracted by pulling negative pressure on the Rhizon with a 10 mL sterile syringe and holding the syringe plunger in place with a small wooden block placed between the syringe body and the plunger. Once pore water was extracted in the syringe, it was removed from the Rhizon, dispensed into a 15 mL centrifuge tube, and 250 uL of pore water was immediately transferred to 250 uL of Ferrozine buffer (10 mM in 50 mM HEPES buffer) and read on a MultiSkan MCC plate reader at 562 nm absorbance. The sediment core was then extruded and sliced into 1 cm intervals and dried in an oven at 70-80 degrees C for 24 hours, and then poorly-crystalline iron oxides (i.e., ferrihydrite and lepidocrocite) were extracted with 1 M hydroxylamine HCl in 25 % acetic acid (v/v) for 48 hours on a rotating shaker at 200 rpm. The extractions were allowed to settle for a few hours, then 10 uL was diluted into 990 uL (1:100 dilution) of distilled water containing 100 uL of Ferrozine buffer. The samples were read as above at 562 nm on the Multiskan MCC plate reader.

Note: data were not collected for months of August and October .