See Hardisty et al., 2021 for a detailed description. A brief description is provided below.

Samples were collected onboard the R/V Falkor at the ETNP ODZ in June and July of 2018 (cruise number FK180624). For the present study, we targeted five depths (95, 105, 145, 168, and 475 meters) at 14N, 110W and one depth (151 meters) at 14N, 115W. Briefly, seawater from the Niskin bottles was collected within <0.5 hours of retrieval into ground-glass reagent bottles and prioritized prior to that necessary for other chemical analyses. The bottles were first rinsed 3x with the sample seawater which was discarded. The bottle was allowed to overflow its volume 3x while seawater was continuously added. A glass stopper was added so that it displaced seawater at the neck of the bottle. This entire process lasted <5 minutes for each sample collected. The samples were then immediately stored in a refrigerator at 4 degrees C prior to setting up the experiments, which were within 3 hours of sample collection. Unfiltered seawater was used for the experimental incubations at all depths. For one depth, 151 meters, we performed an additional experiment with 0.2 um filtered seawater. All tracer addition and seawater aliquoting for the experimental setup was performed in a glove bag flushed with N2 at 3x its volume and then episodically flushed throughout the experimental setup. For each bottle collected, a pipette was used to remove water from the top at a volume similar to that of the tracer to be added in the subsequent step. A 127IO−3 spike and additional I− radiotracer with a known 129I/127I ratio.

The amended seawater was then aliquoted in triplicate into individual 30-60 mL glass serum bottles representing each time point. To remove potential O2 contamination relicts from the sampling process, the seawater was immediately bubbled for 20-30 minutes with helium through a needle with vented headspace. The samples were incubated in the dark at 11 degrees C for up to 60 hours. Triplicate sample bottles were independently incubated for each time point, which varied between incubations but were generally at 0, 12, 24, 48, and 60 hours. Upon harvesting, samples were immediately filtered through a 0.2 um filter and stored in the dark at 4 degrees C.

The concentrations of I− from the incubations were determined shipboard <2 weeks after sample collection via the methods outlined in Moriyasu et al. (2020) via a hanging mercury drop (Hg) electrode and a calomel reference electrode.

Iodine isotope ratios were determined via the methods outlined in Hardisty et al., (2020) and Hardisty et al., (2021) using chromatographic separation and subsequent analysis via multi-collector ICPMS.