The Distribution and Redox Speciation of Iodine in the Eastern Tropical North Pacific Ocean

Abstract

The distributions of iodate (IO3−), iodide (I−), nitrite (NO2−), and oxygen (O2) were determined on two zonal transects and one meridional transect in the Eastern Tropical North Pacific (ETNP) in 2018. Iodine is a useful tracer of in situ redox transformations and inputs within the water column from continental margins. In oxygenated waters, iodine is predominantly present as oxidized iodate. In the oxygen deficient zone (ODZ) in the ETNP, a substantial fraction is reduced to iodide, with the highest iodide concentrations coincident with the secondary nitrite maxima. These features resemble ODZs in the Arabian Sea and Eastern Tropical South Pacific (ETSP). Maxima in iodide and nitrite were associated with a specific water mass, referred to as the 13 °C Water, the same water mass that contains the highest concentrations of iodide within the ETSP. Physical processes leading to patchiness in the 13 °C Water relative to other water masses could account for the patchiness frequently observed in iodide and nitrite, probably reflecting subsurface mesoscale features such as eddies. Throughout much of the ETNP ODZ, iodine concentrations were higher than the mean oceanic value. This “excess iodine” is attributed to lateral inputs from sedimentary margins. Excess iodine maxima are centered within a potential density of 26.2–26.6 kg/m3, a density range that intersects with reducing shelf sediments and is almost identical to the ETSP. Evidently, margin input processes are significant throughout the basin and can influence the nitrogen and iron cycles as well, as in the ETSP.

The role of water masses in shaping the distribution of redox active compounds in the Eastern Tropical North Pacific oxygen deficient zone and influencing low oxygen concentrations in the eastern Pacific Ocean

“…Within the ETNP ODZ, there are water parcels with zero iodate (Moriyasu et al 2019), unlike in the ETSP ODZ (Cutter et al 2018). Figure 8c demonstrates that these parcels exist around and between the secondary nitrite maxima in the ETNP ODZ, since these nitrite maxima are shaped by subsurface mesoscale eddies.…”

“…1). While the appearance of iodide would be expected as an indicator for the absence of oxygen, lateral advection of iodide from reducing shelf sediments complicates this analysis (Moriyasu et al 2019;Cutter et al 2018;Zhou et al 2018a). The distribution of iodide within the oxygen deficient layer is depicted in Fig.…”

“…A station from a previous cruise demonstrates that the 13CW shoals onto the continental shelf (Section S6 in Supporting Information), so this water mass contains the highest iodide concentrations from this additional source and its inherent low oxygen concentration. Since oxygen is required for iodide to convert into iodate (Luther et al 1995), the turnover of iodide is slower than nitrite and influenced primarily by mixing processes, and these factors cause a more patchy distribution of iodide than nitrite (Moriyasu et al 2019).…”

Surface Inorganic Iodine Speciation in the Indian and Southern Oceans From 12°N to 70°S

“…Comparable high iodide concentrations have previously been observed in low oxygen sub-surface waters in the north western part of the Arabian Sea (Farrenkopf and Luther, 2002); these were attributed to advection from shelf sediments. More recently, plumes of very high iodide sub-surface concentrations (∼1000 nM) have also been reported in sub-oxic waters in the Eastern Tropical South Pacific (ETSP) (Cutter et al, 2018) and the Eastern Tropical North Pacific (ETNP) (Moriyasu et al, 2020). In both cases some outcropping of elevated iodide concentrations at the ocean surface was observed (Cutter et al, 2018;Moriyasu et al, 2020).…”

“…Iodine is naturally present in the ocean, predominantly as the inorganic ions iodide (I − ) and iodate (IO 3 − ). Iodine speciation is linked to many aspects of ocean biogeochemistry, and has been proposed as a tracer of primary productivity (Wong, 2001;Ducklow et al, 2018), sedimentary inputs and oxygen status (Lu et al, 2018;Moriyasu et al, 2020). In addition, the concentration of iodide at the sea surface has recently attracted renewed interest from atmospheric chemists because of its impact on atmospheric composition and air quality e.g., (Sherwen et al, 2017;Cuevas et al, 2018).…”