Volume 34, Issue 2 e2019GB006302
Research Article
Free Access

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

Rintaro Moriyasu

Rintaro Moriyasu

Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA

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Natalya Evans

Natalya Evans

Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA

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Kenneth M. Bolster

Kenneth M. Bolster

Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA

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Dalton S. Hardisty

Dalton S. Hardisty

Department of Earth and Environmental Sciences, Michigan State University, East Lansing, MI, USA

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James W. Moffett

Corresponding Author

James W. Moffett

Department of Biological Sciences, University of Southern California, Los Angeles, CA, USA

Correspondence to:

J. W. Moffett,

[email protected]

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First published: 28 January 2020
Citations: 36

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.

Key Points

  • The Eastern Tropical North Pacific oxygen deficient zone is strongly enriched in iodide relative to iodate
  • Enriched iodide indicates robust transport from shelf sediments to the interior basin
  • Similarity to oxygen deficient zones in the Eastern Tropic South Pacific and Arabian Sea reveals importance of common processes

1 Introduction

1.1 Iodine Redox Chemistry

Iodine, a biologically important trace element, is found in most of the world's ocean at concentrations averaging 470 nM (Luther et al., 1995). It exists primarily in the form of iodate (IO3), which is thermodynamically favored, but it is also found as iodide (I). Much attention has been focused on its redox chemistry in surface waters which is driven by biology and photochemistry. An important product of these reactions are methyl-iodide and other volatile, halogenated methyl species; these organic-iodine species are the main transport of iodine from seawater into the atmosphere (Lovelock et al., 1973; Moore & Tokarczyk, 1992). In surface waters, IO3 is biologically and photochemically reduced to I (Luther et al., 1995; Tsunogai & Sase, 1969; Wong et al., 1976; Farrenkopf & Luther, 2002; Chance et al., 2014), which accumulates to 50–250 nM. Remarkably little is known about how I- is oxidized back to IO3. The abiotic oxidation of I to IO3 does not occur readily in nature because the oxidation by oxygen (O2) is kinetically unfavorable. Oxidation by reactive oxygen species, peroxides, hydroxyl anion, and hydroxyl radicals are also possible and produce hypoiodous acid (HOI), which reforms to I or reacts readily with organic matter to form organo-iodine compounds (Luther et al., 1995).

Oxygen deficient zones (ODZs) are defined in this paper as regions where O2 is absent or negligible and nitrate (NO3 ) is the primary terminal electron acceptor (Devol, 2015). There are three large, persistent ODZs in the Eastern Tropical North Pacific (ETNP) off the coast of Mexico, the Eastern Tropical South Pacific (ETSP) off Peru, and the Arabian Sea (Codispoti et al., 2001). Oxygen is at nanomolar levels and effectively zero under some conditions (Thanmdrup et al., 2012), and there is a characteristic enrichment of nitrite (NO2-) associated with denitrification. In ODZs, IO3 is utilized as a terminal electron acceptor in respiration and possibly as an oxidant by chemoautotrophic bacteria. Since IO3 is nearly as strong an oxidant as O2, and much stronger than NO3, there is no way to produce it in the absence of O2 (Cutter et al., 2018). Therefore, any IO3 in an anaerobic system must be preformed when that water parcel was oxygenated. Farrenkopf et al. (1997) observed reduction of IO3 during shipboard incubation in the Arabian Sea, and two separate studies have observed dissimilatory IO3 reduction in seawater mediums inoculated with bacteria (Amachi et al., 2007; Farrenkopf et al., 1997). Babbin et al. (2017) hypothesized that IO3 can act as an oxidant for NO2 at the top of the ODZ; this reaction forms NO3 and I. Cutter et al. (2018) showed that IO3 persisted at low concentrations throughout the ETSP ODZ, an important observation discussed throughout this paper.

1.2 Iodide and Iodate Distributions in ODZs

Iodide concentrations are relatively low in comparison to IO3 in the oxygenated water column, but I dominates iodine speciation in the ODZ of the Arabian Sea and the ETSP (Farrenkopf & Luther, 2002; Cutter et al., 2018). Compared to the other two ODZs, where detailed transects have been evaluated, iodine speciation in the ETNP is less well studied. Rue et al., (1997) reported a single depth profile showing quantitative conversion of IO3 to I, within the ODZ, suggesting that the iodine redox couple is a sensitive indicator of its redox environment; in this case, the total iodine was unchanged throughout the profile. In contrast, the Arabian Sea (Farrenkopf & Luther, 2002) and the ETSP (Cutter et al., 2018) exhibited strong local maxima in total iodine within their ODZs. This “excess” in total iodine is defined as the difference between the measured total iodine and the expected total iodine concentration. Several approaches have been used to determine the “expected” iodine concentration. Farrenkopf & Luther, (2002) used an empirical relationship between I and salinity to obtain their expected value within the ODZ. Cutter et al., (2018) calculated an expected IO3, within the ODZ, based on an empirical relationship between phosphate (PO43−) and IO3 derived below the ODZ depths.

1.3 A Margin Source for Excess Iodine

Farrenkopf & Luther, (2002) argued that excess iodine in the Arabian Sea must be derived from reducing shelf sediments rather than remineralization of sinking organic particles because the I:C ratio has a much higher sedimentary organic matter than particulate organic matter in the water column. They reported an excess in iodine concentrations throughout the Arabian Sea. Similarly, Cutter et al. (2018) found, in the ETSP, iodine concentrations exceeding total inorganic iodine found in oxic marine waters. These excesses in I concentrations cannot be explained by in situ IO3 reduction (Cutter et al., 2018).

1.4 Comparative Assessment of Iodine Behavior in Different ODZs

The single iodine profile from the ETNP, reported by Rue et al. (1997), showed that IO3 was undetectable throughout the ODZ where O2 was absent. This implied that in the absence of O2, IO3 is quickly consumed, presumably as a terminal electron acceptor in respiration, which is consistent with experimental observations of IO3 reduction in seawater (Amachi et al., 2007; Farrenkopf et al., 1997). However, in the ETSP, residual IO3 was detected throughout the ODZ in all stations, which suggests that IO3 utilization may be under kinetic control. The data from the ETNP and ETSP suggest that iodine distribution in the ODZ cannot be expressed as a simple function of O2. In some cases, as in Rue et al. (1997), the redox speciation appears to be controlled by O2 distribution. However, in the ETSP, the approach to the equilibrium predicted distribution is incomplete, and the sharp local maxima seem similar to those observed in the profiles for the nitrogen cycle (i.e., NO2) rather than for O2. The Arabian Sea has features common to both of these end members (Farrenkopf & Luther, 2002).

We conducted a survey of iodine chemistry throughout the ODZ of the ETNP. Since there is currently only one reported profile from VERTEX III, 15°N, 107°W (shown as “X” in Figure 1) (Rue et al., 1997), it was the least studied of all three ODZs with respect to iodine. This study is the most extensive measurements of iodine in a single study to date, and it utilized a broad array of hydrographic and nitrogen cycle parameters to evaluate relationships between ocean physics, O2, NO2, and iodine distribution. We also used the data to verify a water mass analysis, reported in a companion paper (Evans et al., 2020), which provide insight into our results.

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Map of combined Revelle and Falkor cruises. Stations prefixed by “RR” connotes stations taken aboard the Revelle; stations prefixed by “F” connotes stations taken aboard the Falkor. X on map denotes location of VERTEX III (Vertical Transport and Exchange 15°N, 107°W, 19–26 November 1982).

2 Sampling

Samples were collected from two cruises on the R/V Roger Revelle from 26 March to 2 May 2018, and the R/V Falkor from 24 June to 17 July 2018 (Figure 1).

Samples were collected, using a Seabird CTD, with calibrated sensors for temperature, conductivity, pressure, fluorometer, transmissometer, and O2 concentrations. The latter was made using an SBE 43 sensor. Each one of these CTD carousels held twenty-four 12 L GO-FLO bottles, which were closed at selected depths.

Samples were filtered using a 0.2 μm (AcroPakTM 200) filter within 2 hr of sample collection using a masterflex pump. Filtered samples were measured typically within 6 hr of filtration and were frozen when this was not feasible. Frozen samples were brought back to the laboratory, left frozen for 1–3 months, and then measured within 24 hr after defrosting. Previous studies have shown that iodine is stable on the timescales of months to years in filtered seawater (Campos, 1997). One study of IO3 reduction, in the Arabian Sea, found that samples collected from the ODZ (quantitatively I) were found to show no oxidation of I (Farrenkopf et al., 1997). This was even after prolonged exposure of the samples to atmospheric O2 while sitting on the lab bench.

3 Methods

Iodide was analyzed using cathodic square wave stripping voltammetry with a hanging mercury drop electrode (HMDE) and a calomel reference electrode (Rue et al., 1997), adapted from Luther et al. (1988). A 10 mL seawater sample is treated with 150 μL of 0.2% Triton X 100 (Sigma Aldrich—BioX grade) and purged under argon for five minutes. Then 50 μL of 2 M anhydrous sodium sulfite (0.1 M in sample) (Millipore GR ACS) is added to avoid O2 interference (Tian & Nicolas, 1995), and the sample is purged for an additional minute. Duplicate measurements were taken for each sample with a drop size of 6, deposition time of 30 s, and 5 s of quiet time. Scan increments were set to 2 mV with scan range set between −140 to −700 mV; the square wave amplitude and frequency were set respectively to 25 mV and 125 Hz. Each sample was measured twice and I- determined through the method of standard additions using potassium iodide (KI) (Sigma Aldrich—ACS grade). Precision was determined to be ±4 nM (standard deviation with 95% confidence for a sample found to be 243 nM in concentration after five replicates) in the laboratory, but this precision was also found to fluctuate to an order of magnitude higher depending on the wear of the capillary and swell of the ocean when measurements were done shipboard.

Iodate determination was also adapted from Rue et al. (1997) who adapted their method from Wong and Brewer, (1977). Measurements were made on a spectrophotometer (Perkin Elmer Lambda 35) using a 10 cm quartz cuvette. One milliliter of 0.12 M sulfanilamide (Sigma Aldrich—ACS grade) in 1% sulfuric acid (Macron Fine Chemicals) was added to each 25 mL sample to prevent NO2 interference. The sample sits for 5 min after which 1 mL I solution (0.12 M KI in deionized water) is added to the sample to form triiodide (I3). Absorbance of I3 is measured at 353 nm within one minute of iodide addition. IO3 concentrations were determined by the method of standard additions using KIO3 (Baker Analyzed—ACS Reagent). Precision, based on five replicates, was ±12 nM for a seawater sample found to be 286 nM.

4 Results

Figure 2 shows NO2 and salinity versus potential density for the entire data set. Nitrite within the secondary nitrite maximum is clustered within a narrow density range with a potential density anomaly from 26.2–26.6 kg/m3. The ODZ (operationally defined here as a zone where O2 was undetectable) spans a much wider range of potential density anomalies of 25.5–27.25 kg/m3. Therefore, the salinity feature probably contributes to the strong gradients in chemistry and biology observed within this narrow density horizon. This feature is also associated with the highest iodine accumulation (Figure 3), particularly the excess iodine (in this case, the amount exceeding the mean concentration of 476 nM). Remarkably, the relationship between density and iodine is very similar off of the ETSP (Figure 3) for the ETNP and reflects the importance of the 13 °C Water (13CW) at both locations. The importance of this water mass in both systems may reflect the salinity gradient enhancing the pycnocline at the base of the oxycline (Evans et al., 2020).

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A scatterplot of salinity, and nitrite at various isopycnals at all stations measured from both cruises.
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Scatterplot on the left is from of all iodide concentrations (nM) at all isopycnals from both cruises from the ETNP. Scatterplot on the right consists of data taken from the ETSP.

4.1 Sectional Data for All Parameters on Each Transect

Figures 4-6 show sectional data for O2, NO2, I, IO3, and excess iodine for each transect. Each section reveals a large plume of I in the upper ODZ that is roughly coincident with NO2 and associated with a large deficit in IO3. For the two zonal transects (Figures 4 and 5), I is highest near the coast and spans a broader range of depths. Iodide concentrations decrease offshore with the plume of I becoming much thinner. These transects also reveal that the most significant depletion of IO3 occurs in the upper ODZ closest to the coast. However, both transects also reveal isolated pockets of high NO2, I, and low IO3 extending as far as 125°W. These features are largely centered between the 26.2 and 26.6 kg/m3 potential density anomaly contours shown on the map (Figures 2 and 3).

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Transect profiles of oxygen, iodide, iodate, and excess iodine along the western transect from 105.6–128°W.
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Transect profiles of oxygen, nitrite, iodide, iodate, and excess iodine along the western transect from 102°W, 14°N to 119°W, 18°N.
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Transect profiles of oxygen, iodide, iodate, and excess iodine along 110°W from 22.6°N to 14°N.

The meridional transect (Figure 6) shows that NO2 and I, as well as IO3 depletion, do not extend north of 19°N. This is evidently the northern boundary of the denitrifying ODZ. Stations north of this all had detectable O2 with the exception of Station 3, which had <1 μM O2 at one depth, 200 m (26.4 kg/m3 isopycnal). There is a strong maximum in I and NO2 in the southern section of this meridional transect as it crosses the two zonal transects in the core of the ODZ.

4.2 Excess Iodine

Excess iodine was calculated by taking the sum of measured I and IO3 and subtracting a mean value of total iodine (476 ± 9 nM) below ODZ depths from both cruises. Our mean agrees within error with the mean global average of 470 nM determined by Luther et al. (1995) but within the standard deviation. That estimate is generally consistent with observations from individual reports from the Arabian Sea, ETNP, and ETSP (Cutter et al., 2018; Farrenkopf & Luther, 2002; Rue et al., 1997).

Our calculation differs from Cutter et al., (2018) who used empirical ratios between IO3 to PO43− to determine an expected value. However, we found IO3 to PO43− correlations were poor in our data as well as in previously published work, and this poor correlation resulted in large uncertainty propagation in the excess iodine calculation (see supporting information). The differences between the Cutter et al., (2018) and our approach inevitably yield differences in surface waters where PO43- is depleted, but only minor differences in the ODZ and do not influence our conclusions. Farrenkopf & Luther, (2002) used salinity to calculate a conservative IO3 value to derive excess iodine. The salinity range in our study was not sufficient for this correlation to have an impact and resulted in additional error propagation as with PO43−.

Excess iodine was highest at the eastern end of the zonal transects and decreased moving offshore, presumably reflecting a benthic source. In addition, the Revelle zonal transect (Figure 4) revealed a striking maximum in excess iodine well offshore. Presumably, this excess was derived from the margin and separated from the larger maxima to the east by a water mass centered at around 120°W, which is probably Northern Equatorial Pacific Intermediate Water (NEPIW) (Evans et al., 2020) and not influenced as directly by the margin. Figure 5 shows a general decline in excess iodine going offshore but interspersed with local minima at 108°W and 113°W. This fine structure was also observed in the NO2 data. Excess iodine was detected throughout the basin, which suggests that the impact of the margin extends throughout the ODZ to at least 110°W and potentially further west in isolated pockets. Clearly, physical processes influence the patchiness in iodine and NO2 distribution.

4.3 Representative Stations Within and Outside the Transect

Further details are revealed by examining depth profiles for representative stations within and outside of the transect. Revelle Station 1 is outside of the ODZ but is close enough to reducing margin sediments that it has elevated I and excess iodine (Figure 7). Revelle Station 3 has the highest concentration of I observed in this study in a pronounced surface maximum (Figures 8 and 9). Revelle Station 34 was not on any of the transects but was the only shelf station with high I throughout the water column (Figure 10). Falkor Station 9 and Revelle Station 17 were crossover stations with similar distributions of iodine species despite a lengthy interval between sampling. Falkor Station 2 was chosen because two separate CTD casts were deployed spanning a 2 day period with surprisingly different features. Data, for all stations, are included in the supporting information.

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Profiles of iodine species and oxygen at Revelle Station 1 (110°W, 22.7°N).
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Plot of temperature (°C) versus salinity (PSU) of Revelle Station 3. The numerical values above each point refers to depth of sampling in meters. Values between 80 and 120 m show a distinct minima in salinity and does not match local water masses.
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Profiles of chemical species at Revelle Station 3 (110°W, 21.6°N).
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Profiles of chemical species at Revelle Station 34 (105.7°W, 21.3°N).

4.3.1 Revelle Station 1 (110°W, 22.7°N)

Revelle Station 1 (Figure 7) was north of the ODZ boundary, and O2 was detectable throughout the water column. We might expect there to be no I in the subsurface waters. Instead, there was a well-developed I maximum and IO3 minimum (Figure 7), where O2 concentration drops to its lowest point just below the IO3 minimum. There appear to be consistently 150–200 nM concentrations of I and ~400 nM IO3 at this station. The exception is at 400 m where I- and IO3 concentrations are equal. Since a subsurface I maximum remains detectable, even in the presence of O2, oxidation rates must be slow enough for it to persist during advection from coastal sediments and denitrifying zones within the ODZ to the south and east.

4.3.2 Revelle Station 3 (110°W, 21.6°N)

The highest I/total iodine/excess iodine was observed at Revelle Station 3 (Figure 9). At the surface, we observed 1,188 nM I and 211 nM IO3. Total iodine was 1,400 nM, and excess iodine was 925 nM. Most likely, this high total iodine concentration reflects lateral inputs from reducing shallow sediments since this station is close to several broad shelf areas. The relatively low IO3 concentration at the surface is unusual for oxygenated water and suggests that biological reduction by phytoplankton may be occurring. Iodide increases to >1,000 nM at 200 m where the nominal O2 concentration was below or near the nominal detection limit of the SBE 43 sensor. The most difficult feature to explain is the minimum in I between 100 and 150 m. This station managed to capture Pacific Subarctic Upper Water advected via the California Current from approximately 80 to 120 m as seen by the distinctly lower salinity of these samples (Figure 8). This water mass originates from the Gulf of Alaska (Thomson & Krassovski, 2010) and is distinct from local water masses.

4.3.3 Revelle Station 34 (105.7°W, 21.3°N)

Station 34 was the depth profile taken above the reducing sediment at the coastal shelf station (Figure 10). From 65–450 m, all O2 concentrations were < 1 μM. Iodide and IO3 were measured in the water column from the surface to bottom. At the surface, we measured 147 nM IO3 and 429 nM I. The rest of the water column from 65–450 m (bottom depth = 455 m) had >700 nM total iodine which was predominantly I; the average IO3 concentration below the surface was only 48 nM. The water overlying the sediment, sampled using a multicorer, had an I concentration of >900 nM. There was no detectable IO3 found in this bottom water sample.

4.3.4 Falkor Station 9/Revelle Station 17 (110°W, 14°N)

Revelle Station 17 was measured as well on the Falkor cruise as Station 9 and was a crossover station. Profiles from Figure 11 were measured aboard the Revelle on 2 April 2018. Profiles from Figure 12 were measured aboard the Falkor on 6 July 2018. Similar profiles for IO3 and I are detected on both cruises. The I- maximum occurred at 120 m depth (corresponding to the 26.0 kg/m3 isopycnal) aboard the Revelle and at 145 m depth (corresponding to the 26.2 kg/m3 isopycnal) aboard the Falkor. A smaller, secondary I maximum occurred aboard the Revelle at 300 m (corresponding to the 26.6 kg/m3 isopycnal) and at 250 m (corresponding to the 26.6 kg/m3 isopycnal) aboard the Falkor (Figures 13 and 14).

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Profiles of chemical species at Revelle Station 17 (110°W, 14°N).
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Profiles of chemical species at Falkor Station 9 (110°W, 14°N).
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Iodide measurements from 110°W, 14°N measured aboard both the Falkor and Revelle plotted against potential density.
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Casts 1 and 2 of Falkor Station 2, respectively. Excess iodine profiles plotted against potential density.

4.3.5 Falkor Station 2 (103°W, 14°N)

At Falkor Station 2, Cast 1 was taken on 30 June, and Cast 2 was taken approximately 48 hr later on 2 July, 2018 (Figure 15 for Cast 1 and Figure 16 for Cast 2). While the first cast showed a modest excess iodine within the ODZ, the second cast showed a pronounced excess with a sharp maximum of 813 nM I at a depth of 365 m (see supporting information); this is an excess of 338 nM I above our average concentration. This sharp maximum is attributed to excess iodine transported via a subsurface mesoscale feature from the shelf, which propagated westward while we were at this station. Using satellite altimetry, as described in Evans et al. (2020), the first cast had a sea surface height anomaly of 0.807 m, whereas the second cast had a sea surface height anomaly of 0.815 m, which indicates that the core of the anticyclonic eddy was approaching from Cast 1 to Cast 2. This trend can also be seen in Figures 6a and 6b in Evans et al. (2020). Although Evans et al. (2020) focused primarily on mesoscale eddies within the 26.2–26.5 kg/m3 isopycnals, and this excess iodine feature is centered at the 26.8 kg/m3, the core of the NEPIW water mass, subsurface eddies could also influence that isopycnal as well.

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Profiles of chemical species at Falkor Station 2, Cast 1 (103°W, 14°N). A second oxygen profile is provided that is on a smaller scale.
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Profiles of chemical species at Falkor Station 2, Cast 2 (103°W, 14°N). A second oxygen profile is provided that is on a smaller scale.

5 Discussion

5.1 Iodine Redox Chemistry in the Absence of Oxygen

The results from this study represent the largest survey of iodine speciation to date in an ODZ. Previous works have evaluated at details of iodine speciation in other low O2 settings in marginal marine basins, such as the Chesapeake Bay (Wong & Zhang, 2003), Cariaco Basin (Wong & Brewer, 1977), Mediterranean Sea (Ullman et al., 1990), Orca Basin (Wong et al., 1985), Baltic Sea (Truesdale et al., 2013), Black Sea (Luther & Campbell, 1991), and Saanich Inlet (Emerson et al., 1979). In addition to this study of the ETNP, one previous work has looked at a single profile from a station nearby our transect (Rue et al., 1997), and studies have looked at vertical transects through the ETSP (Cutter et al., 2018), Arabian Sea (Farrenkopf et al., 1997; Farrenkopf & Luther, 2002), and Benguela (Chapman, 1983) ODZs. Despite the large spatial, chemical, and physical differences between each of these settings, they each reveal similar patterns of IO3 minima overlapping with minima in O2, and NO2 maxima. These patterns are generally interpreted to reflect dissimilatory IO3 reduction and denitrification under low O2 conditions which have similar Gibbs Free Energy Yields (Farrenkopf et al., 1997). However, because of important physical differences between estuarine anoxic basins and marginal upwelling systems, we restrict our discussion below to comparisons of the ETNP, ETSP, and the Arabian Sea ODZs. Thermodynamically, I is favored in the absence of O2. Previously, Rue et al., (1997) reported a single profile from a station located within our sampling region (Figure 1) that showed I and IO3 profiles with no excess iodine, which was analogous to many stations we measured offshore. We observed in many, but not all, stations that IO3 is completely absent when O2 is undetectable, in sharp contrast to the ETSP where IO3 concentrations were greater than 200 nM throughout the core of the ODZ even where O2 concentrations were below detection (Cutter et al., 2018). Presumably, low levels of IO3 are persistent in the ODZ and require a significant time, perhaps decades, to be completely removed. Karstensen et al. (2008) determined that the “age” of the water in the most reducing part of the ETNP was around 90 years, versus 50–60 years in the ETSP. Thus, the additional 30 years might have been required for complete removal of IO3-.

We observed many depth profiles with sharp maxima in I concentrations coinciding with the secondary nitrite maximum (between 26.2 and 26.6 kg/m3) which was noted previously by Farrenkopf et al. (1997; Farrenkopf & Luther, 2002).

The similarities between I and NO2 indicate that I accumulation may be linked to nitrogen cycling in the short term since IO3 is a useful terminal electron acceptor in bacterial respiration. Laboratory studies have shown that IO3 is directly reduced by bacteria during the oxidation of organic matter in low O2 waters (Amachi et al., 2007; Farrenkopf et al., 1997), and it has been recently proposed that nitrite-oxidizing bacteria can also utilize IO3 during the oxidation of NO2 (Babbin et al., 2017). Ultimately, these processes lead to the complete removal of IO3 in the absence of O2.

The most unexpected and potentially important finding is that I and NO2 maxima are associated with the same water mass, the 13CW, in the ETNP and ETSP. This water mass arises in the western Pacific and spreads eastward (Evans et al., 2020). It is associated with a local salinity maximum. This enhances the pycnocline at the base of the oxycline and may contribute to the persistence of the upper boundary of the ODZ. Currently, there are not enough data to firmly establish the importance of the 13CW Water Mass in controlling the boundaries of the ODZ relative to other processes such as primary production in the overlying water. However, it is important to learn more about how this water mass is formed, and how its properties may change in the future.

Near the margin, the elevated I- feature is present over a much broader depth range and extends well below the deep boundary of the 13CW. This may be due to a larger flux of sinking organic matter near the coast, with IO3 reduction to I driven by respiration on sinking particles occurring at greater depths.

In the northern Arabian Sea, NO2 and I are also centered on the 26.5 kg/m3 density surface. There is a local maximum in salinity immediately above this feature that is associated with the Persian Gulf Water (Acharya & Panigrahi, 2016). It is surprising that the properties of this water mass are very similar to 13CW despite its very different source. Persian Gulf Water appears to be enriched in both Fe and iodine that are derived from the continental margin, but it is unclear if the source is margin sediments off the India/Pakistani coast or the Persian Gulf itself.

5.2 A Margin Source of Excess Iodine

Excess iodine was present throughout the ETNP. Hence, all three ODZs (ETNP, ETSP, and Arabian Sea) have a substantial excess iodine signal which indicate that margin to basin transport is important for all of them. However, it is important to critically evaluate the assumptions made by Farrenkopf & Luther, (2002) and Cutter et al. (2018) that led to this conclusion.

The enrichments we observed in excess iodine coincident with NO2 could come from either release of iodine from sinking particles during remineralization or lateral advection from the margins. Farrenkopf & Luther, (2002) argued that a margin source was more likely because the I:C ratio in sedimentary organic matter is 100-fold higher than in sinking particles. Further evidence for a margin source was provided by Cutter et al. (2018). They constrained the supply of iodine from the shelf waters using Ra-228, a tracer of margin inputs that was measured along with iodine as part of the GEOTRACES program. They showed that the supply of iodine along a density surface interval spanning the continental shelf was sufficient to maintain the gradient in excess iodine that was observed relative to vertical mixing which would tend to diminish the feature. Vertical mixing was constrained using 7Be which was also measured. It is important to note that in both the ETNP and the ETSP, the density range between 26.2 and 26.6 intersects a large fraction of the shelf where the benthic iodine is derived.

The argument put forward in these earlier studies is straightforward and compelling, but it requires critical evaluation. That is because the mechanism of iodine incorporation and release from organic matter at the sediment/water interface is fundamentally different than in sinking particles. Kennedy and Elderfield (1987a, 1987b) hypothesized that I:C ratios were high at the sediment/water interface because I released during remineralization was reincorporated into organic matter by nucleophilic substitution following the oxidation of I to HOI (Francois, 1987). Thus, a high I:C does not necessarily mean high benthic fluxes of I into the water column. This process does not occur in reducing regimes because sulfide is a much stronger nucleophile than I. Indeed, Francois et al. (1987) showed that sulfide displaces I when oxidized sediments become anoxic. Specifically, I:C ratios at the sediment water interface of anoxic sediments resemble phytoplankton and POC while sediments directly underlying oxic bottom waters have much higher I:C ratios on the order of 10−3 (Kennedy & Elderfield, 1987a, 1987b; Lu et al., 2008; Price & Calvert, 1973; Price & Calvert, 1977). Thus, it seems likely that excess iodine inputs may ultimately arise from accumulation of excess iodine at the sediment water interface under oxidizing conditions followed by release under reducing conditions. The overall effect would be an enrichment in iodine in waters overlying shelf sediments that would give rise to the features that we observed.

A pore water I source most prominent in reducing waters is also supported by a number of studies evaluating the speciation of pore water iodine, diffusive fluxes of iodine across the sediment-water interface, and comparisons between I:C in oxic versus reducing sediments. Specifically, studies evaluating pore water iodine speciation reveal that it is dominated by I due both to reducing conditions and the input of I from organic matter during sediment diagenesis (Anschutz et al., 2000; Kennedy & Elderfield, 1987a, 1987b). Similar to ammonia, I increases in pore waters during diagenesis to concentrations in excess of that of seawater by several fold which often reach several μM or higher.

The complexity of iodine cycling in sediments makes it harder to dismiss out of hand the role of sinking particles offshore as a source of excess iodine to the ODZ. Therefore, we reexamined the assumptions about iodine sourced from sinking particles as well. Because the small I:C ratio in sinking particles underlie this important aspect of the excess iodine paradigm, we looked at the I:C ratio in suspended particles more closely since the estimate of 10−4 used by Farrenkopf & Luther, (2002) came from a single noisy correlation of IO3 versus PO43− in the South Atlantic (Wong et al., 1976). Unfortunately, there are no measurements of both POC and iodine in marine particles. Wong et al. (1976) measured particulate iodine in the South Atlantic and reported values ranging from 2–127 ng/kg of seawater. Using estimates of POC from a compilation by Martiny et al. (2014) in this region, we found that the highest possible value of I:C is about 10−5. Recently, I uptake has been studied in phytoplankton (de laCuesta & Manley, 2009; van Bergeijk et al., 2016). From these data, estimates of I:C range from 10−8 to 10−6. Thus, the estimate of 10−4 used by Farrenkopf and Luther, (2002) is quite conservative, in this context, and so a source of excess iodine derived from sinking particles is unlikely.

5.3 Implications for Other Species

The large region containing excess iodine implications for the transport of other species as well. Cutter et al., (2018) reported that I and Fe (II) coincided within the ODZ and were both transported from the margin. It seems likely that Fe transport from the margin to the interior of the ODZ is important in the Arabian Sea and ETSP as well since Fe (II) has also been measured there. Iron accumulates in sediments under oxidizing conditions and is released under reducing conditions, which is probably the case for iodine as well, given the arguments in section 5.2. Thus, excess iodine might be a particularly good tracer for iron derived from the continental margin via a shelf to basin shuttle. Such a shuttle is thought to be an important mechanism supplying Fe from the continental margin into ocean basins (Scholz et al., 2018; Moffett & German, 2019).

5.4 Effects of Local Physics—A Water Mass Analysis

Evans et al. (2020) carried out a water mass analysis of the ETNP based on the sections from these cruises. This work confirms that NO2, I, and excess iodine are primarily associated with the 13CW. Peters et al. (2018) showed that the upper ODZ in the ETSP with active denitrification was mostly contained within this water mass. Peters et al. (2018) called this water mass Equatorial Sub Surface Water. In the ETNP, it is more widely referred to as 13CW (Fiedler & Talley, 2006; Nürnberg et al., 2015), and so we adopted that convention in our work. Evans et al., (2020) showed that most of the small-scale structure in the sections reported in Figures 5-7 can be accounted for by variability in the contribution of the 13CW. For example, in Figure 5, the “hot spot” of I is associated with a high percentage of the 13CW water mass relative to NEPIW, which brings water from outside the ODZ into this density horizon. Consequently, the low I and NO2 at 120°W on this transect is associated with a higher percentage of NEPIW relative to 13CW (Evans et al., 2020). Note that while NEPIW is low in I and NO2, O2 levels remain below the detection limit of the CTD sensor. The temporal variability in iodine distribution, revealed in Figures 13 and 14, can also be explained largely by variability in the relative importance of these two water masses. Evans et al. (2020) argued that subsurface mesoscale features, such as eddies or meanders generated by poleward undercurrents flowing along the continental margin, transport 13CW westward, increasing variability but also moving material (such as I) offshore. Evans et al. (2020) demonstrated that sea surface height anomalies, during these cruises, indicate the presence of mesoscale eddies. The water mass analysis can also explain why such eddies are enriched in I. As discussed previously, the density range between 26.2 and 26.6 intersects a large fraction of the shelf where the benthic iodine is derived. Moreover, a cruise in 2012 included a station on the shelf at 18.8°N, −104.6°W, just east of our transects. The 13CW shoaled over the shelf centered at 150 m which enabled I to be transferred from the shelf sediments and enriched in this water mass (Evans et al., 2020). 13CW may act as a conduit in the shelf-to-basin shuttle for reduced species from shelf sediments to the deep interior of the ETNP.

In recent years, I:Ca ratio in carbonates have been a popular paleoproxy for O2 concentrations (Hardisty et al., 2014; Lu et al., 2010). Iodate is usually reduced to I within the ODZ (where [O2] < 1 μM); however, we found at specific stations (Revelle Stations 5, 21, 24–26; Falkor Station 13, and Station18) that even when O2 is below one micromolar (the detection limit of the CTD), IO3 concentrations ranged from 50–200 nM. Since IO3 can exist at relatively high concentrations even in “anoxic” conditions, these results may be useful in calibrating the I:Ca proxy, which takes advantage of surface water planktonic foraminifera I:Ca ratio (Hoogakker et al., 2018). This proxy assumes that IO3 exists only in oxygenated water while the dominant iodine species is I in oxygen deficient waters (Hoogakker et al., 2018). In anoxic waters, where all of the iodine should be in the form of I, iodine is not incorporated into the carbonate structure (Lu et al., 2010). While most of the IO3 is reduced to I within the ODZ, many stations had ~15–30 nM of IO3 remaining and sometimes higher. Moreover, all the O2 deficient waters of the ETSP had significant remaining IO3- (Cutter et al., 2018). These findings suggest that using the I/Ca ratio as a proxy for oxygenation may need to consider that there is a lag between deoxygenation and IO3 disappearance. The timeframe may be on the order of decades based on the ages of ODZ waters determined by Karstensen et al. (2008) described above.

6 Conclusion

The distribution and speciation of iodine is controlled by a combination of physical, chemical, and biological processes. Iodate reduction within the ODZ leads to I becoming the predominant species with maxima at the same potential density anomaly range (26.2–26.6 kg/m3) in both the ETSP and ETNP. There are also substantial inputs of excess iodine from the continental margin, which extend throughout the basin, and it is significant that water centered near the 26.5 kg/m3 potential density anomaly extends over the shelf benthic boundary layer in both hemispheres. These basic features are modified by physical processes that influence the relative contributions of specific water masses within the ODZ. Eddies generated off the continental margin are probably driving these processes and contribute to the fine structure and variability that we observed. Iodine speciation is clearly linked to the distribution of O2, but the similar distributions of iodine and NO2 indicate that it is linked to the nitrogen cycle as well, perhaps because of the use of IO3 by nitrogen-oxidizing bacteria as a terminal electron acceptor. Iodate is absent in some stations in the ETNP, in contrast to the ETSP, where it is always detectable. This may reflect the greater age of ODZ water in the ETNP and suggests complete removal of IO3- occurs on decadal scales.

Acknowledgments

Nitrite measurements were made possible by both Rick Keil, Allen Devol, Andrew Babbin, and their respective labs. We would also like to acknowledge the cooperation from the captains, crew members, and marine technicians aboard the R/V Roger Revelle and the R/V Falkor for operating the equipment to make this study possible. Funding for this project came from the National Science Foundation OCE1636332 to James Moffett. A Schmidt Ocean Institute grant to Karen Casciotti and Andrew Babbin supported the FK180624 cruise. Additional support provided by the MIT Ally of Nature and Heflinger Funds to Babbin. Dalton Hardisty's participation in and contribution to the project is funded through NSF OCE1829406. The data from this paper is being submitted to the Biological and Chemical Oceanography-Data Management Office (BCO-DMO).