accelerationofoxygendeclineinthetropical ... · engineering, georgia institute of technology,...

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Supplementary Information: 1

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1. Model validation 3

The ability of our atmospheric chemical transport model to simulate sol-Fe 4

concentrations and deposition fluxes is previously examined by Johnson and Meskhidze1 using 5

surface-level in situ cruise measurements and compared to similar model-predicted deposition 6

fluxes. Annual mean global sol-Fe deposition is ~0.26 Tg (1 Tg = 1012 g) which is consistent 7

with recent modeling studies2-5 that predicted annual sol-Fe fluxes ranging from ~0.2 to 0.4 Tg 8

yr-1. When the predictions of total mineral dust and sol-Fe were compared to 5 separate ship-9

based measurement campaigns over the North Atlantic and Pacific Oceans, it was determined 10

that the model could reproduce measurement values reasonably well1. On average the model 11

captured the spatiotemporal variability of measurement data and predicted magnitudes of total Fe 12

and soluble ferrous (Fe(II)) and ferric (Fe(III)) concentrations within in a factor of 2 (normalized 13

mean bias ranging between ~100% to -50% shown in Table 5 of Johnson and Meskhidze1.14

Dissolved iron distribution simulated in the ocean biogeochemistry model is tested 15

against published observations in the upper water column of the Pacific basin (Fig S1). The 16

simulated upper ocean profile closely follows the median of observed dissolved Fe profiles, 17

showing a nutrient-like, downward increasing profile below the surface mixed layer. The 18

simulated Fe profiles are well within the range of published observations6. Omission of 19

hydrothermal sources did not generate a major discrepancy dissolved iron in the upper water 20

column (<600m). Also, the model is in reasonable agreement with observed macronutrient 21

distributions7. Fig S2 shows a comparison between the simulated and modeled oxygen 22

LETTERSPUBLISHED ONLINE: XXMONTH XXXX | DOI: 10.1038/NGEO2717

Acceleration of oxygen decline in the tropicalPacific over the past decades by aerosol pollutantsT. Ito1*, A. Nenes1,2,3,4, M. S. Johnson5, N. Meskhidze6 and C. Deutsch7

Dissolved oxygen in the mid-depth tropical Pacific Oceanhas declined in the past several decades1. The resultingexpansion of the oxygen minimum zone has consequences forthe region’s ecosystem2 and biogeochemical cycles3, but thecauses of the oxygen decline are not yet fully understood.Here we combine models of atmospheric chemistry, oceancirculation and biogeochemical cycling to test the hypothesisthat atmospheric pollution over the Pacific Ocean contributedto the redistribution of oxygen in deeper waters. We simulatethe pollution-induced enhancement of atmospheric solubleiron and fixed nitrogen deposition, as well as its impacts onocean productivity and biogeochemical cycling for the latetwentieth century. The model reproduces the magnitude andlarge-scale pattern of the observed oxygen changes from the1970s to the 1990s, and the sensitivity experiments reveal thereinforcing e�ects of pollution-enhanced iron deposition andnatural climatevariability.Despite theaerosoldepositionbeingthe largest inmid-latitudes, its e�ect onoceanic oxygen ismostpronounced in the tropics, where ocean circulation transportsadded iron to the tropics, leading to an increased regionalproductivity, respiration and subsurface oxygen depletion.These results suggest thatanthropogenicpollutioncan interactand amplify climate-driven impacts on ocean biogeochemistry,even in remote ocean biomes.

The tropical Pacific Ocean contains a high nutrient and lowchlorophyll (HNLC) region, where a scarcity of the micronutrientiron limits biological productivity4. Below the surface of theHNLC region (Fig. 1a) lie some of the most voluminous oxygenminimum zones (OMZs) of the world oceans5. Understandingthe mechanisms that cause these OMZs and their variabilityis important, as benthic and pelagic organisms are sensitive tooxygen concentrations ([O2])6. Furthermore, denitrification undersuboxic conditions ([O2]< 5 µmol l−1) accounts for approximatelyhalf the loss of the bioavailable nitrogen in the oceans, whichimpacts the oceanic inventory of nitrate and produces the potentgreenhouse gas nitrous oxide3. Observed [O2] in the tropicalPacific OMZ has shown a prominent decline in recent decades(Fig. 1b)1. The solubility of gases in seawater decreases withincreasing temperature, but the recent decline of tropical Pacific[O2] was not accompanied by a temperature increase1, indicatingthat the cause of the recent oxygen decline in this region mustinclude changes in biological oxygen consumption and/or oceancirculation. Recent studies7–9 have proposed that the dominantmode of the North Pacific climate variability, the Pacific DecadalOscillation (PDO), and associated increase in the tropical upwelling

120° W 90° W150° W180°150° E120° E

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0 60 120 180 240 300Climatological oxygen at 435 m depth (μmol l−1)

ΔO2 (1990−1970) at 400 m (μmol l−1)−25 −20 −15 −10 −5 0 5 10 15 20 25

Figure 1 | Dissolved oxygen in the thermocline waters of the North Pacific.a, Climatological distribution of [O2] at 435 m depth based on the WorldOcean Atlas 2009 in the units of µmol l−1. b, Oxygen change from the1970s to the 1990s in 20◦ longitude × 10◦ latitude × 100 m depth binsbased on Hydrobase329. The boxed regions indicate statistically significantO2 change (90% confidence interval with two-tailed t-test with at least 20profiles in each bin).

and productivity since the 1980s may have contributed to theexpansion of the tropical Pacific OMZ7,10. However, the excessnitrate in the surface of this region permits an additional

1School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30030, USA. 2School of Chemical and BiomolecularEngineering, Georgia Institute of Technology, Atlanta, Georgia 30030, USA. 3Institute of Chemical Engineering Sciences, Foundation for Research andTechnology-Hellas, Patras GR-26504, Greece. 4Institute for Environmental Research and Sustainable Development, National Observatory of Athens,Palea-Pendeli GR-15236, Greece. 5Biospheric Science Branch, NASA Ames Research Center, Mo�ett Field, California 94035, USA. 6Marine, Earth, andAtmospheric Science, North Carolina State University, Raleigh, North Carolina 27695, USA. 7School of Oceanography, University of Washington, Seattle,Washington 98195, USA. *e-mail: [email protected]

NATURE GEOSCIENCE | VOL 9 | JUNE 2016 | www.nature.com/naturegeoscience 1

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NGEO2717

NATURE GEOSCIENCE | www.nature.com/naturegeoscience 1

© 2016 Macmillan Publishers Limited. All rights reserved.

http://dx.doi.org/10.1038/ngeo2717

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climatology at the depth of 435m. The model overestimates the oxygen concentration of the 23

subpolar North Pacific poleward of 40°N. In the south of 40°N, the overall magnitude and the 24

large-scale distribution of [O2] is reasonably well represented. Similar to a previous study7 the 25

tropical Pacific OMZ is biased towards the eastern part of the basin. 26

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2. Transport pathway 28

We evaluate the pathway through which the pollution-induced enhancement of sol-Fe 29

input affects the interior ocean and eventually increase the biological productivity of the tropical 30

Pacific (Fig S3). The effect of pollution-induced enhancement of sol-Fe deposition is evaluated 31

by taking the difference between the full simulation and the sensitivity run with preindustrial iron 32

deposition. The increase in dissolved iron concentration spreads along the density surfaces of the 33

ventilated thermocline whose outcrop regions coincide with the region of increased sol-Fe 34

deposition in the mid-latitudes (Fig S3A). The meridional section of the increase in dissolved 35

iron shows that the pollution effect increases the dissolved Fe concentration of the northern 36

subtropical thermocline where the excess Fe is transported along the pathway of the North 37

Pacific gyre circulation (Fig S3B). The increased tropical productivity is driven by the increase 38

in the concentration of upwelled iron, which has been fed by the transport from the northern 39

subtropical gyre via the shallow overturning circulation and the equatorial undercurrent (EUC). 40

Another important source of iron is the continental shelf sediments in the western margins of the 41

equatorial Pacific8 which is transported along the EUC. The model includes sedimentary iron 42

sources9 and the circulation driven variability (i) includes the effect of shelf-source iron 43

upwelling into the tropics, modulated by the EUC variability. 44

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3. Evaluating the impact of uncertainty in sol-Fe deposition 46

Estimating past variation in the sol-Fe deposition is subject to large uncertainties even 47

though the model captures present-day condition reasonably well. To evaluate the robustness of 48

our conclusion we performed additional sensitivity experiments by scaling (x4, x2, x0.5, x0.25) 49

the temporal pattern of the soluble iron deposition from 1952 to 2002, holding everything else 50

the same. In terms of the absolute magnitude, this is equivalent of roughly a factor of 2.5 51

variation as shown in Fig S4. Overall, the results of the sensitivity runs support the robustness of 52

our conclusion. Given the varying rates of soluble iron deposition, the simulated oxygen decline 53

varies by about 15%. Averaging oxygen change in the tropical Pacific region bounded by 10°S 54

to 10°N, 150°E to 80°W, 260-710m, the results of the sensitivity runs stay within -4.5 to -5.1 55

μmolO2 L-1. The magnitude of the oxygen decrease does not scale linearly with the iron 56

deposition. Fig S5 shows the time series of sinking organic flux at 100m from the tropical 57

Pacific. Our calculation is broadly similar to the previous study showing the effect of multi-58

decadal climate variability using an independent model10. For all cases the productivity increases 59

during 1960s and decreases during late 1970s, and then it increases again after 1980s. These 60

multi-decadal changes are related to the shifts in tropical upwelling associated with Pacific 61

Decadal Oscillation. 62

The differences between the sensitivity runs are relatively subtle, reflecting the different 63

trajectories of nutrient co-limitation in the tropical Pacific. The net community production (NCP) 64

is parameterized as a product of Monod functions, and the tropical Pacific productivity in our 65

simulation is co-limited by macro-nutrient and dissolved iron. The sensitivity runs with higher 66

iron flux (x4 and x2) tends to show smaller long-term trend of NCP and export production. This 67

is due to the increased macro-nutrient limitation induced by the higher level of iron supply, as 68


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evidenced by the depletion of surface macro-nutrient in these runs. As a result, the oxygen 69

decline from 1970s to 1990s is relatively moderate in x4 and x2 runs as shown in Table S2. 70

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4. Relevance to previously published studies 72

Previous study by Krishnamurthy et al (2009, hereafter K09) performed sensitivity 73

simulations using a different modeling tool (CCSM3 with Biogeochemical Elemental Cycling 74

Model). Similar to our study, increased soluble Fe input increased biological productivity and 75

carbon export over the HNLC regions of the North Pacific Ocean in K09. However, they did not 76

find significant change in the oxygen level of the tropical Pacific OMZ. Subsurface oxygen is 77

controlled by the balance between the physical oxygen supply and the biological oxygen 78

consumption. Even though the temporal change of the biological productivity is similar, the 79

simulated oxygen concentration could significantly differ depending on the representation of 80

physical circulation. It is difficult to determine what exactly caused the different response of 81

tropical Pacific OMZ to increased biological productivity between K09 and our study as there 82

are numerous differences in model configurations including resolution, physical and 83

biogeochemical parameterizations and boundary conditions. Below we point out some important 84

differences between K09 and our work, especially the model resolution and the ocean circulation 85

variability. 86

First, the horizontal resolution of the physical model used in K09 is significantly lower at 87

3.6 degrees. At this resolution, it is difficult to reproduce equatorial current system with 88

relatively narrow circulation features that are important pathways for the ventilation of the 89

tropical thermocline11. The lack of resolution to fully resolve equatorial current system in K09 90

may have led to a strong oxygen depletion in the tropical OMZ. To avoid negative oxygen 91


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concentration, models are programmed to reduce the rate of respiratory oxygen consumption 92

when oxygen concentration approaches a certain threshold. 93

Secondly, K09 model was forced by the climatological wind and buoyancy forcing, and it 94

did not include circulation variability which is an important control for the tropical Pacific OMZ. 95

Our study is based on the data-assimilated physical ocean model and is shown to represent 96

equatorial current variability12. 97

Thirdly, there has been significant advances in our understanding of tropical OMZ since 98

the time K09 was published7,10,11,13-15, and our simulations are analyzed in the light of recent 99

advancements. Our oxygen levels in the tropical Pacific are generally reasonable, dissolved iron 100

levels are also within the range of limited observations available at this time. We performed 101

considerably larger number of sensitivity studies and demonstrated signal robustness. 102

Considering above three factors, our result is likely more robust in simulating oxygen decline 103

under increased export production in the tropical Pacific. 104

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References 106

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1 Johnson, M. S. & Meskhidze, N. Atmospheric dissolved iron deposition to the global 108

oceans: effects of oxalate-promoted Fe dissolution, photochemical redox cycling, and 109

dust mineralogy. Geoscientific Model Development 6, 1137-1155, (2013). 110

2 Luo, C. et al. Combustion iron distribution and deposition. Global Biogeochem Cy 22, 111

GB1012, (2008). 112

3 Luo, C. & Gao, Y. Aeolian iron mobilisation by dust-acid interactions and their 113

implications for soluble iron deposition to the ocean: a test involving potential 114

anthropogenic organic acidic species. Environmental Chemistry 7, 153-161, (2010). 115

4 Okin, G. S. et al. Impacts of atmospheric nutrient deposition on marine productivity: 116

Roles of nitrogen, phosphorus, and iron. Global Biogeochem Cy 25, GB2022, (2011). 117

5 Myriokefalitakis, S. et al. Changes in dissolved Iron deposition to the oceans driven by 118

human activity: a 3-D global modelling study. Global Biogeochem Cy in review, (2015). 119

6 Tagliabue, A. et al. A global compilation of dissolved iron measurements: focus on 120

distributions and processes in the Southern Ocean. Biogeosciences 9, 2333-2349, (2012). 121

7 Ito, T. & Deutsch, C. Variability of the oxygen minimum zone in the tropical North 122

Pacific during the late twentieth century. Global Biogeochem Cy 27, 1119-1128, (2013). 123

8 Slemons, L. O., Murray, J. W., Resing, J., Paul, B. & Dutrieux, P. Western Pacific coastal 124

sources of iron, manganese, and aluminum to the Equatorial Undercurrent. Global 125

Biogeochem Cy 24, GB3024, (2010). 126


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9 Elrod, V. A., Berelson, W. M., Coale, K. H. & Johnson, K. S. The flux of iron from 127

continental shelf sediments: A missing source for global budgets. Geophys Res Lett 31, 128

L12307, (2004). 129

10 Deutsch, C., Brix, H., Ito, T., Frenzel, H. & Thompson, L. Climate-Forced Variability of 130

Ocean Hypoxia. Science 333, 336-339, (2011). 131

11 Stramma, L., Johnson, G. C., Firing, E. & Schmidtko, S. Eastern Pacific oxygen 132

minimum zones: Supply paths and multidecadal changes. J. Geophys. Res. 115, C09011, 133

(2010). 134

12 Schott, F. A., Wang, W. Q. & Stammer, D. Variability of Pacific subtropical cells in the 135

50-year ECCO assimilation. Geophys Res Lett 34, L05604, (2007). 136

13 Czeschel, R., Stramma, L. & Johnson, G. C. Oxygen decreases and variability in the 137

eastern equatorial Pacific. Journal of Geophysical Research: Oceans 117, C11019, 138

(2012). 139

14 Deutsch, C. et al. Centennial changes in North Pacific anoxia linked to tropical trade 140

winds. Science 345, 665-668, (2014). 141

15 Duteil, O., Böning, C. W. & Oschlies, A. Variability in subtropical-tropical cells drives 142

oxygen levels in the tropical Pacific Ocean. Geophys Res Lett 41, 8926-8934, (2014). 143

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Figure S1. Comparison of modeled (magenta) and observed (black) dissolved iron in the 148

tropical Pacific Observed and simulated dissolved iron profiles are averaged over the 149

tropical Pacific (160°W-90°W, 10°S-10°N) in the upper 600m of the water column. Black line is 150

the median of all available observations in the published dissolved iron data compiled by A. 151

Tagliabue and co-authors and downloaded from 152

http://pcwww.liv.ac.uk/~atagliab/LIV_WEB/Data.html. Teal dots are raw data, and the shaded 153

region contains the 5-95 percentile. 154

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Figure S2. Observed and simulated oxygen climatology (A) Climatological distribution of 158

[O2] at the 435m depth based on the World Ocean Atlas 2009 in the units of μmol L-1. (B) The 159

same from the present-day simulation averaged over the period of 1971 to 2000. 160

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A

B

Climatology

Present-day simulation

0 60 120 180 240 300

Oxygen at 435m depth [μmol L-1]


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Figure S3. Dissolved iron increases due to anthropogenic effect in the North Pacific Ocean 164

(A) Perturbation in dissolved iron concentration along the meridional transect at 160°W where 165

solid black lines are the potential density surfaces. The plotted data is an time average for 1990s. 166

(B) Perturbation in dissolved iron centration interpolated onto the potential density surface 167


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σθ=25.5 where solid black lines are the contours of the dissolved iron perturbation. The position 168

of the thick solid line along 160°W indicates the position of the meridional transect in (A). 169

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Figure S4. The time series of integrated soluble iron deposition rate over the global oceans 172

The standard run (x1) is in yellow. In the x4 case (blue), the temporal change of soluble iron 173

deposition relative to 1952 level is scaled up by a factor of 4, reaching to beyond 10GmolFe yr-1 174

by year 2000. In contrast, x0.25 case (green) it is scaled down by a factor of 4. In this case it 175

reaches to only about 4GmolFe yr-1 by year 2000. 176

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Figure S5. Export production time series The area-weighted mean export production (panel 181

A) and its de-trended time series (panel B) are plotted for the the tropical Pacific defined as the 182

region bounded by 10°S to 10°N, 150°E to 80°W. Export production is based on by the sinking 183

organic flux at 100m depth. Thin dash lines indicate annual means, and thick solid lines are the 184

running means with 10-year window. 185

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Δ[O2], μmol L-1 Δ(EPC185), mmol m-2 yr-1

Circ. variability -2.79 84.56 Pollution Fe -4.65 71.20 Pollution N -1.42 17.52

ALL -5.11 77.94 190

Table S1. The breakdown of oxygen and export production change Change in the oxygen 191

concentration (in μmolO2 L-1) and export production (in mmolC m-2 yr-1) at 185m depth 192

(EPC185) are plotted for the the tropical Pacific defined as the region bounded by 10°S to 10°N, 193

150°E to 80°W. Oxygen concentration is averaged over the depth of 260-710m and the export 194

production is evaluated as the sinking organic flux at the depth of 185m. 195

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X4 X2 X1 X0.5 X0.25

Δ[O2] -4.53 -5.02 -5.17 -5.01 -4.87 199

Table S2. Oxygen change from the sensitivity experiments Changes in the oxygen 200

concentration are plotted for the the tropical Pacific defined as the region bounded by 10°S to 201

10°N, 150°E to 80°W. Oxygen concentration is averaged over the depth of 260-710m and the 202

values are in the units of μmol L-1. 203

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