Acceleration of oxygen decline in the tropical Pacific over the past decades by aerosol pollutants

Transcription

1 Acceleration of oxygen decline in the tropical Pacific over the past decades by aerosol pollutants T. Ito 1 *, A. Nenes 1,2,3,4, M. S. Johnson 5, N. Meskhidze 6 and C. Deutsch 7 Dissolved oxygen in the mid-depth tropical Pacific Ocean 3 1. Model validation SUPPLEMENTARY INFORMATION DOI: /NGEO The ability of our atmospheric chemical transport model to simulate sol-fe concentrations and deposition fluxes is previously examined by Johnson and Meskhidze 1 using surface-level in situ cruise measurements and compared to similar model-predicted deposition fluxes. Annual mean global sol-fe deposition is ~0.26 Tg (1 Tg = g) which is consistent with recent modeling studies 2-5 that predicted annual sol-fe fluxes ranging from ~0.2 to 0.4 Tg yr -1. When the predictions of total mineral dust and sol-fe were compared to 5 separate ship- based measurement campaigns over the North Atlantic and Pacific Oceans, it was determined that the model could reproduce measurement values reasonably well 1. On average the model captured the spatiotemporal variability of measurement data and predicted magnitudes of total Fe and soluble ferrous (Fe(II)) and ferric (Fe(III)) concentrations within in a factor of 2 (normalized mean bias ranging between ~100% to -50% shown in Table 5 of Johnson and Meskhidze Dissolved iron distribution simulated in the ocean biogeochemistry model is tested Figure 1 Dissolved oxygen in the thermocline waters of the North Pacific. a, Climatological distribution of [O 2 ] at 435 m depth based on the World Ocean Atlas 2009 in the units of µmol l 1. b, Oxygen change from the 1970s to the 1990s in 20 longitude 10 latitude 100 m depth bins based on Hydrobase3 29. The boxed regions indicate statistically significant O 2 change (90% confidence interval with two-tailed t-test with at least 20 profiles in each bin). against published observations in the upper water column of the Pacific basin (Fig S1). The simulated upper ocean profile closely follows the median of observed dissolved Fe profiles, showing a nutrient-like, downward increasing profile below the surface mixed layer. The and productivity since the 1980s may have contributed to the expansion of the tropical Pacific OMZ 7,10. However, the excess nitrate in the surface of this region permits an additional simulated Fe profiles are well within the range of published observations 6. Omission of hydrothermal sources did not generate a major discrepancy dissolved iron in the upper water 1 School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia 30030, USA. 2 School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30030, USA. 3 Institute of Chemical Engineering Sciences, Foundation for Research and 21 Technology-Hellas, column Patras (<600m). GR-26504, Also, Greece. the 4 Institute model foris Environmental in reasonable Research agreement and Sustainable with Development, observed National macronutrient Observatory of Athens, Palea-Pendeli GR-15236, Greece. 5 Biospheric Science Branch, NASA Ames Research Center, Moffett Field, California 94035, USA. 6 Marine, Earth, and Atmospheric Science, North Carolina State University, Raleigh, North Carolina 27695, USA. 7 School of Oceanography, University of Washington, Seattle, Washington 98195, USA. * taka.ito@eas.gatech.edu distributions 7. Fig S2 shows a comparison between the simulated and modeled oxygen NATURE GEOSCIENCE VOL 9 JUNE

2 climatology at the depth of 435m. The model overestimates the oxygen concentration of the subpolar North Pacific poleward of 40 N. In the south of 40 N, the overall magnitude and the large-scale distribution of [O 2 ] is reasonably well represented. Similar to a previous study 7 the tropical Pacific OMZ is biased towards the eastern part of the basin Transport pathway We evaluate the pathway through which the pollution-induced enhancement of sol-fe input affects the interior ocean and eventually increase the biological productivity of the tropical Pacific (Fig S3). The effect of pollution-induced enhancement of sol-fe deposition is evaluated by taking the difference between the full simulation and the sensitivity run with preindustrial iron deposition. The increase in dissolved iron concentration spreads along the density surfaces of the ventilated thermocline whose outcrop regions coincide with the region of increased sol-fe deposition in the mid-latitudes (Fig S3A). The meridional section of the increase in dissolved iron shows that the pollution effect increases the dissolved Fe concentration of the northern subtropical thermocline where the excess Fe is transported along the pathway of the North Pacific gyre circulation (Fig S3B). The increased tropical productivity is driven by the increase in the concentration of upwelled iron, which has been fed by the transport from the northern subtropical gyre via the shallow overturning circulation and the equatorial undercurrent (EUC). Another important source of iron is the continental shelf sediments in the western margins of the equatorial Pacific 8 which is transported along the EUC. The model includes sedimentary iron sources 9 and the circulation driven variability (i) includes the effect of shelf-source iron upwelling into the tropics, modulated by the EUC variability. 45 2

3 Evaluating the impact of uncertainty in sol-fe deposition Estimating past variation in the sol-fe deposition is subject to large uncertainties even though the model captures present-day condition reasonably well. To evaluate the robustness of our conclusion we performed additional sensitivity experiments by scaling (x4, x2, x0.5, x0.25) the temporal pattern of the soluble iron deposition from 1952 to 2002, holding everything else the same. In terms of the absolute magnitude, this is equivalent of roughly a factor of 2.5 variation as shown in Fig S4. Overall, the results of the sensitivity runs support the robustness of our conclusion. Given the varying rates of soluble iron deposition, the simulated oxygen decline varies by about 15%. Averaging oxygen change in the tropical Pacific region bounded by 10 S to 10 N, 150 E to 80 W, m, the results of the sensitivity runs stay within -4.5 to -5.1 μmolo 2 L -1. The magnitude of the oxygen decrease does not scale linearly with the iron deposition. Fig S5 shows the time series of sinking organic flux at 100m from the tropical Pacific. Our calculation is broadly similar to the previous study showing the effect of multidecadal climate variability using an independent model 10. For all cases the productivity increases during 1960s and decreases during late 1970s, and then it increases again after 1980s. These multi-decadal changes are related to the shifts in tropical upwelling associated with Pacific Decadal Oscillation. The differences between the sensitivity runs are relatively subtle, reflecting the different trajectories of nutrient co-limitation in the tropical Pacific. The net community production (NCP) is parameterized as a product of Monod functions, and the tropical Pacific productivity in our simulation is co-limited by macro-nutrient and dissolved iron. The sensitivity runs with higher iron flux (x4 and x2) tends to show smaller long-term trend of NCP and export production. This is due to the increased macro-nutrient limitation induced by the higher level of iron supply, as 3

4 69 70 evidenced by the depletion of surface macro-nutrient in these runs. As a result, the oxygen decline from 1970s to 1990s is relatively moderate in x4 and x2 runs as shown in Table S Relevance to previously published studies Previous study by Krishnamurthy et al (2009, hereafter K09) performed sensitivity simulations using a different modeling tool (CCSM3 with Biogeochemical Elemental Cycling Model). Similar to our study, increased soluble Fe input increased biological productivity and carbon export over the HNLC regions of the North Pacific Ocean in K09. However, they did not find significant change in the oxygen level of the tropical Pacific OMZ. Subsurface oxygen is controlled by the balance between the physical oxygen supply and the biological oxygen consumption. Even though the temporal change of the biological productivity is similar, the simulated oxygen concentration could significantly differ depending on the representation of physical circulation. It is difficult to determine what exactly caused the different response of tropical Pacific OMZ to increased biological productivity between K09 and our study as there are numerous differences in model configurations including resolution, physical and biogeochemical parameterizations and boundary conditions. Below we point out some important differences between K09 and our work, especially the model resolution and the ocean circulation variability. First, the horizontal resolution of the physical model used in K09 is significantly lower at 3.6 degrees. At this resolution, it is difficult to reproduce equatorial current system with relatively narrow circulation features that are important pathways for the ventilation of the tropical thermocline 11. The lack of resolution to fully resolve equatorial current system in K09 may have led to a strong oxygen depletion in the tropical OMZ. To avoid negative oxygen 4

5 concentration, models are programmed to reduce the rate of respiratory oxygen consumption when oxygen concentration approaches a certain threshold. Secondly, K09 model was forced by the climatological wind and buoyancy forcing, and it did not include circulation variability which is an important control for the tropical Pacific OMZ. Our study is based on the data-assimilated physical ocean model and is shown to represent equatorial current variability 12. Thirdly, there has been significant advances in our understanding of tropical OMZ since the time K09 was published 7,10,11,13-15, and our simulations are analyzed in the light of recent advancements. Our oxygen levels in the tropical Pacific are generally reasonable, dissolved iron levels are also within the range of limited observations available at this time. We performed considerably larger number of sensitivity studies and demonstrated signal robustness. Considering above three factors, our result is likely more robust in simulating oxygen decline under increased export production in the tropical Pacific

6 106 References Johnson, M. S. & Meskhidze, N. Atmospheric dissolved iron deposition to the global oceans: effects of oxalate-promoted Fe dissolution, photochemical redox cycling, and dust mineralogy. Geoscientific Model Development 6, , (2013). 2 Luo, C. et al. Combustion iron distribution and deposition. Global Biogeochem Cy 22, GB1012, (2008). 3 Luo, C. & Gao, Y. Aeolian iron mobilisation by dust-acid interactions and their implications for soluble iron deposition to the ocean: a test involving potential anthropogenic organic acidic species. Environmental Chemistry 7, , (2010). 4 Okin, G. S. et al. Impacts of atmospheric nutrient deposition on marine productivity: Roles of nitrogen, phosphorus, and iron. Global Biogeochem Cy 25, GB2022, (2011). 5 Myriokefalitakis, S. et al. Changes in dissolved Iron deposition to the oceans driven by human activity: a 3-D global modelling study. Global Biogeochem Cy in review, (2015). 6 Tagliabue, A. et al. A global compilation of dissolved iron measurements: focus on distributions and processes in the Southern Ocean. Biogeosciences 9, , (2012). 7 Ito, T. & Deutsch, C. Variability of the oxygen minimum zone in the tropical North Pacific during the late twentieth century. Global Biogeochem Cy 27, , (2013). 8 Slemons, L. O., Murray, J. W., Resing, J., Paul, B. & Dutrieux, P. Western Pacific coastal sources of iron, manganese, and aluminum to the Equatorial Undercurrent. Global Biogeochem Cy 24, GB3024, (2010). 6

7 Elrod, V. A., Berelson, W. M., Coale, K. H. & Johnson, K. S. The flux of iron from continental shelf sediments: A missing source for global budgets. Geophys Res Lett 31, L12307, (2004). 10 Deutsch, C., Brix, H., Ito, T., Frenzel, H. & Thompson, L. Climate-Forced Variability of Ocean Hypoxia. Science 333, , (2011). 11 Stramma, L., Johnson, G. C., Firing, E. & Schmidtko, S. Eastern Pacific oxygen minimum zones: Supply paths and multidecadal changes. J. Geophys. Res. 115, C09011, (2010). 12 Schott, F. A., Wang, W. Q. & Stammer, D. Variability of Pacific subtropical cells in the 50-year ECCO assimilation. Geophys Res Lett 34, L05604, (2007). 13 Czeschel, R., Stramma, L. & Johnson, G. C. Oxygen decreases and variability in the eastern equatorial Pacific. Journal of Geophysical Research: Oceans 117, C11019, (2012). 14 Deutsch, C. et al. Centennial changes in North Pacific anoxia linked to tropical trade winds. Science 345, , (2014). 15 Duteil, O., Böning, C. W. & Oschlies, A. Variability in subtropical-tropical cells drives oxygen levels in the tropical Pacific Ocean. Geophys Res Lett 41, , (2014)

8 Figure S1. Comparison of modeled (magenta) and observed (black) dissolved iron in the 149 tropical Pacific Observed and simulated dissolved iron profiles are averaged over the tropical Pacific (160 W-90 W, 10 S-10 N) in the upper 600m of the water column. Black line is the median of all available observations in the published dissolved iron data compiled by A. Tagliabue and co-authors and downloaded from Teal dots are raw data, and the shaded region contains the 5-95 percentile

9 A Climatology B Present-day simulation Oxygen at 435m depth [μmol L -1 ] Figure S2. Observed and simulated oxygen climatology (A) Climatological distribution of [O 2 ] at the 435m depth based on the World Ocean Atlas 2009 in the units of μmol L -1. (B) The same from the present-day simulation averaged over the period of 1971 to

10 Figure S3. Dissolved iron increases due to anthropogenic effect in the North Pacific Ocean (A) Perturbation in dissolved iron concentration along the meridional transect at 160 W where solid black lines are the potential density surfaces. The plotted data is an time average for 1990s. (B) Perturbation in dissolved iron centration interpolated onto the potential density surface 10

11 σθ=25.5 where solid black lines are the contours of the dissolved iron perturbation. The position of the thick solid line along 160 W indicates the position of the meridional transect in (A) Figure S4. The time series of integrated soluble iron deposition rate over the global oceans The standard run (x1) is in yellow. In the x4 case (blue), the temporal change of soluble iron deposition relative to 1952 level is scaled up by a factor of 4, reaching to beyond 10GmolFe yr -1 by year In contrast, x0.25 case (green) it is scaled down by a factor of 4. In this case it reaches to only about 4GmolFe yr -1 by year

12 Figure S5. Export production time series The area-weighted mean export production (panel A) and its de-trended time series (panel B) are plotted for the the tropical Pacific defined as the region bounded by 10 S to 10 N, 150 E to 80 W. Export production is based on by the sinking organic flux at 100m depth. Thin dash lines indicate annual means, and thick solid lines are the running means with 10-year window

13 Δ[O 2 ], μmol L -1 Δ(EPC185), mmol m -2 yr -1 Circ. variability Pollution Fe Pollution N ALL Table S1. The breakdown of oxygen and export production change Change in the oxygen concentration (in μmolo 2 L -1 ) and export production (in mmolc m -2 yr -1 ) at 185m depth (EPC185) are plotted for the the tropical Pacific defined as the region bounded by 10 S to 10 N, 150 E to 80 W. Oxygen concentration is averaged over the depth of m and the export production is evaluated as the sinking organic flux at the depth of 185m X4 X2 X1 X0.5 X0.25 Δ[O 2 ] Table S2. Oxygen change from the sensitivity experiments Changes in the oxygen concentration are plotted for the the tropical Pacific defined as the region bounded by 10 S to 10 N, 150 E to 80 W. Oxygen concentration is averaged over the depth of m and the values are in the units of μmol L