The paradox of iron demand due to diatom productivity in the Southern Ocean Jacqueline G. Valett, Taka Ito, and Ellery D. Ingall School of Earth and Atmospheric Sciences, Georgia Institute of Technology
Motivation Box model A paradox?
Iron plays an important role in the regulation of biological productivity Iron and Silica Cycling within Southern Ocean We use a simple box model con- • Can we evaluate the overall iron budget of the Southern Ocean? and the carbon cycle in the Southern Ocean. Recently, X-ray spectromi- sisting of a surface and deep box to Given the range of the observed Fe:Si ratio, Rbio, and the circulation rate, croscopic techniques revealed high molar Fe:Si ratios in Antarctic bio- represent overturning circulation source J , we estimate the ‘missing’ source of iron, J (mol Fe/sec), into the South- genic silica from living phytoplankton samples (Ingall et al., 2013). The Fe2 (Ψ) in the Southern Ocean. North Si2 ern Ocean. The area bound by the yellow lines indicates a range of J given export and burial of biogenic silica can be a major removal pathway of A Atlantic Deep Water enters into the by Tagliabue et al. (2010). This iron from the Southern Ocean. Based on this new data, we estimate the n 2 3000 t deep Southern Ocean (box 1) 1.2 range is the sum of observed at- a iron removal rate in the pelagic Southern Ocean using a simple inverse r which then upwells into the surface c 2500 mospheric dust, shelf sediments, Fe1 Si1 1 box model. t ocean (box 2). We neglect the org Fe and iceberg Fe sources. With in- i export c hydrothermal Fe sources for this 2000 • How much iron is removed from the Southern Ocean that is associated 0.8 creased upwelling, more iron is a Si-bound Fe export initial study. We include a general with the burial of biogenic silica? Fe0 exported to the deep ocean, thus 1500 Si0 source term (J) which may include 0.6 • Can the upwelling of deep water satisfy the iron demand due to diatom 1 dissolution requiring more iron from outside atmospheric dust, glacier melt or 1000 productivity? 0.4 sources. • Can we evaluate the overall iron budget of the Southern Ocean? burial shelf sediment sources. 500
Observed biogenic Fe:Si ratio [mmol/mol] 0.2
0 0.5 1 1.5 2 2.5 3 3.5 3 −1 7 Methods Coupled Fe and Si cycling Ψ [m s ] x 10
100 6 Synchrotoron X-ray microscopy • How much iron is removed from 4 x 10 Combining the existing dissolved iron and silicic acid datasets with a simple (Ingall et al., 2013) the Southern Ocean that is associated 3.5 box model we reveal that, even with the most conservative estimates of the 10 )
• Fe content of living diatom -1 with the burial of biogenic silica? 3 Fe:Si ratio, the upwelling of subsurface waters cannot supply enough iron to samples from the Southern Ocean is balance the loss due to diatom production. Given the upwelling rate of 8 to 22 2.5 determined using synchrotoron X-ray 1 Sv (Sallee et al. 2013) and the ob- microscopy. The estimated Fe:Si ratio 2
Fe:Si (mmol mol served Si distribution, we estimate the is in the range of 0.045 to 1.278 0.1 1.5
burial loss of silica into the sediment Silica loss [mol Si / sec] mmol/mol depending on the sample 1 Conclusion with the magnitude of approimately 0.01 processing. • 6 • 6 0.5 6N HCl 30% H2O2 Untreated 0.75 10 to 2 10 mol Si/sec. 0 0 0.5 1 1.5 2 2.5 3 3.5 4 The observed iron concentrations in upwelling subsurface water Large-scale hydrographic survey (WOCE) 3 7 4000 Ψ [m / sec] x 10 • We use the large-scale water mass and silicate distributions from the 1.2 masses within the Southern Ocean do not supply enough iron to WOCE Southern Ocean hydrographic surveys. Silicate concentration was 3500 balance the loss due to diatom productivity. 1 analyzed based on the density thresholds of water masses as defined by 3000 • Could there be another large iron source despite research showing Sallee et al. (2013). 0.8 2500 Using the lowest and highest observed 2000 that the majority of iron supply is from upwelling (Boyd et al., 2012)? 0.6 biogenic Fe:Si ratio of the diatom 1500 frustles, we estimate the associated loss • Are there problems with the simplicity of the model or with the 0.4 1000 of iron to be approximately 33.8 to 3 scarcity of nutrient concentration measurements in the Southern •
Observed biogenic Fe:Si ratio [mmol/mol] 2.56 10 mol Fe/sec. 0.2 500 Ocean?
0.5 1 1.5 2 2.5 3 3.5 3 −1 7 Ψ [m s ] x 10 References • Can the upwelling of deep water satisfy the iron demand due to diatom Boyd P. W. et al. (2012) Mapping phytoplankton iron utilization: Insights into Southern Ocean supply mechanisms, productivity? J. Geophysical Research, 117, C06009, doi:10.1029/2011JC007726. Ingall E. D. et al. (2013) Role of biogenic silica in the removal of iron from the Antarctic seas, Nature Communica- We calculate the Fe:Si supply ratio based tions, 4:1981, doi:10.1038/ncomms2981. Global dissolved Fe database (Tagliabue et al., 2012) Sallee J. B. et al. (2013) Assessment of Southern Ocean water mass circulation and characteristics in CMIP5 models: on the nutrient concentrations flowing into Historical bias and forcing response, J. Geophysical Research: Oceans, 118, 1830-1844, doi:10.1002/jgrc.20135. • We use the global compilation of dissolved Fe distribution by Tagliabue et and out of the Southern Ocean driven by the Strzepek R. F. et al. (2011) Adaptive strategies by Southern Ocean phytoplankton to lessen iron limitation: Uptake of organically complexed iron and reduced cellular iron requirements, Limnology and Oceanography, al., (2012). We associate data points with their longitude, latitude, and depth overturning circulation (MOC). doi:10.4319/lo.2011.56.6.1983. on the density cross section. Tagliabue A. et al. (2012) A global compilation of dissolved iron measurements: focus on distributions and processes in the Southern Ocean, Biogeosciences, doi:10.5194/bg-9-2333 Fe:Si ratio in overturning water mass RMOC = 0.0014 to 0.0036 mmol/mol Tagliabue A. et al. (2010) Hydrothermal contribution to the oceanic dissolved iron inventory, Nature Geoscience Letters, doi: 10.1038/NGEO818. Observed Fe:Si ratio in diatom frustles Rbio= 0.045 to 1.278 mmol/mol Treguer P. J. and De La Rocha C. L. (2013) The World Ocean Silica Cycle, Annual Review of Marine Science, 5:477-501, doi: 10.1146/annurev-marine-121211-172346.
RMOC << Rbio suggests that every mol of diatom export tends to deplete iron We are thankful for the NSF grant OPP-1142009. in the surface Southern Ocean. The upwelling of deep water cannot satisfy the iron demand due to the Fe deficit in upwelling water relative to diatom export. comments and suggestions: [email protected]