Does Iron Fertilization Enhance Carbon Export in the Southern Ocean?

1. Does Iron Fertilization Enhance Carbon Export in the Southern Ocean? Matthew A. Charette and Ken O. Buesseler Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543; mcharette@whoi.edu ABSTRACT The Southern Ocean has the potential to influence climate due to its large inventory of excess macronutrients such as nitrate and phosphate. It has been hypothesized that if the supply of the micronutrient iron increased, it would lead to enhanced uptake of atmospheric CO2 and hence the sequestration of carbon via sinking particles (Kumar et al., 1995; Sarmiento and Toggweiler, 1984). While much has been learned about iron-limitation and low phytoplankton biomass in high-nutrient, low-chlorophyll regions (Martin et al., 1991; Coale et al., 1996), less is know about the effect of Fe on particle export. Here we present results from the first detailed study of particle export during a mesoscale iron fertilization experiment (SOIREE). Measurements of the natural tracer thorium-234 indicate negligible particle export within 14 days after the initial infusion of iron. We attribute this lack of response to colder water temperatures that promote slower cell metabolism in phytoplankton and hence, slower secondary responses of herbivores and particle aggregation. SOUTHERN OCEAN CARBON EXPORT: IMPLICATIONS FOR CLIMATE CHANGE • Potential to influence climate due to its large inventory of excess macronutrients • It has been hypothesized that if the supply of the micronutrient iron increased, it would lead to enhanced uptake of atmospheric CO2 • Paleoceanographic evidence suggests that, indeed, Southern Ocean iron-induced organic carbon export led to past changes in global climate OBJECTIVE Figure 2. Vertical profiles of dissolved, particulate, total (dissolved plus particulate) 234Th, and its parent nuclide 238U collected on day 9 of the experiment inside the iron-fertilized patch. Uranium-238 is chemically non-reactive in seawater and can be determined from salinity (238U (dpm l-1) = 0.07081 x Salinity (‰); Chen et al., 1986). The disequilibrium between the parent and daughter nuclides is due to scavenging and vertical export of 234Th on rapidly-sinking particles. Since the biological pump is often the major source of these particles in open ocean waters, 234Th:238U disequilibria is mainly confined to the euphotic zone (upper ~100 m). Error bars are propagated from counting statistics (1s) and measurement uncertainties. Utilize particle-tracer thorium-234 to study export response during a mesoscale iron addition experiment in the Southern Ocean. Figure 3. Time-series measurements of vertically-integrated 234Th (0-100 m) during SOIREE at stations located in the center of the iron-fertilized patch (solid symbols) and at control stations outside the patch (open symbols). Uranium-238 (solid dashed line) was determined from the average salinity over the 0-100 m. The +Fe symbols along the x-axis indicate the approximate timing of the four iron enrichments. Figure 5. Discrete-depth samples of thorium-234 at 30 m. Similar 234Th activities both inside and outside the patch indicate negligible response from the biological pump at this depth late in the experiment. Figure 1. Map showing the location of the SOIREE iron addition experiment. IN SITU, INTEGRATED WATER COLUMN THORIUM-234 SAMPLING CONCLUSIONS • Based on (a) no observable change in 234Th inside vs. outside the patch and (b) a 234Th flux indistinguishable from zero, we conclude that particle export during SOIREE was negligible • The magnitude of export following SOIREE remains unconstrained, especially considering new evidence of SOIREE bloom longevity (based on SeaWiFS images) • An iron-induced response in chlorophyll (increasing) or pCO2 (decreasing) does not necessarily lead to a proportional response from the biological pump in the Southern Ocean UPPER-LIMIT SOIREE POC EXPORT • Typical lower-limit PTh = 200-600 dpm m-2 d-1 (NE Pac. 1997–Charette et al., 1999); translated to a PPOC of ~2-3 mmol C m-2 d-1.   • Given the potential upper-limit SOIREE PTh = 720 dpm m-2 d-1, PPOC = 7.2 mmol C m-2 d-1 (using a measured POC/Th ratio of 10) • Invoking steady-state assumption on initial Th data gives pre-SOIREE PTh = 2600 dpm m-2 d-1; this translates to a PPOC = 26 mmol C m-2 d-1. Therefore, the potential SOIREE-induced POC export is minor. • Pump collects 20 L sample every 5 m (0-100 m) for a total sample volume of 400 L • Samples collected for particulate (> 70 micron, 1-70 micron) an dissolved thorium-234 Fig. 4. Methods used for collection of thorium-234 during SOIREE. Acknowledgements For their support of this research, we thank the captain and crew of the NIWA Vessel Tangaroa. We thank Phil Boyd, Andy Watson, Cliff Law, Tom Trull, and countless others for their efforts in organizing SOIREE. Ed Abraham kindly provided statistical information on mixed-layer depth variation. Finally, we thank Sheila Clifford for designing the poster. This work was supported by funds from the National Science Foundation (to K.O.B) NIWA, and the Vetlesen Foundation (Postdoctoral Fellowship to M.A.C).