1. Changes in POC Concentration and  13 C during Mesoscale Iron Fertilization in the Southern Ocean 1 Mark A. Altabet, David Timothy, Matt McIlvin, and Peng Feng 1 School for Marine Science and Technology, U. Massachusetts Dartmouth, 706 S. Rodney French Blvd., New Bedford, MA 02744-1221. United States Acknowledgements : Rekha Singh provided technical assistance. Kenneth Coale and Craig Hunter provided bottle data. Ken Buesseler and Bob Bidigare provided in situ pump samples and data. Funding is from the DOE Carbon Sequestration Program and the NSF. Contact: maltabet@umassd.edu, dtimothyn@umassd.edu or http://www.cmast.umassd.edu/ Introduction and Background Although  13 C of marine sediments has been used to infer past changes in [CO 2 ] aq the roles of physiological state (manifested through growth rate), cell size and species assemblage also have strong influences on  13 C of phytoplankton and sinking particulates. As part of the Southern Ocean Fe Experiment (SOFeX), we tracked  13 C of particulate matter during Fe-induced blooms in high-nitrate, low-chlorophyll (HNLC) and low-silicate waters north of the polar front (52 o S, 167 o W), and in HNLC and high-silicate waters south of the polar front (66 o S, 172 o W). A previous study (AESOPS; Fig 1) found that  13 C of POC increased as pCO 2 decreased along a latitudinal transect in the SOFeX study area, and during another recent Fe-enrichment in the Southern Ocean (EisenEx; Fig 2), a significant increase in  13 C of POC was observed. We therefore anticipated the possibility that  13 C of POC would increase during the progression of the Fe-enduced blooms at the SOFeX sites. Indeed, very low organic  13 C (as low as -30‰) is commonly observed in the polar Southern Ocean due to large fractionation factors for DIC incorporation (  ) by phytoplankton.  was expected to decrease and organic  13 C increase in response to increasing phytoplankton growth rate, cell size, and decreasing [CO 2 ] aq. As an extension, we have considered the use of isotopic signals to trace the fate (e.g export vs. remineralization) of newly produced organic C during SOFeX. Fig. 2 Results from the EisenEx Fe fertilization experiment in the South Atlantic sector of the Southern Ocean. A ring feature in the ACC was chosen to do the experiment and it took place in austral spring. POC increased by less than 2x over a 3-week observation period.  13 C increase by at least 3‰ in response to a combination of reduced [CO 2 ] aq and/or increased phytoplankton growth rates. Fig. 4 In-patch (southern) time series for mixed layer average POC concentration and  13 C. Niskin 1 (Altabet lab) and Niskin 2 (courtesy Kenneth Coale) are results for Niskin-collected water samples. LVP refers to results from a submerged large-volume pump (courtesy Bob Bidigare). These results generally support a large increase in patch POC compared to out- stations. The three data sets generally agree except for high POC for the pump data in the latter half of the experiment. Compared to EisenEx, there is surprisingly little change in  13 C. The variation in  13 C observed is also not monotonic, suggesting multiple influences. Fig. 5 Southern Patch time series for size-fractionated, pump- collected particulate data (samples courtesy Ken Buesseler). While the 1-54 µm fraction shows a clear temporal change in  13 C, these data fall within the range shown in Fig. 4. However, the >54 µm fraction shows substantial isotopic enrichment in the latter half of the experiment. This may reflect the influence of large, relatively fast growing diatoms. Fig. 6 There is no clear relationship between POC concentration and  13 C throughout the SOFeX experiment. Preliminary Conclusions 1) For SOFeX, there was a ~3-fold increase in POC in the southern patch. In contrast, EisenEx saw less than a doubling, perhaps due to a combination of deeper mixed layer and shorter observation period. 2)  hough SOFeX had greater POC accumulation, there was substantially less change in  13 C. While actual comparisons with CO 2 system parameters and phytoplankton growth rate await availability of those data, it is clear that there is no simple relationship between POC accumulation and  13 C. 3) Nevertheless, the size-fractionated  13 C data is consistent with the expected role of large diatoms during Fe fertilization. Fig. 1 Transect of near-surface pCO 2 and  13 C in POC across the JGOFS AESOPS study region in late Austral Summer 1998. The trap data are the flux-weighted annual average of moored sediment collections. Large latitudinal gradients are observed with lowest  13 C and highest pCO 2 values in the polar waters of the northern Ross Sea. Surface POC  13 C data show a general correspondence with the trap data. The SOFeX experiment took place in 2002 along this transect near 66°S APF Fig. 3 Representative POC concentration profiles for SOFeX in and out stations for the southern patch in high silicate, polar water. POC increased by almost 3-fold within a 40 to 50 m surface layer for the in- patch stations. In contrast, the out-patch station showed no clear progressive change. These data were for samples collected by Niskin bottles and filtered onto GF/F filters. SOFeX Niskin 1 Niskin 2 LVP Niskin 1 LVP SOFeX Niskin 1 LVP pCO2 (uatm) 500 400 300 200