Changes in POC Concentration and d 13 C during Mesoscale Iron Fertilization in the Southern Ocean

1. Niskin 1 LVP Changes in POC Concentration and d13C during Mesoscale Iron Fertilization in the Southern Ocean 1Mark A. Altabet, David Timothy, Matt McIlvin, and Peng Feng 1School for Marine Science and Technology, U. Massachusetts Dartmouth, 706 S. Rodney French Blvd., New Bedford, MA 02744-1221. United States Introduction and Background Although d13C of marine sediments has been used to infer past changes in [CO2]aq the roles of physiological state (manifested through growth rate), cell size and species assemblage also have strong influences on d13C of phytoplankton and sinking particulates. As part of the Southern Ocean Fe Experiment (SOFeX), we tracked d13C of particulate matter during Fe-induced blooms in high-nitrate, low-chlorophyll (HNLC) and low-silicate waters north of the polar front (52oS, 167oW), and in HNLC and high-silicate waters south of the polar front (66oS, 172oW). A previous study (AESOPS; Fig 1) found that d13C of POC increased as pCO2 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 d13C of POC was observed. We therefore anticipated the possibility that d13C of POC would increase during the progression of the Fe-enduced blooms at the SOFeX sites. Indeed, very low organic d13C (as low as -30‰) is commonly observed in the polar Southern Ocean due to large fractionation factors for DIC incorporation (e) by phytoplankton. e was expected to decrease and organic d13C increase in response to increasing phytoplankton growth rate, cell size, and decreasing [CO2]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. Niskin 1 LVP 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. d13C increase by at least 3‰ in response to a combination of reduced [CO2]aq and/or increased phytoplankton growth rates. 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. Fig. 6 There is no clear relationship between POC concentration and d13C 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) Though SOFeX had greater POC accumulation, there was substantially less change in d13C. While actual comparisons with CO2 system parameters and phytoplankton growth rate await availability of those data, it is clear that there is no simple relationship between POC accumulation and d13C. 3) Nevertheless, the size-fractionated d13C data is consistent with the expected role of large diatoms during Fe fertilization. SOFeX SOFeX Niskin 1 Niskin 2 LVP 500 400 300 200 pCO2 (uatm) APF 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/ Fig. 4 In-patch (southern) time series for mixed layer average POC concentration and d13C. 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 d13C. The variation in d13C observed is also not monotonic, suggesting multiple influences. Fig. 1 Transect of near-surface pCO2 and 13C 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 13C and highest pCO2 values in the polar waters of the northern Ross Sea. Surface POC 13C data show a general correspondence with the trap data. The SOFeX experiment took place in 2002 along this transect near 66°S 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 d13C, 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.