Tracing the Fate of Enhanced Organic Carbon Production during a Southern Ocean Fe Fertilization Experiment using Natural Variations in Carbon and Nitrogen Isotopic Composition

• Main Author: Altabet, M.A.
• Other Authors: United States. Department of Energy. Office of Science.
• Format: Report
• Language: English
• Published: University of Massachusetts Dartmouth (United States) 2005
• Subjects: Ocean Science; Carbon Sequestration; Stable Isotopes; Iron Fertilization; Particulates; Stable Isotopes Ocean Science; Chlorophyll; Productivity; Nitrogen; Exports; Nutrients; Iron; Diatoms; 58 Geosciences; 59 Basic Biological Sciences; Production; 54 Environmental Sciences; Fractionation; Carbon; Fertilization; Phytoplankton; Retention; Antarctic; New Zealand; Southern Ocean; The Antarctic; Antarc*
• Online Access: https://doi.org/10.2172/837333; https://digital.library.unt.edu/ark:/67531/metadc785104/

This project focused on the N and C natural stable isotope response during SOFeX--a purposeful iron (Fe) addition experiment in the Fe limited Southern Ocean. One purpose of the study was to determine if relief of phytoplankton Fe stress would increase productivity sufficiently to enhance C export from surface to deep waters. We proposed that N and C stable isotopes would be useful for tracing this export. Iron was added to waters north and south of the Antarctic Polar Front in waters to the southwest of New Zealand. While both sites have high-nutrient, low chlorophyll conditions (HNLC) typical of Fe limitation, [SiO4] a required nutrient for diatoms was low at the northerly site and high at the southern location. The most extensive coverage occurred at the southern site. Here, FeSO4 was added four different times over an {approx}two week period. We found that: (1) Particulate organic nitrogen and carbon in the mixed layer increased by a factor of 2-3 in response to the Fe addition in the southern patch. (2) PN accumulation and NO3- drawdown were both 1-2 {micro}M during the occupation of the bloom, suggesting retention of particulates within the mixed layer of the southern patch. (3) {sub 15}N of PN and of NO{sub 3}{sup -} increased by 1-2{per_thousand} as [NO{sub 3}{sup -}] decreased, and there is a clear contrast between in- and out-patch stations with respect to particulate {sub 15}N. The isotopic fractionation factor for NO{sub 3}{sup -} was near 5-6{per_thousand} and appears to have been unaffected by Fe fertilization. In contrast, there was little change in {delta}{sup 13}C. (4) The > 54 {micro}m size fraction was typically lighter than the 1-54 {micro}m size fraction by about 0.5 {per_thousand} in {delta}{sup 13}C. In the south patch, this difference increased as the bloom progressed, and with increasing PN concentration. This result may have been caused by large chain-forming diatoms responded to the Fe addition and were likely isotopically lighter than smaller flagellates. Similar observations were made for {delta}{sup 13}C.