Cristian A Vargas1,2,3, Sebastian I Cantarero4, Julio Sepúlveda5,4, Alexander Galán6,7, Ricardo De Pol-Holz8, Brett Walker9, Wolfgang Schneider5,10, Laura Farías11,10,12, Marcela Cornejo D'Ottone5,13, Jennifer Walker14, Xiaomei Xu14, Joe Salisbury15. 1. Department of Aquatic System, Faculty of Environmental Sciences & Environmental Sciences Center EULA Chile, Universidad de Concepción, Concepción, Chile. crvargas@udec.cl. 2. Millennium Institute of Oceanography (IMO), Universidad de Concepción, Concepción, Chile. crvargas@udec.cl. 3. Coastal Social-Ecological Millennium Institute (SECOS), Universidad de Concepción, Concepción, Chile. crvargas@udec.cl. 4. Department of Geological Sciences and Institute of Arctic and Alpine Research (INSTAAR), University of Colorado Boulder, Boulder, CO, USA. 5. Millennium Institute of Oceanography (IMO), Universidad de Concepción, Concepción, Chile. 6. Centro de Investigación de Estudios Avanzados del Maule (CIEAM), Departamento de Obras Civiles, Facultad de Ciencias de la Ingeniería, Universidad Católica del Maule, Talca, Chile. 7. Centro Regional de Estudios Ambientales (CREA), Universidad Católica de la Santísima Concepción, Concepción, Chile. 8. Centro de Investigación GAIA-Antártica (CIGA) and Network for Extreme Environment Research (NEXER), Universidad de Magallanes, Punta Arenas, Chile. 9. Department of Earth and Environmental Sciences, University of Ottawa, Ottawa, Canada. 10. Department of Oceanography, Universidad de Concepcion, Concepcion, Chile. 11. Coastal Social-Ecological Millennium Institute (SECOS), Universidad de Concepción, Concepción, Chile. 12. Center for Climate and Resilience Research (CR)2, Santiago, Chile. 13. Escuela de Ciencias del Mar, P. Universidad Católica de Valparaíso, Valparaíso, Chile. 14. Department of Earth System Science, University of California, Irvine, CA, USA. 15. Ocean Process Analysis Lab, University of New Hampshire, Durham, NC, USA.
Abstract
Geochemical and stable isotope measurements in the anoxic marine zone (AMZ) off northern Chile during periods of contrasting oceanographic conditions indicate that microbial processes mediating sulfur and nitrogen cycling exert a significant control on the carbonate chemistry (pH, AT, DIC and pCO2) of this region. Here we show that in 2015, a large isotopic fractionation between DIC and POC, a DIC and N deficit in AMZ waters indicate the predominance of in situ dark carbon fixation by sulfur-driven autotrophic denitrification in addition to anammox. In 2018, however, the fractionation between DIC and POC was significantly lower, while the total alkalinity increased in the low-pH AMZ core, suggesting a predominance of heterotrophic processes. An isotope mass-balance model demonstrates that variations in the rates of sulfur- and nitrogen-mediated carbon fixation in AMZ waters contribute ~7-35% of the POC exported to deeper waters. Thus, dark carbon fixation should be included in assessments of future changes in carbon cycling and carbonate chemistry due to AMZ expansion.
Geochemical and stable isotope measurements in the anoxic marine zone (AMZ) off northern Chile during periods of contrasting oceanographic conditions inn class="Gene">dicate that microbial processes mediating sulfur and nitrogen cycling exert a significant control on the carbonate chemistry (pH, AT, DIC and pCO2) of this region. Here we show that in 2015, a large isotopic fractionation between DIC and POC, a DIC and N deficit in AMZ waters indicate the predominance of in situ dark carbon fixation by sulfur-driven autotrophic denitrification in addition to anammox. In 2018, however, the fractionation between DIC and POC was significantly lower, while the total alkalinity increased in the low-pH AMZ core, suggesting a predominance of heterotrophic processes. An isotope mass-balance model demonstrates that variations in the rates of sulfur- and nitrogen-mediated carbon fixation in AMZ waters contribute ~7-35% of the POC exported to deeper waters. Thus, dark carbon fixation should be included in assessments of future changes in carbon cycling and carbonate chemistry due to AMZ expansion.
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