Assessment of C, N, and Si Isotopes as Tracers of Past Ocean Nutrient and Carbon Cycling. Issue 7 (2nd July 2021)

• Record Type: Journal Article
• Authors: Farmer, J. R.; Hertzberg, J. E.; Cardinal, D.; Fietz, S.; Hendry, K.; Jaccard, S. L.; Paytan, A.; Rafter, P. A.; Ren, H.; Somes, C. J.; Sutton, J. N.
• Is Part Of: Global biogeochemical cycles. Volume 35:Issue 7(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-07-02
• Subjects: marine chemistry -- carbon -- nitrogen -- silicon -- paleoceanography -- stable isotopes; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1029/2020GB006775
• Languages: English
• ISSNs: 0886-6236

Abstract

Abstract: Biological productivity in the ocean directly influences the partitioning of carbon between the atmosphere and ocean interior. Through this carbon cycle feedback, changing ocean productivity has long been hypothesized as a key pathway for modulating past atmospheric carbon dioxide levels and hence global climate. Because phytoplankton preferentially assimilate the light isotopes of carbon and the major nutrients nitrate and silicic acid, stable isotopes of carbon (C), nitrogen (N), and silicon (Si) in seawater and marine sediments can inform on ocean carbon and nutrient cycling, and by extension the relationship with biological productivity and global climate. Here, we compile water column C, N, and Si stable isotopes from GEOTRACES‐era data in four key ocean regions to review geochemical proxies of oceanic carbon and nutrient cycling based on the C, N, and Si isotopic composition of marine sediments. External sources and sinks as well as internal cycling (including assimilation, particulate matter export, and regeneration) are discussed as likely drivers of observed C, N, and Si isotope distributions in the ocean. The potential for C, N, and Si isotope measurements in sedimentary archives to record aspects of past ocean C and nutrient cycling is evaluated, along with key uncertainties and limitations associated with each proxy. Constraints on ocean C and nutrient cycling during late Quaternary glacial‐interglacial cycles and over the Cenozoic are examined. This review highlights opportunities for future research using multielement stable isotope proxy applications and emphasizes the importance of such applications to reconstructing past changes in the oceans and climate system. Plain Language Summary: The ability of marine phytoplankton to fix carbon—and hence influence the air‐sea partitioning of the greenhouse gas carbon dioxide—highlights the potential for these organisms to influence global climate in the past and future. In addition to C, phytoplankton require nutrients including inorganic N and for certain groups, Si. Because nutrients fuel phytoplankton growth, tracing past nutrient uptake can inform on important aspects of past biological production. Phytoplankton preferentially incorporate the light isotopes of C, N, and Si into their cells and metabolic products. These isotopic signatures can be preserved in marine sediments, providing a means to reconstruct past changes in biological activity. Here we use new data to illuminate processes driving the stable isotopic composition of C, N, and Si in the water column and in marine sediments. We evaluate the processes that lead to changes in the concentration of these elements and their isotopes in the ocean. We discuss scientific caveats and the extent of uncertainty relevant for interpreting past records of these isotopes. We then discuss examples of representative geochemical reconstructions using sediment records from the last ice age and over the last 70 million years. We use this knowledge to highlight directions for future research. Key Points: Review of oceanic distribution, controlling processes, and sedimentary archives of carbon (C), nitrogen (N), and silicon (Si) isotopes Late Quaternary C, N, and Si sedimentary isotope records demonstrate coupling between ocean carbon and nutrient cycling and atmospheric CO2 levels Cenozoic C, N, and Si sedimentary isotope records indicate large‐scale changes in nutrient sources, concentrations, and the carbon cycle