Assessment of C, N and Si isotopes as tracers of past ocean nutrient and
carbon cycling
Jill Sutton
Univ Brest, CNRS, IRD, Ifremer, Institut Universitaire Européen de la Mer, LEMAR, Univ Brest, CNRS, IRD, Ifremer, Institut Universitaire Européen de la Mer, LEMAR
Author ProfileAbstract
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.