CHANGES IN THE ISOTOPIC VALUE OF SEDIMENTARY NITROGEN AT CALIFORNIA MARGIN SITE 1010: A RECORD OF DENITRIFICATION CHANGES OVER THE LAST FIVE MILLION YEARS

Abstract:

Global cooling over the last 5 myrs, from the warm Pliocene to the cold ice ages, is associated with changes in circulation and atmospheric carbon dioxide concentrations. Since primary productivity is limited by bioavailable nitrogen (N), fluctuations in the oceanic fixed N inventory potentially played an important role in these cooling trends. This inventory of fixed N is a balance of fixation and denitrification; both processes impart N isotope effects. Published records of increasing N isotopic values of bulk sediment have been interpreted as indicating intensification of denitrification with global cooling. However, most data, such as from California margin ODP Site 1012 (Liu et al., 2007), are from upwelling regions of the east Pacific where changes in N utilization may have occurred, obscuring the signature of changes in the N isotopic value of source nitrate which is affected by denitrification. Our study focuses on Site 1010, located off-shore from Site 1012 in a region of compete N utilization where we can obtain a record of isotopic values of source N, and thus of the evolution of denitrification during global cooling.