Nitrogen isotope dynamics of the Cariaco Basin, Venezuela

Abstract

We report 15N/14N measurements of water column nitrate and ammonium, sinking particles, and sediments from the Cariaco Basin, an anoxic marine basin off the coast of Venezuela. Water column denitrification occurring in the basin has only a very small isotopic imprint on nitrate in the basin because nitrate consumption is nearly complete in the actively denitrifying water near the oxic/anoxic interface (∼275 m). Being free of a large denitrification signal, the δ15N of shallow thermocline nitrate is ∼3.5‰, significantly lower than the mean deep ocean nitrate δ15N of 5‰. This may be due to the nitrification of newly fixed N, whether it occurs within the basin or in open Atlantic waters that flow into the Cariaco over the sill. The 15N/14N of the sinking flux in the deepest trap (∼1250 m) is similar to that of thermocline nitrate, as expected given the complete consumption of nitrate in the surface layer. Moreover, the 15N/14N of the seafloor sediment is similar to that of the sinking flux, as is common in environments of high export production, low O2, and good organic matter preservation. Thus the modern Cariaco Basin records the 15N/14N of the thermocline nitrate, which, in turn, may record the input of newly fixed N to the upper ocean, be it local or more regional in origin.