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Nitrogen isotope dynamics of the Cariaco Basin, Venezuela
Author(s) -
Thunell Robert C.,
Sigman Daniel M.,
MullerKarger Frank,
Astor Yrene,
Varela Ramon
Publication year - 2004
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/2003gb002185
Subject(s) - thermocline , oceanography , anoxic waters , nitrate , water column , geology , denitrification , structural basin , sill , sediment trap , environmental science , nitrogen , geochemistry , geomorphology , chemistry , organic chemistry
We report 15 N/ 14 N 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 δ 15 N of shallow thermocline nitrate is ∼3.5‰, significantly lower than the mean deep ocean nitrate δ 15 N 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 15 N/ 14 N 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 15 N/ 14 N of the seafloor sediment is similar to that of the sinking flux, as is common in environments of high export production, low O 2 , and good organic matter preservation. Thus the modern Cariaco Basin records the 15 N/ 14 N 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.