Benthic-Pelagic Coupling in the East Siberian Sea from Nitrate Isotopes

Over the East Siberian Sea and common to the western
Arctic Ocean, a subsurface nutrient maximum is reported in
the halocline, and generally attributed to both nutrient-rich
Pacific inflow and intensive remineralization in shelf bottom
waters being advected into the central Arctic basin. We report
nitrogen and oxygen isotopic measurement of nitrate in the
water column. A large decoupling between nitrate δ15N and δ18
O is reported, increasing and decreasing upward from the
Atlantic T°C maximum into the halocline, respectively.
Nitrate δ18O follows the decrease in water δ18O, harboring
low-δ18O from large Arctic river discharge. This imprint is
transmitted with nitrification to the ambient nitrate δ18O,
suggesting that most of the nitrate being supplied into the
Arctic Ocean has been reprocessed at least once within the
Arctic.
The associated increase in nitrate δ15N, correlated with
the fixed N deficit, indicates that a significant share of
benthic denitrification is supplied from nitrate produced by
partial nitrification in the reactive sediment layer. Following
an imbalance between remineralization and nitrification, the
residual high-δ15N ammonium is accumulated in pore waters
and diffuses into shelf bottom waters to be ultimately
nitrified. Following the advection into the central basin, this
processes could explain both the nutrient maximum, with
high-δ15N and low-δ18O nitrate, and the accentuated fixed N
deficit in the western Arctic halocline in comparison to the
Pacific inflow. A sedimentary isotope effect is reported for
benthic denitrification, 2.3-3.1‰, in the middle range given
in the literature.