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A Test of the Diatom‐Bound Paleoproxy: Tracing the Isotopic Composition of Nutrient‐Nitrogen Into Southern Ocean Particles and Sediments
Author(s) -
Robinson Rebecca S.,
Jones Colin A.,
Kelly Roger P.,
Love Amanda,
Closset Ivia,
Rafter Patrick A.,
Brzezinski Mark
Publication year - 2020
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 - 10.1029/2019gb006508
Subject(s) - diatom , nitrate , nitrogen , isotopes of nitrogen , oceanography , nutrient , biogenic silica , environmental chemistry , sediment , biogeochemistry , geology , environmental science , chemistry , paleontology , organic chemistry
Sedimentary nitrogen isotope (as δ 15 N) records from the Southern Ocean provide critical constraints on surface nutrient consumption in the past and the role of Southern Ocean biophysical changes in setting atmospheric p CO 2 . We present a field assessment of how surface nitrate consumption is reflected in δ 15 N values of total nitrogen and diatom‐bound nitrogen pools of particles and sediments across the Southern Ocean along 170°W during late austral summer. Mixed layer nitrate δ 15 N values increase northwards associated with greater nitrate drawdown. Particles and sediments are expected to follow this trend. Contrary to expectations, surface ocean particle total nitrogen and diatom‐bound δ 15 N values decreased northward during the late summer, likely due to recycling of nitrogen and the assimilation of regenerated ammonium, as well as nitrate. The relationship between δ 15 N values of the total nitrogen and diatom‐bound pools remains relatively constant across this Southern Ocean transect, suggesting that the isotopic composition of these two surface ocean nitrogen pools are largely set by the δ 15 N value(s) of the assimilated nutrient(s). Surface sediment δ 15 N values do increase away from the region of maximum biogenic silica deposition, suggesting that the recycled nitrogen isotopic signal observed in late summer particles may not significantly impact the sedimentary record. However, the enrichment in δ 15 N values of the diatom‐bound pool is greater than what is expected from progressive utilization of the surface nitrate alone and not yet explained.