TY - JOUR
T1 - The Nitrogen Isotopic Composition of Tissue and Shell-Bound Organic Matter of Planktic Foraminifera in Southern Ocean Surface Waters
AU - Smart, Sandi M.
AU - Fawcett, Sarah E.
AU - Ren, Haojia
AU - Schiebel, Ralf
AU - Tompkins, Emily M.
AU - Martínez-García, Alfredo
AU - Stirnimann, Luca
AU - Roychoudhury, Alakendra
AU - Haug, Gerald H.
AU - Sigman, Daniel M.
N1 - Funding Information:
The data presented here are available online (http://www.bco-dmo.org). This work was supported by the South African NRF (Grant 111090, S. M. S.); US NSF Grants 0922345, 1060947, and 1401489 (D. M. S.); the Max Planck Institute for Chemistry, Germany (S. M. S.); the South African National Antarctic Programme (Grant 93069, A. R.; Grants 105539 and 110735, S. E. F.); the South African NRF CSUR fund (Grant 105895, S. E. F); and the Swiss Polar Institute Antarctic Circumnavigation Expedition Grant, Project 12 (S. E. F.). We are grateful to the Department of Environmental Affairs, South Africa, for providing equipment and technical support for net tow deployments, as well as the to the Captain and crew of the R/V S.A. Agulhas II for safe voyages in winter 2015 and late summer 2016. We thank the members of the Sigman Lab (Princeton; particularly E. Kast and V. Luu) and Martínez-García Lab (MPIC; particularly N. Duprey and A. Foreman) for laboratory assistance and method development discussions; A. Plattner for help with creating Figure in GMT; and H. Spero for insightful discussions. S. M. S. gratefully acknowledges hosting institutions, the Department of Earth and Environmental Sciences at Fresno State and the Department of Geological Sciences at the University of Alabama.
Publisher Copyright:
©2020. The Authors.
PY - 2020/2/1
Y1 - 2020/2/1
N2 - We present the first nitrogen isotope (δ15N) measurements of planktic foraminifera, paleoceanographically important zooplankton, from the nutrient-rich waters of the modern Southern Ocean. Foraminifera were collected from net tows in the Subantarctic and Polar Frontal Zones (SAZ and PFZ, respectively) south of Africa during winter 2015 and late summer 2016. In late summer, consistent with preferential uptake of 14N-nitrate and the progressive, northward depletion of nitrate by phytoplankton across the Southern Ocean, foraminifer tissue and shell-bound δ15N rise equatorward along with nitrate δ15N. However, foraminifer δ15N is ~3‰ lower than expected for heterotrophs relying on photosynthetic biomass generated directly from nitrate. This discrepancy appears to originate with the particulate organic N (PON) in late-summer surface waters, the δ15N of which is lowered by ammonium recycling. In winter, when overall productivity and foraminifer production are reduced, foraminifer δ15N is higher (by 4.6 ± 0.8‰ for tissue and by 4.0 ± 1.5‰ for shell-bound N compared to late summer) and exhibits no clear north-south trend. These characteristics can also be explained by the feeding-driven connection of foraminifera to PON, which is elevated in δ15N by net degradation in winter. Therefore, foraminifer δ15N is more closely tied to PON δ15N than to nitrate δ15N in the Southern Ocean mixed layer. Combining our isotope data with previously reported sediment trap fluxes from the western Pacific SAZ/PFZ suggests that, under modern conditions, the late-summer ammonium recycling signal outweighs that of wintertime decomposition on the annually integrated δ15N of sinking foraminifera.
AB - We present the first nitrogen isotope (δ15N) measurements of planktic foraminifera, paleoceanographically important zooplankton, from the nutrient-rich waters of the modern Southern Ocean. Foraminifera were collected from net tows in the Subantarctic and Polar Frontal Zones (SAZ and PFZ, respectively) south of Africa during winter 2015 and late summer 2016. In late summer, consistent with preferential uptake of 14N-nitrate and the progressive, northward depletion of nitrate by phytoplankton across the Southern Ocean, foraminifer tissue and shell-bound δ15N rise equatorward along with nitrate δ15N. However, foraminifer δ15N is ~3‰ lower than expected for heterotrophs relying on photosynthetic biomass generated directly from nitrate. This discrepancy appears to originate with the particulate organic N (PON) in late-summer surface waters, the δ15N of which is lowered by ammonium recycling. In winter, when overall productivity and foraminifer production are reduced, foraminifer δ15N is higher (by 4.6 ± 0.8‰ for tissue and by 4.0 ± 1.5‰ for shell-bound N compared to late summer) and exhibits no clear north-south trend. These characteristics can also be explained by the feeding-driven connection of foraminifera to PON, which is elevated in δ15N by net degradation in winter. Therefore, foraminifer δ15N is more closely tied to PON δ15N than to nitrate δ15N in the Southern Ocean mixed layer. Combining our isotope data with previously reported sediment trap fluxes from the western Pacific SAZ/PFZ suggests that, under modern conditions, the late-summer ammonium recycling signal outweighs that of wintertime decomposition on the annually integrated δ15N of sinking foraminifera.
KW - Southern Ocean
KW - nitrogen isotopes
KW - paleo-proxy
KW - planktic foraminifera
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U2 - 10.1029/2019GC008440
DO - 10.1029/2019GC008440
M3 - Article
AN - SCOPUS:85080082820
SN - 1525-2027
VL - 21
JO - Geochemistry, Geophysics, Geosystems
JF - Geochemistry, Geophysics, Geosystems
IS - 2
M1 - e2019GC008440
ER -