Seasonal characteristics of the nitrogen isotope biogeochemistry of settling particles in the western subarctic Pacific: A model study

Autor: Shinichiro Noriki, Yasuhiro Yamanaka, Nobuhiro Maeda, Yutaka W. Watanabe, Masahito Shigemitsu
Rok vydání: 2010
Předmět:
Zdroj: Earth and Planetary Science Letters. 293:180-190
ISSN: 0012-821X
DOI: 10.1016/j.epsl.2010.01.043
Popis: article Weused mooredtime-series sediment traps tocollectsettling particlesatstation KNOT(44°N, 155°E; trapdepth 770 m) in the western subarctic Pacific (WSAP) from October 1999 to May 2006. Particulate nitrogen content (PN) and isotope ratios (δ 15 NPN) were measured in the samples collected. The general pattern of variation in δ 15 NPN results showed lower values during the spring bloom periods and summer, and higher values during winter. To interpret the processes controlling such variations quantitatively and reveal some implications for paleoceanographic use of δ 15 NPN, we developed an ecosystemmodel thatincluded nitrogen isotopes. Thismodel was validated with an observed data set and successfully reproduced the seasonal variations of δ 15 NPN .I n simulations, the lower δ 15 NPN during the spring bloom period was caused mainly by the highest proportion of dead large phytoplankton (diatom) in PN within a year, the highest f-ratio of the year, and phytoplankton assimilation of nitrate with the lowest δ 15 N of the year. The lower δ 15 NPNin summer was due to the high relative proportion of dead non-diatom small phytoplankton and microzooplankton fecal pellet with the lowest δ 15 N valuesamongallthePNcomponentsinourmodel.The higher δ 15 NPN inwinterwasmainlycausedbythe highest proportion of zooplankton components in PN, with higher δ 15 N valuesthan phytoplankton components, and the enhanced δ 15 N values of ammonium induced by nitrification and its subsequent assimilation by phytoplankton. Our identification of nitrification as one cause of higher δ 15 NPN in winter is consistent with previous findings in a proximal marginal sea, the Okhotsk Sea, with an ecosystem model simpler than our model. This might indicate thatthecauseofhigher δ 15 NPNinwinteriscommonintheWSAP.Inourmodel,weoptimizedtheisotopeeffectof
Databáze: OpenAIRE
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