Computer Science
Scientific paper
Apr 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010e%26psl.293..180s&link_type=abstract
Earth and Planetary Science Letters, Volume 293, Issue 1-2, p. 180-190.
Computer Science
Scientific paper
We used moored time-series sediment traps to collect settling particles at station KNOT (44°N, 155°E; trap depth 770 m) in the western subarctic Pacific (WSAP) from October 1999 to May 2006. Particulate nitrogen content (PN) and isotope ratios (δ15NPN) were measured in the samples collected. The general pattern of variation in δ15NPN 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 δ15NPN, we developed an ecosystem model that included nitrogen isotopes. This model was validated with an observed data set and successfully reproduced the seasonal variations of δ15NPN. In simulations, the lower δ15NPN 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 δ15N of the year. The lower δ15NPN in summer was due to the high relative proportion of dead non-diatom small phytoplankton and microzooplankton fecal pellet with the lowest δ15N values among all the PN components in our model. The higher δ15NPN in winter was mainly caused by the highest proportion of zooplankton components in PN, with higher δ15N values than phytoplankton components, and the enhanced δ15N values of ammonium induced by nitrification and its subsequent assimilation by phytoplankton. Our identification of nitrification as one cause of higher δ15NPN 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 that the cause of higher δ15NPN in winter is common in the WSAP. In our model, we optimized the isotope effect of each process using our observational data of δ15NPN and δ15N of nitrate published elsewhere as constraints, and investigated the sensitivity of the annual flux-weighted mean of δ15NPN to the isotopic fractionation effects. As a result, we found that the isotope effects of nitrate assimilation appear to be different for non-diatom small phytoplankton and large diatom, and the annual flux-weighted mean of δ15NPN can be influenced to some extent by the isotope effect of nitrification.
Maeda Nobuhiro
Noriki Shinichiro
Shigemitsu Masahito
Watanabe Yutaka W.
Yamanaka Yasuhiro
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