Nitrogen isotope ratios of nitrate and N* anomalies in the subtropical South Pacific

作者：Yoshikawa Chisato^*; Makabe Akiko; Shiozaki Takuhei; Toyoda Sakae; Yoshida Osamu; Furuya Ken; Yoshida Naohiro

来源：Geochemistry Geophysics Geosystems, 2015, 16(5): 1439-1448.

DOI：10.1002/2014GC005678

• nitrogen isotopes
• nitrogen fixation
• denitrification
• South Pacific

Nitrogen isotopic ratios of nitrate (delta N-15-NO3-) were analyzed above 1000 m water depth along 17 degrees S in the subtropical South Pacific during the revisit WOCE P21 cruise in 2009. The delta N-15-NO3- and N* values were as high as 17 parts per thousand and as low as -18 mu mol N L-1, respectively, at depths around 250 m east of 115 degrees W, but were as low as 5 parts per thousand and as high as +1 mu mol N L-1, respectively, in subsurface waters west of 170 degrees W. The relationships among NO3- concentrations, N* values, delta N-15-NO3- values, and oxygen and nitrite concentrations suggest that a few samples east of 90 degrees W were from suboxic and nitrite-accumulated conditions and were possibly affected by in situ water column denitrification. Most of the high-delta N-15- NO3- and negative-N* waters were probably generated by mixing between Subantarctic Mode Water from the Southern Ocean and Oxygen Deficit Zone Water from the eastern tropical South Pacific, with remineralization of organic matter occurring during transportation. Moreover, the relationship between delta N-15-NO3- and N* values, as well as Trichodesmium abundances and size-specific nitrogen fixation rates at the surface, suggest that the low-delta N-15-NO3- and positive-N* subsurface waters between 160 degrees E and 170 degrees W were generated by the input of remineralized particles created by in situ nitrogen fixation, mainly by Trichodesmium spp. Therefore, the delta N-15 values of sediments in this region are expected to reveal past changes in nitrogen fixation or denitrification rates in the subtropical South Pacific.

• 出版日期2015-5