Open AccessTime variations of δ 15 N of organic nitrogen in deep western subarctic Pacific sediment over the last 145 ka
Author(s) -
Shigemitsu Masahito,
Watanabe Yutaka W.,
Narita Hisashi
Publication year - 2008
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2008gc001999
Subject(s) - subarctic climate , interglacial , glacial period , geology , sediment , iron fertilization , aeolian processes , diagenesis , nitrogen , oceanography , isotopes of nitrogen , new production , physical geography , nutrient , geochemistry , paleontology , phytoplankton , ecology , chemistry , organic chemistry , biology , geography
We present δ 15 N data of organic nitrogen ( δ 15 N ON ) from a deep western subarctic Pacific (WSAP) sediment core over the last 145 ka. To remove the effect of inorganic nitrogen (IN) on bulk nitrogen (BN), we used the contents of IN and BN, and the isotopic ratios of IN ( δ 15 N IN ) and BN ( δ 15 N BN ). The contribution of IN to BN was largest 59% and δ 15 N IN had lower values than δ 15 N BN . However, the large contributions of IN to BN and the distinct isotopic values did not largely change any variability associated with δ 15 N ON in the δ 15 N BN . Thus, it is possible to use δ 15 N BN in carrying out paleoceanographic research with δ 15 N at least in the open WSAP. The δ 15 N ON values have been affected by long‐term diagenesis making the values lower with depth in the sediment, but the values during less productive colder periods were generally lower compared to those during neighboring productive warmer periods. The results might indicate that δ 15 N ON during the less productive colder periods (i.e., glacial periods) were lower than that during the productive warmer periods (i.e., interglacial periods). Previous studies have reported that the biological production in WSAP, where it is limited by iron at present, during glacial periods was at more reduced level than during interglacials despite there being more eolian dust supply during glacial periods. This indicates that the increase of biological production with enhanced iron supply derived from eolian dust during the glacial periods did not occur. If this is the case and alternatively the iron input from the subsurface layer to the surface‐mixed layer by ventilation process is more important as suggested by recent works, the variations in δ 15 N ON would be explained by the variations in the Fe/NO 3 − ratio in the upwelled water. Although δ 15 N ON has been affected by long‐term diagenesis in the sediment, the usage of this isotopic ratio provides new implication regarding the past ocean circulation in the open WSAP.