Time variations of δ15N of organic nitrogen in deep western subarctic Pacific sediment over the last 145 ka

Abstract

We present δ15N data of organic nitrogen (δ15NON) 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 (δ15NIN) and BN (δ15NBN). The contribution of IN to BN was largest 59% and δ15NIN had lower values than δ15NBN. However, the large contributions of IN to BN and the distinct isotopic values did not largely change any variability associated with δ15NON in the δ 15NBN. Thus, it is possible to use δ 15NBN in carrying out paleoceanographic research with δ15N at least in the open WSAP. The δ15N 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 15Nδ15 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 δ15NON would be explained by the variations in the Fe/NO3- ratio in the upwelled water. Although δ15NON 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. © 2008 by the American Geophysical Union.