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Spatiotemporal variations of nitrogen isotopic records in the Arabian Sea

Authors :
X. H. Sean Wan
C.-T. Arthur Chen
Min Nina Xu
Li-Wei Zheng
Shuh-Ji Kao
Shih-Chieh Hsu
Kandasamy Selvaraj
Binju Wang
Source :
Biogeosciences, Vol 12, Iss 1, Pp 1-14 (2015)
Publication Year :
2015
Publisher :
Copernicus Publications, 2015.

Abstract

Available reports of dissolved oxygen, δ15N of nitrate (δ 15NNO3) and δ15N of total nitrogen (δ15Nbulk) for trap material and surface/downcore sediments from the Arabian Sea (AS) were synthesized to explore the AS' past nitrogen dynamics. Based on 25 μmol kg−1 dissolved oxygen isopleth at a depth of 150 m, we classified all reported data into northern and southern groups. By using δ15Nbulk of the sediments, we obtained geographically distinctive bottom-depth effects for the northern and southern AS at different climate stages. After eliminating the bias caused by bottom depth, the modern-day sedimentary δ15Nbulk values largely reflect the δ15NNO3 supply from the bottom of the euphotic zone. Additionally to the data set, nitrogen and carbon contents vs. their isotopic compositions of a sediment core (SK177/11) collected from the most southeastern part of the AS were measured for comparison. We found a one-step increase in δ15Nbulk starting at the deglaciation with a corresponding decrease in δ13CTOC similar to reports elsewhere revealing a global coherence. By synthesizing and reanalyzing all reported down core δ15Nbulk, we derived bottom-depth correction factors at different climate stages, respectively, for the northern and southern AS. The diffusive sedimentary δ15Nbulk values in compiled cores became confined after bias correction revealing a more consistent pattern except recent 6 ka. Such high similarity to the global temporal pattern indicates that the nitrogen cycle in the entire AS had responded to open-ocean changes until 6 ka BP. Since 6 ka BP, further enhanced denitrification (i.e., increase in δ15Nbulk) in the northern AS had occurred and was likely driven by monsoon, while, in the southern AS, we observed a synchronous reduction in δ15Nbulk, implying that nitrogen fixation was promoted correspondingly as the intensification of local denitrification at the northern AS basin.

Details

Language :
English
ISSN :
17264189 and 17264170
Volume :
12
Issue :
1
Database :
OpenAIRE
Journal :
Biogeosciences
Accession number :
edsair.doi.dedup.....36780bde2f62745c7de9e01e2d9f6ba2