Spatiotemporal variations of nitrogen isotopic records in the Arabian Sea
Available reports of dissolved oxygen, δ<sup>15</sup>N of nitrate (δ <sup>15</sup>N<sub>NO<sub>3</sub></sub>) and δ<sup>15</sup>N of total nitrogen (δ<sup>15</sup>N<sub>bulk</sub>) for trap material and sur...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-01-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/12/1/2015/bg-12-1-2015.pdf |
| Summary: | Available reports of dissolved oxygen, δ<sup>15</sup>N of nitrate (δ
<sup>15</sup>N<sub>NO<sub>3</sub></sub>) and δ<sup>15</sup>N of total nitrogen (δ<sup>15</sup>N<sub>bulk</sub>) 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<sup>−1</sup> dissolved oxygen isopleth at
a depth of 150 m, we classified all reported data into northern and southern
groups. By using δ<sup>15</sup>N<sub>bulk</sub> 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 δ<sup>15</sup>N<sub>bulk</sub> values
largely reflect the δ<sup>15</sup>N<sub>NO<sub>3</sub></sub> 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 δ<sup>15</sup>N<sub>bulk</sub>
starting at the deglaciation with a corresponding decrease in
δ<sup>13</sup>C<sub>TOC</sub> similar to reports elsewhere revealing a global
coherence. By synthesizing and reanalyzing all reported down core
δ<sup>15</sup>N<sub>bulk</sub>, we derived bottom-depth correction factors at
different climate stages, respectively, for the northern and southern AS. The
diffusive sedimentary δ<sup>15</sup>N<sub>bulk</sub> 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 δ<sup>15</sup>N<sub>bulk</sub>) in the
northern AS had occurred and was likely driven by monsoon, while, in the
southern AS, we observed a synchronous reduction in
δ<sup>15</sup>N<sub>bulk</sub>, implying that nitrogen fixation was promoted correspondingly as the
intensification of local denitrification at the northern AS basin. |
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| ISSN: | 1726-4170 1726-4189 |