Contrasting composition of terrigenous organic matter in the dissolved, particulate and sedimentary organic carbon pools on the outer East Siberian Arctic Shelf
Fluvial discharge and coastal erosion of the permafrost-dominated East Siberian Arctic delivers large quantities of terrigenous organic carbon (Terr-OC) to marine waters. The composition and fate of the remobilized Terr-OC needs to be better constrained as it impacts the potential for a climate–...
Main Authors: | , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2016-11-01
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Series: | Biogeosciences |
Online Access: | https://www.biogeosciences.net/13/6121/2016/bg-13-6121-2016.pdf |
Summary: | Fluvial discharge and coastal erosion of the permafrost-dominated East
Siberian Arctic delivers large quantities of terrigenous organic carbon
(Terr-OC) to marine waters. The composition and fate of the remobilized
Terr-OC needs to be better constrained as it impacts the potential for a
climate–carbon feedback. In the present study, the bulk isotope (<i>δ</i><sup>13</sup>C and Δ<sup>14</sup>C) and macromolecular (lignin-derived phenols)
composition of the cross-shelf exported organic carbon (OC) in different
marine pools is evaluated. For this purpose, as part of the SWERUS-C3
expedition (July–September 2014), sediment organic carbon (SOC) as well as
water column (from surface and near-bottom seawater) dissolved organic
carbon (DOC) and particulate organic carbon (POC) samples were collected
along the outer shelves of the Kara Sea, Laptev Sea and East Siberian Sea.
The results show that the Lena River and the DOC may have a preferential
role in the transport of Terr-OC to the outer shelf. DOC concentrations
(740–3600 µg L<sup>−1</sup>) were 1 order of magnitude higher than POC (20–360 µg L<sup>−1</sup>),
with higher concentrations towards the Lena River plume. The
<i>δ</i><sup>13</sup>C signatures in the three carbon pools varied from
−23.9 ± 1.9 ‰ in the SOC, −26.1 ± 1.2 ‰ in the DOC and −27.1 ± 1.9 ‰
in the POC. The Δ<sup>14</sup>C values ranged
between −395 ± 83 (SOC), −226 ± 92 (DOC) and −113 ± 122 ‰
(POC). These stable and radiocarbon isotopes were also different between the
Laptev Sea and the East Siberian Sea. Both DOC and POC showed a depleted and
younger trend off the Lena River plume. Further, the Pacific inflow and the
sea-ice coverage, which works as a barrier preventing the input of “young”
DOC and POC, seem to have a strong influence in these carbon pools,
presenting older and more enriched <i>δ</i><sup>13</sup>C signatures under the
sea-ice extent. Lignin phenols exhibited higher OC-normalized concentrations
in the SOC (0.10–2.34 mg g<sup>−1</sup> OC) and DOC (0.08–2.40 mg g<sup>−1</sup> OC) than in the POC
(0.03–1.14 mg g<sup>−1</sup> OC). The good relationship between lignin and Δ<sup>14</sup>C signatures in the DOC suggests that a significant fraction of the
outer-shelf DOC comes from “young” Terr-OC. By contrast, the slightly
negative correlation between lignin phenols and Δ<sup>14</sup>C signatures
in POC, with higher lignin concentrations in older POC from near-bottom
waters, may reflect the off-shelf transport of OC from remobilized
permafrost in the nepheloid layer. Syringyl ∕ vanillyl and cinnamyl ∕ vannillyl
phenol ratios presented distinct clustering between DOC, POC and SOC,
implying that those pools may be carrying different Terr-OC of partially
different origin. Moreover, 3,5-dihydroxybenzoic acid to vanillyl phenol
ratios and <i>p</i>-coumaric acid to ferulic acid ratios, used as a diagenetic
indicators, enhanced in POC and SOC, suggesting more degradation within
these pools. Overall, the key contrast between enhanced lignin yields both
in the youngest DOC and the oldest POC samples reflects a significant
decoupling of terrestrial OC sources and pathways. |
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ISSN: | 1726-4170 1726-4189 |