Dissolved greenhouse gases (nitrous oxide and methane) associated with the naturally iron-fertilized Kerguelen region (KEOPS 2 cruise) in the Southern Ocean
The concentrations of greenhouse gases (GHGs), such as nitrous oxide (N<sub>2</sub>O) and methane (CH<sub>4</sub>), were measured in the Kerguelen Plateau region (KPR). The KPR is affected by an annual microalgal bloom caused by natural iron fertilization, and this may stimul...
Main Authors: | , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2015-03-01
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Series: | Biogeosciences |
Online Access: | http://www.biogeosciences.net/12/1925/2015/bg-12-1925-2015.pdf |
Summary: | The concentrations of greenhouse gases (GHGs), such as nitrous oxide
(N<sub>2</sub>O) and methane (CH<sub>4</sub>), were measured in the Kerguelen Plateau
region (KPR). The KPR is affected by an annual microalgal bloom caused by
natural iron fertilization, and this may stimulate the microbes involved in
GHG cycling. This study was carried out during the KEOPS 2 cruise during the
austral spring of 2011. Oceanographic variables, including N<sub>2</sub>O and
CH<sub>4</sub>, were sampled (from the surface to 500 m depth) in two transects along
and across the KRP, the north–south (TNS) transect (46°–51° S, ~ 72° E) and the east–west (TEW)
transect (66°–75° E, ~ 48.3° S), both associated with the presence of a plateau, polar front (PF) and
other mesoscale features. The TEW presented N<sub>2</sub>O levels ranging from
equilibrium (105%) to slightly supersaturated (120%) with respect to
the atmosphere, whereas CH<sub>4</sub> levels fluctuated dramatically, being highly
supersaturated (120–970%) in areas close to the coastal waters of the
Kerguelen Islands and in the PF. The TNS showed a more homogenous
distribution for both gases, with N<sub>2</sub>O and CH<sub>4</sub> levels ranging from
88 to 171% and 45 to 666% saturation, respectively. Surface
CH<sub>4</sub> peaked at southeastern stations of the KPR (A3 stations), where a
phytoplankton bloom was observed. Both gases responded significantly, but in
contrasting ways (CH<sub>4</sub> accumulation and N<sub>2</sub>O depletion), to the
patchy distribution of chlorophyll <i>a</i>. This seems to be associated to the
supply of iron from various sources. Air–sea fluxes for N<sub>2</sub>O (from −10.5
to 8.65, mean 1.25 ± 4.04 μmol m<sup>−2</sup> d<sup>−1</sup>) and for
CH<sub>4</sub> (from 0.32 to 38.1, mean 10.01 ± 9.97 μmol<sup>−2</sup> d<sup>−1</sup>) indicated that the KPR is both a sink and a source for N<sub>2</sub>O,
as well as a considerable and variable source of CH<sub>4</sub>. This appears to be
associated with biological factors, as well as the transport of water masses
enriched with Fe and CH<sub>4</sub> from the coastal area of the Kerguelen
Islands. These previously unreported results for the Southern Ocean suggest
an intense microbial CH<sub>4</sub> production in the study area. |
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ISSN: | 1726-4170 1726-4189 |