Sarah Wakelin , Jason Holt , John Huthnance , Momme Butenschön

Carbon exchange between the northwest European
continental shelf and the North Atlantic Ocean
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1
1
Sarah Wakelin , Jason Holt , John Huthnance ,
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2
2
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Momme Butenschön , Yuri Artioli , Jerry Blackford and Icarus Allen
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2
National Oceanography Centre, Liverpool, Plymouth Marine Laboratory
NCEO Science Conference 17-20 September 2012
1. Introduction
2. Light absorption
In surface waters, photosynthesis converts dissolved inorganic carbon
into organic carbon, reducing the partial pressure of CO2 and prompting
CO2 draw down from the atmosphere. Deeper water becomes relatively
rich in carbon through the sinking of detritus and the excess of respiration
over photosynthesis under lower light conditions. For the NW European
continental shelf to be an effective sink of atmospheric CO2, carbon
sequestered during each growing season must be removed from the
shelf before the water column becomes well mixed by autumn storms.
For this carbon to remain isolated from release back into the atmosphere,
it must be transported off the shelf in the
lowest part of the water column.
air-sea
sea surface
Modelling of non-biotic light absorption due to suspended sediment
and coloured dissolved organic matter is problematic due to
uncertainties in the sources and sinks of material and in the
chemical processes involved. As an alternative, we implement a
simple interpolation-based assimilation scheme using a mean
annual cycle of 8-day composite non-biotic absorption based on
1998-2007 SeaWiFS data (figures 2 and 3). A new model variable
for non-biotic absorption, adet, is initialised with SeaWiFS data,
transported by the model dynamics and, at each model timestep,
relaxed to the SeaWiFS data with a timescale of 7 days.
rivers
Here we use a coupled hydrodynamicecosystem model (POLCOMS-ERSEM)
to study the exchange of carbon between
the shelf and the North Atlantic (Wakelin
et al., 2012). The simulations use a mean
annual cycle of non-biotic Inherent
Optical Property from SeaWiFS to
constrain the light field, and hence
phytoplankton growth.
photosynthesis
advection mixing
thermocline
respiration
sinking
shelf sea
open ocean
The total diffuse light attenuation Kd is then defined as
Kd = adet + aphyt + aw, where
aphyt = phytoplankton absorption, calculated from ERSEM and
aw = constant pure water absorption.
Figure 2: Non-biotic absorption
from SeaWiFS; day 148 of the
1998-2007 mean annual cycle.
resuspension
benthic
respiration
sea bed
Figure 1: The carbon cycle.
Figure 3: The mean
annual cycle of
SeaWiFS non-biotic
absorption at selected
points (Figure 2).
3. Ocean/shelf carbon exchange
There is a net flux of carbon onto the shelf in the upper layer (above
180m) and a net flux off the shelf below 180m (figure 4). The cross-slope
flux is dominated by the dissolved inorganic carbon (DIC) component.
The main routes for the export of carbon from the shelf are by flow
through the Norwegian Trench
Top Below
Net
and the off-shelf circulation north
180m 180m
flux
and west of Scotland. The largest
-0.01 -0.00
-0.01
Phytoplankton
component in the export of
-0.03
Zooplankton+bacteria -0.01 -0.02
organic carbon from the shelf is
-0.03 -0.29
-0.32
Detritus
112.05 -117.53
-5.48
Inorganic carbon
the transport of detritus (table 1),
especially near the bed across
Table 1: Total fluxes of carbon onto the NW
the continental slope to the west
European Shelf (×10 mol C yr ).
of Scotland.
12
a)
b)
0.02 - P
0.89 - Z
0.00 - D
78.89 - I
w eg
Nor
0.01 - P
0.47 - Z
-0.00 - D
64.27 - I
ian
n
Tre
ch
0.02 - P
0.89 - Z
-0.01 - D
127.86 - I
0.00 - P
0.05 - Z
-0.00 - D
22.34 - I
top 180 m
0.02 - P
0.55 - Z
0.00 - D
23.52 - I
-1
-0.03 - P
0.00 - Z
0.06 - D
22.02 - I
4. Shelf-wide pelagic carbon budget
The region acts as a sink for atmospheric CO2 (figure 5). There is
little carbon burial: the loss of organic carbon through settling is in
near balance with gains from erosion of the sea bed and diffusion of
benthic dissolved inorganic carbon into the water column. Carbon
inputs from the atmosphere, the Baltic Sea and river outflows are
largely balanced by horizontal transport (advection) out of the
region. There is a small mean increase in the amount of DIC (the
trend, dCi/dt > 0) indicating possible acidification.
0.06 - P
0.00 - Z
0.04 - D
13.94 - I
-0.05 - P
0.00 - Z
0.18 - D
77.46 - I
-0.00 - P
0.00 - Z
0.01 - D
5.03 - I
below 180 m
-0.03 - P
-0.00 - Z
-0.00 - D
0.92 - I
12
-1
Figure 4: Carbon fluxes (×10 mol C yr ) across the 200m depth contour a) above and b)
below 180m, averaged over 1989 to 2004; for P - total phytoplankton, Z - total zooplankton +
bacteria, D - detritus and I - dissolved inorganic carbon (DIC).
Reference
Wakelin, S.L., Holt, J.T., Blackford, J.C., Allen, J.I., Butenschön, M., Artioli, Y., (2012). Modeling the carbon
fluxes of the northwest European continental shelf: validation and budgets. J. Geophys. Res. 117, C05020,
doi:10.1029/2011JC007402.
Figure 5: NW European shelf pelagic carbon budget for 1989 to 2004. Baltic
Sea outflows are included in the river data. Positive/negative values denote an
increase/decrease in the carbon pool due to each component; the lengths of the
blue bars are one standard deviation of the annual mean fluxes.
Summary
1. Carbon from river sources and air-sea CO2 flux is exported from
the NW European Continental Shelf.
2. The primary export pathway for carbon is flow through the
Norwegian Trench.
3. Near bed export is largest in the Norwegian Trench and to the
north and west of Scotland.
Contact: Sarah Wakelin, National Oceanography Centre, Joseph Proudman Building, 6 Brownlow Street,
Liverpool, L3 5DA, (slwa@noc.ac.uk)