Transcript No Slide Title

Changes in POC Concentration and
13
d C
1Mark A. Altabet,
School for Marine Science and Technology,
U. Massachusetts Dartmouth,
706 S. Rodney French Blvd., New Bedford,
MA 02744-1221. United States
1
during Mesoscale Iron Fertilization in the Southern Ocean
David Timothy, Matt McIlvin, and Peng Feng
Introduction and Background
Although d13C of marine sediments has been used to
infer past changes in [CO2]aq the roles of physiological state
(manifested through growth rate), cell size and species
assemblage also have strong influences on d13C of
phytoplankton and sinking particulates. As part of the
Southern Ocean Fe Experiment (SOFeX), we tracked d13C of
particulate matter during Fe-induced blooms in high-nitrate,
low-chlorophyll (HNLC) and low-silicate waters north of the
polar front (52oS, 167oW), and in HNLC and high-silicate
waters south of the polar front (66oS, 172oW). A previous
study (AESOPS; Fig 1) found that d13C of POC increased as
pCO2 decreased along a latitudinal transect in the SOFeX
study area, and during another recent Fe-enrichment in the
Southern Ocean (EisenEx; Fig 2), a significant increase in
d13C of POC was observed. We therefore anticipated the
possibility that d13C of POC would increase during the
progression of the Fe-enduced blooms at the SOFeX sites.
Indeed, very low organic d13C (as low as -30‰) is commonly
observed in the polar Southern Ocean due to large
fractionation factors for DIC incorporation (e) by
phytoplankton. e was expected to decrease and organic d13C
increase in response to increasing phytoplankton growth rate,
cell size, and decreasing [CO2]aq. As an extension, we have
considered the use of isotopic signals to trace the fate (e.g
export vs. remineralization) of newly produced organic C
during SOFeX.
0
2
4
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8
10
12
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16
0
0
0
40
40
1
2
3
4
5
6
SOUTH PA TCH MIXED LA YER
7
120
Jan 24
120
OUT SOUTH PATCH
Jan 28
160
Jan 31
160
Jan 31
Feb 2
Feb 6
Feb 12
Feb 4
Feb 11
200
-29.5
-30.0
Fig. 2 Results from the EisenEx Fe fertilization experiment in
Fig. 3 Representative POC concentration profiles for SOFeX in and
the South Atlantic sector of the Southern Ocean. A ring feature in
the ACC was chosen to do the experiment and it took place in
austral spring. POC increased by less than 2x over a 3-week
observation period. d13C increase by at least 3‰ in response to a
combination of reduced [CO2]aq and/or increased phytoplankton
growth rates.
out stations for the southern patch in high silicate, polar water. POC
increased by almost 3-fold within a 40 to 50 m surface layer for the inpatch stations. In contrast, the out-patch station showed no clear
progressive change. These data were for samples collected by Niskin
bottles and filtered onto GF/F filters.
SOFeX
SOUTH PATCH MIXED LAYER A VERAGES
20
" out " > 5 4 um
" i n" > 5 4 um
" i n" 1 - 5 4 um
" out " 1 - 5 4 um
-26
in green
out blue
-27
d1 3 C
12
8
JGOFS AESOPS
-28
30
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-28.0
APF
40
45
50
55
CALENDER DAY
1.0
Sout h IN
Sout h OUT
0.5
-24
16
20
POC ( M)
Fig. 6 There is no clear relationship between POC concentration
and d13C throughout the SOFeX experiment.
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2) Though SOFeX had greater POC
accumulation, there was substantially less change in
d13C. While actual comparisons with CO2 system
parameters and phytoplankton growth rate await
availability of those data, it is clear that there is no
simple relationship between POC accumulation and
d13C.
-0.5
-1.0
d
-30
12
13
Bidigare
in green
out blue
C1 m - d C5 4 m
d1 3 C
d1 3 C
Niskin 1
Alt ab et
LVP
-29.5
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3) Nevertheless, the size-fractionated d13C data
is consistent with the expected role of large diatoms
during Fe fertilization.
0.0
-29.0
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1) For SOFeX, there was a ~3-fold increase in
POC in the southern patch. In contrast, EisenEx
saw less than a doubling, perhaps due to a
combination of deeper mixed layer and shorter
observation period.
Buesseler
SOFeX LVP: MIXED LAYER
Niskin
Alt ab et1
Coale 2
Niskin
Bidigare
LVP
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0
Preliminary Conclusions
0
-28
-29.0
Alt
abet1
Niskin
Bidigare
LVP
-30
d C
(‰)-26
blue out
200
Feb 16
-28.5
13
d1 3 C - POC
depth (m)
depth (m)
IN SOUTH PATCH
-18
-22
green in
80
80
4
Annual Avg. Trap
3/98 surface POM
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-28.0
-29
-20
8
-28.5
POC ( M)
pCO2 (uatm)
500
400
300
200
POC (M)
POC (M)
-1.5
-30.0
-32
80
70
60
50
20
40
S. Latitude
25
30
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40
CALENDER DAY
45
50
-2.0
-2.5
Fig. 1 Transect of near-surface pCO2 and
d13C
in POC across the
JGOFS AESOPS study region in late Austral Summer 1998. The
trap data are the flux-weighted annual average of moored
sediment collections. Large latitudinal gradients are observed
with lowest d13C and highest pCO2 values in the polar waters of
the northern Ross Sea. Surface POC d13C data show a general
correspondence with the trap data. The SOFeX experiment took
place in 2002 along this transect near 66°S
Fig. 4
In-patch (southern) time series for mixed layer average POC
concentration and d13C. Niskin 1 (Altabet lab) and Niskin 2 (courtesy
Kenneth Coale) are results for Niskin-collected water samples. LVP refers to
results from a submerged large-volume pump (courtesy Bob Bidigare). These
results generally support a large increase in patch POC compared to outstations. The three data sets generally agree except for high POC for the pump
data in the latter half of the experiment. Compared to EisenEx, there is
surprisingly little change in d13C. The variation in d13C observed is also not
monotonic, suggesting multiple influences.
30
35
40
45
50
55
CALENDER DAY
Fig. 5 Southern Patch time series for size-fractionated, pumpcollected particulate data (samples courtesy Ken Buesseler).
While the 1-54 µm fraction shows a clear temporal change in
d13C, these data fall within the range shown in Fig. 4. However,
the >54 µm fraction shows substantial isotopic enrichment in the
latter half of the experiment. This may reflect the influence of
large, relatively fast growing diatoms.
Acknowledgements: Rekha Singh provided technical
assistance. Kenneth Coale and Craig Hunter provided bottle
data. Ken Buesseler and Bob Bidigare provided in situ pump
samples and data. Funding is from the DOE Carbon
Sequestration Program and the NSF.
Contact: [email protected], [email protected] or
http://www.cmast.umassd.edu/