Carbon isotopic composition of waters
in the South Atlantic, tracing water
masses and biological activity
Alex Piotrowski and Jo Clegg
University of Cambridge
Thanks to Alex Thomas, Malcolm Woodward, Mike Hall, & James Rolf.
Holocene:
Seawater d13C from GEOSECS
and WOCE
Glacial:
Benthic foraminiferal d13C from
various studies
Curry and Oppo, 2005
Ravelo and Hillaire-Marcel, 2007
Integrating Nd and C isotopes
Can Nd isotopes help to deconvolve the different signals
contributing to the benthic d13C record?
Benthic d13C → deep ocean circulation + carbon cycling + air-sea gas exchange
Nd isotopes → deep ocean circulation + REE cycling Charles et al., 1996, Piotrowski et al., 2005
Site of those records…
Lower
Higher
eNd and Higher d
13C
eNd and Lower d
13C
Site of those records…
Mackensen 2012
Mackensen 2012
Methods
• Samples collected early to minimize gas
exchange, glass containers overfilled to
prevent air bubbles.
• Sealed poisoned (with mercuric chloride)
seawater samples stored in refrigerator
• Subsamples run on Thermo MAT253 mass
spectrometer with Gas Bench
d13C with depth for all stations (except 5)
d13C (per mil)
0.40
0
0.90
1.40
1000
depth (m)
2000
3000
station 8
station9
station 1
station 2
station 10
station 3
station 4
station 11
station 6
station 7
Cruise D357 Cape Basin
4000
5000
6000
41S 18E depths 11 to 4395 Kroopnick 1980 has d13C -0.24 to 1.74‰
this is nearest to our station 7 which is (5 to 3531m) d13C -0.426-0.6‰
(so our d13C lower than this of Kroopnick’s; but similar sorts of values)
d13C with depth for all stations (except 5)
d13C (per mil)
0.40
0
near 0.90
coast
1.40
1000
depth (m)
2000
3000
station 8
station9
station 1
station 2
station 10
station 3
station 4
station 11
station 6
station 7
Cruise D357 Cape Basin
4000
5000
6000
41S 18E depths 11 to 4395 Kroopnick 1980 has d13C -0.24 to 1.74‰
this is nearest to our station 7 which is (5 to 3531m) d13C -0.426-0.6‰
(so our d13C lower than this of Kroopnick’s; but similar sorts of values)
13 (0.75)
8 (0.5)
9 (1.5)
test
10 (2.5)
1
2
12 (3.5)
11ss (4.5)
4
7
6ss
First leg
Second leg
5
3ss
The mechanism for removing the biological contribution from the d13C is due
to Broecker and Maier-Reimer 1992. They find the 1.1 slope with PO4 and
use the -2.7 , an arbitrary constant, to bring deep water values from Pacific
and Indian Ocean to 0 ‰.
d13Cas = d13C + (1.1 x PO4) -2.7
(1)
Lynch-Stieglitz and Fairbanks (1994) adopted the notation d13Cas
– ‘the air/sea exchange signature’:
d13Cas = d13C - (2.7 - 1.7 x PO4)
(2)
Lynch-Stieglitz at al (1995) for Antarctic
The numbers in this formula are not agreed. Other possibilities are:
d13Cas = d13C - (1.92 - 0.7 x PO4)
(3)
d13Cas = d13C - (2.4 – 0.93 x PO4)
(4)
Mackensen et al (1993) taking the regression computed by Kroopnick (1985).
But in more recent papers, Mackensen uses the Broecker and Maier-Reimer equation (1).
Charles et al 1993
Here we use: d13Cas = d13C + (1.1 x PO4) -2.7
(1)
and PO4 in μmol/L rather than μmol/kg, to match most studies.
d13C air-sea for all stations
(station 3 no line through as two important data points missing)
depth
near
-2 coast-1.5
-1
-0.5
0
0.5
1
0
1000
station 8
d13C (air-sea)
2000
station9
station 1
station 2
3000
station 10
station 3
station 4
4000
station 11
station 5
station 6
5000
station 7
6000
Southern Ocean surface has highest d13Cas, sub tropical gyre waters usually ~ -1 ‰
We then confirm the water mass identification using conservative properties
Salinity and Potential temperature
Potential temperature
(again leave out station 5 as so much data missing)
0
0
1000
1000
2000
2000
Depth (m)
Depth (m)
Salinity
(again leave out station 5 as so much data missing)
3000
3000
4000
4000
5000
5000
6000
34
34.2
34.4
34.6
34.8
35
Salinity (psu)
35.2
35.4
35.6
35.8
station 8
station 9
station 1
station 2
station 10
station 3
station 4
station 11
station 6
station 7
schneidl max
schneidl min
6000
0.0000
5.0000
10.0000
15.0000
20.0000
Temp (deg C)
Max and Min for NADW:
Pink lines Schmiedl at al 1997
Grey lines Kroopnick 1980a
Max and Min for NADW:
Dashed lines Schmiedl at al 1997
d13C air-sea for all stations
(station 3 no line through as two important data points missing)
horizontal lines: blue NADW, pink AAIW, green LCDW
d13C (air-sea)
-2
-1.5
-1
-0.5
0
1000
AAIW
depth
2000
3000
NADW
4000
AABW
5000
6000
0
0.5
1
Comparison of eastern and western South Atlantic profiles
Red = west of mid-Atlantic
Ridge
Blue = east of mid-Atlantic Ridge
blue - east of MAR; red - west of MAR
d13C (per mil)
0.00
0
1000
depth (m)
2000
3000
4000
5000
6000
0.50
1.00
1.50
2.00
2.50
Comparison of eastern and western South Atlantic profiles
Red = west of mid-Atlantic
Ridge
Blue = east of mid-Atlantic Ridge
blue - east of MAR; red - west of MAR
d13C (per mil)
0.00
0
1000
depth (m)
2000
3000
4000
5000
6000
0.50
1.00
1.50
2.00
2.50
blue - east of MAR; red - west of MAR; triangles - silica
d13C
0.00
0
0.50
1.00
1.50
2.00
2.50
1000
depth (km)
2000
3000
d13C
dissolved silica
4000
5000
6000
0
50
100
Silica
150
200
blue - east of MAR; red - west of MAR PO4 triangles
d13C
0.00
0.50
1.00
1.50
2.00
2.50
0
1000
depth (m)
2000
3000
PO4
d13C
4000
5000
6000
0
0.5
1
1.5
PO4
2
2.5
3
d13C air sea (per mil)
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
depth (m)
0
1000
Blue
East of mid-Atlantic ridge
2000
Yellow
D357 cruise
3000
Red
West of mid-Atlantic ridge
4000
5000
6000
Red = west of mid-Atlantic Ridge
Blue = east of mid-Atlantic Ridge
d13C air-sea
-2.00
-1.00
0.00
1.00
2.00
0
1000
aaiw
depth (m)
2000
3000
4000
5000
6000
nadw
lcdw
weddell sea
deep water?
WSDW from Provost et al, 1990
depth in Argentine Basin 4500-6000
vertical lines potential temp values for cape basin
aaiw max min green nadw violet lcdw pink
depth ranges shown determined for D357
salinity pss
34.0
0
1000
34.5
35.0
35.5
potential temp
36.0
36.5
37.0
-5
4000
10
15
20
25
1000
aaiw
aaiw cape basin
2000
nadw
lcdw
5000
depth
depth
5
0
2000
3000
0
nadw cape basin
3000
4000
5000
6000
lcdw cape basin
6000
Red = west of mid-Atlantic Ridge
Blue = east of mid-Atlantic Ridge
30
Comparison with data of Lynch-Stieglitz et al 1995
1.50
1.00
d13Cas
0.50
0.00
-0.50
1.00
1.50
2.00
2.50
3.00
PO4 (mmol/kg)
Added Mackensen 1993 data as white squares.
WOCE A10 track
25S to 25S
Data from JC068 stations 8-21
Mackensen (2012)
Bottom water values
AAIW
lat/long water d13C
depths only 1000-2000 (AAIW in Cape Basin)
South
30E
390 Africa
-20
120W
240
70W
290
20W
340
-30
long (S)
-40
d13C
-ve
South
America
0-0.2
0.2-0.4
Our coretop values
on benthic forams
are 1.24 ‰ while
overlying seawater
is 0.73 ‰
0-4-0.6
-50
0.6-0.8
0.8-1
-60
ours 0.4--0.6
-70
ours 0.6--0.8
-80
lat (recall 0=360)
lat/long water d13C air-sea
depths only 1000-2000 (AAIW in Cape Basin)
-20
120W
240
d13Cas
70W
290
20W
340
-30
long (S)
-40
-50
South
Africa
30E
390
-ve
South
America
0-0.2
0.2-0.4
0-4-0.6
ours 0.2-0.4
-60
ours 0.4-0.6
-70
Triangles: our values
Squares: Mackensen 2012
-80
lat (recall 0=360)
NADW
lat/long water d13C
depths only 2000-3500 (NADW in Cape Basin)
long (S)
-20
120W
240 100W
260
-30
-40
80W
280
60W
300
40W
320
20W
340
0
360
South
40E
400
Africa
20E
380
60E
420
Our coretop values
on benthic forams
are 0.85 ‰ while
d13C overlying seawater
is 0.90 ‰
-ve
0-0.2
South
America
0.2-0.4
0-4-0.6
-50
0.6-0.8
-60
0.8-1
0.2-0.4 ours
-70
0.6-0.8 ours
-80
0.8-1 ours
lat (recall 0=360)
lat/long water d13Cair-sea
depths only 2000-3500 (NADW in Cape Basin)
-20
120W
100W
240
260
-30
d13Cas
long (S)
-40
-50
80W
280
60W
300
40W
320
20W
340
South
America
0
360
20E
380
40E
60E
400
420
South
Africa
0-0.2
0.2-0.4
0-4-0.6
0--0.2 ours
-60
0.2-0.4 ours
-70
Triangles: our values
Squares: Mackensen 2012
-80
lat (recall 0=360)
LCDW
lat/long water d13C
depths only below 3500 (LCDW in Cape Basin)
-20
240
260
120W 100W
-30
long (S)
-40
-50
280
80W
300
60W
320
40W
340
20W
360
0
South
400
40E
Africa
380
20E
RC11-83 benthic d13C
has a “coretop” (~5ka)
13
d C value of 0 ‰
420
-ve
0-0.2
South
America
0.2-0.4
Cape Basin bottomwater is
clearly more positive
0-4-0.6
0.6-0.8
0.8-1
-60
ours 0.2--0.4
-70
ours 0.4--0.6
ours 0.6--0.8
-80
ours 0.8--1
lat (recall 0=360)
lat/long water d13C air-sea
depths only below 3500 (LCDW in Cape Basin)
-20
120W
100W
240
260
80W
280
60W
300
40W
320
20W
340
-30
-40
long (S)
d13Cas
-50
0
360
20E
380
South
40E
60E
400
420
Africa -ve
0-0.2
South
America
0.2-0.4
0-4-0.6
0.6-0.8
0.8-1
ours 0--0.2
-60
ours 0.2--0.4
ours 0.4--0.6
-70
ours 0.6--0.8
Triangles: our values
Squares: Mackensen 2012
-80
lat (recall 0=360)
Conclusions
• Seawater d13C clearly shows water mass structure, both
surface hydrography and deep water.
• Air-sea exchange component of strong d13C gradient in
surface ocean near Agulhas likely has subtropical
(Indian) source.
• The d13C offset at NADW/AABW boundary in west is
shallower than in east
• Nutrient signal? May not appear in d13C air-sea
exchange component. Need to check potential density.
• Coretop calibration to benthic foraminifera d13C remains
questionable.
-ve
lat/long water d13C
with forams superimposed
0-0.2
0.2-0.4
long (S)
-20
240
-30
0-4-0.6
260
280
300
320
340
360
380
400
420
0.6-0.8
0.8-1
forams -ve
-40
forams 0-0.2
-50
forams 0.2-0.4
-60
forams 0.4-0.6
forams 0.6-0.8
-70
forams 0.8-1
forams >1
-80
lat (recall 0=360)
ours 0.8-1
ours >1
These are all Mackensen 2012 data for d13C of forams in
coretops vs d13C in bottom water.
And now we can add our two more – the big stars!
They don’t seem to match much but then neither do Mackensens.
lat/long water d13C
with forams superimposed
-ve
0-0.2
0.2-0.4
-20
240
-30
0-4-0.6
260
280
300
320
340
360
380
400
0.6-0.8
420
0.8-1
forams -ve
forams 0-0.2
long (S)
-40
forams 0.2-0.4
-50
forams 0.4-0.6
forams 0.6-0.8
-60
forams 0.8-1
forams >1
-70
ours 0.8-1
ours >1
-80
lat (recall 0=360)
our w ater 0.6-0.8
our w ater 0.8-1
And the big circles are our bottom water samples just above the forams.
(Note very few of Mackensens data have water taken just above the forams)
Worksheets provided
•
•
•
•
•
•
‘jo's current look at results-xtra DIC’
‘jo's current look at results’
‘just before Oxford jc068 geotraces’
jc068 geotraces
jc068 geotraces TDD
and try for final
AAIW 500-2000
NADW 2000-3500
LCDW below 3500
So we find d13Cair-sea
AAIW -0.55 to 0.1‰
NADW 0.21 to 0.42‰
LCDW 0.19 to 0.77‰
Comparisons with other data:
Working out d13Cas using PO4 and d13C of Mackensen 1993 at 44S 10.3E
AAIW 0.52 to 0.59‰
NADW 0.003 to 0.053‰
LCDW -0.11 to 0.022‰
The discrepancy in values is disappointing as I worked them out. I have checked
the units and they seem to be the same – umol/L.
Lynch-Stieglitz and Fairbanks 1994 had d13Cair-sea for Atlantic all negative (their
values were from Cd not PO4) and I can’t see where they get their modern ocean
values from. We could perhaps say our values will be higher, being pulled up by
mixing with positive Antarctic. But surely not this much.
( Mackensen 2012 says NADW characterised by negative d13Cair-sea, despite
the values above 0.003 to 0.053‰)
surface (highest value we have)
d13C vs PO4
2.5
d13C
2
1.5
1
0.5
0
0
0.2
0.4
0.6
0.8
1
1.2
PO4
For surface waters our d13C and PO4 values are similar to Broecker and
Maier-Reimer S Atlantic; despite possibly having more anthropogenic effects that are
not accounted for. (We don’t have their very high phosphate level ones.)
This suggests our values of d13Cas are different to other work because I have calculated
the equation wrong. But I’ve checked several times!
Note the values of d13Cas are similar to those of mackensen 2012 –slides at end
If we decide to use this slide it needs redoing and to be put somewhere else as it
uses JC068 cruise data and up to now all we have discussed is D357
d13C vs DIC concentration for all stations
grey shallow, blue deep
1
station 8
station 1 deep
station 1 shallow
station 9 deep
station 9 shallow
station 2 deep
station 2 shallow
station 10 deep
station 10 shallow
station 3 deep
station 3 shallow
station 4 deep
station 4 shallow
station 5 deep
station 5 shallow
station 7 deep
station 7 shallow
station 6 deep
station 6 shallow
0.5
d13C air sea
0
-0.5
-1
-1.5
-2
2000
2100
2200
2300
2400
2500
DIC
As we have not got DIC for second cruise perhaps delete this slide?
Green max min of AAIW; pink of NADW;
blue of LCDW from Schmiedl at al 1997
for Cape Basin
28
26
24
22
20
potential temp
18
16
)
14
12
Figures below
right: Provost et al 1990 for Argentine Basin
Left: Memery 2000 for southwest Atlantic
10
8
6
4
AAIW
AAIW
2
0
-2
33.000
LCDW
NADW
NADW
LCDW
34.000
35.000
salinity pss
36.000
37.000
WOCE A11 track
30S to 45S
WOCE A10 track
25S to 25S
WOCE A10 track
25S to 25S
Data from JC068 stations 8-21
So the PO4 and potential temp data agree quite well with WOCE.
But we have problems that it does not lay on Lynch Steiglitz fig 6 – next slide