Alex Piotrowski and Jo Clegg University of Cambridge

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.

Glacial:

Holocene:

Curry and Oppo, 2005

Seawater 13C from GEOSECSand WOCE

Benthic foraminiferal 13C from various studies

Ravelo and Hillaire-Marcel, 2007

Integrating Nd and C isotopes

Benthic 13C → deep ocean circulation + carbon cycling + air-sea gas exchangeNd isotopes → deep ocean circulation + REE cycling

Can Nd isotopes help to deconvolve the different signals contributing to the benthic 13C record?

Charles et al., 1996, Piotrowski et al., 2005

Site of those records…

Lower Nd and Higher 13C

Higher Nd and Lower 13C

Site of those records…

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)

0

1000

2000

3000

4000

5000

6000

0.40 0.90 1.40

d13C (per mil)

dep

th (

m)

station 8

station9

station 1

station 2

station 10

station 3

station 4

station 11

station 6

station 7

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)

Cruise D357 Cape Basin

d13C with depth for all stations (except 5)

0

1000

2000

3000

4000

5000

6000

0.40 0.90 1.40

d13C (per mil)

dep

th (

m)

station 8

station9

station 1

station 2

station 10

station 3

station 4

station 11

station 6

station 7

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)

Cruise D357 Cape Basin

nearcoast

test

12

3ss

4

56ss7

8 (0.5)

9 (1.5)

10 (2.5)

12 (3.5)

11ss (4.5)

13 (0.75)

First leg

Second leg

The mechanism for removing the biological contribution from the 13C 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 ‰.

13Cas = 13C + (1.1 x PO4) -2.7 (1)

Lynch-Stieglitz and Fairbanks (1994) adopted the notation 13Cas

– ‘the air/sea exchange signature’:

13Cas = 13C - (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:

13Cas = 13C - (1.92 - 0.7 x PO4) (3)Mackensen et al (1993) taking the regression computed by Kroopnick (1985).

13Cas = 13C - (2.4 – 0.93 x PO4) (4)Charles et al 1993

But in more recent papers, Mackensen uses the Broecker and Maier-Reimer equation (1).

Here we use: 13Cas = 13C + (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)

0

1000

2000

3000

4000

5000

6000

-2 -1.5 -1 -0.5 0 0.5 1

depth

d1

3C

(a

ir-s

ea

)

station 8

station9

station 1

station 2

station 10

station 3

station 4

station 11

station 5

station 6

station 7

nearcoast

Southern Ocean surface has highest 13Cas, sub tropical gyre waters usually ~ -1 ‰

We then confirm the water mass identification using conservative propertiesSalinity and Potential temperature

Salinity(again leave out station 5 as so much data missing)

0

1000

2000

3000

4000

5000

6000

34 34.2 34.4 34.6 34.8 35 35.2 35.4 35.6 35.8

Salinity (psu)

Dept

h (m

)

Potential temperature (again leave out station 5 as so much data missing)

0

1000

2000

3000

4000

5000

6000

0.0000 5.0000 10.0000 15.0000 20.0000

Temp (deg C)

Dept

h (m

)

station 8

station 9

station 1

station 2

station 10

station 3

station 4

station 11

station 6

station 7

schneidl max

schneidl min

Max and Min for NADW: Pink lines Schmiedl at al 1997Grey 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

0

1000

2000

3000

4000

5000

6000

-2 -1.5 -1 -0.5 0 0.5 1

d13C (air-sea)

dep

th

AAIW

NADW

AABW

blue - east of MAR; red - west of MAR

0

1000

2000

3000

4000

5000

6000

0.00 0.50 1.00 1.50 2.00 2.50

d13C (per mil)d

ep

th (

m)

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; triangles - silica

0

1000

2000

3000

4000

5000

6000

0.00 0.50 1.00 1.50 2.00 2.50

d13C

de

pth

(k

m)

0 50 100 150 200Silica

dissolved silica13C

blue - east of MAR; red - west of MAR PO4 triangles

0

1000

2000

3000

4000

5000

6000

0.00 0.50 1.00 1.50 2.00 2.50

d13C

de

pth

(m

)

0 0.5 1 1.5 2 2.5 3

PO4

13CPO4

Blue East of mid-Atlantic ridge

Yellow D357 cruise

RedWest of mid-Atlantic ridge

0

1000

2000

3000

4000

5000

6000

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

d13C air sea (per mil)d

ep

th (

m)

aaiw

nadw

lcdw

weddell seadeep water?

0

1000

2000

3000

4000

5000

6000

-2.00 -1.00 0.00 1.00 2.00

d13C air-sea

de

pth

(m

)

WSDW from Provost et al, 1990 depth in Argentine Basin 4500-6000


aaiw

nadw

lcdw

0

1000

2000

3000

4000

5000

6000

34.0 34.5 35.0 35.5 36.0 36.5 37.0

salinity pss

dept

h

vertical lines potential temp values for cape basinaaiw max min green nadw violet lcdw pinkdepth ranges shown determined for D357

nadw cape basin

aaiw cape basin

lcdw cape basin

0

1000

2000

3000

4000

5000

6000

-5 0 5 10 15 20 25 30

potential temp

dept

h


Added Mackensen 1993 data as white squares.

Comparison with data of Lynch-Stieglitz et al 1995

-0.50

0.00

0.50

1.00

1.50

1.00 1.50 2.00 2.50 3.00

13Cas

PO4 (mol/kg)

WOCE A10 track25S to 25S

Data from JC068 stations 8-21

-1

4

9

14

19

24

0 0.5 1 1.5 2 2.5 3

PO4 (uM/kg)

Mackensen (2012)Bottom water values

lat/long water d13Cdepths only 1000-2000 (AAIW in Cape Basin)

-80

-70

-60

-50

-40

-30

-20

240 290 340 390

lat (recall 0=360)

lon

g (

S)

-ve

0-0.2

0.2-0.4

0-4-0.6

0.6-0.8

0.8-1

ours 0.4--0.6

ours 0.6--0.8

lat/long water d13C air-seadepths only 1000-2000 (AAIW in Cape Basin)

-80

-70

-60

-50

-40

-30

-20

240 290 340 390

lat (recall 0=360)

lon

g (

S)

-ve

0-0.2

0.2-0.4

0-4-0.6

ours 0.2-0.4

ours 0.4-0.6

d13C

d13Cas

Triangles: our valuesSquares: Mackensen 2012

AAIW

20W 30E70W120W

20W 30E70W120W

Our coretop valueson benthic foramsare 1.24 ‰ while overlying seawater is 0.73 ‰

South America

South America

SouthAfrica

SouthAfrica

d13C

lat/long water d13Cdepths only 2000-3500 (NADW in Cape Basin)

-80

-70

-60

-50

-40

-30

-20

240 260 280 300 320 340 360 380 400 420

lat (recall 0=360)

lon

g (

S)

-ve

0-0.2

0.2-0.4

0-4-0.6

0.6-0.8

0.8-1

0.2-0.4 ours

0.6-0.8 ours

0.8-1 ours

lat/long water d13Cair-seadepths only 2000-3500 (NADW in Cape Basin)

-80

-70

-60

-50

-40

-30

-20

240 260 280 300 320 340 360 380 400 420

lat (recall 0=360)

lon

g (

S)

0-0.2

0.2-0.4

0-4-0.6

0--0.2 ours

0.2-0.4 ours

NADW

d13Cas


20W 20E60W120W 40E 60E40W80W100W 0

20W 20E60W120W 40E 60E40W80W100W 0

South America

South America

SouthAfrica

SouthAfrica

Our coretop valueson benthic foramsare 0.85 ‰ while overlying seawater is 0.90 ‰

lat/long water d13Cdepths only below 3500 (LCDW in Cape Basin)

-80

-70

-60

-50

-40

-30

-20

240 260 280 300 320 340 360 380 400 420

lat (recall 0=360)

lon

g (

S)

-ve

0-0.2

0.2-0.4

0-4-0.6

0.6-0.8

0.8-1

ours 0.2--0.4

ours 0.4--0.6

ours 0.6--0.8

ours 0.8--1

lat/long water d13C air-seadepths only below 3500 (LCDW in Cape Basin)

-80

-70

-60

-50

-40

-30

-20

240 260 280 300 320 340 360 380 400 420

lat (recall 0=360)

lon

g (

S)

-ve

0-0.2

0.2-0.4

0-4-0.6

0.6-0.8

0.8-1

ours 0--0.2

ours 0.2--0.4

ours 0.4--0.6

ours 0.6--0.8

d13C

d13Cas

LCDW


20W 20E120W 40E40W80W100W 060W

20W 20E120W 40E 60E40W80W100W 060W

RC11-83 benthic d13Chas a “coretop” (~5ka)value of 0 ‰

Cape Basin bottomwater is clearly more positive

South America

South America

SouthAfrica

SouthAfrica

Conclusions

• Seawater 13C clearly shows water mass structure, both surface hydrography and deep water.

• Air-sea exchange component of strong 13C gradient in surface ocean near Agulhas likely has subtropical (Indian) source.

• The 13C offset at NADW/AABW boundary in west is shallower than in east

• Nutrient signal? May not appear in 13C air-sea exchange component. Need to check potential density.

• Coretop calibration to benthic foraminifera 13C remains questionable.

These are all Mackensen 2012 data for d13C of forams incoretops 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 d13Cwith forams superimposed

-80

-70

-60

-50

-40

-30

-20

240 260 280 300 320 340 360 380 400 420

lat (recall 0=360)

lon

g (

S)

-ve

0-0.2

0.2-0.4

0-4-0.6

0.6-0.8

0.8-1

forams -ve

forams 0-0.2

forams 0.2-0.4

forams 0.4-0.6

forams 0.6-0.8

forams 0.8-1

forams >1

ours 0.8-1

ours >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)

lat/long water d13Cwith forams superimposed

-80

-70

-60

-50

-40

-30

-20

240 260 280 300 320 340 360 380 400 420

lat (recall 0=360)

lon

g (

S)

-ve

0-0.2

0.2-0.4

0-4-0.6

0.6-0.8

0.8-1

forams -ve

forams 0-0.2

forams 0.2-0.4

forams 0.4-0.6

forams 0.6-0.8

forams 0.8-1

forams >1

ours 0.8-1

ours >1

our w ater 0.6-0.8

our w ater 0.8-1

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

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‰)

So we find d13Cair-seaAAIW -0.55 to 0.1‰NADW 0.21 to 0.42‰LCDW 0.19 to 0.77‰

AAIW 500-2000NADW 2000-3500LCDW below 3500

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 calculatedthe 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

surface (highest value we have)d13C vs PO4

0

0.5

1

1.5

2

2.5

0 0.2 0.4 0.6 0.8 1 1.2

PO4

d13

C

d13C vs DIC concentration for all stations grey shallow, blue deep

-2

-1.5

-1

-0.5

0

0.5

1

2000 2100 2200 2300 2400 2500

DIC

d13

C a

ir s

ea

station 8station 1 deepstation 1 shallowstation 9 deepstation 9 shallowstation 2 deepstation 2 shallowstation 10 deepstation 10 shallowstation 3 deepstation 3 shallowstation 4 deepstation 4 shallowstation 5 deepstation 5 shallowstation 7 deepstation 7 shallowstation 6 deepstation 6 shallow

As we have not got DIC for second cruise perhaps delete this slide?

NADWAAIW

LCDW

Green max min of AAIW; pink of NADW;blue of LCDW from Schmiedl at al 1997 for Cape Basin

)Figures below right: Provost et al 1990 for Argentine BasinLeft: Memery 2000 for southwest Atlantic

-2

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

33.000 34.000 35.000 36.000 37.000

salinity pss

po

ten

tial

tem

p

AAIWNADW

LCDW


Data from JC068 stations 8-21

-1

4

9

14

19

24

0 0.5 1 1.5 2 2.5 3

PO4 (uM/kg)

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

Alex Piotrowski and Jo Clegg University of Cambridge

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Transcript of Alex Piotrowski and Jo Clegg University of Cambridge