Alex Piotrowski and Jo Clegg University of Cambridge

1. 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.

2. Holocene: Seawater d13C from GEOSECS and WOCE Glacial: Benthic foraminiferal d13C from various studies Curry and Oppo, 2005

3. Ravelo and Hillaire-Marcel, 2007

4. Integrating Nd and C isotopes Can Nd isotopes help to deconvolve the different signals contributing to the benthic d13C record? Benthicd13C → deep ocean circulation + carbon cycling + air-sea gas exchange Nd isotopes → deep ocean circulation + REE cycling Charles et al., 1996, Piotrowski et al., 2005

5. Site of those records… Lower eNd and Higher d13C Higher eNd and Lower d13C

6. Site of those records…

7. Mackensen 2012

8. Mackensen 2012

9. 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

10. Cruise D357 Cape Basin 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)

11. near coast Cruise D357 Cape Basin 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)

12. 13 (0.75) 8 (0.5) 9 (1.5) test 10 (2.5) 1 2 12 (3.5) 3ss 11ss (4.5) 4 7 5 6ss First leg Second leg

13. 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) Mackensen et al (1993) taking the regression computed by Kroopnick (1985). d13Cas = d13C - (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: d13Cas = d13C + (1.1 x PO4) -2.7 (1) and PO4 in μmol/L rather than μmol/kg, to match most studies.

14. near coast Southern Ocean surface has highest d13Cas, sub tropical gyre waters usually ~ -1 ‰

15. We then confirm the water mass identification using conservative properties Salinity and Potential temperature 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

16. AAIW NADW AABW

17. Comparison of eastern and western South Atlantic profiles Red = west of mid-Atlantic RidgeBlue = east of mid-Atlantic Ridge

18. Comparison of eastern and western South Atlantic profiles Red = west of mid-Atlantic RidgeBlue = east of mid-Atlantic Ridge

19. d13C dissolved silica

20. PO4 d13C

21. Blue East of mid-Atlantic ridge Yellow D357 cruise Red West of mid-Atlantic ridge

22. Red = west of mid-Atlantic RidgeBlue = east of mid-Atlantic Ridge WSDW from Provost et al, 1990 depth in Argentine Basin 4500-6000

23. Red = west of mid-Atlantic RidgeBlue = east of mid-Atlantic Ridge

24. Comparison with data of Lynch-Stieglitz et al 1995 d13Cas PO4 (mmol/kg) Added Mackensen 1993 data as white squares.

25. WOCE A10 track 25S to 25S Data from JC068 stations 8-21

26. Mackensen (2012) Bottom water values

27. AAIW Our coretop values on benthic forams are 1.24 ‰ while overlying seawater is 0.73 ‰ South Africa d13C 120W 70W 20W 30E South America South Africa 120W 70W 20W 30E d13Cas South America Triangles: our values Squares: Mackensen 2012

28. NADW Our coretop values on benthic forams are 0.85 ‰ while overlying seawater is 0.90 ‰ South Africa 120W 100W 80W 60W 40W 20W 0 20E 40E 60E d13C South America d13Cas South Africa 120W 100W 80W 60W 40W 20W 0 20E 40E 60E South America Triangles: our values Squares: Mackensen 2012

29. LCDW RC11-83 benthic d13C has a “coretop” (~5ka) value of 0 ‰ Cape Basin bottomwater is clearly more positive South Africa d13C 120W 100W 80W 60W 40W 20W 0 20E 40E South America South Africa 120W 100W 80W 60W 40W 20W 0 20E 40E 60E South America d13Cas Triangles: our values Squares: Mackensen 2012

30. 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.

32. 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.

33. 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)

35. 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

37. So we find d13Cair-sea AAIW -0.55 to 0.1‰ NADW 0.21 to 0.42‰ LCDW 0.19 to 0.77‰ AAIW 500-2000 NADW 2000-3500 LCDW below 3500 Comparisons with other data: Working out d13Casusing 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‰)

38. 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

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

41. 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 Basin Left: Memery 2000 for southwest Atlantic AAIW LCDW AAIW NADW NADW LCDW

43. WOCE A11 track 30S to 45S WOCE A10 track 25S to 25S

44. 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