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  13 C from GEOSECS and WOCE Benthic foraminiferal  13 C from various studies

Ravelo and Hillaire-Marcel, 2007

Integrating Nd and C isotopes Benthic  13 C → deep ocean circulation + carbon cycling + air-sea gas exchange Nd isotopes → deep ocean circulation + REE cycling Can Nd isotopes help to deconvolve the different signals contributing to the benthic  13 C record? Charles et al., 1996, Piotrowski et al., 2005

Site of those records… Lower  Nd and Higher  13 C Higher  Nd and Lower  13 C

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

41S 18E depths 11 to 4395 Kroopnick 1980 has d13C to 1.74‰ this is nearest to our station 7 which is (5 to 3531m) d13C ‰ (so our d13C lower than this of Kroopnick’s; but similar sorts of values) Cruise D357 Cape Basin

41S 18E depths 11 to 4395 Kroopnick 1980 has d13C to 1.74‰ this is nearest to our station 7 which is (5 to 3531m) d13C ‰ (so our d13C lower than this of Kroopnick’s; but similar sorts of values) Cruise D357 Cape Basin near coast

test 1 2 3ss 4 5 6ss 7 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  13 C is due to Broecker and Maier-Reimer They find the 1.1 slope with PO 4 and use the -2.7, an arbitrary constant, to bring deep water values from Pacific and Indian Ocean to 0 ‰.  13 C as =  13 C + (1.1 x PO 4 ) -2.7 (1) Lynch-Stieglitz and Fairbanks (1994) adopted the notation  13 C as – ‘the air/sea exchange signature’:  13 C as =  13 C - ( x PO 4 ) (2) Lynch-Stieglitz at al (1995) for Antarctic The numbers in this formula are not agreed. Other possibilities are:  13 C as =  13 C - ( x PO 4 ) (3) Mackensen et al (1993) taking the regression computed by Kroopnick (1985).  13 C as =  13 C - (2.4 – 0.93 x PO 4 ) (4) Charles et al 1993 But in more recent papers, Mackensen uses the Broecker and Maier-Reimer equation (1). Here we use:  13 C as =  13 C + (1.1 x PO 4 ) -2.7 (1) and PO 4 in μmol/L rather than μmol/kg, to match most studies.

near coast Southern Ocean surface has highest  13 C as, sub tropical gyre waters usually ~ -1 ‰

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

AAIW NADW AABW

Comparison of eastern and western South Atlantic profiles Red = west of mid-Atlantic Ridge Blue = east of mid-Atlantic Ridge

Comparison of eastern and western South Atlantic profiles Red = west of mid-Atlantic Ridge Blue = east of mid-Atlantic Ridge

dissolved silica  13 C

PO 4

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

WSDW from Provost et al, 1990 depth in Argentine Basin Red = west of mid-Atlantic Ridge Blue = east of mid-Atlantic Ridge

Added Mackensen 1993 data as white squares. Comparison with data of Lynch-Stieglitz et al 1995  13 C as PO 4 (  mol/kg)

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

Mackensen (2012) Bottom water values

d 13 C d 13 C as Triangles: our values Squares: Mackensen 2012 AAIW 20W30E70W120W 20W30E70W120W Our coretop values on benthic forams are 1.24 ‰ while overlying seawater is 0.73 ‰ South America South America South Africa South Africa

d13C NADW d 13 C as Triangles: our values Squares: Mackensen W20E60W120W40E60E40W80W100W0 20W20E60W120W40E60E40W80W100W0 South America South America South Africa South Africa Our coretop values on benthic forams are 0.85 ‰ while overlying seawater is 0.90 ‰

d 13 C d 13 C as LCDW Triangles: our values Squares: Mackensen W20E120W40E40W80W100W060W 20W20E120W40E60E40W80W100W060W RC11-83 benthic d13C has a “coretop” (~5ka) value of 0 ‰ Cape Basin bottomwater is clearly more positive South America South America South Africa South Africa

Conclusions Seawater  13 C clearly shows water mass structure, both surface hydrography and deep water. Air-sea exchange component of strong  13 C gradient in surface ocean near Agulhas likely has subtropical (Indian) source. The  13 C offset at NADW/AABW boundary in west is shallower than in east Nutrient signal? May not appear in  13 C air-sea exchange component. Need to check potential density. Coretop calibration to benthic foraminifera  13 C remains questionable.

These are all Mackensen 2012 data for d 13 C of forams in coretops vs d 13 C 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.

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

Comparisons with other data: Working out d 13 C as using PO 4 and d 13 C of Mackensen 1993 at 44S 10.3E AAIW 0.52 to 0.59‰ NADW to 0.053‰ LCDW 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 to 0.053‰) So we find d13Cair-sea AAIW to 0.1‰ NADW 0.21 to 0.42‰ LCDW 0.19 to 0.77‰ AAIW NADW LCDW 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 d13C as 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

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

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

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