Reconstruction of paleo-ph in the sub-equatorial Pacific Ocean using boron isotopes in recifal corals Porites

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1 Reconstruction of paleo-ph in the sub-equatorial Pacific Ocean using boron isotopes in recifal corals Porites: preliminary results of a seasonal field calibration in the New Caledonia Lagoon Eric Douville (eric.douville@lsce.ipsl.fr) Collaborations: LSCE/IPSL - France : Martine Paterne & Anne Juillet-Leclerc IRD-Noumea - New Caledonia : Guy Cabioch & John Butscher, IPG-Paris - France : Pascale Louvat & Jérôme Gaillardet FP7 European project French project INSU LEFE/CYBER

2 Distribution of Boron species in seawater (%) B( OH ) Boron speciation ~ ph = 8.2 pk B K ( ) + + H O B fn T, S 3 2 B( OH ) 4 + H 0 10 pk B ph B(OH) 4 - B(OH) 3 δ B ( o /oo) Corals (25.9) B( OH) OH 10 α B( OH) 4 B( OH) 3+ B( ) 4 B(OH) 3 α ε = 1000 ln α SeaWater ( δ B SW ) (ph 8.2) ph B(OH) 4 - B(OH ) 4 Incorporation of borate ions during the calcification of biogenic carbonates (bio-mineralization of corals, foraminifera) ph SW δ B SW δ Bcarbonates= borates log 1 1 α δ B4 δ BSW ( α 1) = pk B Dickson (1990) α = (Kakihana et al., 1977) fn (T C, S) Empirical value!

3 Precise measurement of boron isotopes using MC-ICPMS Neptune Advantages of MC-ICPMS for boron isotope analysis of carbonates Rapid and simplified chemistry ; Direct introduction of solutions (50 µl /min) ; Reproducible and accurate analysis ; High analytical capacity: ~ 20 samples per day. δ B ( o /oo) NBS Seawater 35 Coral Sand 5 Standard NBS δ B ( o /oo) Samples Analytical sequence Results: * External reproducibility is about 0.2 o / oo at 2σ (100ppb boron solutions of NBS 951) ; * Seawater ph is calculated with an analytical uncertainty of +/ ph-unit (Porites sp.) ; * New developments: d-dihen direct injection: high reproducibility, low memory effects & high capacity of measurement Louvat et al. (2007 and submitted)

4 Seasonal field calibration of the technique «δ B coral - ph SW» is developed from seawater samples collected every month and modern corals from two sites in the New Caledonia lagoon off Noumea (inside: Laregnière and outside: Fausse passe de Uitoé) Porites ph outside the lagoon (Fp.Uitoé) 9.0 ph inside the lagoon (Laregnère) T C outside (Fp.Utioé) 8.9 T C inside (Laregnère) T C ph NBS May-05 June-05 July-05 August-05 September-05 October-05 November-05 December-05 January-06 February-06 March-06 April-06 May-06 June-06 July-06 August-06 September Seasonal variability of seawater temperature and ph measured since May 2005

5 Boron isotopes in New Caledonia seawater : δ B SW = 39.9 External reproducibility of +/- 0.2 (2σ, n=36) using MC-ICPMS Neptune ph SW δ B δ 40.7 SW carbonates = borates = pk B log 1 1 α δ B4 δ BSW ( α 40.5 B 1) Site outside the lagoon (Fp Uitoé) Site inside the lagoon (Laregnère) New Caledonia lagoon (average, n=36) North Atlantic Standard Seawater - NASS II (average, n=25) 40.3 δ BSW ( ) April-05 May-05 June-05 July-05 August-05 September-05 October-05 November-05 December-05 January-06 February-06 March-06 April-06 May-06 June-06 July-06 August-06 September-06 October-06

6 Boron isotopes in modern Porites sp. Seasonal sea surface ph calculated from δ B measurements in a from New Caledonia (work in progress) 26 Outsite Modern Porites «Fausse Passe d Uitoé» (New Caledonia) δ B ( ) Distance from top (mm)

7 Seasonal calibration in the New Caledonia Lagoon (in progress) phsws Temperature (T C) Spectrophotometric ph measurements Including AT + ΣCO December-04 August-04 April-04 July-05 March-05 December-03 March-07 November-06 July-06 July-07 March-06 November-05 November-08 July-08 March-08 November-07

8 Ocean acidification off Fiji Islands Industrial era ( ) From models and direct ph measurements, sea surface acidification off Fiji Islands is about 0.07 ph-unit

9 Abrupt ph change in the central Pacific at the end of the Younger Dryas Porites & Acropora sp. from Marquesas and Tahiti Douville et al. (revision for EPSL)

10 Conclusions Boron isotopes in tropical corals Porites sp. are a powerful proxy of paleo-ph ; Field calibration highlights its potential at seasonal timescale; Interest for past reconstruction of Ocean acidification due to industrial era ; Interest for past CO 2 exchange between the Ocean and Atmosphere since the Last Glacial Period and beyond. Future objective would be to continue work calibration in progress and extend paleo-ph reconstruction on different sites along the Equatorial Pacific Ocean in order better constrain oceanic circulation : - future (inter)national project, - We recently obtained a CEA funding for a PhD position and we looking for a candidate. Thank you