CT4: Biogeochemical feedbacks on the oceanic carbon sink

1. CarboOcean annual meeting 2009 Sølstrand, October 05-09 CT4: Biogeochemical feedbacks on the oceanic carbon sink Marion Gehlen Laboratoire des Sciences du Climat et de l’Environnement IPSL, CEA-CNRS-UVSQ, Gif-sur-Yvette, France L.G. Anderson, G.J. Bellerby, J. Bendtsen, L. Bopp, L. Chou, J.P. Gattuso, R.J. Geider, C. Heinze, T. Johansen, F. Joos, C. Klaas, J. La Roche, S. Martin, C.M. Moore, U. Riebesell, J. Segschneider, C. Völker, D. Wolf-Gladrow and others

2. Figures from Friedlingstein et al. (2006) Earth System INPUT OUTPUT Processes: physical biogeochemical

3. Physical-chemical feedbacks athigh latitudes

4. The Arctic • Study the impact of the change in sea ice cover on the CO2 sources and sinks in the northern North Atlantic and the Arctic Ocean • 2. Assessment of the evolution of the CO2 system in the Arctic Mediterranean and Nordic Seas over the period from 1990-2060 using a coupled physical-biogeochemical ocean model

8. Surface ocean pH reduction over this century will exhibit large regional variability Reductions in pH over this century will be greatest in the Arctic Larger Change in surface ocean pH in the 21st Century Large Bellerby, et al, 2005.

9. Wintertime climate scenarios for WArag at the Røst cold water coral system W Model scenario PIcntrl: If there had been no anthropogenic carbon release SRESA1B Coral depth range Model based on proxies for the CO2 system (specific alkalinity and scenarios of dpCO2) applied to hydrographic output from the Bergen Climate Model

10. The SouthernOcean Investigation of air-sea exchange of CO2 under changing CO2, temperature, wind stress, precipitation, and ice cover by integration and analysis of results of a fine resolution circulation model

11. weakening of the Southern Ocean CO2 sink: 0.08 PgC /yr per decade in response to increased winds south of 45° (positive SAM phase) attributed to (1) to an increase in outgassing of natural carbon which (2) overcompensates the increase in the uptake of anthropogenic CO2 mean SO sink 0.1 to 0.6 PgC/yr

12. Reaction of Southern Ocean nutrients and phytoplankton to interannual variability in atmospheric forcing Method: calculate anomalies by susbtracting mean seasonal cycle, regress anomalies onto normalized Southern Annular Mode (SAM) index Physics: higher than usual SAM: equatorward Ekman transports, upwelling -> cooling and slightly deeper mixed layer depth in band around Antarctica Is there a biological reaction to increased upwelling of nutrients?

13. Reaction of Southern Ocean nutrients and phytoplankton to interannual variability in atmospheric forcing Nutrients: increased upwelling increases silicate and dissolved iron; iron increase is 10-20% of background value Biology: no clear reaction over most of the Southern Ocean, except in subpolar Pacific: Other factors (cooling, deeper mixed layer) outweigh increase in growth rate through nutrients Völker, Hohn, Losch, Wolf-Gladrow, in preparation

14. Biogeochemical feedbacks The concept of ocean carbon pumps • definition: “An ocean carbon pump is defined as a process that depletes the ocean surface of SCO2 relative to deep-water SCO2” Volk and Hoffert (1985) • relevant quantity = export flux below winter mixed layer • paradigm: pre-industrial C pump was neutral to air-sea gas exchange • drivers of change: climate change (T-effect) atmospheric CO2 (acidification) boundary fluxes (external input of nutrients)

15. Climate change (Temperatureeffect) Temperature effect on remineralization talk on Tuesday afternoon by J. Bendsten « Temperature dependent remineralisation of organic matter and its influence on the oceanic CO2-uptake »

16. Changes in input of externalnutrients • Process understanding: • ballast hypothesis • CO2 – dust – N2 fixation • 2. Future projections: changes in dust input and CO2 • talk on Tuesday afternoon by J. Segschneider • «  Remote input of nutrients in a changing climate »

17. QUESTION Why are deep POC fluxes associated to mineral fluxes? Do deep-sea fluxes reflect: Processes occurring at the source? => Relation between POC flux and minerals in the upper layers of the water column Processes occurring at depth? => No relation between POC flux and minerals in the upper layers of the water column Christine Klaas & Dieter Wolf Gladrow

18. POC REMINERALIZATION MODEL APPLIED TO CARRYING COEFFICIENTS (ƒi) Mi: mass flux of mineral i µ: remineralization length POC (z) = S Mi ƒi(z) w: sinking speed ƒi: mineral-associated POC µ ∂ƒi ƒi - = ∂z w ƒMi: mineral-associated POC „protected, glued or non degradable“ µ ) ( z - w ƒi(z) = ƒMi+ ƒ0i * e ƒ0i: mineral-associated POC at depth z0

19. Exponential decay model: POC coefficient (ƒi)vs. depth Data-model comparison (mg C m-2 d-1) R2 = 0.69

20. CONCLUSIONS • POC and mineral fluxes are also tightly correlated in the upper water column suggesting an influence of minerals on export and particle properties in and below the mixed layer • Results also support the notion of an asymptotic value at depth (non labile OM or glue effect) ESTIMATED RAIN RATIO PIC/POC at 100m: ~ 120 Tmol Bsi y-1 Tréguer et al. [1995] <=> ~ 7.2 Pg SiO2 0.1 => Corg export at 100 m: 10 Pg y-1 ~1.1 Pg C y-1 in CaCO3 Lee [2001] <=> ~ 9.2 Pg CaCO3

21. AMT 17 Striking correlation between tracer of dust input (DAl), DFe and N2 fixation. Very low N2 fixation in S. Atl. where DFe <50pM and DIP > 200nM LaRoche et al.

22. The interconnection of C/N cycles N2 N2 Possible future forcings, expected direction and likely response of N2 fixation: Temperature ↑↑ CO2 ↑↑? Fe deposition ↑?↑ N deposition ↑↓? Stratification (light) ↑↑ O2 Fe↑ N↓ Light T, CO2 ↑ Phytoplankton Diazotrophs P Thermocline Exported organic matter N-loss OMZ N:P ~ 16 N:P<<16 Internal forcings/feedbacks: O2 ↓ → Denitrifcation ↑ → N2 fixation ↑ N2 fixation ↑ → Export ↑ → O2 ↓ ….

23. Changes in external nutrient fluxes Dust/Fe Future changes in dust deposition: Mahowald et al., 2006 predicts a scale decline in Fe deposition by 2100 Approach Run from 1860-2100 forced by observed and projected atmospheric CO2 Linearly interpolate dust fields from 1860, 2000 and 2100 PISCES OBM [Aumont and Bopp, 2006] => Examine response of NPP and FCO2 Tagliabue et al. Biogeosciences [2008]

24. Changes in external nutrient fluxes Dust/Fe Large response in areas directly impacted by reduced Fe deposition But a retroaction in ‘downstream’ regions Unused nutrients can fuel additional NPP and drive extra CO2 uptake

25. Changes in external nutrient fluxes Dust/Fe • Global impacts • Fe deposition 60% • NPP 3.3% • CEX 5% change in dominant phyto groups • FCO2 3.2% - 22.8 GtC decrease of ocean sink •  small positive feedback but:  uncertainty with respect to sign of future changes in dust deposition  impact on marine ecosystems : N-fixation Why this low sensitivity to Fe input? Previous models only had an atm source (no seds) and might therefore have an unrealistically high sensitivity Sediment Fe included in PISCES and now also hydrothermal Fe input

26. CO2effect on seawaterchemistry • Export stoichiometry • Carbonate production and ballast effect • First order quantification of feedback

27. Changes in ocean chemistry: Export stoichiometry Pelagic Ecosystem CO2Enrichment Study (PeECE) (Bergen May 10 – June 12, 2005) Photosynthetic C drawdown present CO2 C/N=6.7 2xCO2 C/N=7.9 3xCO2 C/N=8.9 “Assuming the observed response can be extrapolated to new production systems in the ocean, we calculate an excess CO2 sequestration potential by the biological carbon pump of 116 Pg C until 2100” Riebesell et al. (2007)

28. Changes in ocean chemistry: Export stoichiometry C:N = f(pCO2) projected (SRES A2) • Assumption: Mesocosm results can be extrapolated to global ocean … • UVic Earth system model (Schmittner et al., subm.) • 1.8 x 3.6 degree resolution, 19 levels • NPZD + diazotrophs ecosystem, (N,P,C,O2) • Simulations from 1765 to 2100 • forced by CO2 emissions (historical+SRES A2) observed C:N=const. C:N=f(pCO2) Oschlies et al. (2008)

29. Cumulative signal Int(del EP) Int(del C storage) Changes in ocean chemistry: Export stoichiometry Export production C:N=const. C:N=f(pCO2) Oceanic carbon storage Increase in export production: 104 GtC Direct impact on marine C uptake: + 34 GtC small negative feedback Oschlies et al. (2008)

30. But increasing Corg supply to a DOC limited system resulted in less NCP and less POC as bacteria outcompeted autotrophs for nutrients Increasing C:N in a high CO2 world? D16.17 “Report on results of an arctic mesocosm experiment focusing on the competition between phytoplankton and bacteria”

31. Changes in ocean chemistry: CaCO3 production MODEL APPLICATION forcing 1 x pCO2 to 4 x pCO2, 1% increase/year constant circulation simulations CAL01: calcification and dissolution dependent on Ωc CAL02: calcification independent, dissolution dependent of Ωc CAL03: calcification and dissolution constant at pre-industrial values Gehlen et al. (2007)

32. Difference between cumulative CO2 uptake:  CaCO3 PRODUCTION and DISSOLUTION CAL01 - CAL03: + 5.9 GtC  CaCO3 DISSOLUTION onlyCAL01 - CAL03: + 1.2 GtC small negative feedback Changes in ocean chemistry: CaCO3 production production - 27 % CAL01 - 16 % CAL02 + 19 % export - 29 % black: CAL01 red: CAL02 blue: CAL03 dashed: control Gehlen et al. (2007)

33. CONCLUSIONS • CarboOcean allowed to: • assess the contribution of Arctic Seas to the global marine C cycle • evaluate the Cant inventory of Arctic Seas • a forecast of changes in the CO2 system of Arctic seas in response to environmental changes (sea ice cover, river run-off, ocean acidification …) • assess the variability of the SO CO2 sink in response to climate change, as well as to forecast impacts on marine productivity • explore the potential for changes in the biological pump in response to ecosystem reorganization driven by external input of nutrients, as well as physical and chemical changes in the ocean: but a start only … • However : Feedbacks still poorly quantified, mostly negative (biogeochemical)

34. OUTLOOK • high latitudes identified as regions of rapid environmental change • what about the tropical ocean ? • continuous effort on process understanding • new targeted experiments ? • synthesis of existing data … • improved synergy between experimentalists & modellers • critical assessment of processes in terms of impact on air-sea fluxes • model development: interconnexion of biogeochemical cycles: N & C !! • what might be gained from a negative FB in terms of atm CO2, might be lost in terms of radiative effect of N2O • So far the feedbacks that were quantified are small … existence of thresholds??

35. The evolution of the marine CO2 system as seen by Clara BARATANGE early career scientist at LSCE – born in 2007 ‘unhappy’ coccolithophore pHtot ‘happy’ coccolithophore 1886 2100 Time