Decoupling of Iron and Phosphate in the Global Ocean

1. Decoupling of Iron and Phosphate in the Global Ocean Payal Parekh MIT/WHOI Joint Program April 18, 2003 In collaboration with Mick Follows, Ed Boyle and John Marshall

2. Thesis Aims • Develop a mechanistic model of iron cycling to: • reproduce observed [Fe] • predict rates of biogeochemical processes • understand controls on Fe in upwelled waters • gain insight on decoupling of Fe and PO4 Motivation: Understand iron’ s role in possible pCO2 drawdown.

3. High Nutrient, Low Chlorophyll Regions Latitude µM Longitude Conkright et al. (1994)

4. Chlorophyll Distribution SeaWiFS image

5. Could Lack of Iron be the Culprit? • Sources of Iron to surface waters: • Upwelling • Aeolian Dust Flux mg Fe m -2 yr -1 Gao et al. (2001)

6. Link between dust flux and pCO2? Atmospheric CO2 (ppm) Dust Flux (mg m -2 yr -1) Age (kyr)

7. Iron Addition Experiment SOIREE – S. Ocean

8. Why are Fe and PO4 decoupled? R=Fe:PO4 biological uptake ratio

9. DUST surface dissolved Fe (< 0.4 mm) biological loop lateral transport and mixing Fe’ + L’  FeL scavenging & desorption mixed layer bottom biogenic export upwelling and vertical mixing refractory dust Fe’ + L’  FeL scavenging & desorption remineralization lateral transport mixing sediment-water interface sedimentary deposition Iron Biogeochemistry Figure from Bergquist

10. Iron Observations 1000m Surface Compiled from the literature and Boyle (unpub).

11. Is it possible to reproduce and understand controls on observed deep water Fe distributions? • Perform sensitivity tests to constrain rates of biogeochemical processes using an idealized multi box model • Test iron parameterization in the MITGCM, resolve intrabasinal differences and vertical profile of iron

12. Box Model Simulations • Volume transport chosen to fit Radiocarbon, Broecker/Peng (86,87) • Tracers: PO4, DOP, Fe • Biological Uptake: µ*[Fe/(Fe+Ks)] • Michaelis-Menten kinetics – Fe, light limited

13. Iron Parameterization • K= binding strength of ligand • Log(K) ranges 9.5-13.2 • FeL=K[Fe’][L’] • LT=L’+FeL • LT = 1 nM, based on Rue/Bruland (1995) and others

14. Results: Complexation Case 0.6 nM Atlantic 0.3 nM S. Ocean 0.4 nM Indo-Pac. log K Ksc yr-1

15. Summary: Box Model Results • Iron parameterization successfully reproduces deep water Fe gradients • Sensitivity study constrains scavenging rate over wide range of K (ligand binding strengths).

16. Explorations with an Ocean General Circulation Model • Same parameterization as in box model • Forced seasonally with Gao et al. (2001) dust flux estimate

17. Modeled Fe Surface 935m • Fe exceeds solubility (Liu and Millero, 2002) in surface waters • Successfully reproduces observed Fe gradients at depth

18. Observations: PO4 Surface 1000m Conkright et al. (1994)

19. Modeled PO4 Surface • Fe limitation term [µ*(Fe/Fe+Ks)] successfully reproduces HNLC region • Previously, models either restored to surface or adjusted export time scale • Model has difficulty in subtropical Pacific surface waters (nutrient trapping) • Deep water PO4 agrees well with observations 1000m

20. Fe*: Measure of decoupling between Fe and PO4 • Fe* = Fe-RPO4 • Interpretation of Fe*: • Positive Fe*: Adequate Fe to support biological utilization of PO4 • Negative Fe*: Deficit in Fe • Fe:PO4 ratio (R) must be specified

21. Positive at surface in northern hemisphere As NADW travels southward, scavenging depletes Fe* AAIW/AABW both negative due to low aeolian dust flux in S. Ocean Fe*: Zonally averaged section in the Atlantic Calculated residence time for GCM: 285 yr

22. Fe*: Indo-Pacific • Positive Fe* at surface between 25-40° N • Decoupling of Fe and PO4 greatest in deep, old waters of N. Pacific.

23. North Pacific vs. North Atlantic Aeolian Fe flux (mg Fe m-2 yr-1) • Both basins have high dust flux and are regions of upwelling • Why is the N. Pacific Fe limited?

24. Fe* below the mixed layer Fe* at 290m • Aeolian dust flux is not enough to compensate for the low Fe, high PO4 waters of the old, upwelled waters in the N. Pacific

25. GCM Simulations: Summary • Mechanistic parameterization of Fe that includes complexation and scavenging reproduces deepwater Fe gradients • The model exceeds solubility (Liu and Millero, 2002) at the surface in high dust flux regions • Fe* is a useful tracer for understanding how/why Fe and PO4 become decoupled in HNLC regions

26. Utility of mechanistic Fe model • A mechanistic Fe model allows for exploration of important climate questions, such as Does increased dust flux lead to increased efficiency of the biological pump and subsequent drawdown in pCO2?

27. Surface PO4 Sensitivity to Increased Dust Flux: Preliminary Results • ‘Paleo’ dust estimate from Mahowald et al. (1999) • Dust flux greater nine times globally • 63 fold increase in the Southern Ocean

28. Southern Ocean Surface PO4 Response • Drawdown of ~ 0.5 µM • Due to low surface PO4,results should be viewed as a lower bound estimate • Watson et al. (2000) estimated 0.6 µM drawdown of PO4 - ~ 30 ppm drawdown PO4 (µM) ΔPO4

29. Thesis Conclusions • Developed a mechanistic model of Iron cycling for the global ocean: • Sensitivity study using box model suggests iron scavenging rate ranges between 0.1-2 yr-1 log(K) between 10-13 • Complexation and scavenging description successfully reproduces deep water Fe gradients in the GCM • Fe* is a useful tracer for understanding the decoupling of Fe and PO4 • Dust flux is not enough to compensate the low Fe* of upwelled waters in HNLC regions • A nine fold increase in dust flux results in ~ 0.5 µM drawdown of PO4

30. Research Recommendations • A pressing need for Fe measurements, especially in the deep waters • Identification of source(s), sink(s) and chemical characterization of Fe-binding ligands • Experiments aimed at studying processes affecting Fe in surface waters

31. Works in Progress • Improving surface iron distribution in model– in collaboration with M. Follows, E. Boyle • Coupling iron model with a more sophisticated ecosystem model – in collaboration with S. Dutkiewicz, M. Follows • Exploring more closely iron cycling in the S. Ocean – in collaboration with T. Ito, J. Marshall

32. Acknowledgements • J. Marshall for inviting me to join his group • M. Follows for being a patient and encouraging advisor • E. Boyle for sharing his insights on iron chemistry and the intricacies of making iron measurements • J. Moffett for enlightening me on iron’s role in ocean ecology • M. Bacon for pointing me to the parallels between iron and thorium • T. Voelker for acting like a committee member and being a supportive female faculty member of the Joint Program

33. Acknowledgements • S. Dutkiewicz and T. Ito for many stimulating discussions on ocean biogeochemistry • J. Campin for teaching me how to program (and debug) in FORTRAN • C. Hill for introducing me to the joys of running on parallel computers • M. Losch and D. Ferreira for sharing with me their encyclopedic knowledge of MATLAB • The folks on the upper floors of the Green Building for their friendship and assistance at various times for various tasks

34. Acknowledgements • Thesis Crisis Intervention Committee: Bridget Bergquist, Mick Follows, Chris Hill, Taka Ito, Alex Rae, Steph Dutkiewicz , Ariane Verdy, Fanny Monteiro, Oren Weinrib • Friends that fed and housed me during the dark days of thesis writing: Anke, Jen and Pedro, Fernanda, Nico, Anne, Vijay family, Patrick, Ariane, Fanny • To the many friends (old and new) I have made at MIT, WHOI and around the city for reminding me that there is a world beyond the Green building • The MIT/WHOI Joint Program in Oceanography • To my family for their love and support

35. Funding Acknowledgement • NASA Global Change Fellowship

36. Scavenging/Desorption Case • Thorium, a metal with similar properties to Fe, is known to desorb from particles (Bacon and Anderson, 1982). • Perhaps Fe is also involved in this process.

37. Scav/Desorption Results Atlantic S. Ocean kb yr-1) Indo-Pac Scav. Rate (yr-1)

38. Observations: Iron Surface 1000m Below 2500m Compiled from literature, unpub. results from E. Boyle

39. GCM Simulations • In the context of the more sophisticated physical description of the MITGCM, net scavenging and desorption case are not adequate to describe the iron cycle • Complexation case agrees well with measurements below the surface • explains role of transport and scavenging in decoupling Fe and PO4

40. Iron Observations: Surface Compiled from literature and Boyle (unpub.)

41. Iron Observations: At depth 1000m Z> 2500m Compiled from literature, Boyle (unpub.)

42. Net Scavenging Case • Simplest Description • Net scavenging term (K_sc) is the sum of the various geochemical processes Fe is involved in • Biological export is Fe, light limited

43. Results: Scavenging Case • For 0.004 < knetsc < 0.006, observed gradients are produced • Description does not resolve various biogeochemical processes

44. Iron Biogeochemistry Figure courtesy of B. Bergquist

45. Comparison of observed and modeled profiles Pacific: 3°S, 140°W Atlantic:10°N, 45°W Boyle et al. (unpub.) Johnson et al. (1997)