Unresolved Issues

1. Unresolved Issues • Cuffy and Vimeux (2001) show that • 90% of DT can be explained by variations in CO2 and CH4 • Reasonably firm grasp on causes of CH4 variations (Monsoon forcing) • What produced CO2 variations? • Variations are large – 30% • Show rapid changes – drop of 90 ppm from interglacial to glacial

2. Physical Oceanographic Changes in CO2 90% of the CO2 decrease unexplained by physical processes • During glaciations physical properties change • Temperature and salinity • Affect solubility of CO2(aq) and thus pCO2

3. Exchange of Carbon • Carbon in rock reservoir exchanges slowly • Cannot account for 90 ppm change in 103 y • Rapid exchange of carbon must involve near-surface reservoirs

4. Changes in Soil Carbon • Expansion of ice sheets • Covered or displaced forests • Coniferous and deciduous trees • Displaced forests replaced by steppes and grasslands • Have lower carbon biomass • Pollen records in lakes • Indicate glacial times were dryer and less vegetated than interglacial • Estimates of total vegetation reduced by 25% (15-30%) during glacial maxima • CO2 removed from atmosphere did not go into vegetation on land!

5. Where is the Missing Carbon? • Carbon from reduced CO2 during glacial times • Not explained by physical properties of surface ocean • Did not go into biomass on land • Must have gone into oceans • Surface ocean not likely • Exchanges carbon with atmosphere too rapidly • Most areas of ocean within 30 ppm of atmosphere • Glacial surface ocean must also have been lower, like atmosphere • Deep ocean only likely remaining reservoir

6. Interglacial-Glacial Change in Carbon Ice core data indicate atmospheric CO2 30% lower Terrestrial vegetation 25% lower Mass balance indicates 2.7% increase in deep ocean Ocean mixed layer in equilibrium with atmosphere so it too was lower by 30% • At LGM, reduction of carbon occurred in atmosphere, vegetation and soils on land and in surface ocean • This carbon (1010 gigatons) must have been moved to deep ocean

7. Tracking Carbon • d13C values can be used to determine how carbon moved from surface reservoirs to deep ocean • Major carbon reservoirs have different amounts of organic and inorganic carbon • Each with characteristic d13C values

8. d13C Changes During Photosynthesis • Large KIE during carbon fixation by plants • Magnitude depends on C-fixation pathway

9. d13C Tracks Carbon Transfer • Isotope mass balance quantifies transfer of terrestrial Corg to deep ocean • Cinorg*d13Cinorg + Corg*d13Corg = Ctot*d13C • (38,000*0‰) + (530*-25‰) = (38530*x) • Solving for x = -0.34‰ • Just this transfer predicts a shift in deep ocean DIC of –0.34‰ • Isotopic change recorded in benthic foraminifera

10. Change in Benthic d13C • Oscillations in benthic d13C correspond to benthic d18O • 100,000 and 41,000 year cycles • Confirm transfer of organic carbon to deep ocean during ice sheet expansion • d13C shifts greater than ~0.4‰ • Suggesting additional factors have affected oceanic d13C values

11. Increase the Ocean Carbon Pump • If biological productivity and Corg export were higher in surface waters during glacial intervals • Atmospheric CO2 could be fixed in shallow ocean by phytoplankton • Sinking dead organic matter transfers that carbon to the deep ocean • Biological productivity and export can only increase if essential nutrients increase in surface ocean • Increases in wind-driven upwelling of deep, nutrient-rich water • Increases in the nutrient concentration of deep water that is already upwelling

12. The Iron Hypothesis • In the 1980s, the late John Martin suggested that • Carbon uptake during plankton growth in many regions of the world's surface ocean • Was limited not by light or the nutrients N and P • But by the lack of the trace metal iron • Iron is typically added to the open ocean as a component of dust particles

13. The Iron Hypothesis • Correlations between dust and atmospheric carbon dioxide levels in ancient ice core records • Suggest that the ocean would respond to natural changes in iron inputs • Higher glacial winds would increase the amount of windblown dust containing Fe to oceans • Stimulate phytoplankton growth • Increasing carbon uptake and decrease atmospheric CO2 • Alter the greenhouse gas balance and climate of the earth

14. Evidence for Iron hypothesis • Some areas of the ocean contain high amounts of essential nutrients (N, P) • Yet low amounts of chlorophyll (HNLC) • Phytoplankton require Fe in small amounts for growth • “Bottle experiments” demonstrate conclusively that addition of Fe stimulates phytoplankton growth • CO2 uptake If Iron Hypothesis increased biological pump, iron addition must increase production and export

15. Open-Ocean Iron Enrichment • "Give me half a tanker full of iron and I'll give you an ice age“ (John Martin) • Results of “fertilizing” large patches of the ocean with iron • Showed strong biological response and chemical draw-down of CO2 in the water column • But what was the fate of this carbon? • Plant uptake of carbon in the ocean is generally followed by zooplankton bloom • Grazers respond to the increased food supply • Producing a blizzard of fecal pellets that descend through the water column • Exporting the carbon to the deep sea

16. Quantifying Carbon Export • Thorium is a naturally occurring element that by its chemical nature is "sticky" • Due to its natural radioactive properties, relatively easy to measure. • Analysis of a series of samples collected during the 1995 FeEx II • Indicated that as iron was added • Plant biomass increased • Total thorium levels decreased indicating carbon export

17. Quantifying Carbon Export • After some delay • Particulate organic carbon export increased in the equatorial Pacific • Relationship between uptake and export not 1:1 • The iron-stimulated biological community showed • Very high ratios of export relative to carbon uptake • Thus the efficiency of the biological pump had increased dramatically

18. Quantifying Carbon Export • Results of similar iron fertilization of Southern Ocean • Slower biological response • Total thorium levels never responded • The biological pump was not activated • Speculate that difference • Slowness of the biological community's response to stimulation in colder waters • Biological pump may have turned on later

19. Persistence of Patch • Sea surface color satellite image taken 32 days after the addition of Fe • Colored ring indicates area of high chlorophyll • Believed to be a result of the increased Fe

20. Iron Fertilization is Hot Topic • Iron fertilization of the ocean captured attention of entrepreneurs and venture capitalists • See potential for enhancing fisheries and gaining “C credits” through large-scale ocean manipulations

21. Marshall Islands • Territorial waters of the Marshall Islands • Leased to conduct an iron fertilization experiment • The new businesses involved suggest that • Iron fertilization process will reduce atmospheric CO2 levels • Allowing Marshall Islands to profit by trading carbon credits with more industrialized nations • Increased fisheries as a consequence of enhanced phytoplankton production • Iron additions could alter the ocean in unforeseen ways • Creating a polluted ocean with new opportunistic species that do not support enhanced fisheries

22. d13CDIC Tracks Productivity • Photosynthesis removes 12C from surface ocean and exports it to deep ocean • Close correlations between d13CDIC and nutrients

23. Measuring Changes in the Carbon Pump • Greater productivity during glaciations pumps more Corg to deep sea, reduces atmospheric CO2 • Past changes in strength of carbon pump • Recorded in planktic and benthic foraminifer

24. Past Changes in the Ocean Carbon Pump • Dd13C planktic-benthic are tantalizingly large when CO2 is low and small when CO2 is high • Correlation not perfect • May explain as much as 25 ppm CO2 lowering • Best documented in equatorial regions • Worse in Southern Ocean • Even HNLC regions • Detailed records lacking

25. Changes in Deep Water Circulation • d13C can be used to trace carbon transfer • Photosynthetic rate • Sets d13C and nutrient levels in surface waters • Water gets down-welled and carry the signals • These factors can produce regional differences in the d13CDIC • Deep waters in different ocean basins • Monitors changes in deep water circulation with time

26. Modern Deep Ocean Circulation • High d13C values in N. Atlantic results from • High production in surface waters in subtropical latitudes • Transported north and sinks • In contrast, intermediate waters originate in Antarctica • Seasonal production produces lower 13C enrichment • These contrasts allows water masses to be tracked

27. Atlantic Deep Water d13C • Deep water formed in N. Atlantic have high d13C values and low nutrient concentrations • Intermediate waters formed in the Southern Ocean have low d13C values and high nutrient concentrations

28. d13C Aging • As the Corg in deep water is gradually oxidized • 12C-rich CO2 released lowering d13CDIC • Particularly evident in deep Pacific waters

29. Past Changes in d13CDIC • d13C of benthic foraminifera indicate changes in Atlantic deep water flow at the LGM • Northern water did not sink as deeply, not as dense • Relative increase in water flowing from Antarctica • Knowing the d13C of the source region (planktic foraminifera) • Percent contribution from each region can be determined

30. Changing Sources of Atlantic Deep Water • Long records of d13C indicate cyclic changes in deep water sources • North sources dominate during interglacial • Southern sources dominate during glacial • 100,000 year cycle • During glacial • Low d13C water from Antarctica • Increase flux of 12C carbon from continents • Additive effects explains large shifts noted earlier

31. Summary • d13C results indicate an important link • Size of N. Hemisphere ice sheets • Formation of deep water in N. Atlantic • Less deep water formed in the N. Atlantic • Every time ice sheets grew at a 100,000 year cycle • Must have affected atmospheric CO2 concentrations • But how?

32. Changes in Ocean Chemistry • CO2 levels in surface waters sensitive to carbonate ion concentration • CO32- produced when corrosive bottom waters dissolve CaCO3 • When CO32- returned to surface waters • Combine with CO2 to form HCO3- • Thus reducing CO2 content of surface ocean • The corrosiveness of deep water determined by the weight of foraminifer shells • Depth of the CCD • Southern Ocean particularly vulnerable to changes in carbon ion concentration

33. Carbon System Controls on CO2 Change chemistry of shallow Southern Ocean subsurface water Increase carbon pump in Antarctic Increase biologic carbon pump in coastal and tropical ocean Change chemistry of Antarctic surface water