Biogeochemistry of the Earth System QMS Lecture 6 Dr Zanna Chase 16 June 2015

Transcription

1 Biogeochemistry of the Earth System QMS Lecture 6 Dr Zanna Chase 16 June 2015

2 Lecture 5: Glacial-interglacial CO2 changes Outline Ocean carbon pumps Climate-driven changes in ocean carbon pumps Revelle buffer factor Glacial-interglacial CO 2 change IMAS 2

3 Carbon pumps maintain this gradient in DIC against mixing

4 Biological ocean pump carbon pumps Imaged adapted from Heinz et al. (1991) and Chisholm (2000)

5 Physical ocean pump carbon pump 100 m High Solubility Low Solubility > 3000 m Imaged adapted from Heinz et al. (1991) and Chisholm (2000)

6 Which pump is more important? correlation with phosphate suggests biology is important correlation with temperature suggests physics is important This line describes how photosynthesis changes DIC and phosphate (Redfield ratio) This line describes the expected variations in DIC based on temperature alone

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8 Deep North Pacific water is old - it s been ~1000 years since it was at the surface

9 Air-Sea CO 2 Flux Reds: flux from sea to air Blues: flux from air to sea flux = k(pco 2w pco 2a ) Takahashi et al 2009 Annual flux represents influence of biology driving undersaturation, upwelling driving supersaturation, and wind-speed facilitating the transfer across the interface

10 How will the pumps operate in the future? If pumps are less effective in a high CO2 future, that means even more CO2 ends up in the atmosphere. This is a positive feedback If pumps are more effective in a high CO2 future, that means less CO2 ends up in the atmosphere. This is a negative feedback Need to know what controls the efficiency of the pumps

11 Biological pump- some important variables Supply of limiting nutrients: Runoff of nutrients from land may increase, due to human influence. Supply of iron from dust might increase, or decrease... Sunlight: not expected to change UV damage: Ozone hole hopefully stabilized Complex food web effects: Some food webs promote carbon export, others promote recycling Ocean acidification may negatively impact biological pump

12 Physical pump- important variables Surface temperature in areas of deep water formation: warmer water holds less CO2; solubility of CO2 is greater in cold water Ocean stratification: If deep waters aren t formed, less CO2 will be taken up via the solubility pump The concentration of CO3 2- with which CO2 can react. This is finite. The ability of the ocean to hold CO2 will decrease since the ocean has a limited buffering capacity

13 Revelle Factor: buffer capacity ( [CO 2 ]/[CO 2 ])/( [DIC]/[DIC]) Carbon dioxide entering the ocean is buffered due to scavenging by the CO 3 2 ions and conversion to HCO 3, that is, the resulting increase in gaseous seawater CO 2 concentration is smaller than the amount of CO 2 added per unit of seawater volume. Carbon dioxide buffering in seawater is quantified by the Revelle factor, relating the fractional change in seawater pco 2 to the fractional change in total DIC after reequilibration (Revelle and Suess, 1957; Zeebe and Wolf-Gladrow, 2001):

14 Revelle Factor: buffer capacity continued ( [CO 2 ]/[CO 2 ])/( [DIC]/[DIC]) A high Revelle factor means a low ability to take up atmospheric CO 2 Geographic variability relates mainly to the CO 3 2- content of the water The ocean s ability to take up CO 2 will decrease (Revelle factor increase) as atm CO 2 increases and removes CO

15 Kinetics are also important On the time scales of several thousands of years, it is estimated that 90% of the anthropogenic CO 2 emissions will end up in the ocean. Because of the slow mixing time of the ocean, however, the current oceanic uptake fraction is only about one-third of this value Sabine et al. 2004

16 Fate of fossil fuel CO 2 on different timescales eventually the ocean uptake is reduced because the buffering capacity (CO 3 2- ) has been used up

17 Natural atmospheric CO 2 variability Magnitude of anthropogenic perturbation almost as large as natural variability Rate of change much greater for anthropogenic CO 2 Recent concentrations much higher than observed in at least the past 1 million years Consistent, cyclical pattern in natural variability What caused this variability??

18 Natural atmospheric CO 2 variability coupled to climate During cold periods, atmospheric CO2 consistently lower than during warm periods. Responsible for 1/2 of the cooling. Amplifies orbital forcing

19 Which carbon reservoirs and fluxes changed during glacial periods?

20 Why the low atm CO 2 during glacial periods? Atm CO ppmv lower during glacial periods Obvious explanations include: 1. Terrestrial biosphere: Fewer land plants (ice sheets) atm CO 2 higher by ~15 ppm (transfer of C to atm) 2. Temperature: Surface ocean ~ 3 C colder. atm CO 2 lower by ~30 ppm (increased solubility) 3. Salinity: Ocean volume (ice sheets), salinity 3% atm CO 2 higher by ~6.5 ppm (decreased solubility)

21 Known changes account for less than 10% of the decrease in atm CO 2 during glacial times see Sigman and Boyle, Nature 2000 for a good review

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23 The effect of the solubility and biological pumps on atmospheric CO 2 results of a simple 2-box model Emerson and Hedges 2008"

24 High latitude dominance High latitude oceans dictate atmospheric CO 2 Knox and McElroy Harvard Sarmiento and Toggweiler Princeton Siegenthaler and Wenk Bern (Switzerland) = Harvardton-Bears! connection between high latitude ocean and deep ocean via deep water formation and isopycnal ventilation

25 The 3-box model again: High latitude dominance

26 Southern Ocean window DIC h =DIC d r C:PO4 [PO 4 ] d T + Φ h P High latitude box has a large leverage on atm CO 2 T + f hd

27 Two ways to close the window: 1. Increase stratification (ie decrease f hd ) 2. Increase productivity (ie increase Φ p ) DIC h =DIC d r C:PO4 [PO 4 ] d T + Φ h P T + f hd

28 Southern Ocean mechanisms for low glacial CO 2 1. Increased strength of the biological pump- Fe fertilisation? 2. Decreased upwelling in the Southern Ocean- stratification/winds? 3. Increased sea-ice acted as a barrier to CO 2 exchange 4. Silicic acid leakage Sigman and Boyle 2010

29 Iron fertilisation: One mechanism to increase export production temperature CO 2 dust flux sea ice Antarctic (EDC) records

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32 Note: Pacific sector does not show enhanced glacial productivity. Why? Kohfeld and Ridgwell

33 But maybe the high latitudes aren t dominant. Equilibration effect Start with: alk of 2340 µmol kg -1 and DIC of 2150 µmol kg -1 in all boxes producing pco 2 of 280 µatm in high lat surface box (2 C) and 630 µatm in low lat surface box (21.5 C) abiotic ocean Run till it reaches a new steady state

34 HBEI describes degree of equilibration in the box models, the high latitude surface box effectively equilibrates any changes in atmospheric CO 2, so surface pco 2 is essentially set my the high latitude box

35 Is the real ocean like a box model, or like a 3D GCM?? Understanding mixing in the real ocean in critical! A large HBEI Harvardton Bear Equilibration Index means less high lat sensitivity

36 Example of a nutrient inventory change hypothesis: change in N inventory driven by Fe availability (Broecker and Henderson 1998) 1. Nitrogen fixation in modern ocean is Fe limited 2. During glacial times, increased Fe availability lead to more N fixation 3. The increase in N inventory then allowed more export production and lower pco 2 Assuming a fixed C:N:P ratio, and no change in ocean phosphate, this mechanism can decrease atmospheric CO 2 by only 10 ppm in the 3-box model, but if the true HBEI is higher, the impact could be as much as 40 ppm.

37 Calcium carbonate compensation Input: rivers output: sediment accumulation If input from weathering increases, CO 3 2- increases, so dissolution of sediments decreases until accumulation = input This feedback helps maintain ocean alkalinity within tight bounds

38 Any process that decreases the ratio of CaCO 3 : organic carbon production has the potential to draw down atmospheric CO 2 through 2 effects: 1. Maintaining alkalinity in surface waters, which lowers surface ocean pco 2 ( closed system response ) 2. Decreasing ocean burial of CaCO 3, which results in whole-ocean increase in alkalinity, and associated decrease in pco 2 ( open system response )

39 Impact of CaCO 3 :orgc rain ratio on pco 2 Fig 8.3.5; 20C and 35 salinity

40 Decrease CaCO 3 export from surface waters, e.g. less coccolithophores Initially alk input > burial, as rivers unchanged. Surface ocean and mean Alk will increase Lysocline deepens so a greater fraction of produced CaCO 3 is buried, until burial = river input The increase in alkalinity leads to low ocean pco 2 and invasion of atmospheric CO 2

41 Response to increase in organic matter production increase soft-tissue pump, leads initially to lower CO 3 = (because of the increase in regenerated DIC at depth) and shoaling of lysocline, leading to more dissolution of CaCO 3 and increasing CO 3 = and deepening of lysocline and ultimately increase in whole ocean alkalinity and drawdown of atm CO 2 The open system response acts to magnify atmospheric CO 2 changes in response to changes in the biological pump (but model dependent)

42 Carbonate compensation- summary The carbonate compensation mechanism represents a positive feedback for decreasing atmospheric CO 2 by increasing the soft tissue pump It also means that atmospheric CO 2 is sensitive to changes in the ratio of the carbonate and soft tissue pumps, ie the CaCO 3 :orgc rain ratio Sensitivity is model dependent

43 Silicic Acid normally, diatoms take up Si:N, 1:1 when Fe is low, diatoms take up Si:N > 1 Sarmiento et al Si* = Si(OH)4 - NO3

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45 Silicic Acid Leakage Hypothesis 1. Fe limited diatoms have a high Si:N uptake ratio 2. Fe fertilisation during glacial times leads to decreased uptake of Si relative to N, and export of high Si:N waters from the Southern Ocean to the low latitudes 3. This infusion of Si to low latitudes via intermediate water leads to growth of diatoms at the expense of CaCO 3 -producing organisms 4. This decreases the CaCO 3 :orgc rain ratio to the deep ocean (carbonate pump), which leads to a decrease in atmospheric CO 2 through carbonate compensation

46 Isotopes are commonly-used tools in paleoceanography Isotopes: nuclides with same # of protons, different number of neutrons number of protons Isotopes of oxygen Isotopes of nitrogen Isotopes of carbon 14N: 7 protons + 7 neutrons- most common nuclide of nitrogen (99.6%) number of neutrons 15N: 7 protons + 8 neutrons (0.4%)

47 Nitrate utilisation from nitrogen isotopes Nitrate utilisation: Fraction of available nitrate that is used by phytoplankton. Utilisation is less than 100% when iron or light is limiting. Utilisation is high when nitrate is limiting Nitrogen comes in two stable isotopes: 14 N (~99.6%) and 15 N (~0.4%) Phytoplankton preferentially take up 14 N. Phytoplankton sink to the sediment As nitrate utilisation increases 15 N/ 14 N in the sediment increases This is because if nitrate supply is unlimited, phytoplankton can be selective and only take the 14 N. As nitrate supply gets smaller, phytoplankton have to consume more of the 15 N. 47

48 N and Si isotopes from the Southern Ocean suggest during the glacial periods (white bars) N utilisation was high, and Si utilisation was low Brzezinski et al. 2002

49 Summary During glacial times atm CO 2 is ppm lower than during interglacial times such as the pre-industrial Theory and observations point to the Southern Ocean as playing a key role in glacial-interglacial CO 2 change The Southern Ocean is a region where the deep ocean is in direct contact with the atmosphere via upwelling The SO window can be closed either by increasing stratification, increasing productivity, or capping (ice) G-IG CO 2 change likely not caused by a single mechanism rather a number of mechanisms acting consistently across glacial cycles. Uncertainties remain in terms of the proxy record as well as the importance of the Southern Ocean window in the real ocean (HBEI issue in box models versus 3D OGCMs)