Lecture # 4b- Stable Isotopes Part II

1. Lecture # 4b- Stable Isotopes Part II 1) Stable Isotopes in Geochemistry: Background, Reprise 2) Intro to Carbon & N Isotopes – More detail

2. recall: δ notation H = (H/L)spl - (H/L)std x1000 (H/L)std

3. Aside: how talked about: “Heavy” = enriched in heavy isotope ( 13C, 15n ETC) note: for carbon, all values are usually negative – so δ 13C value of – 15 is “really heavy” vs. typical marine value (-21) or if δ 13C changes from -21 to -20 it is “becoming enriched” (or “getting heavier”)

4. Intro to Stable C isotopes Recall Major Use: as Original Source (C-fixation) indicators Why: Main δ 13C signature is a measure of carbon fixation pathway Further food-chain & Diagenetic transformations don’t alter that signature all that much.

5. Basic process: “Photosynthetic Isotope Effect”(epsilon) Defined as: Isotopic difference between CO2 (or DIC) and Biomass δd = dissolved CO2 , δp = photosynthetic biomass CO2 in aquatic systems is dissolved CO2- recall only part of carbonate buffer system.

6. Cartoon of basic process in aquatic cell :(recall, final δ = due to total δ of a chain of events) δ of uptake CO2(aq) δ of Enzyme process (Rubisco) (=> simple sugars)

7. δ of uptake CO2(aq) δ of fixation Basic process Process is: Dependent on starting δ of CO2 pool different for land vs. ocean plants- largely due to uptake step different for major kinds of autotrophic C-fixation biochemistries eg: C3-plant, C4-plant, chemo-autotrophs

8. In ocean starting CO2 δ values are pretty similar (~ near the ref std, = zero) Ocean δ 13C does across globe in predictable ways- BUT in big picture is very very little..

9. What about in Atm?

10. summer= relatively Heavy (why?) Winter = relatively Light (why?) Highly Cyclic- due to Seasonal plant growth Atm δ of C02 is well mixed (vs. ocean) – and very small vs. ocean bicarbonate– so driven by land plant cycles

11. In reality, many smaller variables can also effect epsilon values- these can be important if trying to understand precise changes in a given region, or back in time. CO2 conc. vs. species “cell size matters” Examples from work by Ed Laws and Brian Popp (at UH)

12. For land plants, complicated also: in addition to plant type, turns out recycling of CO2 and forest structure is important

13. Some overall generalizations for δ 13C: Organic C is “lighter” (more “negative”, more “13C depleted”) than inorganic C. (again - why? Could it ever be heavier?) 2. Land Plants: C3 = light vs. C4 pathways (much heavier!) C3 ~ -27 to -29(vascular plants) lighter than C4 (grasses, eg: corn- -15 to -18? ). 3. Marine plankton (on average) are intermediate between C3 and C4 plants. (canonical value: “-21.5;” but in reality also vary) 4. Microbial 13-C compositional ranges can be very broad, especially for chemotrophs! (stereotypical for free living : very light, -30 to -50, BUT turns out that diverse chemoautotroph bug types are very differnet- some endosymbionts are actually heavy!

14. Approximate Source Endmember values:

15. Note: utility as a tracer for given question depends on ratio: “endmember” differences / measurable accuracy (Recall: can measure accurately to 0.1 ‰ or better!) C3 vs C4 plants: Δδ (“delta del”) ~ 12 –15 ‰ = sensitivity factor of 120-150! Marine vs Terrestrial OM: Δδ ~ 6 ‰ = sensitivity factor of 60

16. Note: utility as a tracer for given question depends on ratio: “endmember” difference / accuracy can measure 13C to 0.1 ‰ (or better!) Eg: C3 vs C4 plants: Δδ (“delta del”) ~ 12 –15 ‰ If can measure to 0.1 ‰ = sensitivity factor of 120-150! Marine vs Terrestrial OM: Δδ ~ 6 ‰ = sensitivity factor of ~60

17. Important: C-stable isotopes vs. Trophic transfer have very weak relationship  For Carbon Isotopes: “you are what you eat!” (+/- ~ 1 ‰ or less)

18. A Basic Marine Example: Bulk Marine Organic Carbon Close to -21 ‰ (exact value depends..) Marine bicarbonate (in principle giant reservoir) Close to +1 ‰

19. N B: • In order to get this fractionation, you MUST have only a partial reaction! • Why? What would a time vs. fractionation plot look like?

20. Thus Can get some unexpected effects: Eg: under conditions of very high production..observed fractionation falls! Marine bicarbonate (in principle giant reservoir) Close to +1 ‰ Bulk Marine Organic Carbon << -20 ‰ (exact value depends..) Why? Consider: in extreme (theoretical) case, where 100% of biocarb is used fractionation must be 0 ‰ ! Other extreme: where starting material is “infinite”, fractionation  free to approach maximum set by reaction series.

21. Q: What would a time vs. fractionation plot of a plankton bloom box model look like? (think box model.. Reactant/ product..)

22. Aside: How can CO2 ever be limiting? Aside II: this sort of thing is not so central with C isotopes, where starting material is usually in excess.. but for example with N isotopes – where starting material may often be totally used up- it becomes a key consideration.

23. Rau, 1998 DSRII: Montery Bay – δ 15 N of plankton vs N03 conc. what is going on here?

24. Ended about here in 2009, wk 3 2nd lecture (now way behind..) • In 2009 lect 1 of wk 4: start with review of factionation vs. substrate limitation examples Review - two cases on board: • Unlimited substrate • Limited substrate (plank. Bloom) • RESULT: a) Carbon, typically get the epsillon fractionation in OM ( 19 per mil vs. DIC) b) BUT for nitrogen- often get NO FRACT. In OM. (but if you do.. Can get some very odd values) To do: * redo this lecture to really stress above points- find some graph to show? • Add some values for marcophyte kelp project. ? Stress that values of coastal • Kelps are NOT near “marine plankton”…

25. A N example, but same idea: Montoya, 2007 N example NOTES: Early in the bloom,isotopic fractionation during NO3 - uptake by phytoplankton produces PN with a low δ15N. As the bloom progresses, the δ15N of the residual NO3 - increases, leading in turn to an increase in the δ15N of PN formed. If the bloom is rapid with little material lost through sedimentation or grazing, the δ15N of PN will converge on the δ15N of the initial pool of NO3 available to support growth (dashed line). If significant losses occur through grazing or sedimentation, the δ15N of PN may overshoot and exceed the initial δ15N of NO3

26. C-isotopes use example: source inference from “endmember”

27. δ 13C of galapagos rift zone hydrothermal vent mussels IF “You are what you eat”( ± ~1‰). Can use endmembers directly  First Proof of hydrothermal vent macro-fauna not tied to surface! Rau, 1979 Science Article- first definitive proof of bacterial-based ecosystem

28. C-isotopes Complexity level I: fractionation differences between biochemical classes

29. Why?Sum of biosynthetic pathways CO2(aq) Lipids δ reflective of Avg. lipid –family pathways Uptake fract. (epsilon) Carbos C fixation fract. (simple sugars) Amino Acids δ reflective of Avg. AA-skeleton pathways

30. * Lipid is “light”(sometimes very light) * protein is “heavy” Total carbos ~ average. C-isotopes Complexity level I: biochemical classes 1. Different biochemical constituents of living organisms have consistent patterns of stable carbon isotope offset Take-home info: The upside: can be a proxy for composition.

31. C-isotopes Complexity level I: Example: if particle falling through ocean water column changes from –21.2 to –26.0 between 100 to 1000 meters, what different things could you hypothesize are going on? (and how to test?) And: what other information would you want to put some context on this observation?

32. Does it work in real world? Wang & Druffel, 1998 GCA: Station M Plankton Tows Yes- More or less- but as with everything, lots of variation.

33. Add Jenny Figure

34. III: Nitrogen Stable isotopes:The second dimension Major Use: Trophic Level indicators. • Why? Unlike C Average δ15N trophic offset strongly- ~3 ‰ per trophic level! • Ie: “ you are what you eat + 3 ‰”

35. Why? Observation: light isotope is preferentially enriched in excreted by ANIMALS- as ammonia. (note: not bacteria..) Ammonia excreted Vs body 15N is Offset by…3 ! Checkey, 1989- DSR

36. Example of N isotopes and trophic levels

37. BUT What is “base value” for N ? And why is there this large range in the previous plot?

38. Marine N cycle: mucho complexity! • But Overall: • Atm N2 = 0 • N-fixation = δ 15N of ~ zero • NO3 (major pool in ocean) = heavy (positive), ~ + 4 to +8 • Often assume total Nitrate utilization- therefore NO fractionation! (unlike Carbon!)

39. Simplified (but still not that simple..)Marine N isotopes plankton trophic levels • N fixation vs. Nitrate dominated ecosystems differ strongly on del 15N of plankton. • In N-dominated systems with lots of recycling negative 15N of plankton are actually possible!

40. Major Problem/ Complexity: ~ all del 15N values in nature are postive- BUT any given N value (in an animal) is due to TWO things: • Value of starting N source in food web. • Trophic level (Number of trophic transfers) •  How can you tell the difference?

41. Examples of “classical” del 15N uses: trophic structure & diet reconstruction in ecology/ archeology, etc.

42. Basic Trophic structure Easter Island

43. Question: what did ancient humans societies on Easter Island subsist on?

44. Result: ALL values high. Humans must have ate fish. But, did they also (inadvertently?) feed fish meal to Rats? Chickens?

45. But what about Microbial food webs? Does the classic increase hold? And, what about Microbial degradation of OM? • Turns out answer is : “very unclear” • Bacteria have many sources of N (DON, DIN)-while animals have only their food- thus it would in principle depend on what else is available. • Protists ( Hoch et al., MEPS, 1996) showed flaggelates and cilliates 15N enrichment depended on growth conditions- in particular, degree of coupling to bacterial production- Would this hold?

46. END

47. Mol Level slides (NEXT 3- MOVE TO AA’s lecture)

48. One example of how you can use this.. Investigation of what controls butterfly reproduction. Experiment:(O’brien, PNAS 2001) • Larva is fed only C4 sugar  pupate • Adult Butterfly, fed only C3 sugar  grind them up. * What will amino acids in adult butterfly look like? `

49. Appendix: Wang, Druffel- paper- 13C & 14C of plankton classes

50. Appendix: Wang, Druffel- paper- C breakdown- major cmdp-classes plankton into seds. Bit Hard to see These figs- Look at table.