Lecture # 4b- Stable Isotopes Part II. 1) More Focus on C & N Isotopes 2) Brief bit on molecular-level isotopes?

Transcription

1 Lecture # 4b- Stable Isotopes Part II 1) More Focus on C & N Isotopes 2) Brief bit on molecular-level isotopes?

2 recall: δ notation H = (H/L)spl - (H/L)std x1000 (H/L)std Primary Standards Isotope Ratios Ratios x 10-6 Standard mean ocean water 2 H/ 1 H O/ 16 O O/ 16 O 373 PeeDee belemnite (PDB) 13 C/ 12 C Air 15 N/ 14 N Canyon Diablo meteorite 32 S/ 34 S 22.22

3 Aside: how talked about: Heavy = enriched in heavy isotope ( 13C, 15n ETC) Note on possible confusion 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: 1) Main δ 13 C signature is a measure of carbon fixation pathway 2) Further food-chain & Diagenetic transformations don t alter that signature - (all that much..)

5 Basic process: Photosynthetic Isotope Effect ( epsilon factor) Defined as: 1) Isotopic difference between CO 2 (or DIC) and fixed carbon (=Biomass) 2) δ d = dissolved CO 2, δ p = photosynthetic biomass 3) CO 2 in aquatic systems is dissolved CO2- recall only one 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) CO 2 (aq) δ of uptake δ of Enzyme process (Rubisco) (=> simple sugars)

7 Basic process CO 2 (aq) δ of uptake δ of fixation Process is: 1) Dependent on starting δ of CO 2 pool 2) different for land vs. ocean plants- largely due to uptake step 3) different for major kinds of autotrophic C-fixation biochemistries eg: C3-plant, C4-plant, chemoautotrophs

8 In ocean starting CO 2 δ values are pretty (~ near the ref std, = zero) but Ocean δ 13C does vary across globe in predictable ways primarily as function of Temperature

9 ALSO many smaller variables can also effect epsilon valuesthese can be important if trying to understand precise changes in a given region, or back in time. CO2 uptake & species cell size matters Examples from work by Ed Laws and Brian Popp (at UH)

10 However: despite these kinds of variations, overall a lot of open ocean mid-latitude OC is characterized by very similar del 13 C value ranges ~ -21 to 22 per mil

11 What about Land Plants?

12 C source: Atm del 13 C Highly Cyclic- due to Seasonal plant growth summer= relatively Heavy (why?) Winter = relatively Light (why?) Atm δ of C0 2 is well mixed (vs. ocean) and very small vs. ocean bicarbonate so driven by land plant cycles

13 1) Plant species Mechanism of CO2 uptake more diverse: can have very important effects on a given plant s overall del 13C values *Basic principle: if have a closed system, there can be no isotopic fractionation * If you have open system, can express maximum kinetic fractionation. Schlesinger fig. 5.2: stoma conductance vs. del 13C Ability of CO2 to diffuse freely has major effect on carbon isotopic fractionation.

14 2) For land plant in forests, also complicated by physical partioning: in addition to plant type, recycling of CO2 and forest structure is important Free Troposphere C m (mixed layer) r a Canopy Air space Photosynthesis (PSN) C a Z ca Respiration Soil Respiration (Resp)

15 All that said: Some overall generalizations for δ 13C: 1. 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!

16 Some approximate Source Endmember values: Marine carbonate Marine bicarbonate Atmospheric CO2 C3 vascular plants C4 vascular plants Photosynthetic bacteria Eukarotic Algae Methanogenic bacteria C, o/oo 0

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

18 Note: utility as a tracer for given question depends on ratio: endmember difference / accuracy can measure 13C to 0.1 (or better!) Marine carbonate Marine bicarbonate Atmospheric CO2 C3 vascular plants C4 vascular plants Photosynthetic bacteria Eukarotic Algae Methanogenic bacteria C, o/oo 0 Eg: C3 vs C4 plants: Δδ ( delta del ) ~ If can measure to 0.1 = sensitivity factor of ! Marine vs Terrestrial OM: Δδ ~ 6 = sensitivity factor of ~60

19 C-isotopes use example: source inference from endmember

20 δ 13 C 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

21 C-isotopes Complexity level I: fractionation differences between biochemical classes?

22 Why? Sum of biosynthetic pathways CO 2 (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

23 C-isotopes Complexity level I: biochemical classes 1. Different biochemical constituents of living organisms have consistent patterns of stable carbon isotope offset C, o/oo Fractions of a marine plankton Pectin Protein Hemicellulose Take-home info: * Lipid is light (sometimes very light) * protein is heavy Total Carbohydrate Total Organic Matter Cellulose Lignin-like material Lipids Total carbos ~ average. The upside: can be a proxy for composition. Marine Sediment Degens, 1969

24 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?) 13 C, o/oo Fractions of a marine plankton Pectin Protein Hemicellulose Total Carbohydrate Total Organic Matter And: what other information would you want to put some context on this observation? Cellulose Lignin-like material Lipids Marine Sediment Degens, 1969

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

26 What would be implication of this for interpreting changes in bulk OM 13 C values?

27 II: Nitrogen Stable isotopes: A second dimension Major Use: Trophic Level indicators. Why? Unlike C Average δ 15 N trophic offset strongly- ~3 per trophic level! Ie: you are what you eat + 3

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

29 Example of N isotopes and trophic levels Schoeninger and DeNiro (1984) GCA 48,

30 BUT What is base value for N? And why is there this large range in the previous plot? Schoeninger and DeNiro (1984) GCA 48,

31 Marine N cycle: mucho complexity! But Overall: Atm N 2 = 0 N-fixation = δ 15N of ~0 NO 3 (major pool in ocean) = heavy (positive), ~ + 4 to +8 BUT denitrification creates very heavy 15N.. Often assume total Nitrate utilization- therefore NO fractionation! (unlike Carbon!)

32 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!

33 Major Problem/ Complexity: ~ all del 15N values in nature are postive- BUT any given N value (in an animal) is due to TWO things: 1. Value of starting N source in food web. 2. Trophic level (Number of trophic transfers) How can you tell the difference?

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

35 Basic Trophic structure Easter Island

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

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

38 But what about Microbial food webs? Does the classic increase hold? And, what about Microbial degradation of OM? Turns out answer is : very unclear Would this hold? 1) 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. 2) Protists ( Hoch et al., MEPS, 1996) showed flaggelates and cilliates 15N enrichment depended on growth conditions- in particular, degree of coupling to bacterial production-

39 Finally: A basic point to remember for all stable isotope signatures:

40 In order to get this C fractionation, you MUST have only a partial reaction! Why? What would a time vs. fractionation plot look like?

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

42 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.

43 Aside: How can CO2 ever be limiting? 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.

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

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

46 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

47 END..