Stable Isotopes in the Hydrosphere and Biosphere

1. Stable Isotopes in the Hydrosphere and Biosphere Lecture 36

2. Isotopic Fractionation in the Hydrosphere • δ18O in the oceans is nearly constant at 0. However, as a consequence of Rayleigh fractionation and temperature dependence of the water-vapor fractionation factor, the isotopic composition of precipitation varies widely. • There is a strong correlation with temperature.

3. Isotopic Fractionation in the Hydrosphere • Distance from source and orographic effects also affect δ18O of precipitation. • δD is strongly correlated with δ18O in precipitation. This is known as the Meteoric Water Line.

4. Biological Fractionations • Relatively large carbon isotope fractionations occur during photosynthesis as a result of: • More rapid diffusion of 12CO2 into the leaf (∆ ≈4.4‰) and • The 12C-O bond being weaker than the 13C-O bond, 12CO2 is preferentially fixed. • Key step in ‘C3’ plants is reaction of ribulosebis-phosphate with CO2 to make 2 molecules of a 3-carbon chain 3-phosophglyserate (which is subsequently converted to sugar and the phosphate recycled). • The result is a fractionation of -20- to -30‰. • In ‘C4’ plants, including grasses, sugar cane and maize, CO2 is initially fixed into a 4-carbon chain and then transported into an interior cell where it is converted to 3-phosophglyserate. This process more efficiently utilizes CO2 and thus results in less isotopic fractionation. • C4 plants are heavier on average, δ ≈ -13‰ • Only small subsequent fractionations occur as sugars are converted into other organic substances and during respiration. As a consequence, the isotopic composition of plants reflects the photosynthetic fractionation and this isotopic composition is maintained up the food chain (with 1 or 2‰ fractionation at each step. • The isotopically light signature of photosynthesis is also retained in sedimentary organic matter, including oil and gas (although fractionation during catagensis results in ‘thermogenic’ methane being even lighter).

5. δ13C in the Oceans • There is a fractionation with dissolution of CO2 in water so that dissolved CO2 is heavier. • δ13C is high in dissolved inorganic carbon (DIC, which is mainly ?) in surface waters due to preferential photosynthetic utilization of 12C. • δ13C is lower in deep water as organic matter is ‘remineralized’ (metabolized). • δ13C of deep water decreases with age, so it can be used as a water mass tracer. Incorporation into carbonate shells allows for use in paleo-oceanography. • Variable productivity results in somewhat variable δ13C surface water.

6. Nitrogen & Sulfur Isotope Fractionation • Biological utilization results in N isotope variations. Many are related to redox reaction between N in 5 different valence states. • N incorporated into living matter as amine groups (NH2–) but can be taken up in either reduced (NH3) or oxidized (NO3-) form - not as N2 (except by bacteria). • Terrestrial plants on the whole has slightly ‘heavy’ nitrogen - but contamination by artificial fertilizer has screwed this up. • Extent of fractionation depends on availability. • Not much fractionation when it is scarce as essentially all is utilized. • There are particularly large fractionations, 20 to 40‰, between sulfide and sulfate. These are often biologically mediated.

7. Isotopic Fingerprints • Plants can be grouped based on their carbon and isotopic compositions due to different fractionations of marine, C4, and C3 autotrophs and nitrogen fixers (cyanobacteria and legumes) and non-nitrogen fixers (everything else).

8. You are what you eat • Carbon isotopes undergo 1-2‰ fractionation with each step up the food chain. Given the large variation in autotrophs, these changes are small. • N isotopes also undergo 1-2‰ fractionation with each step. These are relative large compared to differences in autotrophs. • Marine food chains tend to be longer, so isotopic differences at the top are greater.

9. Appearance of C4 Plants • C4 plants (again, mainly warm climate grasses) first become important in the Miocene. • This is documented by, among other things, a shift is δ13C in soil carbonates in Pakistan (similar shifts seen in N. America). • Question is why? • C4 pathway more efficient at low atmospheric CO2 levels (otherwise not), so it may be an adaptation to lower Neogene CO2 levels.

10. Isotopes in Archeology • Pioneering work by DeNiro (not that one) and his advisor Epstein (a student of Urey) has led to widespread use of isotopes in archeology and paleontology. • Bone collagen, teeth, and cooking residues on pottery shards retain original δ13C and δ15N signatures. This allows reconstruction of diets. • 87Sr/86Sr and δ18O can “fingerprint” the region an individual lived in as these too are preserved in bone.

11. Isotopic Fossils • The earliest evidence (still somewhat disputed) of life is low δ13C values in graphite inclusions in apatite found in 3.8 Ga metasediments from Isua, Greenland. • δ13C as low as -19‰ occur. There is no demonstrated natural mechanism (so far) to produce such isotopically light carbon abiologically.