1. 2. understand how stable isotope rations are affected by transformations in the environment
1.1. 1. mixing
1.1.1. process which combines sources with different stable isotopes
1.1.2. combines
1.1.3. pools into mixture of a homogenous Stable Isotope value
1.1.4. processes that **mix** reservoirs in the **water cycle**
1.1.4.1. *infiltration * Rainwater + groundwater
1.1.4.2. *snowmelt runoff * ice + streams/rivers
1.1.4.3. *streamflow * streams/rivers + ocean
1.1.5. Stable isotope consequence
1.1.5.1. 2 source mixing
1.1.5.1.1. 2 source mixing fractional contributions
1.1.5.1.2. 2 source mixing fractional contribution
1.1.5.2. 3 source mixing
1.2. 2. fractionation
1.2.1. process separates mixtures into reservoirs with different stable isotopes
1.2.1.1. mixing v fractionation
1.2.2. separates
1.2.3. separates mixtures into reservoirs with different stable isotope values
1.2.4. processes that **fractionate reservoirs** in the **water cycle**
1.2.4.1. evaporation and precipitation are processes that fractionate delta18O
1.2.4.1.1. evaporation
1.2.4.1.2. precipitation
1.2.4.1.3. calculation. evaporation and precipitation have fractionated the ocean water delta 18O by -8permil
1.2.5. fractionation reactant (source) vs product and residual reactant stable isotope values
1.2.6. e.g., **photosynthesis**
1.2.6.1. carbon dioxide in the air is relatively light compared to the standard (-5permil) but biomass (plants) are even lighter (-29permil) this means biomass prefers the lighter isotope and fractionates the source! **therefore photosynthesis fractionates deltaC-13 by -24permil**
1.2.6.2. simple fractionation calculation
1.2.7. carbon cycle fractionation volcanics = organic + carbonates volcanic = (f x organic) + (f x carbonates) -5 = (f x -29) + (f x 0) forg = -5/-29 = 1/6 therefore in the phanerozoic carbon cycle organic carbon is 6x lighter than volcanic carbon outflux = sign for life (the relative proportion of carbon that is being buried in ocean sediments = 1/6)
1.2.7.1. delta C13 record though time (in rocks) 4Ga years
1.2.7.1.1. changes in carbonate carbon burial **GOE and snowball earth**
1.2.7.1.2. change in organic carbon burial increased input of very depleted methane
1.2.7.1.3. the Ccarb is effectively just reflecting the C-13 of the dissolved inorganic pool
1.2.7.1.4. excursions are typically changes in the amount of organic carbon burial
1.2.8. can help constrain presence of life
2. 3. understand how these signatures get transferred to the rock record, and can provide clues to ancient environments, on the earth and beyond
2.1. colour change = grey white = microfossils and nannofossils (i.e., calcium carbonate shells) red= dissolution event (marine carbonate dissolved and/or not produced in surface ocean) **ocean acidification**
2.2. Cenozoic changes in carbon cycle come from C-13 record
2.2.1. carbon mass balance organic c = fractionation carbonate carbon = DIC
2.2.2. bury more organic carbon (photosynthesis which preferentially takes up C-12) therefore DIC (CARBONATE CARBON) reservoir will get heavier and visa vera
2.2.3. miocene high 13C values (i.e., increased organic matter burial)
2.2.4. decrease in C13 values towards the end of the miocene
2.2.4.1. linked to evolution of C4 grasses which does photosynthesis in a slightly different way resulting in a change in the fractionation of organic carbon
2.2.4.1.1. C3 = 20-30per mil fractionation C4 = 10-20per mil fractionation (pre concentrates CO2 therefore less discrimination against the C13 isotope) - desert plants
2.3. carbon isotope in carbonates (rocks and ocean sediment microfossils) can be used to infer changes in the relative amount of organic carbon burial (forg) 13C in carbonates assumed to be a global signal (bulk seawater value), so can be used to correlate different sections of rock = **carbon isotope stratigraphy**
2.4. carbon isotopes in the modern environment
2.4.1. Used to identify processes in the carbon cycle
2.4.1.1. canada (annual cycle)
2.4.1.1.1. NORTHERN HEMISPHERE (more land and therefore trees)- **seasonal cycle in productivity** and photosynthesis/respiration
2.4.1.1.2. increased C13 value in summer = peak production therefore lots of fractionation due to preferentially take up of C12- peak photosynthesis
2.4.1.2. antarctica (decrease)
2.4.1.2.1. less pronounced seasonal cycle in the southern hemisphere because there are less trees
2.4.1.2.2. long term decrease - **anthropogenic activity** - input of CO2 from another source - mixing!! - fossil fuels depleted in C13 as is formed from organic carbon.
2.4.1.3. water column profile C13 more photosynthesis in surface results in a more positive C13 in surface DIC during respiration O16 is preferentially used resulting in a more positive delta18O in water (at o2 minima)
2.4.1.3.1. increase in DIC at depth due to respiration decrease in DIC at surface due to photosynthesis
2.4.1.4. global overturning circulation
2.4.1.4.1. north Atlantic Deep Water forms at high latitudes from warm, salty subtropical waters that cool and become dense
2.4.1.4.2. high C13 of NADW (enriched in 13C because of photosynthesis) low C13 of AABW
3. 1. understand general principles of stable isotopes and isotope notation
3.1. **Isotope** = atoms of the same element with the same number of protons but a different number of neutrons (different mass)
3.1.1. referred to by **mass number** (protons +neutrons)
3.1.2. differ in **mass** not in *chemistry *
3.1.3. components of an atom - ALWAYS SAME NUMBER OF PROTONS (Z) - DIFFERENT NUMBER OF NEUTRONS (A) = DIFFERENT MASS - N = A-Z
3.1.4. E.g., isotopes of Carbon (atomic number (protons) = 6) mass (P+N): 12 (12-6=6 neutrons) 13 (13-6=7 neutrons) 14 (14-6 = 8 neutrons)
3.1.5. e.g., sulphur isotope
3.1.6. e.g., Hydrogen isotope
3.1.7. **stable** isotopes do not decay radioactively
3.1.8. the stable isotopes for common elements are most abundantly the low mass isotope
3.1.9. isotope ratio mass spectrometry
3.1.9.1. sample is ionised
3.1.9.2. charges particles are deflected by a magnet
3.1.9.3. how far they are deflected depends on their mass
3.1.10. isotope notation (used to express and compare stable isotope variations)
3.1.10.1. Isotope ratios (R) are always expressed as a ratio of the **rare** to **abundant** isotope (remember abundant is the lighter one)
3.1.10.1.1. e.g., C-12 and C-13
3.1.10.2. isotope values are generally reported relative to a standard using the **delta notation**
3.1.10.2.1. common standard ratios
3.1.10.2.2. sample calculation
3.1.10.2.3. why a ratio of ratios?
3.2. why are stable isotopes important?
3.2.1. provides some constraints on the composition of natural systems and reactions that took place
3.2.2. elements are **cycled** through many different **reservoirs) via many different transformations/ **fluxes** in the environment
3.2.2.1. e.g., **the long term carbon cycle** **reservoirs**: atmosphere (co2 gas) oceans (HCO3-) sediments (CaCO3) and organic carbon in the deep earth **fluxes** degassing, weathering, CO2 fixation, burial, subduction
3.2.2.1.1. unravel using **stable isotopes**
3.2.2.2. e.g., **the water cycle** **reservoirs** atmosphere, oceans, groundwater, lakes/streams, ice and snow **fluxes** precipitation, infiltration, runoff, streamflow, evaporation/transpiration **mixing processes** infiltration, runoff, streamflow **fractionation processes** precipitation, evaporation/transpiration