| what is an isotope | same element, same amount of protons, but different amount of neutrons and tus different masses |
| stable isotopes do not | decay over time |
| What is is the hydrogen (d2H) reference standard called? | VSLAP / VSMOW |
| What is the carbon (d13C) reference standard called | VPDB |
| What is the nitrogen (d15N) reference standard called? | AIR |
| What is the oxygen (d18O) reference standard called? | VSMOW = Vienna Standard Mean Ocean Water |
| Why do we need isotope reference standards? | Absolute isotope quantity is difficult to measure accurately, however, relative differences can be measured accurately. Common reference point/baseline to ensure accuracy, reliability and reproducibility of isotopes data from different laboratories and studies. Calibration, interlaboratory comparison, and quality control of procedures. |
| The belemnite (VPDB) is inorganic carbon, meaning that organic carbons will always be more .... as it contains less 13C than inorganic carbons | negative |
| ... is the partial separation of isotopes during physical or chemical processes | isotopic fractionation |
| The two most relevant types of isotopic fractionation are: | kinetic (results when rates or reactions or physical processes differ, e.g., biosynthetic pathways) and equilibrium (occurs because the thermodynamic properies of isotopically substituted species differ e.g., fractionation betwee inorganic carbon species in water) |
| The product is often isotopically ..... why the unused part is often isotopically .. | depleted/more negative & enriched/more positive |
| If 100% of substrate is converted, then the isotopic composition of the product is ...to the starting substrate | identical |
| during kinetic isotopic fractionation, eutotrophic organisms fractionate ... 13C durig inorganic carbon uptake (CO2) in different degrees | against (epsilon value works in the negative direction, larger number is larger fractionation) |
| What is rubisco ? | Earth's most abundant enzyme, used by autotrophic organisms to convert CO2 into organic compounds via the Calvin-Benson pathway |
| What factors influence fractionation in autotrophs | growth rate (fractionation is reduced at faster growth rates, more product), concentration of CO2 (more CO2 = pickier, more fractionation) & cell geometry (smaller, pickier, more fractoination) |
| fractionation is inconsistent due to limitations by for example .... Growth conditions are important! | light & nutrients |
| Additional isotopic fractionation takes place during biosynthesis of lipids. Every step can encounter isotopic fractionation – the more downstream you are the more ... you will be – every enzyme can fractionate | depleted |
| 13C-contents depends on ......... and environmental factors (e.g., pCO2 and growth rate for Rubisco) | carbon acquisition pathway |
| Fractionation for rubisco: | (straight chain) lipids (alkyl) most depleted, then isoprenoids and sugars are most positive (unused substrate) |
| Biomass is always depleted compared to | atmosphereic CO2 |
| Reversed TCA - occurs in some bacteria and some archaea. Fractionation: | reverses tge way they produce compound. So sugars most depleted, then isoprenoid lipids then straight chain lipids/alkyl |
| reverse RCA is less impacted by CO2 thus it is generally also subjected to .... fractionation compared to atmospheric CO2 | less |
| Reductive acetyl CoA pathway, not cyclic pathway - used by some bacteria (eg sulphate reducers ) and archaea (methanogens). Fractionation: | Very large fractionation compared to CO2, straigh chain lipids/alkyl & isoprenoids is super depleted vs biomass. Methanogens super depleted in d13C and dD(deuterium/hydrogen) as methane is aswell |
| Anammox fractionation by bacteria | relatively large fractionation, isoprenoid and alkyl lipids (straight chain) strongly depleted in 13C versus biomass (NH3 & NO2- into N2) |
| 3-Hydroxy propionate pathway, used e.g. by phototrophic green non-sulphur bacteria. Fractionation | Uses bicarbonate instead of CO2, it is more positive by about 7-8 permille than CO2 – so bacteria using this will already be more positive from the start -> biomass lil. depleted vs CO2, alkyl enriched vs CO2, isoprenoids more depleted compared to alkyl and biomass |
| 3-Hydroxypropionate/4-Hydroxybutyrate pathway -> - Occurs in some hyperthermophilic Crenarchaeota and also in Thaumarchaeota (producers of crenarchaeol biomarker lipid) | isoprenoids similar to biomass, DIC and sugars are enriched |
| We can use the known fractionations to determine the carbon acquisition pathway of a microbe | Analyse and compare CO2, biomass and lipids |
| C3 plants live in ... regions and are generally ... compared to C4 plants | humid & depleted (they depend on pCO2) |
| C4 plants live in ... regions and are generally ... compared to C3 plants | dry (predominantly grasses on savannahs) & enriched -> C4 plants have concentrating mechanism (limited exchange with atmosphere) -> fractionation does not depend on CO2 |
| There are three types of carbon fixation mechanism in terrestrial higher plants: | C3, C4, CAM |
| Fractionation in C3 plants is dependent of precipitation – fractionation will become ... during more dry weather-climate | less |
| We can measure our biomarker lipid and then reconstruct C3/C4 plant
n-alkanes can be used to reconstruct higher vegetation and differentiate between C3/C4, but | Different contributions of different plant types with different 13C contents may bias the overall n-alkane isotope patterns -> no quantification possible, but at least reconstruct some changes in vegetation |
| What determines the isotopic composition of heterotrophs ? you are what you eat +- a few permille | d13C of food source & biosynthetic pathways |
| methanotrophs | very depleted in d13C and dD |
| nitrogen isotopes are used for ... studies while carbon isotopes are used for ... studies | food web studies (position)(because nitrogen increases by 3.5 permillle per trophic level and 13C much less) & food source |
| When amino acids are changed more strongly with the trophic level the N-isotopic composition is changed ... | more strongly aswell |
| N2 -fixation | hardly any isotopic fractionation, slightly negative N isotopes |
| negative n-isotopes of bulk sediment may point to | N2-fixing cyanobacteria |
| Sedimentary 15N can give paleoenvironmental information on relative importance of
N2-fixation (......15N)
denitrification (......15N) | negative & positive |
| What is this and what is it a biomarker of? | porphyrins - nitrogen isotopes |
