L4 - 13C and 14C analysis Flashcards
(32 cards)
how is CO2 fixed in biological systems
photosynthetic organisms fix CO2 via different pathways (e.g. C3, C4 and CAM). Incorporating C into organic molecules with distinctive δ13C signatures
How do biosynthetic pathways affect δ13C values?
different metabolic pathways (e.g. C3 vs C4 photosynthesis) fractionate carbon isotopes differently, leading to distinct δ13C values in biomass
what competing processes can alter δ13C during blooms
bloom conditions can change isotope signatures due to shifts in C availability and selective uptake e.g. preferential use of 12C during rapid growth
what does “you are what you eat” mean in isotope ecology
organisms retain the isotopic signature of their food source, allowing δ13C to trace trophic relationships and source materials
how do n-alkanes indicate C3 vs C4 vegetation?
C3 plants: δ13C = -24 to -34%
C4 plants: δ13C = -10 to -16%
differences help to identify vegetation sources in sediments
how can 24-ethylcholesterol indicate origin
marine and terrestrial sources differ in δ13C:
- found in both diatoms (marine) and higher plants (terrestrial)
- δ13C from -20 to -25%, with marine dominance often inferred from context
what is isorenieratene and what does it indicate?
a biomarker from green sulphur bacteria (GSB), indicating photic zone euxinia (anoxic and sulphidic light-penetrated environments)
how are δ13C labelling experiments used
introduce labeled 13C substrates (e.g. methane) to trace carbon flow through microbial communities or biological systems
can organisms use methane as a C source
yes
methane oxidising microbes (aerobic or anaerobic) can convert CH4 into biomass; this is tracked via δ13C-PLFA analysis
why is 14C formation limited in the biosphere
it is produced in the upper atmos via cosmic rays, then incorporated into the C cycle
limited input makes it a useful tracer
what are the systematics and half-life of 14C
14C has a half-life of 5730 years
its decay helps date organic materials up to 50,000 years old
what factors influence 14C abundance
- solar activity
- climate change
- volcanic eruptions
- the Suess effect (fossil fuel dilution)
- the bomb spike (post-1950s nuclear testing)
what is the source/reservoir effect in 14C dating?
carbon in some reservoirs (e.g. deep ocean) is older than in the atmosphere due to slower exchange, altering apparent 14C age
how is 14C used to trace transport to marine environments
by analysing 14C in compounds or particulates (POM) we can track the age and origin of organic material entering the ocean
how can 14C identify fossil Vs modern carbon sources?
fossil carbon 14C-dead (no remaining 14C), while modern biogenic carbon contains measurable 14C
how is 14C used in archaeology
used to date bones, charcoal, wood and artifacts up to 50,000 years old
how does compound specific isotope analysis (CSIA) distinguish C3 vs C4 plants
measures δ13C of individual compounds (e.g.n-alkanes) to identify C3 (cool, moist) vs C4 (warm,arid) plant contributions
why does the sahara show C4 dominance
C4 plants thrive in arid, high light environments, they show enriched δ13C signatures compared to tropical C3-dominant regions
what makes isorenieratene a diagnostic biomarker
its biosynthesis is unique to green sulphur bacteria; finding it or its derivatives in sediments signals ancient exuinic conditions
what is the significance of 24-ethylcholesterol in Peru waters
dominated by marine diatoms but also found in higher plants
δ13C values help infer primary production sources
how are 13C labeling experiments applied to methane
by adding labeled CH4 and tracking δ13C in PLFAs, scientists study microbial methane utilisation and transformation
what are the origins of methane (CH4)?
- biogenic: δ13C = 60 to 90% (methonogenesis)
- thermogenic: δ13C = 40% (fossil/petroleum)
can CH4 be consumed at depth
yes
in anoxic marine sediments, CH4 is removed by anaerobic oxidation of methane
what evidence supports AOM in marine sediments
mixed 13C signatures in crocetane and phytane indicate active methane consumption by microbes in places like the Kattegat Strait
