Isotopes

isotopes

different types of atoms of the same chemical element same number but different mass number due to variation in number of neutrons

3 isotopes of carbon

C-12, C-13, C-14

equilibrium isotope-exchange reactions

Due to different thermodynamic properties e.g. evaporation where heavier isotopes require more energy, therefore heavy isotopes preferentially in liquid, light isotope in gas.

Kinetic isotope fractionations

Differences in rate of chemical reaction between isotopes. Typical for biological reactions. Organisms preferentially use the lighter isotope species because of the lower energy 'costs'.

delta notation

the isotope signature is expressed relative to a standard

why is delta notation used

often fractionation of isotopes between 2 phases is quite small

ocean delta 18 variation recording

by biological structures built of calcium carbonate

foraminifera

unicellular, microscopic marine organisms

planktonic

live near ocean surface

benthic

live at the seafloor

what does foraminiferal calcite delta 18 do

preserves past seawater

5 million years of climate variability from oxygen isotopes

Long term increase in global ice volume over the past 5 million years. Cycles between glacial and interglacial conditions throughout these cycles increase in amplitude (more intense glacial conditions) and duration (41 kyr -> 100 kyr) over time.

other sources of oxygen isotope climate records

• Ice cores

• Plant waxes

• Corals

• Speleothems

what can oxygen and hydrogen isotopes act as

temperature/water isotope proxies across a range of different settings

where is progressive fractionation of isotopes in organic matter seen

marine and terrestrial ecosystems

3 main photosynthetic pathways followed by modern plants

C3, C4 and CAM plants

C3 plants

95% of all plant species on the planet

C4 plants

adapted to efficiently carry out photosynthesis in hot and dry environments. They also thrive in low CO2 climates

C3 and C4 carbon isotope signatures

very distinct

How will the record of the standing vegetation be retained in the geological record

The signal needs to be locked in to the sediments and/or fossils.

biomineralized tissues

isotopic signals are incorporated into tissues secreted by organisms

is the isotopic signature preserved

yes (usually)

isotopic signals may be offset from the original vegetation

due to fractionation effects but the offset between C3 and C4 vegetation is preserved

distinctive cranio-dental features

• Powerful mastication muscles

• Large low-cusped post canine dentition i.e. large flat molars and premolars, diminutive anterior teeth, and thick enamel caps

prediction for diet based on this combination

Forceful oral processing, nuts + seeds + hard fruit

Carbon isotope and microwear analysis of P. boisei teeth

support a diet dominated by C4 grasses and sedges