Geology Evaluative

OXYGEN

NORMAL: 16O in the evaporated water is returned to the ocean after falling as rain or snow and draining or melting through the river system

GLACIATION:

Light oxygen is trapped in the ice caps and glaciers

Ratio of 18O is present in colder temperatures that in warmer temperatures

Carbonate formed at lower temp = higher 18O content

RETURNS TO NORMAL:

Only evidence of the change remains in the shells of organisms living in the sea

The oxygen in the calcium carbonate shells of bivalves, belemnites and microfossils e.g. foraminifera comes from the sea water and CO2.

USES ZONE FOSSILS

good zone fossils = rapid evolution, abundance

e.g. graptolites - silurian+ordovician, shales+greywackes

ammonites - jurassic+cretaceous, use suture lines - sensitive indicators to change and evolve quickly

PROBLEMS:

• restricted to particular environments - reefs and sandy sea floor
• some kids of fossils are very long ranged
• good zone fossils are very delicate - only preserved in quiet environments
• not all sedimentary rocks contain fossils
• derived fossils confuse the sequence of beds

USES SEQUENCE, THICKNESS, SIMILAR STRUCTURES (faulting, folding way up structures - desication cracks), COMPOSITION

PROBLEMS:

• cannot recognise nature of deposition
• limited
• changes
• diachronous beds - same beds, different ages - misleading (e.g. DELTAS)

USEFUL WHEN SAME DEPOSIT OVER BIG AREAS :

• volcanic eruptions - big area of tuff - pyroclastic (because unusual and one off event)

TRIASSIC, CRETACEOUS BEST TO USE

DEEP SEA SHALES BEST TO USE - VERY CONSISTENT, LATERAL CONTINUITY

INVOLUTE: covers over the old parts of the shell

EVOLUTE: can see all details

12C accounts for the majority of the carbon in the global cycle, but there is a small amount of heavy isotope 13C

13C vaalues from pelagic and benthonic fossils can be correlated with climate change

PLANTS prefentially take up 12C

- during glacials when terrestrial biomass greatly reduced, ocean was relatively depleted in 13C

CARBON isotopes MORE USEFUL than OXYGEN isotopes, for statigraphic purposes in the remote past up to 10 MILLION YEARS ago because they are MORE RESISTANT to diagenetic change.

WHAT FEATURES DO THEY HAVE IN COMMON?

• five fold symmentry
• tests made of calcite plates
• possession of tube feet