Hi Uriah,

Sedimentary "fingerprinting" and correlation is notoriously difficult and needs to be done with some care. Unfortunately, the results sometimes are limited by which tools you have at your disposal. Sometimes simple trace element ratios can be useful to indicate whether the sediment is derived froom a granitic or mafic hinterland.

Sr isotopes can be useful (particularly for carbonates), but are very readily remobilised during diagenesis and orogenesis, and so will not necessarily reflect the primary depositional characteristics. Low grade marine carbonates might record the seawater Sr isotope composition at the time of deposition, and so can form a basis of correlation.

For clastic sediments whole rock Nd isotopes have enjoyed application, as they are not as mobile in the sedimentary pile and are often reflective of the average source region eroding to supply detiritus. To that end, many studies in the '90's applied Nd ro shales (in particular) to investigate source regions and thus also potentially correlate between packages of stratigraphy.

More recently detrital zircons have become the most powerful tool for correlating sedimentary packages. They record both the Pb isotopes recording the age of zircon growth, and therefore the distribution of detrital zircon ages in a sediment can be a basis of correlation- So too the Hf isotopes of the same grains can add a further level of discrimination, allowing you to pick between source regions supplying sediment of the same age but different character, and thus potentially you can match detrital patterns between packages of sediments to form a basis of correlation.

However all of these approaches involving detrital grains or fingerprinting the source area are only as good as your understanding of the basin dynamics- a sedimentary unit may be sourced from several distinct terrains on either side of a basin, and hence will not contain detritus uniformly from all areas across it, making correlation difficult even though it is the same formation.

To summarise:

1) Young, low metamorphic grade marine carbonates can be correlated on the basis of Sr isotopes by match them with the Phanerozoic sea water curve, however care must be exercised.

2) Terrigenous sediments (particularly fine grained ones) can be cautiously correlated on the basis of whole rock Nd isotopes which average large areas of exposed continental crust.

3) detrital zircons are very powerful, both from generating age spectra (Pb isotopes) and Hf isotopes, however the interior basin dynamics may mean that different zircon populations are preserved in different portions of the same sedimentary sequence....

I hope this helps as an overview... it really depends on the nature of the sediments and their age!

cheers,

Bruce

Hi Bruce! Many thanks. This has shed more light on what geochemical tool should be used and on what and why.

Care must also be taken in using strontium ratios in marine carbonates. While they may reflect oceanic composition this is not always the case. Many marine organisms with carbonate skeletons (aragonite, calcite, or high mg-calcite) now appear to use biological preferences in their intake of Sr. This may also be influenced by local paleo environmental conditions; wind, turbidity, temperature, sunlight etc.

John Paul Jones

Dear all,

I think the Sr content in sediment reflect what type of water was occurred within the pore during diagenesis. therefore, High Sr value reveals sediment altered in marine pore water. However, two marine system. one is open system where surrounding rock is permeable ( Sr value will be low), and closed system where surrounding sediments impermeable( Sr value is high). cement produced in hypersaline environment also has high Sr value.

Regards,

Benaafi

Ideal combination would be Sr- and Nd-isotopes. This isotopic pair provides convincing evidences for source formations.

among the answers I do not see the classic study of isotope techniques carbonates Rb-Sr and 13C