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Strontium isotopes in biogenic phosphates from a Neogene marine formation: implications for palaeoseawater studies

Strontium isotopes in biogenic phosphates from a Neogene marine formation: implications for palaeoseawater studies
Strontium isotopes in biogenic phosphates from a Neogene marine formation: implications for palaeoseawater studies
Neogene marine deposits of NW France ("Faluns") are rich in a variable array of well preserved phosphatic vertebrate fragments. Many of these are of the same age as the host sediments and consist of the remains of fish, mammals and reptiles whilst others are clearly Mesozoic (Turonian) in age. We have determined the Sr isotopic composition of 24 phosphatic samples in order to study the efficacy of using the method to date the Neogene materials and to investigate the effects of diagenesis and seawater alteration on the terrestrial material.

The Sr isotope results obtained on the ichthyoliths and the other Neogene marine vertebrates suggest an age of 11 Ma for the base of the host sediments (Tortonian), which corresponds precisely with the palaeontological data. In contrast, the initial isotopic composition of the contemporaneous terrestrial mammals and reptiles, which should record higher (continental) Sr ratios are close to the value of their marine counterparts. The same trend is observed for a sample of shark's tooth of Mesozoic age where the Sr isotopic composition approaches a Neogene value. The results demonstrate that alteration by seawater or interstitial waters is able to introduce significant quantities of Sr into the phosphate. Leaching experiments carried out on the materials does not remove this secondary strontium and we were unable to establish the original values of the terrestrial material. Our conclusion is that the use of Sr isotopes to characterise phosphatic materials must be viewed with caution particularly if the materials are reworked.
SEA WATER, PHOSPHATES, BIOGENIC MATERIALS, STRONTIUM ISOTOPES, GEOCHEMISTRY, NEOGENE, FRANCE, FOSSILS, PALAEONTOLOGY
0009-2541
325-332
Barrat, J.A.
acfa951b-8140-4574-bb54-9b30f9d75113
Taylor, R.N.
094be7fd-ef61-4acd-a795-7daba2bc6183
Andre, J.P.
1b8a83cf-98c8-4dbe-a73a-0948cb533a42
Nesbitt, R.W.
6a124ad1-4e6d-4407-b92f-592f7fd682e4
Lecuyer, C.
e6b3e513-6039-4c18-94cc-3f7a93d54ffd
Barrat, J.A.
acfa951b-8140-4574-bb54-9b30f9d75113
Taylor, R.N.
094be7fd-ef61-4acd-a795-7daba2bc6183
Andre, J.P.
1b8a83cf-98c8-4dbe-a73a-0948cb533a42
Nesbitt, R.W.
6a124ad1-4e6d-4407-b92f-592f7fd682e4
Lecuyer, C.
e6b3e513-6039-4c18-94cc-3f7a93d54ffd

Barrat, J.A., Taylor, R.N., Andre, J.P., Nesbitt, R.W. and Lecuyer, C. (2000) Strontium isotopes in biogenic phosphates from a Neogene marine formation: implications for palaeoseawater studies. Chemical Geology, 168 (3-4), 325-332. (doi:10.1016/S0009-2541(00)00200-X).

Record type: Article

Abstract

Neogene marine deposits of NW France ("Faluns") are rich in a variable array of well preserved phosphatic vertebrate fragments. Many of these are of the same age as the host sediments and consist of the remains of fish, mammals and reptiles whilst others are clearly Mesozoic (Turonian) in age. We have determined the Sr isotopic composition of 24 phosphatic samples in order to study the efficacy of using the method to date the Neogene materials and to investigate the effects of diagenesis and seawater alteration on the terrestrial material.

The Sr isotope results obtained on the ichthyoliths and the other Neogene marine vertebrates suggest an age of 11 Ma for the base of the host sediments (Tortonian), which corresponds precisely with the palaeontological data. In contrast, the initial isotopic composition of the contemporaneous terrestrial mammals and reptiles, which should record higher (continental) Sr ratios are close to the value of their marine counterparts. The same trend is observed for a sample of shark's tooth of Mesozoic age where the Sr isotopic composition approaches a Neogene value. The results demonstrate that alteration by seawater or interstitial waters is able to introduce significant quantities of Sr into the phosphate. Leaching experiments carried out on the materials does not remove this secondary strontium and we were unable to establish the original values of the terrestrial material. Our conclusion is that the use of Sr isotopes to characterise phosphatic materials must be viewed with caution particularly if the materials are reworked.

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More information

Published date: 2000
Keywords: SEA WATER, PHOSPHATES, BIOGENIC MATERIALS, STRONTIUM ISOTOPES, GEOCHEMISTRY, NEOGENE, FRANCE, FOSSILS, PALAEONTOLOGY

Identifiers

Local EPrints ID: 8976
URI: https://eprints.soton.ac.uk/id/eprint/8976
ISSN: 0009-2541
PURE UUID: 9c1ebccb-37f0-4b1f-a7e9-3166303e8bd9

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Date deposited: 08 Sep 2004
Last modified: 17 Jul 2017 17:12

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