Computer Science
Scientific paper
Jan 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992e%26psl.108..119a&link_type=abstract
Earth and Planetary Science Letters, Volume 108, Issue 1-3, p. 119-129.
Computer Science
1
Scientific paper
In an attempt to test the hypothesis that insular phosphorites on high islands (e.g., Nauru) formed by chemical reactions between seabird guano and carbonate, the oxygen-isotope compositions of phosphate (δ18Op) in modern guano and in the apatite component of phosphorites of various ages and locations have been investigated.
Guano from several species of seabird appears to have δ18Op values that are in equilibrium with the ambient seawater temperature at each site. However, recent apatites closely associated with modern guano can have δ18Op values that are as much as 2‰ higher than their inferred guano phosphate source. This shift in δ18Op values could be the result of isotopic re-equilibration with percolating meteoric waters which have been enriched by evaporative processes in the upper part of the soil profile. Bacterial degradation of the guano in the soil zone is a possible mechanism for resetting the δ18Op values in this environment.
Ancient apatites on high islands such as Nauru, Ocean Island and Makatea have δ18Op values that are significantly lower than the δ18Op values of recently formed apatites. The results indicate that ancient apatite deposits could not have formed by processes similar to those forming recent apatites without unreasonably high palaeotemperatures and/or equilibration with water with much lower δ18O values, such as meteoric water in a karstic terrain at the time of maximum infiltration. If the δ18Op values in apatite are subject to modification during subaerial weathering and diagenesis, perhaps aided by the metabolic activity of micro-organisms, the application of δ18Op signatures of insular phosphorites to palaeoenvironmental reconstructions is limited to post-depositional history and provides few clues to original setting.
Ayliffe Linda K.
Chivas Allan R.
Herbert Veeh H.
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