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Damon Bassett is currently finishing his thesis project which involves the use of d¹⁸O values measured from bulk carbonate and biogenic apatite (conodonts and inarticulate brachiopods) to generate paleotemperature estimates. Lower Ordovician samples were collected in an area of central Texas where preservation of conodonts is very good to excellent. During the early Ordovician this region was equatorial and covered by a shallow sea. Temperatures estimated from d¹⁸O of carbonates average 43.0°C, significantly hotter than modern tropical oceans. These values can be explained in three ways:

1. Tropical temperatures during the early Ordovician were considerably higher than present.
2. The d¹⁸O value of seawater has changed since the early Paleozoic (more so than can be explained by the melting of all glacial ice).
3. The d¹⁸O values preserved in the carbonate samples have been altered by diagenesis.

Damon tested these alternatives by analyzing biogenic apatite which has a different susceptibility to diagenesis than carbonate. d¹⁸O values measured from apatite gave significantly lower paleotemperature estimates (avg. = 36.1°C) than the carbonate analyses. These values are slightly higher than modern tropical temperatures, but similar to recent paleotropical temperature estimates for greenhouse times like the Eocene and Cretaceous. Thus, we conclude that d¹⁸O values of the bulk carbonate samples have been slightly altered, that early Ordovician tropics may have been somewhat warmer than comparable modern environments, but there is no strong evidence for large secular changes in seawater d¹⁸O values of the world's water through time.

Conodonts isolated from the study site [ top ]

Histogram of paleotemperature estimate
based on analyses of
bulk carbonate and of biogenic apatite.

Click for larger historgram.