Permian Radiolarian Faunal Variations Correlated to Stable Carbon and Oxygen Isotopes in the Lamar Limestone, Delaware Basin, West Texas: Implications for Radiolarian Paleoecology

The Lamar Limestone of West Texas is a basinal carbonate that contains a near continuous well-preserved late Guadalupian (Middle Permian) radiolarian record, and reveals eight marked faunal oscillations in a 9-m interval. Radiolarian assemblages alternate between Albaillelid-dominated and sphaerellarian-dominated faunas. The relative abundance of Follicucullus (Albaillellaria), an important zone fossil, varies from nearly zero to as high as 90%. Geochemical proxies are being used to determine which environmental factors contribute to the faunal changes. Inorganic carbon and oxygen isotope results on 135 samples through the section show δ¹³C_carb (VPDB) values varying from -1.2 to +6.0‰, and δ¹⁸O (VPDB) values varying from -5.9 to -0.3‰. Lighter δ¹⁸O values are correlated with higher concentrations of silt in the samples.

Correlation analysis indicates that the absolute abundance of total radiolarians (AAT, for 38 samples to date) in the dataset is negatively correlated with δ¹⁸O values of the samples at the 5% significance level. Furthermore, this correlation is mainly due to the significant (at the 2% level) negative correlation between the absolute abundance of sphaerellarians and the δ¹⁸O values. There is a less well developed correlation (significant at the 10% level) between the AAT and the δ¹³C_carb values. Since carbonate δ¹⁸O values are lighter in freshwater than in seawater, a negative excursion of δ¹⁸O may represent a decline of paleosalinity through increased riverine input. Hence, these correlations may indicate that sphaerellarians are more sensitive to salinity changes than Follicucullus, and are more prosperous at lower salinities. However, due to the various perturbing factors on oxygen isotope values of old rocks, other potential proxies for paleosalinity such as Sr/Ca, Mg/Ca, and Ba/Ca will be analyzed in relationship to the radiolarian shifts in order to further test this hypothesis.