UNDERSTANDING PERMIAN RADIOLARIAN ECOLOGY BY USE OF GEOCHEMICAL PROXIES IN THE LAMAR LIMESTONE, DELAWARE BASIN, WEST TEXAS

Radiolarian faunal fluctuations, subtle lithologic changes and changes in several geochemical proxies (total organic carbon, and stable isotopes d¹³C_carb, d¹⁸O_carb, and d¹³C_org) have been documented in the late Guadalupian Lamar Limestone, West Texas. The radiolarians collected bed-by-bed from the top 5.5-m of the Lamar Limestone are alternatively dominated by two radiolarian groups, sphaerellarians and Follicucullus. Positive correlations exist between sphaerellarian richness and total organic carbon (TOC), and also between TOC and clay content. Negative correlations exist between clay content and d¹⁸O_carb values, and also between TOC and d¹³C_org values. In contrast, Follicucullus richness is not correlated to any measured geochemical parameter. Partial correlation analysis reveals the correlations between sphaerellarian richness, d¹⁸O_carb values, and d¹³C_org are indeed spurious correlations, which are caused by their strong but independent covariation with TOC. We suggest one possible scenario where variation in riverine input may explain the combination of observed faunal, lithologic, and geochemical characters. The correlation between TOC and clay content could be due to favorable preservation of organic matter in fine-grained siliciclastics, which may be delivered by rivers. Likewise, riverine input may also create more favorable oceanographic conditions for high spumellarian richness through addition of nutrients and/or optimization of salinity in an otherwise restricted basin. Additionally, accompanying influx of terrigenous organic matter would alter the signature of bulk organic matter and cause the d¹³C_org value change by introducing isotopically light d¹³C_org. Lighter d¹⁸O_carb may record the impact of meteoric water influence in beds rich in fine-grained siliciclastics. Alternatively, transport of material from the upper slope environment may also explain observed correlations and cannot be ruled out. To prove or reject this model, further research will be conducted on biomarkers and elementary geochemistry.