NO DICE? INCONSISTENCIES WITH THE CAMBRIAN DRUMIAN CARBON ISOTOPE EXCURSION AT THE DRUMIAN GSSP, WEST-CENTRAL UTAH

The International Commission on Stratigraphy defines the base of the Cambrian Drumian Stage at Stratotype Ridge in Utah’s Drum Mountains with the first appearance of the cosmopolitan trilobite Ptychagnostus atavus in a conspicuous calcisiltite unit. Situated ~10m above the Drumian Stage boundary is a -2.5‰ carbon isotope (δ¹³C; VPDB) excursion in carbonate, which is used to locate the Drumian Stage boundary in the absence of age-diagnostic fauna. However, previous research at Stratotype Ridge has defined the DICE with only 4 data points over a <3m interval.

To refine the DICE record at the type section for the Drumian Stage, we sampled carbonates at the Stratotype Ridge section at a centimeter-scale interval for δ¹³C_carb analysis. With substantial calcite veining and small-scale faulting present in the section, we conducted δ¹³C_carb analysis on calcite veins. Lastly, we measured organic carbon (δ¹³C_org) from interbedded black shale within the same interval and paired those values with the carbonate and calcite δ¹³C_carb values, an approach used in other global DICE sections.

Results highlight that the DICE at the GSSP is a complex geochemical signature that does not solely reflect primary ocean chemistry. δ¹³C_carb values of carbonates range from 1‰ to -6‰. Vein δ¹³C_carb values range from 0‰ to -6‰, mirroring the values recorded in the carbonates. Black shale δ¹³C_org values range from -25‰ to -29‰ and do not co-vary with either carbonate or vein δ¹³C_carb values. The similarity between carbonate and vein δ¹³C_carb data and decoupling of δ¹³C_carb and δ¹³C_org suggest diagenetic overprinting of primary δ¹³C_carb values in the DICE interval. Considering the DICE defined at the Drumian GSSP shows complicated structural and diagenetic overprinting, we urge caution using it and other small-scale negative δ¹³C excursions as a chronostratigraphic correlation tool for the Miaolingian in the absence of age-diagnostic fauna or other GSSP proxies.