INVESTIGATING THE CAUSES FOR PERTURBATIONS IN THE GLOBAL CYCLING OF CARBON: INSIGHTS FROM STRONTIUM ISOTOPES

In the southern Appalachians, USA, the maximum Steptoean (Late Cambrian) positive carbon-isotope excursion (4-5‰ VPDB) is recorded in non-fossiliferous carbonate strata with sandy siliciclastic detritus deposited during a late Steptoean sea-level fall. Positive correlation between d¹³C and ⁸⁷Sr/⁸⁶Sr records might indicate that enhanced continental erosion and burial of organic matter during this sea-level fall influenced the cycling of carbon.

The variable and elevated pre-excursion ⁸⁷Sr/⁸⁶Sr values reflect the diagenesis of limestone in the presence of shale. The start of the excursion and a decreased amount of shale upsection are associated with low and consistent ⁸⁷Sr/⁸⁶Sr values indicative of seawater composition. The maximum excursion is accompanied by an increase in ⁸⁷Sr/⁸⁶Sr values, representing the delayed Sr-response to a common forcing factor and/or diagenesis in the presence of siliciclastic detritus. The end of the excursion is associated with scattered ⁸⁷Sr/⁸⁶Sr values, which reflect diagenetic alteration and the presence of dolomite. A concurrent decrease in d¹³C and ⁸⁷Sr/⁸⁶Sr values, without the predicted delayed Sr-response, is suggested by the lowest ⁸⁷Sr/⁸⁶Sr values.

Determination of seawater ⁸⁷Sr/⁸⁶Sr variations and their relationship to the d¹³C record was complicated by diagenesis. Carbonate diagenesis and ⁸⁷Sr/⁸⁶Sr signatures were influenced by a long-term Late Cambrian sea-level fall, which led to a change from shale- to limestone- to dolomite-dominated strata, and by the short-term Steptoean sea-level fall, which caused the deposition of siliciclastic detritus. Enhanced continental erosion and burial of organic material during the Steptoean sea-level fall was not likely the main cause for the carbon-isotope excursion. The onset of the excursion during high sea-level may reflect enhanced preservation of organic material by marine anoxia, whereas the relatively rapid end of the excursion may indicate an oceanic overturn. Thus, the Steptoean perturbations in global carbon cycling were likely controlled by changes in oceanic circulation patterns.