One of the largest known carbon isotope excursions in the Phanerozoic (up to +5% shift in (18C values) has been recently documented in early Late Cambrian marine carbonates in Australia, China, Kazakhstan and North America. and has been defined the SPICE event. In strong contrast to most coupled positive carbon isotope excursions and faunal crises documented in the Phanerozoic rock record, initiation of the SPICE event is interpreted to be synchronous with mass extinction of marine invertebrates rather than postdate the initiation of the carbon isotope excursion. This inferred causal relationship suggests that mass extinction was associated with the cause of the major change in carbon cycling. As a consequence of the PI's NSF-supported collaborative research with Drs. David Osleger and Jay Banner on integrated sequence stratigraphic and Sr isotope analysis of Middle to early Late Cambrian carbonates of North America, we have identified the basal portion of the SPICE event in several sections from the southern Great Basin and southern Canadian Rockies suggesting that the SPICE event has great potential as a chronostratigraphic tool of unprecedented resolution for global correlation. Significantly, however, the results of our study (1) define a relationship between inferred sea-level changes, marine 87Sr/86Sr and (18C values, and the trilobite extinction that differs significantly from that proposed by the discoverers of the SPICE event, and (2) implies that the Processes that produced the trilobite mass extinction. Additionally, our preliminary data suggest that a positive isotope excursion of similar magnitude to SPICE may be recorded in latest Lower Cambrian to early Middle Cambrian carbonates, and that this isotope excursion exhibits a relationship with marine 87Sr/86Sr values, sea-level and possibly an earlier biomere boundary that is similar to that defined by our study for the early SPICE event.
These initial observations, which are a direct outgrowth of the PI's currently NSF-funded research (EAR-9628382), indicate that a complex relationship exists between Cambrian biologic, chemical and physical phenomena, and lead to three research objectives: (1) to more fully address the nature of the temporal relationship between the SPICE event, the trilobite mass extinction, sea-level fluctuations, and inferred changes in seawater 87Sr/86Sr values during the late Middle to early Late Cambrian, (2) to test to what degree a positive carbon isotope excursion occurs in the latest Early Cambrian to early Middle Cambrian within a stratigraphic interval that records a similar magnitude sea-level fall to that associated with peak SPICE values and contains a biomere boundary at its base, and (3) to use different geochemical proxies to establish phase relationships and relative magnitude of physical, biologic and chemical 'events' relative to each other, as well as to critically evaluate and constrain the potential biogeochemical processes that could have linked these Cambrian physical, biologic and chemical phenomena. These objectives will be addressed by studying four localities in the southern Great Basin and two localities in the southern Canadian Rockies from which much of the preliminary data come, and for which the greatest amount of biostratigraphic control is available. The stratigraphic distribution of coupled stable ((13Ccarb and (13Corg) and radiogenic (87Sr/86Sr) isotope data will be coupled with trace element data in order to best accomplish the research objectives. This study should contribute significantly towards better constraining the nature of the biogeochemical processes linking Cambrian biologic, physical and chemical phenomena, as well as further developing the potential of Cambrian positive isotope excursion events as high-resolution chronostratigraphic tools for global correlation and for locating major subdivisions of the Cambrian Systems.
The proposed project contributes to the fulfillment of the PI's research, teaching and professional goals by providing her with additional 'hands-on' experience in unraveling complex paleoenvironmental records, and providing research experience and mentoring to promising women and/or minority undergraduate students. The proposed research incorporates several summer internships through potential cost-sharing with a DuPont Minority Educational Award; internships will be offered to promising women and/or minority undergraduates who will be recruited by the PI from geology programs throughout North America.
