THE HERB EVENT: END OF THE CAMBRIAN CARBON CYCLE PARADIGM?

The Cambrian Period was marked by a series of large-scale excursions away from a slowly-evolving long-term trend in marine carbonate d¹³C values. The frequency of excursions diminished after the frenzy of the Early Cambrian, culminating in a broad couplet of positive and negative d¹³C excursions during the Late Cambrian. This Late Cambrian couplet occurs in conjunction with the last two of the classic Cambrian "biomeres", providing important insights into the linkages between trilobite evolution, sea level change, and the marine carbon cycle.

The Sunwaptan-aged HERB d¹³C event provides an intriguing level of insight into the Cambrian carbon cycle through the juxtaposition of key geologic factors. Identification of the event in Australia, Newfoundland, and western North America has definitively established the strong correlation between lowering d¹³C values and falling sea level. In North America, falling sea level during the mid-Sunwaptan exposed a broad carbonate platform that had been dominated by microbiolite activity. Weathering and erosion of this platform probably provided a major source of the net excess in organic carbon weathering needed to rapidly drive marine d¹³C towards strongly negative values. At the same time, the Ptychaspid biomere fauna was rapidly diversifying. This provides a direct contrast to the Steptoean-aged SPICE event, where Pterocephaliid faunas were diversifying rapidly during a dramatic positive d¹³C excursion. This strongly suggests that the "biomere" pattern of trilobite evolution may represent a synonymous response to different environmental factors.

No Early Ordovician analogues to the SPICE/HERB d¹³C couplet have been identified. Differences between the Ibexian-aged Symphysurina biomere and the classic Cambrian biomeres hints that the oceanographic conditions that permitted the couplet (and biomeres) to develop did not persist into the Early Ordovician. If so, then it is plausible to speculate that the unique nature of the Cambrian fauna was due in part to a fairly unique oceanographic regime as evidenced by the d¹³C record.