Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
APPENDIX 70 rence of benthic foraminifers after >700,000 yr of environmental exclusion from the benthic zone. There is a general decrease in benthic molluscan diversity. (10) 140,000-yr duration: Oceanic environments continued to show dynamic fluctuations as southern water masses expanded their rapid northward immigration, and global sea-level approached its highest Mesozoic-Cenozoic stand. The value of δ13Corg reached its first of four positive late Cenomanian peaks (-24.5%o), δ18O declined sharply, U-Th became enriched in benthic sediments, and TOC decreased rapidly to near zero after a million years at high levels, reflecting a major short-term oxygenation event in the benthic zone. Moderate increases in molluscan and planktic foraminiferal diversity, a major diversification among benthic foraminifers, and a final origination event among warm water molluscs, characterize this interval. Thus, conditions for life seemed highly favorable, with increasing benthic and pelagic diversity, just prior to the abrupt initiation of the C-T mass extinction interval during the next 50,000 yr interval. Late Cenomanian Mass Extinction High-resolution geochemical and paleobiological data suggest that, after the widespread elimination of reef ecosystems in the Caribbean Province by middle late Cenomanian time (Johnson and Kauffman, 1990), the last 520,000 yr of the Cenomanian was characterized by chaotic, short-term, large-scale perturbations in the ocean-climate system. The rate and magnitude of these changes exceeded the adaptive ranges of diverse, extinction-prone marine organisms, narrowly adapted to an equable, maritime-dominated, warm greenhouse world, near one of the highest peaks of Phanerozoic eustatic sea-level rise and global warming. These successive perturbations, most of less than 100,000-yr duration, caused steps of mass extinction that were ecologically graded, first affecting tropical to subtropical taxa, subsequently warm temperate lineages, and finally the cooler temperate and more cosmopolitan elements of marine ecosystems. These environmental perturbations, and resultant C-T extinction events, are as follows. (11) 50,000-yr duration: Stable isotope and TOC values are similar to (10) and atypical of late Cenomanian background conditions. This interval is characterized by the first of 11 successive phases of trace element enrichment (Ir, C, Ni, Sc peaks) above background levels; these are interpreted as a dramatic series of oceanic advection events associated with benthic touchdown of the oceanic oxygen minimum zone during OAE II, remobilization of sequestered trace elements in oxygen-deficient waters, and their reprecipitation on oxygenated substrates above the redoxcline. This trace element enrichment and the numerous volcanic ash falls changed marine chemistry and nutrient levels. Nutrient enhancement may have initially caused increases in diversity and abundance of planktic and benthic foraminifers, and molluscs. The initial regional step of the C-T mass extinction (MX1A), however, eliminated several subtropical to warm-temperate molluscan lineages, and was correlative with the first trace element enrichment peak, including iridium. Contouring of trace element values at numerous localities in the basin (Orth et al., 1993) shows a proto-Caribbean source, with values diminishing northward into Canada. A second extinction event, characterized by the abrupt loss of keeled rotaliporid foraminifers (PFX), occurs at the boundary between this and the succeeding interval, with initiation of a second zone of Ir and other trace element enrichment (see 12), and possible microtektite concentrations (in Colombia). (12) 80,000-yr duration: A second short-term trace element spike (C, Mn, Ni, Sc advection) characterizes this interval. An abrupt positive δ18O excursion is coupled with further drop in TOC values and continued active volcanism. Nutrient enrichment may have led to increases in benthic foraminifer diversity and abundance, but cannot explain decline in both molluscs and planktic foraminifers, reflecting the effects of the planktic foraminifer extinction (PFX) event (''Rotalipora extinction") at the base of the interval. (13) 40,000-yr duration: One of the most geochemically dynamic intervals of the C-T extinction, associated with high sea-level and active volcanism. Two successive trace element enrichment levels, the first characterized by Ir and C, the second by Ni and Sc spikes, may record a rapid trace element advection during downward expansion of OAE II to the seafloor. Modest negative excursions of δ13C and δ18O, and a small positive Corg spike, are associated with the trace element excursions, describing dynamic ocean-climate systems. Significant increases in abundance and diversity of molluscs, and planktic and benthic foraminifers, may reflect increased nutrient levels. A short-term extinction step (MX1B), during which some subtropical and warm temperate molluscs disappeared, was associated with iridium enrichment. (14) 20,000-yr duration: Rapid, large-scale geochemical fluctuations continue as perturbations of the ocean-climate system intensify. Two short-term trace element enrichment levels, with enhanced Ir, Pt, Sc and C followed by Mn and C enrichment, record additional trace element advection from the seafloor. A second major positive δ13C spike within the global δ13C excursion reaches maximum Cretaceous levels in this region (-24.2%o), and is associated with sharp positive followed by rapid negative δ18O
