National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)

Chapter: Late Cenomanian Background Conditions

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Suggested Citation:"Late Cenomanian Background Conditions." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
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Page 69

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APPENDIX 69 Appendix RATES, PATTERNS, AND TIMING OF THE C-T MASS EXTINCTION AND ASSOCIATED ENVIRONMENTAL PERTURBATIONS (SEE FIGURE 3.6) Late Cenomanian Background Conditions The extinction of Caribbean reef communities reached its peak prior to the events described below for the subtropical to temperate Western Interior Province. Precise correlation between the tropics and the north temperate realm is difficult at present. Oxygen isotope values are relative to the Pedee belemnite standard (PDB). (1) 150,000-yr duration: A moderate negative shift in δ13Corg to lowest Cenomanian values (-27.7%o); positive shifts in δ18O and total organic carbon (TOC; wt%) to average late Cenomanian values (-8%o and 3.5-4 wt%, respectively). Several moderately elevated peaks in molluscan diversity and abundance. Organic carbon storage occurs in the benthic zone of dysoxic stratified seas, but with dynamic, episodically oxygenated bottom waters. Falling sea-level culminates in a third-order sequence boundary at the top of this interval. (2) 50,000-yr duration: A moderate positive shift in δ13Corg to average late Cenomanian values (-26.5 to -27%o) a negative shift in δ18O values to a late Cenomanian low of -10.5%o, and major decrease in TOC to 1.2 wt%. An oxygenated interval in the benthic zone during early sea-level rise leads to moderate benthic molluscan diversity and sequestering of Mn in benthic sediments. A modest extinction among low-oxygen adapted benthic molluscs and ammonites is followed by an origination and/or immigration event among benthic molluscs adapted to somewhat higher oxygen levels. (3) 60,000-yr duration: Return of δ18O values to normal late Cenomanian values; a major increase in TOC to a late Cenomanian high of 4.5 wt%, and sharp increase in U-Th values, both indicating strong benthic oxygen restriction and a stratified water column during late sea-level rise. High surface productivity leads to organic carbon storage. Despite this, a significant origination event occurs, especially among low-oxygen adapted benthic molluscs and pelagic ammonites, leading to a modest increase in molluscan diversity. (4) 120,000-yr duration: Relatively stable oceanographic conditions associated with third-order sea-level highstand and active volcanism in the basin. Organic carbon levels remain very high, reflecting a stratified, dysoxic middle to lower water column. An origination interval among mainly benthic molluscs leads to moderate abundance and diversity values. (5) 50,000-yr duration: Two abrupt, major trace element (Ti, V, U) enrichment events occur against otherwise stable background geochemistry during a modest third-order sea-level fall. These trace element spikes are associated with major short-term increases in molluscan abundance and diversity, and a moderate origination event, mainly among benthic bivalves. Biological trends possibly reflect elevated nutrient levels. (6) 30,000-yr duration: Stable oceanographic background conditions are associated with late phases of a third-order relative sea-level fall and a molluscan origination event. A third-order sequence boundary, reflecting maximum relative sea-level fall, caps the sequence. (7) 90,000-yr duration: Relatively stable oceanographic background conditions during early sea-level rise are associated with active explosive volcanism and ash falls, and with an abrupt U-Th enrichment. Significant increases in molluscan diversity and abundance may reflect nutrient enrichment. (8) 90,000-yr duration: The final stable phase of oceanographic background conditions prior to the C-T perturbed interval is associated with active explosive volcanism; numerous ash falls: and successive, short-term, trace element enrichment events (Mn-Co followed by U). An unexplained decrease in molluscan diversity and abundance (extinction?) is followed by a modest origination event. Planktonic foraminifers show a marked diversity increase (10-11 species) after a long period of stable background levels. Lowering of relative sea-level continues. (9) 220,000-yr duration: Initiation of the geochemically perturbed interval of the C-T boundary sequence, 880,000 yr below the boundary, is associated with increased volcanism and rapid northward immigration of southern warm water masses; major positive excursions of δ13Corg (initiating a global oceanographic event), δ18O, and TOC reflect a trend toward normalization of marine surface waters in the basin, but significant stratification of the underlying water column, lowering of benthic oxygen levels, and possible immigration and expansion of the oceanic oxygen-minimum zone. These events mark initiation of the global oceanic anoxic event (OAE II) that characterizes the C-T boundary interval worldwide. They are associated with rapid northward immigration of warm-temperate to subtropical molluscs and microbiotas from Gulf Coast and Caribbean sources, a marked origination event among shallow water benthic and pelagic lineages, and the first reoccur

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What can we expect as global change progresses? Will there be thresholds that trigger sudden shifts in environmental conditions—or that cause catastrophic destruction of life?

Effects of Past Global Change on Life explores what earth scientists are learning about the impact of large-scale environmental changes on ancient life—and how these findings may help us resolve today's environmental controversies.

Leading authorities discuss historical climate trends and what can be learned from the mass extinctions and other critical periods about the rise and fall of plant and animal species in response to global change. The volume develops a picture of how environmental change has closed some evolutionary doors while opening others—including profound effects on the early members of the human family.

An expert panel offers specific recommendations on expanding research and improving investigative tools—and targets historical periods and geological and biological patterns with the most promise of shedding light on future developments.

This readable and informative book will be of special interest to professionals in the earth sciences and the environmental community as well as concerned policymakers.

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