National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)

Chapter: End Of Sampling Interval

« Previous: Late Cenomanian Mass Extinction
Suggested Citation:"End Of Sampling Interval." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 71

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APPENDIX 71 excursions, and depletion of Corg to near zero. Benthic foraminifers show a significant diversity increase: a modest origination event (OM9) occurs among molluscs. The major effects of these geochemical perturbations are two successive, large-scale mass extinction events—a first among subtropical and warm temperate molluscan lineages (MX2), directly correlative with the Ir-rich level, and shortly thereafter (15), a major extinction among southern, warm water elements (MX3). (15) 20,000-yr duration: As in the stratigraphically lower sections (13, 14), rapid geochemical fluctuations in trace elements, stable isotopes, and Corg levels describe a highly perturbed, unstable ocean-climate system. Significant positive excursions occur in δ13C, δ18O, and Corg values, and a final major trace element enrichment layer (advection event) containing Ir, Pt, and C is directly associated with one of the largest warm temperate molluscan extinction events of the C-T interval (MX3). This is followed rapidly by a modest molluscan radiation (OM10), but the diversity and abundance of molluscs and benthic foraminifers decrease sharply through the interval. (16) 170,000-yr duration: Geochemical fluctuations wane, but a strong positive δ13C spike marks the last regionally correlative spike within the global positive δ13C interval. It is associated with sharp negative δ18O and Corg excursions, reduction in molluscan and planktic foraminifer diversity and abundance, and a moderately strong temperate molluscan extinction step (MX4). Highest Cenomanian Events (17) 140,000-yr duration: Dynamic changes in the oceanclimate system return, with very rapid positive to negative δ13C excursions, negative to positive δ18O excursions, very high global sea-level, and a moderate U, Th, and trace element enrichment level. The rapidity and large scale of these oceanographic fluctuations exceeded the adaptive ranges of a large diversity of warm temperate molluscan genera and species. This produced the largest (terminal) Cenomanian extinction event in the C-T mass extinction interval, predominantly among temperate and cosmopolitan ammonite and bivalve lineages (MX5), which disappeared at or just below the Cenomanian- Turonian boundary. Basal Turonian Events (18) 80,000-yr duration: The last extinction steps of the CT boundary interval occur in association with population expansion among surviving clades, and early radiations of new lineages in the basal Turonian (18-20 herein). This interval is characterized by active volcanism, a sharp negative excursion in δ13C, and rapid negative- positive-negative excursions in δ18O. Marked decline in molluscan abundance and diversity is followed shortly by the first Turonian origination event (OM12). Sea-level is nearly at its highest Mesozoic stand (>300 m above present stand). (19) 150,000-yr duration: Maximum eustatic highstand (>300 m above present stand) was accompanied by sharp increase, then decrease in δ18O values; a sharp decrease, then increase in TOC levels; and the final two trace element enrichment levels (Co, U, Th, followed by Mn, U, and V enrichment) of the C-T oceanic advection interval. A modest buildup in diversity and abundance among temperate molluscs was interrupted by a small extinction event (MX6). Final Phases Of The C-T Mass Extinction (20) 230,000-yr duration: Strong negative excursions of δ13C and TOC, and a strong positive excursion of δ18O, record the final major perturbations of the C-T boundary interval. Molluscan diversity and abundance fluctuate wildly within the interval, from low to high values. An important molluscan radiation (OM13) is sandwiched between a major nannoplankton extinction (NX) and a moderate temperate molluscan extinction step (MX7). Above this, extinction events no longer cancel out radiation events, and overall diversity builds gradually as ecosystems recover. Early Recovery Interval (21) 110,000-yr duration: Despite active volcanism and ash fall, sharp reductions in δ13C and δ18O values, and a major increase in TOC, the low diversity survivor and recovery faunas of the early Turonian remain fairly stable, with exceptionally low diversity levels noted only among benthic foraminfers. (22) 370,000-yr duration: The final phases of geochemical perturbations associated with the C-T mass extinction interval occur at this level. The δ13C and δ18O values show strong positive excursions, whereas TOC drops dramatically to near zero. Increases in abundance and diversity among molluscs reflect two major origination events (OM14, OM15) during broad early Turonian radiations. These are associated with the final small extinction step (MX8) of the 1.5- to 2-m.y. long C-T mass extinction interval including the paleotropics). This small extinction affects predominantly mid- to north-temperate and cosmopolitan molluscan lineages. End Of Sampling Interval

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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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