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).
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 d13C, d18O, 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.
170,000-yr duration: Geochemical fluctuations wane, but a strong positive d13C spike marks the last regionally correlative spike within the global positive d13C interval. It is associated with sharp negative d18O and Corg excursions, reduction in molluscan and planktic foraminifer diversity and abundance, and a moderately strong temperate molluscan extinction step (MX4).
140,000-yr duration: Dynamic changes in the oceanclimate system return, with very rapid positive to negative d13C excursions, negative to positive d18O 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.
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 d13C, and rapid negative-positive-negative excursions in d18O. 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).
150,000-yr duration: Maximum eustatic highstand (>300 m above present stand) was accompanied by sharp increase, then decrease in d18O 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).
230,000-yr duration: Strong negative excursions of d13C and TOC, and a strong positive excursion of d18O, 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.
110,000-yr duration: Despite active volcanism and ash fall, sharp reductions in d13C and d18O 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.
370,000-yr duration: The final phases of geochemical perturbations associated with the C-T mass extinction interval occur at this level. The d13C and d18O 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.