Biotic signals from nannoflora across the iridium anomaly in the upper Eocene of the Massignano section: evidence from statistical analysis

Strong evidence for a major late Eocene impact, including an iridium-rich l ayer and shocked quartz, has been found in the Massignano section (central Italy), stratotype of the Eocene/Oligocene boundary. Quantitative calcareou s nannofossils analyses have been performed and classical and multivariate statistical techniques have been applied to assess the paleonvironmental ef fect of this impact on the nannoflora. Abundance patterns indicate: (1) a w arming episode just above the iridium layer; (2) a subsequent slight coolin g trend, probably related to the global cooling during late Eocene to early Oligocene; (3) marked fluctuations in the trophic resources and an increas e in near-shore taxa in the upper part of the interval; (4) increase in low -nutrient taxa in biotite-rich layers, just above the iridium level, possib ly related to lowered productivity linked to volcanic activity. All these s ignals appear most likely to be the result of long-term climatic changes ra ther than to short-term effects of extraterrestrial impact. In addition, du ster analysis was performed on the assemblage data. This delineated similar ities among different taxa and grouped samples with similar assemblages. Se veral groups related to temperature-, trophic- and unknown-preference condi tions have been distinguished. A consistent difference in the abundances of the taxa and in their correlation has been detected between samples above and below the iridium level. This change appears to be related to two impor tant sedimentary features which are found in the interval above the iridium occurrence: (1) the presence of biotite-rich layers linked to increasing v olcanism and (2) a conspicuous color transition from reddish to greenish re lated to changes in sedimentary environments (e.g. variations in trace elem ent abundance). These environmental changes rather than the impact may have affected the nannoflora assemblages. (C) 2000 Elsevier Science B.V. All ri ghts reserved.