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

Abstract

Strong evidence for a major late Eocene impact, including an iridium-rich layer and shocked quartz, has been found in the Massignano section (central Italy), stratotype of the Eocene/Oligocene boundary. Quantitative calcareous nannofossils analyses have been performed and classical and multivariate statistical techniques have been applied to assess the paleonvironmental effect of this impact on the nannoflora.Abundance patterns indicate: (1) a warming episode just above the iridium layer; (2) a subsequent slight cooling trend, probably related to the global cooling during late Eocene to early Oligocene; (3) marked fluctuations in the trophic resources and an increase 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, possibly related to lowered productivity linked to volcanic activity. All these signals appear most likely to be the result of long-term climatic changes rather than to short-term effects of extraterrestrial impact.In addition, cluster analysis was performed on the assemblage data. This delineated similarities among different taxa and grouped samples with similar assemblages. Several groups related to temperature-, trophic- and unknown-preference conditions 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 important sedimentary features which are found in the interval above the iridium occurrence: (1) the presence of biotite-rich layers linked to increasing volcanism and (2) a conspicuous color transition from reddish to greenish related to changes in sedimentary environments (e.g. variations in trace element abundance). These environmental changes rather than the impact may have affected the nannoflora assemblages.