SANTONIAN-CAMPANIAN PLANKTONIC FORAMINIFERAL BIOSTRATIGRAPHY, TAXONOMY AND ISOTOPE PALEOECOLOGY.

Current biozonations based on Santonian-Campanian (Late Cretaceous) planktonic foraminiferal bioevents do not provide a high stratigraphic resolution, when compared to other time intervals. Moreover, some of the marker species of the Tethyan biozonation show a diachronous lowest occurrence across latitudes and a restricted geographic distribution. Thus, paleoclimatic and paleoceanographic inferences are poorly constrained by biostratigraphy and, in addition, are often based on diagenetically altered material. This study is aimed to 1) improve the biostratigraphic resolution of Santonian-Campanian zonal schemes based on planktonic foraminiferal bioevents; 2) reconstruct the evolutionary history of key biostratigraphic species and revise the taxonomy of problematic groups (planispiral and globigeriniform taxa) often overlooked in the literature; 3) infer the paleoecological preferences of selected planktonic foraminiferal species based on stable isotope analyses, and 4) reconstruct the paleoclimatic context of the Santonian-Campanian time interval. The Santonian-Campanian samples analyzed in this study were selected from four different localities: 1) Tanzania - western Indian Ocean (Tanzanian Drilling Project - TDP Sites 23, 28, 32 and 35); 2) Shatsky Rise - northwestern Pacific Ocean (Ocean Drilling Program - ODP Leg 198, Hole 1210B); 3) Exmouth Plateau - eastern Indian Ocean (ODP Leg 122, Hole 762C); 4) Bottaccione section outcropping in the Umbro-Marchigian Apennines near Gubbio (Italy). Samples were prepared following the standard methodologies and considering the different lithologies. A scanning electron microscope (SEM) was used to document the species, observe and describe the wall texture of the specimens at higher magnification. Cathodoluminescence analyses on selected specimens were applied to assess the degree of wall alteration. Well-preserved specimens were finally analyzed to obtain reliable δ18O and δ13C values and provide paleotemperature estimations. Results allow description of the mid Campanian Contusotruncana plummerae Zone, defined as the stratigraphic interval between the appearance of the nominate species and of Radotruncana calcarata and suggest that Globotruncanita atlantica is a promising marker species useful to correlate early Campanian stratigraphic sections especially at low latitudes. In addition, several bioevents (i.e., the lowest occurrence of Contusotruncana morozovae, Rugoglobigerina hexacamerata, Radotruncana subspinosa and the extinction of Hendersonites carinatus) also appear reliable for detailed correlations across latitudes. Stable isotope results suggest that Santonian Marginotruncanids behave as spring/autumn dwellers in the surface water column. During the Campanian, C. plummerae, Contusotruncana fornicata and Globotruncana bulloides inhabited the summer mixed layer, whereas Globotruncanita stuartiformis and Globotruncanita elevata proliferate in the cool winter sea surface. Globotruncana orientalis, Globotruncana mariei, Globotruncana arca and Gublerina rajagopalani were more adapted to stable and deeper environments close to the permanent thermocline. Interestingly, data show that Heterohelix sphenoides and C. plummerae were likely adapted to particularly 13C-enriched environments. High δ13C values have been interpreted as the isotopic signal indicating a symbiotic relationship between these species and photoautotrophic algae. However, results show that this kind of relationship is not supported by clear evidences and should be investigated by further and more detailed isotopic analyses. Paleoclimatic reconstructions inferred from Tanzanian pristinely preserved planktonic foraminifera suggest that Santonian-Campanian subtropical sea-surface temperatures reached 30°C, hence, 4° to 5°C warmer than modern mean values. Oxygen isotopic values measured on planktonic foraminifera from Shatsky Rise and Exmouth Plateau indicate up to 10°C cooler and 20°C warmer temperatures, respectively, when compared to modern mean values. These results would support the “cool tropic paradox” hypothesis, that Late Cretaceous equatorial and tropical sea surface temperatures were several degrees lower than modern mean values. This hypothesis implies a different climatic scenario with respect to the traditional interpretation of the greenhouse world that would predict an Earth global warming. However, the observed secondary recrystallization of planktonic foraminiferal shells from Shatsky Rise and Exmouth Plateau led to an overprinting of the original isotopic signal and thus to erroneous paleotemperature estimations, whose extent is, however, still unknown. Taxonomic analyses of well preserved and diversified planktonic foraminiferal assemblages allowed description of one new species (Globotruncana neotricarinata) and identification of three new morphotypes (Globigerinelloides n. sp. 1, Muricohedbergella n. sp. 1 and Costellagerina n. sp. 1) that are surely new species. The formal description of these taxa is in progress. Stratophenetic observations and isotopic analyses on Rugoglobigerina species indicate that the development of the meridional ornamentation represents an adaptation to different ecological parameters of the water masses. This study suggests that more reliable diagnostic criteria, other than the development of the ornamentation, should be applied to discriminate among Costellagerina, Rugoglobigerina, and Archaeoglobigerina.