Paleoenvironmental conditions across the Santonian/Campanian boundary: Inferences by calcareous nannofossils from Kurdistan region, northeastern Iraq

Abstract

The Santonian/Campanian boundary interval (calcareous nannofossil zones CC16 to CC18) sediments in the Kurdistan region (northeastern Iraq) is analyzed for paleoenvironmental analysis. Based on the stratigraphic distribution of calcareous nannofossil taxa, four intervals were quantitatively identified, two within the Zone CC16 and two corresponding to CC17 and CC18, respectively. The assemblages are dominated by Watznaueria barnesiae (that forms 58% of the total population) and yielded a statistically significant and negative relationship with species diversity (Shannon Index) suggesting digenetic overprinting of the primary ecological signal. However, the presence of dissolution-susceptible taxa of Eiffellithus, Lucianorhabdus, Tranolitus, Lithastrinus and Zeugrhabdotus (albeit in low relative abundances), the ease of species-level identifications, the relatively high species diversity index (up to 2.6) and species richness (up to 22) suggests that the primary ecological signal is still preserved and can be used to infer the broad paleoenvironment. The entire studied interval is marked by a warm and oligotrophic water mass that is punctuated by cooler waters in the middle of Zone CC16 (Interval 2), beginning of zones CC17 (Interval 3) and CC18 (Interval 4; at the Santonian/Campanian boundary). These brief intervals are marked by increased relative abundances of cooler water species (Micula decussata, Eiffellithus turriseiffelii and Broinsonia enormis) and are interpreted as brief upwelling events associated by higher species diversity and species richness. As in the present study, globally also, the Santonian/Campanian boundary is marked by cooler waters.