Recovery of marine primary producers after the Cretaceous–Tertiary mass extinction: Paleocene calcareous red algae from the Iberian Peninsula
Abstract Calcareous red algae (corallines, ‘solenoporaceans’ and peyssonneliaceans) are major contributors to Paleocene platform deposits in the Pyrenean Basin (N Spain) and Prebetic Zone (Betic Cordillera, SE Spain). The earliest Danian–middle Thanetian time interval in the Pyrenean Basin is represented by four depositional sequences, the Ma-Da (earliest Danian), Da-1 (early Danian), Da-2 (late Danian), and Th-1 (Selandian–mid Thanetian) sequences, from bottom to top. This precise chronostratigraphic framework allows an accurate assessment of the recovery of these calcareous algal groups after the Cretaceous–Tertiary (K/T) mass extinction. Calcareous algae are recorded shortly after the K/T boundary (∼ 0.2–0.3 Ma after the K/T), although the richness of identifiable species remains low during the Danian: 9 species in the earliest Danian, 9 species in the early Danian, and 11 species in the late Danian. In addition, no change in taxonomic composition is observed in this interval since approximately the same algal species occur in all three Danian sequences. The same low species richness (11 species) and species composition is observed in the Danian algal assemblages of the Betic Cordillera. However, this low level of diversification changes radically in the Selandian–mid Thanetian interval, when 29 species are recorded. Together with the increase in species richness, the taxonomic composition also changes, since 19 species of corallines are recorded for the first time in the Th-1 sequence. Therefore, it can be deduced that species diversity recovery due to a significant radiation of corallines was delayed ∼ 6 Ma after the end-Cretaceous event. The recovery of corallines during the Paleocene shows a characteristic depth-dependent pattern. Throughout the Danian–middle Thanetian interval, representatives of the family Hapalidiaceae subfamily Melobesioideae and plants of the family Sporolithaceae largely dominate the algal assemblages in the deep-water, outer-platform settings, as they do in present-day seas. Members of these two groups are also the most abundant coralline algae in the Paleocene reef and back-reef paleoenvironments, indicating that the shallowest and best-illuminated areas were occupied by algal groups that in modern tropical seas mainly occur in relatively deep habitats. This suggests that all platform habitats were re-occupied after the end-Cretaceous event by the groups that selectively survived the extinction. Only after a time interval of about 6 Ma did the habitat distribution of coralline groups change to its modern patterns. The subfamilies most abundant in present-day shallow tropical waters, that is, Mastophoroideae and Lithophylloideae, for the first time account for more than 1% of the algal assemblages in the Thanetian and even then they remain very minor components. These shallow-water coralline groups were affected more intensively by the K/T extinction event, accounting for the delay in their recovery in their preferred shallow-water habitats. Shallow-water lithophylloids and geniculate corallines have not been found in Danian sequences.