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Evidence of an abrupt environmental disruption during the mid-Paleocene biotic event (Zumaia section, western Pyrenees)

Gilen Bernaola^,1, Juan Ignacio Baceta^,1, Xabier Orue-Etxebarria^#,1, Laia Alegret^*,2, Maite Martín-Rubio^,3, Javier Arostegui^##,4 and Jaume Dinarès-Turell^**,5

¹ Departamento de Estratigrafía y Paleontología, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado de correos 644, 48080 Bilbao, Spain
² Área de Estratigrafía, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, E-50009 Zaragoza, Spain
³ Departamento de Estratigrafía y Paleontología, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado de correos 644, 48080 Bilbao, Spain
⁴ Departamento de Mineralogía y Petrología, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apartado de correos 644, 48080 Bilbao, Spain
⁵ Istituto Nazionale di Geofisica e Vulcanologia, Via de Vigna Murata 605, 00143 Rome, Italy

An abrupt environmental disruption occurred in the photic zone and at the seafloor during the mid-Paleocene biotic event (MPBE). Calcareous nannoplankton, planktic foraminifer, and benthic foraminifer assemblages at Zumaia section (western Pyrenees) underwent a rapid and remarkable transformation. The major calcareous plankton assemblage changes suggest a shift from relatively cooler mesotrophic to warmer, more oligotrophic conditions, indicating a disturbed environment due to the warming of the ocean. Benthic foraminifer assemblages were also significantly affected by the MPBE; diversity of the assemblages and buliminids show net decline and the low food and opportunistic taxa increase in abundance. The reorganization of the planktic ecosystem possibly involved changes in the food flux (type and quantity) to the seafloor, thus triggering changes in the benthic communities.

A 1 negative ¹³C shift and a 30% carbonate content decrease are recorded in connection with the biotic event. This suggests that during the MPBE, as in the Paleocene-Eocene Thermal Maximum (PETM), an input of a large mass of isotopically depleted carbon into the ocean and atmosphere could have lowered the deep-sea pH, triggering a rapid shoaling of the lysocline and contributing to greenhouse warming.

The MPBE was short lived: according to the counting of limestonemarl couplets, the stratigraphic expression of precession cycles throughout the Zumaia section, the MPBE lasted for 52–53 k.y., with the core of the event representing 10–11 k.y.

The Zumaia section is the first land-based locality in which the MPBE is recognized and described in detail. Due to its expanded character and excellent paleontological record, this section may prove to be a global reference section for the study of this short-lived event.

Key Words: mid-Paleocene biotic event • calcareous nannofossils • planktic foraminifera • benthic foraminifera • Zumaia • Pyrenees

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