PALEOCEANOGRAPHIC CHANGES ACROSS THE MIDDLE CENOMANIAN CARBON-ISOTOPE EXCURSION (MCE 1) IN SOUTH-EAST ENGLAND, UK

Planktonic foraminifera were studied across the Mid-Cenomanian Event 1 (MCE 1) at Lydden Spout, near Folkestone (Kent, south-east England, UK), the reference section of the MCE 1 characterized by a prominent double-peak δ¹³C excursion of 1‰ identified in different ocean basins and considered a global event. Biostratigraphic and quantitative analysis of planktonic foraminifera are correlated to the carbon cycle perturbation that identifies the MCE 1, to the positive δ¹⁸O shifts identified within the MCE 1, and to the occurrence of Boreal macrofossils (Chlamys arlesiensis, Oxytoma seminudum, and Praectinocamax primus).

Planktonic foraminifera population show moderate preservation and is mainly composed by small-sized specimens (< 250 µm), whereas large-sized specimens become more common up-section after the termination of MCE 1. The stratigraphic interval studied is assigned to the Thalmanninella greenhornensis and Rotalipora cushmani Zones.

Variations in abundance and species richness of planktonic foraminifera are correlated with the known paleoecological preferences of taxa and permit the identification of distinct paleoenvironmental settings. The MCE 1 interval is characterized by the absence of single keeled oligotrophic rotaliporids, by the evolutionary appearance of double keeled meso-eutrophic dicarinellids, and by the appearance of Muricohedbergella portsdownensis.

The species is interpreted as a cold-water taxon that first appears at the same level of Boreal macrofossils and correlates with a positive δ¹⁸O shift of bulk carbonate within the lower part of MCE 1 that translates into a decrease in paleotemperature of 4°C in the surface waters. These observations point to a paleoceanographic scenario characterized by reduced stratification of surface waters and absence/disruption of the thermocline in a dominantly eutrophic regime during MCE 1.

Evidence provided by planktonic foraminifera, Boreal macrofossils and δ¹⁸O records documented for the late Cenomanian Plenus Cold Event (PCE) at Eastbourne (UK) reveal similarities with the MCE 1 that confirm the periodic inflow in the Anglo-Paris Basin of cold Boreal seawater originating in the Norwegian Sea. The southerly extension of this water mass may be related to the cyclic re-organization of circulation driven by the long eccentricity cycle.