Orbitally driven nannoplankton evidence of surface ocean cooling and productivity in the Late Cretaceous greenhouse world

The Mid-Cenomanian Event (MCE) was a period of significant climatic and oceanographic perturbation, marked by a global carbon isotope excursion (CIE) and ecological shifts in marine systems. Current evidence from macrofauna and neodymium isotopes highlights increased Boreal influences in northwest European epicontinental seas during this interval. Here we use calcareous nannoplankton from the Lydden Spout section (Dover, east Kent, United Kingdom) to reconstruct the surface water conditions and evaluate their response to MCE. We found a notable increase in the high productivity taxa Biscutum constans and small Zeugrhabdotus spp., coinciding with the double-peaked CIE that characterises the MCE. The most significant changes in the productivity and stratification indices also occur at these CIE levels, suggesting enhanced nutrient availability, possibly driven by intensified upwelling or terrestrial nutrient runoff. A relative decline in the generalist taxon Watznaureia barnesiae underscores reduced water column stratification. This change in surface water nutrient dynamics and stratification is also evident in planktonic foraminifera, which show the disappearance of thermocline-dwelling oligotrophic rotaliporids for the duration of the CIE and emergence of meso-eutrophic taxa (Petrizzo and Gale, 2023).

The nannofossil temperature indices indicate transient warming maxima immediately preceding and midway through the MCE (between the two CIE peaks), a short cooling interval just above the MCE onset and a step down to cooler conditions from the upper MCE.  The cold-water species Repagulum parvidentatum increased to higher levels and then peak values coincident with the two CIE maxima, and a second cold water/high latitude specialist, Seribiscutum primitivum has a consistent first appearance coincident with the lower CIE peak and persists until just after the event termination. This surface water cooling is also indicated by bulk sediment δ¹⁸O records showing episodes of cooler sea surface temperatures (SSTs; Petrizzo and Gale, 2023), which coincide with precession-paced high-amplitude cooling cycles during the low to mid-MCE. Additionally, the occurrences of Boreal-affiliated macrofossil taxa, such as the belemnite Praectinocamax and the bivalves Chlamys arlesiensis and Oxytoma seminudum, are interpreted as marking the incursion of cooler Boreal waters into the proto-Atlantic during this interval (Gale and Kennedy, 2022).

Our analysis suggests that orbital forcing, and particularly precession and eccentricity cycles, played a significant role in modulating climatic and oceanographic conditions during the MCE. Orbital forcing, particularly 100 kyr and 405 kyr eccentricity cycles, modulated carbon cycle, climatic, and ecological changes, with precession-driven variations influencing terrigenous input and productivity. The interplay between orbital forcing and nannoplankton diversity further supports the influence of orbital pacing on the carbon cycle. The 405 kyr eccentricity maxima correspond with minima in nannoplankton diversity and communities indicative of cooler SSTs and higher productivity.  This suggests that eccentricity maxima were associated with enhanced nutrient availability, fostering increased primary productivity and communities dominated by fewer opportunistic taxa, resulting in lower overall diversity. These findings highlight the complexity of climatic and oceanographic dynamics during the MCE, revealing transient cooling episodes that disrupt the predominantly warm conditions of the Cenomanian.