The Benthonic foraminiferal response to changing substrate in Cenomanian (Cretaceous) rhythms induced by orbitally-forced surface water productivity

Abstract. The rhythmic sedimentation of Cenomanian age in Southern England is most likely the result of orbital forcing (precession). The oxygen isotope signature and the foraminiferal assemblage from detailed (5cms), continuous samples through three of these rhythms show systematic variation with lithology. The marls yield cooler surface water palaeotemepratures with high benthonic diversities, low planktonic:benthonic ratios and the planktonic assemblage contains around 5–10% rotaliporids. The chalks yield warmer surface water palaeotemperatures with low benthonic diversities, high planktonic:benthonic ratios and the planktonic assemblage contains 1–4% rotaliporids. This indicates that the chalks represent enhanced surface water productivity induced by increased surface water palaeotemperature. Three groups (Tritaxia pyramidata Reuss, Gavelinella spp. and Lenticulina spp.) together form over 90% of the benthonic assemblage. Each >250μm assemblage exhibits a systematic variation in size with lithology. The mean sizes are smallest and the maximum sizes of the moving populations decrease in the middle of the chalks. This may be a consequence of three different factors, (a) during intervals of increased productivity (chalks) the benthonic foraminifera may be achieving gametogenesis at an enhanced rate due to the increased availability of food supplies, or (b) the increased productivity may cause dwarfism within the micro-benthos, or (c) the effects of current winnowing prior to burial. The relative merits of each of these possibilities are discussed.