Calcareous nannofossils and Milankovitch cycles: the example of the Albian Gault Clay Formation (southern England)
A quantitative study of calcareous nannofossil assemblages was carried out on the Albian Gault Clay Formation (England). This unit consists of grey marly claystones and marlstones with phosphatic nodules and glauconitic layers as minor lithologies. 
 
Two cores were analyzed in detail to focus on changes in nannofloral assemblages and consider their significance. Preservation of calcareous nannofossils is excellent and therefore diagenesis cannot be invoked to explain, not even partially, the changes registered in assemblages. 
 
Fast Fourier spectral analysis was performed on percentages of selected nannofossil species. Fluctuations in abundance of both Biscutum constans and Watznaueria barnesae, regarded as fertility and non-fertility indices, respectively, appear to be orbitally driven because spectral analysis showed periodicities of approximately 41 kyr (obliquity cycle), and 100 kyr (short eccentricity cycle). In all the spectra, the axial obliquityc cycle is a strong signal, as expected at these high latitude sites from the Boreal realm. The same pattern was observed in another fertility controlled taxon, Zygodiscus cf. erectus, but cross-correlation shows that its abundance plot is displaced with respect to that of B. constans. 
 
Orbital perturbations induce variations in climate and ocean circulation because they influence the Earth's insolation and seasonality. During times of high seasonality, strong gradients cause vigorous oceanic circulation, a better cycling of nutrients and higher fertility, whereas during times of low seasonality, low gradients induce more sluggish circulation and lower fertility. 
 
The carbonate content of the Gault Clay cores is not correlatable to the higher fertility indices, as the abundance curves of B. constans, W. barnesae, and Z. cf. erectus are displaced with respect to CaCO3 content variations. This also means that cyclicity in nannofossil assemblages is not a result of diagenesis but an original signal. 
 
Abundance fluctuations of Repagulum parvidentatum and combined Parhabdolithus asper/P. splendens, colder and warmer water indicators, respectively, provide evidence for orbital periodicities suggesting that also cyclic variations of surface water temperature might have influenced the nannofossil assemblage patterns within the Gault Clay.