Annual layering in the Upper Jurassic Kimmeridge clay formation, UK, quantified using an ultra-high resolution SEM-EDX investigation

This paper reports the results of an ultra-high resolution investigation of the geochemistry of the Late Jurassic Kimmeridge Clay Formation (KCF) usin g scanning electron microscope/energy dispersive X-ray analysis (SEM-EDX). Sub-millimetre banding is revealed in which each band (approximately 112 mu m thick) comprises a couplet of coccolith-rich and coccolith-poor sediment. Major element analysis reveals that regularly-spaced peaks in the relative amount of calcium are associated with coccolith-rich bands. It is argued t hat each couplet represents an annual layer, the calcium peaks correspondin g to an annual bloom of coccolithophorids. There is also a broad correlatio n between the calcium/aluminium ratio and the ratios of sodium/aluminium, p hosphorus/aluminium and potassium/aluminium to calcium/aluminium ratios. We tentatively suggest that there may be a link here, sodium, phosphorous, an d potassium today being key nutrients for coccolith productivity. Aluminium is taken as representing the background terrigenous (clay) fraction. If ea ch banded coupler is indeed annual and if the thickness of such bands were to be representative of the KCF as a whole, then the post-compactional thic kness is broadly in agreement with sedimentation rates of 100 mum per year calculated by Oschmann (1990). This study confirms the view that the KCF in southern England accumulated in a predominantly anoxic environment that fa voured the preservation of organic material within the bottom sediment. Bun dles of bands, clustering in groups of about 8-10 are tentatively related t o short-term changes in insolation (possibly sun-spot cycles). (C) 2000 Els evier Science B.V. All rights reserved.