The geochemistry of the Plenus Marls at Dover, England: evidence of fluctuating oceanographic conditions and of glacial control during the development of the Cenomanian–Turonian δ¹³C anomaly

Abstract

Abstract The sediments and organic calcite geochemistry (Fe 2+ , Mn 2+ , Sr, Mg, δ 18 O, δ 13 C) of the Plenus Marls at Dover (southeast England) reflect the changing oceanographic conditions associated with the development of the late Cenomanian-early Turonian S1SC excursion. The Plenus Marls of Dover consist of five marl-chalk couplets with a total thickness of up to 3 m. The sediments were deposited in an increasingly shallowing sea during a major regressional phase of the Chalk Sea over Europe. Each marl horizon was deposited in slightly cooler water of lower productivity and higher energy level than the overlying chalk, and represents a phase of enhanced regression. Inoceramus prisms and shell fragments from the marls have enhanced Mg values, whereas all the biogenic calcite from the chalks is enriched in Fe 2+ . The high clay content of the Plenus Marls probably originated from the post-depositional argillization of unstable silicate detritus (possibly of volcanoclastic origin) introduced into the regressing sea by rejuvenated river systems. The δ 18 O values of individual planktic and benthic foraminifera species have been lightened by diagenetic reaction; however they demonstrate (1) the presence of a constant temperature gradient in the water column with warmer surface waters relative to the bottom waters, and (2) the association of a marked temperature drop with the appearance of Jefferies' cold water occidental fauna and the development of a Mn 2+ anomaly (Bed 5) in the average calcite skeletons of all particle size fractions. The δ 13 C values of individual planktic and benthic foraminifera species show that a normal oceanic gradient existed except at two levels (Beds 3, 5), where this is reversed with heavier values shown by the benthic forms. The normal δ 13 C gradient steepened rapidly as the water shallowed and the δ 13 C excursion reached maximum values. Dysaerobic and anaerobic water conditions were completely absent and the level of oxygenation (macrofaunal diversity) increased in parallel with the shallowing and the enhanced δ 13 C excursion. Comparison with the Plenus Marls section at Flixton (east Yorkshire), which shows evidence of restricted faunas and deposition under anoxic conditions (black bands rich in terrestrial organic matter), demonstrates that between these two localities there is unequivocal correlation of the δ 13 C excursion and the temperature drop associated with Jefferies' cold water occidental fauna. Anomalous geochemical features of the Dover Plenus Marls are explained by the redeposition of Flixton-type coccolith-rich chalk (very low Fe 2+ content) and by the occasional introduction of cold northern bottom waters enriched in 13 C. The status of the Cenomanian-Turonian oceanic anoxic event and its supporting evidence is discussed and is considered to be untenable on present evidence. An alternative hypothesis based upon a glacial mechanism is put forward; this is supported by evidence of widespread regression, restricted ocean circulation, lower ocean temperatures, enhanced input of terrestrial organic matter, and the presence of dropstones.