David H. Keen

A chronological framework for the British Quaternary based on Bithynia opercula

Marine and ice-core records show that the Earth has experienced a succession of glacials and interglacials during the Quaternary (last ∼2.6 million years), although it is often difficult to correlate fragmentary terrestrial records with specific cycles. Aminostratigraphy is a method potentially able to link terrestrial sequences to the marine isotope stages (MIS) of the deep-sea record. We have used new methods of extraction and analysis of amino acids, preserved within the calcitic opercula of the freshwater gastropod Bithynia, to provide the most comprehensive data set for the British Pleistocene based on a single dating technique. A total of 470 opercula from 74 sites spanning the entire Quaternary are ranked in order of relative age based on the extent of protein degradation, using aspartic acid/asparagine (Asx), glutamic acid/glutamine (Glx), serine (Ser), alanine (Ala) and valine (Val). This new aminostratigraphy is consistent with the stratigraphical relationships of stratotypes, sites with independent geochronology, biostratigraphy and terrace stratigraphy. The method corroborates the existence of four interglacial stages between the Anglian (MIS 12) and the Holocene in the terrestrial succession. It establishes human occupation of Britain in most interglacial stages after MIS 15, but supports the notion of human absence during the Last Interglacial (MIS 5e). Suspicions that the treeless ‘optimum of the Upton Warren interstadial’ at Isleworth pre-dates MIS 3 are confirmed. This new aminostratigraphy provides a robust framework against which climatic, biostratigraphical and archaeological models can be tested.

An aminostratigraphy for the British Quaternary based on Bithynia opercula

Aminostratigraphies of Quaternary non-marine deposits in Europe have been previously based on the racemization of a single amino acid in aragonitic shells from land and freshwater molluscs. The value of analysing multiple amino acids from the opercula of the freshwater gastropod Bithynia, which are composed of calcite, has been demonstrated. The protocol used for the isolation of intra-crystalline proteins from shells has been applied to these calcitic opercula, which have been shown to more closely approximate a closed system for indigenous protein residues. Original amino acids are even preserved in bithyniid opercula from the Eocene, showing persistence of indigenous organics for over 30 million years. Geochronological data from opercula are superior to those from shells in two respects: first, in showing less natural variability, and second, in the far better preservation of the intra-crystalline proteins, possibly resulting from the greater stability of calcite. These features allow greater temporal resolution and an extension of the dating range beyond the early Middle Pleistocene. Here we provide full details of the analyses for 480 samples from 100 horizons (75 sites), ranging from Late Pliocene to modern. These show that the dating technique is applicable to the entire Quaternary. Data are provided from all the stratotypes from British stages to have yielded opercula, which are shown to be clearly separable using this revised method. Further checks on the data are provided by reference to other type-sites for different stages (including some not formally defined). Additional tests are provided by sites with independent geochronology, or which can be associated with a terrace stratigraphy or biostratigraphy. This new aminostratigraphy for the non-marine Quaternary deposits of southern Britain provides a framework for understanding the regional geological and archaeological record. Comparison with reference to sites yielding independent geochronology, in combination with other lines of evidence, allows tentative correlation with the marine oxygen isotope record.

Age-estimate evidence for Middle-Late Pleistocene aggradation of River Nene 1st Terrace deposits at Whittlesey, eastern England

At Whittlesey, eastern England, Pleistocene interglacial sediments (unit G3) deposited in marine oxygen isotope stage (MIS) 7 appear to be truncated by a sequence comprising: (i) a limestone-rich gravel containing organic mud beds (unit F1); (ii) vertically aggrading gravel beds with sand-clay-lined bases (unit F2); and (iii) interbedded sands and gravels (units F4–F6) with associated overbank deposits (unit G4). Preliminary investigations of the floral and faunal assemblages of the organic muds were consistent in providing evidence for deposition under cool conditions. This apparent single cool/cold-phase sequence therefore could have been deposited in either MIS 6 or MIS 5d−2. The presence of sand beds in units F5, F6 and G4 and of molluscs in the organic mud beds of unit F1 provided the opportunity for obtaining age estimates using optically stimulated luminescence (OSL) and amino acid racemization (AAR). Rather than a single cool/cold-phase sequence the age-estimate data revealed multiphase aggradation, with the AAR data indicating the possibility that the organic muds in unit F1 were deposited earlier in MIS 7 than the interglacial deposits of unit G3. Therefore, the succession could be: unit F1 was truncated initially by unit G3, with unit F2 subsequently incising both unit F1 and unit G3. The OSL age estimates indicate that units F5, F6 and G4 are Early to Middle Devensian in age (MIS 5d-2), and therefore unit F2 was deposited sometime between late MIS 7 and MIS 5d. This paper has demonstrated the utility of using dual age-estimate techniques in dating complex fluvial sequences. Furthermore, the organic mud beds of unit F1 could provide important new information on the complex character of the MIS 7 interglacial. In addition, the OSL data and the fluvial style recorded by units F4–F6 and G4 allow comparisons to be made with recent investigations of nearby Devensian deposits.

Chironomid responses (Insect: Diptera) to Younger Dryas and Holocene environmental changes in a river floodplain from northern France (St-Momelin, St-Omer basin)

Lateglacial to Holocene subfossil chironomids from the St-Omer basin at St-Momelin (northern France) were analysed. At the end of the Lateglacial, stratigraphic and chironomid sequences indicate a shallow, oligotrophic and slow-flowing freshwater stream, with abundant aquatic macrophytic vegetation. Cold-water adapted taxa indicate a cold climate. During the early Holocene, from about 9500 to 7700 conventional radiocarbon yr BP, climate improvement is marked by an increase in warm-water adapted taxa and a decrease in cold-water fauna. However, the cold-water fauna persist, probably because of both the instability and continentality of the climate. During the mid Holocene, from about 7700 to 6000 BP, the marine transgression strongly influenced the rivers hydrological regime and consequently the chironomid assemblages. At that time, eutrophic, warm and shallow stagnant-water conditions occured throughout the basin. From 6000 to 3200 BP, the St-Momelin basin was subjected to a marine regression. Chironomids indicate a hydrological regime marked by an alternance of phases with either a dominance of rheophilous or limnophilous fauna. During this period, equivalent percentages of warm- and cold-adapted taxa may be explained by cold-water input in context of a temperate climate.