Aminostratigraphy of the Netherlands.
Correlations & Implications

T. Meijer & P. Cleveringa
TNO-NITG - National Geological Survey of the Netherlands

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INTRODUCTION
Amino Acid Razemisation (= AAR) in calcareous fossils provides an independent dating method applicable in Quaternary deposits with a maximum age of c. 500 Ka. In the method the remaining L-isomere resulting from the decay of proteins included in the skeleton is a relative measure of age. Results are expressed as D/L ratios.
Amino acids are present between (=intracrystalline), as well as included in (=intercrystalline) the calcium carbonate crystals of the shell. Contrary to intercrystalline organic material, intracrystalline organic material may interact with agents from outside the shell. Therefore, intracrystalline amino acids may be contaminated with younger amino acids arriving from outside the shell, but these (original as well as contaminated material) may also be oxidized resulting in less amino acids. Obviously, several D/L ratios in a sample may show a certain degree of scatter. If so, the highest ratios are considered to represent the most reliable data and highly differing low ratios should therefore be excluded from the mean.
Analyses were performed at the laboratories of Aberystwyth/Cardiff (UK) and INSTAAR (Boulder, USA) according to standard methods. Material was submitted without providing the stratigraphy.
From 61 Dutch sites molluscan shells, mostly originating from Middle and Late Pleistocene marine and fluvial deposits, were analyzed for their AAR. Because of species dependency of results 4 standard species were used (Valvata piscinalis, Corbicula fluminalis, Macoma balthica, Spisula subtruncata). Sample size is 5 shells on average. If present, more than one species was analyzed from a single sample (multi species samples = MSS). MSS analyses provide an opportunity to link the individual data sets.
Frequency counts according to size classes of 0.020 were applied to each of the four data sets of mean D/L ratios. Results are presented in Fig. 1.



RESULTS
Because the standard species are characteristic of a temperate climate and hence are absent during cold phases, accumulations of D/L ratios, appearing as peaks in the graphs, are considered to represent temperate stages. The four data sets are linked with help of 20 MSS data. These are indicated in Fig. 1 with arrows at both sides pointing to the D/L ratios of different species obtained from the same sample. The resulting pattern is highly significant and almost naturally leads to connection of corresponding peaks. Corresponding peaks are mentioned an AAR zone which is considered to represent an individual temperate stage.
It seems reasonable that the zonation represents an uninterrupted sequence of interglacial stages and this enables a rather straightforward correlation with the marine isotope stratigraphy. This correlation doesn't conflict with the absolute ages obtained by other dating methods (Figs. 2 & 3).



DISCUSSION
The longly discussed stratigraphical position of the marine deposits in SW Netherlands ('Schouwen' deposits) seems to be clarified. Besides obviously present reworking during cold stages, this 'unit' includes temperate marine molluscan assemblages from different interglacials: Eemian and at least one Middle Pleistocene interglacial.
The transgression from the interglacial immediately preceding the Eemian is represented from the Flemish Valley (Zelzate) to the Northern Netherlands and further to Germany (Wacken, see below). This widespread transgression occurred during an interglacial that we name herewith the Oostermeer Interglacial.
From the Belvédère Interglacial no D/L ratios from marine onshore deposits are observed, so we assume that a transgression during this stage did not reach the present mainland. We did find this, however, in offshore boreholes in the present North Sea.
The available AAR data leave us no possibility to distinguish the Holsteinian of the fluvial deposits of Neede from the estuarine deposits of the Noordbergum Interglacial at Noordbergum. The latter deposits were considered to represent a 'fourth Cromerian Interglacial'. On the basis of AAR this is highly questionable. It should be kept in mind that asignment to separate interglacials was never justified on the basis of faunal data (molluscs and micro mammals). We consider the Noordbergum deposits to be of Holsteinian age. A reinvestigation of both Noordbergum and Dutch Holsteinian sites is therefore necessary.
Published data from about 80 sites outside the Netherlands have been collected. We compared these data with our results. Here, we only have a few remarks. Strikingly, elsewhere in the North Sea Basin a very low number of sites which can be assigned to Zone B is known, contrary to sites which are attributable to zone C.
The stratotypes Hoxne, Hummelsbüttel (for Hoxnian and Holsteinian respectively, which are considered to represent the same interglacial) are in different zones (D and E respectively). The German site of Wacken is in zone C. This means that the marine deposits in the Northern Netherlands which we assign to the Oostermeer Interglacial and the Holsteinian have their counterparts in Northern Germany. The Dutch marine Holsteinian deposits of Noordbergum are of similar age as the Holsteinian stratotype at Hummelsbüttel.