J. Wallinga

The single-aliquot regenerative-dose (SAR) protocol applied to coarse-grain feldspar

The application of the single-aliquot regenerative-dose (SAR) protocol to equivalent dose determination in coarse-grain feldspar is investigated. An outline of the protocol is presented, and basic tests of its performance are discussed. It is shown that the method is robust and yields results similar to those obtained with a single-aliquot additive-dose (SAAD) procedure. The use of the SAR procedure for fading tests is also discussed. We conclude that the SAR protocol for coarse-grain feldspar yields several advantages over previously used multiple-aliquot and single-aliquot methods. The method is fast and precise, the equivalent dose is obtained by interpolation, and the method corrects for sensitivity changes. The luminescence age obtained by SAR and additive-dose dating of the coarse-grain feldspar separates used in this study do, however, underestimate the independently estimated geological age of the samples.

Optical dating of young coastal dunes on a decadal time scale

We explore the use of quartz optically stimulated luminescence (OSL) dating for reconstructing coastal evolution on a time scale of decades to a few hundred years. Samples are taken from the accretionary south-west coast of Texel, a barrier island just offshore of the northern Netherlands. The ages of dune ridges are known from historical sources; an excellent chronology with a decadal accuracy exists for the past 260 years. OSL ages of less than 10 years on the youngest samples indicate that the OSL signal of the quartz grains is very well zeroed prior to deposition and burial. OSL ages of five samples from a 250-year-old dune ridge are indistinguishable, and the OSL ages on 17 out of 20 samples are in excellent agreement with the well-known independent age controls. Our results highlight the potential of OSL dating for high-resolution reconstruction of coastal evolution over the past few centuries.

Underestimation of equivalent dose in single-aliquot optical dating of feldspars caused by preheating

The ability of single-aliquot techniques to recover a known laboratory dose in coarse-grain potassium-rich feldspar separates is tested. It is shown that, for the samples under study, single-aliquot methods underestimate the known dose. It is deduced that this phenomenon is caused by an increase in electron trapping probability as a consequence of heating. The same phenomenon would lead to an underestimation in the equivalent dose in a natural sample. Multiple-aliquot techniques for equivalent dose determination may not suffer from this problem, but are not suitable for samples that might not have been completely bleached at the time of deposition. It is argued that testing for changes in electron trapping probability should become routine practice in the single-aliquot dating of feldspar.

Testing optically stimulated luminescence dating of sand-sized quartz and feldspar from fluvial deposits

Abstract We apply single-aliquot optically stimulated luminescence (OSL) dating to quartz- and feldspar-rich extracts from fluvial channel deposits of the Rhine–Meuse system in The Netherlands. The time of deposition of these deposits is tightly constrained by radiocarbon dating or historical sources. This allows us to compare OSL ages obtained on quartz and infrared OSL (IR-OSL) ages obtained on potassium-rich feldspar with independent ages over the range of 0.3–13 ka. We show that the quartz OSL ages are in good agreement with the expected age. Using IR-OSL dating of feldspar, we find a slight age overestimate for the youngest sample, whereas for older samples the age is significantly underestimated. We also apply OSL dating to older fluvial and estuarine channel deposits with limited independent chronological constraints. Comparison of feldspar IR-OSL ages with the quartz OSL ages up to ∼200 ka shows a clear trend, where the former severely underestimates the latter. This trend is similar to that found for the samples with independent age control, indicating that the feldspar IR-OSL ages are erroneously young for the entire age range. In the youngest samples, incomplete resetting of the IR-OSL signal prior to deposition probably masks the age underestimation. We show that the IR-OSL age underestimation is partly caused by changes in trapping probability due to preheating. Correction for this phenomenon improves the IR-OSL ages slightly, but does not provide a complete solution to the discrepancy. We suggest that, in the light of the problems encountered in the IR-OSL dating of feldspar, quartz is the mineral of choice for OSL dating of these deposits. However, feldspar dating should continue to be investigated, because it has potential application to longer time scales.

Response of the Rhine–Meuse system (west-central Netherlands) to the last Quaternary glacio-eustatic cycles: a first assessment

Abstract An almost 50-m-deep core from the Weichselian Rhine–Meuse palaeovalley, near the present Dutch coast, reveals new insights into how this continental-scale fluvial system responded to relative sea-level fluctuations associated with the last Quaternary glaciations. A multidisciplinary study of this core included sedimentological and stratigraphic analysis augmented with data on shell, diatom and pollen content to infer depositional environments. Optically stimulated luminescence dating provides a first numerical chronostratigraphy for these strata. Net fluvial incision due to relative sea-level fall associated with the Weichselian glaciation (notably oxygen-isotope stage 4) is estimated at >10 m, and we argue that this amount of incision decreases both updip and downdip, because our study area is located near the thickest part of the Eemian/Early Weichselian (oxygen-isotope stage 5) highstand coastal prisms that were particularly sensitive to erosion during ensuing relative sea-level falls. Coastal prism geometry, with a relatively steep upper shoreface, is extremely important in promoting erosion, as demonstrated by the Rhine–Meuse system that borders an exceptionally wide, low-gradient continental shelf. Our results show that fluvial deposits associated with relative sea-level fall (80–40 ka) can constitute a considerable part of preserved strata (‘falling-stage systems tract’). Interglacial transgressive and highstand systems tracts tend to have a relatively low preservation potential; in our core these are represented by estuarine deposits scoured into underlying fluvial strata. Furthermore, we note that sequence boundaries in such settings may be relatively undistinct, whereas tidal ravinement surfaces can be more conspicuous and may represent considerably longer time gaps.

Timing of the last sequence boundary in a fluvial setting near the highstand shoreline—Insights from optical dating

We investigated, by means of optical dating, the chronostratigraphic nature of the sequence boundary associated with the last glacial in a sandy to gravelly compound paleovalley fill, just landward of the highstand shoreline in the Rhine-Meuse Delta (Netherlands). Laterally extensive fluvial strata deposited during oxygen isotope stage 4, coeval with a major sea-level fall, unconformably overlie estuarine deposits from stage 5 or fluvial deposits from the penultimate glacial (stage 6). These chronostratigraphic relationships differ substantially from widely used models and indicate (1) that sequence-boundary formation in this setting was associated with the onset of pronounced sea-level fall, shortly after 80 ka; (2) that the time gap represented by the sequence boundary may be extremely small (,10 k.y.); (3) that the age of the sequence boundary may decrease both updip and downdip of the highstand shoreline; and (4) that our study does not provide viable diagnostic criteria for a sea-level‐controlled sequence boundary above the falling-stage systems tract. Despite the high-frequency, high-amplitude glacio-eustatic regime that might be considered ideal for the formation of an unambiguous unconformity, the last sequence boundary in this setting is commonly cryptic.

Sedimentary architecture and optical dating of Middle and Late Pleistocene Rhine-Meuse deposits – fluvial response to climate change, sea-level fluctuation and glaciation

Eight continuous corings in the west-central Netherlands show a 15 to 25 m thick stacked sequence of sandy to gravelly channel-belt deposits of the Rhine-Meuse system. This succession of fluvial sediments was deposited under net subsiding conditions in the southern part of the North Sea Basin and documents the response of the Rhine-Meuse river system to climate and sea-level change and to the glaciation history. On the basis of grain size characteristics, sedimentological structures, nature and extent of bounding surfaces and palaeo-ecological data, the sequence was subdivided into five fluvial units, an estuarine and an aeolian unit. Optical dating of 34 quartz samples showed that the units have intra Saalian to Weichselian ages (Marine Isotope Stages 8 to 2). Coarse-grained fluvial sediments primarily deposited under cold climatic conditions, with low vegetation cover and continuous permafrost. Finer-grained sediments generally deposited during more temperate climatic conditions with continuous vegetation cover and/or periods of sea-level highstand. Most of the sedimentary units are bounded by unconformities that represent erosion during periods of climate instability, sea-level fall and/or glacio-isostatic uplift.

Sensitivity of Rhonegletscher, Switzerland, to climate change : experiments with a one-dimensional flowline model

A one-dimensional time-dependent flowline model of Rhonegletscher, Switzerland, has been used to test the glaciers response to climatic warming. Mass-balance variations over the last 100 years are obtained from observations of the equilibrium-line altitude (ELA) and a reconstruction of the ELA based on a statistical correlation between temperature and ELA. For the period prior to AD 1882, for which no reliable climate data exist, we chose equilibrium-line altitudes that enabled us to simulate accurately the glacier length from AD 1602. The model simulates the historical glacier length almost perfectly and glacier geometry very well. It underestimates glacier-surface velocities by 1-18%. Following these reference experiments, we investigated the response of Rhonegletscher to a number of climate-change scenarios for the period AD 1990-2100. For a constant climate equal to the 1961-90 mean, the model predicts a 6% decrease in glacier volume by AD 2100. Rhonegletscher will retreat by almost 1 km over the next 100 years at this scenario. At a warming rate of 0.04 K a-1, only 4% of the glacier volume will be left by AD 2100.

Expectations of scatter in equivalent-dose distributions when using multi-grain aliquots for OSL dating

In the OSL dating of sediment, the scatter in equivalent dose (De) between grains is almost always larger than would be expected due to counting statistics alone. Some scatter may be caused by insufficient (partial) bleaching of some of the grains prior to deposition. In order to date partially bleached sediment, it is essential to estimate the amount of scatter caused by other processes (e.g. grain-to-grain variability in the natural dose rate). Measurements of such scatter are performed at the single-grain level; by contrast, most OSL dating is performed on multi-grain subsamples, for which grain-to-grain scatter is reduced through averaging.Here we provide a model for estimating the expected scatter (i.e. excluding that caused by partial bleaching) for multi-grain aliquots. The model requires as input the single-grain sensitivity distribution, the number of grains in the sub-samples, and the expected scatter at the single-grain level, all of which can be estimated to an adequate degree. The model compares well with measured values of scatter in De, determined using aliquots of various sizes, and can be used to help produce a minimum-age De from multi-grain subsamples that is consistent with single-grain data.

Extracting storm-surge data from coastal dunes for improved assessment of flood risk

Future changes in climate and sea level are likely to increase the threat from storm surges in many coastal regions. Mitigation of this threat requires an understanding of storm surge magnitude and frequency, and the relationship of these variables to climate parameters. This understanding is currently limited by the brevity of instrumental records, which rarely predate the twentieth century. However, evidence of former storm surges can be recorded in coastal dunes, because the dune topography may trap high-magnitude deposits at elevated locations. Here we combine a range of techniques to extract storm-surge data from coastal dune sediment. The sediment is tracked in the subsurface with ground-penetrating radar to assess its height and extent, and its age is determined with good precision through optically stimulated luminescence dating. The probable age of the sediment (A.D. 1775 or 1776) is within a period of increased storminess in northwest Europe, and the local magnitude of the event is likely to be greater than any on instrumental record. By utilizing coastal dunes for storm surge analysis, our approach provides a valuable new source of information for understanding storm surge risk, which is vital for the protection of coastal regions.