K. van der Borg

Late Quaternary central Mediterranean biochronology

Jorissen, F.J., Asioli, A., Borsetti, A.M., Capotondi, L., De Visser, J.P., Hilgen, F.J., Rohling, E.J., Van der Borg, K., Vergnaud Grazzini, C. and Zachariasse, W.J., 1993. Late Quaternary central Mediterranean biochronology. Mar. MicropaleontoL, 21: 169-189. A high-resolution biochronology is presented for the Late Quaternary of the central Mediterranean. In the Late Pleistocene-Holocene successions three assemblage zones are distinguished on the basis of frequency patterns of planktic foraminifera. The age of these zones is determined by Accelerator Mass Spectrometry (AMS) t4C dating. The zonal boundaries are dated at 12,700 yr B.P. (the end of Termination la) and 9600 yr B.P. (the start of Termination Ib), respectively. The AMS dates show that major changes in the planktic and benthic realms occurred synchronously over wide areas, although records of individual species may show important regional differences. In the studied areas, resedimentation processes revealed by anomalous successions of 14C dates, play a far more important role than indicated by the sedimentological and micropaleontological data. Possibly these processes contribute to the very high accumulation rates in the glacial Zone IlL Although the AMS technique has increased the accuracy of laC-measurements, of older carbonate may still lead to substantial age differences between areas with different sedimentary regimes.

Verification of annual growth increments in Arctica islandica L. from the North Sea by means of oxygen and carbon isotopes

Abstract 14 C analysis of material from the shells of Arctica islandica supports the hypothesis that the clear and definite bandings of these shells are annual rings. The pulse of 14 C around 1960, resulting from atmospheric nuclear-bomb testing was recorded in the shell at a location in concurrence with that expected from band counting. The observed cyclic variation in stable isotopes of oxygen and carbon coincided with growth bands. This variation, at least for 18 O, agreed with annual temperature variations and suggests that growth bands are a reflection of seasonally determined differential growth rates. The longevity of individuals of this species, coupled with variations in band width, may provide important information regarding growth and productivity, as well as a record of past environmental conditions.

Accelerator mass spectrometry with 14C and 10Be in utrecht

Abstract The Utrecht facility for accelerator mass spectrometry is now in operation for routine measurements of 14C and 10Be in natural samples. Sample preparation techniques have been introduced. A 1% precision for 14 C 12 C ratios is routinely achieved. In the last year, more than 500 samples have been prepared and measured for various applications in archeology, geology and paleontology.

A cooling event during the Younger Dryas chron in Costa Rica

Abstract A fossil pollen record from the Costa Rican Cordillera de Talamanca presents the first AMS radiocarbon-dated evidence of a temperature decrease during the Younger Dryas Chron. This cooling event is named La Chonta stadial after the bog at 2310 m altitude, located at the actual lower to upper montane forest boundary. High-resolution pollen analysis revealed that between 11,070±130 (AMS 14C age) and 10,400 yr B.P. (interpolated age) vegetation comparable to present-day subalpine forest occurred about 300–400 m lower than in the previous warmer interval, and 600 to 700 m lower than at present time. The radiocarbon dates are close to the earlier estimated ages of 11,080-10,500 yr B.P. for this interval, based on interpolation and pollen concentration rates (Hooghiemstra et al., 1992). The downslope shift of the upper forest line indicates an estimated temperature drop of 2–3°C during the La Chonta stadial. The local vegetation development is indicative of a drier climate. From 10,400 to 9800±120 yr B.P. (AMS 14C age) subalpine rain forest was replaced by upper montane forest, a transitional period to Holocene environmental conditions.

Precision and mass fractionation in 14C analysis with AMS

Modifications of the Utrecht AMS set-up have resulted in 14C analysis with a routine 0.4% analytical precision and a detection limit of 2.0 × 10−15. In analysis with 10Be, 26Al and 36Cl the precision is a few percent and the detection limit 10−14. Measurements on small carbon samples (< 0.4 mg) revealed a sample-mass dependent isotope fractionation effect, which together with the background limits the precision for the smallest samples (0.02 mg) to a few percent.

The late Quaternary sedimentary record of Reykjanes Ridge, North Atlantic

Variability in surface and deep ocean circulation in the North Atlantic is inferred from grain-size characteristics and the composition of terrigenous sediments from a deep-sea core taken on Reykjanes Ridge, south of Iceland. End-member modeling of grain size data shows that deep-ocean circulation in this area decreased significantly during periods of maximum iceberg discharge. The episodes of reduced circulation correlate with the cold and abrupt warming phases of the Dansgaard-Oeschger cycles as recognized in the Greenland ice cores.

Dry extraction of 14CO2 and 14CO from Antarctic ice

Abstract A dry extraction method was used to obtain trapped CO2 of 2–5 kg ice samples from a blue ice zone in East Antarctica. In situ produced 14C was also extracted in 14CO2 and 14CO concentrations at a ratio of 3.4 ± 0.9. Correction of trapped 14CO2 from in situ resulted in ice dates in the range 5–15 ka. The realistic rates for accumulation and ablation of ice indicate about total efficiency of extraction.

A novel approach for developing high-resolution sub-fossil peat chronologies with 14C dating

Sub-fossil sections from a Florida wetland were accelerator mass spectrometry (AMS) dated and the sedimen- tological conditions were determined. 14C data were calibrated using a combined wiggle-match and 14C bomb-pulse approach. Repeatable results were obtained providing accurate peat chronologies for the last 130 calendar yr. Assessment of the different errors involved led to age models with 3-5 yr precision. This allows direct calibration of paleoenvironmental proxies with meteorological data. The time frame in which 14C dating is commonly applied can possibly be extended to include the 20th century.

In situ produced 14C by cosmic ray muons in ablating Antarctic ice

Abstract Samples of a core (52 m) of ablating Antarctic ice were analysed for 14CO and 14CO2 by accelerator mass spectrometry. The data were compared with a 14C in situ production model that includes muon capture in addition to oxygen spallation by neutrons. The analysis reveals significant in situ 14C at depths below 10 m, which we attribute to 14C production by cosmic ray muons. The age of the ice was determined as 9.3±0.4 14C ka BP.

Kau Bay, Halmahera, a late quaternary palaeoenvironmental record of a poorly ventilated basin

Abstract Kau Bay is a small marine basin in between the two northern arms of the Island of Halmahera (Indonesia), separated from the adjacent Philippine Sea by a sill of 40 m deep. Microfossil and radiocarbon data of piston cores indicate that Kau Bay was a freshwater lake in Weichselian times. At 10 ka BP, the Bay became reconnected with the open ocean. Present-day deep water in Kau Bay is poorly ventilated because the sill limits water exchange with the adjacent ocean. Ventilation occurs through occasional inflow of denser oceanic water, but when this fails oxygen depleted conditions rapidly arise. The permanent presence of oxygen tolerant deep water benthic foraminifers over the past 10 ka indicates that dysoxic conditions prevalied and that anoxic conditions, as observed during the Snellius-I Expedition (May 1930), apparently existed for a short time only and left no trace that could be resolved with our time-resolution of 35 years. Slight and long-term variations in the degree of oxygen deficiency most likely are caused by long-term changes in the density of inflowing oceanic water, possibly in consequence of climatic oscillations. The homothermal surface water of Kau Bay shelters a low-diversity planktonic foraminiferal fauna of primarily shallow-dwelling species. This specific association extends back over the past 8 ka indicating that surface waters remained homothermal over much of the Holocene. Surface water productivity most likely changed with variations in river discharge.