Irena Hajdas

AMS radiocarbon dating and varve chronology of Lake Soppensee: 6000 to 12000 14C years BP

For the extension of the radiocarbon calibration curve beyond 10000 14C y BP, laminated sediment from Lake Soppensee (central Switzerland) was dated. The radiocarbon time scale was obtained using accelerator mass spectrometry (AMS) dating of terrestrial macrofossils selected from the Soppensee sediment. Because of an unlaminated sediment section during the Younger Dryas (10000–11000 14C y BP), the absolute time scale, based on counting annual layers (varves), had to be corrected for missing varves. The Soppensee radiocarbon-verve chronology covers the time period from 6000 to 12000 14C y BP on the radiocarbon time scale and 7000 to 13000 calendar y BP on the absolute time scale. The good agreement with the tree ring curve in the interval from 7000 to 11450 cal y BP (cal y indicates calendar year) proves the annual character of the laminations. The ash layer of the Vasset/Killian Tephra (Massif Central, France) is dated at 8230±140 14C y BP and 9407±44 cal y BP. The boundaries of the Younger Dryas biozone are placed at 10986±69 cal y BP (Younger Dryas/Preboreal) and 1212±86 cal y BP (Alleröd/Younger Dryas) on the absolute time scale. The absolute age of the Laacher See Tephra layer, dated with the radiocarbon method at 10 800 to 11200 14C y BP, is estimated at 12350 ± 135 cal y BP. The oldest radiocarbon age of 14190±120 14C y BP was obtained on macrofossils of pioneer vegetation which were found in the lowermost part of the sediment profile. For the late Glacial, the offset between the radiocarbon (10000–12000 14C y BP) and the absolute time scale (11400–13000 cal y BP) in the Soppensee chronology is not greater than 1000 years, which differs from the trend of the U/Th-radiocarbon curve derived from corals.

Ams Radiocarbon Dating of Annually Laminated Sediments From Lake Holzmaar, Germany

AMS radiocarbon ages have been determined on terrestrial macrofossils selected from the annually laminated sediments of lake Holzmaar (Germany). The radiocarbon chronology of this lake covers the last 12.6 ka. Comparison of the radiocarbon dated varve chronology with tree ring data shows that an additional 878 years have to be added to the varve chronology. The corrected C-14 varve chronology of Holzmaar reaches back to ca. 13.8 ka cal. BP and compares favourably with the results from Soppensee (Switzerland) (Hajdas et al., 1993). The corrected ages for the onset and the end of the Younger Dryas biozone are 11,940 cal. BP and 11,490 cal. BP, respectively. The ash layer of the Laacher See volcanic eruption is dated at 12,201 +/- 224 cal. BP and the Ulmener Tephra layer is dated at 10,904 cal. BP.

Evidence for a reduction in the carbonate ion content of the deep sea during the course of the Holocene

The paleo carbonate ion proxy proposed by Broecker et al. [1999] is applied in a search for trends in the Holocene acidity of waters in the transition zone between North Atlantic Deep Water and Antarctic Bottom Water (AABW). A clear signal emerges that the carbonate ion content of waters in this zone declined during the past 8000 years. In order to determine whether this decline represents a strengthening of the northward penetrating tongue of low CO3= content AABW or a global reduction of CO3=ion, measurements were made on a core from the Ontong Java Plateau in the western equatorial Pacific. Evidence for a similar decline in CO3=ion over the course of the Holocene was obtained lending support of the latter explanation. Such a drop is consistent with the recent finding by Indermuhle et al. [1999] that the CO2 content of the atmosphere (as recorded in the Taylor Dome Antarctica ice core) rose by 20–25 ppm during the past 8000 years.

AMS 14C age determinations of tissue, bone and grass samples from the Ötztal ice man

14 C ages of samples from the Otztal Ice Man, found on the Hauslabjoch in the Tyrolean Alps in September 1991, were determined using accelerator mass spectrometry (AMS). Uncalibrated 14 C ages of 4555 ± 34 bp, 4560 ± 65 bp and 4535 ± 60 bp were measured on tissue (mean of four samples), bone and grass, respectively, from the Ice Man. The mean of all of our measurements is 4550 ± 27 bp.

Radiocarbon age of late glacial deep water from the equatorial Pacific

Radiocarbon age differences for pairs of coexisting late glacial age benthic and planktic foraminifera shells handpicked from 10 sediment samples from a core from a depth of 2.8 km in the western equatorial Pacific are not significantly different from that of 1600 years calculated from measurements on prenuclear seawater. This places a lower limit on the depth of the interface for the hypothetical radiocarbon‐depleted glacial age seawater reservoir required to explain the 190‰ drop in the 14C/C for atmospheric CO2, which occurred during the mystery interval (17.5 to 14.5 calendar years ago). These measurements restrict the volume of this reservoir to be no more than 35% that of the ocean. Further, 14C measurements on a single Last Glacial Maximum age sample from a central equatorial Pacific core from a depth of 4.4 km water fail to reveal evidence for the required 5‐ to 7‐kyr age difference between benthic and planktic foraminifera shells if the isolated reservoir occupied only one third of the ocean. Nor does the 13C record for benthic forams from this abyssal core yield any evidence for the excess respiration CO2 expected to be produced during thousands of years of isolation. Nor, as indicated by the presence of benthic foraminifera, was the dissolved oxygen used up in this abyssal water.

Late glacial diatom accumulation at 9°S in the Indian Ocean

A continuous 10-m-long section consisting of roughly two thirds Ethmodiscus rex (a diatom) and one third mixed planktonic foraminifera was identified in a core from 3800 m depth at 9°S on the Indian Oceans 90°E Ridge. Radiocarbon dates place the onset of deposition of this layer at >30,000 years B.P. and its termination at close to 11,000 years B.P. However, precise dating of the foraminifera from the Ethmodiscus layer itself proved to be impossible owing to the presence of secondary calcite presumably precipitated from the pore waters. During the Holocene, high calcium carbonate content ooze free of diatoms was deposited at this locale. As the site currently lies beneath the pathway taken by upper ocean waters entering the Indian Ocean from the Pacific (via the Indonesian Straits), it appears that during glacial time, thermocline waters moving along this same path provided the silica and other nutrients required by these diatoms.

THE EFFECT OF TILLAGE ON SOIL ORGANIC MATTER USING 14C: A CASE STUDY

We compared four adjacent soil plots in an effort to determine the effect of land use on soil carbon storage The plots were located at the High Plains Agricultural Research Laboratory near Sidney, Nebraska. We measured 14 C total carbon, total nitrogen and 137 C to determine the size and turnover times of rapid and stable soil organic matter (SOM) pools, and their relation to land-use practices. Results were consistent with the model produced by Harrison, Broecker and Bonani (1993a) in that the 14 C surface soil data fell on the time trend plots of world 14 C surface soil data, indicating that the natural sod and non-tilled plots had a rapidly turning over SOM pool, comprising ca. 75% of surface soil carbon, and the tilled plots had a rapidly turning over SOM pool, comprising only 50% of surface soil carbon.

Radiocarbon dating the Holocene in the Gosciaz Lake floating varve chronology.

Des macrofossiles terrestres selectionnes a partir de sediments lamines provenant du lac Gościaz ont ete dates par AMS. Les resultats chronologiques obtenus par flotaison sont compares a ceux du C14, apres calibration. Un affinement de la chronologie est ainsi obtenu

Assessing AMS 14C ages of detrital organics from Holocene and late-Pleistocene moraines, east-central Sierra Nevada, California, USA

Obtaining age estimates of moraines which have insufficient carbon for conventional 14C dating may be possible by AMS 14C dating of detrital organic material in the till matrix. Sufficient detrital carbon was available in the till matrix of Holocene and late-Pleistocene moraines of upper Laurel Creek Canyon, east-central Sierra Nevada, California, for AMS 14C measurements. Radiocarbon analysis of till matrix organics from the moraine closest to the headwall, M1, reveals an age of 2875 ± 60 BP (ETH 13550). The next oldest moraine, M2, yields 4450 ± 60 BP (ETH 12810). M3, the oldest moraine in the Bloody Mountain Cirque, yielded organics with a radiocarbon age of 13 600 ± 120 BP (ETH 12812). While results are preliminary, these ages are among the first on till matrix in the Sierra Nevada and are consistent with geomorphic context, other palaeoecologic records in the Sierra Nevada, and palaeolake records in the western United States.