Christine Hatté

High-resolution chronologies for loess: comparing AMS 14C and optical dating results

The NuXloch loess section in South West Germany is famous in Central Europe for its thick deposits from the Last Glacial Maximum. It has therefore been intensively studied during the past few years and offers an excellent opportunity to compare the performance of different dating techniques covering the period 15–45 ka. Here we present results fromoptical and AMS 14 C dating. The silt-sized polymineral fraction of the clastic sediments and a multiple-aliquot approach was used for IR-OSL dating. 14 C dating was carried out on organic macro-remnants and humin fractions extracted fromthe sedim ents. With the exception of samples taken from the uppermost 1.5 m of the section, IR-OSL and calibrated 14 C-AMS ages are consistent over the entire period. The inconsistencies in the upper meters are thought to be due to disturbances during soil formation in the Holocene. The good agreement obtained on the remaining sequence demonstrates the high accuracy of both methods when studying loess sections: for the IR-OSL ages no significant age underestimations are obvious. The results clearly document that accurate chronologies can be developed for such continental sedimentary sequences. This gives access to archives that can now be studied with a high temporal resolution and allows the establishment of new paleoclimatic proxies for the study of terrestrial responses to past climatic changes. r 2003 Elsevier Science Ltd. All rights reserved.

δ13C variations of loess organic matter as a record of the vegetation response to climatic changes during the Weichselian

This paper presents high-resolution records of 13 C/ 12 C ratios of organic matter from two loess sequences in northwestern Europe. Our analysis is the first attempt to use organic matter δ 13 C as a record of the response of vegetation to climatic variations in an area where climatic changes were not strong enough to induce a radical change in vegetation cover. Over the last climatic cycle, the vegetation of the Rhine Valley showed a strong predominance of C3 plants. Thus, the small δ 13 C variations, with an amplitude of only 1.5‰ to 3‰, are interpreted as corresponding to fluctuations in water supply and atmospheric CO 2 concentration variations rather than to the ratio of C4 to C3 vegetation. Furthermore, loess sequences accumulated at high rates and allow high correlation with climatic proxy data, like the Greenland Ice Sheet Project 2 (GISP2) δ 18 O and the variations in CO 2 concentration recorded in the Vostok ice core. The δ 13 C constitutes a reliable and complementary proxy to study small climatic stresses endured by vegetation during the Weichselian in northwestern Europe. Moreover, by using absolute age control and correlations between global (Vostok-CO 2 ) or semi-global (GISP2-δ 18 O) climate effects, δ 13 C values of organic matter in loess sequences offer a new tool to establish a refined chronology in continental sequences.

The C37 alkenone record of seawater temperature during seasonal thermocline stratification

Abstract The concentrations of suspended particulate pigments, C37–C38 alkenones, total organic carbon and nitrogen in the Ligurian Sea (northwestern Mediterranean) have been studied at 5 and 30 m depth during well defined thermocline conditions. An accurate description of the short term changes of these compounds has been achieved by means of four 36-h sampling cycles each encompassing consecutive filtration periods of 4 h. During sampling the thermocline changes were followed closely by simultaneous measurements of water column temperature, salinity and other physical parameters. The analysis of the collected samples indicates that the Haptophyte pigments and alkenones are essentially synthesized at the levels of highest primary production and therefore the C37 alkenone record reflects the seawater temperature at this depth level. The study also shows that part of these alkenones are distributed throughout the water column in association to the suspended particles. This process results in C37 alkenone distributions that, due to their high resistance to chemical and microbial degradation, record the temperature of the highest primary productivity layers even at shallow (e.g., 5 m depth) or deep (e.g., 1100 m depth) waters.

Is Classical Acid-Alkali-Acid Treatment Responsible For Contamination? An Alternative Proposition

It is well known that, during the widely used AAA pretreatment (de Vries and Barendsen 1954), alkali treatment is responsible for the incorporation of modem carbon due to the precipitation of atmospheric CO 2 as carbonate. Until now, the last step of the experiment, consisting in acid treatment (most of the time with hydrochloric acid) was considered to be sufficient to eliminate all of lab contamination. But wood, peat and sediment present a complex molecular structure. During radiocarbon chemical treatments, functional groups still present in the molecules are likely to form ionic bonds with modern carbonates. These new chemical bonds resist a classical acid treatment and are responsible for rejuvenation. This short paper presents preliminary results for two common 14 C cases: rejuvenation of a 0.4 pMC wood and of an Oxygen Isotope Stage 3 (OIS3) paleosol. For both cases, contamination due to incorporation of modern carbon during chemical treatment is evaluated and an alternative protocol is proposed.

Northeastern Atlantic cold-water coral reefs and climate

U-series age patterns obtained on reef framework-forming cold-water corals collected over a nearly 6000-km-long continental margin sector, extending from off Mauritania (17 degrees N; northwest Africa) to the southwestern Barents Sea (70 degrees N; northeastern Europe), reveal strong climate influences on the geographical distribution and sustained development of these ecosystems. Over the past three glacial-interglacial cycles, framework-forming cold-water corals (Lophelia pertusa and Madrepora oculata) seem to have predominantly populated reefs, canyons, and patches in the temperate East Atlantic and the Mediterranean Sea. Above 50 degrees N corals colonize reefs in the northern East Atlantic primarily during warm climate periods with the biogeographic limit advancing from similar to 50 degrees N to similar to 70 degrees N. We propose that north-south oscillations of the biogeographic limit of reef developments are paced by ice ages and may occur synchronously with north-south displacement of cold nutrient-rich intermediate waters and surface productivity related to changes of the polar front.

Development of accurate and reliable 14C chronologies for loess deposits: Application to the loess sequence of Nussloch (Rhine Valley, Germany)

Due to very high accumulation rates, loess sequences are best suited archives for the continental paleoclimate of glacial periods. Accurate chronologies cannot be easily established by radiocarbon-dating, because of the lack of organic macrorests, the only material for reliable 14 C dating so far. A chemical protocol is reported to extract the organic matter of loess (organic carbon content lower than 0.1% by weight) for accelerator mass spectrometry (AMS) 14 C dating. Sediments were taken from the loess sequence of Nussloch, for which a large dataset of luminescence ages (TL, IRSL/OSL) is available. The 14 C chronology of the organic matter extracted from loess is in good agreement with the corresponding luminescence ages. It allows high resolution correlations with climatic proxy signals (magnetic susceptibility, malacological assemblages, δ 13 C on organic matter, etc.) derived from the loess sequence and global environmental proxy records.

Magma-derived CO2 emissions recorded in 14C and 13C content of plants growing in Furnas caldera, Azores

Abstract The environmental impact of fumarolic and soil emanations of magma-derived carbon dioxide across Furnas caldera has been investigated by measuring the 14 C and 13 C content of 40 specimens of different C3 plants (leaves) growing within and outside the degassing areas. The results demonstrate a significant to large 14 C depletion in many of the plants due to assimilation of 14 C -free endogenous CO2 during photosynthesis and leading to artificial radiocarbon ageing of up to 4400 years. The extent of 14 C ageing broadly correlates with the intensity of gas manifestations at the sampling sites, as inferred from field observations and measurements of excess CO2 concentrations in the volcanic ground. It also provides a time-integrated measure of the amount of volcanic CO2 locally admixed to the ambient air; at several sites this accounts for 15 to 40% of total CO2 (420 to 600 ppm) in enriched air. In some of the plant species (Azalea, Camellia and fern) 14 C depletion is correlated with an enrichment of 13 C due to assimilation of magma-derived CO2 with a 4‰ higher δ 13 C than normal atmosphere. The rate of 13 C enrichment averages ca. 0.18‰ by percent of volcanic carbon fixed in the plant and includes enhanced 13 C discrimination during photosynthesis as a consequence of increased ambient pCO2 (inferred at −0.0306‰ per added ppm of volcanic CO2). Furnas is one of the few volcanoes where clear 13 C enrichment in plants due to endogenous degassing has been evidenced. Our results can be used to estimate the local intensity of volcanic soil gas fluxes in the emanating areas of Furnas caldera. They also have implications for radiocarbon dating of past eruptive events in the caldera, since plants artificially aged by previous degassing could be trapped in volcanic deposits.

New chronology and organic matter paleoclimatic significance of Nußloch loess sequence (Rhine Valley, Germany)

Abstract Due to its high accumulation rate, the loess sequence of Nusloch (Rhine Valley, Germany) is well suited to a high-resolution study of continental paleoenvironmental changes over the last climatic cycle for northwestern Europe. As carbon isotope ratios of plants depend on environmental variations, δ 13 C of loess-derived organic matter constitute reliable paleoclimatic proxies. We present here a new chronology of Nusloch loess sequence based on several 14 C and TL-dates. Our data show high correlations with global climate effects (Vostok-[CO2]) or semi-global (GISP2- δ 18 O ) and thus offer, in this way, a new approach to establish accurate chronologies in continental sequences. Furthermore, using a deconvolution of the δ 13 C signal would provide a quantitative proxy of moisture supply and permit paleoprecipitation reconstructions.

Identification and dating of tephra layers from Quaternary sedimentary sequences of Inner Anatolia, Turkey

Abstract Results presented here are part of a research programme to investigate the environmental evolution in central Anatolia during the Pleistocene. In this study, tephra are used as chronostratigraphical markers between lacustrine sequences studied in cores or in sections surrounding eruptive centres of the Central Anatolian Province. At different locations in the Konya Plain and Cappadocia, tephra have been identified, characterized and linked to eruptions of the nearby volcanoes. Seven different tephra have been clearly identified in lacustrine sequences of the Konya Plain. 14 C dates on shells or organic matter, as well as 230 Th / 234 U ages on gypsum, give a chronological framework to the tephra layers. In addition, the study of the sediment fill of the Eski Acigol maar near Nevsehir has allowed the recognition of several eruptions during Late Glacial and Holocene in the vicinity of Acigol. The results show the first tephra to be identified in lacustrine sequences of Anatolia; they also show the Cappadocian volcanism to be very young (≤35 ka). This young volcanism is often of a basaltic type and does not include thick differenciated pyroclastic deposits.

Radiocarbon calibration by means of varves versus (super 14) C ages of terrestrial macrofossils from Lake Gosciaz and Lake Perespilno, Poland.

This paper presents radiocarbon dates of terrestrial macrofossils from Lakes Gosciaz and Perespilno, Poland. These data agree very well with most of the German pine calibration curve. In the Late Glacial, they generally agree with the data from Lake Suigetsu, Japan, and indicate constant or even increasing (super 14) C age between 12.9 and 12.7 ka BP, rapid decline of (super 14) C age around 12.6 ka BP, and a long plateau 10,400 (super 14) C BP around 12 ka BP. Correlation with corals and data from the Cariaco basin seems to support the concept of site-speficic, constant values of reservoir correction, in contradiction to those introduced in the INTCAL98 calibration. Around the Allerod/Younger Dryas boundary our data strongly disagree with those from the Cariaco basin, which reflects large discrepancy between calendar chronologies at that period. The older sequence from Lake Perespilno indicates two periods of rapid decline in (super 14) C age, around 14.2 and 13.9 ka BP.