Axel Suckow

TOWARDS A DEEPER UNDERSTANDING OF HOW CARBONATE ISOTOPES (14C, 13C,18O) REFLECT ENVIRONMENTAL CHANGES : A STUDY WITH RECENT 210Pb-DATED SEDIMENTS OF THE PLITVICE LAKES, CROATIA

Five short cores (top 40-45 cm of sediment) from 4 lakes of the Plitvice Lakes system (Croatia) were measured for 210Pb, 137Cs, a14C, δ13C, and δ18O in order to study the influence of environmental changes on the sediment system in small and large lakes. Sediment chronology based on the constant flux (CF) 210Pb model was the most reliable. Lake sediments consisted mainly of autochthonous carbonates with higher sedimentation rates in small lakes. Sediments from 2 large lakes, Prosce and Kozjak, showed constant stable isotope profiles for the carbonate fraction and full agreement between the 137Cs and 210Pb chronologies. Sediments from 2 small lakes, Gradinsko and Kaluderovac, showed synchronous increases in 14C and δ13C and disturbed 137Cs records. All lakes showed an increase in a14C in the carbonate sediments above the first occurrence of 137Cs, which was interpreted as a damped (~10 pMC increase in a14C) and decades-delayed consequence of the bomb-induced increase in a14C in atmospheric CO2. For the small lakes, increased δ13C in the last 2 decades and part of the a14C increase is probably due to an increase in primary productivity, which enhanced biologically induced calcite precipitation with concomitant changes in the carbon isotopic composition of carbonate sediments. δ13C values of a near-shore sediment core close to the confluence of one of the tributaries of Lake Kozjak showed that the carbonates in this core are a mixture of autochthonous and eroded allochthonous mineral carbonate. This core had a higher fraction of organic material. The sedimentation rate at this core site was high, but rates could not be quantified by 210Pb, 137Cs, or 14C.

Isotope records in submarine speleothems from the Adriatic coast, Croatia

Isotope studies, using 14C dating, [sigma]13C and [sigma]18O measurements, were performed at eight speleothems taken from three submerged caves situated along the eastern Adriatic coast, Croatia. The speleothems were taken from 17 m to 38.5 m depth below mean sea level. The samples consist of four stalagmites and four stalactites in position of growth, covered with marine biogenic overgrowth, and the length of speleothems ranges from ~80 mm to ~190 mm.nnThe youngest (surface) and the oldest (base) layers of speleothems were radiocarbon dated and the 14C ages range from 21,600 cal B.P. to 37,000 yr B.P. During that period the global sea level was more than 40 m below the recent one, so presently submerged objects were under the subaerial conditions necessary for speleothem deposition. 14C ages of the youngest layer range from 21,600 to 32,200 cal B.P. for different submerged speleothems. This indicates the time when the speleothem growth ceased, most probably due to flooding of the cave with either fresh or brackish water. Speleothem growth during the Last Glacial Maximum (30-19 kyr ago) and different time of growth cessation for the different speleothem samples suggest that climate change was not the reason for cessation of deposition.nnSamples for [sigma]13C and [sigma]18O measurements were taken from six submerged speleothems with sampling distances of ca. 5-10 mm from the surface to the base of speleothems. Most of the [sigma]13C values are in the range from -10.5%o to -8.5%o, with few exceptions to -6%o. These values are typical for Dinaric karst, and very different values for marine biogenic overgrowth indicate that no isotopic exchange took place during the submerged period. [sigma]18O values range from -6.7%o to -4.1%o. A weak correlation between [sigma]13C and [sigma]18O values indicates possible kinetic isotope fractionation during the calcite precipitation. If the [sigma]18O record is interpreted as climatic signal, it suggests similar climatic conditions for the late Pleistocene and the Holocene, especially no significant differences in temperature and/or moisture transport.

Chapter 9 Analysis of Radionuclides

Publisher Summary This chapter introduces many of the basic concepts involved in the measurement of radionuclides. It describes the methods by which radionuclides are detected and quantified. There are different ways to measure radioactive nuclides in environmental samples. The oldest and still most common methods use the radioactive decay itself. They use techniques of detecting the ionizing particles released during radioactive decay. This ‘‘activity,’’ the number of disintegrations per unit time, is evaluated quantitatively in terms of how many atoms disintegrate per gram or cubic centimeter of sample and time, which is called ‘‘specific activity.’’ Normally, but not necessarily, these methods are specialized to the kind of radioactivity (alpha, beta, and gamma radiation), and they use certain properties of the radiation, such as the kind of radiation or particle energy. This field of laboratory techniques is known as “radiometry.”