Age and chemical composition of apatites from five DSDP and ODP Sites in the Pacific, Atlantic and Indian Oceans

Since studies on deep-sea cores were carried out in the early 1990s it has been known that ambient temperature may have a marked affect on apatite fission track annealing. Due to sluggish annealing kinetics, this effect cannot be quantified by laboratory annealing experiments. The unknown amount of low-temperature annealing remains one of the main uncertainties for extracting thermal histories from fission track data, particularly for samples which experienced slow cooling in shallow crustal levels. To further elucidate these uncertainties, we studied volcanogenic sediments from five deep-sea drill cores, that were exposed to maximum temperatures between ~10° and 70°C over geological time scales of ~15-120 Ma. Mean track lengths (MTL) and etch pit diameters (Dpar) of all samples were measured, and the chemical composition of each grain analyzed for age and track length measurements was determined by electron microprobe analysis. Thermal histories of the sampled sites were independently reconstructed, based on vitrinite reflectance measurements and/or 1D numerical modelling. These reconstructions were used to test the most widely used annealing models for their ability to predict low-temperature annealing. Our results show that long-term exposure to temperatures below the temperature range of the nominal apatite fission track partial annealing zone results in track shortening ranging between 4 and 11%. Both chlorine content and Dpar values explain the downhole annealing patterns equally well. Low chlorine apatite from one drill core revealed a systematic relation between Si-content and Dpar value. The question whether Si-substitution in apatite has direct and systematic effects on annealing properties however, cannot be addressed by our data. For samples, which remained at temperatures <30°C, and which are low in chlorine, the Laslett et al. [Laslett G., Green P., Duddy I. and Gleadow A. (1987) Thermal annealing of fission tracks in apatite. Chem. Geol. 65, 1-13] annealing model predicts MTL up to 0.6 µm longer than those actually measured, whereas for apatites with intermediate to high chlorine content, which experienced temperatures >30°C, the predictions of the Laslett et al. (1987) model agree with the measured MTL data within error levels. With few exceptions, predictions by the Ketcham et al. [Ketcham R., Donelick R. and Carlson W. (1999) Variability of apatite fission-track annealing kinetics. III: Extrapolation to geological time scales. Am. Mineral. 84/9, 1235-1255] annealing model are consistent with the measured data for samples which remained at temperatures below ~30°C. For samples which experienced maximum temperatures between ~30 and 70°C, and which are rich in chlorine, the Ketcham et al. (1999) model overestimates track annealing.

Oligocene to Pliocene palaeoclimate reconstructions from 18 sites in Hungary

A survey of the Neogene flora and vegetation pattern of the Pannonian domain based on 18 selected fossil plant assemblages is given. Flora and vegetation patterns are based on well-documented and studied fossil plant assemblages (macrofloras, primarily leaves). A general but slow cooling trend is definitely observable after the Early Miocene as reflected by both quantitative climate reconstructions and floristic change, i.e. decrease of diversity, slow disappearance of thermophilous and exotic elements, as well as decrease in the variety of vegetation types. A significant decline of coldest month temperatures (as compared to warmest month temperatures) must have played a defining role in forming flora and vegetation through the Neogene.

Table 3: Reproductive viability of Hemichloris antarctica after cycles of cooling und warming

Laboratory experiments show that undercooling to about -5°C occurs in colonized Beacon sandstones of the Ross Desert, Antarctica. High-frequency temperature oscillations between 5°C and -5°C or -10°C (which occur in nature on the rock surface) did not damage Hemichloris antarctica. In a cryomicroscope, H. antarctica appeared to be undamaged after slow or rapid cooling to -50°C. l4CO2 incorporation after freezing to -20°C was unaffected in H. antarctica or in Trebouxia sp. but slightly depressed in Stichococcus sp. (isolated from a less extreme Antarctic habitat). These results suggest that the freezing regime in the Antarctic desert is not injurious to endolithic algae. It is likely that the freezing-point depression inside the rock makes available liquid water for metabolic activity at subzero temperatures. Freezing may occur more frequently on the rock surface and contribute to the abiotic nature of the surface.