A review of the geochemical applications of the amino acid racemization reaction

A method of geochronology based on the chemical racemization of amino acids has been developed within the last few years. The various amino acids that make up the proteins of all living organisms consist virtually entirely of thel-enantiomer. After death, thel-enantiomer for each amino acid is slowly racemized over geological time and eventually forms an equilibrium mixture consisting of equal amounts of thed- andl-enantiometer. The increase in D/L ratio can be used to obtain a measure of the time that has elapsed since the organism died. The range of applicability of this method is the Pleistocene and may eventually be useful throughout the Pliocene in some cases. This paper presents a review of the literature on these applications as well as several suggested areas for future research.Unlike radionuclide decay rates, chemical reaction rates are sensitive to changes in such common environmental parameters as temperature, pH, solvent-medium, etc. For this reason, kinetic studies have been conducted at elevated temperatures in various “fossil-types” in order to simulate the changes that occur over long periods of time at the low temperatures found on the surface of the earth. Such studies, while of somewhat limited value for precise extrapolation, do nevertheless provide valuable information on which to base the theoretical concepts necessary for a complete understanding of the geochemical implications of the racemization reaction.Skeletal remains form the most suitable fossils for chronological study. Proteinaceous material is found embedded within the carbonate exoskeletons of invertebrates and the phosphatic endoskeletons of vertebrates. Most of the geochemical racemization studies have been conducted on foraminiferal shells and on bones. However, some work has also been reported on shells of other invertebrates, marine and lacustrine clays, and a few abiogenic concretions.Since the racemization reaction is temperature dependent, it has been used as a paleothermometer to estimate the average temperature to which bones and shells of independently known age have been exposed since their deposition. These average temperatures as a function of time have then been converted into estimates of the magnitude of the Holocene postglacial/Wisconsin glacial temperature change on land. They have also been used to obtain estimates of the “time-averaged” thermal gradient in deep-sea sediments using foraminifera isolated from the sediments.     

A strategy for frequency spectra of quaternary climate records

The paleoclimatic variability at frequencies ranging from 10^–4 cycle per year (cpy) to 10^–5 cpy is investigated using a set of four deep-sea cores from the Atlantic, Pacific and Indian Oceans. Dominant features are the presence of orbital frequencies corresponding to mean periods of 117.7, 43.6, 24.9 and 19.3 kyr. These are statistically significant according to such advanced spectral tools as Blackman-Tukey, maximum entropy and the highly efficient Thomson technique. However, the main purpose of this paper is methodological, describing the statistical analyses of time series with modern methods in order to stress their relative power, advantages and disadvantages. The more advanced statistical methods confirm the coincidence of the dominant periods in the deep sea cores and those in the astronomical elements, including combination tones. Three frequency bands of high paleoclimatic variability centred at 15.4, 13 and 10.8 kyr are indeed also detected. These two last periods are very close to those predicted by the climatic non-linear model of Ghil and Le Treut and found by Pestiaux et al. and Yiou et al.     

Kinetics and mechanism of the epimerization and decomposition of threonine in fossil foraminifera

Determination of alloThr/Thr ratios in foraminiferal tests ranging in age from Recent to ～ 700,000 yr B.P. has shown that threonine epimerization ratios approach a steady-state value of ～0.2. This is well below the equilibrium ratio of ～0.9 as measured in buffered aqueous solutions. It has also been found that the initial rate of decomposition of threonine in fossil foraminfera is about an order of magnitude faster than would be predicted on the basis of extrapolation from high temperature kinetic studies on free threonine in buffered aqueous solution. These results are apparently a consequence of metal cation catalysis of both the decomposition and the epimerization of free threonine, which is being continually released by the slow hydrolysis of peptide and proteinaceous material present in the fossil shells.     

Racemization of amino acids in fossil bones and teeth from the Olduvai Gorge region,Tanzania, East Africa

Investigation of amino acid racemization in fossil bones and teeth from the Olduvai Gorge region, Tanzania, indicates that aspartic acid racemization can be used to date samples which are less than ～80,000–100,000 years old in this area. In older samples, significant secondary aspartic acid is apparently present and thus these samples have lower than expectedD/L aspartic acid ratios. Isoleucine in older samples, however, is apparently syngenetic with the samples, so the epimerization of isoleucine to alloisoleucine can be used with certain limitations to date fossil bones and teeth from the older stratigraphic sections in the Olduvai region.     

Middle to late Pleistocene uplift rate of the Hungarian Mountain Range at the Danube Bend, (Pannonian Basin) using in situ produced He

Topography of the terraced Danube Bend area indicates fast incision of the Danube River, which was followed by its tributaries dissecting deeply the former terrace levels. These surfaces are vertically bended along the river course, indicating antecedent incision of the Danube into the SW–NE trending Hungarian Mountain Range (HMR). Timing and rate of the incision of the Danube into the HMR and consequently, the rate of vertical motions have remained unresolved so far. This study aims at quantifying the landscape evolution and neotectonic deformation of the central part of the HMR. We used terrace levels along the antecedent section of the Danube River to constrain its incision rate, which is a measure for the uplift rate of the HMR.

Here we use ³He, a terrestrial in situ produced cosmogenic nuclide (TCN), to date uplifted geomorphologic levels along in the Danube Bend gorge. This method, first applied in the Carpathian–Pannonian system in the framework of present study, proved to be suitable for the quantification of landscape evolution in this area. Our ³He exposure age data suggest a maximum incision rate of 2.7 ± 0.1 mm/y for the last 170 ky. Considering likely effect of erosion a more conservative value of 1.6 mm/y for the last 270 ky, was obtained. Both rates are significantly higher than the incision rate of 0.41 mm/y of the Danube derived from previous geologic and geomorphic data for the last 360 ky. The formation of the terrace levels in the Danube Bend probably occurred during the last two glacial cycles (OIS 1–8). According to the exposure age data, there is no direct relationship between the terrace formation and climate in the Danube Bend. Incision of the Danube appears to be connected to the uplift of the HMR, obtained incision rate values can be taken as valid approximations of the uplift rate in the Danube Bend area.     

New strategies to detect life on Mars

If we are to find unequivocal evidence for life on Mars, we will need new ways to search for it. Jeff L Bada and the MOD team describe the innovative strategy developed for the ExoMars mission.     

Review of the current distribution of southern elephant seals in the eastern South Pacific

The eastern South Pacific southern elephant seal population was extinguished by extensive hunting and the current presence of this species in this area is not well understood. We reviewed existing records from the 1900s to 2014 in the eastern South Pacific, as well as tagged seals and movement records in order to assess the potential immigration source of these individuals. A total of 409 confirmed sightings were compiled, ranging from the southern tip of South America (Cape Horn Archipelago) as far north as Ecuador, including some oceanic island groups. The evidence shows also that recolonisation of its former Pacific distribution has started, with slow population growth in three incipient pupping sites in southern Chile (south of 51°S). Although the source of all of the individuals in the growing colonies is unknown, evidence from tags and satellite tagging indicates that some seals come from subpopulations on the Peninsula Valdes and the Falkland/Malvinas Islands, although mtDNA analysis showed, preliminarily, a greater genetic relationship only with the Falkland/Malvinas Islands rookery.     

Prebiotic synthesis of adenine and amino acids under Europa-like conditions.

In order to simulate prebiotic synthetic processes on Europa and other ice-covered planets and satellites, we have investigated the prebiotic synthesis of organic compounds from dilute solutions of NH4CN frozen for 25 years at -20 and -78 degrees C. In addition, the aqueous products of spark discharge reactions from a reducing atmosphere were frozen for 5 years at -20 degrees C. We find that both adenine and guanine, as well as a simple set of amino acids dominated by glycine, are produced in substantial yields under these conditions. These results indicate that some of the key components necessary for the origin of life may have been available on Europa throughout its history and suggest that the circumstellar zone where life might arise may be wider than previously thought.