Amino Acid Racemization Research Articles

Radiocarbon-calibrated amino acid racemization ages from Holocene sand dollars (Peronella peronii)

Amino acid racemization (AAR) is widely used as a cost-effective method to date molluscs in time-averaging and taphonomic studies, but it has not been attempted for echinoderms despite their paleobiological importance and distinct biomineralization. Here we demonstrate the applicability of AAR geochronology for dating Holocene Peronella peronii (Echinodermata: Echinoidea) collected from Sydney Harbour (Australia). Using standard HPLC methods we determined the extent of AAR in 74 Peronella tests and performed replicate analyses on 23 tests. Replicate analyses from the outer edge of 23 tests spanning the observed range of D/L values yielded median coefficients of variation <4% for Asp, Glu, and Phe D/L values, which is comparable to the analytical precision. Correlations between THAA D/L values across 178 independently prepared subsamples of 74 individuals are also very high (Spearman ρ ≥ 0.95) for these three amino acids. The ages of 20 individuals spanning the observed range of D/L values were determined using 14C analyses, and Bayesian model averaging was used to determine the best AAR age model. Only three models fit to Glu D/L contributed to the final averaged age model. Modeled ages ranged from 14 to 5496 years, and the median 95% confidence interval for the 54 AAR ages was ±29% of the modelled age. In comparison, the median 2σ confidence interval for nine graphite target 14C ages was ±8% of the median age estimate and the median 2σ confidence interval for 20 carbonate target 14C ages was ±26% of the median age estimate. Overall Peronella yield high-quality D/L values and appear to be a good target for AAR geochronology.

TIME-AVERAGING AND STRATIGRAPHIC RESOLUTION IN DEATH ASSEMBLAGES AND HOLOCENE DEPOSITS: SYDNEY HARBOUR'S MOLLUSCAN RECORD

Abstract: Time-averaging has evolved from an unrecognized variable in paleoecological analyses to a key concept in understanding the dynamics of sedimentary systems and the formation of fossil deposits. Here we used radiocarbon-calibrated amino acid racemization ages from 173 Fulvia tenuicostata shells collected from Sydney Harbour (NSW, Australia) to quantify time-averaging in surficial and excavated death assemblages. A novel approach to estimating the total age-estimate variability of a collection of specimens is presented that integrates the age-estimation error associated with the AAR calibration model and the effect of time-averaging variability on the age distribution. Fulvia collected from a single 1.6-m deep excavation were used to quantify changes in time-averaging with burial depth. Fulvia collected from surficial death assemblages at six sites were used to quantify spatial variation in time-averaging. The median shell age increased from ∼ 150 yr to ∼ 4230 yr and time-averaging from ∼ 40 yr to ...

Synthesis and performance of chiral ferrocene modified silica gel for mixed-mode chromatography

A brush-type chiral stationary phase, N-ferrocenyl benzoyl-(1S, 2R)-1, 2-diphenyl ethanol-bonded on the silica gel (NFcBEs) for high performance liquid chromatography (HPLC), was prepared using γ-glycidoxypropyltrimethoxysilane as coupling reagent. The structure of this novel material was characterized by infrared spectroscopy, elemental analysis and thermogravimetric analysis. Mechanism involved in the chromatographic separation is the multi-interaction including hydrophobic, π-π, hydrogen-bonding, π-charge transfer, dipole-dipole and acid-base equilibrium interactions. Based on these interactions, successful separation could be achieved among polycyclic aromatic hydrocarbons, mono-substituted benzenes, aromatic amines, quinolines, nucleosides, phenols and 5-nitroimidazoles drugs in reversed phase liquid chromatography (RPLC). Good resolutions for substituted amine isomers were also obtained with NFcBEs. Racemates of amino acids and drug carvedilol mixtures were well separated on NFcBEs in the normal phase liquid chromatography (NPLC) mode. Such stationary phase with characteristics of multi-interaction mechanism and mixed-mode separation is potential for the analysis of complex samples. The retention behaviors of R- and S-carvedilol on NFcBEs column were investigated with the assistance of quantum chemistry calculation using the density functional theory (DFT) B3LYP method.

Isoaspartic acid is present at specific sites in myelin basic protein from multiple sclerosis patients: could this represent a trigger for disease onset?

Multiple sclerosis (MS) is associated with breakdown of the myelin sheath that coats neurons in the central nervous system. The cause of MS is not known, although the pathogenesis involves destruction of myelin by the immune system. It was the aim of this study to examine the abundant myelin protein, myelin basic protein (MBP), to determine if there are sites of modification that may be characteristic for MS. MBP from the cerebellum was examined from controls and MS patients across the age range using mass spectrometry and amino acid analysis. Amino acid racemization data indicated that myelin basic protein is long-lived and proteomic analysis of MBP showed it to be highly modified. A common modification of MBP was racemization of Asp and this was significantly greater in MS patients. In long-lived proteins, L-Asp and L-Asn can racemize to three other isomers, D-isoAsp, L-isoAsp and D-Asp and this is significant because isoAsp formation in peptides renders them immunogenic.Proteomic analysis revealed widespread modifications of MBP with two surface regions that are altered in MS. In particular, isoAsp was significantly elevated at these sites in MS patients. The generation of isoAsp could be responsible for eliciting an immune response to modified MBP and therefore be implicated in the etiology of MS.Electronic supplementary materialThe online version of this article (doi:10.1186/s40478-016-0348-x) contains supplementary material, which is available to authorized users.

Last interglacial (MIS 5e) sea-level determined from a tectonically stable, far-field location, Eyre Peninsula, southern Australia

ABSTRACTThe last interglacial maximum (Marine Isotope Substage 5e [MIS 5e], 128–116 ka) is a distinctive event in recent Earth history. Shoreline successions of this age are important for calibrating climate models and defining the overall behaviour of the crust–mantle system to fluctuating ice and ocean-water volumes. In a global context, the recently intensified interest in last interglacial shoreline successions has revealed considerable variability in the magnitude of sea-level rise during this time interval and highlighted the need to examine paleosea-level evidence from tectonically stable, far-field settings. Situated in the far-field of continental ice sheets and on the tectonically stable Gawler Craton, the 300 km coastal sector of western Eyre Peninsula between Fowlers Bay and Lake Newland in southern Australia represents an important region for defining the glacio-eustatic (ice-equivalent) sea-level attained during the last interglacial maximum based on the relative sea-level observations from this region. Low-energy, shoaling upward, peritidal bioclastic carbonate successions of the last interglacial (locally termed Glanville Formation) formed within back-barrier, estuarine–lagoonal environments in the lee of eolianite barrier complexes (locally termed Bridgewater Formation) along this coastline. The well-preserved shelly successions (coquinas) contain diverse molluscan fossil assemblages including species no longer living in the coastal waters of South Australia (e.g. the Sydney cockle Anadara trapezia and the benthic foraminifer Marginopora vertebralis). The extent of amino acid racemisation (a measure of fossil age based on increasing d/l value) in a range of species, and in particular A. trapezia and Katelysia sp., confirms the time equivalence of the isolated embayment-fill successions, correlated with the informal type section of the Glanville Formation at Dry Creek, north of Adelaide. Preliminary U-series analyses on A. trapezia also suggest a correlation with the last interglacial maximum, but further highlight the complexity in dating fossil molluscs by the U-series method in view of their open-system behaviour. The shelly successions of the Glanville Formation occur at elevations higher than attained by sea-level in the current, Holocene interglacial. A higher sea-level of between 2.1 ± 0.5 and 4 ± 0.5 m above present sea-level is inferred for the last interglacial maximum (MIS 5e) along this coastline based on the elevation of sedimentary successions host to the shallow subtidal–intertidal fossil molluscs Katelysia sp., and Anadara trapezia. The paleosea-level observations place a lower limit on the sea-level attained during the last interglacial maximum and suggest that caution be exercised in the definition of the upper limit of sea-level during this interglacial.

One‐Pot Synthesis of Phosphanylbis(N‐arylglycines) and Spontaneous Diastereoselective Lactamization of P‐Alkyl Derivatives To Form Five‐Membered P,N‐Heterocyclic Amino Acids

The one‐pot, three‐component reaction of phenylphosphane with two equivalents each of 4‐acetylaniline and glyoxylic acid hydrate (GAH) in diethyl ether at room temperature furnished phosphanyl‐bis(glycine) 1 as a rac/meso mixture (molar ratio 7:3) whereas condensation of primary alkylphosphanes (cHexPH2, iBuPH2) with p‐toluidine and GAH was followed by additional spontaneous and highly diastereoselective lactamization to form five‐membered P,N‐heterocyclic racemic amino acids 2 and 3. Compound 2, with three centres of asymmetry, displayed only one set of NMR signals, whereas for compound 3 one set was strongly preferred. Crystal structure analysis of 2 confirmed the presence of a racemate, with the asymmetric unit consisting of the 2R,3R,4R isomer; the molecules are linked by hydrogen bonds. The trans(eq‐eq)‐position of the P‐cyclohexyl group towards the substituents at the adjacent C2 and C4 atoms is maintained in [D8]THF solution, as shown by small 2JPH coupling constants. Large 2JPH couplings in compound 3 hint at the opposite preference, i.e. cis orientation, of the less spatially demanding isobutyl group at phosphorus towards the C2 and C4 substituents. The trans isomers of 3 were detected only in trace amounts on the basis of characteristic PCH proton doublets with small 2JPH values. This facile route to novel trialkylphosphane‐type hybrid ligands 2 and 3 may pave the way for further studies of their properties and use as ligands in transition metal compounds and catalysis.

A phase transition from monoclinic C2 with Z' = 1 to triclinic P1 with Z' = 4 for the quasiracemate L-2-aminobutyric acid-D-methionine (1/1).

Racemates of hydrophobic amino acids with linear side chains are known to undergo a unique series of solid-state phase transitions that involve sliding of molecular bilayers upon heating or cooling. Recently, this behaviour was shown to extend also to quasiracemates of two different amino acids with opposite handedness [Görbitz & Karen (2015). J. Phys. Chem. B, 119, 4975-4984]. Previous investigations are here extended to an L-2-aminobutyric acid-D-methionine (1/1) co-crystal, C4H9NO2·C5H11NO2S. The significant difference in size between the -CH2CH3 and -CH2CH2SCH3 side chains leads to extensive disorder at room temperature, which is essentially resolved after a phase transition at 229 K to an unprecedented triclinic form where all four D-methionine molecules in the asymmetric unit have different side-chain conformations and all three side-chain rotamers are used for the four partner L-2-aminobutyric acid molecules.

High-resolution estimates of temporal mixing within shell beds: the evils and virtues of time-averaging

This study quantifies the fine structure of time-averaging by using large samples of dated shells collected from within individual strata. Time-averaging results in both good and bad news for interpreting bioclastic deposits.Nine samples of shells were collected from four Holocene cheniers on the Colorado Delta (Gulf of California) and 165 shells of the bivalve Chione fluctifraga were dated using 14C-calibrated amino acid racemization (D-alloisoleucine/L-isoleucine). The age range of shells within samples averages 661 years and, in seven out of nine samples, exceeds 500 years. The sample standard deviation ranges from 73 to 294 years and averages 203 years, far exceeding the dating errors (≪100 years) and potential variation in the life span of Chione (<10 years). Time-averaging is homogeneous among strata within cheniers but varies significantly among cheniers. Age-distributions of dated shells indicate that at 50-year resolution, the samples provide a continuous and uniform record for the entire interval. The actual sample completeness (63.6%) is very close to that predicted by simulations of sampling a 100% complete, uniform record (67.3%).The bad news is that, no matter how carefully collected, data from shell beds may not be suitable for studying processes on timescales shorter than 102 to 103 years; explanations for faunal change that invoke reasoning or models derived from a strictly ecological point of view may rarely be justifiable. Also, notable differences in temporal resolution between the shell beds of seemingly identical origin imply that paleontological patterns (e.g., species diversity) may be affected by cryptic variation in time-averaging. The comparison of our data with time-averaging estimates obtained from other cheniers at coarser sampling resolutions indicates that pooling of samples (analytical time-averaging) can significantly reduce the temporal resolution of paleontological data.The good news is that shell beds can record the optimal type of time-averaging: where paleobiological data are a time-weighted average of the faunal composition from the spectrum of environments that existed during the entire interval of time. Samples from single strata provide a long-term record that is representative of the predominating environments. Within the range of 14C dating, shell beds can provide a complete, high-resolution record, and thus may offer exceptional insights into the environmental and climatic changes of the last 40 thousand years.

On the Centenary of Emanuel Gil‐Av, Former Professor of the Weizmann Institute of Science and Pioneer of Enantioselective Chromatography

AbstractEmanuel Gil‐Av (Zimkin) was born in 1916 and passed away in 1996. The introduction of chemical selectivity into the chromatographic separation process constitutes his main scientific contribution. This approach culminated in the first enantiomeric separation of racemic amino acids by gas chromatography on an optically active chiral stationary phase in 1966, together with Binyamin Feibush and Rosita Charles‐Sigler. Thus the year 2016 marks an important date to remember and to appreciate the invaluable achievements of Gil‐Av in the realm of chirality carried out at the Weizmann Institute of Science, Israel.

Deuteration protects asparagine residues against racemization.

Racemization in proteins and peptides at sites of L-asparaginyl and L-aspartyl residues contributes to their spontaneous degradation, especially in the biological aging process. Amino acid racemization involves deprotonation of the alpha carbon and replacement of the proton in the opposite stereoconfiguration; this reaction is much faster for aspartate/asparagine than for other amino acids because these residues form a succinimide ring in which resonance stabilizes the carbanion resulting from proton loss. To determine if the replacement of the hydrogen atom on the alpha carbon with a deuterium atom might decrease the rate of racemization and thus stabilize polypeptides, we synthesized a hexapeptide, VYPNGA, in which the three carbon-bound protons in the asparaginyl residue were replaced with deuterium atoms. Upon incubation of this peptide in pH 7.4 buffer at 37°C, we found that the rate of deamidation via the succinimide intermediate was unchanged by the presence of the deuterium atoms. However, the accumulation of the D-aspartyl and D-isoaspartyl-forms resulting from racemization and hydrolysis of the succinimide was decreased more than five-fold in the deuterated peptide over a 20day incubation at physiological temperature and pH. Additionally, we found that the succinimide intermediate arising from the degradation of the deuterated asparaginyl peptide was slightly less likely to open to the isoaspartyl configuration than was the protonated succinimide. These findings suggest that the kinetic isotope effect resulting from the presence of deuteriums in asparagine residues can limit the accumulation of at least some of the degradation products that arise as peptides and proteins age.

Temporal and bathymetric resolution of nautiloid death assemblages in stratigraphically condensed oozes (New Caledonia)

AbstractCephalopod shells can be affected by postmortem transport and biostratigraphic condensation, but direct estimates of the temporal and spatial resolutions of cephalopod assemblages are missing. Amino acid racemisation calibrated by 14C demonstrates a centennial‐scale time averaging (&lt;500 years) of Nautilus macromphalus in sediment‐starved, epi‐ and mesobathyal pelagic environments. The few shells that are thousands of years old are highly degraded. The median occurrence of dead shells is at 445 m depth, close to the 300–400 m depth where living N. macromphalus are most abundant. Therefore, dead shells of this species accumulate at a centennial temporal resolution and with excellent bathymetric fidelity. Dead Nautilus shells exist for only a few hundred years on the seafloor, in contrast to the biostratigraphically condensed mixture of extant foraminifers and foraminifers that went extinct during the Pleistocene. Cephalopod shells that do not show any signs of early diagenetic cementation are unlikely to be biostratigraphically condensed.

The role of skeletal micro-architecture in diagenesis and dating of Acropora palmata

Past variations in global sea-level reflect continental ice volume, a crucial factor for understanding the Earth’s climate system. The Caribbean coral Acropora palmata typically forms dense stands in very shallow water and therefore fossil samples mark past sea-level. Uranium-series methods are commonly used to establish a chronology for fossil coral reefs, but are compromised by post mortem diagenetic changes to coral skeleton. Current screening approaches are unable to identify all altered samples, whilst models that attempt to correct for ‘open-system’ behaviour are not applicable across all diagenetic scenarios. In order to better understand how U-series geochemistry varies spatially with respect to diagenetic textures, we examine these aspects in relation to skeletal micro-structure and intra-crystalline amino acids, comparing an unaltered modern coral with a fossil A. palmata colony containing zones of diagenetic alteration (secondary overgrowth of aragonite, calcite cement and dissolution features). We demonstrate that the process of skeletogenesis in A. palmata causes heterogeneity in porosity, which can account for the observed spatial distribution of diagenetic features; this in turn explains the spatially-systematic trends in U-series geochemistry and consequently, U-series age. We propose a scenario that emphasises the importance of through-flow of meteoric waters, invoking both U-loss and absorption of mobilised U and Th daughter isotopes. We recommend selective sampling of low porosity A. palmata skeleton to obtain the most reliable U-series ages. We demonstrate that intra-crystalline amino acid racemisation (AAR) can be applied as a relative dating tool in Pleistocene A. palmata samples that have suffered heavy dissolution and are therefore unsuitable for U-series analyses. Based on relatively high intra-crystalline concentrations and appropriate racemisation rates, glutamic acid and valine are most suited to dating mid-late Pleistocene A. palmata. Significantly, the best-preserved material in the fossil specimen yields a U-series age of 165±8ka, recording a paleo sea-level of −35±7 msl during the MIS 6.5 interstadial on Barbados.

Mechanisms of d-Amino Acid Formation During Maturation of Sweet Rice Wine &lt;i&gt;(mirin)&lt;/i&gt;

Ripened sweet rice wines (mirins) are stored at room temperature for several years to produce a well-balanced sweet flavor. We performed reverse-phase HPLC quantitative analysis of d-amino acids in eight ripened mirins by using derivatization reagents, and investigated the influence of the Maillard reaction on amino acid racemization in mirin during maturation. The relative quantities of d-enantiomers (%d) of Asp, Glu, and Ser in mirins matured for seven years or more were higher than in non-ripened mirins. Based on the calculated correlation coefficient between %D and the amount of Amadori rearrangement products (ARPs) analyzed by LC-MS in ripened mirins for each amino acid, Asp showed a statistically significant strong correlation. Finally, we conducted heating experiments using synthetic ARPs. Our results revealed for the first time that Asp was racemized via ARPs under the influence of pH during mirin maturation.

Oil platforms in the Persian (Arabian) Gulf: Living and death assemblages reveal no effects

Environmental impact assessments require pre-impact data to discriminate between natural and anthropogenic sources of community shifts. True pre-impact data, however, are rarely available because the onset of ocean biomonitoring in the 20th century frequently postdates the timing of such shifts. Death assemblages (DAs) sampled on a sea-floor capture previous community states and represent archives of past pre-impact conditions. This is because they change more slowly (10–10,000 years) than the corresponding living assemblage (LA) (yearly scales of turnover). Strong and rapid directional changes such as those due to human activities are therefore not immediately captured by DAs, leading to a greater live-dead (LD) mismatch than under natural processes alone. We used this approach to infer disturbance of benthic habitats around platforms in two major oilfields in the Persian (Arabian) Gulf: Umm Al Dalkh (UA) and Zakum (ZK). We found that, first, LD metrics showed no correlation with contamination levels or distance from the platform. Second, although LAs and DAs significantly differed in species composition and did not overlap in multivariate space, the difference was generated by a single species, the semelid deposit-feeding bivalve Ervilia purpurea. This bivalve was not found alive, but constituted 30–50% of the DAs at the field scale and up to 92% at some stations. Radiocarbon-calibrated amino acid racemization dating of valves of this species showed considerable spatial and temporal heterogeneity in production over the last decades and centuries, implying a boom-and-bust population dynamics; its absence in LAs is thus likely unrelated to the onset of oil platform production in the 20th century. The remaining LD differences probably reflect within- and multiple-habitat temporal or spatial averaging, and taxonomic similarity and rank-order agreement in species abundance at the field scale are comparable with values recorded in pristine areas. Therefore, we found no evidence of major community shifts in this oilfield area.

Structural evidence of a ketimine as the product of an amino acid with an aldehyde. An intermediate in the racemization and transamination of amino acids

The aldimine and ketimine forms of two Schiff base complexes formed by the condensation of two isomeric imidazole carboxaldehydes with an amino acid are reported. Reaction of L1, the Schiff base condensate of 5-methyl-4-imidazolecarboxaldehyde (5Me4Im) and valine, with copper(II) perchlorate results in the isolation of [Cu(L1)(5Me4Im)(ClO4)] while the analogous reaction of L2, the Schiff base condensate of 1-methyl-2-imidazolecarboxaldehyde (1Me2Im) with alanine, and nickel(II) results in the isolation of [Ni(L2)2]. L1 exhibits the expected aldimine form of the amino acid derived Schiff base, 5Me4Im-CH=N-CH(R)CO2−, while L2 exhibits the tautomeric ketimine form, 1Me2Im-CH2–N=C(R’)CO2−. Structural data clearly support the two tautomeric forms. The ketimine form, observed in [Ni(L2)2], has been proposed as an intermediate in the racemization and transamination of amino acids.

Raman spectroscopic study of DL valine under pressure up to 20 GPa

dl-valine crystal was studied by Raman spectroscopy under hydrostatic pressure using a diamond anvil cell from ambient pressure up to 19.4 GPa in the spectral range from 40 to 3300 cm−1. Modifications in the spectra furnished evidence of the occurrence of two structural phase transitions undergone by this racemic amino acid crystal. The classification of the vibrational modes, the behavior of their wavenumber as a function of the pressure and the reversibility of the phase transitions are discussed.

Immobilized chiral tropine ionic liquid on silica gel as adsorbent for separation of metal ions and racemic amino acids

Tropine-type chiral ionic liquid with proline anion was immobilized on silica gel by chemical modification method for the first time, which was proved by elemental, infrared spectrum and thermogravimetric analysis. Secondly, the performance of this kind of ionic liquid-modified silica gel was investigated in the adsorption of some metal ions, which included Cu2+, Fe3+, Mn2+ and Ni2+. Then the effects of time, initial concentration and temperature on adsorption for Cu2+ ions were studied in detail, which was followed by the further research of adsorption kinetics and thermodynamics. The adsorption could be better described by pseudo-second-order kinetics model and that the process was spontaneous, exothermic and entropy decreasing. In the mode of ‘reuse after adsorption’, the ionic liquid-modified silica gel with saturated adsorption of Cu2+ was finally used in resolution of racemic amino acids for the first time. The static experiment showed that adsorption rate of two enantiomers was obviously different. Inspired by this, the complex was packed in chromatographic column for the separation of racemic amino acids and d-enantiomers were firstly eluted by water or ethanol. Steric hindrance was found as one of key influencing factors for its effect on the stability of the complex.

The history of Danube loess research

The loess-paleosol sequences of the Danube basin contain the longest and most complete terrestrial records of Middle and Late Pleistocene paleoclimates in Europe. Consequently it is no surprise that scientific research into European loess began in this region and has a long and distinguished history. European loess research was initiated by Count Marsigli in the Serbian Vojvodina in the late 17th century, formed the focus of the international Loess Commission which straddled both sides of the Iron Curtain from 1961, and culminated with George Kukla's seminal studies in the 1970s correlating the loess archives with deep-sea sediments. The rise in popularity of the longer Chinese loess sequences in the early 1980s shifted the international research focus away from Europe, and it was not until the change of century that scientific investigation intensified in the region – this time with much more sophisticated tools for analysis.Despite its potential as one of Europe's most valuable Quaternary archives, scientific research into Danube loess has until recently been hindered both by landscape heterogeneity and the large number of countries it covers. These have presented a major limiting factor in developing a unified research approach. For decades, regional/national loess stratigraphic models were defined separately. Difficulties in correlating such schemes persist, especially since these often change significantly with advances in dating methodologies. A cooperative effort is necessary to synthesize the available data, which requires not only the insights from but also the perspective provided by an appreciation of the history of the research.In this study we present a critical historical overview of loess research in the Danube region. We classify the major research phases throughout the long tradition of Danube loess investigations into five categories: (1) the Early period (late 17th century until 1920) extending from Marsigli to Penck; (2) the Classic period (1920–1961) encompassing the contributions of Milanković and the development of pedostratigraphy; (3) the Modern period (1961-present) which oversaw Czech innovations and developments in geochronologic methods, in particular Amino Acid Racemisation (AAR) and luminescence dating; and (4) Contemporary research from Bronger's unified pedostratigraphy to orbitally-tuned timescales. Finally, (5) we present a vision of future Danube loess research.

Climatic implications of the Quaternary fluvial tufa record in the NE Iberian Peninsula over the last 500 ka

The drainage area of the Iberian Ranges (NE Spain) houses one of the most extensive Quaternary fluvial tufaceous records in Europe. In this study, tufa deposits in the Añamaza, Mesa, Piedra and Ebrón river valleys were mapped, stratigraphically described and chronologically referenced from U/Th disequilibrium series, amino acid racemization and radiocarbon methods. Tufa deposits accumulated in cascades, barrage-cascades and related damming areas developed in stepped fluvial systems. The maximum frequency of tufa deposition was identified at 120ka (Marine Oxygen Isotope Stage [MIS] 5e), 102ka (MIS 5c), 85ka (~MIS 5a) and 7ka (MIS 1), probably under warmer and wetter conditions than today. Additional phases of tufa deposition appear at ~353ka (~end of MIS 11), 258–180ka (MIS 7) and 171–154ka (MIS 6). Although most tufa deposition episodes are clearly correlated with interstadial periods, the occurrence of tufa deposits during the penultimate glaciation (MIS 6) is remarkable, indicating that the onset of this stage was climatically favourable in the Iberian Peninsula. Biostatic conditions and the dynamics of karstic systems regulating tufa deposition seem to be sensitive to the precipitation regime, controlled by shifts in the position of North Atlantic atmospheric belts, and summer insolation, regulated by orbital forcing.

Pecten as a new substrate for IcPD dating: The quaternary raised beaches in the Gulf of Corinth, Greece

Intra-crystalline protein diagenesis (IcPD), a recent development of amino acid racemization dating (AAR), is now established as a reliable geochronological tool for the Quaternary. However, extending the method to new biominerals requires extensive testing in order to provide evidence for the closed-system behaviour of the intra-crystalline proteins and to assess the temporal span that can be covered.Here we present results from high-temperature experiments on the IcPD of the bivalve Pecten, demonstrating that a fraction of proteins can be isolated from a bleach-resistant mineral matrix, which effectively operates as a closed system under conditions of accelerated diagenesis in the laboratory. Analyses of Pecten from the well-dated terrace system of the Gulf of Corinth (Greece) provided a pilot test for the integrity of the intra-crystalline fraction in subfossil shells. The small sample sizes in this preliminary study preclude a full assessment of the aminostratigraphic power of Pecten IcPD, but a concordance is observed between the extent of IcPD and sites dating from between MIS 5 and MIS 11.We conclude that Pecten is a potentially good substrate for IcPD dating in the Mediterranean, and that the temporal limit of the technique in this area lies beyond MIS 11.