Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz

This paper presents an improved method for the optical dating of Holocene sediments from a variety of geomorphic settings. We have measured the equivalent dose (D_e) in individual grains of quartz, using green laser light for optical stimulation, and have simulated the D_e distributions for multiple-grain ‘synthetic’ aliquots using the single-grain data. For 12 samples of known (independent) age, we show that application of a ‘minimum age model’ to the single-grain and ‘small’ (10-grain) aliquot D_e data provides the most accurate estimate of the burial dose for nine of the samples examined (3 aeolian, 5 fluvial, and 1 marine). The weighted mean D_e (as obtained using the ‘central age model’) gives rise to burial age overestimates of up to a factor of 10 for these nine samples, whether single grains, small aliquots, or ‘large’ (100-grain) aliquots are used. For the other three samples (two aeolian and one fluvial), application of either the minimum age model or the central age model to the single-grain, small aliquot, and large aliquot D_e data yields burial ages in accord with the independent age control. We infer that these three samples were well bleached at the time of deposition. These results show that heterogeneous bleaching of the optical dating signal is commonplace in nature, and that aeolian transport offers no guarantee that the sample will be well bleached at the time of deposition. We also show that grains sensitive to infrared (IR) stimulation can give rise to low D_e values, which will result in significant underestimation of the burial dose and, hence, of the age of deposition. We demonstrate that use of a modified single-aliquot regenerative-dose protocol incorporating IR stimulation prior to green light stimulation deals effectively with contamination by IR-sensitive grains. We conclude that application of the modified protocol to single grains or small aliquots of quartz, using the lowest D_e population to estimate the burial dose, is the best means of obtaining reliable ages for Holocene sediments from a wide range of depositional environments.     

Racemization of amino acids in marine sediments determined by gas chromatography

Ratios of d- to l-amino acids in acid hydrolysates from foraminifera of two deep-sea cores from the Caribbean Sea and Atlantic Ocean increase with depth and consequently with age over a span from 40,000 to 2,000,000 yr. The changing ratios do not seem to follow first-order reversible rate laws. Valine, leucine and glutamic acid apparently racemize (isoleucine epimerizes) at slower rates than do phenylalanine, alanine, aspartic acid and proline. The general relative order for rates of racemization of total (free and bound) amino acids may depend on the electron-withdrawing capacity of the R substituents of the amino acids and on the rates with which the amino acids are naturally hydrolyzed. In contrast to the total amino acids, the free amino acids in these samples are more extensively racemized, probably as a result of various catalytic and hydrolytic reactions.Previous related work based on ion-exchange chromatography has considered only ratios of alloisoleucine to isoleucine. With the gas chromatographic method used here, d/l ratios of all common asymmetric amino acids can be estimated. Measurement of the extent of racemization of amino acids in marine sediments seems to provide the basis for a geochronological tool covering the last few million years.     

The application of single zircon evaporation and model Nd ages to the interpretation of polymetamorphic terrains: an example from the Proterozoic mobile belt of south India

The technique of single zircon dating from the thermal evaporation of ²⁰⁷Pb/²⁰⁶Pb (Kober 1986, 1987) provides a means of dating successive periods of growth and nucleation of zircons in polymetamorphic assemblages. In contrast Nd model ages may provide a measure of the period of crustal residency for the sample or its protolith. These two techniques have been combined to elucidate the tectonic history of the Proterozoic mobile belt of southern India, exposed south of the Palghat-Cauvery Shear Zone that marks the southern boundary of the Archaean craton of Karnataka. The two main tectonic units of this mobile belt comprise the Madurai and Trivandrum Blocks, both of which are characterised by massive charnockite uplands and low-lying polymetamorphic metasedimentary belts that have undergone a complex tectonic history throughout the Proterozoic. Evidence for early Palaeoproterozoic magmatism is restricted to the Madurai Block where single zircon evaporation ages from a metagranite (2436 ± 4 Ma) are similar to model Nd ages from a range of lithologies suggesting crustal growth at that time. The Trivandrum Block, to the south of the Achankovil shear zone, is comprised of the Kerala Khondalite Belt, the Nagercoil charnockites and the Achankovil metasediments. Single zircon evaporation ages, together with conventional zircon and garnet chronometry, suggest that all three units underwent upper-amphibolite facies metamorphism at ∼1800 Ma, an event unrecorded in the metagranite from the Madurai Block. This implies that the Madurai and Trivandrum blocks represent distinct terrains throughout the Palaeoproterozoic. Model Nd ages from the Achankovil metasediments are much younger (1500–1200 Ma) than those from the adjacent Kerala Khondalite Belt and Madurai Blocks (3000–2100 Ma), but there is no evidence for zircon growth in these metasediments during the Mesoproterozoic. Hence the comparatively young model Nd ages of the metasediments are indicative of a mixed provenance rather than a discrete period of crustal growth. Zircon overgrowths from the Madurai Block (547 ± 17 Ma) and Achankovil metasediments (530 ± 21 Ma) suggest that all tectonic units of the Proterozoic mobile belt of South India shared the same metamorphic history from the early Palaeozoic. This event has been recognised in the basement lithologies of Sri Lanka and East Antarctica, confirming that the constituent terrains of East Gondwana had assembled by this time. Received: 10 October 1995 / Accepted: 27 October 1997