Richard M. Mitterer

U-series and amino-acid racemization geochronology of Bermuda: implications for eustatic sea-level fluctuation over the past 250,000 years

Abstract Bermuda is a stable, mid-oceanic carbonate platform for which a particularly complete record of Late Pleistocene eustatic sea-level fluctuation has been reconstructed from a detailed study of geological field relationships combined with an extensive programme of U-series and amino-acid racemization geochronology. Only twice in the past 250,000 yr. has sea level in Bermuda been above its present level, once at approximately 200 k.y. when it stood at about + 2 m and most recently at 125 ± 4 k.y. when it stood at 5 ± 1 m. These times of interglacial high sea level are characterized by the development of patch reefs and marine calcarenites at elevations above present sea level. Episodes of lower sea stand onto the Bermuda platform at elevations higher than −20 m are observed within the two interglacial periods and are characterized by the deposition of eolianites. By contrast glacial periods are times of residual soil formation and deposition of speleothems in caves at elevations below present sea level. Excellent correlation is observed between the Bermuda glacio-eustatic sea-level record and other marine and terrestrial paleoclimate records.

Amino acid composition of organic matter associated with carbonate and non-carbonate sediments☆

Abstract Amino acids comprise from 15 to 36% by weight of humic substances from carbonate and non-carbonate sediments. Humic and fulvic acids extracted from carbonate sediments are characterized by an amino acid composition consisting primarily of the acidic amino acids, aspartic and glutamic acid. Humic substances from non-carbonate sediments have a distinctly different amino acid composition consisting primarily of glycine and alanine. Amino acid analyses of various molecular weight fractions of fulvic acids extracted from carbonates show that lower molecular weight fractions have appreciably higher relative abundances of the acidic amino acids compared to higher molecular weight fractions. Based on typical values for carboxyl group content in humic substances, acidic amino acids may be a significant contributor of these functional groups. Carbonate surfaces appear to selectively adsorb aspartic acid-enriched organic matter while non-carbonates do not have this property.

Calculation of amino acid racemization ages based on apparent parabolic kinetics

Abstract Amino acid racemization and epimerization reactions in carbonate fossils are non-linear and follow reversible first-order kinetics only to a d l -amino acid value of about 0.3 to 0.5. Although relative age relationships are not affected by non-linearity of the reaction rate, deviation from reversible first-order kinetics prevents simple linear extrapolation of the kinetic expression in order to estimate ages of fossils with high ratios. Instead of applying reversible first-order kinetics, the racemization and epimerization reactions can be modeled in terms of apparent parabolic kinetics, a procedure that generates a linear relationship between square root of age of samples and d l values. Data from a variety of speciments show that the relationship between square root of age and d l -amino acid is linear to a d l value of about 1.0. This approach provides a simple expression for calculating ages of fossils over a greater portion of the racemization and epimerization reaction range.

Ages and diagenetic temperatures of pleistocene deposits of Florida based on isoleucine epimerization inMercenaria

Abstract The epimerization reaction of isoleucine in skeletal carbonates provides a basis for estimating the age and the average diagenetic temperature experienced by fossils. The kinetics of this reaction inMercenaria, a bivalve, are established by a combination of elevated temperature heating experiments and analysis of Holocene fossils of known age and diagenetic temperature. The Arrhenius activation energy for the reaction is 29.4 kcal/mole. Application of the kinetic relations to Pleistocene marine deposits of Florida yields ages of about 134,000, 180,000, 236,000 and 324,000 years for these deposits. These ages correspond to the times of eustatic high sea levels dated radiometrically throughout the world. Average diagenetic temperatures for the Late Pleistocene in Florida were about 12°C less than the present values; however, the average temperature gradient was close to the present gradient.

Isoleucine epimerization in peptides and proteins: kinetic factors and application to fossil proteins.

The observed rate of isoleucine epimerization in peptides and proteins is dependent on (in addition to time and temperature) (i) the position of isoleucine in the peptide chain, (ii) the nature of adjacent amino acids, and (iii) the stability of the isoleucine peptide bonds. The relative rate is: NH2-terminal > COOH-terminal >> interior ≥ free amino acid. The gradual hydrolysis of peptides and proteins to the more slowly epimerizing free amino acid causes a decrease in the apparent first-order rate constant with time. These results explain the isoleucine kinetics observed in fossil shells.

Biogeochemistry of aragonite mad and oolites

Abstract Aragonite needle mud and oolites from different localities contain a protein matrix comprised in large part of the acidic amino acids, aspartic and glutamic acid. At least one-third of the total organic matter is proteinaceous. The amino acid composition of the organic matter in all of the non-skeletal muds and oolites is nearly identical and is very similar to that in many skeletal carbonates. Treatment of samples with various oxidizing agents indicates that protein is within the non-skeletal carbonates and not just a coating on the outer surface. Adsorption experiments demonstrate that carbonate surfaces are able to selectively adsorb aspartic acid-rich protein. Based on the minor degree of amino acid diagenesis all muds and oolites appear to be essentially contemporaneous and not reworked. In terms of protein composition, skeletal and non-skeletal carbonates are virtually indistinguishable. In skeletal carbonates it is generally recognized that the protein matrix is an important aspect of the calcification process. Organic matter may also play an important role in the formation of oolites and aragonite needles.

Comparison of rates and degrees of isoleucine epimerization in dipeptides and tripeptides

Abstract The degrees and rates of isoleucine epimerization in heating experiments with pure di- and tripeptides are dependent on: (1) the position of isoleucine in the peptide chain; (2) the formation from dipeptides of diketopiperazines, cyclic intermediates that are in equilbrium with, and hydrolyze to, both the parent and inverted dipeptides as follows: NH 2 -terminal⇌diketopiperazine⇌COOH-terminal. The results of heating experiments show that isoleucine residues in diketopiperazine are the most highly epimerized form in both di- and tripeptide solutions. NH 2 -terminal isoleucine also undergoes relatively rapid epimerization, while the rates of epimerization of COOH-terminal and free isoleucine are much slower. Through hydrolysis reactions, high degrees of isoleucine epimerization are transferred to slower epimerizing species. Consequently, the relative rate of isoleucine epimerization in the various positions differs from the relative degree of epimerization. The relative rates of isoleucine epimerization are: NH 2 ⩾ diketopiperazine ⪢ COOH ≈ interior ≈ free, while the relative degrees of isoleucine epimerization are: diketopiperazine > NH 2 > COOH ⪢ interior. The high degree of epimerization exhibited by COOH-terminal isoleucine in fossil dipeptides is due to prior preferential epimerization in diketopiperazine. The high degree of epimerization of free isoleucine in carbonate fossils is due to the hydrolysis of highly epimerized terminal isoleucine. These results are applicable to peptides both in fossil shells and in aqueous solutions and form the basis for a revised model of isoleucine epimerization in fossil shells.

The Diagenesis of Proteins and Amino Acids in Fossil Shells

Proteins are one of the major classes of biopolymers in organisms. These nitrogen-containing macromolecules, comprised of amino acid building blocks, constitute more than 50 percent of the dry weight of most animal tissues. Proteins fulfill a variety of biochemical roles including strengthening of connective tissues, muscles, and membranes and regulation of metabolic activities and immunological functions. Geological interest in these compounds focuses on the role of proteins in invertebrate biomineralization and on the application of amino acid racemization reactions in fossil shells and bones to chronostratigraphy.

Late quaternary land snails from the north coast of Jamaica: Local extinctions and climatic change

Abstract Analysis of the land snail faunas and chronology (14C and amino acid epimer ratios) of six late Quaternary deposits from a site on the north coast of Jamaica provides a basis for reconstruction of local extinction events and their relation to paleoclimate. Paleotemperature estimates are derived from epimerization rates at an interior site and indicate an average Holocene-late last glacial temperature difference of c.4–5°C. Size variation in land snails indicates relatively dry conditions during the late last glacial and moist conditions in the late Holocene, followed by drier conditions beginning sometime during the last millennium. The rich faunas of the latter half of the last glacial are represented today at the same site by only a relatively few remnant species. Two major periods of extinction are identified. During latest last glacial to early Holocene time, a number of species became locally extinct, probably due to increased temperatures; many survive today in the cooler interior of the island. Extensive extinctions occurred again during the last millennium. These apparently resulted largely from habitat disturbance by man. A similar pattern of late Holocene extinctions has also been documented for vertebrates on other tropical islands and is also thought to result from human disturbance.

Hydrogen sulfide–hydrates and saline fluids in the continental margin of South Australia

During the drilling of the southern Australian continental margin (Leg 182 of the Ocean Drilling Program), fluids with unusually high salinities (to 106‰) were encountered in Miocene to Pleistocene sediments. At three sites (1127, 1129, and 1131), high contents of H2S (to 15%), CH4 (50%), and CO2 (70%) were also encountered. These levels of H2S are the highest yet reported during the history of either the Deep Sea Drilling Project or the Ocean Drilling Program. The high concentrations of H2S and CH4 are associated with anomalous Na+/Cl− ratios in the pore waters. Although hydrates were not recovered, and despite the shallow water depth of these sites (200–400 m) and relative warm bottom water temperatures (11–14 °C), we believe that these sites possess disseminated H2S-dominated hydrates. This contention is supported by calculations using the measured gas concentrations and temperatures of the cores, and depths of recovery. High concentrations of H2S necessary for the formation of hydrates under these conditions were provided by the abundant SO42− caused by the high salinities of the pore fluids, and the high concentrations of organic material. One hypothesis for the origin of these fluids is that they were formed on the adjacent continental shelf during previous lowstands of sea level and were forced into the sediments under the influence of hydrostatic head.