Aspartic acid racemisation

Amino acid racemisation is a chemical dating method that measures the relative abundance of amino acid isomers preserved within organic materials (expressed as a D/L ratio). It has been successfully applied to the dating of marine and terrestrial molluscs, teeth, bone, and aeolanites [1]. Traditionally, amino acid racemisation has been used in the dating of Quaternary sedimentary successions [2,3]. The fast racemising acid, aspartic acid (Asp) may be applied in the dating of recent sedimentary successions younger than 600 years [4,5,6]. Thus, the racemisation of aspartic acid has the potential to provide a geochronology for Late Holocene sedimentary successions that are otherwise difficult to date using more conventional dating techniques such as radiocarbon dating. Aspartic acid racemisation can provide a chronology for sedimentary successions that are >120 years old, where ²¹⁰Pb has reached its limit, and <600 years old where dating marine and estuarine material with the radiocarbon method is complicated by the marine reservoir effect.

The potential of aspartic acid to provide a chronology for geologically-young sedimentary successions was confirmed by comparing the kinetic trend of Asp observed in laboratory-induced racemisation established by simulated aging (heating) experiments (Fig. 1a) and the degree of Asp racemisation observed in radiocarbon dated fossil specimens of Anadara trapezia. This was achieved using seven fossil samples of A. trapezia that were analysed by the radiocarbon and amino acid racemisation methods (Fig 1b). This permitted a direct comparison between the degree of aspartic acid racemisation under ambient diagenetic temperatures and fossil age established by the radiocarbon dating method (Fig. 1a & b).

Figure 1. The results from the time-series experiments on fossil molluscs, together with the initial modern aspartic acid D/L value, support the apparent parabolic kinetic trend of aspartic acid for A. trapezia (Fig. 1b). The apparent parabolic nature of the time series experiment can also be seen in the relationship between D/L ratios and the square-root of radiocarbon ages which yielded a near-linear relationship within the reaction range (Fig. 1c) [7]. The relatively high R² value of 0.98 indicates that there is only a minor deviation from the trend line. This deviation is partly the result of the small number of analysis, but can also be attributed to natural variations.

Using the fossil time series data for A. trapezia and similar results for the estuarine mollusc Notospisula trigonella, numeric ages based on the degree of aspartic acid racemisation were determined using an apparent parabolic kinetic model [2,7]. Accordingly, for both A. trapezia and N. trigonella, numeric ages based on the degree of aspartic acid racemisation were calculated using the following formula [7]:

t = [(D/Ls – D/Lm) / Mc]²

Where:
t is age;
D/Ls is the average D/L ratio of the sample of unknown age;
D/Lm is the D/L ratio for a modern sample of the same species as D/Ls; and
Mc is the slope.

Using the above formula a total of 28 aspartic ages calibrated by the radiocarbon method were determined to establish a geochronology for the Holocene sedimentary successions of Lake Illawarra (Fig. 2).

Figure 2. Example of the aminostratigraphy of the Holocene sedimentary successions in Lake Illawarra using aspartic acid racemisation.

When examined within a lithostratigraphic framework, racemisation data permit the comparison of pre- and post-European sedimentation rates for Lake Illawarra (Fig. 3a & b). For example, sedimentation rates using the top 50 cm of a core collected from the central lagoon of Lake Illawarra have been calculated using Asp-derived ages obtained from in situ N. trigonella. The results indicate that the rate of sedimentation was ca 0.33 mm/a for 500 years prior to European settlement. The period between 130 and 40 years BP, which corresponds with primary land clearing for agricultural areas, shows an increase in sedimentation rates to 0.66 mm/year. The period from 40 years to the present, corresponding with an increase in urban and industrial development within the Lake Illawarra catchment, shows a dramatic increase in sedimentation rate to 4.5 mm/a, however some of this can be attributed to lower compaction within the upper portion of the core (Fig.3).

Figure 3. Asp D/L ratio and calculated age down core from central lagoon facies, Lake Illawarra.

Using Lake Illawarra on the south coast of New South Wales, Australia, as a case study, this research highlights the potential of aspartic acid for dating the Holocene marginal marine record. Several conclusions have been identified:

1. This study has shown that the application of aspartic acid racemisation dating, in conjunction with more traditionally utilised Holocene dating techniques, can provide a time framework for the Holocene marginal marine record of the Lake Illawarra region. This will enable further study of the Holocene evolution of marginal marine environments by providing a chronology for the lithostratigraphy associated with estuary infill. It should also aid in the investigation of Holocene sealevel fluctuations and their impact on sedimentation within the marginal marine record.
2. This study has also shown that aspartic acid racemisation dating can provide a viable alternative for age determinations of Holocene marginal marine environments, where more traditionally utilised dating techniques have reached their limits. For example, results from this study have shown that aspartic acid racemisation has the ability to date material less than 500 years of age where radiocarbon dating is ineffective due to the marine reservoir effect, and greater than 120 years of age where methods of dating younger material have reached their limit (e.g. ²¹⁰Pb). In essence, aspartic acid racemisation as a dating technique can fill the gap between more traditionally used dating methods, and when used in conjunction with these traditional dating methods, can provide a dating technique that is applicable for the entire Holocene.
3. Results from this study have shown that an aspartic acid-derived chronology on relatively recent sedimentary successions allows the quantification of sedimentation rates, and assists in assessing the impact of anthropogenic activities on sedimentation within estuarine environments. For the Lake Illawarra barrier estuary the results indicate that the sedimentation rate prior to European settlement was less than or equal to ca 1 mm/year or less, and that significant acceleration in sedimentation rates is observed post-European settlement to a rate of ca 4 mm/year.

These results show that the application of aspartic acid racemisation can provide a chronology for Holocene sedimentary successions in the estuarine environments, as well as quantifying sedimentation rates for geologically recent successions. This method provides a framework for the comparison of pre- and post European sedimentation rates in lagoonal successions and can assist in identifying potential adverse affects of urban, industrial and agricultural development on the marginal marine environment.

Acknowledgements

This study constitutes a portion of a PhD degree undertaken by C.R. Sloss in the School of Geosciences at the University of Wollongong. Financial and fieldwork support was provided by the School of Geosciences and the Research Centre for Landscape Change (University of Wollongong); Australian Research Council UOW; the Australian Institute of Geoscientists (AIG); and the Australian Institute of Mining and Metallurgy (AUSIMM).

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Author

Craig Sloss, University of Wollongong.