Quaternary Geochronology 10 (2012) 175e179
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Research paper
OSL and sediment accumulation rate models: Understanding the history of sediment deposition Gloria I. López a, b, c, *, Jeroen W. Thompson d a
Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem 95501, Israel Leon H. Charney School of Marine Sciences, University of Haifa, Mt. Carmel, Haifa 31905, Israel c School of Geography & Earth Sciences, McMaster University, 1280 Main St. W., Hamilton, ON, Canada L8S 4K1 d Department of Medical Physics & Applied Radiation Sciences, McMaster University, 1280 Main St. W., Hamilton, ON, Canada L8S 4K1 b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2011 Received in revised form 27 February 2012 Accepted 29 February 2012 Available online 12 March 2012
Coastal deposits are very dynamic systems that may not have constant sediment accumulation rates over time. High-resolution reconstruction and dating of coastal environments, e.g. timing of vertical accretion of dune and/or beach ridges, can be achieved when collection of samples is closely-spaced, although this method is not always economically viable or time-effective. The low terrestrial dose rate environment leads to an unusual situation in which sample ages depend on the rate of accumulation of the overburden. By mathematically coupling successive sample ages obtained from vertical sediment cores, we are able to provide constraints on the model accumulation profiles. The dependence of sample age on the accumulation model is presented, and sample ages are compared with leading accumulation models of dune and beach ridge formation. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Optically stimulated luminescence (OSL) Cosmic dose rate models Accumulation rate models Sediment accumulation over time Coastal beach ridges Coastal dune ridges
1. Introduction One of the key assumptions in Optically Stimulated Luminescence (OSL) dating is that the targeted sedimentary deposit should have known, average and definable dose rate. The dose rate is assumed constant over time due to the long half-lives of the radionuclides involved (238U, 232Th, and 40K), and these radionuclides are expected not to migrate throughout the sedimentary deposit during its burial history (Aitken, 1998). The cosmic (extraterrestrial) dose rate may be calculated as a function of latitude, longitude and altitude (Prescott and Hutton, 1994). However, variations in the thickness of the overburden through time can negatively impact the accuracy of the calculated cosmic dose rate. For near-pure quartz sand dune and beach ridge coastal deposits, the cosmic dose rate dominates. For more than 55 samples taken from 28 long vertical sediment cores on the Gulf shores of North-western Florida U.S.A. (see Fig. S1), the cosmic dose rate was found to be 50e80% of the total annual dose rate (López * Corresponding author. Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem 95501, Israel. Tel.: þ972 52 225 2746; fax: þ 972 4 828 8267. E-mail addresses:
[email protected],
[email protected] (G.I. López),
[email protected] (J.W. Thompson). 1871-1014/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2012.02.026
and Rink, 2007, 2008; Rink and López, 2010). In such cases it is imperative to determine the history of each sample’s burial depth, in order to determine the total integrated cosmic dose over time and enable a more accurate calculation of the optical age of each sample. Unfortunately, the sedimentological history of a given sedimentary deposit is not known a priori. In active coastal settings, rapid erosion and/or slow deposition (or vice-versa) are uncontrollable factors directly affecting the overburden thickness. In the absence of known geological markers (e.g. a precise event datum, known feature or erosional boundary), it is necessary to apply arbitrary models for the rate of overburden accumulation. Previously, we have used (López, 2007) the following two models (which we here term the “conventional” models e see Fig. S1): IA: instant accumulation of the overburden contemporaneously with the deposition of the target sample. LA: linear accumulation of the overburden (i.e. constant rate of accumulation). In addition, we may also consider a third model which assumes full accumulation of the overburden immediately before sampling (minimal accumulation, or MA; see also Madsen et al., 2005).
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G.I. López, J.W. Thompson / Quaternary Geochronology 10 (2012) 175e179
Each model yields an integrated cosmic dose, and each model can have a major impact on the inferred age of the sample (in cases for which the cosmic dose rate is dominant, as indicated previously). Whether the IA or LA model is used depends on the hypothesized geological evolution of the deposit, but overall the LA model has been generally preferred as it is more likely to appropriately represent the natural process of sedimentation: periods of accumulation, stabilization and erosion that gradually result in a long-term increase of the overburden. It is becoming recognized that these accumulation models require additional scrutiny. In general, authors should give more details as to what type of accumulation model is being used in the age calculation. Just to mention that correction for changes in overburden through time is not enough for the reader to understand the model’s methodology and probably a standardization of models among the Luminescence Community should be considered. Munyikwa, 2000 indicated a stepwise approach when determining sample ages based on the ages of overlying samples (and hence inferred burial depth history). Similar approaches have been utilized by Carr et al. (2010), Telfer and Thomas, 2007 and Roberts et al. (2008), among which the latter did provide explicit details to the model methodology used. In order to more appropriately determine the burial history of certain sedimentary environments (i.e. the cored stratigraphical sequence), we developed a computational model that assumes instant, linear, and minimal accumulation for each burial period between each pair of adjacent samples in a vertical core (Fig. 1). We discuss the ages, sedimentary history, and accumulation rates generated by this stepwise accumulation model.
Model ages (Harvey 4) SIA SLA SMA Stepwise (constrained) Conventional IA LA MA 2
2.5
3 OSL age (ka)
3.5
4
Model ages (Wild 4) SIA SLA SMA Stepwise (constrained) Conventional IA LA MA
2. Study area
0.6 The Apalachicola Barrier Island Complex (ABIC) is situated on the NE margin of the Gulf of Mexico, in a region known as the Florida Panhandle (U.S.A.). It is composed of a group of Holocene barrier islands decorated by a series of parallel to sub-parallel beach and dune ridges and swells and textbook examples of strandplains (Fig. S1). López (2007) cored 26 individual beach and dune ridges throughout the ABIC (Fig. S2) with penetrations up to 4 m. All cores were sampled for OSL dating at minimum two different depth intervals in an attempt to get two distinct lithosomes: the lower water-lain sedimentological units and the upper aeolian component (usually capping the majority of the ridges in the region). Details on sampling strategies, laboratory methodologies and OSL dating protocols can be extracted from López (2007), López and Rink (2007, 2008) and Rink and López (2010). Typically, ABIC sand ridges show no marked geological or biological markers (i.e. evident timelines or datum) which could be used to argue in favour of a particular model of sediment accumulation. Previously calculated Average Sediment Accumulation Rates (ASAR) based on single-aliquot regenerative-dose (SAR) OSL ages for beach/dune ridges on St. Joseph Peninsula (SJP; Figs. S1 and S3; Rink and López, 2010) show a generally rapid rate (0.40 cm/a) accumulation rates are associated with more constricted PSD plots (i.e. tight modal normal distributions) as opposed to lower accumulation rates (50% of the total dose rate) in the age calculation. Moreover, the stepwise age calculations provide improved constraints on the accumulation history with additional, closely-spaced samples. Further investigations and fine-tuning of this mathematical model are being pursued by the authors and hopefully soon be published. Acknowledgements The authors are grateful to S.A. Mahan, T. Rittenour, and J. Wallinga for helpful discussions over the original poster presentation at the 13th International Conference LED 2011. G.I.L. is grateful to the Azrieli Foundation (Canada-Israel) for the award of an
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International Azrieli Fellowship that enabled her participation in the LED 2011 Conference. Editorial handling by: R. Grun Appendix A. Supplemetary data Supplemetary data related to this article can be found online at doi:10.1016/j.quageo.2012.02.026. References Aitken, M.J., 1998. An Introduction to Optical Dating: The Dating of Quaternary Sediments by the Use of Photo-Stimulated Luminescence. Oxford University Press, Oxford. Carr, A.S., Bateman, M.D., Roberts, D.L., Murray-Wallace, C.V., Jacobs, Z., Holmes, P.J., 2010. The last interglacial sea-level high stand on the Southern Cape coastline of South Africa. Quat. Res. 73, 351e363. López, G.I., 2007. The Late Quaternary Evolution of the Apalachicola Barrier Island Complex, North-East Gulf of Mexico, as determined from Optical Dating. Ph.D. Dissertation, McMaster University, Hamilton, Canada, p. 264, Available as PDF document at: http://www.aber.ac.uk/ancient-tl/ López, G.I., Rink, W.J., 2007. Characteristics of the burial environment related to quartz SAR-OSL dating at St. Vincent island, NW Florida, U.S.A. Quat. Geochronol 2, 65e70. López, G.I., Rink, W.J., 2008. New Quartz-OSL ages for beach ridges on the St. Vincent island Holocene strandplain, Florida, U.S.A. J. Coastal Res. 24, 49e62. Madsen, A.T., Murray, A.S., Andersen, T.J., Pejrup, M., Breuning-Madsen, H., 2005. Optically stimulated luminescence dating of young estuarine sediments: a comparison with 210Pb and 137Cs dating. Mar. Geol. 214, 251e268. Munyikwa, K., 2000. Cosmic ray contribution to environmental dose rates with varying overburden thickness. Ancient TL 18, 27e34. Prescott, J.R., Hutton, J.T., 1994. Cosmic ray contributions to dose rates for luminescence and ESR dating: large depths and long-term time variations. Radiat. Meas. 23, 497e500. Rink, W.J., López, G.I., 2010. OSL-based lateral progradation and aeolian sediment accumulation rates for the Apalachicola barrier island Complex, North Gulf of Mexico, Florida. Geomorphology 123, 330e342. Roberts, D.L., Bateman, M.D., Murray-Wallace, C.V., Carr, A.S., Holmes, P.J., 2008. Last Interglacial fossil elephant trackways dated by OSL/AAR in coastal aeolianites, Still Bay, South Africa. Palaeogeo., Palaeoclimat., Palaeoecol 257, 261e279. Telfer, M.W., Thomas, D.S.G., 2007. Late Quaternary linear dune accumulation and chronostratigraphy of the southwestern Kalahari: implications for aeolian palaeoclimatic reconstructions and predictions of future dynamics. Quat. Sci. Rev. 26, 2617e2630.
