Luminescence chronologies for coastal and marine sediments

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BOREAS Luminescence chronologies for coastal and marine sediments 517 2007) and MIS 6 at 160 kyr and 175–180 kyr. As a first pass comparison with the mean global sea-level curve of Waelbroeck et al. (2002), it is interesting to note that many of these dune pulses are associated with smaller-scale fluctuations in relative sea level. The continental shelf around the southern Cape coast is gently sloping and shallow, so any changes in sea level, wind direction and increased windiness may result in massive entrainment of large quantities of sand, resulting in dune formation. The southern Cape coast of South Africa is also an important region archaeologically, where cave deposits dated using luminescence dating techniques, contain evidence of the presence of Homo sapiens (e.g. Feathers & Bush 2000; Feathers 2002) and their ability to behave in a symbolic fashion (e.g. Henshilwood et al. 2002, 2004; Jacobs et al. 2006; Tribolo et al. 2006; Marean et al. 2007). The presence of Homo sapiens and other animals, such as elephants, is also known from footprints preserved in some of the aeolianite units (Roberts & Berger 1997; Roberts et al. 2008; Jacobs & Roberts in press). Understanding the interplay between dune formation, fluctuating sea levels and human populations living in the coastal environment provides insights into when these cave sites would have been occupied and why archaeological materials have been preserved. For example, at both Blombos Cave and Pinnacle Point Cave 13B, dunes sealed the caves at different times for considerable durations (many tens of thousands of years), making the sites unavailable as living spaces for humans (Jacobs et al. 2003a, b; Marean et al. 2007). This, to some extent, explains the considerable occupational hiatuses that can be observed in the archaeological record and, at the same time, provides reasons for its good preservation. Australia The vast majority of luminescence studies that involve Australian coastal and marine sediments have used TL dating to derive the chronologies. There are 33 TL studies (see Table 1), 1 IRSL (Huntley et al. 1993b) and 10 OSL studies (e.g. Huntley et al. 1985b, 1994b, 1996; Yoshida et al. 2000; Murray-Wallace et al. 2002a; Olley et al. 2004a, b; Brooke et al. 2006; Switzer et al. 2006; Bostock et al. 2007). These studies primarily address two broad themes: Pleistocene and Holocene dune formation and their relation to glacial–interglacial cycles, and the timing and frequency of tsunami events. The geographic area that has received most attention with regard to investigation of the broader chronological history of Pleistocene and Holocene dune formation in Australia is the coastal plain between Coorong and Mount Gambier in southeast South Australia (Fig. 2B). Dunes, in the form of Holocene relict foredunes and Pleistocene relict barrier dunes, span almost the entire last 800 000 years or 25 marine A δ 18 O ( ) B –2 –1 0 Robe Woakwine West Dairy East Dairy Reedy Creek West Avenue East Avenue Ardune Baker Peacock Woolumbool Stewart Harper West Naracoorte East Naracoorte 100 75 50 25 0 0 200 400 600 800 1000 Age (kyr) WA NT Coorong SA QLD NSW V C Luminescence age (kyr) 800 600 400 200 isotope stages (Fig. 2A). The Holocene dune sequences formed as a result of progradation under conditions of rapid sediment supply (Murray-Wallace et al. 2002a), whereas the Pleistocene sequence represents deposition during successive sea-level highstands, with each barrier dune spatially separated as a result of steady tectonic uplift of the land surface (Murray-Wallace et al. 1996, 2002a) (Fig. 2A). Dating the Holocene relict dunes provides an opportunity to test how well the traps that give rise to the OSL and TL signals are bleached, and how well one can resolve ages towards the lower range of luminescence dating techniques. By contrast, the Pleistocene dune sequence offers the opportunity to test the upper limit of the dating technique, alongside its accuracy. TL and IRSL methods were applied to Holocene foredunes at Robe I (Huntley et al. 1993a, b, 1994a) and Sea level (m) 0 0 200 600 1000 Independent age (kyr) Fig. 2. A. A cross-section of the Coorong coastal plain from Robe on the coast, inland to Naracoorte, showing the major dune cordons, some of which were dated using TL, IRSL and OSL. Also shown is a d 18 O time curve showing the matching between sea-level high stands and corresponding dune cordons (modified from Huntley & Prescott 2001). B. An outline map of Australia showing the location of the Coorong coastal plain in southeast South Australia. C. A plot of TL ages versus independent ages.
518 Zenobia Jacobs BOREAS the SAR OSL method to a series of relict foredunes on the Coorong coastal plain at Guichen Bay (Murray- Wallace et al. 2002a). Effectively, modern ages were obtained for the Robe I dune using both TL and IRSL, while an age of 515 yr was measured for the modern foredune at Long Beach using OSL. Sunlight bleaching during aeolian transport therefore seems sufficient in this region. To test the ability to precisely resolve the age of young dunes, OSL dating was applied to a series of the Guichen Bay relict foredunes. Ages were largely in sequential order and demonstrated a very fast initial rate of progradation (between 5 kyr and 4 kyr), followed by a more gradual and linear rate (after 4 kyr). This study, therefore, indicated the potential of OSL dating to provide accurate and precise chronologies, resulting in useful insights into the rate and nature of foredune development. Geological information linking the Pleistocene dunes to successive high sea-level stands, and chronological information in the form of some U-series ages (Schwebel 1984), amino acid racemization ages (Murray- Wallace et al. 2001) and a palaeomagnetic reversal (Brunhes-Matuyama) found between the West and East Naracoorte ranges (Idnurm & Cook 1980), provided an excellent testing ground for the performance of a range of luminescence methods over a significant time range. Several luminescence studies have now used this dune sequence to explicitly test the reliability of various luminescence dating methods as applied to coastal sediments. Many of these studies resulted in important methodological advances that subsequently influenced the field for many years. Huntley et al. (1993a, 1994a, b) demonstrated satisfactory agreement between the MAAD TL ages on quartz for dunes deposited between 0 and 500 kyr and the independent ages and d 18 O record, but uncertainties associated with both D e and dose rate for the samples from the West and East Naracoorte dunes (the oldest in the sequence) produced indefinite results (Fig. 2C). Huntley & Prescott (2001) later applied an improved methodology involving a 33 h preheat at 1601C to empty the traps that give rise to the 2801C TL peak. This adjustment to their measurement procedures resulted in superior preheat plateaus and, thus, improved reliability of the TL ages, but still did not resolve those ages older than 500 kyr. This led them to suggest that the upper limit of TL dating is 250 Gy. To better estimate the duration of the last interglacial Woakwine barrier dune, Murray-Wallace et al. (1999) also applied TL dating, but to samples that were collected explicitly from sedimentary facies that represent the transgressive phase, last interglacial maximum and the regressive phases, as well as an older aeolianite in the core of the barrier dune. They obtained ages consistent with previous TL estimates and with expectations for the transgressive and maximum phases, but the ages for the samples associated with the regressive phases and the dune core were problematic. This demonstrates the complications arising from different modes of formation of these dunes, which is important when improved accuracy is required alongside greater precision. Huntley et al. (1993b) also applied a MAAD-IRSL method to potassium-rich feldspar inclusions in quartz for a number of those samples that they had measured using TL (Huntley et al. 1993a). The ages were consistent within errors, but systematically younger (by less than 10%), which they suggest may have been due to anomalous fading for which they did not correct. Banerjee et al. (2003) applied the now widely used SAR procedure to quartz grains and also found good agreement with independent estimates over the last 250 kyr. They attempted to date the much older West Naracoorte dune, but age underestimates and significantly different results depending on the methodology that was used were obtained. Finally, Yoshida et al. (2000) used the SAR OSL method and applied it to single grains from the Baker (460–490 kyr), West Naracoorte (800–870 kyr) and East Naracoorte (780–990 kyr) dunes. They identified a small number of ‘supergrains’, which are quartz grains that show unusually high saturation doses, and obtained ages that were in agreement with independent ages for the Baker and West Naracoorte samples, but that underestimated the expected age of the East Naracoorte sample. A problem associated with these grains is that it will be difficult to discern, in the absence of independent age control, whether or not the D e values obtained are reliable, given that so few grains yielded realistic age estimates. In summary, therefore, the luminescence chronologies obtained using various different methods applied to the southeast South Australian relict barrier dunes confirm a dune sequence increasing in age inland from the coast, controlled by changes in Quaternary sea levels. Extensive tracts of aeolianite and barrier dunes are also found elsewhere along the Australian coast. TL dating applied to these deposits in locations such as the continental coast near Perth, Western Australia (Price et al. 2001), Point Richie in Victoria (Sherwood et al. 1994), the Nepean Peninsula, Victoria (Zhou et al. 1994) and Lord Howe Island (Woodroffe et al. 1995; Price et al. 2001) also suggests that aeolianite formation occurred at the end of, or slightly after, the last interglacial, thereby linking aeolian deposition to changes in sea level. In other locations, the TL chronologies suggest more complex depositional patterns. Along the northwestern coast (Kendrick et al. 1991), Rottnest Island (Price et al. 2001) and most of the east coast (e.g. Bryant et al. 1990, 1997a; Bryant & Price 1997), aeolian deposition has also been observed during the last glacial period (MIS 2 and 3). Dune formation during the Pleistocene along the entire Australian coast was thus influenced by changes in sea level and provides valuable information about palaeoenvironments during glacial periods when sea levels were lower.
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