DATING HOLOCENE NATURAL LEVEE AND CREVASSE SPLAY DEPOSITS IN THE MISSISSIPPI DELTA WITH OPTICALLY-STIMULATED LUMINESCENCE (OSL) FROM QUARTZ

In a study aiming to quantify late Holocene wetland accretion rates in the Mississippi Delta, natural levee and crevasse splay deposits of Bayou Lafourche, a former Mississippi River course, were dated by quartz OSL using a single-aliquot regenerative-dose (SAR) protocol. An age constraint is provided for the natural levee sediments by an underlying radiocarbon-dated peat. Quartz samples were tested for thermal transfer, recycling ratio and dose recovery ratio. In order to understand the degree of OSL signal resetting in levee and slay sediments both fine silt quartz and sandy quartz were studied. 1mm aliquots containing ~80 grains are used for the sandy quartz dating and the resulting equivalent doses are analysed using statistic methods following Bailey and Arnold (2006) and Arnold and Roberts (2009).

The natural levee sediments measured show a relatively good OSL signal to background (s/b) ratio and the test results suggest a standard SAR protocol with thermal treatments at 220°C for 10 s for natural/regeneration doses and 180°C for test doses are able to recover a known laboratory dose. Bimodal dose distribution of sandy quartz suggests that grains were partially bleached prior burial. OSL ages derived from statistical treatment of such a bimodal distribution (minimum age model statistics, Galbraith et al., 1999) are younger than those ages derived from statistically untreated fine silt quartz, but by no more than a few hundred years. The OSL ages show internal stratigraphic consistency and are in stratigraphic order with the underlying peat. The OSL chronology suggest that the average sedimentation rate of the natural levee deposits is ~3.5 cm/a.

The crevasse splay deposits, situated ~8 km downstream of the natural levee deposits are expected to be younger than the peat bed mentioned above. Indeed, luminescence properties of the quartz are different, showing a poor s/b ratio. To increase the ratio, room temperature OSL stimulation was carried out. But the best suitable SAR protocol has yet to be found for both sandy quartz and fine silt quartz, which will be presented. Wetland accretion rates in the crevasse-splay deposits based on OSL dating will be reported.

Arnold, L.J., and Roberts, R.G., 2009, Quaternary Geochronology, v. 4, p. 204-230.

Bailey, R.M., and Arnold, L.J., 2006, QSR, v. 25, p. 2475-2502.

Galbraith, R.F., Roberts, R.G., Laslett, G.M.Y., H., and Olley, J.M., 1999, Archaeometry, v. 41, p. 339-364.