A new method for measuring bioturbation rates in sandy tidal flat sediments based on luminescence dating

The rates of post-depositional mixing by bioturbation have been investigated using Optically Stimulated Luminescence (OSL) dating in two sediment cores (BAL2 and BAL5), retrieved from a sandy tidal flat in the Danish part of the Wadden Sea. A high-resolution chronology, consisting of thirty-six OSL ages with ages ranging between 2 ± 4 and 410 ± 20 years, is presented. Slices of sediment (1–2 cm thick) have been dated at least every 5 cm, and from these data mixing depths of 20 cm and 22 cm (BAL5 and BAL2, respectively) are readily identified. Below the mixing zone there is a significant decrease in the apparent sedimentation rate at BAL5 (from 6.4 to 0.3 mm a−1) and an abrupt increase in OSL age at the other site, BAL2 (from ∼19 years–∼290 years). Apparent sedimentation rates in the upper ∼20 cm are more rapid than at greater depths, due to the age underestimation arising from the mixing processes. The significant change in sedimentation rates at BAL5 may indicate an offset in OSL age of up to 620 years. This paper uses a simple conceptual model for vertical mixing in which all the sediment excreted at the surface by lugworms is assumed to be completely reset before being re-incorporated in the sediment column. Using this approach, we conclude that the upward transport rate due to lugworms at BAL5 is 5–6 mm a−1, and that the net sedimentation rate is ∼0.3 mm a−1, whereas at BAL2 the upward transport rate is currently ∼16 mm a−1 and that the site is probably eroding. The situation below the mixing depth (22 cm) is however unclear because of a marked 300 year hiatus. This is the first time that OSL has been used to determine bioturbation rates and our data clearly show the potential of such measurements to provide retrospective field estimates of this important environmental parameter.