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

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Anni Tindahl Madsen, Andrew S. Murray, A jain, Thorbjørn Joest Andersen, Morten Pejrup

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 (1e2 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 a1) and an abrupt increase in OSL age at the other site, BAL2 (from w19 yearsew290 years). Apparent sedimentation rates in the upper w20 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 5e6 mm a1, and that the net sedimentation rate is w0.3 mm a1, whereas at BAL2 the upward transport rate is currently w16 mm a1 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.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 5e6 mm a1, and that the net sedimentation rate is w0.3 mm a1, whereas at BAL2 the upward transport rate is currently w16 mm a1 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.
Original languageEnglish
JournalEstuarine, Coastal and Shelf Science
Volume92
Issue number3
Pages (from-to)464-471
Number of pages8
ISSN0272-7714
Publication statusPublished - 1 May 2011

ID: 34209558