Sediment mixing and accumulation in a mangrove ecosystem: Evidence from 210Pb, 234Th and 7Be.

Abstract

210Pb, 234Th and 7Be activities were measured to establish sediment accumulation rates, estimate sediment mixing rates, and determine the depth of the sediment mixed layer in the Sepetiba Bay mangrove ecosystem near Rio de Janeiro City, Brazil. Three sediment cores were collected from Enseada das Garcas, a typical exposed tidal flat region with a sequence of sedimentary features. The seaward edge of this sequence is a mud flat with the landward portion covered with Spartina alterniflora followed by mangrove vegetation. An additional core was collected on an overwash island near Barra de Guaratiba, which is cover with mangroves without a mud flat or Spartina alterniflora sequence. Sediment accumulation rates were determined to range up to 1.8 cm/yr with the Spartina alterniflora having the maximum rate. Mixing rates were estimated for the Spartina alternifloracore at 40 cm2/yr based on 210Pb and 7Be from the upper mixed region of the core. The 234Th activity in this core suggested that either mixing or the input of 234Th were not in steady state. The sediment mixed region depth ranged from 4 cm to greater than 30 cm. At the Enseada das Garcas site the mixing depth decreased in the landward direction (i.e. mud flats > 30 cm, Spartina alterniflora 11 cm, mangroves 4 cm). Along with this decrease in sediment mixing depth was a shift from physical to biological mixing. The Barra de Guaratiba core had a sediment mixed layer of 13 cm as a result of physical and intense biological activity.