Annual and interannual variability of sedimentary recycling studied with anon-steady-state model: application to the North Atlantic Ocean (BENGAL site)

Using a time dependent model, we investigate seasonal and interannual varia tions of oxygen, nitrate and organic carbon profiles in sediment in which t he supply of particulate organic matter (POM) exhibits temporal variations, Model parameters are estimated using sediment and porewater composition ob tained during the BENGAL program at a deepsea site in the North Atlantic. T ime-dependent simulations are performed using the seasonal pattern of fluxe s recorded in sediment traps as forcing functions. With the diagenetic conditions recorded at the BENGAL site, i.e deep oxygen penetration, low denitrification rate, the model indicates that sediment s olute fluxes (oxygen and nitrate) should not exhibit large variation (less than +/- 20%). Little variation of the sediment and porewater composition i s calculated by the model for the seasonal variation or the POM flux. Oxyge n concentration varies by 14 mu mol/l in the upper layer of the sediment wh ile nitrate concentration changes by less than 2 mu mol/l over the year. Wh en time-variable bioturbation is introduced in the model, our simulations i ndicate that enhanced bioturbation during high flux periods could generate larger changes in porewater composition. Oxygen concentration would vary by 25 mu mol/l at 1.5 cm depth in the sediment. The model with variable biotu rbation reproduces the main feature of the data set obtained during the BEN GAL programme where large pulses of phytodetritus were not observed, i.e. a relative constancy of oxygen fluxes from the sediment and of the oxygen an d organic carbon profiles. A large event of phytodetritus deposition is simulated by imposing a flux o f POM in summer which is six times greater than the flux recorded. The resp onse of oxygen and nitrate fluxes is much larger, as these fluxes increase by a factor of 3 after the pulse deposition. The simulated concentration pr ofiles of oxygen and nitrate also show large variations after the depositio n event. The model with large supply of phytodetritus captures the features observed on the labile organic carbon data, i.e. a continuous quasi-expone ntial decrease of the labile (bioavailable) carbon over the course of the p rogram. This simulation also provides steeper oxygen gradients in the porew aters after the deposition such as observed during a cruise in September 19 96. This certainly points towards large deposition of phytodetritus before the programme began and suggests that the diagenetic system in this region might be dominated by interannual, rather that seasonal, variability. (C) 2 001 Elsevier Science Ltd. All rights reserved.