Imbalance in the carbonate budget of surficial sediments in the North Atlantic Ocean: variations, over the last millenium?

Fluxes contributing to the particulate carbonate system in deep-sea sedimen ts were investigated at the BENGAL site in the Porcupine Abyssal Plain (Nor theast Atlantic). Deposition fluxes were estimated using sediment traps at a nominal depth of 3000 m and amounted to 0.37 +/- 0.1 mmol C m (2) d (- 1) . Dissolution of carbonate was determined using flux of total alkalinity fr om in situ benthic chambers, is 0.4 +/- 0.1 mmol C m (2) d (-1). Burial of carbonate was calculated from data on the carbonate content of the sediment and sedimentation rates from a model age based on C-14 dating on foraminif era (0.66 +/- 0.1 mmol C m(-2) d(-1)). Burial plus dissolution was three ti mes larger than particle deposition flux which indicates that steady-state is not achieved in these sediments. Mass balances for other components (BSi , (210) Pb), and calculations of the focusing factor using Th-230, show tha t lateral inputs play only a minor role in this imbalance. Decadal variatio ns of annual particle fluxes are also within the uncertainty of our average . Long-term change in dissolution may contribute to the imbalance, but can not be the main reason because burial alone is greater than the input flux. The observed imbalance is thus the consequence of a large change of carbon ate input flux which has occured in the recent past. A box model is used to check the response time of the solid carbonate system in these sediments a nd the time to reach a new steady-state is in the order of 3 kyr. Thus it i s likely that the system has been perturbed recently and that large dissolu tion and burial rates reflect the previously larger particulate carbonate d eposition rates. We estimate that particulate carbonate fluxes have certain ly decreased by a factor of at least 3 and that this change has occurred du ring the last few centuries. (C) 2001 Elsevier Science Ltd. All rights rese rved.