Export production in the Bay of Biscay as estimated from barium - barite in settling material: a comparison with new production

We present barium data for sediment traps deployed in a northeast Atlantic margin environment (Bay of Biscay). Fluxes of excess barium were measured w ith the objective of calculating carbon export production rates from the su rface mixed layer and thus contribute to the understanding of organic carbo n transport in a margin environment. Therefore, it was necessary to properl y understand the different processes that affected the barium fluxes in thi s margin environment. Seasonal variability of POC/Ba flux ratios and decrea se of barium solubilisation in the trap cups with increasing depth in the w ater column probably indicate that the efficiency of barite formation in th e organic micro-environment varies with season and that the process is rela tively slow and not yet completed in the upper 600 m of water column. Thus barite presence in biogenic aggregates will significantly depend on water c olumn transit time of these aggregates. Furthermore, it was observed that s ignificant lateral input of excess-Ba can occur, probably associated with r esidual currents leaving the margin. This advected excess-Ba affected espec ially the recorded fluxes in the deeper traps (>1000 m) of the outer slope region. We have attempted to correct for this advected excess-Ba component, using Th (reported by others for the same samples) as an indicator of enha nced lateral flux and assigning a characteristic Ba/Th ratio to advected ma terial, Using transfer functions relating excess-Ba flux with export produc tion characteristic of margin areas, observed Ba fluxes indicate an export production between 7 and 18 g C m(-2) yr(-1). Such values are 3-7 times low er than estimates based on N-nutrient uptake and nutrient mass balances, bu t larger and more realistic than is obtained when a transfer function chara cteristic of open ocean systems is applied. The discrepancy between export production estimates based on excess-Ba fluxes and nutrient uptake could be resolved if part of the carbon is exported as dissolved organic matter. Re sults suggest that margin systems function differently from open ocean syst ems, and therefore Ba-proxy rationales developed for open ocean sites might not be applicable in margin areas. (C) 2000 Elsevier Science Ltd. All righ ts reserved.