Advective excess Ba transport as shown from sediment and trap geochemical signatures

We report the results of a geochemical study of sediment and trap material. Major and trace elements (Zr, Ba, rare earth elements, and Th) were analyz ed on bulk sedimentary material collected along the NE Atlantic margin. Our aim is to test the widespread use of Ba-barite as a proxy for paleoproduct ivity in a continental margin area. This environment is of great interest b ecause atmospheric-oceanic exchanges are important. In sediments, the geoch emical signatures remain close to an upper crust reference, with flat shale -normalized rare earth elements patterns and constant elementary ratios. Th e calculated biogenic fraction of Ba or excess Pa (20-45%) remains lower th an the excess Ba record in trap material (80-99%). The evolution of the geo chemical signature along the margin reflects variable dilution of a detrita l Post Archean Australian Shale-like component by a biogenic carbonaceous s eawater-derived component. The trap material displays a wide range of varia tion in its trace element content (e.g., Ba similar to 150-3000 ppm, Zr sim ilar to 2-100 ppm), except for the abyssal site, which is characterized by constant signature. In the two other sites, all of the trace element conten ts increase with water depth and present pronounced seasonal changes at eac h sampled water depth. The amount of excess Ba also increases in the deepes t traps, and its evolution throughout the year mimics the change of the oth er analyzed trace elements. In contrast, its relationships with particulate organic carbon are not obvious. In term of fluxes, two periods of enhanced excess Ba fluxes are observed: (1) excess Pa flux increases with the detri tal-like elements like Th especially during winter, and (2) excess Pa flux is enhanced without any change for the other trace elements during spring. To explain the first case, a supply through lateral advection is proposed. Such transient input of significant excess Ba flux will have a great impact on the yearly averaged estimation of the export production. Indeed, only t he second case reflects a bloom in the biological productivity of the water column and must be taken into account in a mean calculation of the export production. Finally, a normalization of the excess Ba by detrital-like elem ent like Th will help to discriminate between a real increase of the excess Ba due to local productivity change (high excess Ba and high excess Ba/Th ratio greater than or equal to 10,000) and any input due to advection proce ss (high excess Ba but low excess Ba/Th ratio < 2000). Copyright (C) 1999 E lsevier Science Ltd.