KeywordsSediment Trap Benthic Foraminifera Barium Content Downcore Variation Mineral Granule
Biogenic barium usually occurs as discrete microcrystals of the refractory mineral, barite (BaSO4). It may be found in the water column (in the tests of both live and dead planktonic species), in benthic foraminifera, in coral skeletons, and in the underlying sediment. The earliest observations of enriched barium (usually identified as barium concentrations exceeding typical shale or sediment concentrations), and attributed to biological processes, are the work of Revelle et al. (1955) working in the equatorial divergence of the Pacific Ocean. Dehairs et al. (1980) and Bishop (1988) showed that barite (BaSO4) was precipitated in decaying suspended marine particulate matter (particularly diatoms) in oceanic waters. Some studies have suggested that biogenic barium may occur in heavy mineral granules functioning as statoliths in statocyst organs and within protozoans such as Xenophyophoria and Loxodes. Biogenic barium distribution and concentration have been studied in benthic foraminifera and corals as a tracer of bottom water nutrients and upwelling, respectively.
Biogenic barium in sediments is often found in water underlying areas of high productivity. Bishop (1988) has studied the barium content of large and small particles in the Gulf Stream, and Schmitz (1987) has illustrated the use of Ba as a tracer of Indian Ocean plate movement beneath the equatorial upwelling zone on a timescale of millions of years. Virtually, all ocean basins display enrichment of biogenic barium where productivity is elevated and with time (paleoproductivity). Shimmield (1992) suggested that barite-secreting organisms may be confined to a rather discrete zone within the coastal upwelling productivity belt, seaward of the shelf break (under a different nutrient regime), and as a consequence shallow-water, organic-rich sediments may receive little biogenic barium, or the sedimentary barite undergoes diagenesis during sulfate reduction (see below). A similar distribution of biogenic barium was noted by Calvert and Price (1983) in their work off Namibia.
The refractory nature of barite was remarked on in the earliest work by Dymond (1981), something he called a “dissolution residue.” The association of biogenic Ba, opal, and biogenic sedimentation has been described by many authors (see reviews in Schmitz, 1987; Gingele et al., 1999). These observations have opened the potential to use biogenic barium downcore distributions as an important proxy for paleoproductivity, given that both organic carbon and opal may suffer from remineralization and dissolution. An important step in quantifying past productivity is to establish the relationship between biogenic barium and organic carbon in sediment traps (Dymond et al., 1992; Francois et al., 1995). Using algorithms developed from empirical observations, downcore variations in biogenic barium flux have been converted to paleo-primary production. This approach has a number of assumptions and potential drawbacks. In particular, it is necessary to calculate the biogenic component of the total barium content in the sediment. This is usually achieved by normalization to lithogenic metals such as aluminum or titanium. There may be additional sources of barium to the sediment from hydrothermal systems or benthic organisms such as xenophyophores. Finally, although barite is very refractory under oxic conditions, during sulfate reduction in anaerobic sediments, barite dissolution may occur.
- Calvert, S. E., and Price, N. B., 1983. Geochemistry of Namibian shelf sediments. In Suess, E., and Thiede, J. (eds.), Coastal Upwelling, Part A. New York: Plenum, pp. 337–375.Google Scholar
- Shimmield, G. B., 1992. Can sediment geochemistry record changes in coastal upwelling paleoproductivity? Evidence from northwest Africa and the Arabian Sea. In Summerhayes, C., Prell, W., and Emeis, K. (eds.), Upwelling Systems Since the Early Miocene. London: Geological Society. Geology Society Special Publication, Vol. 63, pp. 29–46.Google Scholar