Sr/Ba in barite: a proxy of barite preservation in marine sediments?
Introduction
Over the last decades, many studies have focused on the search for reliable geochemical tracers to reconstruct past oceanic conditions and productivity changes that could bring insight into the role played by the oceans in the glacial/interglacial variations of atmospheric CO2 concentrations as recorded in ice cores (Barnola et al., 1987, Neftel et al., 1988). From this point of view, marine barite (BaSO4) appears to be a promising tracer that covers a wide range of applications. Barite crystals constitute a universal component of suspended matter that carries most of the particulate barium in the water column (Dehairs et al., 1980, Bishop, 1988). Higher barite fluxes characterize intermediate waters and deep-sea sediments underlying areas of high productivity (Schmitz, 1987, Dehairs et al., 1992, Gingele and Dahmke, 1994, Paytan et al., 1996a). From this observation, relationships between barite fluxes derived from sediment trap data, export of organic matter and the associated productivity in surface oceans have been proposed (Dymond et al., 1992, François et al., 1995), thus allowing barite accumulated in sediments to be used as a proxy for paleoproductivity reconstructions (Schmitz, 1987, Gingele and Dahmke, 1994, Paytan et al., 1996a, Nürnberg et al., 1997). In addition, because of its formation in the water column, barite provides the opportunity to record seawater elemental composition, like Sr (Paytan et al., 1993, Martin et al., 1995, Bertram and Cowen, 1997), rare earths (Guichard et al., 1979, Martin et al., 1995) and S (Paytan et al., 1998). Being chemical analogues of barium, radium isotopes (in particular, 226Ra, T1/2=1602 a and 228Ra, T1/2=5.75 a) are also incorporated in barite crystals. 226Ra/Ba (Moore and Dymond, 1991) and 228Ra/226Ra (Legeleux and Reyss, 1996) ratios measured in particulate matter collected with sediment traps have therefore been used to trace both the formation and settling of barite in the water column. Finally, the decay of 226Ra activities in barite that accumulates in deep-sea sediments provides a means to date Holocene sediments (Paytan et al., 1996b, van Beek and Reyss, 2001, van Beek et al., 2002).
Paradoxically, although sedimentary barite has been widely used for paleoceanographic investigations, many unknowns remain. The mechanism of barite formation is still poorly known. Because the water column is mostly undersaturated with respect to barite (Church and Wolgemuth, 1972), barite crystals are assumed to form within microenvironments of settling flocs in the upper water column (Dehairs et al., 1980, Moore and Dymond, 1991, Legeleux and Reyss, 1996). It has been proposed that barite supersaturation within microenvironments was achieved by Ba and S release from decomposing organic matter exported from the euphotic layer (Chow and Goldberg, 1960, Dehairs et al., 1980, Dehairs et al., 1990, Dehairs et al., 1991, Bishop, 1988, Stroobants et al., 1991). The dissolution of acantharian-derived celestite may also constitute a significant source of Ba and S for barite formation (Bernstein et al., 1992, Bernstein et al., 1998). In addition, the fate of barite crystals (preservation versus dissolution), either within the water column or during early diagenesis is poorly characterized leading to uncertainties in interpreting sedimentary barite records.
The oceanic distribution of dissolved Ba indicates biological uptake in the upper water column and regeneration at depth (Wolgemuth and Broecker, 1970). Because sediment porewaters are generally at saturation with respect to barite under oxic and suboxic conditions, barite seems to be well preserved relative to most of the other biogenic phases. Estimates indicate that ca. 30% of the barite fluxes to the ocean floor is buried in the sediment (Dymond et al., 1992, Paytan and Kastner, 1996). Studies carried out using benthic chambers nevertheless point to a geographic heterogeneity in barite burial efficiencies (McManus et al., 1999). Dymond et al. (1992) suggested that barite burial efficiency was likely to vary in relationship with the mass accumulation rate: in sediments displaying a high accumulation rate barite would have a shorter period of time to efficiently communicate with undersaturated bottom waters, thus enhancing barite preservation. Following this argument, a higher barite preservation is expected in continental margin environments, a trend that is not always found as shown by Kumar et al. (1996). Moreover, it has been recently proposed through thermodynamic calculations that equilibrium between barite and seawater may be reached in several places of the world’s oceans (Monnin et al., 1999, Rushdi et al, 2000). These findings have led these authors to define barite saturation horizons in the different regions investigated. The resulting implication is that barite preservation in marine sediments may thus depend on both the geographic location and the depth of the sediments in the water column. A better understanding of the geochemical behavior of barite, especially during early diagenesis, is therefore required before barite can be used with confidence in paleoceanographic studies.
Strontium, chemical analogue of barium, is incorporated in barite during its formation in the water column. Strontium concentration in barite reaches a few percent (Dehairs et al., 1980, Bishop, 1988). Bertram and Cowen (1997) reported a lower Sr content in barite crystals from superficial sediments relative to those from the water column, which they attributed to barite dissolution in undersaturated waters. Here we report Sr/Ba ratios of barite separated from several cores collected between 2000 and 5000 m in the equatorial Pacific and in the Southern Ocean to assess the heterogeneity of the Sr/Ba ratio in these two regions and with changing water depth. Ratios are reported for the Holocene period for all cores and up to about 600 000 years for two locations in the equatorial Pacific. These investigations are designed to provide information on the geochemical behavior (preservation versus dissolution) of barite crystals during early diagenesis and to test the potential of the Sr/Ba ratio in barite of being an indicator of the preservation state of barite crystals.
Section snippets
Ba and Sr contents in barite
Barite crystals were chemically separated from equatorial Pacific cores (Fig. 1 and Table 1) using the protocol proposed by Paytan et al. (1993), slightly modified (van Beek and Reyss, 2001). This protocol consists of sequential leaching steps that remove the different Ba-rich phases – other than barite – of the sediment (i.e. calcium carbonate, oxides and hydroxides, organic matter, opal and alumino-silicates). For cores from the Southern Ocean (Fig. 1 and Table 1), a heavy liquid separation
Sr/Ba ratio in marine barite
Sr/Ba ratios measured in the separated barite samples are reported in Table 3. Samples separated from the sediment may contain impurities – mainly heavy minerals, such as TiO2 minerals, monazite, zircon – that resist, like barite, the chemical treatment (Paytan et al., 1993, Martin et al., 1995, van Beek and Reyss, 2001). This is particularly true (1) for cores from the Southern Ocean because the heavy liquid technique employed does not allow the separation of barite from the other dense phases
Conclusion
Profiles of Sr/Ba ratios in barite separated from sediment cores from the equatorial Pacific and the Southern Ocean display little downcore variation. The Sr/Ba ratios in sedimentary barite appear to fall at the low end of the range found in barite collected in the water column, indicating that Sr-rich barite crystals dissolve preferentially in the water column during settling to the deep-sea floor and/or at the sediment–water interface.
Our results suggest that the Sr/Ba ratios in barite from
Acknowledgements
We thank Adina Paytan and an anonymous reviewer for their comments that improved significantly the quality of the manuscript. We are grateful to the SCRIPPS Institution of Oceanography, La Jolla, and especially to Warren Smith for providing sediment samples from ERDC and PLDS cores, as well as to the Alfred Wegener Institute, Bremerhaven, and especially to Rainer Gersonde, Michiel Rutgers van der Loeff, Gerhard Kuhn, and Hanes Grobe for providing sediment samples from cores PS2102-2, PS1768-1
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