Effect of barite-bound Sr on detrital Sr isotope systematics in marine sediments
Résumé
In marine sediments, the Sr content and isotope composition (87Sr/86Sr) of the terrigenous detrital component are widely used to track changes in provenance and related transport and weathering processes. Accurately separating detrital-Sr from other sedimentary Sr-phases is a prerequisite for such studies. Conventionally, it is assumed that Sr in the carbonate-free residue corresponds to detrital Sr alone. However, the decarbonated residue may contain barite with significant Sr content and a non-detrital 87Sr/86Sr composition; this may substantially affect the measured Sr signal. To examine this chronically overlooked phenomenon, the Mediterranean Sea is an ideal area because 1) detailed provenance studies have been done using Sr and 87Sr/86Sr of the residual fraction, and 2) enhanced levels of barite repeatedly occurred in association with distinct, organic-rich sapropel sediments.
Here, we use the most-recent sapropel S1 interval to evaluate the effect of barite-bound Sr in the residual fraction after decarbonation. A total of 130 samples were taken from 10 cores in the eastern Mediterranean Sea (EMS) and 1 core in the western Mediterranean Sea. This selection represents a geographic and bathymetric coverage of the EMS and permits the basin-wide comparison between organic-rich and -lean sediments. After decarbonation using 1 M HCl solution, the residual sediments were subject to NH4Cl extraction (2 M, pH 7), known to selectively dissolve barite. Our results demonstrate the presence of Sr-bearing barite after traditional carbonate removal and its effect on the derived “detrital” Sr signature. This barite-Sr effect is considerable for samples with barite-Ba >400 μg/g in bulk sediment. The impact of barite is prominent if accompanied by a detrital provenance background of high 87Sr/86Sr (>0.713) or low Sr/Al (<1.0 mg/g). In such cases, removal of remaining barite is required to obtain an unbiased detrital Sr signal. We recommend an improved procedure for detrital Sr separation in marine sediments, with an additional NH4Cl leaching step to eliminate any remaining barite after decarbonation. This approach is particularly important for areas/times of high biological productivity, where sediments are often characterized by abundant barite content.