Biogenic barium and the detrital Ba/Al ratio: a comparison of their direct and indirect determination

Biogenic barium concentrations obtained from direct determination by a three-step sequential extraction procedure are compared to those obtained from the widely used indirect normative calculation based on total digestion. A comparison of the biogenic barium from the direct/sequential extraction and the indirect/normative calculation clearly shows that the detrital Ba/Al ratio is the critical factor in the normative approach, and that erroneous assumptions based on this ratio may introduce significant errors to the calculated biogenic barium. Overall, the crustal average Ba/Al ratio of 0.005–0.01 is much higher than our directly determined global average of 0.0037. This would result in an underestimation of the biogenic barium and thus of the primary productivity calculated using the Ba-flux obtained from the normatively calculated Ba record. Using our (Ba/Al)det ratio of ∼0.0037 leads to normatively calculated biogenic barium results that are in reasonable agreement with the biogenic barium from sequential extraction for samples of the Atlantic, Pacific and Indian Ocean. Our directly determined ‘regional’ (Ba/Al)det ratios deviate from those calculated or assumed from hinterland ratios and our global average (Ba/Al)det is lower than the one commonly reported. Therefore, in sediments with a significant terrigenous fraction, the sequential extraction technique is always required.