Distribution of various extracted phosphorus compounds in the interbedded turbiditic/pelagic sediments of the Madeira Abyssal Plain, eastern North Atlantic

Abstract

A sequential extraction scheme was used to differentiate different P fractions in the interbedded turbiditic/pelagic sediments of the Madeira Abyssal Plain. These fractions were: ‘carbonate-associated + exchangeable phosphate’ (P ex), ‘iron- and aluminium-associated phosphate’ (P OH), ‘calcium-associated phosphate/apatite’ (P HCl), ‘organic phosphorus’ (P org), and ‘remaining phosphorus’ (P R). The P ex forms a minor fraction of the total sedimentary phosphorus (P tot), whereas for most samples P HCl represents near 80% of the P tot. The amount of P in most fractions appears to be determined by the composition and provenance of the various turbidites. The relatively high P ex and the moderately high ‘apatite-P’ (P HCl) for the sediment of turbidite e (‘organicrich’ turbidite provenance group) may point to a source area that is possibly upwelling-related. On the other hand, the high P HCl for the sediments of the turbidites b,g,n,o,p corresponds nicely to their ‘volcanic’ and near-coastal provenance area. In contrast to this major impact of inherited P, only a small contribution comes from P that is associated to post-depositional re-allocation of Fe (P OH). This results in a P OH concentration that is slightly higher in the upper post-depositionally oxidized layer of a turbidite than it is in the lower still un-oxidized layer of the same turbidite. In addition, this postdepositional oxidation of the upper layer of a turbidite may also result in enhanced Fe and P levels in the overlying pelagic interval. The C/P molar ratio for organic matter in these sediments is much higher than that reported for more recent sediments, and for average marine plankton (1200 versus 500, and 106, respectively), which may be caused by postdepositional differential decomposition of organic matter.