Geochemical and early diagenetic aspects of interbedded pelagic/turbiditic sediments in two N. Atlantic Abyssal Plains

Abstract

An adequate shipboard system has been developed for the handling and analysis of sUboxic sediments and pore waters. The results obtained with this system have been compared with those obtained using another system. Between these two systems large variations have been found in the concentration of some pore-water constituents. These variations could be explained not only by contamination with oxygen, but also by temperatures deviating from the in-situ temperature during the pore-water extraction. The sediments that have been studied originate from two North Atlantic abyssal plains, namely the Madeira and Nares Abyssal Plains (from hereon referred to as MAP and NAP respectively). Both plains are characterized by interbedded pelagic/turbiditic sediments. The average sedimentation rate for the sediments in the NAP is 10 times higher, and the average turbidite thickness 10 times lower, than those in the MAP. The post-depositional early diagenetic processes that occur in the sediments of both plains are mainly determined by the organic matter content. The organic matter content is in general higher in turbiditic sediments than in pelagic. Therefore, in the former sediments the concentration of N03- generally goes to zero within a sediment depth of 1 m, whereas at 8 m in a pelagic core from the same area (NAP) the concentration of N03- is still higher than it is in the bottom water. The pore-water concentration of Mn2 + in the turbiditic sediments of the NAP increases sharply down to a depth of approximately 3 m, and from thereon remains nearly constant due to saturation with respect to Mn,Ca-C03. The pore water of the turbiditic sediments is also saturated with respect to calcite. The few "diagenetic spikes" in the pore-water concentration of N03- and Mn2 + and the concentration/depth profile of dissolved iron, H4Si04 and phosphate all clearly demonstrate the inhomogeneous nature of interbedded pelagic and turbiditic sediments. The simultaneous occurrence of peaks of dissolved iron/silica and of sediment intervals with a relatively high organic carbon content is attributed to enhanced early diagenetic reactions associated with the decomposition of organic matter in these specific intervals. Due to the unique combination of low porosity and relatively high sedimentation rates, the sediments from the NAP are an ideal basis for the study of the pore-water chemistry in such interbedded sequences of pelagic and turbiditic deposits.