Cycling of phosphorus and Manganese in the Arabian Sea during the Late Quaternary

This thesis investigates phosphorus and manganese cycling in the Arabian Sea during the Late Quaternary, using sediment records recovered during the Netherlands Indian Ocean Programme (NIOP) in 1992. The first part is concerned with the reconstruction of Late Quaternary variability in paleoproductivity and oxygen minimum zone intensity. Special attention is given to manganese and sulphur accumulation patterns in order to trace back variations in bottom water oxygen content of the deep basin and continental slope. In the second part of this thesis P cycling in the Arabian Sea sediments is investigated. Pore water fluxes and solid-phase P speciation results are combined to assess the diagenetic processes controlling the benthic regeneration and burial of P. The contrasting environmental conditions that 'characterized the Arabian Sea in past and present provide an excellent opportunity to elucidate the mechanisms that govern P removal from the oceans, and to determine the potential role ofP cycling on changes in paleoproductivity over geological time scales

Phosphogenesis and active phosphorite formation in sediments from the Arabian Sea oxygen minimum zone

In this study, porewater chemistry, solid-phase analysis and microscopic observations were combined to evaluate phosphogenesis in three boxcores located within the intensive oxygen minimum zone of the Arabian Sea. Three parameters, namely a decrease of the dissolved phosphate and fluoride concentrations with depth, saturation with respect to carbonate fluorapatite, and the presence of a solid-phase Ca-phosphate mineral, all indicate that phosphogenesis is currently taking place at all three sites. Authigenic apatite precipitation rates vary between 0.076 and 1.04 μmolP cm−2 yr−1, and are of the same order of magnitude as reported for other high productivity areas. Precipitation of an intermediate precursor precedes francolite formation in the continental slope sediments on the Karachi Margin. Results of a diagenetic P model indicate that phosphogenesis is induced by high rates of organic matter degradation. Dissolution of fish debris is likely to provide a substantial additional source of phosphate. Redox iron cycling does not influence phosphogenesis in these environments. Model results suggest that sediment mixing is essential in promoting early diagenetic phosphogenesis. The highest rate of francolite formation was observed in a boxcore taken on the Oman Margin, where it contributes to the formation of a Holocene phosphorite deposit. This observation contrasts with previous reports of only old phosphorites in this area. Phosphorites are presently forming on the Oman Margin as a result of: (a) deposition of older, reworked material from the continental shelf, which has undergone an earlier phase of phosphogenesis; (b) a high input of reactive P (fish debris and degradable organic matter); (c) a relatively low sediment accumulation rate; and (d) the absence of winnowing on this location. Holocene phosphorite deposits may be less common on the Oman Margin than in other coastal upwelling areas because of the narrowness of the shelf and the steepness of the slope, which limit the area where phosphorite formation may occur

Phosphorus burial as a function of paleoproductivity and redox conditions in Arabian Sea sediments

Abstract—In this study the response of sedimentary phosphorus (P) burial to changes in primary productivity and bottom water oxygen concentrations during the Late Quaternary is investigated, using two sediment cores from the Arabian Sea, one recovered from the continental slope and the other from the deep basin. The average solid-phase P speciation in both cores is similar, authigenic and biogenic (fish debris) apatite make up the bulk of the P inventory (ca. 70%);whereas P adsorbed to iron oxides, organic P, and detrital apatite constitute minor fractions. Postdepositional redistribution has not significantly altered the downcore distribution of total solid-phase P. Phosphorus burial efficiencies are generally lower during periods of increased paleoproductivity. This is caused by (a) partial decoupling of the P export flux, consisting primarily of particulate organic P, and the P burial flux, consisting primarily of biogenic and authigenic apatite; and (b) the lack of increased rates of authigenic CFA formation during periods of higher P deposition. In addition, fluctuations in bottom water oxygen concentrations may have affected P burial in continental slope sediments. The results of this study indicate that higher primary productivity induces more efficient P cycling. On time scales exceeding the oceanic P residence time, this process may induce higher surface water productivity, thus creating a positive feedback loop. In the Arabian Sea, this feedback mechanism may have contributed to changes in sea surface productivity on sub-Milankovitch time scales because P, regenerated on the continental slopes of the Oman and Somalian coastal upwelling zones, is reintroduced into the photic zone relatively fast

Ba accumulation in the Arabian Sea: Controls on barite preservation in marine sediments

In this study, a new sequential extraction technique is used to investigate the particulate barium (Ba) phases in sediments from the Arabian Sea to determine the processes controlling Ba accumulation in marine sediments. The total solid-phase Ba concentration in Arabian Sea surface sediments increases with water depth from ∼200 ppm at 500 meters below sea surface (mbss) to ∼1000 ppm at 3000 mbss. The sedimentary Ba composition consist of three major fractions: Barite, Ba incorporated in aluminosilicates, and Ba associated with Mn/Fe oxides. Accumulation of barite, which is the most important Ba fraction in sediments located below 2000 mbss, increases gradually with water depth. The Ba/Al ratio of the terrigenous fraction varies significantly across the Arabian Basin as the result of differences in grain size and provenance of the terrigenous sediment. Ba associated with Mn oxides is a relatively minor fraction compared with bulk Ba concentrations, and it only accumulates in well-oxygenated sediments below the present-day oxygen minimum zone. The water-depth-dependent accumulation of barite in the Arabian Sea is not related to the continuous formation of barite in settling organic particles or Ba scavenging by Mn oxyhydroxides but is primarily controlled by differences in Ba preservation upon deposition. A good correlation between the barite saturation index and the barite accumulation rate in the upper 2000 m of the water column may indicate that the degree of barite saturation of the bottom water is the main environmental factor regulating the burial efficiency of barite. Organic matter degradation, bioturbation, diagenetic Mn cycling, and the crystallinity of the accumulating barite may play additional roles. Copyright © 2001 Elsevier Science Ltd

Erratum to ''Synchroneity of oxygen minimum zone intensity on the Oman and Pakistan Margins at sub-Milankovitch time scales'' - [Marine Geology 185 (2002) 283-302]

In the above paper the wrong figure was printed as Fig. 6 on page 411. The correct figure is given below. The Publisher would like to apologise for any inconvenience caused by this error

Ice skating

What we are proposing in this chapter is an overview of the ice skating. We are now not considering the figurative ice skating, but only the ice speed-skating. Furthermore, only few hints will be provided about the outdoor skating, focusing then on the indoor competitions. After an historical reconstruction of the origins of this sport, the analysis will be divided into two main blocks: the dynamical model and the aerodynamic analysis of a speed skater (both numerical and experimental). Few notes about the track will be discussed along the chapter