Redox-dependent phosphorus burial in modern and ancient marine sediments. Geologica Ultraiectina (334)

Phosphorus (P) is an essential nutrient in Earth’s biosphere that helps regulate marine primary productivity. Burial in sediments is the only pathway for long-term removal of P from the marine reservoir, the efficiency of which depends strongly on the redox state of the bottom waters; oxygen-depleted conditions at the sediment-water interface enhance the recycling of P from the sediment back to the water column. In this thesis, the diagenesis and burial of P in relation to temporally and spatially variable redox conditions are investigated in modern and ancient sediments. The impact of sample treatment and long-term diagenesis on sedimentary P records is of special interest. It is shown that exposure of reducing, carbonate-poor sediments to oxygen can strongly alter the sedimentary P speciation through the effects of pyrite oxidation, rendering the artifact P speciation of little use to reconstruct redox conditions during sediment deposition. In addition, long-term diagenesis may impact P speciation in ancient, 94 Myrs-old sediments. Apart from these limitations, Cretaceous sedimentary records show a strong coupling between the burial efficiency of P and deposition of organic-rich sediments during oceanic anoxic event (OAE) 2, ~ 94 Myrs ago, suggesting that enhanced P availability may have helped to sustain high productivity. In younger, Quaternary sediments in long cores from the Mediterranean Sea, the in situ P speciation and organic carbon to phosphorus ratios have been preserved and can be used to reconstruct paleoceanographic conditions during formation of the investigated sediments. Detailed investigation of both the Cretaceous and Quaternary sediments suggest that fish debris can constitute an important sedimentary P sink during periods of high primary productivity and bottom water oxygen depletion in Earth’s past. Due to rapid deposition and burial, the abundance of fish debris in marine sediments can provide valuable information on surface water productivity. The techniques applied to investigate the chemical forms of P in sediments become ever more advanced. This development is incorporated in this thesis in the form of a study employing both classic sequential P extractions and advanced X-ray P spectroscopy to quantify P speciation in marine sediments. The results underline the reliability of sequential extraction as a fast, economic tool to quantify sedimentary P speciation and highlight the added advantage of X-ray spectroscopy, which enables not only bulk P speciation but also provides information on the mineralogy and crystallinity of the sedimentary P phases. Finally, P cycling in modern sediments from a depth transect across the oxygen minimum zone (OMZ) in the northern Arabian Sea was investigated. The results show that formation of authigenic calcium-phosphate minerals (Ca-P) is largely restricted to sediments in the OMZ, where the increased input of labile P phases compensates for enhanced recycling of P under more oxygen-depleted conditions. Iron redox cycling is important to sedimentary P retention and authigenic Ca-P formation. Furthermore, dust deposition may be an important source of calcium phosphate minerals in the investigated sediments. Accounting for dust-derived, detrital P results in the downward correction of the reactive, non-detrital, P burial efficiency in Arabian Sea sediment