Sulphur geochemistry and sapropel formation : syngenetic and diagenetic signals in eastern Mediterranean sediments

Abstract

In this thesis the sulphur geochemistry of eastern Mediterranean sediments is studied. The sediments discussed were recovered during the 1987 ABC cruise with R/V Tyro (core ABC27), the 1988 BAMO-3 expedition of R/V Bannock (cores GC17 and GC21), the 1991 Marflux cruise with R/V Marion Dufresne (cores KC01B and KC19C), the 1993 Marflux cruise with R/V Tyro (core MT1), the 1994 Palaeoflux cruise with R/V Urania (core UM26) and ODP Leg 160 in 1995 (Sites 964, 966, 967, and 969) (Fig. 1.1). Chapter 2 deals with the most eye-catching feature in the sedimentary sulphur chemistry around sapropels: apart from enrichments of solid phase sulphur and iron within sapropels, sulphur and iron are also enriched in a zone of decimetres thickness, directly below each organic-rich layer, where organic carbon is not enriched. The sulphur and iron enrichments reflect the presence of abundant pyrite (FeS 2 ). Stable sulphur isotopic compositions of bulk sediments show that the reduced sulphur within as well as below sapropels formed through bacterial sulphate reduction at or close to the sediment surface, i.e. during or shortly after sapropel deposition. Pyrite formation within the sapropels was iron-limited and consequently, bisulphide was able to migrate downwards. This resulted in pyrite formation below each sapropel by reaction of bisulphide with solid phase reactive iron and upward diffusing dissolved Fe(II). This downward sulphidisation mechanism allowed burial of twice as much sulphur in alternating organic-rich- Chapter 1 12 anoxic / organic-poor-suboxic sediments compared to homogeneous organic-rich-anoxic sediments. In Chapter 3 the speciation of sulphur and contents of reactive iron in sediments with sapropels are discussed in detail. Pyrite is the dominant sulphur species within and immediately below each sapropel. Directly above sapropels, sulphur is hardly present in the solid phase, but occurs as porewater sulphate. Large scale formation of organic sulphur compounds occurred only in the most organic-rich sapropel that was investigated (maximum organic carbon content = 23.5 wt%). The presence of iron sulphides other than pyrite indicate that sulphate reduction probably still continues in this exceptionally organic-rich sapropel, whereas in other sapropels no sulphate reduction occurs at present. Chapters 4 and 5 investigate detailed pyrite properties within and below sapropels, in order to gain further insight into the formation of pyrite and sapropels. The pyrite characteristics (contents, microtextures and isotopic compositions) were gouverned by the relative rates of bisulphide production and iron liberation and supply in the sapropels. These rates were both temporally and laterally variable during sapropel deposition. At times of relatively high sulphate reduction, bisulphide escaped from the sapropel and pyrite was formed in the underlying sediments. The sources of iron for pyrite formation comprised detrital reactive solid phase iron and diagenetically liberated dissolved Fe(II) from sapropel-underlying sediments. In exceptionally organic-rich sapropels, input of dissolved Fe(II) from the water column via iron sulphide formation in the water may have been important. Rapid pyrite formation at high saturation levels of iron and bisulphide resulted in the formation of framboidal pyrite within the sapropels, whereas directly below each sapropel slow euhedral pyrite formation at low saturation levels occurred. Stable sulphur isotopes in pyrite in sapropels are strongly enriched in the light sulphur isotope 32 S. Below the sapropels, sulphur isotopes are even more enriched in 32 S than within sapropels. This is a result of increased bisulphide reoxidation at times of relatively high bisulphide production, when bisulphide could escape from the sediment into the water column. The reoxidation may have effected up to 80% of the produced sulphide. The effect of extensive sulphide reoxidation on organic carbon burial efficiencies and paleoproductivity estimations are considered in Chapter 5. The enhanced accumulation of OM in sapropels appears to have been caused by both increased paleoproduction of OM in the surface waters and enhanced preservation of the OM after settling. Chapter 6 explores the origin of sulphur in OM within and around sapropels by studying the stable sulphur isotopic compositions and sulphur to carbon ratios in the OM. The organic sulphur in the sediments is a mixture of sulphur derived from (1) inorganic reduced sulphur produced in microbial sulphate reduction, and (2) biosynthetic sulphur. The uptake of reduced sulphur into OM was most pronounced within the sapropels, where pyrite formation was iron-limited, so that reactive iron was no longer competing with OM for the uptake of reduced sulphur and dissolved sulphide concentrations increased. In Chapter 7 results from organic geochemical analyses, trace metal chemistry and pyrite study in some exceptionally organic-rich Pliocene sapropels are integrated. These data prove that euxinic (sulphidic) water column conditions must have existed throughout the eastern Mediterranean basin over substantial periods of time during the formation of these sapropels. Introduction and summary 13 Chapter 8 discusses the sediment chemistry and magnetic properties in a 19.6-metre-long core from an exceptional site in the abyssal eastern Mediterranean. This core contains a large number of sapropels which were used to construct a time frame for the sediment record at this site. The exceptional feature is that, in contrast with comparable sites, porewater contained sulphide down from a few metres below seafloor (mbsf). This sulphide was possibly produced by bacterial sulphate reduction combined with methane oxidation at a depth of about 17.5 mbsf. Upward migrating sulphide has pyritized all reactive iron up to a depth of 2 mbsf. As a consequence, the paleomagnetic signal of iron minerals has been destroyed, and no reliable paleomagnetic data can be obtained in the lower half of the core. Summarizing, the work described in this thesis illustrates that chemical processes in marine sediments may be highly dynamic, especially when different redox systems are forced to coexist. This may happen when sediments with different OM contents are superimposed, as a result of periodically changing environmental conditions, or when processes in the subsurface, such as the input of methane, disturb the chemical system in the sediments. As a result of the redox imbalances, sediments may be chemically altered after deposition. On the one hand, this alteration can obscure and even destroy geological information stored in the sedimentary record. On the other hand, relicts of geochemical processes may reveal the paleoceanographic and diagenetic history of the sediments.