Dissolution of titanomagnetite and sulphidization in sediments from Lake Kinneret, Israel

Abstract

S U M M A R YMagnetic extracts obtained from sediment cores recovered from Lake Kinneret, Israel, were subjected to comprehensive scanning electron microscope and in-line energy dispersive Xray spectroscopy analyses. A variety of ferrimagnetic iron oxides and iron sulphide particles were identified, along with apparently non-magnetic iron sulphides. The majority of the iron oxides are titanomagnetites that have undergone pervasive dissolution, and in many cases were only preserved as skeletal, 3-D lattices of ilmenite lamellae. Often the spaces between the submicron ilmenite lamellae are empty. Also sponge-like dissolution features are observed in some residual titanomagnetite grains. Another group of titanomagnetite grains is characterized by superficial shrinkage-cracks, which indicate low-temperature maghemitization. In some cases, these cracks appear to be where dissolution started. Most titanomagnetites contain impurities of Al, Mg, Si and Mn on the order of 1 per cent. Chromium-magnetites, with minor Ti, Mg and Al content on the order of a few percent, but without Mn, were also found. Chromium-magnetites mostly occur as octahedra in a well-preserved state, which indicates that they are less prone to dissolution. Magnetite spherules of 40-60 μm in size, which were mostly hollow, were only found in material collected from the outer zone of the core section that was in contact with the steel core barrel. These spherules were not found in material collected from the centre of the core. They are therefore interpreted as contaminants that were produced during fabrication of the steel core barrel. Iron sulphides are present as pyrite, either as larger aggregates of cubic crystals, each of up to 20 μm in size, or as 5-10 μm framboidal aggregates, consisting mostly about 0.3 μm idiomorphic octahedra. The cubic pyrite crystals have heterogeneous compositions. Pure pyrite should be non-magnetic, but even aggregates of up to 150 μm in size were magnetically separated. They probably therefore contain a magnetic core of greigite or overgrown titanomagnetite. The latter possibility is supported by observations of residual titanomagnetites with individual pyrite crystals residing directly on their surfaces, or filling space between ilmenite lamellae. Irregular aggregates of flaky, submicron iron sulphides were also found, which are interpreted to be greigite. The greigite has a low degree of visible crystallization. Pyrite and/or greigite grains were found inside the aggregates, or in agglomerates with organic remnants. Together, visual observations and elemental analyses provide clear evidence of pervasive dissolution of primary titanomagnetite and concurrent sulphidization of sediments from Lake Kinneret.

Magnetic mineral diagenesis

“…Much iron oxide dissolution in lake sediments could occur via iron reduction; nevertheless, widespread iron sulphide formation indicates that sulphidic diagenesis is not a negligible process in lake sediments. For example, greigite formation and preservation has been widely reported in lake sediments (Skinner et A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 55 al., 1964;Dell, 1972;Giovanoli, 1979;Snowball and Thompson, 1988,b, 1992Hilton, 1990;Snowball, 1991Snowball, , 1996Roberts et al, 1996;Turner, 1997;Hu et al, 1999;Peters and Turner, 1999;Reynolds et al, 1999;Stockhausen and Zolitschka, 1999;Demory et al, 2005;Frank et al, 2007a,b;Ron et al, 2007;Nowaczyk, 2011). These observations demonstrate that sulphidic diagenesis cannot be ignored in lake sediments.…”

“…Nevertheless, sulphate values are not zero and magnetite dissolution is a widely reported process in organic-rich lake sediments (e.g., Anderson and Rippey, 1988;Snowball, 1993a, b;Thouveny et al, 1994;Rosenbaum et al, 1996;Snowball, 1996;Stockhausen and Zolitschka, 1999;Demory et al, 2005;Ao et al, 2010;Nowaczyk, 2011;Su et al, 2013;Fu et al, 2015). Much iron oxide dissolution in lake sediments could occur via iron reduction; nevertheless, widespread iron sulphide formation indicates that sulphidic diagenesis is not a negligible process in lake sediments.…”

“…Konhauser, 1998), for which Watson et al (2000) provided photomicrographic evidence (Figure 7). Iron sulphides often also occur as mineralized replacements of organic matter (Figure 8), including the interior of microfossil chambers ( Figure 9) (e.g., Raiswell, 1982;Roberts and Turner, 1993;Stumm and Morgan, 1996;van Dongen et al, 2007;Nowaczyk, 2011). Grimes et al (2001) demonstrated experimentally that iron sulphide precipitation starts on the cell walls of plant matter.…”

GEOMAGIA50.v3: 2. A new paleomagnetic database for lake and marine sediments

“…Climate may affect weathering, erosion, and sedimentation processes, resulting in distinct magnetominerological facies. In situ processes may be deduced from the identification of oxidation, dissolution, and growth of new magnetic minerals, such as sulphides (e.g., Frank et al 2007;Nowaczyk 2011;Roberts et al 2011;Snowball and Thompson 1988).…”

Mechanism of variations in environmental magnetic proxies of lake sediments from Nam Co, Tibet during the Holocene

“…Susceptibility is low in the whole Holocene lake sediment ( lf < 12×10 8 m 3 kg 1 ) for two reasons: on one hand, the water of Nam Co is mainly supplied from south and southwest shore mainly composed of sandstone, which contains less magnetic minerals and is weakly magnetic; on the other hand, there are probably iron sulfides in the whole core (probably greigite contained in coarse pyrite particles [28,32,37]), which is suggested by 3-axis thermal demagnetization curves and -T curves. This suggests that magnetic minerals are in reducing conditions.…”