Lithium isotope geochemistry of marine pore waters - Insights from cold seep fluids

Lithium concentration and isotope data (δ⁷Li) are reported for pore fluids from 18 cold seep locations together with reference fluids from shallow marine environments, a sediment-hosted hydrothermal system and two Mediterranean brine basins. The new reference data and literature data of hydrothermal fluids and pore fluids from the Ocean Drilling Program follow an empirical relationship between Li concentration and δ⁷Li (δ⁷Li = −6.0(±0.3) · ln[Li] + 51(±1.2)) reflecting Li release from sediment or rocks and/or uptake of Li during mineral authigenesis. Cold seep fluids display δ⁷Li values between +7.5‰ and +45.7‰, mostly in agreement with this general relationship. Ubiquitous diagenetic signals of clay dehydration in all cold seep fluids indicate that authigenic smectite-illite is the major sink for light pore water Li in deeply buried continental margin sediments. Deviations from the general relationship are attributed to the varying provenance and composition of sediments or to transport-related fractionation trends. Pore fluids on passive margins receive disproportionally high amounts of Li from intensely weathered and transported terrigenous matter. By contrast, on convergent margins and in other settings with strong volcanogenic input, Li concentrations in pore water are lower because of intense Li uptake by alteration minerals and, most notably, adsorption of Li onto smectite. The latter process is not accompanied by isotope fractionation, as revealed from a separate study on shallow sediments. A numerical transport-reaction model was applied to simulate Li isotope fractionation during upwelling of pore fluids. It is demonstrated that slow pore water advection (order of mm a⁻¹) suffices to convey much of the deep-seated diagenetic Li signal into shallow sediments. If carefully applied, Li isotope systematics may, thus, provide a valuable record of fluid/mineral interaction that has been inherited several hundreds or thousands of meters below the actual seafloor fluid escape structure.     

Cretaceous reactivation and intensified erosion in the Archean–Proterozoic Limpopo Belt,demonstrated by apatite fission track thermochronology

Cratons are generally assumed to be regions of long-lasting tectonic stability. In particular the study of the Phanerozoic exhumation history of cratons has been largely hampered by the scarcity of suitable stratigraphic controls onshore. This fact is even more pronounced in terranes lacking Mesozoic or younger penetrative structural fabrics and metamorphic overprinting. Our study in the Limpopo belt shows that modern apatite fission track thermochronology provides a hitherto unavailable perspective in the study of these rocks, and has profound implications for the crustal evolution of the Zimbabwe Craton.Apatite fission track data from 35 samples taken along two transects, in the southern edge of the Zimbabwe Craton and in the Central Zone of the Limpopo Belt, suggest that extensive regions experienced kilometer-scale exhumation in two discrete events, as recently as the Cretaceous. The first occurred at around 130 Ma, and the second at around 90 Ma. Basin subsidence and sedimentation loads on the Mozambique margin support the timing of these events and provide strong indications of the source and pathways for the eroded material. Further, the results indicate that young and old “surfaces” (in a geomorphological sense) may be structurally juxtaposed in regions of high elevation in Zimbabwe. This is contrary to early ideas of surface chronologies based on summit accordances or invoking pediplanation.     

Sponges (Porifera) as living metazoan witnesses from the Neoproterozoic: biomineralization and the concept of their evolutionary success

The emergence of the Metazoa can be dated back to the Neoproterozoic Era which comprises the Cryogenian Period during which two major glaciations occurred, the Sturtian and the Varanger-Marinoan. At that time, the phylum Porifera (sponges) evolved as the first animals and developed a hard skeleton. The two classes of siliceous sponges, the Hexactinellida and the Demospongiae, are already provided with the major genetic repertoire and gene regulatory networks that also exist in modern multicellular animals. Besides these metazoan innovations, the siliceous sponges display one autapomorphic character, silicatein, an enzyme which mediates bio-silica formation. Well preserved siliceous sponge fossils have been excavated from the Cambrian Burgess Shale- and Chengjiang deposits. It is concluded that it was the hard skeleton of the siliceous sponges that contributed to the successful evolution and survival of the Porifera during the last 500 Ma.