No abstract

Biosilicification has driven variation in the global Si cycle over geologic time. The evolution of different eukaryotic lineages that convert dissolved Si (DSi) into mineralized structures (higher plants, siliceous sponges, radiolarians, and diatoms) has driven a secular decrease in DSi in the global ocean leading to the low DSi concentrations seen today. Recent studies, however, have questioned the timing previously proposed for the DSi decreases and the concentration changes through deep time, which would have major implications for the cycling of carbon and other key nutrients in the ocean. Here, we combine relevant genomic data with geological data and present new hypotheses regarding the impact of the evolution of biosilicifying organisms on the DSi inventory of the oceans throughout deep time. Although there is no fossil evidence for true silica biomineralization until the late Precambrian, the timing of the evolution of silica transporter genes suggests that bacterial silicon-related metabolism has been present in the oceans since the Archean with eukaryotic silicon metabolism already occurring in the Neoproterozoic. We hypothesize that biological processes have influenced oceanic DSi concentrations since the beginning of oxygenic photosynthesis.

No abstract

Silicon isotope ratios (expressed as δ30Si) in marine microfossils can provide insights into silica cycling over geologic time. Here we used δ30Si of sponge spicules and radiolarian tests from the Paleogene Equatorial Transect (Ocean Drilling Program Leg 199) spanning the Eocene and Oligocene (~50–23 Ma) to reconstruct dissolved silica (DSi) concentrations in deep waters and to examine upper ocean δ30Si. The δ30Si values range from −3.16 to +0.18‰ and from −0.07 to +1.42‰ for the sponge and radiolarian records, respectively. Both records show a transition toward lower δ30Si values around 37 Ma. The shift in radiolarian δ30Si is interpreted as a consequence of changes in the δ30Si of source DSi to the region. The decrease in sponge δ30Si is interpreted as a transition from low DSi concentrations to higher DSi concentrations, most likely related to the shift toward a solely Southern Ocean source of deep water in the Pacific during the Paleogene that has been suggested by results from paleoceanographic tracers such as neodymium and carbon isotopes. Sponge δ30Si provides relatively direct information about the nutrient content of deep water and is a useful complement to other tracers of deep water circulation in the oceans of the past.

International audienceThe silicon isotopic composition (δ30Si) of the headwaters of the Ganges River, in the Himalaya, ranged from +0.49±0.01 to +2.17±0.04 at dissolved silicon (DSi) concentrations of 38 to 239μM. Both the concentration and isotopic composition of DSi in the tributaries increased between the highest elevations to where the Ganges leaves the Himlayas at Rishikesh. The tributaries exhibit a linear correlation between δ30Si and DSi that may represent mixing between a low DSi, low δ30Si (e.g., 40μM, +0.5 ) component potentially reflecting fractionation during adsorption of a small fraction of silicon on to iron oxides and a high DSi, high δ30Si component (e.g., 240μM, +1.7 ) produced during higher intensity weathering with a greater proportional sequestration of weathered silicon into secondary minerals or biogenicsiica. On the Ganges alluvial plain, in the Ganges and the Yamuna, Gomati, and their tributaries, DSi ranged from 122 to 218μM while δ30Si ranged from +1.03±0.03 to +2.46±0.06 . Highest values of δ30Si occurred in the Gomati and its tributaries. In general, the lower DSi and higher δ30Si of DSi in these rivers suggests control of both by removal of DSi by secondary mineral formation and/or biogenic silica production. A simple 1-dimensional model with flow through a porous medium is introduced and provides a useful framework for understanding these results