Discuss the main sources of silicon to the ocean
Rivers >> aeolian > basalt weathering > hydrothermal
From land: weathering of quartz, Olivine, Kaoline (minerals combined with Fe, Mg, Ca)
In ocean: weathering of Quartz, feldspar, clay minerals
Discuss the major process driving the silicon cycle in the ocean
The largest part of dissolved Si stays in surface layer-remineralization loop or export-upwelling loop. The most of the terrigenous input accumulates in sediment, as it is not easily dissolved because lithogenic silica is in a mineral crystal lattice -> bad dissolution. Most biogenic silica stays in biological production loop because it is amorphous opal and easily diluted. Key players in the biogenic silica cycle are diatoms. The outcompete most other phytoplankton species, once c(Si) has reached a threshold of 2μmol/l. They play a major role in upwelling regions.
Where are the major regions with opal-rich sediments and what are the reasons for their occurrence?
Major regions for opal-rich sediments are dependent on the overlying productivity and silicic acid supply, the degree of preservation during sinking (type of opal → frustule thickness, also species differences, sinking speed, temperature, bacterial activity, distance of sinking, concentration in underlying waters), amount of non-silicious sedimentation → rate of dilution and burial. This usually aligns with upwelling areas → diatoms outcompete + strong CaCO3 dissolution due to OMZ beneath and in the Southern Ocean (High productivity, efficient sinking + very little dissolution (Tlow), high export flux + little non- silicious sedimentation) -> exceptionally large amount of Si-Sediments
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