Biogenic silica
This is opposed to the other major biogenic minerals, comprising carbonate and phosphate, which occur in nature as crystalline iono-covalent solids (e.g. salts) whose precipitation is dictated by solubility equilibria.
These organisms extract dissolved silicate from open ocean surface waters for the buildup of their particulate silica (SiO2), or opaline, skeletal structures (i.e. the biota's hard parts).
[2][3] Some of the most common siliceous structures observed at the cell surface of silica-secreting organisms include: spicules, scales, solid plates, granules, frustules, and other elaborate geometric forms, depending on the species considered.
[4] The silicon cycle has gained increasingly in scientific attention the past decade for several reasons: Firstly, the modern marine silica cycle is widely believed to be dominated by diatoms for the fixation and export of particulate matter (including organic carbon), from the euphotic zone to the deep ocean, via a process known as the biological pump.
As a result, diatoms, and other silica-secreting organisms, play a crucial role in the global carbon cycle, and have the ability to affect atmospheric CO2 concentrations on a variety of time scales, by sequestering CO2 in the ocean.
Secondly, biogenic silica accumulation on the sea floor contains lot of information about where in the ocean export production has occurred on time scales ranging from hundreds to millions of years.
For this reason, opal deposition records provide valuable information regarding large-scale oceanographic reorganizations in the geological past, as well as paleoproductivity.
This relative short residence time, makes oceanic silicate concentrations and fluxes sensitive to glacial/interglacial perturbations, and thus an excellent proxy for evaluating climate changes.
[3][5] Increasingly, isotope ratios of oxygen (O18:O16) and silicon (Si30:Si28) are analysed from biogenic silica preserved in lake and marine sediments to derive records of past climate change and nutrient cycling (De La Rocha, 2006; Leng and Barker, 2006).
In addition, isotope analyses from BSi are useful for tracing past climate changes in regions such as in the Southern Ocean, where few biogenic carbonates are preserved.
[6] This preferential preservation of biogenic silica relative to organic carbon is evident in the steadily increasing ratio of silica/organic C as function of depth in the water column.
About thirty-five percent of the biogenic silica produced in the euphotic zone survives dissolution within the surface layer; whereas only 4% of the organic carbon escapes microbial degradation in these near-surface waters.
As a result, polar sediments account for most of the ocean's biogenic silica accumulation, but only a small amount of the sedimentary organic-carbon flux.
This gradual increase in the importance of silicate (Si) relative to nitrogen (N) has tremendous consequences for the ocean biological production.
This lower interbasin difference in DSi has the effect of decreasing the preservation potential of opal in the Atlantic compared to its Pacific and Southern ocean counterparts.
Continental margin upwelling areas, such as the Gulf of California, the Peru and Chile coast, are characteristic for some of the highest biogenic silica accumulation rates in the world.
