Why study the marine silicon cycle? Major nutrient needed by Si mineralizers Diatoms Role in biogeochemical cycles Link to carbon cycle diatoms account for 75% of marine primary productivity…in high nutrient and coastal regions Diatoms couple the silicon cycle to the carbon cycle important in transferring carbon from surface waters to the deep sea Many rocks are formed from a combination of calcium and silicon When these rocks weather, the silicon atoms combine with oxygen to form quartz-like minerals (silicon dioxide) and the calcium ions become available to form limestone. The formation of limestone has a net effect of removing carbon atoms from other reservoirs, including the atmosphere.

Silicate Structural material Diatoms Radiolarians Silicoflagellates Sponges Silicon = Silicon is one of the most common elements on Earth in the Earth's crust, it's second in mass only to oxygen and can be found in any quartz crystal. Beach sand is largely silicon. Silicon is also the semiconductor material out of which almost all modern transistors are made. Silicate = Any of a large group of minerals, forming over 90 percent of the earth's crust, that consist of silicon, oxygen, and one or more metals, (and sometimes hydrogen). Opaline skeletons

Silica in Water Silicon dioxide SiO2 + 2H2O H4SiO4 Three forms: H4SiO4 (monosilicic acid) Three forms: Reactive Colloidal Suspended particles Young seawaters that are highly undersaturated with H4SiO4 are far more corrosive to SiO4 (there is more water to combine with it). Older seawaters that have been dissolving and accumulating H4SiO4 over hundreds of thousands of years are less corrosive. Reactive = SiO2 dissolved in water, creating monosilicic acid (H4SiO4); generally un-ionized in this form at most natural pH levels Collodial = either silicon that has polymerized with multiple units of silicon dioxide or silicon that has formed loose bonds with organic compounds Suspended particles (quartz = sand)

Dissolution of seafloor basalts = 7% Inputs: Riverine = 80% Aeolian deposition = 7% Dissolution of seafloor basalts = 7% Hydrothermal vents = 6% Inputs: Riverine = 80% Aeolian deposition = 7% (pertaining to the action or effect of the wind) Dissolution of seafloor basalts = 7% Seafloor hydrothermal inputs = 6% Outputs: Sedimentation of biogenic opal

Bloom ‘n’ Bust Lifestyle Spring High nutrients and light Bust Depletion of nutrients, esp. silicon Sink Resting spores Sinking is advantageous b/c removing from grazer populations and higher temps. Some lost to the deep, others held at thermocline until vertical mixing entrains them. This results in replenishment of nutrients to the upper layers, leading to a bloom round once again.

What’s so good about this? Sinking diatoms export from the atmosphere into the ocean interior Diatoms play a key role in the biogeochemical cycle of silicon in the ocean Biogenic silica in diatom skeletons acts as an effective pH buffer, facilitating the conversion of bicarbonate to dissolved CO2 CO2 is more readily assimilated In seawater, the principle natural buffers are, in order of importance, bicarbonate, borate, and silicate. The large buffering capacity of polymerized silica would add to the energetic economy that may come from making the cell wall out of inorganic rather than organic material. From this, diatoms may potentially influence the concentration of CO2 in the atmosphere. In the Eocene (40 mya) there was a radiation in diatoms and evidence for an atmospheric low of CO2. These may be linked.