Ocean Chemistry Flashcards
Photosynthesis is a redox reaction. Write out the individual oxidation and reduction equations and give their ‘nicknames’.
Oxidation (aka ‘the light equation’ as occurs using light energy) = 2H20 —–> O2 + 4H+ + 4e-
Reduction (aka ‘the dark equation’) = CO2 + 4H+ + 4e- —> CH20 + H2O
What is decomposition and what is it mediated by?
The conversion of organic matter into inorganic matter, mediated by bacteria using oxygen.
Give the reaction for decomposition and identify what is oxidised and what is reduced.
Organic matter + O2 —–> H2O + CO2
Organic matter is oxidised to CO2, whilst O2 is reduced to H2O
Organic matter reaches the ocean as marine snow - what is this?
A continuous shower of largely organic detritus primarily as aggregated dead cells and faecal pellets
What fraction of primary production escapes the surface ocean, and what percentage reaches the ocean floor?
20% escapes surface ocean, with only 0.5% of this reaching the ocean floor.
How do phytoplankton contribute to differences in enrichment between the surface and deep oceans?
Phytoplankton grow in the surface ocean, but not at depth, therefore decomposition dominates at depth and enriches nutrients here, relative to the surface ocean.
Nutrients are brought to the surface ocean by upwelling. Where does this occur?
- Coastal upwelling = particularly strong in the peru current. El Nino / La Nina events reverse areas of upwelling.
- Equatorial upwelling = in the middle of the subtropical gyres.
- Island effect upwelling = as currents encounter islands, deep nutrient rich water is forced to surface - helps formation of coral reefs.
By what other process are nutrients brought to the surface? Where does this occur?
Winter mixing in the high latitudes.
Which nutrient cycles are coupled in the ocean? What are the exceptions?
PO4, Silicate (SiO4), carbon (mostly coupled to silicate), nitrogen. Difference in that CO2, HCO3- and CO32- available to phytoplankton in form of DIC, as well as the surface exchange of CO2. Oxygen is anticoupled to phosphorus.
What is the concentration of oxygen in the deep ocean controlled by? How does this explain its ‘anticoupling’ to the phosphorus (and closely linked carbon) cycle?
Controlled by decomposition and ocean circulation. Oxygen is used in the decomposition of matter, as opposed to released by it - like CO2 - so its cycle flows in the opposite direction.
Define Eutrophication.
The increased rate of supply of organic matter due to a higher nutrient supply.
What are the effects of eutrophication and what is it attributed to?
Changes species composition and causes hypoxia - attributed to human PO4 and NO3 release from intensive agriculture, industry and population growth. (Humans caused a doubling of nitrogen and tripling of phosphorus flows to the environment).
What are the additional reactions that the marine nitrogen cycle has, in comparison to the phosphorus cycle.
Nitrogen fixation - whereby N2 is used by some nirtogen fixers to build organic matter, as opposed to NO3- or NH4+.
Denitrification - occurs when oxygen levels are too low for bacteria to decompose organic material using O2. Instead they use NO3- and release N2 and NO2.
What is denitrification an example of? Show this using an equation.
A redox reaction.
Organic matter + NO3- —> 1/2N2 + CO2
NO3- reduced to N2 and organic matter oxidised to CO2.
Decomposition of organic matter produces NH4+. Bacteria can convert - but what into? And what is the process called?
Bacteria can transform ammonium into nitrate (NO3-) by nitrification.
Describe the characteristics of the marine nitrogen cycle?
Characterise by ‘Scavenging’ (by sinking particles that then remove it from the water column) and ‘Dust’ input (the main source of iron for the surface ocean.
Describe the ‘dust’ associated with the marine iron cycle. What is its distribution related to?
1-10μm particles - emitted globally at a rate of 1000-2000Tg/yr - Typical lifetime in atmosphere of a couple to hours to a few weeks - can include viruses and pollen grains, as well as anything else in soils of a suitable size. It’s flux to the oceans corresponds to latitudes of deserts.
What is the connection between ‘dust’ and HNLC regions?
As HNLC regions are thought to be cause by a limitation of iron, areas without dust are often HNLC regions.
