Oceans Flashcards
What is salinity and why is it an important measurement of ocean chemistry?
Salinity: total amount of dissolved salts in seawater
- affects physical and chemical properties of seawater, such as density and freezing point
also plays important role in global water cycle and the distribution of marine organisms.
- average ocean salinity is 35%o
Conservative elements in oceans
Conservative: don’t undergo significant biological or chemical reactions, remain constant in concentration
- Na, Cl, Mg, SO4
conservative because there are so many of them in seawater
Non-conservative elements in Oceans
undergo biological or chemical reactions, varying concentrations
- N, CO2, O2, Fe,Zn Cu
short residence times relative to ocean mixing
- biological processes deplete them
Processes controlling the chemical composition of the ocean
- biological processes: dissolution, decomposition, death etc.
- river inputs of non-conservative elements
- ocean mixing and circulation
-gas exchanges with atm (increase in dissolved inorganic Carbon) - Pressure: high pressure causes dissolution of biogenic calcium carbonate falling to the bottom.
-Temperature: decreasing temperature with depth restricts vertical mixing due to thermally induced density stratification - volcanoes (Deposition of gases)
- burial in sediments
Effect of increased CO2 in the atm on the ocean
- Increased atm Co2= increased Co2 absorption by seawater–> reduction in pH
- h2O+CO2–> H2CO3
H2CO3–> H+ + CO3 - increase in H= decrease in pH
- if pH decreases by 0.3 units as expected, will decrease the saturation state of seawater with respect to CaCO3 to the point where CaCO3 would become undersaturated and begin to dissolve CaCO3 secreting organisms that fix carbonate
Important biological processes that influence oceanic chemistry?
- Photosynthesis/respiration by phytoplankton
- Calcite formation and dissolution by marine organisms
- nitrogen fixation: n2–> NO3 or N2O via organisms
- nutrient cycling of N, P, Fe
How deep can sunlight penetrate into the ocean?
~200m
- varies due to water clarity and presence of particles/dissolved substances
Generalized equation for photosynthesis
106CO2 + 16NO3−
+ HPO4−− + 122H2O + 18H+ → C106H263O110N16P + 138O2
Redfield Ratio
C:N:P of 106:16:1
- limits/stimulates primary production depending on depletion state
Common limiting nutrients in lakes
N, P
Ocean circulation impact on primary productivity
Processes that bring water to zone of light penetration will aid in photosynthesis:
1. upwelling and downwelling regions cause increased primary productivity
- upwelling brings deep, nutrient-rich water to the surface
- downwelling can transport organic matter and nutrients to deep water
- surface circulation redistributes nutrients across different regions of the ocean
Phytoplankton
organisms that drift in the water and perform photosynthesis
Zooplankton
small animals that drift in water column and consume phytoplankton
Why are nutrients enriched in deep waters relative to shallow waters?
Sinking organic material from surface results in decomposition at depth and deposition of nutrients into deep water
What is the biological pump
Absorption of CO2 from atm via photosynthesis and deposition downwards; formation of H2CO3, CaCO3 etc.
What is unique about the limiting nutrient in the waters around Antarctica?
Limiting nutrient around Antarctica is Fe
- extensive resupply of N and P to surface waters results in exhaustion of Fe
What is the typical depth in sediments at which oxygen is exhausted by oxidation of organic material?
10cm of the sediment-water interface
How is organic material oxidized in the sediments at depths greater than that of oxygen exhaustion?
anaerobic bacteria use oxygen bound to other compounds such as NO2, SO4, CO2 to oxidize the organic matter in sediments to CO2, while the original oxygen containing compounds become reduced.
What is bacterial sulfate reduction and why is it important?
performed by anaerobes
- 2CH2O + SO4– –> H2S + 2HCO3-
CH2O is o.m
- occurs in sediments with appreciable concentrations of organic matter
hydrogen sulphide produced migrates out of sediment and is oxidized
How is pyrite formed in the sediments and what are the important reactions for its formation?
- bacterial sulfate reduction produces H2S, most of which migrates out of sediments and is oxidized back into SO4, while the remainder stays and reacts with the detrital iron minerals in the sediment to form a series of iron sulfides that are ultimately transformed to pyrite
- CH2O + SO4 –> H2S + 2HCO3
- Formation of pyrite in sediments constitutes a major mechanism for removal of sulfate from seawater:
What is the influence of skeletal hard parts on the geochemistry of the oceans and their sediments?
- Organisms with skeletal hard parts live in surface layer of ocean and when they die they sink they dissolve
How do sediments formed beneath shallow seas (< 200 m water depth) differ from those formed beneath deeper seas?
- deep sea sediments:
- contain mainly coccolith and planktonic foraminiferal calcite plus biogenic silica.
- plankton debris in deep sea floor since plankton can live at any depth in sea
- much of coccolith and foram calcite are dissolved by the time they reach bottom - Shallow seas
- most benthic skeletal debris accumulates in sediments overlain by shallow waters
- little to no dissolution of calcite in shallow warm waters
How is opaline silica precipitated from ocean waters that are undersaturated with respect to opaline silica?
- opaline silica is secreted by diatoms and radiolaria
- photosynthetic energy from plankton allows plankton to remove opaline silica from surrounding (Shallow) seawater
- after this opaline silica is removed by plankton the plankton die and sink, causing most of the opaline silica to be returned to solution via dissolution at depth
What are the f and g parameters in Broecker’s (1971) model and why are they important?
f= particle flux into sediments/particle flux from shallow to deep water
- important because it represents the fraction of a biogenic element falling into deep water that survives decomposition and dissolution
g=particle flux from shallow to deep water/(river input flux + upwelling input flux)
- represents fraction of an element delivered to surface water by rivers and upwelling that is removed by biological secretion plus particle fallout. (g=1 means all the element carried into shallow water is removed by bio processes)
How do magmatic processes on and below the seafloor influence ocean chemistry?
- seafloor spreading creates new seafloor consisting of basalt
- underwater volcanism creates convection current above it
- due to chemical reactions between seawater and basalt, the convection current changes in the composition of both the basalt and circulating seawater
What is the importance of smectite formation during high-temperature and low-temperature alteration of basalt under the ocean?
What is reverse weathering and why is it important?
- process where silicate minerals carried to oceans by rivers undergo a chemical reaction which forms a new, generally more cation-rich mineral than the original mineral.
What role do cation exchange reactions on clay surfaces play in the composition of seawater?
- adsorbing cations such as ca, mg, K depending on ph and ionic strength of water
What does the abbreviation mEq mean?
milliequivalent, a unit used to express the concentration of ions in solution based on their charge
What is K-fixation and how is it different from cation exchange?
cation exchange involves replacement of one hydrated cation on clay surfaces or interlayer
in k-fixation the addition of K to the clay involves dehydration of the added K ion, replacement of hydrated cations by K only in interlayer positions, and a consequent loss of exchangeability
How do the concentrations of major ions in modern river waters compare to those elements’ concentrations in “pristine” river water?
The concentrations are much higher due to the impacts of humans
Explain and discuss the budgets of these elements and species in the ocean: Cl, Na, S, Mg, K, Ca, HCO3 - , Si, P, and N?
The budgets of the elements and species in the ocean are determined by a complex set of processes, including weathering of rocks on land, oceanic circulation, biological uptake, and sedimentation. Chloride (Cl) and sodium (Na) are conservative species, meaning they do not undergo significant biological uptake or sedimentation and are primarily introduced into the ocean through rivers and volcanic emissions. Sulfur (S) is also introduced into the ocean through rivers, but it is also released into the atmosphere by volcanic eruptions and anthropogenic activities such as the burning of fossil fuels. Magnesium (Mg) and calcium (Ca) are primarily introduced into the ocean through weathering of rocks on land, while potassium (K) is largely recycled within the ocean by biological processes. Bicarbonate (HCO3-) is introduced into the ocean through weathering of rocks on land and is a product of biological processes such as photosynthesis and respiration. Silica (Si) and phosphorus (P) are primarily introduced into the ocean through rivers and are important nutrients for marine organisms. Nitrogen (N) is also introduced into the ocean through rivers, but it is primarily recycled within the ocean by biological processes.
What is the effect of an evaporite basin on ocean chemistry?
- evaporite basin: area where seawater evaporates, leaving behind deposits of salts such as NaCl, CaSO4
- removes large amounts of ions from seawater such as NaCl and CaSO4, leading to increase in concentrations of other ions, like mg and so4, impacting pH
What is the lysocline?
depth in ocean where rate of dissolution of CaCO3 exceeds rate of supply. ~4000m in open ocean
- sudden
What is the carbonate compensation depth?
the depth where the rate of supply of calcareous skeletal debris from above is equaled by the downward increasing rate of dissolution
Sea to air transport
Bubbles burst at surface, and while most material returns to water, some contribute to rain and freshwater chemistry (Na and Cl only)
saturation state
Omega=Ion activity Product / Equilibrium constant
