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At which temperature does pure liquid water have it’s highest density? Explain the processes that determine the density of water.
at T=4°C -> lowest vibration before hexagons start to form -> highest density
at T>or< 4°C -> abundance of hexagons increases/vibration increases -> density increases
at T<=°C -> all H2O held in hexagons (ice crystal lettice structure) -> all H2O molecules farthest apart --> lowest possible density .> ice 8% larger -> floats
Explain the consequences for ocean circulation of the density characteristics of SW.
fresh and warm waters are the lightest whilst cold and saline waters are the heaviest
deep water formation occurs when warm surfce water cools down, e.g. in the North Atlantic
driver of the ocean conveyor belt
Generally: warm and fresher water floats (lower density) and cold and saline water sinks (higher density). Deep water formation occurs at high latitudes e.g.: NADW/AAIW/AABW, when it cools down during its journey to colder polar waters. Additionally, the formation of ice, and the rejection of brine, might further increase the waters density, causing it to “drop” and form deep-waters. The thereby created “vacuum” is replenished with warmer and less saline surface waters, resulting in a global ocean circulation. The overall driver of this is the differing radiation budget on a global scale (e.g.: equator higher solar energy input, than in polar regions –> affects temperature).
Name 4 cations and anions that belong to the major constituents of SW.
Cations:
Na+
Ca2+
K+
Mg2+
Anions:
Cl-
HCO3-
Br-
SO4-
Which ion has the longer residence time in the ocean: Potassium or Calcium? Explain why.
Potassium, as Ca is involved in CaCO3-formation
Remember residence times: τ = mean conc. in ocean * (total ocean vol. / inflow per yr)
Although inflow per year of Ca2+ is higher (major cation of river discharge), mean concentration of Ca2+ in ocean is way lower (used in calcification→CaCO3)→K+ has the higher residence time, it is not subject to biological depletion, it behaves conservative.
Provide two reasons why some chemical elements do not behave in a conservative manner in the ocean.
Conservatice = long residence times (RT>>>ocean mixing)
Uptake
Precipitation
Scavenging
Additional supply by sediments
Aerosols
Rivers
What is the difference between molality and the molinity concentration scale (provide definitions)
molality = mol/kg H2O
molinity = mol/kg SW
Molinity incorporates the density differences occurring for seawater of different salinity.
Which is the preferred concentration scale used by marine chemists and what is the major advantage?
Molinity (mol/kgSW)
-> calculation with SW have less error
-> conservative with changes in pressure and temperature
Define residence time.
-> time that a molecule spends in the ocean
-> defined by reservoir and flux (in and out)
-> time of one turnover
Explain which basic assumptions must be made in order to apply this residence time concept.
influx = outflux
reservoir = constant
What has the longer residence time in the ocean: iron or phosphate? Explain why.
Iron: 200-500 yrs
Phosphate: 69.000 yrs
-> scavanginng leads to iron fallout
-> P is repeatedly reused
PO43- has the longer residence time in the ocean. Phosphate, although being an essential component for building up biomass is almost never limiting. However, iron, as an important co-factor for many enzymatic processes is quickly taken up and stripped out of the seawater, once it is dissolved. Furthermore, iron is quickly precipitated under oxic conditions.
Which has the longer residence time in the ocean: bromide or water?
Br-: 100 Mio. yrs
H2O: 40.000 yrs
Mention and discuss three different processes that are affected by the tltal dissolved salt concentration in the ocean.
-> freezing: dissolved salts function as a structure breaker, no H-bonds, less hexagons
-> circulation: saline water is heavier than fresh water
-> solubility of gases: saline water has less dissolved gases
What are the processes that alter seawater salinity at the ocean boundaries?
-> river input
-> mineral input (weathering)
-> evaporation
-> precipitation (direct)
-> groundwater flow
-> ice formation and brine rejection
-> ice melting
Which of the three relationships between the concentration of chemical constituent and seawater salinity (figure below) indicates conservative behavior?
Provide possible causes for non-conservative behavior.
-> precipitation
-> biological activity
-> external input
At which temperature does pure liquid water have its highest density? Explain the process that determine density of water.
At 3.98°C.
Vibration energy controls distance between molecules. At 3.98°C the minimal distance is reached and the molecules start forming hexagonal structures which reduces the density. Salt is a structure breaker and allows higher density at lower temperatures as well as lower freezing point.
Last changed2 years ago