Oceans

Question 1

What is salinity and why is it an important measurement of ocean chemistry?

Answer 1

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

Question 2

Conservative elements in oceans

Answer 2

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

Question 3

Non-conservative elements in Oceans

Answer 3

undergo biological or chemical reactions, varying concentrations
- N, CO2, O2, Fe,Zn Cu
short residence times relative to ocean mixing
- biological processes deplete them

Question 4

Processes controlling the chemical composition of the ocean

Answer 4

• 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

Question 5

Effect of increased CO2 in the atm on the ocean

Answer 5

• 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

Question 6

Important biological processes that influence oceanic chemistry?

Answer 6

• 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

Question 7

How deep can sunlight penetrate into the ocean?

Answer 7

~200m
- varies due to water clarity and presence of particles/dissolved substances

Question 8

Generalized equation for photosynthesis

Answer 8

106CO2 + 16NO3−
+ HPO4−− + 122H2O + 18H+ → C106H263O110N16P + 138O2

Question 9

Redfield Ratio

Answer 9

C:N:P of 106:16:1
- limits/stimulates primary production depending on depletion state

Question 10

Common limiting nutrients in lakes

Answer 10

N, P

Question 11

Ocean circulation impact on primary productivity

Answer 11

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

2. surface circulation redistributes nutrients across different regions of the ocean

Question 12

Phytoplankton

Answer 12

organisms that drift in the water and perform photosynthesis

Question 13

Zooplankton

Answer 13

small animals that drift in water column and consume phytoplankton

Question 14

Why are nutrients enriched in deep waters relative to shallow waters?

Answer 14

Sinking organic material from surface results in decomposition at depth and deposition of nutrients into deep water

Question 15

What is the biological pump

Answer 15

Absorption of CO2 from atm via photosynthesis and deposition downwards; formation of H2CO3, CaCO3 etc.

Question 16

What is unique about the limiting nutrient in the waters around Antarctica?

Answer 16

Limiting nutrient around Antarctica is Fe
- extensive resupply of N and P to surface waters results in exhaustion of Fe

Question 17

What is the typical depth in sediments at which oxygen is exhausted by oxidation of organic material?

Answer 17

10cm of the sediment-water interface

Question 18

How is organic material oxidized in the sediments at depths greater than that of oxygen exhaustion?

Answer 18

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.

Question 19

What is bacterial sulfate reduction and why is it important?

Answer 19

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

Question 20

How is pyrite formed in the sediments and what are the important reactions for its formation?

Answer 20

• 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:

Question 21

What is the influence of skeletal hard parts on the geochemistry of the oceans and their sediments?

Answer 21

• Organisms with skeletal hard parts live in surface layer of ocean and when they die they sink they dissolve

Question 22

How do sediments formed beneath shallow seas (< 200 m water depth) differ from those formed beneath deeper seas?

Answer 22

1. 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
2. Shallow seas
   - most benthic skeletal debris accumulates in sediments overlain by shallow waters
   - little to no dissolution of calcite in shallow warm waters

Question 23

How is opaline silica precipitated from ocean waters that are undersaturated with respect to opaline silica?

Answer 23

• 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

Question 24

What are the f and g parameters in Broecker’s (1971) model and why are they important?

Answer 24

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)

Question 25

How do magmatic processes on and below the seafloor influence ocean chemistry?

Answer 25

• 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

Question 26

What is the importance of smectite formation during high-temperature and low-temperature alteration of basalt under the ocean?

Question 27

What is reverse weathering and why is it important?

Answer 27

• 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.

Question 28

What role do cation exchange reactions on clay surfaces play in the composition of seawater?

Answer 28

• adsorbing cations such as ca, mg, K depending on ph and ionic strength of water

Question 29

What does the abbreviation mEq mean?

Answer 29

milliequivalent, a unit used to express the concentration of ions in solution based on their charge

Question 30

What is K-fixation and how is it different from cation exchange?

Answer 30

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

Question 31

How do the concentrations of major ions in modern river waters compare to those elements’ concentrations in “pristine” river water?

Answer 31

The concentrations are much higher due to the impacts of humans

Question 32

Explain and discuss the budgets of these elements and species in the ocean: Cl, Na, S, Mg, K, Ca, HCO3 - , Si, P, and N?

Answer 32

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.

Question 33

What is the effect of an evaporite basin on ocean chemistry?

Answer 33

• 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

Question 34

What is the lysocline?

Answer 34

depth in ocean where rate of dissolution of CaCO3 exceeds rate of supply. ~4000m in open ocean
- sudden

Question 35

What is the carbonate compensation depth?

Answer 35

the depth where the rate of supply of calcareous skeletal debris from above is equaled by the downward increasing rate of dissolution

Question 36

Sea to air transport

Answer 36

Bubbles burst at surface, and while most material returns to water, some contribute to rain and freshwater chemistry (Na and Cl only)

Question 37

saturation state

Answer 37

Omega=Ion activity Product / Equilibrium constant