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Continuation of the second exam notes

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Origin of the Ocean and the Atmosphere - Keynote pdf
Hydrological Cycle - Keynote pdf
Seawater Salinity - Keynote pdf
Variations in Seawater Chemistry - Keynote pdf

Seawater

Origin of the Ocean and Atmosphere

Earth and the Solar System formed 4.6 billion years ago.
Its likely that Earth originally was molten or partially molten because of the energy released during its formation.
Earth was too hot to have liquid water when it formed.
Within a few hundred million years, the outside of Earth cooled, forming the crust.
Evidence indicates that the ocean existed by at least 4.0 billion years ago.

Likely Earth originally cooled from a magma to form solid rock.
Therefore the ocean, the sediments, and the atmosphere formed later.
Most of the chemical constituents in the ocean, sediment, and the atmosphere came from the weathering of the rocks at Earth's surface.
For example, the amount of Ca in the weathered rocks equals the amount of Ca in the ocean, sediments, and atmosphere.

The total amounts of some chemical constituents in the ocean, atmosphere, and sediments, however, are too high to have come solely from the weathering of the rocks at Earth's surface.
These constituents are called excess volatiles.
Their concentrations are in excess, and they readily enter the gas phase at Earth surface temperature and pressure, so they form gases in the atmosphere.
The major excess volatiles are H2O, CO2, Cl, N, S.

Because H2O forms most of the ocean and N2 forms most of the atmosphere, both the ocean and the atmosphere could not have come primarily from elements weathered from rocks at Earth's surface.

The most-accepted hypothesis is that the ocean and the atmosphere formed from gases released from volcanoes.
Early in the history of Earth, it likely was much hotter and had much more volcanic activity.
Magma and its gases come from the mantle, so the ocean and the atmosphere come from deep in the earth.

Hydrologic Cycle

The hydrologic cycle describes the cycling of water at Earth's surface.
The hydrologic cycle is driven by solar energy.

97% of water at Earth's surface is in the ocean.
Most of the rest, 2%, is in the polar ice caps.

Water entering the ocean is recycled sea water:

  1. Most water is removed from the ocean by evaporation
  2. 90% of the evaporated water returns to the ocean as rain
  3. The remaining 10% precipitates on land and flows back into the ocean

Then the cycle begins again.
Water runnng into the ocean from the land is called runoff.

Seawater Salinity

Major Constituents

Seawater is 96.5% H2O (solvent) and 3.5% sea salts (solutes).
Most of the salinity variations in open-ocean seawater are relatively small, so oceanographers move the decimal place over one place.
Salinity concentrations are given in parts per thousand (ppt).
The average salinity of seawater is 34.7 ppt (35 ppt)

The dissolved constituents of seawater are divided into four categories:

  1. Major constituents
  2. Minor constituents
  3. Trace elements
  4. Gases

All of the major constituents are ions.
The major constituents constitute >99% of all dissolved substances.
The six major constituents are Cl, Na, SO4, Mg, Ca, and K
Know the first two in order: Cl is the most abundant, followed by Na.
Know the other four in any order.

The concentrations of the six major constituents essentially determine the salinity of sea water.

Scientists once believed that seawater salinity was constant, but better measurements show that salinity varies (a little).
In the open ocean, salinity generally varies from 33 ppt to 37 ppt.

Marcet's Principle

Despite the small variations in seawater salinity, the relative proportions of the major constituents are constant.
Salinity averages 35 ppt, but varies from 33-37 ppt.

Because the principle of constant proportions holds for the major constituents (Cl, Na, SO4, Mg, Ca, and K).
For example, the Na/Ca, K/Mg, or Cl/SO4 are constant for the most part in the open ocean.
This concept is termed Marcet's principle or the principle of constant proportions.

To determine salinity within <1%, one must know the concentrations of only the major constituents.
The major constituents' concentrations determine salinity, as the major constituents are >99% of salinity.
However, to determine the concentrations of the six major constituents, one only needs to measure the concentration of a sinlge major constituent.
Generally, chloride (Cl-) is measured, as it has the highest concentration.
The concentration of the other major constituents are calculated.

Conservative Properties of Seawater

In the ocean, salinity is altered primarily at the ocean surface by the

  1. addition of water (precipitation, ice melting, or river flow)
  2. removal of water (evaporation or ice formation)

At some latitudes, it rains a lot, and salinity decreases.
At other latitudes, evaporation is dominate, and salinity increases.
In both instances, salt was not added nor removed, so salinity will change, but not the relative proportions of the major constituents.

As the major constituents are 99% of salinity, salinity can be determined closely from the concentrations of the major constituents only.
Marcet's principle is valid, because salinity varies in the open ocean when water is added via precipitation or removed via evaporation.
Salts are not added nor removed, so the relative proportions of the major constituents remain constant.

The concentrations of the major constituents are conservative properties of sea water.
Conservative properties are:

  1. altered primarily at the ocean's surface
  2. not affected significantly by biological processes

The salinity of seawater is relatively uniform because the ocean is well mixed.
Most water sinks from the surface in the high latitudes of the Atlantic Ocean.
The mixing pattern of the deep ocean is from the Atlantic to the Indian to the Pacific.
Everywhere water rises back to the surface.
At the surface the return flow is back to the Atlantic.
The mixing time is approximately 1000 yr.

The ocean is well mixed, as the residence times of the major constituents are much longer than the mixing time of the ocean.
Major constituents have very long residence times, all >1,000,000 yr.
In 1,000,000 years, the ocean mixes itself 1000 times.
The ocean is well mixed, therefore salinity is relatively uniform.

Minor Constituents and Trace Elements

The minor constituents are measured in ppm, parts per million.
The trace elements are measured in ppb, parts per billion, or pptr, parts per trillion.
In general,

  1. Marcet's principle does not apply to the minor constituents and trace elements
  2. these constituents tend to be nonconservative
  3. these constituents tend to have shorter residence times

The primary biolimiting nutrients generally are in trace amounts in seawater.
The primary biolimiting nutrients, those most responsible for limiting primary production, are N, P, Si, and Fe.
Nutrient concentrations in the surface waters of the open ocean generally are low, so surface water tends to be clear.

Gases

The atmosphere primarily is composed of two gases:

The ocean has three primarily gases:

  1. N2
  2. O2
  3. CO2

N2 is conservative, whereas O2 and CO2 are nonconservative.
The concentrations of oxygen and carbon dioxide are both affected significantly by biological activity and can be altered below the surface.

Variations in Seawater Chemistry

Major constituents are conservative, so their concentrations are relatively uniform.
The ocean is well mixed, so the concentrations of the conservative constituents are about the same everywhere in seawater.
However, the concentrations of most of other constituents can vary significantly.
Large variations in the concentrations of most minor constituents, trace elements, and gases are measured.

Oceanographers observe two types of variations in the seawater chemistry of most minor constituents, trace elements , and gases:

  1. vertical variations
  2. horizontal variations

Vertical Variations

The most dramatic changes in the concentrations of these constituents occurs in upper 1 km of the ocean.

The most commonly observed pattern of vertical variation is

These variations of the concentrations of most minor constituents, trace elements, and gases primarily result from biological processes.
The two dominant biological processes are:

  1. photosynthesis
  2. respiration

Photosynthesis:

CO2 + H2O + (N, P, etc) => CH2O(N, P, etc) + O2

Respiration:

CH2O(N, P, etc) + O2 => CO2 + H2O + (N, P, etc)

Plants live in the sunlit surface waters where they incorporate dissolved constituents.
Photosynthesis dominates in surface waters, therefore the constituents consumed by photosynthesis are depleted.
Dead organic matter and fecal material sinks into deeper water where most decays.
The deep waters are enriched in the constituents released during respiration, the dominant process in deep waters.
The primary decomposers are bacteria.

A major exception to the primary pattern of variation, depleted (lower concentrations) in surface waters and enrichment (higher concentrations) in deep waters, is oxygen, O2.
Oxygen tends to be enriched (higher concentrations) in surface waters and depleted (lower concentrations) in deep waters.
This pattern results because O2 is released during photosynthesis and consumed during respiration.

Photosynthesis is the dominant biological process in surface waters, whereas respiration dominates in deep waters.
There is no photosynthesis below a few hundred meters because of a lack of visible light.
Therefore the concentration of O2 varies inversely compared to those of N, P, and CO2.

Horizontal Variations

The concentrations of the major constituents are approximately the same in the deep waters of all ocean, approximately 35 ppt.
The major constituents are conservative and the ocean is well mixed.
However, significant horizontal variations in the concentrations of most of the minor constituents, trace elements, and gases are observed.

The primary pattern of horizontal variation is low in the deep waters of the Atlantic Ocean, higher in the deep waters of the Indian Ocean, and highest in the deep waters of the Pacific Ocean.
This pattern is a function of the general circulation of deep water, i.e. deep-water currents.
The general pattern of flow starts with sinking from the surface in the high latitudes of the Atlantic Ocean, then flowing into the Indian Ocean, and ultimately into the Pacific Ocean.

During the hundreds of years that it takes the water to flow from the Atlantic Ocean to the Indian Ocean and ultimately to the Pacific Ocean, the deep waters collect dissolved constituents released by respiration.

Respiration:

CH2O(N, P, etc) + O2 => CO2 + H2O + (N, P, etc)

Each day organic matter sinks into the deep ocean where it is respired.
Bacteria are the primary decomposers.
By the time the water reaches the deep Pacific Ocean, it concentrations of N, P, and CO2 are significantly higher.

Therefore the concentrations of most minor constituent, trace elements, and gases, like N, P, and CO2, tend to be lower in the deep Atlantic Ocean, higher in the deep Indian Ocean, and highest in the deep Pacific Ocean.

However, O2 is a major exception to this primary pattern of horizontal variation.
Oxygen is highest in the deep Atlantic Ocean, lower in the deep Indian Ocean, and lowest in the deep Pacific Ocean.
This pattern results because O2 is consumed during respiration.
The concentration of O2 varies inversely compared to those of N, P, CO2.

The second exam notes continue in Air-Sea Interactions

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