11 Marine Chemistry

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Marine Chemistry I

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OutlinePart I

Physical Properties of Water

Salinity

Part 2 Geochemical Cycles

Conservative ConstituentsNon-Conservative Constituents

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Physical Properties of Water

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Cl-

Na +

Dissolving Power of Water1. Highest of any substance known

2. Solvation effects must consider the molecular interactions between water (solvent) and dissolved substances (solutes)

Simple case: NaCl solid " Na +solvated + Cl-solvated

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Relatively Weak (but highly important) Hydrogen Bonding

#+

#+

# -

# - #+#+

# -# -

an electrostatic attraction between partial + and charges on separate polar molecules

Hydrogen Bond

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Three States (Phases) of Water

H-bonds relatively weak bond energy holding adjacent water molecules together unchanging strength

Thermal (kinetic) Energy pushes/breaks adjacent molecules apart increases with increasing temperature

solid, liquid and gas

Two opposing forces determine the structure of water in its three phases:

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1. Maximum number of H-bonds

2. Maximum order, low thermal motion

3. Regular lattice structure of iceice (solid)

Low Temperature Limit:

E H-bond > E thermal

Solid Ice

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1. Clusters of H-bonded water(structural water)

2. Interspersed non-H-bonded water(free water)

liquid water

The Intermediate Case:E H-bond " E thermal

Liquid Water

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1. minimum number of H-bonds

2. minimum order, rapid thermal motion

3. independent, non-interacting gasmolecules

High Temperature Limit:E H-bond < E thermal

water vapor (gas)

Water Vapor (gas)

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1. The exceptionally high specic heat capacity of water means that it takes anexceptionally large amount of heat energy to change ocean temperatures

2. Conversely, relatively small observed changes in ocean temperature represent very large changes in heat content

3. All things being equal, if you have more heat energy coming into the earth than leaving the earth, then you should observed a steady rise in global temperature.

However , if you had the same amount of excess heat coming into the earth,

but you shifted more of the excess heat to the ocean versus the atmosphere (or land), then the rise in temperature would not be as great (since the sameexcess heat added to water does not change the water temperature all thatmuch because of waters high specic heat capacity)

An Important Consequence of Waters Exceptionally High

Specic Heat Capacity...

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Evaporation Moves Latent Heat From the Ocean to the

Atmosphere

Evaporation: Latent Heat removed from the ocean and stored in the atmospherein the form of water vapor

Condensation: Latent Heat released

into the atmosphere by condensation of water vapor to form clouds and rain

1.

2.

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Conclusions: Molecular Properties of Water

1. Strong polar nature of the water molecule makes it a very good solvent for ionicconstituents (salt ions) i.e., it can dissolve a lot of salt

2. Hydrogen bonds are weak, but below 100 oC they are strong enough to allowindividual water molecules to bond temporarily with other water molecules to form

liquid water. Below 0 oC they are strong enough to hold/lock all water molecules intosolid crystalline ice.

3. High specic heat capacity means a given heat addition does not change ocean temperatures as much as would occur if the same amount of heat was added to theatmosphere (or land)

4. High latent heat of vaporization allows large amounts of heat to be removed from theocean, stored at latent heat in the form of water vapor and then transported by winds to other parts of planet where it can then be released to the atmosphere as sensible heatupon precipitation

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Salinity of Seawater

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Salinity is a measure of the salt concentration (total weight of salt) in a seawatersample. It is often expressed as the number of grams of salt contained in a

thousand grams of seawater and expressed as parts per thousand and denoted by the symbol . 35 grams of salt in 1000 grams of seawater has a salinity of 35

Denition: Salinity

Side Note: A more modern unit of salinity is used in ofcial oceanographic research called the practical salinity unit ( psu ), that is based on electrical conductivity measurements rather than the mass of saltmeasurements. Both methods give essentially the same numerical values (i.e., 35 35 psu).

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Input: Weathering of continental rock, and subsequent transport by rivers, constantly brings new salt ions to the ocean each year

Output: Mineral precipitation (e.g., calcium carbonate CaCO 3 shell formation anddeposition into sediments) within the ocean constantly removes salt ions from solutioneach year

The magnitude of the input and output rates have been roughly equal for millions of year - i.e. steady state conditions have been achieved

NOTE: While the total amount of salt in the ocean does not vary, the unequal

addition/removal of freshwater over the global oceans surface creates large regionaldifferences in surface ocean salt concentration (i.e., in surface ocean salinity)

The Total Amount of Salt Contained in the Entire Ocean is Essentially Constant

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Cl- (chloride) 55%

Na + (sodium) 31%

SO 4-(sulfate) 8%

Mg 2+ (magnesium) 4%

Ca2+ (calcium) 1%

K + (potassium)

1%

HCO 3-(bicarbonate) 0.4%

All other ions


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1. All salinities in the ocean (deep ocean included!) are/were set at the air-sea interface

2. Evaporation at the ocean surface removes only freshwater and leaves behind salt - thus increasing surface ocean salinity

3. Atmospheric precipitation adds freshwater to the surface ocean - thus reducing surface ocean salinity

4. Overall, salinity is a direct function of evaporation minus precipitation

Variation in Surface Salinity

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Global Pattern of Precipitation andEvaporation

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uestionGiven that surface salinity is a function of evaporation and precipitation , and given the

basic Hadley Circulation pattern (below), what do you expect the surface salinity to bein the subtropical gyres?

a) salinity is relatively low in the subtropics

b) salinity is relatively high in the subtropics

The correct answer is (b) - dry winds produce high evaporation and low precipitation

subtropical gyre

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Idealized Hadley Circulation

Low SLPHigh SLP High SLPLow SLP

1. Moist surface air at the equator warms and rises aloft. Air aloft spreads north/south and becomes more dense as it cools and dries (due to precipitation) and then sinks at about 30 latitude.

2. Dry air aloft descends and warms and spreads out over the sea surface at 30 to the north and

south. The surface air picks up moisture and by 60 latitude it has warmed and moistened to the point where it rises, cools, precipitates and spreads out aloft north/south.3. Near the poles the dry air aloft becomes very cold and very dense so it sinks over the poles and

spreads out toward the equator

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Millimeters Per Day

Annual Average Precipitation Pattern

Hadley circulation produces upward convection and high precipitationalong the equator and also at about 60 Latitude

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Idealized Hadley Circulation

Low SLPHigh SLP High SLPLow SLP

1. Moist surface air at the equator warms and rises aloft. Air aloft spreads north/south and becomesmore dense as it cools and dries (due to precipitation) and then sinks at about 30 latitude.

2. Dry air aloft descends and warms and spreads out over the sea surface at 30 to the north and

south. The surface air picks up moisture and by 60 latitude it has warmed and moistened to the point where it rises, cools, precipitates and spreads out aloft north/south.

3. Near the poles the dry air aloft becomes very cold and very dense so it sinks over the poles andspreads out toward the equator

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Annual Average Evaporation Pattern

Millimeters Per Day

Hadley circulation at around 30 latitude is where cold dry air aloft descends and warms and spreads outnorth/south (and is turned by Coriolis) over the earths surface.

The warm and dry surface winds are conducive to strong evaporation in the subtropics regions centeredat 30 latitude.

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P a r t s p e r T h o u s a n d

Ocean Surface Salinity

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Vertical Distribution ofSalinity

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Atlantic Ocean Salinity

The salinity of seawater in the ocean interior is set at the ocean surface in the regions of deep waterformation (North Atlantic and Antarctica) by the combined effects of precipitation and evaporation in

these regions. Once removed from the surface the salinity remains constant unless it mixes withother water masses.

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Pacic Ocean Salinity

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Indian Ocean Salinity

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Surface Salinity Variation Summary1. Surface salinity varies widely and is a function of evaporation minus precipitation

High latitudes have low surface salinity High precipitation Low evaporation

Tropics have high surface salinity High evaporation Low precipitation

Equator has a dip in surface salinity High precipitation offsets high evaporation

2. While salinity may vary considerably in different regions, the relative proportion of one ion to another does not vary .

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Marine Chemistry II

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Outline

1. Spatial patterns of conservative constituents2. Spatial patterns of non-conservative constituents

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Geochemical Cycles Keeping Track of Elemental Inputs, Chemical Transformations and Outputs

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Spatial Distribution of Conservative andNon-Conservative Constituents in Seawater

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Conservative and Non-conservative PropertiesConservative Constituents of seawater are those that are only varied by

physical exchange processes at the sea surface ( or else mixing at depth).Once the water leaves the surface, these properties are conserved .1. salinity ()

2. temperature

3. inert gas concentration (e.g., Argon)

Non-conservative Constituents of seawater are those that are varied by processes (other than mixing) that occur anywhere in the watercolumn. For example:

1. biological processes (e.g. nutrient uptake and remineralization)

2. geochemical processes (e.g., radioactive decay)

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Phytoplankton Nutrients (Nonconservative Constituents)

Nutrients - elements or compounds required by phytoplankton togrow and reproduce

nitrogen NO 3- (nitrate), NH 4+ (ammonium)

phosphorus PO 43- (phosphate)

silicon SiO 42- (silicate)

trace metals Fe, Zn, Mo, Cu, Co, etc

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Non-conservative Constituents: Plant Nutrients

d e e p w a

t e r

sea surface

thermocline

sediments

dissolved nutrient

Particulate Organicsphotosynthesis

respiration

sinking

burial

[Nutrient Conc.] !

" d

e p

t h

dissolved nutrient = NO 3 or PO 4 or SiO 2

dissolved nutrient

Particulate Organics

1.Low in surface layer because of rapid uptake by phytoplankton in the presence of sunlight

2. High at depth because of respiration/remineralization and no uptake by phytoplankton in

the dark

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Atlantic Ocean Nitrate While plant nutrients like nitrate and phosphate and silica are low in most of thesurface ocean (except in the iron-limited Southern Ocean), the generally higherdeep nutrient concentrations can vary greatly due to horizontal advection . Note

the high southern ocean nitrate moving with Antarctic Intermediate Waternorthward under the south and north subtropical gyres.

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Global Pattern of Nutrient Concentrationin the Deep Ocean

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Conveyor Belt Circulation

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Phosphate at 4000 Meters

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Pattern of Oxygen Concentrationin the Ocean

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Dissolved O 2

(Non-conservative Constituent)

CO 2 + H2O inorganic N (NO 3-, NH 4+) inorganic P (PO 43-)

organic materials

+ O 2

photosynthesis

respiration

1. Vertical diffusion across the air sea interface 2. Horizontal advection from nearby regions

1.Photosynthesis produces oxygen 2.Respiration consumes oxygen

Biological Sources and Sinks

Physical Sources and Sinks

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Non-conservative Constituent: Dissolved O 2

m i x e

d

l a y e r

d e e p w a

t e r

sea surface

Thermocline

O 2

O 2

O 2

atmosphere

particulate organics

particulate organics

[O2] !

" d

e p

t hoxygen

minimum

Photosynthesis produces O 2

respiration consumes O 2

Little respiration occurs at depth due to low organics needed to

fuel O 2 consumption combined with horizontal advection ofhigh O 2 water from other locations

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Atlantic Ocean Oxygen

Notice the southward movement of high O 2 water within the North Atlantic Deep Water (NADW).

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[O 2] !

" d

e p

t h

oxygenminimum

[O 2] !

" d

e p

t h oxygenminimum

relatively easy

mixing and diffusion(thick red arrow) ofoxygen down into

the oxygen minimumzone to moderate

the extent of the minimum

Weak Thermoclinerelatively difcult

mixing anddiffusion (thin redarrow) of oxygendown into theoxygen minimumzone so minimum is

made even stronger

Strong Thermocline

Global warming is expected to increase the strength of the thermocline and thereby reduced vertical mixing and diffusion across this boundary and make the oxygen minimum zone even lower

Today Future

weak

thermocline

strong

thermocline

strong diffusion/mixing

weak diffusion/mixing

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Conveyor Belt Circulation

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Oxygen at 4000 Meters

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Carbon Dioxide (Non-conservative Constituent)

CO 2 + H2O inorganic N (NO 3-, NH 4+) inorganic P (PO 43-)

organic materials + O 2

photosynthesis

respiration

1. Photosynthesis Consumes CO 2 2. Respiration Produces CO 2

4. Chemical Reaction With Water Makes Hydrogen Ions:

CO 2 + H2O # H 2CO 3 # H + + HCO 3- # H + + CO 3-2 carbonic acid bicarbonate carbonate

CO 2 + H2O # H 2CO 3 carbonic acid H 2CO 3 # H + + HCO 3- bicarbonate

HCO 3- # H + + CO 3-2 carbonate

3 Diffusion Across the Air-Sea Interface

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Non-conservative Constituent: CO 2

sediments

CO 2gas

CO 2organic C + H 2O H2CO 3 HCO 3- CO 32-

organic C deposition

photosynthesis takes up (consumes) some CO 2 and makes organic carbon and thereby lowers CO2 in the upper ocean

below the euphotic zone (the sun lit zone of surface ocean) the respiration by bacteria thatdegrade dead organic carbon produces CO 2 and thereby increases CO 2 at depth

+ H + + H +

diffusion across air/sea interface

respiration --> reaction of CO 2 with water --------------------------------------->

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Non-conservative Constituent: CO 2

ocean

Photosynthesis Consumes CO2 in the surface ocean to form particulate organic carbon

Respiration Produces CO2 in thedeep ocean

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Pacic Meridional Section of Inorganic Carbon Species

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Carbon Dioxide in the Deep Ocean (below the thermocline) is by Far theLargest Active/Mobile Reservoir of Carbon Dioxide on Earth.

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1. Red and Yellow are regions where CO2 uxes out of the ocean 2. Purple and Blue are regions where CO2 uxes into the ocean

Bringing deep ocean water that is rich in CO 2 into contact with the atmospherecauses CO 2 to ux out of the ocean note the equatorial upwelling region

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pH of the Ocean Surface Layer

1.Purple and Blue are regions that are more acidic 2.Red and Yellow are regions that are less acidic

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Coastal Upwelling brings deep water that is cold and rich in CO2 (and therefore more acidic) up to the surface in coastal regions...

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Conclusions1. The eld of chemical oceanography is concerned with the geochemical

(i.e., global-scale elemental) cycles that take place at least in part within the ocean.

2. A main approach to identifying geochemical cycles is to identify a particular elements principal sources into the ocean, its major avenues ofremoval from the ocean and any signicant chemical reactions that it

participates in while in the ocean.

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Conclusions

1. Conservative Properties (e.g., temperature and salinity) do not change value once the water leaves the surface ocean ( except when different water masses mix together at great depth)

2. Non-Conservative Properties (e.g., nitrate, phosphate, oxygen, carbondioxide) can change value after the water leaves the surface ocean.

3. The Conveyor Belt Circulation explains why nitrate and phosphate getmore concentrated as the deep water moves from the Deep North

Atlantic and gradually into the Deep Pacic - organic matter rains down intothe deep water, and it is remineralized to nitrate and phosphate, as the deep water slowly moves toward the Pacic

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Conclusions1. Oxygen in the deep ocean become lower as the deep circulation slowly carries

water from the North Atlantic to the Pacic because the remineralization process, that caused nitrate and phosphate to increase along the way, consumes oxygen.

2. Carbon Dioxide that enters the ocean by diffusion across the air-sea interface, or

from biological respiration, undergoes a chemical reaction with water to formother inorganic carbon compounds (e.g., carbonate and bicarbonate) and thisreaction needs to be considered when examining the overall cycling of carbondioxide in the ocean - for this class, the details of the reactions are not important,

just knowing that other reactions need to be considered is enough - you do,however, need to know that increases in CO 2 leads to more acidic ocean

3. Sinking organic carbon and biogenic/mineral precipitation produces a strong vertical gradient of CO 2 in the ocean.

11 Marine Chemistry

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