Lectures 5 Geosc 040Chemical Properties of Water --The Wonder Substance! Inputs, Outputs and Residence Time

Today’s music:

Into the Ocean Blue OctoberThanks to Darien

H.

Ocean Salinity Map

The Elements Lyrics by Tom

Lehrer

Ocean ManBy Ween

Thanks to David D.

• Lecture Review Questions:

• TA Office Hours (T & W 11-12)

• Homework 1 due on Thurs. (30th) by 11pm

• Cell Phone Recycling

A landcane over Australia

Thanks to Justin D.http://earthsky.org/earth/a-landcane-over-australia

…warm-core cyclones

Hurricane Tracks

Sea Surface Temperature & Latent heat are key factors in Hurricane development and

sustainability.

Sea Surface Temperature & Latent heat are key factors in Hurricane development and

sustainability.

Warm water is the energy source for hurricanes

energy

Warm water is the energy source for hurricanes

energy 0 200 400 600 800

Heat input (cal/gram)

Tem

pera

ture

(°C

)

150

100

-100

50

0

-50

Liquid water

Ice

Vapor

Latent heat of evaporation 540cal/gm

Chemical Properties of Water --The Wonder Substance! Inputs, Outputs and Residence Time

Cl-, Na+, S04-2, Mg+2, Ca+2, K+ >99% of salt

in sea water

HC03-2, Br-, Sr-2, B+2, F- (with these,

99.99%)

http://www.webelements.com/

Seawater is essentially an NaCl solution (saltwater)

For example, exchange of Magnesium (Mg) in seawater for Ca in ocean crust supplies excess Calcium

•Difference in chemical compositions between rivers and ocean

--reflects sedimentation (precipitation) processes--other inputs/exchanges, such as basalt-seawater reactions at midocean ridges

Rivers vs. Other Sources

Evaporation plays a big role in Surface Water Salinity

ocean chemistry: A balance of inputs and outputs

37 ppt30 ppt Surface water salinity

We’ve already examined why water is a powerfulSolvent, now let’s look at the whole picture

Can we explain ocean chemistry usingthe inputs of rivers alone?

ocean

rivers

atmosphererock

weathering

Outputs compete with Inputs to shape the chemistry of seawater

Ocean Chemistry and the Geochemical Cycle

The Ocean has Both Inputs and OutputsOutputs include:1--sea salt aerosols 2--biogenic sediments (biological processes);

deposited on ocean floor (CaCO3, SiO2)3--inorganic sediments (precipitates, evaporites;

adsorption)4--interaction of seawater with hot basalt (Mg and SO4

"sink”)

Outputs: Seawater Evaporation in isolated basins.These sediments (“evaporites”) provide a record of

seawater chemistry

Salt from the Sea

Dead Sea Salt, Evaporation! At work

Pay attention to the sequence, from the open sea, first calcite … and eventually Halite!

The Grand Geochemical Cycle: Residence time

The average time that a substance remains dissolved in seawaterWe call this the “residence time” of an element or substance

where Input rate= average concentration in rivers (kg/km3) x river discharge (km3/yr)

Let’s consider:

We will see how this works: first for water, then for total salt, and, finally, for some individual elements. These calculations give us

insights into how the system works

Residence Time (yrs.) = Total amount in seawater (kg)

Input rate (kg/yr)

How long does it take to cycle ocean water through rivers and back again?

Residence time of water in the ocean

Volume = 1.4 x 109 km3

River Influx = 3.7 x 105 km3 /yr

t = Volume / Influx

1.4 x 109 km3

3.7 x 105 km3

t ≈ 4000 years

t =

Residence Time: total amount divided by input rate

For Coconuts on the beach:• 10 coconuts fall on the beach each day• There are generally 20 coconuts on the beach

The residence time for coconuts on this beach is:

A) 1 dayB) 2 daysC) 0.5 daysD) 20 days

The Grand Geochemical Cycle

•How much time to make the ocean salty?

• about 5 x 1022 grams of dissolved solids in ocean• rivers bring in about 2.5 x 1015 gm dissolved solids per

year--think about it!

•Should only take about 2 x 107 years (20 million yrs.) to bring oceans to present salinity

--but we know oceans are 3.8 billion yrs. old

Assuming:• rivers have kept approx. same input through time • oceans have kept approx. same composition through

time

•This confirms that there must be output of material from ocean!!

Cl 80 million yrs. SO4 9 million yrs.Na 60 million yrs. Ca 1 million yrs.Mg 10 million yrs. PO4 100 thousand yrs.

Typical Element Residence Times

The Grand Geochemical Cycle

Don’t worry too much about absolute numbers,but be able to explain why Cl residence time is so much longer than, say, that of phosphate

The Grand Geochemical Cycle

Residence time is inversely related to extent of involvement in chemical reactions in the ocean

• Na and Cl primarily precipitate as evaporite deposits (infrequent events over geologic history). Bio-inert

• Ca used by organisms to make CaCO3 (calcium carbonate) skeletons

• PO4 used in biological cycle (organic matter production)--this is a nutrient element. Biolimiting

Was the Chemistry of the Ancient Oceans the Same as Today?

We can use ancient evaporite deposits to tell us how ocean chemistry changed through time (different sequence of minerals precipitated)

We also can use the chemistry of “brine” inclusions in the evaporites to constrain elemental ratios of major elements in seawater through time.

New data suggest that ocean chemistry has changed a bit through time. Perhaps this reflects changes in ocean spreading rates and cycling of seawater through hydrothermal systems!

Time (billions of yrs)O

cean

Salin

ity

35 0/00

Phosphate has a shorter residence time than most major elements in seawater because…

A. It has an atomic mass that is much less than all other elements

B. It preferentially forms inorganic minerals when seawater evaporates

C. It is quite volatile and goes easily into the atmosphere

D. It is cycled very rapidly by organisms because it is essential to their metabolism

E. None of the above

Ocean’s Chemistry and The hydrologic and geochemical

cycles Outputs compete with Inputs to shape the chemistry of seawater

Ocean Chemistry is a balance between inputs and outputs:

A) True or B) False

Seawater salinity:

Which is/are True?

A. The most abundant element dissolved in seawater is Cl

B. The element sodium is present in table salt and it is dissolved in seawater

C. The salinity of ocean surface water is generally higher than that of deepwater

D. All of the above

E. A and B

Seawater ChemistryWhich of the following are true:A. Evaporation reduces

the salinity of seawater

B. The average salinity of seawater is 70 oo/o

C. Mineral deposits, such as halite and gypsum represent outputs for chemicals in the ocean

D. The geochemical cycle includes wash, spin, and rinse

E. B and C

Gases in Seawater

•Nitrogen (N2) 78.08•Oxygen (O2) 20.95•Carbon dioxide (CO2) 0.03 (365 ppm)•Argon, Helium, Neon (Ar, He, Ne) 0.95

% in atmosphere

Gases Dissolved in Seawater

• Gases are soluble in seawater in proportion to their atmospheric concentration --Gas Exchange

Gas exchange is enhanced by mixing and biologic processes

Solubility (amount that can be dissolved)1) Depends on temperature and salinity of seawater• Decreases with increasing Temperature• Decreases with increasing Salinity

2) Helped by wind-mixing of surface layer

• Important for oxygenation of seawater and CO2 uptake from the atmosphere

• Important for fishes and other aerobic critters. Gasp! They need Oxygen to Breathe!

For Gases Dissolved in Seawater

Solubility of gases in seawater is controlled by temperature (and also by salinity)

These curves reflect maximum amount that can be held in solution under these conditions

Cold water holds more gas in

solution

Pepsi

Solubility of Gases in Seawater

A) Gases dissolve more readily in warmer water

B) Cold freshwater can dissolve more gas than warm saltwater

C) Fishes need oxygen to live and they get that by breaking O molecules from H20

D) A and BE) B and C

What controls variations in oxygen and carbon dioxide with depth in the ocean?

What controls variations in oxygen and

carbon dioxide with depth in the ocean?

Dissolved Gases in Seawater

•more oxygen in surface waters

• less oxygen in deep waters

Indicates consumption of dissolved oxygen below surface waters

•less carbon dioxide in surface waters

•more carbon dioxide in deep waters

Indicates a process that creates carbon dioxide in deep waters

Depth profiles of dissolved oxygen and dissolved carbon dioxide

General Classification of Water Masses

Why is ocean water stratified into these layers?What are the characteristics of each of these regions?Why do we care?

Density of Seawater

Salinity (S) - Temperature (T) - Density Diagram• note dual effects of S & T• typical range of seawater S & T

Define Sigma t :

t = (Density-1.0) x 1000

For example: seawater with a density of 1.026

would have t = 26

We'll use this diagram to differentiate water masses

Increasing

Density