As marine scientists we should know: • why the oceans are salty• why they contain the salts that they do• why the composition of the oceans has been stable for very long periods of time• the processes that govern ocean stability

Why is the sea salty, and how does it stay that way?

Origin and Steady State Composition of Seawater – Geochemical Mass Balance of the Oceans

The earth is about 4.8 x 109 y old.

Micro fossil records (stromatolites) going back 3.8 x 109 y, indicate that Earth quickly evolved conditions not too different than those of today.

http://www2.nature.nps.gov/geology/parks/glac/car0484.jpg from http://www2.nature.nps.gov/geology/parks/glac/

Modern Stromatolites –

Shark Bay Australia

Ancient stromatolites

Ocean Origin - Quick soak theory

Highly acidic conditions caused rapid leaching of Juvenile or primordial rock material - first circuit through crustal-ocean-atmosphere factory - yielding many of the cations (Na+, K+, Mg2+, Ca2+ etc) and alkalinity in the primitive ocean.

Rapid outgassing of volatiles (H2O, CO2, H2S, HCl, HBr, HF, B, Ar) from the primitive earth. As Earth cooled water condensed and volatiles dissolved, leading to acid ocean.

These early weathering reactions consumed acid

After quick soak, the ocean composition was maintained in a pseudo-steady state, implying that:

salt inputs = salt exports.

Thermodynamic equilibrium model of Sillen (ca 1960)

Major ion concentrations are controlled by reactions that reach equilibrium with the various phases, rock, sediment and ocean waters.

Flux mass balance model of MacKenzie and Garrels (1966)

Inputs of materials to ocean are balanced by outputs through various mechanisms. Their theory invoked a process of reverse weathering to remove certain elements and alkalinity

What are the present day sources of matter to the oceans?:

- Rivers- Wind (aeolian transport)- Bedrock and sediment leaching- Hydrothermal reactions at vents

Rivers are by far the largest source for most materials

What are the sinks?- Evaporite salt precipitation- Sedimentation (e.g. CaCO3)- Pore water burial with sediments- Hydrothermal reactions at vents

Sources and sinks for major salt elements in seawater

These same mechanisms affect many minor elements as well

Review - the major salt ions in seawater

Six ions constitute > 99.8% of all dissolved solids in seawater

Cl-chloride

SO42-

sulfate

Na+

sodium

Ca2+

calcium

Mg2+

magnesium

K+

potasium

Cations

Anions

Residence time vs. concentration of different elements in seawater.

For the most part - the elements with the longest residence times have the highest concentrations - Na+ and Cl- are VERY unreactive chemicals in aqueous solution!

Residence Time (y)

Con

cent

rati

on (

M)

C

Mg

C

SCa

Cu

K

P

Fe

108102 106

Major ion elements

Rivers are the major source of material to the ocean.

Is the ocean merely evaporated river water? Not quite. Rivers contain the same salts as seawater but the proportions are not the same in river vs. ocean water.

Per gram of salt, river water contains more carbonate and bicarbonate than seawater and thus evaporated river water would have a much higher alkalinity than the ocean.

Mono Lake

Most lakes aren’t salty Most lakes aren’t salty because they are geologically because they are geologically young. Ancient lakes like young. Ancient lakes like Mono Lake, CA are indeed Mono Lake, CA are indeed salty and highly alkaline (soda salty and highly alkaline (soda lakes; pH’s ~9.5).lakes; pH’s ~9.5).

The ionic composition of seawater is distinct from that of river water

Per gram of dissolved salt, river water has a much higher amount of Ca2+ and HCO3

- than seawater. Rivers also contain very little Na+ or Cl-, whereas seawater has little H2SiO4

-.

Ionic composition of natural waters normalized per mg of total dissolved

solids

0.000

0.100

0.200

0.300

0.400

0.500

0.600

Na+ K+ Mg2+ Ca2+ Cl- HCO3- SO42- H2SiO4-

Ion

mg

ion

/ m

g t

ota

l dis

so

lved

so

lids

River

Seawater

These are averages - River water varies considerably depending on drainage basin

Source: Open University

Pore water burial is a significant sink for Cl- and Na+ but a relatively insignificant sink for other major ions and alkalinity

Evaporite Basins: Where net evaporation of water takes place, salts become more concentrated. If concentrations of some ion species become high enough to exceed the solubility products of the least soluble mineral forms, precipitation will occur and selectively remove those species.

The sequence of precipitation

due to evaporation of a fixed

volume of sea water:

1) Calcite (CaCO3) (least soluble)

2) Aragonite (CaCO3)

3) Gypsum (CaSO4.2H2O)

4) Anhydrite (CaSO4)

5) Halite (NaCl) and some Mg salts.

6) Sylvite (KCl) (most soluble)

Den

sity

(g/

cm3 )

Proportion of original volume remaining

Lines perpendicular to curve represent concentration of salts precipitated

Evaporite salt formation – reverse estuary system

Continuous input of salts from ocean allow continuous precipitation of salts in basin

Do marginal seas evaporate to dryness and deposit salt? No, not really.

EvaporationFluctuating sea level

Glacial sea level

Interglacial sea level

Mediterranean Sill

Salt deposits

Atlantic Ocean

Precipitation of evaporite salts during low sea levels

sw

Mud layers deposited during non-salt precipitating periods

Continuous input model. Mediterranean example.

Mediterranean Sea

Evaporites - Contain a large fraction of the crustal

Na, Ca, Cl, SO42-, K and Mg.

The amount of NaCl in evaporites (as halite) might be as much as 2-4 times the amount in seawater presently.

Much of the river runoff of ions is from sedimentary deposits and evaporites (a form of sedimentary deposit).

Evaporites are the main mechanism of removing chloride (also some pore water losses which are significant for Na and Cl).

Sulfate is also removed in large amounts - therefore

evaporites are the major anion sink.

Weathering:

The simplest weathering reaction can be viewed as the dissolution of calcium carbonate rock by acid solution (acid derived from CO2 in atmosphere).

CO2 + H2O <=> H2CO3

H2CO3 + CaCO3 <=> Ca2+ + 2HCO3-

Note this reaction: consumes CO2 (e.g. carbonic acid) releases cations (e.g. Ca2+) releases alkalinity in the form of bicarbonate

Weathering of continental igneous rocks often refers to alteration of aluminosilicates (clays):

2CO2 + 3 H2O + CaAl2Si2O8 --> Al2Si2O5(OH)4 + Ca2+ + 2 HCO3-

Weathering: consumes CO2 releases cations generates alkalinity

The dissolution of carbonate rocks during weathering supplies >80% of the alkalinity of river water with the remainder coming from weathering of shales and igneous silicates.

If igneous rock weathering goes to completion, all the cations will be

removed leaving quartz (SiO2) - hence our sand beaches!

From Emerson & Hedges

Reverse weathering: Reverse weathering (RW) is a misnomer in that the original products are not reformed - rather the reactions are similar to the reverse of the weathering reaction. Cation-poor alumino silicate + silicic acid + bicarbonate + cations ---> Cation rich alumino silicate + CO2

RW Forms illites, montmorilinites and chlorites (types of aluminosilicate clays

Reverse weathering: generates acid (CO2) consumes cations consumes alkalinity

Weathering and reverse weathering - long term balance of elements, pH and alkalinity in the oceans (Feedbacks in the ocean-atmosphere-crust connection)

With these feedbacks, the whole system is balanced within relatively narrow limits.

Decreased pCO2 in seawater (and hence atmosphere) would slow weathering because pH of rain would rise.

Increased [OH-] would shift the carbonate equilibrium toward precipitation of carbonates, thereby removing cations like Ca2+ (returning to steady state). It would also lower pCO2 in ocean.

Seawater remains electronically neutral. Weathering releases alkalinity (HCO3

-) which can absorb protons, causing OH- and pH to go up.

Clays: Crystalline solids made of aluminosilicate with diameters of < 1 m. Sheet like structure - think of something like mica or vermiculite.

Two layered clays (alternating octahedron and tetrahedron. - Kaolinites. The octahedral layer contains Al or Mg while the tetrahedral layer contains Si.

Three layered clays have an octahedral layer sandwiched between two tetrahedral layers. - Montmorilinites, illites and chlorites.

Vermiculite is a 3 layer clay which can expand due to formation of a water layer between the sheets of clay. This is why vermiculite is good to have in the garden and why it is used for spills (holds water).

**Important Characteristics of Clay minerals**

High surface area

High cation exchange capacity

Net negative charge on surface

Small size – highly mobile (winds & currents;

highly compressible in sediments)

Some types are rich in Fe

Cool namesClays are important “chemicals” in marine systems

Clay AssociatedMetals

Characteristics

Montmorillonite Ca2+ -richAluminosilicate

Formed at hydrothermal sites, found onridges. 3 layer

Illite K+-richAluminosilicate

3 Layer

Chlorite Mg2+-richAluminosilicate

Low temperature formation found athigh latitudes. 3 Layer.

Kaolinite Cation-poorAluminosilicate

2 layer, low Cation exchange capacity –topical latitudes

Glauconite Fe-rich illite Formed authigenically in coastal areaswhere organic matter is high

Characteristics of some Clay minerals(aside of having some great names)

Clay Transport

- River transport - most deposited on shelf or slope areas, but some is transported offshore.

- Wind transport is significant due to the small particle size. Aeolian transport of clays is important from the standpoint of clay geochemistry but also because of the trace elements associated with it, particularly Fe. Some regions of the open ocean have sediments composed mainly of atmospheric-derived clay!

- Ice transport. - Glaciers (ice rafted detrius deposited in central ocean basin – includes clays and larger particles that can only get offshore by transport in floating ice)

McKenzie & Garrels Mass Balance model for ocean

Total bar length is total input per 108 years

Missing sinks

Incorrect assumptions about location of sinks in original model – e.g. FeS2 ppt in sediments

Removal of these ions occurs at Hydrothermal vents

Hydrothermal Vent Systems

History: Geochemists and geophysicists predicted some sort of hydrothermal input to the deep sea as far back as the mid to early sixties (Elder, 1965).

Evidence included extensive metaliferous sediments, altered mid-ocean ridge basalts and temperature anomalies. 3He/4He anomalies pointed toward conductive and convective heat flow from ridges.

Discovery of hydrothermal vents in the 1970’s revolutionized understanding of ocean geochemistry

Hydrothermal vent systems and sea-floor spreading centers.

Here reactions of seawater constituents with basalts at high and low temperature accelerate reverse weathering and other reactions. There is now good evidence that Mg2+ is completely removed from circulating water at these vents.

Location of confirmed (circles) and probable (triangles) hydrothermal vents

3 5 0 d e g r e e w a t e r

s p r e a d i n g c e n t e r

2 - 5 c m / y

C o n v e c t i v e f l o w a n d h e a t t r a n s f e r a t v e n t s

c o n d u c t i v e f l o w

Magma (Hot!)

Seawater is drawn through fissures in the rock/sediment. It is heated thereby becoming less dense and buoyant, therefore it rises by convection, exiting through vents. Meanwhile the water chemistry is changed dramatically, with addition of some compounds and removal of others.

Within the fissure of vent systems Heated seawater interacts with basaltic rocks at 350oC and 500 atm pressure. Some water is incorporated into the rock but a host of chemical reactions occur.

• Reduction of metals and sulfate, forming highly soluble metal ions and hydrogen sulfide

The sulfide originates primarily from seawater sulfate that is reduced as it is drawn through the circulation cells in the rock/vent

system.

• Precipitation of gypsum and anhydrite - removes calcium

• Reverse weathering type reactions consume alkalinity and Mg2+

• Heated seawater dissolves quartz leading to saturated H2SiO4 at 350 oC end-member temperature

• Acidic vent water (CO2 rich) leaches metals and other minerals

http://oceanexplorer.noaa.gov/explorations/06fire/background/chemistry/media/arcvolcano.html

http://www.oceanexplorer.noaa.gov/explorations/02fire/background/vent_chem/ventchem.html

Hydrothermal vent processes and chemistry

Black smoker vents

Acidic hot water circulating through rocks is rich in H2S and Fe2+. When this water mixes with the cold, relatively alkaline (pH 8) seawater, metal sulfides precipitate.

Fe2+ + S2- -> FeS (s)

Reduced iron + sulfide -> pyrhotite The vent chimneys are often made of metal sulfides and CaSO4 (gypsum and anhydrite)

Clouds of metal sulfide precipitates

Pore water evidence that Mg2+ ions are consumed (removed from solution) in sediments near mid ocean ridges. The number near each line represent the upward convective flow of pore water.

Removal of Mg2+ occurs below these depths at the interface of hot basalt and sediments

Seawater end-member

Chemosynthetic communities at vents

Food chain based on chemical energy - not light

Oxidation of reduced chemicals (H2S, CH4, Fe2+, Mn2

+) by chemoautotrophic bacteria forms the basis of the food chain.

Chemosynthetic bacteria often live in symbiosis with higher organisms

Black smoker vents (photo by Emory Kristof - National Geographic Society

Vent community (photo by Emory Kristof - National Geographic Society

Vent Crab on chiminey (photo by Emory Kristof - National Geographic Society

Types and locations of “vents”:

Hot vents - spreading centers (Galapogos, East Pacific Rise, Juan deFuca ridge)

- seamount volcanos (Loihi)-Cool vents/diffuse flow

Aegean Sea (accessible by snorkeling!)

Seeps - cold - CH4-rich (Gulf of Mexico)- Mud volcanoes (CH4 seeps)- brine seeps (Florida escarpment)- oil seeps - Santa Barbara

Loihi seamount vent – Fe

rich

Vents are:

-Sources of gases and solutes-sites of heat input and dissipation-sources of polymetalic sulfides-sites of geochemical exchange

In Review

End

Origin and Steady State Composition of Seawater – Geochemical Mass Balance of the Oceans (Outline)

Origin and Evolution of the Oceans

Where did the water and major dissolved constituents come from?

Sources and Sinks for Material in the Present Day Oceans

Sources - Rivers, Atmosphere, Vents/seeps Sinks - Sediments, Vents, Evaporites Weathering/Reverse Weathering

How is pseudo-steady state composition of the oceans maintained?

The McKenzie and Garrels (1966) flux balance model was generally accepted despite questionable assumptions about significant removal of dolomite (CaMgCO3) and pyrite (FeS2) as sedimentary deposits - evidence does not support large current deposition of these minerals.

Later discovery of hydrothermal vents makes these assumptions unnecessary. At vents, sufficient Mg and S are removed to eliminate the need to invoke dolomite and pyrite sink proposed by MacKenzie and Garrels.

Geological evidence for the long-term stability of ocean composition:

sedimentary rocks nearly the same composition

evaporites nearly the same composition

over geologic time (though some significant

variations)

organisms appear to have been similar

over time

Seawater Salts – The major ions of seawater include 4 cations (Na+, K+,Ca2+, Mg2+), and two anions (Cl-, SO4

2-), and together make up > 99.8% of salinity. Chloride is by far the major anion, with a concentration of ~540 mM compared to sulfate 28 mM. Seawater is about 86% NaCl in terms of mass.

Charge balance - Seawater, like all solutions, is electronically neutral. The negative charge of anions is balanced by the positive charges of cations. This must be satisfied everywhere.

Why are major ions so abundant?

Because they do not undergo appreciable reaction on the time scale of mixing in the ocean.

• The Ocean mixing time is approximately 1000 y (about the time for one cycle of the Ocean Conveyor). Elements with residence times longer than several mixing cycles (say 104 y) will be homogenized with respect to the other elements - yielding a constant ratio amongst these elements.

• Some biologically active elements do not fit this pattern - N & P for example have long residence times (due to biological recycling), but are heterogeneously distributed (again due to biology).

Why are major ions in constant proportion?

0.37

37% of initial amount remains after 1 renewal time

Tau (τ) = fill time, renewal time, replacement time, turnover time.

Different names for the same thing!

Should be 10-3

mol kg-1

Note: River water concentrations in Table 2.3 in Emerson & Hedges should be in micromoles (10-6 mol) per kg, not mol/kg

Residence time (tau) = pool/flux or reservoir/input. In this context we are considering the amount (mass) of a particular element in the entire ocean divided by the sum of all the inputs (and exports) i.e. net input. We assume steady state, that is, the amount of the element in the ocean does not change appreciably over time.

Ocean(mass of Na+)

Riverine input of Na+

Loss of Na in sedimentary pore waters

Loss of Na in evaporites

Atmospheric dust input of Na+

At steady state:

Inputs = outputs (sources = sinks)

influx

pool

(Mass/time)

(Mass/time) (Mass/time)

(Mass/time)Example of Na+ residence time in ocean

Important!

Steady State is a relative concept – it depends on the time frame of reference. The ocean composition is in steady state from year to year, but on billion year time scales it is not.

Time

Pro

pert

y

Apparent steady state - over short time interval

Time

Pro

pert

y

Apparent non-steady state condition

It is always important to specify the time frame of the steady state assumption!

Seawater end member

Seawater-Hot Basalt end member

Graph implies nearly conservative mixing between seawater end member and water in contact with hot basalt