Carbonate Flashcards
Importance of ocean chemistry
Controls marine life distribution
Critical control on atmospheric gas concentrations and therefore climate and therefore sedimentary rock deposition
Salinity is a driver for ocean circulation
Neritic zone =
relatively shallow part of ocean above the drop off of the continental shelf
Pelagic zone =
Water column above open ocean, further divided by depth:
Epipelagic Mesopelagic Bathypelagic Abyssalpelagic Hadadlpelagic
Aphotic zone =
Little/no light
Littoral zone =
Intertidal zone
Sublittoral zone =
Permanently covered by seawater
Benthic zone =
Ecological region at the lowest level of the ocean
Species here = benthos
What do the rivers transport?
1) organic carbon
2) chemical weathering by-products
3) particulates
DEPENDS ON BEDROCK/EROSION TYPE DUE TO CLIMATE AND LATITUDE
Types of river
Precipitation dominated
Weathering dominated
Evaporation dominated
Precipitation dominated river
Rainfall controls composition
In low relief areas, can be far from sea
E.g. tropical rivers in Africa and S America
Weathering dominated rivers
Lots of dissolved species
In equilibrium with basins
E.g. tropical/subtropical rivers with moderate rainfall like Congo/Orinoco/Niger
Evaporation dominated rivers
Concentrated rainwater and dissolved species (high concentrations)
E.g. arid regions
Estuary =
Mixing zone of fresh water and seawater
An extreme salinity change on the system
- causes PRECIPITATION
- slow flow increases reaction time
Conservative behaviour =
Simple mixing
Straight line relationship
Non conservative behaviour =
Elements with a higher chemical reactivity have addition/subtraction FROM SOLUTION
Very high concentrations of some species found in flocculants
Non conservative - subtraction
Sorption
Flocculation
Precipitation
Biological activity
Non conservative - addition
Desorption
Dissolution
Atmospheric inputs
Aerosols
- fine particles of liquid or solid in the air
Gases
Deposition
- wet = overland atmospheric water dissolves gas and particles
- dry = particles in the air deposit without rain’s influence
DISSOLUTION OF GASES DIRECTLY FROM ATMOSPHERE - MOST IMPORTANT
Atmospheric inputs - examples
Canary Islands = lots of volcanic rocks
- dust fluxes from the desert with lots of nutrients
Saharan dust increases phytoplankton in the oceans
Hydrothermal inputs
Large input of material into oceans due to magma a high temperatures and percolation of sea water into hot sediments and rocks
Relative importance of sources
Rivers: surfaces and margins (dominate in coastal and open oceans)
Atmosphere: surface
Hydrothermal systems: deep water and mid ocean ridges
Henry’s law
At a constant temperature, the amount of given gas dissolved in a given volume of liquid is proportional to the partial pressure of that gas in equilibrium with that liquid
Carbonate species distribution in the oceans
Low pH = H2CO3
Medium pH = HCO3-
High pH = CO32-
Thermocline =
zone where there is a rapid temperature drop with depth
Lysocline =
Depth below which the dissolution of calcite increases dramatically
What does hydrogen bonding in seawater cause?
Higher boiling point and freezing point than expected
Can dissolve salts into ionic solution
- breaks hydrogen bonding
- increases ionic content
- decreases volume
Largest SHC of any substance
Specific heat capacity =
The amount of energy required to raise the temperature of 1kg of a substance by 1 degree
- if large can absorb and release more energy with small temperature changes
Latent heat =
Heat absorbed during changes of state
- large for water!!!
Physical properties of seawater
DENSITY
- increases until 4 degrees C then decreases
SALT
- dissolved salts lower the temperature of maximum density and the freezing point
PH
- slightly above 8 due to carbonate buffering effect
What causes variations in salinity?
Convection
Mixing
Evaporation
Precipitation
How does salinity vary?
Surface maximum at low and mid latitudes
Surface minimum at high latitudes
Low in tropics
Atlantic > Pacific (high T)
Fairly homogeneous in the deep sea
Potential temperature =
Temperature that would be acquired if a substance was adiabatically brought to standard reference pressure (usually 1 bar)
- with increasing depth comes increasing pressure and the water is compressed
- this exerts work as heat
- small increase in T
Why is temperature important?
Controls reaction rates
Controls biological process rates
Controls water density
Controls the concentration of dissolved gases
Which are faster, ocean surface currents or deep ocean currents?
Surface
Conservative behaviour =
Residence time > mixing of ocean water
Thoroughly mixed = constant concentration with depth
Non conservative behaviour
EXAMPLES
Concentration changes with depth DUE TO:
Biological activity Decaying organic matter Dissolution Sorption Hydrothermal inputs
EXAMPLE: Sodium
Non conservative behaviour - recycled
EXAMPLES
Surface depletion
Depth enrichment
E.g. biolimiting constituents (photosynthesis/respiration/decomposition)
EXAMPLE: cadmium/phosphate/zinc/silicate
Non conservative - scavenged
EXAMPLES
Surface enrichment
Depth depletion
E.g. atmosphere/river sources removed faster than the ocean’s circulation
EXAMPLE: lead
Aluminium
Mid depth minimum
Strong atmosphere/river source
Removed into siliceous shells
Deep water source as sediments dissolve and flux of elements from hydrothermal systems
Reductant =
Electron donor
Gets oxidised
Oxidant =
Electron acceptor
Gets reduced
Positive standard potential…
As reduction equations
Negative gibb’s free energy
Exothermic
»> RHS
STRONG OXIDISING AGENT
Negative standard potential…
As reduction equations
Positive gibb’s free energy
Natural water; positive standard potential
New species oxidised
Existing reduced
OXIDISING ENVIRONMENT
Natural waters; negative standard potential
New species reduced
Existing oxidised
REDUCING ENVIRONMENT
Oxic =
Measureable dissolved oxygen
Suboxic =
Lack measurable oxygen or S2–, does have dissolved Fe
Anoxic =
Has dissolved Fe and S2-
Metal mobility
Redox states of metals and ligands determine solubility
Species distribution is a function of pH and pe
Mn profiles in the oceans
Mn4+ = insoluble and easily scavenged Mn2+ = soluble
Dissolved manganese peaks at an oxygen minima i.e. in a reducing environment
Why does oxygen increase at depth?
Due to mixing with cold waters containing more dissolved oxygen
BUT NOT ALWAYS THE CASE
- oceanic anoxia
Oceanic anoxia
Due to Caribbean eruptions
- increase nutrients
- increase plankton
- decay
- food for organisms
- respire and use oxygen
Can form a meromictic lake
Also done anthropogenically with fertilisers containing nitrates and phosphates
Meromictic lake =
Highly stratified body of water Algae/nutrients/decay/respiration at the top Anoxic at the bottom - Black strata = organic matter - preserved COAL/OIL/GAS
What limits productivity?
Carbon dioxide
- carbonate equilibrium supplies
Light
- allows photosynthesis to take place
Nutrients
- redfield ratio
The Redfield Ratio =
Algae, as the most abundant organism and one of fixed elemental composition…
Represent the formula most life will want to operate at
C:N:P = 106:16:1
Major nutrients
NITROGEN
inorganic (nitrate/ammonium)
Organic (organic compounds/particles)
PHOSPHOROUS
inorganic (orthophosphate PO4)
Organic (sorbed to particles)
SILICON
many diatoms require this for their shells
Gyre =
Centre with little mixing and low productivity
Upwelling =
Offshore currents causing high nutrient concentrations at the thermocline, brought from the bottom to the surface
Productivity in polar lands vs polar oceans
Polar oceans most productive (no thermocline so nutrients are the same throughout)
Polar lands the least (less rapid nutrient release from Soil Organic Matter)
What is productivity in the ocean affected by
Ocean upwelling
Nutrient provision
Latitude
Where are some upwelling zones found?
Peru
Africa outer banks
North Pacific
California
North Africa
Antarctica
Forms of ocean sedimentation
1) aeolian
2) fluvial
3) coastal erosion
4) volcanic ash clouds
5) biogenic debris
6) authigenesis
7) ice rafting
8) mass gravity flows
9) hydrothermal activity
10) submarine volcanism
11) high altitude jet streams
12) micrometeorites
Sediment classification
GEOGRAPHIC DISTRIBUTION
WATER DEPTH
GRAIN SIZE
ORIGIN
SEDIMENTATION RATE
Classification: geographic distribution
Neritic = on continental margin
Oceanic = overlaying oceanic crust
Sediment classification: water depth
Neritic = continental shelf/coastal environments
Hemi pelagic = 200-3000m
Pelagic = >3000m
Sediment classification: grain size
Clay: 0.12-3.9um
Silt: 3.9-125um
Sand: 0.125-2mm
Sediment classification: origin
AUTHOGENIC/AUTOCHRONOUS
- precipitate from solution
1) hydrogenous (abiogenic)
2) biogenous
ALLOCHRONOUS
- carried into the sea as a solid phase
1) lithogenous/terrigenous
2) cosmogenous/extra terrestrial
Sediment classification: sedimentation rate
Non pelagic = >1cm/1000yr
Pelagic = <1cm/1000yr
Relic = 0/less (NET dissolution)
Types of deep sea sediments (A)
TRUE PELAGIC:
1: median <5um (except authigenic/biogenic)
2: less than 25% of particles >5um are terrigenous/volcanogenic/neritic
HEMIPELAGIC
- resedimented deep sea sediments
Types of true pelagic sediments
Lithogenous
Biogenous
Hydrogenous
Cosmogenous
Resedimentation processes
Slides and slumps
Debris flows
Turbidity currents
Lithogenous
Terrigenous muds
RED CLAY
GREY MUD
From rivers and deserts
Types of terrigenous muds
RED CLAY
- montmorillonite/kaolinite/chlorite
- 4000-5000m
- nearly 1/2 of earth’s surface
GREY MUD
- has traces in it left by acorn worms and sea cucumbers
Clays
Kaolinite and chlorite composition and location
Kaolinite
- basic
- tropical weathering
- lower latitudes
Chlorite
- physical weathering
- higher latitudes
WEATHERING OF FELDSPARS
Biogenous
Calcareous oozes
- coccolithophore
- foraminifers
- “periplatform ooze”
Siliceous oozes
- radiolarians
- diatoms
N.B. Unlike carbonate, surface waters not supersaturated wrt silica so dissolution occurs more rapidly
Fecal pellets bring down faster than dissolution so they survive
Cosmogenous sediments
Cosmic dust found in red clay
Common in South Pacific
Iron nickel and magnetite
50-200microns diameter
~300x10^3 tonnes fall on earth’s surface each year
Hydrogenous
Formed directly from seawater in the pelagic zone (an oxygen environment)
Ion exchange and precipitation
E.g. ferromanganese nodules
Turbidity currents =
Main agent for transporting shallow water sediment to deep waters
High density, sediment laden fluids
Slides =
Move on bedding planes
Little internal deformation of moving mass
Slumps =
Cut across bedding in rotational failures
Little internal deformation of moving mass
Debris flows/mud flows =
Cohesive, viscous flows depositing debrites
Submarine canyon =
Turbidity currents trigger flow and erodes surfaces forming a canyon with a fan at the bottom
Bouma sequence
SANDS/LARGER GRAINS
- slow energy drop = graded
PARALLEL LAMINATED SANDS
- upper flow regime
- traction = flute casts
CROSS LAMINATED SANDS
- lower flow regime
- enough energy for saltation
PARALLEL LAMINATED SILTS
- slight current
MUD OFTEN BIOTURBATED
- suspension settling with no current
Ice rafting, forms…
Morain deposits and U-shaped valleys
- deposits a significant amount of material
- associates with global climate events
How do water’s physical properties help to regulate the earth’s climate?
Liquid to gas transition ABSORBS a lot of latent heat
Absorbs heat (evaporation) at low latitudes Releases heat (condensation) at high latitudes
W/O this the contrast in temps would be large at poles vs equator
