Exam preperation Flashcards
What is a system? Features of a system?
A system is … a collection of components that work together to perform a function.
.1. It is made up of component parts
Can be studied on different scales (e.g., ecosystem vs individual organism)
2. Parts work together
Implies presence of linking structure, inter-relationships and dependency
Alteration or removal of components leads to changes in the system
3. System serves some purpose
Energy Flows and Cycles of Material?
-Flows of Energy drive environmental systems
External infinite source is the Sun (supplemented by radioactive decay)
-Cannot be cycled (cf. material)
Different types of energy flows in environment?
-Rock fragment falling from cliff top (potential energy kinetic energy)
-Exothermic reactions in stratosphere involving ozone (chemical energy heat energy)
-Lightning (kinetic energy electrical energy)
Photosynthesis (radiative energy chemical energy)
Features of cycling material in environment?
- Water, gases, dissolved and particulate substances
- Finite sources means recycling is a natural feature of environmental systems
Properties of environmental systems?
- Physical boundaries define limit within which components interact
Sharp boundaries (e.g., coastline, catchment drainage divide, weather front)
Transitional boundaries (e.g., gradual change in vegetation towards a desert margin, seaward boundary of an estuary) - Boundary exchanges determine type of system
Isolated systems: inward but no outward flow of energy across system boundaries; no exchange of material
Closed systems: flows of energy but no exchange of material across system boundaries
Open systems: flows of energy and exchanges of material across system boundaries - Systems can be defined on a variety of scales, can overlap, and exist within other systems, e.g.,
Global water cycle vs individual drainage basin
Estuarine system overlaps with riverine and marine systems
Headwater streams nested within larger streams (nested hierarchy)
Structure of environmental systems? With definitions?
Inputs - Precipitation (including dissolved substances and particulate material
Outputs - Evaporation and transpiration of water to the atmosphere
Water, dissolved substances and sediment discharge to the sea or a lake
Flows (or fluxes) - Water, dissolved substances and sediment transport downslope and along river channel
Stores (or reservoirs) - Short-term storage of water in soil, vegetation and river channel
Long-term storage of water in groundwater and lakes
Sediment stored in hill slopes, floodplains, river/lake beds and deltas.
Types of systems?
Isolated systems: inward but no outward flow of energy across system boundaries; no exchange of material
Closed systems: flows of energy but no exchange of material across system boundaries
Open systems: flows of energy and exchanges of material across system boundaries.
What does increased input (rainfall) lead to?
- Changes in flows, storage and outputs
- Inter-dependency of components
- Interaction between systems
Features of steady state equilibrium?
- Constant balance between inputs and outputs of energy/material
- No net change in storage
- Short-term changes superimposed on unchanging average state
What is dynamic equilibrium?
Short-term changes superimposed on slow progressive adjustment over time
Concept of feedback and its effects in biogeochemical cycles?
-Interrelationships between components means disturbance to one component has knock-on effect
Amplifying effect = positive feedback leads to increased destabilisation
Dampening effect = negative feedback leads to restabilisation (system regains original equilibrium)
What does initial disturbance in biogeochemical cycles lead to?
Lower temperatures to continental ice sheet growth to higher albedo to lower radiation reciepts and this leads to lower temperatures as a cycle.
Why is snowball earth unlikely?
Lower temperatures leads to lower evaporation leads to lower snow.
What is resilience and what is its state in natural systems?
- The ability of a system to withstand or recover from a disturbance
- Natural systems are often very resilient (e.g., recovery of vegetation after a prolonged drought)
What is threshold and what is its state in natural systems?
- Critical point at which system responds abruptly to disturbance
- Characteristic of systems whose response is sporadic or discontinuous (e.g., slope stability)
- Not always easy to identify or predict (e.g., climate change)
What is lag and what is its state in natural systems?
-Time delay of system’s response to
disturbance
-May reflect system complexity or scale (e.g.,
response of small stream vs major river to
heavy/prolonged rainfall
-Establishing clear cause and effect difficult
What is a Systems Approach to Studying the Environment?
-Holistic
-Interdisciplinary
-Emphasizes inter-relationships/interdependence
Provides a framework for recognizing, interpreting and responding to signs of global change
Features of biochemical cycles?
- A systems approach from an element perspective
- Describe the cyclical movement of elements around the global environment as a result of activity within environmental systems
- Crust, ocean and atmosphere are defined as the major reservoirs for an element
- Movement between reservoirs (e.g., volcanic emissions, precipitation, river flow) defined as fluxes
Types of diagram cycle representations?
- pictorial/qualitative
- diagrammatic/semi-quantitative
- box models/quantitative
Features of box models?
-Transfer of subject material of cycle shown by arrows going between boxes.
-They indicate the relative importance of different parts of the cycle
Non-volatile (or sedimentary or imperfect) cycles have a minor atmospheric component to the cycle (metals, Si, P)
-Volatile (or gaseous or perfect) cycles have an important atmospheric component (most semi-metals and non-metals)
For many elements, the flux represented by river transport dominates the global cycle
-They indicate the extent to which cycling operates through the biosphere
-They indicate the extent, scale and (sometimes) consequences of human activities
-They indicate the sensitivity of different environments to change, particularly as a result of human activities
Features of residence time in box models?
-Sensitivity to Change = Residence Time
-Residence time = reservoir burden/fluxes in or out
-If the reservoir size is not changing, fluxes should be the same (steady state)
- A large residence time means a substance remains in a reservoir for a long time
-A short residence time means a reservoir is sensitive to changes in fluxes as a result of human activity
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Key features of biogeochemical cycles?
- Alterations in oxidation state often biologically mediated
- Biological processes account for 95 % of fluxes
- Relatively small number of processes responsible for major fluxes between reservoirs
The Major Biological Transformations and Fluxes of N Compounds?
- Atmosphere is most significant reservoir of N (as N2)
- Abstraction from atmosphere is critical process for life
Definition and features of nitrogen fixation?
-Conversion of atmospheric N2 into ammonia
-Performed by blue-green algae, some bacteria
-May be free-living or symbiotic (e.g., legumes)
Requirements:
-High energy to break triple bond in N2 molecule
-Reductive enzyme nitrogenase
-Anaerobic environment
3H2O + 3CH2O + 2N2 3CO2 + 4NH3
Fate of ammonium in soil and aquatic systems?
-Uptake or assimilation by plants ( amino acids), or
-Nitrification in aerobic soil
Oxidation to nitrite by bacteria of the Nitrosamonas genus:
2NH4+ + 3O2 to 2NO2 + 2H2O + 4H+
Oxidation to nitrate by bacteria of the Nitrobacter genus:
2NO2 + O2 to 2NO3
Two major pathways of nitrate?
- Assimilation
- Denitrification in anaerobic soil ( N2O, N2)
Ammonification?
-Decomposition of organic matter by heterotrophic bacteria returns ammonium to soil/aquatic system .
Interferences in nitrogen cycle? Consequences?
-Industrial fixation
5-fold increase in fertiliser application since 1950
Now of similar magnitude to natural fixation
-Atmospheric deposition of NO3 (combustion) & NH4+ (livestock wastes).
-removal of steady-state condition from inorganic reservoir:
- Increased denitrification (N2O more likely to be the end product in a fertiliser-rich soil)
- NO3- leaching and run-off to eutrophication of streams, rivers, lakes & coastal seas
Compare natural nitrogen cycle to post industrial nitrogen cycle?
Natural:
-Fixation above ground
- Nitrification, Assimilation, Ammonification, and denitrification underground.
Post industrial nitrogen cycle:
- natural & industrial fixation, livestock wastes above ground.
- Nitrification, Assimilation, Ammonification, and denitrification underground
- Combustion occurs from above ground to underground
- Leaching and run-off occur underground.
Sources of material to ocean?
-Rivers dominates
75% of total dissolved input
95 % of total particulate input
-Particulate to dissolved ratio = 4:1, but
particulates largely unreactive (weathered aluminosilicates)
-Particulates deposited in coastal regions
also dissolved material has greater impact
Difference between river and seawater sediments?
Mean dissolved composition of river-water and seawater differ.
True or false? Residence time of dissolved constituents gives timescale for measurable change?
True.
Evidence of Constancy of Composition over last 108 – 109 years?
- General similarity of ancient and modern marine sediments
- Similar mineral abundances of key minerals
- Chemical similarity of skeletons of key species
- Families (sometimes species) have been in existence over last 5*10^8 years
- Assumed biogeochemical processes must be responsible for maintaining steady state
Features of bubble bursting in maintaining a steady state?
- Important sink for Na+ and Cl
- Associated with breaking waves
- Ejects jet and film drops of seawater into atmosphere
- Evaporation of water content generates seasalt microparticles (incorporated into cloud and rainwater)
Features of evaporite formation?
-Evaporation of seawater leads to precipitation of constituent salts in a predictable sequence:
CaCO3, CaSO4.2H2O, NaCl, … bittern salts
Important process for removal of Na+, Ca2+, Cl- and SO42-.
What are the high levels of evaporation needed for mineral saturation in precipitation?
-47 % evaporation for precipitation of CaCO3
Ca2+(aq) + 2HCO3-(aq) (reversable) CaCO3 (s) + CO2 (g) + H2O(l)
-75 % evaporation for precipitation of gypsum
Ca2+(aq) + SO42-(aq) + 2H2O(l) (reversable) CaSO4.2H2O(s)
-90 % evaporation for precipitation of halite
Na+(aq) + Cl(aq) (reversable) NaCl(s)
Features of a barred basin? Example?
- Requirement for wholly or partially enclosed seawater body (predominance of evaporation over supply)
- Restricted connection to sea
- Important in geological past (separating continents)
- Evaporite record suggests such conditions are uncommon
- Important for maintaining long-term steady state
- e.g Red Sea is nearest modern analogue
Features of supertidal flats? Example?
- Periodic tidal incursions and evaporation lead to precipitation of evaporites
- Relatively minor process in maintaining steady state
- Persian Gulf, fringed by carbonate sediments 25 km wide and 1 m above sea level
Dissolved particulate ineractions?
-Cation exchange on riverborne colloids when riverwater and seawater mix (equilibrium adjustment)
-Most important for Na+, K+ and Mg2+ (replacing Ca2+), e.g.,
clay-Ca2+(s) + 2Na+(aq) (exchangeable) clay-2Na+(s) + Ca2+(aq) to sink for Na+, K+ and Mg2+, source for Ca2+
Features of carbonate deposition?
-Organisms primarily responsible for precipitation of CaCO3
-Removes Ca2+, some Mg2+ (isomorphous substitution) and HCO3-
Ca2+(aq) + 2HCO3(aq) (reversable) CaCO3 (s) + CO2 (aq) + H2O(l)
-Death leads to sedimentation, but dissolution may occur
What controls carbonate dissolution? Features of controls?
- Oceans predominantly undersaturated with respect to CaCO3 at all depths below thermocline
- Dissolution of sedimenting carbonate
- Level where dissolution rates increase markedly with depth is shallower than level where dissolution rate = rate of supply from overlying water.
- CCD generally < 4,000 m little preservation of sedimenting carbonate in deep oceans
Opaline Silica Dissolution?
-Diatoms responsible for precipitation of SiO2 (formation of skeletal material)
-Important removal process for Si
H4SiO4 (aq) (reversable) SiO2 (s) + 2H2O (l)
-Seawater undersaturated with respect to Si leads to 95 % dissolves during sedimentation
True or false? Preservation in sediments only occurs where burial is rapid?
True.
Features of sediment microbial processes?
-Important sink for SO42-, source for HCO3
-Respiration of organic matter by sulphate reduction
CH2O(s) +SO42(aq) (reversable) 2HCO3(aq) + HS(aq) + H+(aq)
-About 10 % of HS reacts with Fe2+ to precipitate FeS (which converts to FeS2)
Features of hydrothermal processes?
-Hydrothermal cycling of seawater through mid ocean ridges is important in the budget of major and trace species.
-Most important sink for Mg2+ in modern ocean, e.g., reaction with basalt:
11Fe2SiO4 (s) + 18H2O(l) + 2Mg2+(aq) + 2SO42(aq) (reversible)
Mg2Si3O6(OH)4 (s) + 7Fe3O4 (s) + FeS2 (s) + 8H4SiO4 (aq)
-Source of Ca2+ (leaching from calcium feldspars) and Si (leached from basalt)
True or false? Some reactions (particularly between seawater and sediments) not yet identified?
True
Features of primary production and nutrient cycling?
- Oceans account for 50 % global 1 degree production
- Highest rates in coastal & upwelling regions.
- Open ocean accounts for 80 % of total
- Supported by rapid & efficient recycling (90 % organic matter) in the photic zone
- Grazing and excretion by zooplankton
- Bacterial respiration of organic matter
- 5 % reaches sediments in deep ocean
- < 1 % buried
Features of carbon in ocean?
-Dissolution of CO2 in surface ocean leads to theoretical equilibrium.
- Uptake of CO2 (as HCO3-) by phytoplankton leads to surface oceans undersaturated
Removal of CO2 in sinking particles promotes further dissolution (carbon pump)
CO2 returned to atmosphere on 103 year timescales in upwelling areas.
Features of nitrogen in ocean?
-River and atmospheric inputs roughly balanced by denitrification losses to atmosphere, e.g.,
4NO3 + 5CH2O + 4H+ 2N2 + 5CO2 + 7H2O
-Sinking particles = flocculations of organic matter leads to anaerobic microzones
-Respiration in freshly deposited sediment
-Little burial
Features of phosphorous in ocean?
- New input dominated by river particulates
- Some desorption (anion exchange with sulphate)
- Iron hydroxide minerals release adsorbed phosphate at high pH
- Balanced by burial in sediments.
Relationship between marine Sulphur and climate?
Some phytoplankton synthesise dimethylsulphoniopropionate (DMSP) to dimethylsulphide (DMS).
What is important negative feedback with climate change?
Increased nutrients & CO2 in the oceans leads to
increased primary production leads to increased dimethysulphide levels leads to increased cloudiness which potentially leads to cooling.
Features of behaviour of Other Minor Constituents in biogeochemistry?
-Variety of sources for trace elements:
Sediments (e.g., release of Mn & Fe as a result of redox processes)
-Rivers
-Atmosphere
-Often involved in complex cycling processes
-Three classes of behaviour: conservative, nutrient-like and scavenged
Features of conservative behavior in biogeochemistry?
-Low or high ionic potential
Simple hydrated ions (e.g., Cs+, Br)
Hydrated complex oxyanions (e.g., MoO42, WO42)
-Characterised by vertical profiles that show little variation with depth
-Behave like major ions (long RTs, well mixed)
-Little interaction with biological cycles
Features of nutrient like behavior in biogeochemistry?
-Biological processes leads to removal from surface waters
-Death leads to sinking and decomposition
-Return to surface by slow diffusion & upwelling
-Vertical profiles show surface water depletion & deep water maxima
-N & P recycling efficient leads to sharp gradients near surface
Recycling of Ca & Si slower (skeletal material) leads to shallower gradients near surface
Examples of nutrient transformations in biogeochemisrty?
Reduction of IO3- to I- by phytoplankton
- IO3- has a nutrient-like profile
2. NO3- removed but remineralised as NH4+
NH4+ is preferred N source for phytoplankton
Nitrification of NH4+ leads to NO3-
- Very low NH4+ in surface waters
Biological uptake does not necessarily mean biological function because?
- Zn has biological function
- Cd shows nutrient-like behavior because it often substitutes for Zn (similar charge/size)
Features of scavenged behavior?
-Particle-reactive elements (intermediate ionic potential) adsorb to particles
-River inputs removed in estuaries
-Atmosphere is principal source
Wind-blown dust (e.g., Al, Fe)
Particulate material from human activities (e.g., combustion)
Precipitation
-Surface maxima & decline with depth due to scavenging (adsorption)
-Short residence times (< few hundred years)
Features of marine pollution?
-Coastal environments often most threatened, for example:
Oil spills
-E.g. Torrey Canyon (1967, Cornwall)
-Minamata Bay, Japan (mercury)
-High levels of DDT in Baltic fish
-Eutrophication in the southern North Sea
Features of plastic pollution?
-Sources
Dumping at sea (e.g., discarded fishing gear)
Beach litter
Laundering synthetic material (microfibres)
Plastic beads in exfoliants & toothpastes
Transport via rivers
macroplastic, microplastic (direct sources & physical degradation)
-Impacts
Entanglement
Ingestion
-Solutions
Mechanical removal in areas of high concentration (e.g, ‘Great Pacific Garbage Patch’, intercepting river-borne plastic waste) – theoceancleanup.com
Improved waste management
Improvements in recycling
Reducing dependence on plastic
What is an esturary?
- The region where the river meets the sea
- No indication of boundaries
- Estuaries influence and are influenced by events outside defined area
- Have an upper. mid, and lower region.
How do estuaries form?
- Last post-glacial rise in sea level leads to drowned mouths of river valleys
- Geologically very young
- Transient: filling up with sediment
- High sediment discharge + limited tidal action leads to rapid filling & seaward growth of delta.
Causes of estuary variation?
- Tidal range (strength of tidal current)
- Magnitude of river discharge
Types of estuary?
Salt wedge
Partially mixed
Well mixed
Features of salt wedge estuaries?
-Low tidal range
-Currents dominated by out-flowing river-water
-Sheer stresses at freshwater/seawater interface leads to some mixing
-Sharp density & salinity gradients (halocline)
-Can only form where sediment load is low
High sediment load leads to delta (e.g., Rhone, Nile, Mississippi).
Features of partially mixed estuaries?
- Moderate tidal range
- Greater turbulence greater mixing
- Less marked halocline
- Increase in surface salinity seawards
e. g., Mersey, Thames
Features of well mixed estuaries?
- Broad, shallow, high tidal range whole body of water moves upstream with flood tide & downstream with ebb tide
- Completely mixed water column
- e.g., Severn, Firth of Forth, Humber, most UK estuaries
Examples of esturary data?
Total number of UK estuarine systems 104 Total number of UK estuaries 134 Well mixed 59 Partially mixed 8 Salt wedge 3
Features of estuarine biogeochemistry?
-Region of mixing between two aqueous solutions of Very different chemical composition
Most important physico-chemical differences:
Ionic strength (salinity)
pH (riverwater 5 – 8; seawater 8.2)
leads to large gradients
-Twice daily tidal reversal (zero water velocities at high & low tide)
Trap for riverborne particulate material
Extensive sediment resuspension (high tidal energy & shallow depths)
-Opportunity for dissolved/particulate interactions
-Relatively long water residence times
-river-transported materials are subject to a variety of physical, chemical and biological processes in estuarine zone
-Estuary = filter of river-transported material (i.e., material can emerge from mixing zone in highly modified form)
What is used to predict behavior of material in estuary?
Predicting the behaviour and fate of material entering the head of the estuary
requires detailed knowledge of:
-Physical, chemical & biological processes
-Their kinetics
-Interactions in a particular estuary
-Modelling
Why is it useful to measure net processes in estuaries? Process of measuring?
-Budgeting (calculating net fluxes to sea)
-Validating process study models
1.Estuary is sampled at high tide at intervals along its length to maximise the geographic distribution of salinity
2. Samples are analysed for substances of interest
3. Salinity is conservative (i.e., the salinity measured at any point exactly reflects the relative proportions of seawater and riverwater mixed together at that point)
-A straight line (substance vs salinity) indicates conservative behaviour (i.e., estuarine processes are not affecting that constituent)
-A curved plot indicates that processes are adding or removing that substance during mixing
4. Removal or input quantities are calculated
-Actual riverwater concentration is read off the graph
-Slope at seawater end of plot is extrapolated back through the y axis (zero salinity) effective riverwater concentration.
5. Multiply concentrations by river flow rate to calculate fluxes
actual flux = amount transported to estuary
effective flux = amount transported to sea
actual flux – effective flux = amount removed (added) during mixing
% removed (added) = (actual flux-effective flux/actual flux)*100.
Assumptions and limitations of measuring net processes in estuaries?
- Estuary should be in steady state
- Processes vary over the tidal cycle
- Composition of freshwater within the estuary may be variable (e.g., effect of recent rain)
- Deviations from conservative mixing line may indicate mid-estuarine inputs
What are the estuarine processes?
Flocculation
Sediment water exchanges
Biological uptake
Features of flocculation in estuaries?
- Negative charges on riverborne colloids only partly balanced by adsorbed cations leads to forces of repulsion keep colloids apart
- Increased ionic strength leads to excess charges neutralised by seawater cations leads to colloids collide, aggregate (flocculate) leads to sedimentation
- Adsorption of phosphate and metals to sedimenting particles leads to removal from dissolved phase.
Examples of sediment water exchanges?
- Cation exchange on clay particles due to transport from Ca2+ dominated riverwater to Na+ dominated seawater leads to some Ca2+ substituted for Na+, K+, Mg2+ (halmyrolysis)
- Ammonification in sediments leads to NH4+ released to overlying water leads to broad mid-estuarine peak in NH4+ concentrations.
Features of biological uptake?
- Low turbidity estuaries leads to phytoplankton growth
- High water residence times leads to development of large populations
- Removal of NO3-, PO43- and SiO44- in spring & summer
Pollution?
The introduction of matter or energy into the environment where it results in harm.
Types of harm?
Hazard to human health Harm to ecosystems Harm to living resources Damage to structures Interference with legitimate use Damage to amenity Consequence of human activity or natural process
Contamination?
No evidence of harm.
Important properties of water?
Maximum density at 4 °C
Thermal stratification
Ice floats
High specific heat
Water heats up and cools down slowly
Powerful solvent
Dipole molecule strong attraction for ions on crystal surface
Ionic potential determines solubility of ions
Catalyst
Water increases the probability of reaction
Sources of water pollution?
Direct inputs
Land contamination
Weathering of spoil heaps leads to oxidation of low-grade ore minerals leads to acidic leachate containing soluble salts of toxic metals
Leachate from landfills
Nutrients leached from farmland (NB: not land contamination)
Atmospheric deposition
Features of acidification?
-Atmospheric deposition of nitric & sulphuric acids (following atmospheric oxidation of combustion products & biogenic gases)
-AMD (Acid Mine Drainage):
metal-S + H2O + O2 leads to metal n+ + 2H+ + SO42-
Impacts:
Aquatic fauna sensitive to pH changes:
< 5.5 leads to severe stress
< 5.0 leads to few survive
Affects mobility of metals (often toxic)
Features of Sensitivity of Aquatic Environments to pH Changes?
-Catchment bedrock
Sedimentary rocks easily weathered leads to high rates of soil formation leads to high buffering capacity:
replacement of cations adsorbed to clay mineral surfaces
bicarbonate in soil & river water
H+ + HCO3 (reversible) H2CO3
Igneous rocks resistant to weathering leads to low rates of soil formation leads to low buffering capacity
-Slope
Steep slopes
Erosion of weathered rock leads to low soil accumulation rates (weathering-limited regimes)
Low precipitation/catchment interaction
-Elevation
Low rates of organic matter decomposition leads to peat.
-Vegetation type
Low rates of pine needle decomposition leads to organic acids
Most vulnerable catchments in freshwater environments?
Igneous bedrock
Steep watershed
High elevation
Coniferous woodland
Features of heavy metals in freshwater environments?
-May be toxic (e.g., Pb, Cd, Hg) or essential (e.g., Cu, Zn)
-Sources
Industrial activities (e.g., smelting)
Combustion of fuels (e.g., Pb)
Disused mine workings
-pH
-Dissolved organic matter concentrations
-Organic methylation
e.g., methylmercury (CH3Hg) & Minamata Bay
-Impacts
Disrupt enzyme function
high affinity for S
Zn replaced by Cd
Bind proteins & cell membranes
disrupts transport
Features of nitrate and phosphate in freshwater environments?
-Sources
Farming & agriculture
Sewage treatment works (phosphate)
-Impacts
‘Blue Baby’ Syndrome & stomach cancer (nitrate)
Eutrophication (leads to reduced diversity, blue-green algae, oxygen depletion)
Features of organic substances in freshwater environments?
-Gross Organic Matter
From sewage, farm or industrial wastes, or eutrophication
Microbial respiration deoxygenation of water
Aquatic fauna sensitive to dissolved oxygen concentrations
Affects solubility of metals
-Causes:
Infrastructure upgrades not keeping pace with house building
Storm water collected with waste water
Climate change ( rainfall intensity)
Brexit & Covid-19 disrupting supply of chemicals for water treatment
Maintenance issues?
Poor/indifferent practice/management?
Types of organic substances in freshwater environments?
-Oil From effluent and urban/road run-off Smothering effects Soluble components poisonous/carcinogenic -Persistent organic pollutants Industrial chemicals (e.g., solvents, cleaners, degreasers, flame retardants, stain & water resistant finishes, by-products from plastics manufacture), pesticides Characterised by: high toxicity persistence potential for bioaccumulation capacity for long-range transport
-UN Stockholm Convention agreed in 2001 (came into force in 2004:
‘Dirty dozen’ defined
nine substances banned
use of DDT limited to malaria control
unintentional production of dioxins & furans to be curtailed
-May 2009: nine further substances added to convention
Two approaches for freshwater pollution control?
- Environmental Quality Objectives/Standards (EQO/EQS)
Use of receiving waters defines EQO
EQS = upper concentration limit of dangerous substances to secure EQO
Dilution capacity of receiving waters leads to emission limits for dangerous substances. - Uniform Emission Standards (UES) or limit values
Dilution capacity of receiving waters & presence of other inputs not considered
Acts against pollution examples?
-Dangerous Substances Directive (76/464/EEC)
First key piece of legislation focusing on eliminating/reducing pollution to inland waters by particularly dangerous substances
-List 1 (Black List):
129 substances (published in 1982)
Identification based on production volume, toxicity, persistence, bioaccumulation
UES & EQS agreed at community level
-List 2 (Grey List):
Less harmful substances
Quality standards (EQS approach in UK) set nationally
Member states required to establish programmes to reduce pollution
-Integrated Pollution Control (Environmental Protection Act 1990)
Represents a more holistic pollution control philosophy
Embodies the precautionary principle
Integrated approach (air, water, land)
BATNEEC applied to prevent/minimise emissions
contentious: seeking balance between cost to industry & cost to environment.
-All aspects of process examined:
nature of raw materials
process technology
treatment of wastes/abatement
training of operators
BPEO applied where more than one medium affected
-IPPC Directive (96/61/EC) required similar systems to be implemented across EU from 1999 (based on List 1)
-Water Framework Directive (2000/60/EC)
River-basin management approach with objective to:
protect and enhance status of aquatic ecosystems
ensure quality & quantity of resource
promote sustainable use of water resources
Now transposed into UK law as Water Environment (Water Framework Directive) (England and Wales) Regulations, 2017
33 priority substances identified for control, selected because
toxic at low concentrations
mutagenic or carcinogenic
bioaccumulate
persistent
frequently found in monitoring programmes
-River basin management plans drawn up every 6 years (2009, 2015, 2021)
-Environment Act 2021
Provides framework for setting targets for ‘recovery of the natural world’ in 4 priority areas (air quality, biodiversity, water & waste)
Overseen by new Office for Environmental Protection
Generally welcomed: ambitions of 25-year Environment Plan more likely to be realised.
Methods of diffusing sources of pollution in pollution control?
Pesticides:
Product registration
Lists of active ingredients that may be used
Bans/restriction of marketing/use of some pesticides
Classification, packaging, labelling requirements (including application methods, timing & rates, disposal methods)
-Controlling Land Use: Water Protection Zones
Certain activities prohibited or restricted in areas requiring extra protection
e.g., Nitrates Directive (91/692/EEC) defined Nitrate Vulnerable Zones (NVZs)
Farmers must conform to specified agricultural practice to reduce nitrate leaching
annual limits of fertilizer application
follow codes of good practice (timing, application practices, precautions)
Designated land = 70% of England
Features of environmental agency?
-Set up in 1995 (replaced the National Rivers Authority) to:
Prevent deterioration of the environment
Improve the quality of the environment
Monitor implementation of EU & UK legislation
-A: Monitoring and Classification
1. General Quality Assessment (GQA) employed until 2011
Rivers and canals surveyed quarterly
Biological monitoring of invertebrates
Chemical monitoring of DO, BOD & ammonia
Categorisation from A (excellent) to F (bad)
English rivers in category A in 2008: 72% (biological quality); 79% (chemical quality)
-2. Water Framework Directive (from 2008)
Covers rivers, canals, groundwater, lakes, estuaries & coastal waters
Ecological status based on a wider range of assessments (biological, chemical & physical)
Classification based on the principle of ‘one out, all out’ (i.e., determined by poorest individual result)
-Percentage of surface water bodies in England awarded each status classification in 2020 (modified from JNCC, 2021)
-B: Discharges to the Aquatic Environment
Issuing consents
Monitoring discharges
-C: Pollution Incidents
Monitoring, protection & clean-up
Investigation & prosecution
Importance of river transport?
-Rivers responsible for transporting products of physical & chemical weathering to sea:
Largest single flux for many elements
Particulate component from
physical weathering & erosion
soil biological processes & vegetation (leaves & detritus)
-Dissolved component from:
precipitation
chemical weathering
soil biological processes
leaching
in situ river biogeochemical processes (including sediment-water exchanges
Importance of nutrient cycling in river biogeochemistry?
Concentrations of dissolved N & P naturally low:
Rapid recycling during heterotrophic respiration
release of CO2, retention of N & P (immobilisation)
Decreased C:N and C:P in particulates downstream
P adsorbed on sediments
-particulate forms dominate N & P transport in pristine environments
-Effect of human activities important:
N in fertilisers riverine dissolved N is 2 leads to pre-industrial levels
P in detergents riverine dissolved P is 3 leads to pre-industrial levels
-1° production in freshwaters particularly sensitive to P levels ( eutrophication)
Factors Affecting Concentrations of Dissolved Constituents?
-Variations in Discharge
Increased Precipitation means increased proportion of drainage waters from surface run-off leads to decreased interaction with soil
Most major ions show inverse relationship between concentration & discharge (e.g., Ca2+, Mg2+, Na+, H4SiO4, Cl, HCO3).
-Some limiting nutrients may show positive relationship with discharge (e.g., NO3).
Features of precipitation/catchment interactions in river biogeochemisrty?
-Rainwater = dilute seawater
-Ca2+ is most abundant cation from weathering
Sodium/sodium+calcium shows relative importance of weathering or precipitation.
-Precipitation dominated when sodium/sodium+calcium approaches 1
- Weathering dominated when sodium/sodium+calcium approaches 0
Features of a precipitation dominated river?
-Sodium/sodium+calcium approaches 1
-TDS very low
-lower right arm of figure
-Rivers characterised by:
-high rainfall
-weathering resistant or extensively weathered bedrock
-E.g. Rio Negro (tributary of Amazon)
Drains highly weathered tropical soils
Features of a weathering dominated river?
-Lower ratios and intermediate TDS
-Position on diagram dependent on minerals weathered:
low ratios = sedimentary minerals
higher ratios = igneous minerals
Features of a evaporation dominated river?
- Hot arid regions where evaporation > precipitation
- High ratios & TDS (evaporation leads to precipitation of CaCO3)
- lie in upper right arm of diagram
The Mean Composition of Riverwaters of the World?
-20 largest rivers carry 40% of continental run-off ( best indication of global average riverwater composition) Total dissolved transport = 4*10^15 g a-1 1. Ca2+, Mg2+, K+, H4SiO4 Rock weathering = dominant source sedimentary: Ca2+ igneous: Mg2+, K+, H4SiO4 2. HCO3- Most acid hydrolysis weathering reactions Respiration Weathering of carbonates 3. Na+, Cl- Significant marine source Weathering of evaporites 4. NO3-, SO42- Atmospheric deposition: NO3-, SO42- Agricultural run-off: NO3-
What affects particulate load in river biogeochemistry?
Sediment transport affected by:
-1. Elevation & relief
Rivers draining southern Asia carry 70% of global sediment input to oceans
Amazon carries 9% (low elevation, limited relief)
-2. Vegetation
Transport increases when vegetation is removed
-3. Run-off
Sediment transport during episodes of extreme flows > accumulative total dissolved & particulate over periods of normal flow
Land-sea transport of poorly soluble elements dominated by particulate load:
Cu, Fe, Mn, P
Features of sediment?
-Sediments found in freshwater (rivers & lakes), brackish (salt marshes & estuaries) & marine environments:
Contain valuable record of past environments (e.g., evolution of atmosphere, climate variation)
Formation & diagenesis important in biogeochemical cycling of elements.
-Composition highly variable:
Clays & quartz from weathering
Biogenic material (tests & organic matter) from run-off & local biological activity
But similar microbial processes important in each environment.
Influence of redox controls in sediment?
-Microbial decomposition of organic matter = oxidation:
Organic matter loses electrons & is thereby oxidised
Oxidising agent (electron acceptor) gains electrons and is thereby reduced
-Nutrient cycling in sediments is controlled by the tendency of an environment to accept (oxidising) or donate (reducing) electrons
Electrode measurement = redox potential (Eh) = voltage required to prevent flow of electrons
high when oxidising
low when reducing (becomes negative)
Microbial Transformations in Sediments?
Aerobic conditions leads to high redox potential:
Electron acceptor = O2 (leads to H2O)
Oxidising agent (electron acceptor) gains electrons and is thereby reduced
Heterotrophic respiration of organic matter in sediments leads to rapid consumption of O2 with depth leads to anaerobic conditions leads to decreased Eh with depth.
-Decline in Eh with depth corresponds with a series of reactions involving weakly oxidising constituents that sequentially accept electrons from organic matter:
NO3, Mn4+, Fe3+, SO42-
Gradient in redox potential dependents on?
Concentration of organic matter
Concentration of oxidising constituents
Diffusion processes
Features of denitrification in sedimentary environments?
- Becomes dominant process when Eh falls to 421 mV
- More important in freshwater environments (including waterlogged soils)
- Denitrifying bacteria use nitrate as alternative electron acceptor
Features of manganese reduction in sedimentary environments?
- Important when Eh < 396 mV
- Denitrification & manganese reduction zones may overlap
- Denitrification & manganese reduction both performed by facultative anaerobes.
Features of iron reduction in sedimentary environments?
-Obligate anaerobes from now on!
-Dominant oxidation process when Eh < 182 mV
-Convenient indicator of transition between mildly oxidising & strongly reducing conditions
Fe3+ = red colour in sediments
Fe2+ = black colour in sediments
Features of sulphate reduction in sedimentary environments?
- Dominant oxidation process when Eh < 215 mV
- Responsible for up to 50% of total respiration in marine coastal areas
- Dominant natural source of S gases to atmosphere
Features of methanogenesis in sedimentary environments?
- Dominant oxidation process when Eh < 244 mV
- Important in freshwater environments; limited in marine environments (high sulphate)
- Marine sources of CH4 to atmosphere minor
- Rice paddies = 50% of global emissions
Examples of metabolic pathways in sedimentary environments?
-Disproportionation reaction involving organic matter
Dominant in freshwater environments
-CO2 reduction
Dominant in marine environments
Energy yields in sedimentary environments?
- Aerobic respiration (3,000 kJ mol1) > denitrification > manganese reduction > iron reduction > sulphate reduction > methanogenesis (400 kJ mol1)
- Order represents declining energy yield
Biogeochemical Cycling of Nitrogen in Sediments?
- Ammonification: NH4+
- Adsorbs to sediment clays in freshwater environments (& some diffusion into overlying water)
- Diffuses into overlying water in marine sediments
Biogeochemical Cycling in the aerobic zone?
- Aerobic zone
- NH4+ in water & sediments stimulates nitrifying bacteria
- Nitrosamonas bacteria convert NH4+ to NO2-
- Nitrobacter bacteria convert NO2- to NO3-
- Diffusion: NO2- & NO3- in water & sediments
Biogeochemical cycling in the anaerobic zone?
- Anaerobic zone
- NO3- & NO2- denitrified leads to N2O & N2
Biogeochemical Cycling of phospherous in Sediments?
-Not involved in redox processes
-Presence in sediments:
In organic matter
Adsorbed on oxides/hydroxides (Fe & Mn)
Adsorbed on clay minerals
Precipitated
- Sediment = sink for P in oxidising conditions
-Solubilisation of Mn & Fe in reducing conditions release of P:
-Diffusion & re-precipitation in oxic layer
-Diffusion into overlying waters if surface sediment & bottom waters are anoxic
True or false? Remineralization processes are important for recycling of nutrients? Features?
True
-Stimulates primary production
Death and sedimentation of organic matter remineralization, etc.
-N & P released from marine sediments upwelled to surface waters at western margins of continents.
Difference between sediment composition of open ocean, coastal ocean, and upwelling area?
Open ocean - anoxic and oxic sediment - oxic seawater
Coastal ocean - anoxic sediment - oxic seawater
Upwelling area - anoxic sediment - oxic and anoxic seawater.
Relationship between sediments and atmospheric composition?
- Permanent burial of reduced compounds (particularly orgC & pyrite) regulates O2 in atmosphere
- Increased atmospheric oxygen leads to decreased area and depth of anoxic sediments leads to decreased atmospheric oxygen leads to increased area and depth of anoxic sediments leads to increased atmopsheric oxygen. - Carries on as cycle.
Climate change effect on sediment?
-Globally, 25% of soil organic matter is contained in saturated soils of tundra & boreal forest
-Drainage & warmer climates lead to greater decomposition leads to lower carbon storage
leads to significant source of CO2 & CH4 to atmosphere.
Features of soil development?
- Soil-forming processes development of distinct stable layers, or soil horizons, with diagnostic features
- Internationally agreed descriptions/abbreviations
- May be sub-divided
- Number, thickness and character of each horizon varies with soil type
- Basis of soil classification.
General soil profile?
- O Horizon – Surface Litter: fallen leaves and organic debris.
- A Horizon – Top Soil: organic matter (humus), living organisms, inorganic minerals.
- E Horizon – Eluvial (Leaching) Zone: dissolved and suspended material moves downwards.
- B Horizon – Illuvial (Accumulation) Zone: sub-soil – Fe, Al, humic compounds and clay leached from A and E horizons; more altered material than C horizon.
- C Horizon – Weathered Parent Material: partially broken down inorganic minerals.
- R Horizon – Bedrock: impenetrable layer.
Soil characteristics depend on?
Type of parent material
Degree of weathering
Slope
Climate
Systems of soil classification?
US Department of Agriculture (USDA)
UNESCO FAO
Features of soil texture?
- Affects water retention, infiltration & nutrient availability (CEC)
- Determined by the mixture of particle sizes (USDA classification)
- Sand (0.05 – 2.0 mm)
- Silt (0.002 – 0.05 mm)
- Clay (<0.002 mm)
- Loam = mixture of sand, silt and clay
Loam?
Micture of sand, silt, and clay.
CEC values for soil textures?
Sands (light colour, extremely poor organic matter 3-5 meq 100g^-1
Sands (dark colour, poor organic matter) 10-20
Loams 10-15
Silt loams 15-25
Clay loams and clay 20-50
Organic soils 50-100
Features of nutrient cycling in soils?
-Most plant annual nutrient requirement met by decomposition (= recycling) by fungi & bacteria
-Decomposition of fresh litter converts orgC to CO2
-N & P initially retained (= immobilisation)
-High C:N & C:P ratios promote microbial growth (demand for N & P)
-reduced C:N & C:P ratios leads to slower microbial growth leads to mineralisation:
orgN = NH4+
orgP = PO43-
-Immobilisation dominates in fresh litter; mineralisation more important in lower soil horizons
Mean Residence Time (in years) of Organic Matter and Nutrients in Litter of Forest and Woodland Ecosystems?
Boreal forest organic matter: 353 Nitrogen: 230 Phosphate: 324
temperate coniferous forest organic matter: 17 Nitrogen: 17.9 Phosphate: 15.3
temperate deciduous forest organic matter: 4 Nitrogen: 5.5 Phosphate: 5.8
Mediterranean organic matter: 3.8 Nitrogen: 4.2 Phosphate: 3.6
Tropical rainforest organic matter: 0.4 Nitrogen: 2.0 Phosphate: 1.6.
Organic matter of soil?
Vegetation (above ground)
Detritus (dead plant & animal matter; cellular fraction)
Soil microbes (bacteria & fungi)
Humus (non-cellular organic matter; resistant to decay)
Features of soil organic matter?
- Humus»_space; detritus + soil microbes + vegetation in most systems (except tropical soils)
- Global pool of N in vegetation = 3.8*10^15 kg
- Global pool of N in humus + detritus + soil microbes = 95 – 140 1015 kg
- Large nutrient pool but slow turnover (1 – 3 % per year) subject to active uptake by plant roots
Fate of mineralized nitrogen in soil?
Nitrogen (mineralisation of organic matter to NH4+)
Two pathways for ammonium:
1. Uptake or assimilation by plants (into amino acids)
2. Nitrification in aerobic soil
Oxidation to nitrite by bacteria of the Nitrosamonas genus:
2NH4+ + 3O2 2NO2 + 2H2O + 4H+
Oxidation to nitrate by bacteria of the Nitrobacter genus:
2NO2 + O2 2NO3
Two pathways for nitrate:
1. Assimilation
2. Denitrification in anaerobic soil (into N2O, N2)
Performed by facultative anaerobic bacteria that use NO3- as an oxidising agent in the absence of O2
Fate of mineralized phosphorous in soil?
- Phosphorus (mineralisation of organic matter PO43)
- Orthophosphate readily precipitated:
- FePO4 or AlPO4 in acid soils
- Ca3(PO4)2 in alkaline soils
- Maximum utilisable concentrations at near-neutral pH
- Low availability
How does chemical contamination degrade soil?
-Local – particularly in proximity to landfill sites & heavy industry (oil & heavy metals)
-Diffuse – use of sewage sludge as fertiliser (contamination with metals, pathogens & organic pollutants)
-Salinisation (accumulation of soluble salts from irrigation)
-Acidification (atmosphere)
This impacts biodiversity (sensitivity of taxa, nutrient loss).
How does compaction and soil loss degrade soil?
-Compaction
Farm machinery, animals & tillage
This affects air capacity & permeability; increases waterlogging & run-off; impacts soil biological activity & root development
-Soil Loss
Erosion, principally by water (cropping systems where soil is left bare for long periods)
Compaction (increases surface run-off).
How does decline in soil organic matter degrade soil?
- Conversion of grassland, forests and natural vegetation to arable
- Deep ploughing, extensive tillage, drainage, inorganic fertilizer use
- This increases compaction; impacts soil biodiversity & biomass; reduces carbon storage & nutrient cycling.
Features of conservation farming?
Minimal mechanical disturbance
Permanent soil cover
Crop residues left in the field
Diversification of crop species grown in the same field
Features of EU Common Agricultural Policy (CAP)?
-Designed to:
Guarantee food supply
Support price of agricultural products
Provide farmers with an acceptable level of income
-But:
Led to over-production of some agricultural goods in 1970s and 1980s
Accelerated intensification
- After brexit?
CAP payments (£3.5 billion in 2018) to be phased out over 7 years from 2021, to be replaced by schemes that pay farmers for ‘public goods’ such as environmental improvements.
What are the environmental protection measures introduced since 1999?
-Statutory management requirements (‘good agricultural practice’) for farmers receiving CAP subsidies
-Latest reforms (2014 – 2020) reward farmers for:
maintenance of permanent grassland
maintenance of Ecological Focus Areas
crop diversification.
What was DEFRA’s response to agriculture act in 2020? Features of response?
-Environmental Land Management Scheme (ELMS)
- 1. Sustainable Farming Incentive (SFI)
Farmers paid for taking actions to promote wildlife diversity, use water efficiently, enhance hedgerows & manage croplands & grasslands
- 2. Local Nature Recovery Programme
Payment for creating, managing and restoring natural habitats.
Formation of solar system?
-Gravity
-1. Smaller masses attracted to larger ones
leads to contraction leads to acceleration leads to flattening of cloud to disk.
-2. Drifting of material to centre
Accumulation & compression leads to heat leads to proto-Sun
Energy release from nuclear fusion once temperatures reached 106 K
How did the planets form?
-Cooling of cloud leads to condensation
-Gravitational attraction leads to small planetismals
-Larger planetismals attracted smaller ones (9 planets)
-.Inner terrestrial planets too hot for light gases to be retained (e.g., H2, He, H2O)
Mercury, Venus, Earth & Mars composed of heavier metals (e.g., Fe)
99% of Earth’s mass made up of 8 elements (Al, Ca, S, Ni, Mg, Si, O, Fe)
Volatile substances (e.g., H2, He, H2O, CH4, NH3) carried to cold outer reaches of solar system to accumulate on gaseous planets
Jupiter, Saturn, Uranus, Neptune (& Pluto) dominated by H2 & He
Earths planetary evolution?
-Differentiation into core, mantle & crust caused by heating:
-Collision of planetismals with primitive Earth
-Compression
-Radioactive decay
-Internal temperatures > 2,000 °C melting & distillation
-Dense Fe accumulated at centre core
Less dense elements migrated to surface primitive crust
Intermediate density elements formed mantle
-Differences in elemental abundance between crust & whole Earth reflect density differences.
How was the earth’s crust formed?
- Distillation of continents separation of lighter materials from heavier ones
- Escape of volatiles formation of early atmosphere & oceans
Earth’s Surface 4.6 Ga BP?
-Temperatures too high to allow an ocean
-Evidence of little/no atmosphere
e.g., accounting for presence of 20Ne in today’s atmosphere:
-Not produced by radioactive decay
-Inert
-Too heavy to have escaped to space
Abundance today = initial abundance
Ratio of Ne to other gases in cosmic abundance mass of primitive atmosphere, e.g.,
-Initial abundance of Ne = 6.510^16 g
-Initial abundance of N = 3510^16 g
= 0.1 % present day
How were primitive atmosphere and oceans created?
-Crustal outgassing of volatiles:
dominated by water vapour (WV) & CO2
also SO2, H2S, CH4, NH3, CO, H2, HCl & N2
-Cooling of surface to <100 °C condensation of WV, formation of oceans & dissolution of soluble gases
oldest preserved rocks (3.8 Ga BP) indicate water present.
- Less than 1% of volatiles have remained in atmosphere
Composition of primitive atmosphere?
Dominated by N2 Water vapour No O2 Relatively high CO2 Moderately reducing (presence of olivine, FeMgSiO4) H2 escaped NH3 photolysis led to H2 lost to space
Composition of primitive oceans?
Major constituents similar to today
Higher CO2 leads to lower pH
Higher Ca2+ (CaCO3 found in earliest deposits)
Some SO42- (gypsum, CaSO4.2H2O, found in earliest deposits)
Higher concentrations of reduced metals.
Origin of life?
-Good correlation between solubility/abundance of elements in seawater & their concentration in living tissues:
-Fe, Al, Si
low solubility/abundance leads to low concentrations
-Na, Ca, K, Mg
very soluble/abundant leads to high concentrations
- C, N, P, S form soluble oxyanions in seawater (HCO3-, NO3-, PO43-, SO42-) leads to high concentrations in living tissues
-Rare elements in seawater (e.g., As, Cd, Hg, Pb) often poisons.
What leads to simple organic molecules in a primitive atmosphere?
Lightning & UV light
Heterotrophic metabolism?
-Earliest metabolic pathway (4.2 – 3.8 Ga BP) involved splitting simple organic molecules?
-e.g., CH3COOH leads to CO2 + CH4
-More elaborate = oxidation + reduction
-Sulphate reduction also occurring by 2.4 Ga BP:
Nitrogen fixation also probable, but no direct evidence:
Autotrophic metabolism?
-Earliest photosynthesis reaction probably based on S rather than water (requires less energy):
-CO2 + 2H2S CH2O + 2S + H2O
2S + 3CO2 + 5H2O 3CH2O + 2SO42 + 4H+
Oxygen-evolving photosynthesis by 3.5 Ga BP:
H2O + CO2 CH2O + O2
-Oxygen rapidly consumed in oxidation reactions (particularly Fe leads to Fe2O3) appearance of Banded Iron Formations.
How did oxygen appear in the atmosphere?
-Decline in soluble reduced metals in ocean O2 accumulation in oceans & diffusion to atmosphere
Reaction of O2 with reduced atmospheric gases & exposed minerals (e.g., FeS2 Fe2O3)
-Transport to ocean Red Beds (from c. 2 Ga BP)
-O2 accumulation in atmosphere when rate of diffusion from ocean > rate of consumption
-Present ‘steady state’ from 400 Ma B.
How has the environment adapted with oxygen evolving metabolism?
Poisoning/marginalisation of anaerobic pathways
2. Formation of ozone layer leads to development of higher organisms
Marine multicellular organisms from 680 Ma BP
Appearance of terrestrial vascular plants from 400 Ma BP
3. Development of important new biochemical pathways (e.g., nitrification leads to NO3-)
Influence of the Biosphere on Surface Conditions and Processes?
- Presence of O2 in the atmosphere.
- Simple model for Earth as a biogeochemical system illustrates central role of biosphere.
Features of Schlesinger model made in 1997?
-7 major minerals.
-Describes interactions between crust, oceans, atmosphere & biosphere.
-Model indicates transfers associated with increase in size of biosphere.
- Large areas of swamps during Carboniferous (360 – 290 Ma BP) leads to peat (leads to coal):
-Increased organic matter burial during Carboniferous associated with large deposits of gypsum
Ratio of organic carbon to gypsum fairly constant.
Weathering?
Adjustment of rocks & minerals formed at high T & P at depth to low T & P at Earth’s surface.
What does weathering exert control over?
Composition of the atmosphere
Composition of the oceans
Formation of sedimentary rock
Characteristics of soils
What is important for most of earth’s history?
- Crustal outgassing leads to dissolution of volcanic gases in atmospheric water leads to acids reacting with surface minerals
- Evolution of O2 leads to oxidation of reduced minerals
- Evolution of land plants leads to high CO2 in soils from decomposition
- Human activities leads to acid rain leads to increased weathering.
Role of water in weathering?
Solvent
-In liquid state water has an angular geometry
O is more electronegative than H
Dipole molecule
Strong attraction for ions on a crystal surface
-Ions released from crystal lattice are surrounded by envelope of water molecules
-Ionic potential (charge/radius, Z/r) determines strength of attraction between water molecules & ion (& subsequent behaviour).
- Z/r < 3 (small charge spread over large area)
Water molecules more strongly attracted to each other leads to hydration of ion e.g., K+, Na+.
-3 < Z/r < 12
Water molecules more strongly attracted to ion leads to binding force with one H overcome (expelled into solution) leading to precipitation of insoluble hydroxide
e.g., Fe3+ Fe(OH)3, Mn4+ Mn(OH)4
-Z/r > 12 (large charge spread over small area)
Water molecules very strongly attracted to ion leads to binding forces with both H overcome leads to soluble complex ions e.g., CO32-.
Catalyst
Water allows ions to approach each other closely without spatial constraint leads to increased probability of reaction.
Weathering processes?
Physical (or mechanical) weathering
= fragmentation with little chemical change
Chemical weathering
= reaction with acidic & oxidising substances secondary minerals & dissolved ions.
Types of physical weathering?
1. Pressure release Unloading leads to expansion leads to cracks & joints prised apart by: diurnal cycle of thermal expansion & contraction (e.g., deserts) crystal formation (particularly ice) plant roots 2. Glacial activity 3. Landslides 4. Sand-blasting
Types of chemical weathering?
Releases elements from crust for uptake by biota
Processes may be:
Congruent leads to dissolved products only
e.g., weathering of halite, anhydrite, gypsum, aragonite, calcite, dolomite, quartz
Incongruent leads to dissolved & solid products
HCO3-, H4SiO4, Ca2+, Mg2+, Na+, K+
Clays (cation & Si-depleted secondary minerals)
-Dissolution e.g. halite
-Oxidation - Slow at surface temperatures but catalysed by water e.g., fayalite:
- Acid hydrolysis e.g foresterite
Water contains acid:
from atmosphere (e.g., CO2)
from soils:
decomposition CO2 up to 200 atmospheric concentration
organic acids (plant roots, fungi, microbial decomposition)
- Highest rates of chemical weathering observed in soils
Factors Controlling Weathering Rates?
-Temperature & water
Reaction rates double for every 10 °C rise in temperature
Weathering rates in tropics (20 °C) 2 temperate regions (12 °C)
But related to availability of water:
Weathering limited in hot, arid environments
Weathering rapid in humid tropical climates
-Bedrock resistance/susceptibility
Joints and voids
Igneous rocks: jointing from contraction on cooling & unloading
Sedimentary rocks: bedding play Unconsolidated sediments: voids
-Mineral stability (particularly igneous rocks)
Minerals crystallised from magma/lava at high temperatures (e.g., feldspars) weather more easily than those at low temperatures (e.g., quartz)
- Soil biology
Decomposition of soil organic matter CO2 & organic acids
Mediates in moisture budget
Dense vegetation limited penetration of water
Controls water retention
Organic-rich soils retain more water
-Slope
Steep slopes correlate to weathering-limited regimes:
Weathering controlled by bedrock susceptibility
Thin or no soils (weathering products readily eroded)
Characteristic process = rock falls
Shallow slopes lead to transport-limited regimes:
Weathering controlled by transport
Thick soils (may limit water penetration to un-weathered bedrock).
Global weathering rates?
-Chemical weathering delivers 4 1015 g a1 dissolved substances to oceans
27 % from weathering of igneous rocks
H4SiO4, HCO3, cations
73 % from weathering of sedimentary rocks
high Ca2+, Mg2+, HCO3
-Physical weathering delivers 1.4*10^16 g a1 particulate material to oceans
most of Fe, Al, Si transport (poorly soluble)
large proportion of P and trace metal transport (readily adsorb to particulate surfaces)
Regional variation on weathering?
Chemical weathering is more important in warm, moist regions; physical weathering is more important in cold, dry regions
Contributions from chemical weathering are greatest in regions of much vegetation
Contributions from physical weathering are greatest in steep terrains (and overall weathering rates are higher).
highest weathering rates in humid tropics:
-High temperature
Moist conditions
Luxuriant vegetation
Tectonically active (steep terrain)
Represents 25 % of land surface, but delivers to ocean:
65 % dissolved Si (as H4SiO4)
38 % total ionic load
50 % particulate load
Weathering Rates in the Geologic Past?
Before 400 Ma BP (appearance of vascular plants)
Lower weathering rates meant potentially higher atmospheric CO2?
From 65 Ma BP (major mountain-building episodes)
Higher weathering rates meant potentially lower atmospheric CO2?
Effect of human activities on weathering rates?
-Mining, deforestation, farming lead to increased weathering & erosion
-Physical weathering & erosion has increased by a factor of 2
-Accelerated loss of agricultural soils
increased sediment accumulation in estuaries & river deltas.
Features of concept of soil?
- Most derived from weathered bedrock (silicate minerals)
- Consist of rock fragments, clays, organic matter, living organisms, air & water
- Support many global food webs
- Important role in biogeochemical cycling & climate
Structure of silicate materials?
-Earth’s crust dominated by silicate minerals
-Based on silicon and oxygen
-The most abundant elements in the crust (74.3 % by mass)
-Main building block = silicate tetrahedra (SiO44-)
-Polar-covalent bonds
- Tetrahedra often linked via oxygen atoms to produce a number of different structural arrangements
Surplus negative charges satisfied by metal cations (e.g., Al3+, Mg2+, Fe2+)
Metals may be substituted for others that have a similar ionic radius (isomorphism) lead to mineral ‘types’ with variable composition.
The different silicate arrangements in soil?
Isolated e.g. olvines Single chain e.g. pyroxenes Double chain e.g. amphiboles Sheet e.g. micas, clays Framework e.g. quartz, feldspars?
Structure of clay minerals?
-Sheet silicates
-The solid products of chemical weathering of complex silicate minerals
-Important constituents of most soils
-Important components soil fertility
-Two structural components: tetrahedral sheets and octahedral sheets.
-Tetrahedral sheets
Silicate tetrahedra which share their three basal oxygen atoms with neighbouring tetrahedra
-Octahedral sheets
Cations (usually Al3+ or Mg2+) arranged equidistant from six oxygen or hydroxide (OH) ions, which are shared with neighbouring cations
e.g., gibbsite, Al(OH)3.
- 1:1 clay mineral structure - Octahedral and tetrahedral sheets linked through apical oxygen atoms of tetrahedral sheets (which become part of octahedral sheet).
- Sequences of tetrahedral/octahedral (1:1) layers held together by hydrogen bonds (prevents cations getting between layers) - Example = kaolinite.
- 2:1 clay mineral structure - Octahedral sheet sandwiched between two tetrahedral sheets
Most other clay minerals share this structure
- Illites - One in four tetrahedral Si4+ replaced by Al3+
Some octahedral Al3+ replaced by Fe2+ or Mg2+
Called isomorphic substitution
Strong net negative layer charge neutralised by K+
- K+ bonds ionically with basal oxygen atoms of opposing (neighbouring) tetrahedral sheets
Strong bonds lead to stable lead to abundant in temperate and colder climates.
- Smectites
Less regulation isomorphic substitution led to weaker negative layer charge lead to interlayer cations (+ water) weakly held and exchangeable
Capacity to hold and exchange cations = cation exchange capacity (CEC)
Abundant in in temperate and colder climates
Mixing of illites and smectites in temperate areas mixed layer clays.
Features of leaching controlling clay formation?
- Exerts fundamental influence (preferentially removes soluble cations)
- Controlled by topography, rainfall and drainage
- Intense leaching removes soluble cations & H4SiO4 (lowering Si:Al ratio) leads to 1:1 (kaolinite) or 0:1 (gibbsite) clay structure
- Formation of laterites (impenetrable siliceous Fe & Al layers that inhibit plant growth)
- High Fe & Al toxic to some plants & animals
- Low fertility
- e.g., high altitude tropics (high rainfall; soluble ions carried down slope)
- Poorly drained sites favour 2:1 clay structure (e.g., smectite)
- High fertility
- Swelling and shrinking of clays causes cracking in dry conditions (may cause problems for agriculture)
Relationship between parent
aluminosilicate minerals, smectite, koalinite, and gibbsite?
Parent
aluminosilicate to Smectites to Kaolinite to Gibbsite
minerals
Si:Al: 2:1 1:0 0:1
Time: increases over right
Leaching intensity: increases over right.
Relationship between ion exchange and soil fertility?
- Ability of minerals to hold ions temporarily on their surfaces (weak electrostatic forces)
- Resistant to leaching, but can be replaced by other ions (equilibrium controlled)
- Important component of soil fertility (reservoir of nutrients for plant growth)
- Only Cation Exchange Capacity (CEC) is important in most soils.
Features of cation exchange capacity?
Arises from:
1. Isomorphous substitution
Permanent surface negative charge
e.g., replacement of Si4+ with Al3+ in smectites
2. Edge effects
Edge damage may break bonds leading to uncoordinated oxygen atoms (negative charge).
3. Dissociation of surface hydroxyl functional groups
Greater importance with increasing pH
Mineral OH(s) (reversible) Mineral O-(s) + H+(aq)
4. Organic matter - Carboxyl functional groups dissociate at pH > 5.
Different mineral’s cation exchange capacity and site of ion exchange?
Kaolinite CEC: 3-15 Site of ion exchange: edge effects
Illite CEC: 10-40 Site of ion exchange: mainly edge effects and some interlayer.
Smectite CEC: 80-150 Site of ion exchange: mainly edge effects and some interlayer.
Organic matter CEC: 150-500 Site of ion exchange: disassociation of functional rap groups.
What factors drive exchange in soil?
- Preference for adsorbed state
Al3+ > H+ > Ca2+ > Mg2+ > K+ > NH4+ > Na+ - Equilibria between cations held on exchange sites and soil pore water concentrations
e.g., addition of potassium to an agricultural soil. - pH
-Temperate fertile (neutral or alkaline) agricultural soils can buffer acid rain
-BUT denudes soil of plant nutrients (porewater cations subject to leaching).
-Acidification of gibbsite-based (acid) soils (heavily weathered environments) releases soluble Al3+ (toxic to fish).
