Oceanography Flashcards
What does ocean circulation do?
Key regulator of climate by storing and transporting heat, carbon and nutrients around the globe
Key player in driving glacial-interglacial cycles
What does vertical ocean circulation do?
Ventilates oxygen into deep waters
What drives vertical ocean circulation?
Vertical ocean circulation occurs when the density of water changes, allowing waters to sink or rise
Where do high surface seawater densities occur?
North Atlantic Ocean
The southern ocean (Antarctica)
What drives the circulations of oceans?
Temperature and salinity (thermohaline)
What are the characteristics of Antarctic bottom water (aabw)
Forms at multiple sites (polynyas) and migrates out of the Antarctic region
Very cold and quite saline
Cooling by cold wind reduces surface water temp and promotes ice formation, increasing salinity so it sinks
What are polynyas?
Ice free areas of sea in the ice cover
Ie maud rise polynyas, September 2017
What is the formation of North Atlantic deep water (nadw)?
Forms in Labrador Sea (upper NADW) and seas between Iceland, Greenland and Norway (lower NADW).
North Atlantic drift current carries warm salty Gulf Stream water into they Nordic sea and then to the arctic
Cold fresh polar surface water flows south through the Fram Strait and mixes with the warmer salty Atlantic water in the Nordic seas.
Heat from the Atlantic water is released to the atmosphere and the mixed water is dense enough to sink.
How can NADW and AABW be detected?
Based on temperature and salinity depth profiles
What eventually happens to deep water?
Mixes to become less dense ie in the pacific
What are the conservative properties of seawater?
Potential temperature and salinity
What speed does the ocean move at?
Surface currents up to 1m/s
Deep water up to 1cm/s
How old is the ocean deep waters?
Oldest deep water >2000 yrs old before upwelling in the pacific.
Where are intermediate water masses formed?
North Pacific
Mediterranean
Antarctic
What is simple evidence for ocean circulation?
More heat is emitted by the poles than received
Less heat is emitted by the equator than received
Why are the Antarctic and North Atlantic waters so dense?
Antarctic = high density from cold
North Atlantic = high density from salinity, less dense than Antarctic
What is the depth of the ocean?
Typical = 4km
Up to 11km
What is OCW?
Ocean common water
Mixture of different water bodies
Can’t tell where it formed
What causes the mixing of deep water?”
Turbulence
What are ocean currents principally driven by?
Wind
How fast does the earth rotate?
Equator 1000mph
60 degree latitude 500mph
Why does the earth spin at different speeds?
Because the earth is a sphere and thus at higher latitudes there is a smaller radius of curvature
Where is the coriolis effect at its minimum and maximum?
Minimum at equator ~0
Maximum at poles
In what direction do ocean gyres rotate?
Clockwise in the northern hemisphere
Anticlockwise in the southern hemisphere
What causes ocean gyres to rotate?
Coriolis effect which deflects water/air masses
At what angle did Fridtjof Nansen observe that ice moved to wind direction?
20-40 degrees to the right of wind direction
What is modelled Ekman transport?
Ocean transport is at 90 degrees to the right (in N hemisphere, left in S hemisphere) of wind direction
How deep is the Ekman layer?
<100m
Give examples of gyres that rotate in the opposite direction than expected?
Alaskan and Norwegian currents rotate anticlockwise
What is the result of Ekman transport?
Pile up of water in the centre of ocean gyres
Variations in sea surface height
What do variations in sea surface height generate?
Horizontal pressure gradients
Low pressure/pressure gradient force/ high pressure
What is geostrophic current?
The flow when coriolis force balances the pressure gradient
Where is the apex of the sea slope found in ocean gyres?
In the west
What is westward intensification?
When the apex of the sea slope is in the west of the gyre
Part of the coriolis effect
What are ocean gyres a result of?
Wind currents
Geostrophic currents
Westward intensification
What does oceanic convergences and divergences led to?
Vertical circulation
What do diverging after currents look like?
A sea surface valley \/ with water coming from the depths and spreading outwards. Anticlockwise rotation.
Upwelling
What do convergence currents look like?
Sea surface hill /\ with water coming in from surrounding and being forced downwards
Clockwise rotation
What divergence cause?
Upwelling of nutrient rich deep waters
Where do we find divergence currents?
Alaskan and Norwegian currents
North and south equatorial currents
Antarctic currents
What causes the coriolis effect?
Occurs due to something moving across a surface but isn’t frictionally bound to it ie wind
What can affect Ekman transport?
Shallow water friction with sea bead
What does the pressure gradients force do?
Forces water from the inside of a gyre to the outside
Why does the deep ocean have more nutrients than the surface ocean?
Phytoplankton eat nutrients in the photic zone
Dead phytoplankton sinks and acculmalates
Why must there be mixing in the ocean?
Allows deep waters to gradually decrease in density and upwelling back to the pacific and Indian oceans rather than filling ocean basins
How are counter surface ocean currents created?
By the upwelling of deep waters to the surface in the North Pacific and Indian oceans
What is the scale of molecular diffusion and turbulent diffusion?
Molecular diffusion: cm scale process
Turbulent diffusion: M to 100’s km process
What is turbulent diffusion?
Chaotic flow with irregular fluctuations in speed and direction
What causes turbulent diffusion?
Wind driven waves
Current shear
Movement over an irregular seabed
Movement along an irregular sea coast
Tidal currents
What is current shear?
Interfaces between different water bodies flowing in different directions or at different speeds
How much faster is horizontal diffusion than vertical diffusion?
~10^5 - 10^8 times faster
Why is vertical ocean mixing so much slower than horizontal mixing?
Because of density
What is the effect of stratification on vertical mixing?
Stratified waters mix vertically much more slowly than unstratified waters
What inhibits turbulent mixing?
Stratification
What is meant by stratification?
Two or more distinct layers within the ocean of different densities which prevents turbulent mixing
What drives vertical changes in seawater density?
Temperature (inversely)
Salinity (likewise)
How are sea surface temperatures measured and what does it reflect?
By satellite
Reflect balance of heat gain (solar) and loss (albedo, conduction, evaporation)
Net heat gain below 30 degrees lat
Net heat loss above 30 degrees lat
What is sea surface salinity defined as?
Practical salinity units (psu)
What does sea surface salinity reflect?
Reflect balance of freshwater inputs (precipitation, runoff, ice melt ) and outputs (evaporation, freezing)
Where is sea surface salinity highest?
Atlantic, Mediterranean Sea
In subtropics where evaporation>precipitatiom
What is an isotherm?
Lines joining points of equal temperature
What is a isohaline?
Lines joining points of equal salinity
What is a thermocline?
Section below which temperature changes very rapidly with depth
What and where is the mixed surface layer?
Surface section where temperature has little variation
~10-300m
How far can solar energy penetrate the ocean?
<100mk, mostly first few metres
How can heat be transmitted deep into the water column?
Turbulent mixing
Conduction
Downwelling
Are thermoclines permanent or temporary?
Most oceans have a permanent thermohaline
Seasonal thermoclines can form in mid latitudes
Does salinity increase or decrease with depth?
Typically decreases at low and mid latitudes
Can increase
What is a halocline?
The section of a profile where salinity changes very rapidly
What is a isopycnal?
Lines joining points of equal density
What is a pynocline?
Section of a profile where density changes very rapidly
How does the pynocline change with latitude?
Equator: very horizontal
Sub tropics: 45 degrees angle
High latitude: vertical
How does stratification of the water column affect primary production?
Tropical Atlantic has permanent stratification therefore no nutrient recycling from depth
North Atlantic has seasonal stratification that breaks down in winter, allowing upwelling of nutrient rich deep waters annually
What is the rate of molecular diffusion dependent on?
Temperature
How fast does molecular diffusion travel in seawater?
10^-5 cm2 s-1
Why is heat diffusion faster than salt diffusion?
X100 faster
Warm molecules move faster and bounce off each other, transmitting heat
What is the affect of temperature diffusing faster than salt?
Warm salty water overlies cool, fresh water
What is the impact of double diffusion?
The temperature gradient will diffuse away in 1% of the ice it takes for the salt gradient to diffuse.
Remaining salinity gradient becomes unstable, and overturning occurs
What is salt fingering?
When sinking cells of salty water (that are losing heat) alternate with rising cells of fresher water that are gaining heat.
What is the scale of salt fingering?
1-5 cm wide
10s cm long
What is the result of salt fingering?
Thermohaline staircase in oceanographic profiles
Where does salt fingering occur?
Where warm saline waters overlie cool fresh waters
Subtropics
How does climate change affect the Meridional ocean circulation pattern?
Influx of freshwater from the melting of high latitude ice
Increased evaporation by higher temperatures and increased wind speed
What is the importance of the meridional ocean circulation pattern?
Transports heat from tropics to high latitudes
Ventilates the deep ocean
Redistributes nutrients, oxygen, carbon and pH
What trend was observed in the thickness of AABW over the past twenty years?
Reductions in thickness from 64m/decade (Weddell sea) to 132m/decade (Mawson sea)
What is a possible reason for a slight recovery of Antarctic bottom water formation?
Positive El Niño+Sam -> weakened easterly winds -> reduced sea ice transport -> more space in Ross sea -> increased sea ice formation on the shelf -> saltier water ->increased AABW
How is NADW formation tracked?
Using two arrays (RAPID sub tropics, OSNAP sub polar) measuring temperature, salinity, current velocity
What is the trend of change in the upper and lower sections of AMOC?
Reduced northward flow of surface waters
Reduced southward flow of deep waters
What is the decrease of NADW circulation annually?
~0.5 Sv/year
What is Sv?
Sverdrup
Measure of current volume
1 x10^6 m^3
What is MOC?
Meridional overturning circulation
What is EKM?
Wind driven Ekman component
What is UDW?
Upper deep water
1100-3000m
What is LDW?
Lower deep water
3000-5000m
What happens if NADW formation is reduced?
Velocity of entire thermohaline circulation is reduced
Why is the MOC closing
?
Decrease in high latitude surface water density
High latitude surface waters becoming warmer and more buoyant
Input of freshwater from melting ice sheets = less saline
What is the biological pump?
Phytoplankton creates sugar and takes up nutrients and carbon (co2)
Phytoplankton is consumed and released as fecal matter, or dies and decays.
Aggregate forms from matter and sinks into deep water
Remineralisation, carbon export, digestion
What is the vertical profile of nutrients in the ocean?
Depleted in surface waters
Concentrated at depth
What is the slope where nutrient concentrations increase the most rapidly called?
Nutricline
What is the biolimiting factor?
Where concentrations are zero at the surface
Where are nutrient concentrations the highest and lowest in the surface?
Lowest in open ocean waters
Highest in upwelling regions or in areas of coastal runoff
Where are there high concentrations of nutrients in the deep ocean?
In areas of deep water with great age
What is he impact of climate change on ocean stratification and primary production?
Surface warming leads to:
Increased water stratification
Decreased vertical mixing
Decreased primary production
What is stratification principally controlled by?
Temperature
What has caused the strengthing of stratification around Antarctica?
Salinity changes
What is the effect of off shore wind farm structure on stratification?
Mixing effect
Laminar flow to strong vertical mixing
what is the chemical composition of seawater?
in 1kg. 965.6g water, 35.4g salinity including sodium, chloride, calcium, bicarbonate, magnesium, potassium and sulphate
what is the relationship between seawater and rain?
rainwater is dilute seawater that is derived from sea aerosols
has far lower concentrations of na and cl
what is the chemical difference between seawater and river water?
Seawater NaCl rich, rover water depleted but high in Ca2+ and HCO-3 due to weathering of carbonate r0cks
what is the length of the hydrological cycle?
~4000 years
what does seawater chemistry reflect?
inputs: evaporated river water, oceanic volcanism, aeolian dust, atmospheric gases
outputs: inorganic precipitation, biological processes, export to sediments
what are biological processes?
production of organic matter
biogenic precipitation
how can we classify the elements/ions in seawater?
based on their vertical profiles
conservative, nutrient behaviour, scavenged elements
what are the characteristics of conservative elements?
constant normalised salinity with depth
includes most major seawater ions (Na, Cl), and non-major elements with weak biological reactions that are relatively soluble and not particle reactive
long residence time x10^6 -10^8 years
What elements are minor non-conservative and why?
calcium - small variations due to formation and dissolution of CaCO3 and hydrothermal vent input
magnesium - removed at high temp hydrothermal vents
dissolved inorganic carbon - formation and dissolution of CaCO£ and organic matter
SO2-4 - conservative in oxic basins, not in anoxic basins or sediments
what are the characteristics of nutrient behaviour elements?
concentrations controlled by biological cycling
depleted in surface waters and increase with depth
biologically essential elements macronutrients (P, N, Si) or micronutrients (Fe, Zn)
elements with no or limited biological function (Ba, Cd)
residence time = x10^3 - 10^5 years
what are the characteristics of scavenged elements?
concentration decrease with depth
adsorption of ions onto particles (dust, minerals, biological, bacteria)
overcomes friction and drops through water column as accumulations
very particle reactive
high conc in Atlantic deep waters compared to pacific
ie aluminium supplied by aelion dust
residence time = x10^2 - 10^3 years
what is residence time?
the average time an element spends in the ocean before being removed
=total mass dissolved in ocean/rate of supply or removal
why do scavenged elements have a low residence time?
rapidly exported to sediments
What is a possible application of understanding ocean chemistry?
palaeoceanography
how does palaeoceanography use seawater chemistry?
chemistry of biogenic CaCO3 records seawater chemistry
chemistry of CaCO3 is affected by environment i.e. strontium substitutes for calcium in aragonite and low temperatures
Cadmium substitutes for calcium in aragonite cooler waters in areas of upwelling
what is the ITCZ?
intertropical convergence zone
area where northern and southern trade winds meet
moves seasonally and is divergent causing upwellings
what is the oxygen profile with depth?
high concentration in surface waters
reaches a minima at 300-800m
then increases with depth due to input of cold, oxygen rich deep waters formed at poles
where is dissolved oxygen at its highest?
in cold waters (more soluble) at the poles
what does the oxygen minima reflect?
High respiration associated with the decay of sinking biological matter
what is the photosynthesis and respiration equation?
CO2 + H2O -> (CH2O)n +O2
forward=photosynthesis
reverse=respiration
how does climate change impact ocean dissolved oxygen?
expansion of minima zones
warmer waters
decreased or increased turbulence
increased stratification
what are the classifications of ocean dissolved oxygen?
hypoxic : 60-120 micro mol/kg
suboxic : <10micro mol/kg
anoxic : no dissolved oxygen
what is causes oxygen minima zones (OMZ)
Reduction in convective mixing from surface
reduced in deep water subduction
reduction of O2 solubility in warmer waters
increased O2 consumption from organic decay at depth
why is mauna loa an ideal place to study carbon dioxide?
far away from industrial activities
what is the dissolution of CO2?
CO2 + H2O <-> H2CO3 <-> H+ + HCO-3 <-> 2H+ + CO2-3
carbon dioxide dissolves in water to become carbonic acid which then loses a proton and becomes a bicarbonate which can then lose another proton and become carbonate
what does an equilibrium constant do?
defines the ration of the concentrations of products and reactants in the reaction at equilibrium
what is the equilibrium constant equal to and what is it dependant on?
k = [product1] [product2] / [reactant1] [reactant2]
dependant on temp, salinity and pressure
why do we remove H2O from the equilibrium constant equation?
because it is infinite on both sides of the reaction
what is the concentration of a proton at pH8.1?
[H+] = 7.9 nmol/kg
what is pH?
pH = -log[H+]
ie pH8 = 10^-8 mol/kg
what is ocean pH controlled by?
the total dissolved inorganic carbon (DIC)
total alkalinity
what is total alkalinity?
the abundance of bases which can accept protons
what is the Bronsted-Lowry acid-base theory?
HA <-> H+ + A-
Acid <-> proton + conjugate base
what is an acid described as?
any substance that can donate a proton
what is a conjugate base?
a substance that accepts protons
is HCO-3 a base or an acid?
both
H2CO3 <-> H+ + HCO-3 <-> 2H+ + CO2-3
What is a Bjerrum plot?
the concentrations of the different species of a polyprotic acid in a solution, as a function of the solution pH
what is the average pH of seawater?
8
what percentage of primary production is carried out by coccolithophores?
9%
what is marine snow?
the downward drift of particles i.e. organic matter
what adds to DIC?
CO2: Remineralisation of organic matter
CO2-3 : dissolution of CaCO3
what adds to total alkalinity?
CO2-3: dissolution of CaCO3
What is the relationship between DIC, total alkalinity and dissolution of CaCO3?
increases DIC and total alkalinity in a 1:2 ratio
each CO2-3 can neutralise 2 protons
does DIC increase or decrease in deep waters with age?
increase
is atlantic or pacific water more acidic?
pacific
what are the four sources of particles?
terrigenous/lithogenous
biogenous
hydrogenous
cosmogenous
what is meant by terrigenous/lithogenous?
particles from weathering/erosion of land and volcanic activity
transported by rivers, wind, or glaciers
what is meant by biogenous?
particles from organic materials, biogenic CaCO3, SiO2.nH2O
what is meant by hydrogenous?
particles from reactions of dissolved chemicals in seawater (evaporates, hydrothermal vent deposits)
what is meant by cosmogenous?
particles orginiating in outer space
give an example of a terrigenous and biogenous sediment
terrigenous: red clay
biogenous: carbonate ooze
give an example of a calcareous organism
foraminifera - calcitic
pteropods - aragonitic
coccolithophores - calcitic, abundant in central ocean gyres
give an example of a siliceous organism
diatoms - hydrated silica or opal, abundant in high productivity areas (upwelling)
radiolarians - hydrated silica or opal, abundant at low latitudes
what is particle settling velocity dependant on?
size, shape and density
what allows small particles to settle for efficient?
if they are packed into larger faecal pellets
what is the 30% rule?
if deep ocean sediment contains >30% biogenic material it is either a calcareous or siliceous ooze
if <30% biogenic material, it is a red clay
where do we find the highest abundance of carbonates in the oceans?
atlantic and south pacific
what is the relationship between water depth and particulates?
within a basin carbonates dominate at shallow depths, red clays dominate at deeper depths
what is solute concentration (mi)?
the amount of solute per unit volume
what is solute activity (ai)?
effective concentration of a solute in terms of its observed chemical behaviour
what happens in a low ionic strength solution?
easy for an ion to find other species of interest and react (empty shop)
what happens in a high ionic strength solution?
ions get in the way of finding species of interest, preventing reaction
what is the solubility product for calcite and aragonite?
at surface pressure, Temp 25, Salinity 35
calcite: 4.27x10-7
aragonite: 6.48x10-7
which is more soluble in surface seawater: calcite or aragonite?
aragonite
what is Ksp?
the product of the concentrations (activities) of the dissolution solutes at equilibrium
what is ion activity product (IAP)?
the product of the concentrations (activities) of the dissolution solutes in a sample
what determines saturation state?
IAP/Ksp <1 : solution is undersaturated, dissolution is predicted
IAP/Ksp =1 solution is at equilibrium
IAP/Ksp >1 : solution is supersaturated, precipitation predicted
what is the saturation state of sewater?
saturation state=[Ca2+][CO2-3] / Ksp
what is a lysocline?
the depth at which CaCO3 dissolution begins
what happens at the carbonate compensation depth (CCD)?
no CaCO3 is preserved
what are the factors that determine how much CaCO3 is preserved?
saturation state of seawater (dependant on CO2-3)
temperature (more dissolution at low temp)
salinity and pressure
sedimentation rate (inc sedimentation = inc preservation)
what are CaCO3 sediments derived from?
planktonic and benthic organisms that produce calcite or aragonite structures
What is inferred to regulate biomineral growth, morphology and properties?
Biomineral that contain bio molecules ie proteins, lipids, polysaccharides
what is the saturation state of seawater?
=[Ca2+] [CO2-3] / Ksp
what is the change in carbon uptake by the oceans between 1990s and 2000s?
increased carbon uptake, especially in the north atlantic at 3000m
pacific has less penetration of carbon
where do we see the greatest change in pH and why?
South pacific - linked to the formation of AABW
What do we know about pCO2?
Lot of noise suggesting other factors - El Nino
surface seawater pCO2 not in equilibrium with the atmosphere
how does ocean acidification affect marine biogeochemical processes?
increases atmospheric CO2, seawater CO2, H+ conc, HCO-3, total dissolved inorganic carbon
decreases CO2-3
What is the relationship between seawater CO2 and algal production?
increasing seawater CO2 increases the inorganic carbon taken up per cell therefore primary production is slighty increased under ocean acidification
what is the reaction of calcification?
Ca2+ + CO2-3 –> CaCO3
Where are aragonite concentrations the highest?
highest at equator, lowest at poles
temperature impact
what increases CO2 solubility?
decreasing temperature and increasing pressure
at what saturation state does aragonite shells being to dissolve?
<1
pteropods and coral reefs most at risk
how are corals still able to calcify at an aragonite saturation state <1?
corals increase the pH of the calcification fluid above that of seawater
how can we offset ocean acidification?
use primary production to decrease seawater CO2
What are biomolecules?
proteins, lipids, polysaccharides
regulate biomineral growth, morphology and properties
what is the effect of increased pCO2 on amino acids in corals?
amino acids are significantly higher in coral skeletons under high seawater pCO2
results in different morphology and slower growth
what is aspartic acid?
most common amino acid in corals
reduced aragonite precipitation rate
how do amino acids influence aragonite precipitation rate?
may complex Ca2+ in solution, reducing its activity
may adsorb to the mineral lattice inhibiting attachment of the mineral ions
what is the effect of increased CO2 on aragonite precipitation rates?
increased CO2 = increased effectiveness of amino acids = decreased aragonite precipitation rates
