Goal of this course: What determines the abundance of different elements in the ocean? How does...
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Goal of this course:
• What determines the abundance of different elements in the ocean?
• How does their distribution depend on physical circulation and biological activity?
• How does the chemical environment of the ocean influence marine life?
• How do these factors combine to create the global ocean carbon cycle?
Use chemical distributions to understand how the ocean works
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Why Chemical Oceanography?1) Integrating diverse processes
Chemical composition
of ocean
Fundamental processes in Earth Science are recorded in the chemical composition of the ocean. (We’ll study only a subset!)
Physical
Geological Chemical
Biological
River discharge
Photosynthesis+ respiration
Circulation+ Mixing
Acid-base reactions
Sediment formation
Hydrothermal vents
Scavenging
Gas exchange
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Tools of Chemical Oceanography1) Observations
ourfocus
Enormous information potential, BUT…
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The “biological pump”
For our favorite elements, we had to learn about:
PhotosynthesisSinking particles (“export”)Respiration (“remineralization”)CirculationGas exchange
We’re confronted with a bewildering variety of processes…
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Tools of Chemical Oceanography2) Models
But simple models can yield important insights!
Radioactive Carbon
Biological export = Physical transport
~1000 years!~5 GtC/year!
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Wind-driven circulation
Surface winds set surface currents into motion (in surprising ways!)…
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Upwelling/downwelling
… that induce upward and downward motion…
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Sea Surface Height
H
L
H
…and influence the pressure in deeper layers, causing “gyres”
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The Ocean Conveyor
Areas of deep winter mixing initiate global deep currents.
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Introducing Horizontal Variations
Ocean circulation explains areas of high surface nutrients.But it can’t be the whole story, we need biology too!
[mmol/m3]
0-50 m
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CO2Corg
Phytopl.
Zoopl.Bacteria
Respiration (Heterotrophic)
Photosynthesis “Net Primary Production (NPP)”
Rh
NPP
Export Production
EP
Measures of Productivity
Photosynthesis converts simple molecules to complex organic matter - some of it sinks, some gets recycled by complex food webs.
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Autotrophs: Phytoplankton
Innumerable species contribute to the total photosynthesis of the ocean. They’re all similar in many ways (Redfield’s proportions) but differ in many ways too (chemical function, size).
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Net Primary Production
We can detect the presence of phytoplankton chlorophyll from space and use it to map biological productivity. But what determines this pattern?
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To what degree does Temperature appear to limit productivity?Are there warm (cold) waters with low (high) productivity?
NPP vs SST
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Seasonal light limitation
Where should light never be limiting? Where is it limiting in winter? And why (what combination of two factors?). What about summer?
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Observed surface nitrate
Nitrogen limits photosynthesis throughout most of the surface ocean…
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Iron supply by dust
Mahowald et al. [2006]
Atmospheric Dust Flux(simulated)
But iron also plays a role in limiting plankton growth.
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Export Production
Low exportSmall+warm
High exportBig + cold
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More soluble gasor colder water.
Less soluble gasor warmer water.
pA pB
CBCA
Solubility and Equilibrium
air
water
Gases like O2 and CO2 can be held in solution in seawater, but their solubility depends on strongly on temperature.
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or Warming
SupersaturationCooling or
Undersaturation
Temperature
Gas
Co
nce
ntr
atio
n
SA(T)*pA
Sources of Disequilibrium
Production
Consumption
The ability of gases to go between ocean/atmosphere always pushes them toward exquilibrium. But it is never perfectly reached because its constantly disrupted by heating/cooling or production/consumption of a gas.
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O2 Disequilibrium (∆O2)
For O2, gas exchange is very effective, but not perfect.Supersaturation is a signature of productivity (export), Undersaturation of low-O2 water upwelling -> respiration!!
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Respiration can be shallow, keeping nutrients close to the photic zone, and CO2 close to the atmosphere.
Shallow Respiration
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Deep Remineralization
…or deep, storing nutrients and CO2 in the abyss, far from plankton and the atmosphere.
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Apparent Oxygen Utilization
AOU proves that most of the respiration occurs shallow.But this leads to anoxic conditions in some places!
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Oceanic Oxygen
[mmol/m3]
400 m
How is respiration carried out when oxygen is all gone?
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Nutrient Limitation: NO3 or PO4?
Release
Uptake Nutrient Uptake and ReleaseN:P = 16:1
By removing nitrate from the ocean, which is why nitrogen is limiting!
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Why Chemical Oceanography?3) Impact of human activities
Carbon dioxide is increasing in the atmosphere and oceans. What are the consequences of this?
Anthropogenic CO2 in the ocean
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The Global Carbon Cycle
What if the ocean stopped absorbing so much CO2?
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Influences on pCO2
€
pCO2 =[H 2CO3
* ]
K 0
From Henry’s Law:
Temperature Salinity
Total DICAlkalinity (or pH)
Biology andGas exchangeBiology only
Heating/coolingPrecip/Evap
Depends onDepends on
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pCO2 vs Temperature, Salinity
Temperature
Sal
init
y
Low pCO2
High pCO2Pre-industrial(280 ppm)
The pCO2 of seawater increases by ~4% for each degree of warming, and by ~1% for each 1% increase in salinity.
HeatingCooling
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Carbon pumps and pCO2
Effect of Organic Carbon Pump is to reduce pCO2 of surface ocean, and thus the atmosphere.
Influence of Carbonate Pump counteracts some of that lower pCO2
by removing CO32-.
Net effect of biological pumps is to lower atmospheric CO2 – by a lot!
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Anthropogenic CO2 in the Ocean
ATL
PAC
IND
The “preindustrial” background of DIC can be estimated from a suite of simultaneous physical and chemical measurements, and the anthropogenic CO2 increase can be inferred by difference from observed DIC.
€
C obs =C sfc280 +C bio +C anthro
The total inventory of anthropogenic CO2 (~120 Gt) is nearly half of the historical fossil fuel burning source.
Sabine et al. [2004]
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Ocean acidification
Source: IPCC AR4
The most obvious direct consequence of increasing CO2 is the reduction of pH.The biological consequences are largely unknown, but could be profound.
Eastern North Atlantic
Western North Atlantic
Central North Pacific
pCO2 pH
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Final exam:
Just like the midterm:
Testing Concepts, not calculationsCumulativeShort answer questions
Except:
You have a lot more time (3 hours)Exam is only a little longer (1.5 hours)