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

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

Tools of Chemical Oceanography1) Observations

ourfocus

Enormous information potential, BUT…

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…

Tools of Chemical Oceanography2) Models

But simple models can yield important insights!

Radioactive Carbon

Biological export = Physical transport

~1000 years!~5 GtC/year!

Wind-driven circulation

Surface winds set surface currents into motion (in surprising ways!)…

Upwelling/downwelling

… that induce upward and downward motion…

Sea Surface Height

H

L

H

…and influence the pressure in deeper layers, causing “gyres”

The Ocean Conveyor

Areas of deep winter mixing initiate global deep currents.

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

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.

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).

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?

To what degree does Temperature appear to limit productivity?Are there warm (cold) waters with low (high) productivity?

NPP vs SST

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?

Observed surface nitrate

Nitrogen limits photosynthesis throughout most of the surface ocean…

Iron supply by dust

Mahowald et al. [2006]

Atmospheric Dust Flux(simulated)

But iron also plays a role in limiting plankton growth.

Export Production

Low exportSmall+warm

High exportBig + cold

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.

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.

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!!

Respiration can be shallow, keeping nutrients close to the photic zone, and CO2 close to the atmosphere.

Shallow Respiration

Deep Remineralization

…or deep, storing nutrients and CO2 in the abyss, far from plankton and the atmosphere.

Apparent Oxygen Utilization

AOU proves that most of the respiration occurs shallow.But this leads to anoxic conditions in some places!

Oceanic Oxygen

[mmol/m3]

400 m

How is respiration carried out when oxygen is all gone?

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!

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

The Global Carbon Cycle

What if the ocean stopped absorbing so much CO2?

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

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

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!

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]

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

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)