Biogeochemical cycles
Biogeochemical systems/cycles
The hydrologic cycle
The carbon cycle
Cycles
• Budgets
• Feedbacks
• Rates of change
The hydrologic cycle
Cycle concepts 1
• We use the concept of cycles to calculate
budgets
• Substances in a cycle reside in reservoirs
• These substances move between reservoirs
by processes or mechanisms: the rate at
which they move is called a flux
Some volumes for the
hydrological cycle
Quantifying fluxes….
…is a step towards understanding controls on the system
Cycle concepts 2
• We use the concept of cycles to examine the
dynamics
• Reservoirs can be sources or sinks.
• Flux is the rate at which a substance moves
between reservoirs
• Knowing rates and mass allows us to
calculate residence times…
– At Steady State
What is the residence time of
water in the ocean today?
•  =total amount of substance in a reservoir
rate of supply or removal
Some numbers for the
hydrological cycle
The hydrologic cycle
Residence time of water in the
ocean
• If the volume is: 1370x106 km3
• And the rate of removal is : ~425 x 103 km3 yr-1
• Then what is the residence time of water in the
ocean?
• Any residence time calculation assumes steady
state.
Residence time of water in the
ocean
•  =1370x106 km3 = 3224 yrs
•
425x 103 km3 yr-1
• This calculation assumes steady state.
• If the flux changes – then the size of the
reservoir changes…..
…OR the residence time changes
Timescales
Cycling in the ocean
The residence time
defines the time
scale
The flux controls the
rate of exchange in the
reservoir
If they are not in
balance the system
isn’t in equilibrium
The hydrologic cycle
Changes in
fluxes change
reservoir size:
this can result in:
Glacial build up
and decay
Fluxes control
the rates of
change.
The carbon cycle
Unlike the
hydrological
cycle, the
carbon cycle
involves a lot
of chemistry….
And a lot of
that involves
CO2 and
biological
transformations
The carbon
cycle
Has much
longer
timescales
than the
hydrocycle
a.k.a. the
Wilson Cycle
The carbon cycle: quantified
Residence time of carbon in the
deep ocean
•  =amount in reservoir Gt = ? yrs
flux Gt yr-1
What this tells us is the timescale of the
reservoir
The carbon cycle:
The deep ocean
reservoir is large.
It’s largely
carbonate (DIC)
Residence time of carbon in the
deep ocean
•  =38100 Gtons = 380 yrs
•
100.2 Gt yr-1
• This is a short to midterm timescale
reservoir…..
The carbon cycle:
The sediment
reservoir is huge.
It’s both organic
and carbonate
carbon.
Residence time of carbon in the
sediment reservoir
• How long does carbon stay in the
sediments?
•  =5x107 Gt = 996,016 yrs ~1million yrs
50.2 Gt yr-1
This is a long timescale reservoir…..
Fluxes tend to reflect the reservoir times
How do we apply this concept?
• And how can we use these numbers to
understand the dynamics of the earth as a
system?
• Let’s look at short term system…
The carbon cycle:
The biota reservoir
is tiny. (note the
size)
The flux is huge
(note the values)
Residence time of carbon in the
ocean biota reservoir
•  =3G tons = 0.06 yr = ~22 days
50 Gt yr-1
This is a very short timescale reservoir…..
Carbon cycles very quickly through this
reservoir
Biotic reservoir has short
residence times
Seasonal
fluctuations in
the concentration
of atmospheric
carbon dioxide
are controlled by
the fluctuations
in photosynthesis
how does this cycling relate to vertical
profiles?
Libes
Fig 9.1
How do we apply this concept?
• And how can we use these numbers to
understand the dynamics of longer terms
changes in the earth system?
• Let’s look at a long term change…
• The concept of reservoirs and residence
time is very powerful.
The ocean is both a source and
sink for CO2….
In different places and at different times…
….Thermohaline circulation
Controls on the distribution of CO2
CO2
CO2
CO2
CO2
Corg DIC Corg DIC
Corg
CO2
CO2
DIC
Water downwells:
Low nutrients
High O2
low CO2 content
DIC
Corg DIC
DIC
DIC
content of
Water
increases
with
increasing
age
DIC
DIC
Corg
Corg
DIC
Corg
DIC
Corg
Corg
DIC
Corg
DIC
Carbonate Sediments are a
source and sink of carbon,
both Corg and CO2
The CO2 Climate connection
CO2 is held in the deep ocean.
how much is held is a function of circulation
How long it is held is a function of circulation.
(the overturning rate of the ocean)
What happens if circulation changes?
what if we slow down thermohaline circulation?
160,000 yr record of atmospheric
CO2
The rapidity of this
change meant that
the CO2 must be
exchanging with the
ocean reservoir
-not the sediments
(too fast).
-The bio-pump and
carbonate
dissolution are of
the right magnitude
and timescale
The record of CO2 levels in the atmosphere measured in bubbles trapped in ice cores
The CO2 Climate connection
How much CO2 in the ocean controls the levels
in the atmosphere
How fast the ocean circulates controls can
change the amount of CO2 in the atmosphere to
change climate
The carbon cycle IS the climate cycle.
The hydro and carbon cycle
effect on climate
• The combination of the changes in:
– The hydrologic cycle: how fast the ocean can
cycle (i.e. residence time in the ocean)
– Changes in the carbon cycle (the bio-pump
CaCO3 sediment dissolution)
• Combine to control the rate at which the
CO2 in the atmosphere is changed.
Quantifying fluxes….
…is a step towards understanding controls on the system
The 13C reflects traces the concentration of
CO2 in the deep ocean
Curry and Oppo, 2005
Our tracer is 13C
The 13C reflects the global
thermohaline circulation
NADW became NAIW in the LGM
Curry and Oppo, 2005
Our tracer is 13C