Biogeochemical cycles

Biogeochemical systems/cycles

The hydrologic cycle

The carbon cycle

Cycles

• A cycle is a sequence of events that continuously reoccurs such that matter is transported from one portion of the cycle to another and returned.

• There are physical and chemical cycles, biological cycles and biogeochemical cycles.

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?

• τ =total amount of substance in a reservoirrate 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

• τ =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 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

Cycling in the ocean

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 ΣCO2 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

• τ =amount in reservoir Gt = ? yrsflux 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

• τ =38100 Gtons =• 100.2 Gt yr-1

• This is a short to midterm timescale reservoir…..

380 yrs

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?

• τ =5x107 Gt50.2 Gt yr-1

= 996,016 yrs ~1million yrs

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

• τ =3G tons50 Gt yr-1

= 0.06 yr = ~22 days

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?

LibesFig 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

The biological pump is about CO2

The biological pump moves carbondioxide

from the surface ocean to the deep

water masses

32

Controls on the distribution of ΣCO2

DICDIC

DIC DIC DIC

DIC CorgCorgCorg

Corg

CorgCorgCorgCorg

CorgCorgDICDIC

DIC DIC

DIC

DIC

Water downwells:Low nutrientsHigh O2low CO2 content

DIC content of Water increaseswith increasing age

Carbonate Sediments are a source and sink of carbon, both Corg and ΣCO2

CO2

CO2CO2

CO2CO2

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