ADINA_Global biogeochemical cycles

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Traces Elements and Isotope Geochemistry as 
Environmental Tracers
Adina Paytan - UCSC
We use both chemical and isotopic tracers 
in water, sediments and other archives to 
study present day and past biogeochemical 
processes on Earth over time scales of 
days to millions of years.
The interactions between the various spheres of the earth system
Interactions involving 
biological, geological, and 
chemical processes 
operating on a broad 
range of time scales.In paleoceanography 
we try to study how 
these processes 
operated in the past.
Movement of 
elements between 
these spheres 
• Biogeochemical cycles provide the basic 
framework for investigating the evolution 
of life on earth and earth’s climate. 
• An understanding of biogeochemical 
cycles and anthropogenic impacts on them 
is fundamental for predicting impacts of 
global climate and environmental change. 
Earth is a dynamic system. The study of global 
biogeochemical cycles enables us to understand how 
the world works in the present and under different 
boundary conditions. Helps predicting future change.
Atmospheric CO2 concentrations at 
Mauna Loa, Hawaii
Pagani et al., 2005
Dieter Lüthi, et al, 2008
What controls it.
How it got to that state.
The state of the Earth’s surface 
environment at a given time.
How did it change over Earth’s history.
What are the processes/feedbacks that 
sustain a habitable planet.
Biosphere
HydrosphereGeosphere
Atmosphere
Biogeochemical cycles operate on many 
different spatial and temporal scales
What fuels biogeochemical cycles?
Three basic types of system
A closed system
The ocean an open system
A reservoir is a physically well-defined 
system. A given setting with defined physical 
and/or biological boundaries. In each 
reservoir, the relevant chemical, physical 
and biological properties are assumed to be 
(reasonably) uniform or their distribution 
could be well described.
A reservoir will contain a collection of matter.
EXAMPLES?
Atmosphere
Land/Lithosphere
Ocean/Hydrosphere
Sediments
Deep Earth
Atmosphere
Land/Lithosphere
Ocean
Sediments
Deep Earth
stratosphere
troposphere
clouds
aerosols
surface/deep
Pacific/Atlantic dissolved/particulate
organic/inorganic
ecosystem soil
anthropogenic
lower crust
volcanoes
mantel
quartz carbonateleaf, root, trunk
ice
wetland
lake
coral reefs
hot spot
Fluxes transfer of matter from one reservoir to another. 
A flux into a reservoir is sometimes referred to as a 
source, a flux out of the reservoir as a sink.
For a perfectly mixed reservoir, the concentration of a 
component in the outflow is equal to the uniform 
concentration inside the reservoir.
Biogeochemical Cycles – Matter 
and energy move all the time at 
different rates from one earth 
reservoir to another.
Atmosphere
Land/Lithosphere
Ocean/Hydrosphere
Sediments
Deep Earth
solids, liquid, gases
Matter and energy flow between and within these reservoirs
EXAMPLES?
The two main tasks in depicting a biogeochemical cycle is the 
definition of the reservoirs and the parameterization of the 
fluxes. There are no magic guidelines, other than to clearly 
define the goals of the work and system of investigation and 
to start as simple as possible. Obviously, it is only possible to 
provide direct information on reservoirs, fluxes and 
parameters that are explicitly represented. 
Steady-state is a common assumption made about
the changes in the components of a system (lack of) 
For steady-state:
Fluxin = Fluxout
An
equilibrium
state
– Content – the total amount of any constituent in a 
reservoir (standing stock)
– Capacity – maximum concentration of a substance a 
reservoir can reach/hold before saturation occurs
– Rate of In/Out flow – how much of a substance get 
into/out a reservoir at a given time.
– Residence Time – how quickly a substance cycles 
through a reservoir (exchange rate)
• Residence times for different elements vary widely
• Humans can alter the rate of influx/outflux by our activity 
(pumping, diversion, adding pollution)
The turnover time is defined as
where M is the mass of the reservoir (say, the total number of moles 
of organic carbon in marine biota) and Fout is the total flux out of the 
reservoir (i.e., the sinks) or that in (the sources).
Content = 1,000,000 m3
Input /output = 163 m3/s
Residence Time
1,000,000/163 = 
6000 seconds = 1.66 hours
• In many instances the source (Q) and sink (S) rates are 
not constant with time or they may have been constant 
and suddenly change (transient events or perturbation).
• To describe how the mass in a reservoir changes with 
time after an increase in source (or sink) for a reservoir
Starting with: dM/dt = Q0 – S = Q1 – kM
We let the input change to a new value Q1 and we assume that the 
initial amount at t = 0 is M0 . 
The new equation is: dM/dt = Q1 – kM
and the solution is: M(t) = M1 – (M1 – M0 ) exp (-kt )
This describes how M changes from M0 to the new equilibrium 
value M1 (= Q1 / k) with a response time equal to k-1.
For constant exponential change Q = Q0 exp [m (t – t0 )]
The solution for dM/dt for these conditions is:
M = M0 {(m/m+k) exp [-k (t-t0)] + k / m+k exp [m (t-t0 )]} for t0 < t < t1
Reactive transport models
For a system with a single inlet and a single outlet
where Cin is the concentration of the species in the inflow, 
R is the rate, per unit volume, at which the species is 
produced in the system (note: when the species is being 
consumed R is negative), and tf is the mean residence 
time (or transit time) of the carrier fluid in the system.
Box models - Reservoirs and Fluxes
Reservoir total inventories (in brackets) in units of 106 km3
Fluxes in units of 106 km3yr-1 .
Vapour transport 
0.036 Atmosphere (0.0155)
Precipitation 
0.107
Runoff 0.036
Precipitation 
0.398 Evaporation 
0.434
Evaporation 
0.071
Ice (43.4)
Ocean 
(1400)
Lakes and rivers 
(0.13) 
Groundwater 
(15.3)
Box models - Reservoirs and Fluxes
Vapour transport 
0.036 Atmosphere (0.0155)
Precipitation 
0.107
Runoff 0.036
Precipitation 
0.398 Evaporation 
0.434
Evaporation 
0.071
Ice (43.4)
Ocean 
(1400)
Lakes and rivers 
(0.13) 
Groundwater 
(15.3)
Ocean Flux in = 0.398 (precipitation) + 0.036 (runoff) = 0.434
Flux out = 0.434 (evaporation)
Residence Time 1400/0.434 = 3200 years
Box models - Reservoirs and Fluxes
Oxygen isotope fluctuations in seawater as recorded in marine sediments can 
be indicative of ice volume and thus sea level changes. Temperature effects 
will be included as well. Indicative of exchange among water reservoirs. 
The black numbers indicate how much carbon is stored in various reservoirs, in 
billions of tons (GigaTons, circa 2004). The purple numbers indicate how much 
carbon moves between reservoirs each year. The sediments, as defined in this 
diagram, do not include the ~70 million GtC of carbonate rock and kerogen.
On geologic times 
scales, the carbon cycle 
model can be 
represented by 3 
reservoirs, sedimentary
carbonate, sedimentary 
organic carbon, and the 
mantle, as well as fluxes 
between these 
reservoirs and the 
oceans and atmosphere.
When more C is removed 
into the reduced organic C 
reservoir residual C in the 
ocean and carbonates 
becomes enriched in 13C 
(heavy), atmospheric CO2 
is lower and O2 is higher 
(less consumed for 
oxidation). 
The long-term increases in 13Ccarb (e.g. 
more organic C burial) that began in the 
Mesozoic were accompanied by major 
evolutionary changes among theprimary 
producers in the marine biosphere. Three
groups of eucaryotic marine phytoplankton 
(calcareous nannoplankton, dinoflagellates, 
and diatoms) began their evolutionary 
trajectories to ecological prominence at ~ 200 
Ma. As Pangea fragmented and the Atlantic 
Ocean basin widened, the total length of 
coastline increased and sea level rose, 
flooding continental shelves and low-lying
continental interiors. Nutrients that were 
previously locked up in the large continental 
interior were transported to newly formed 
shallow seas and distributed over wider shelf 
areas and longer continental margins.
Katz et al., 2005
The decline since the late 
Miocene in addition to changes in 
ocean circulation could be related 
to the evolution of C4 plants (less 
fractionation in organic C burial).
Perturbations in the 
global C cycle. 
Strontium 87Sr/86Sr
84Sr = 0.56%
86Sr = 9.86%
87Sr = 7.00%
88Sr = 82.58%
Stable Isotopes of Sr
These abundances are somewhat 
variable because 87Sr is a radiogenic 
isotope generated by emission of a 
negative -particle from 87Rb. 
Sr isotope ratios are not to be confused with other stable isotopes. Differences in Sr 
isotope composition reflect differences in the radiogenic 87Sr component that has grown 
into the various Sr source pools. In order to determine the radiogenic Sr signal, all mass- 
dependent stable isotope variation resulting from either natural or analytical processes 
must be removed from the analyzed isotopic composition using an internal normalization 
routine. Thus any mass-dependent stable isotope fractionation that might occur between 
Sr in sediment, soil, water, organisms etc. is zeroed out by the analytical routine.
Ratio 87Sr/86Sr
Decay 87Rb87Sr
Half Life 4.99 x 1010 yr
Oceanic Residence Time ~2 x 106 yr
Seawater Standard
NBS 987, SrCO3
87Sr/86Sr = .709165
87Sr/86Sr = .710220
Analysis by TIMS
86Sr, 87Sr ,88Sr
normalized to
86Sr/88Sr =0.1194
Evolution of Sr Isotope Ratios
• During early separation of crust and mantle there was significant 
elemental fractionation (fractional crystallization). 
• Incompatible elements like Rb were excluded from basaltic melts and 
incorporated into silicic residues (continental crust).
• Continental crust developed with a larger Rb/Sr ratio than the upper 
mantle
• The present-day quantity of 87Sr depends on its initial 
stock (87Sr at time zero) and the amount of radiogenic Sr 
generated from 87Rb over time.
• The 87Sr enrichment is high in old continental rocks 
(granites) and low 87/86 ratios are found in the mantle 
and rocks derived from it (basalts). 
• Therefore the Sr isotope ratio is characteristic of different 
rock types.
• Marine sedimentary rocks will reflect the seawater 
composition which in turn depends on the relative inputs 
from different weathering and hydrothermal sources (e.g. 
between continental rocks and basalts).
•The 87Sr/86Sr ratio of 
seawater has changed 
significantly with time in 
response to the input of 
varying proportions of Sr 
derived from two main 
sources: 
Continental Crust 
weathering 
Upper mantle 
hydrothermal activity
Oceanic Crust
.703
Continental Crust 
.716
Limestone
.707-.709
Seawater
.7092
R i v
e r s
. 7 1
1
Weathering Hydrothermal 
Circulation
VARIATION IN Sr ISOTOPIC 
COMPOSITION OF SEAWATER 
DURING THE PHANEROZOIC
From high precision measurements of marine carbonates. 
Elderfield et al.,1982
Increase in the 87/86 in the 
past 40Ma related to the uplift 
of the Himalayas.
Increased weathering (and 
continental Sr input) 
consumes CO2 thus cooling 
the Earth and glaciations (e.g 
higher 18O ratios).
Raymo et al., 1988
Sr isotopes used for deciphering change in the global biogeochemical record
N is the total number of moles 
of Sr in the ocean,
Rsw is the 87Sr/86Sr of 
seawater, Jn is the flux of
Sr into the oceans from 
source 'n', which has an
average 87Sr/86Sr of R n, and 
Jout is the total flux
of Sr out of the oceans.
There are three major fluxes of Sr into the oceans, that carried by rivers (Jr), 
that due to the hydrothermal alteration of seafloor basalt (Jh), and that due to 
diagenesis or dissolution of carbonates already on the ocean floor.
Richter et al., 1992
• New application – May help refine our 
understanding of the Sr cycle 
• 86/88 Sr (using double spike).
• Species effect, temperature, 
weathering, seawater changes, 
carbonate deposition…….
Feedbacks in the Climate System
Forcing/
perturbation
Forcing/
perturbation
Water Ice
CO2 Temperature
Precipitation
Weathering
River solute load
Biological Productivity
CO2
Mountain Uplift
Weathering
CO2
Temperature
Weathering
http://www.ess.uci.edu/~reeburgh/figures.html
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	What fuels biogeochemical cycles?
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