Lab: Carbon Cycle, geology homework help

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Lab 10: Sediments and the Global Carbon Cycle
(This exercise was conceived by Dr. Elana Leithold, North Carolina State
University)
Background:
When we talk about a cycle, we are referring to something that starts somewhere and
comes back to the same place. The global carbon cycle refers to the movement of the
element carbon through different storage places, or reservoirs, on Earth. These
reservoirs include the biosphere (living things), the atmosphere, soils, the oceans,
sediments, and sedimentary rocks. Carbon moves (cycles) through these various
reservoirs at different rates. Most carbon near the Earth’s surface, cycles fairly quickly.
The turnover time of carbon in the atmosphere, for example, is about 3 – 5 years while
the average turnover time of carbon in plants is about 50 years. Soil organic matter has
a range of turnover times, but averages about 3000 – 5000 years.
We can think to the “short-term” carbon cycle as having a slow leak. A small amount of
carbon escapes to be stored in marine sediment and sedimentary rocks in either an
inorganic form as carbonates (e.g. limestones) and as disseminated organic matter
(kerogen), primarily in mudrocks. This carbon has a turnover time of millions of years.
Over a geologic time the “slow leak” has had enormous consequences for our humans
and our planet and. It is responsible for the existence of fossil fuels on which our
society depends and for regulating atmospheric chemistry (i.e., CO2 and O2
concentrations) and climate.
This set of exercises focuses on exploring the role of marine sediments in organic
carbon burial and on using the composition of organic carbon preserved in sediments
and sedimentary rocks to reconstruct ancient environments.
The objectives of these exercises are to:
Part I: Compare the amount of organic carbon delivered to the seabed and the
amount that is buried in sediments.
Part II: Explore the importance of the relationship between organic carbon and
mineral surfaces.
Part III: Discuss how stable carbon isotopes can indicate the sources of organic
matter, and be used as a tool to interpret paleoenvironments.
Make sure you read Chapter 10 before you do this lab!
Part I. Burial of organic carbon in marine sediments
The organic carbon buried in marine sediment is produced by photosynthetic organisms
both on land and in the lighted, surface layer of the ocean. Much of the material
produced, however, does not make it to the seabed.
1) Based on the information in Table 1 below, what percentage of the organic
carbon produced on land makes its way to the oceans (primarily via rivers)?
2) Based on the information in Tables 1 and 2, what percentage of global
productivity (both terrestrial and marine) is preserved in marine sediments?
3) What do you think happens to the terrestrial (land) and marine (ocean) carbon
that does not get buried in marine sediments? (Hint: Think about what happens
to leaves that accumulate on the forest floor every Autumn!).
4) Based on the information in Table 2 below, in which marine environments does
most organic carbon get buried and preserved? Name at least two factors
Table 1. Annual production and fate of terrestrial and marine carbon (based on Hedges
and Keil, 1995)
Amount produced
Terrestrial
productivity
60 x 1015 g C yr-1
Marine
productivity
50 x 1015 g C yr-1
Amount delivered to oceans
0.2 x 1015 g C yr-1 as particulate organic matter
0.2 x 1015 g C yr-1 as dissolved organic matter
About 10% remaining at depths of several hundred
meters
About 1% passes to an average ocean depth of 4000 m
Table 2. Organic carbon burial in different ocean environments (after Hedges and Keil,
1995)
OC burial rate in units of 1012 g C yr-1
Environment
Deltas
70
Shelves and upper continental slopes
75 (including 6 in shallow water carbonate
environments, 1 in anoxic basins such as the Black
Sea)
Slopes
7
Deep sea (pelagic) sediments
8
Table 3 lists the main reservoirs of carbon in the Earth’s crust and surface
environments.
Reservoir Type
Active (nearsurface) pools
Amount, in 1018 g C
Inorganic



Marine dissolved inorganic carbon
(DIC)
Soil carbonate
Atmospheric CO2
39
1.1
0.66
Organic




Sedimentary
rocks
Soil humus
Land plant tissue
Sea water dissolved organic
carbon (DOC)
Surface marine sediments
1.6
0.95
0.60
0.15
Inorganic

Carbonates
60,000

Mudrocks (in the form of kerogen),
coal, etc.
15,000
Organic
5) Based on Table 3, what proportion of carbon is contained in sedimentary rocks?
6) Increasing levels of CO2 in our atmosphere and consequent global warming are
one of the most important issues facing our society. What are at least three ways
that humans are causing the movement of carbon into the atmospheric “reservoir”
from other storage places on Earth?
Part II. Organic carbon preservation and mineral surface area
What does organic carbon (OC) in sediments and sedimentary rocks look like? Or put
another way, where in a sample of sediment do we find the organic carbon? Although
our eyes may be drawn to macroscopic organic debris such as wood or leaf fragments,
in actuality only a small amount of the organic matter in sediments is present as
discrete, separable particles. The bulk of the OC in most sediments is strongly bound to
the mineral particles, coating individual grains and sandwiched between collections of
them called aggregates. We can illustrate this with a simple experiment, in which a
sediment sample is submerged in a sodium polytungstate (SPT) solution with a density
of 1.8 gm cm-3. In this liquid, organic fragments (e.g. wood debris) will float, whereas
mineral grains (average density of about 2.7 gm cm-3) will sink. For most marine
sediments, only a small amount of organic matter will float out of the sample, and in the
case of wood or leaf debris, it is approximately 30% organic carbon.
One of the most important realizations in recent years is that the OC content of
sediments and soils is strongly correlated to mineral surface area. The amount of OC in
sediments appears to roughly equivalent to that required to coat the minerals in a
molecule-thick layer of organic compounds, although several studies indicate that the
organic material is actually not evenly spread on the particles, but rather is patchily
distributed. Why are OC content and surface area correlated? Organic carbon appears
to bind to the particles and to “hide out” in spaces on the particles and within
aggregates, thereby protecting it from the bacteria that want to eat it! Let’s explore the
implications of this by doing some simple calculations of the amount of surface area per
given weight of some different size classes of sediment.
The table below gives information about the number of grains of given sizes that would
constitute 1 gram of sample based on an average mineral density of 2.7 gm/cm3.
Assuming that these grains are roughly spherical in shape, fill in the table by calculating
the surface area of a single grain and then the surface area of a gram of sample. Recall
that the surface area of a sphere is given by 4πr2 (where π  3.14 and r is the radius).
Table 4. Surface area of spherical grains
Size class
Diameter
of grains
in
microns
Diameter of
grains in
meters
Number of
grains in a gram
of sample
Coarse sand
1000
0.001
708
Very fine
sand
100
0.0001
7.08 x 105
Medium silt
20
0.00002
8.85 x 107
Fine silt
4
0.000004
1.11 x 1010
Clay
1
0.000001
7.08 x 1011
Surface area
of a single
spherical grain
Surface area of
a gram of these
grains
(in m2)
(in m2 gm-1)
Questions:
1) How much more surface area does a 1 gm sample of 1 micron clay-sized particles
have than a 1 gm sample of 100 micron very fine sand-sized particles?
2) Many sand and silt grains are coated with very fine particles of clay and iron
oxides. These tiny grains have typical surface areas of 200 m 2 gm-1! If a
sediment sample is composed of 98% fine silt by weight and 2% iron-oxide
particles, what would you predict its surface area per gm to be?
3) Based on the results in Table 4 and the observation that the amount of organic
carbon in marine sediments is correlated to surface area, in which types of
sediments and sedimentary rocks would you expect to find the most organic
carbon?
4) It has been proposed that times of extensive mountain building on Earth have
been times of extensive organic carbon burial. Based on what you have learned
about the relationship of organic carbon to mineral surface area, why do think this
would be the case?
Part III. Using organic carbon in sediments to interpret past environments
One of the most common tools used to interpret the sources of organic matter in
sediments and sedimentary rocks is the ratio of the two stable carbon isotopes, 13C and
12C. The ratio of these two isotopes of carbon in a sample of sediment, which can be
measured using an instrument called a mass spectrometer, is usually expressed as a
per mille (‰) deviation ( 13C) from the ratio measured in an internationally recognized
standard (the Pee Dee Belemnite, PDB):
δ13C (per mille, ‰) = [(13C/12C)sample/(13C/12C)standard – 1] x 1000
Values of δ13C that are relatively small (i.e., more negative) indicate a sample that is
depleted in the heavier isotope, 13C relative to the standard. In contrast, greater (more
positive) δ13C values indicate relative enrichment in 13C.
Stable carbon isotopes are useful for the interpretation of sedimentary environments
because they occur in different proportions in the organic matter originating in plants on
land and that originating from plankton near the ocean surface. During photosynthesis,
all plants preferentially take up 12C, resulting in organic matter that is depleted in 13C
compared to the atmosphere (currently atmospheric CO2 has a δ13C of -8‰). Most
(90%) terrestrial plants fix CO2 by the C3 photosynthetic pathway, a process that results
in organic matter with δ13C values ranging from -32 to -21 ‰ and averaging about 27‰. Marine plankton, in contrast, have δ13C values between about -24 and -16‰, with
an average of about -21‰.
Estuaries are one environment where both terrestrial plant and marine algal organic
matter accumulate in sediments. A gradient in the δ13C and C/N of organic matter is
commonly observed in modern estuaries. In the upper reaches of the estuary, organic
matter in sediments typically has δ13C values of around –28 to 25 ‰, indicating a
predominance of terrestrial OM. Further downstream, near the estuary mouth, δ13C
values of about -21 to -19 ‰, reflecting the deposition of marine algal OC. By
determining an average “end-member” δ13C for the terrestrial and marine OC (δ13Cterr
and δ13Cmar) in a particular system, the fractions of material from these sources (Fterr
and Fmar) can be estimated using a simple mass balance where:
(1)
(2)
(δ13C)sample = Fterr(δ13Cterr) + Fmar(δ13Cmar)
Fmar = 1 – Fterr
Combining these two expressions, we get:
(3) Fterr = [δ13Csample – δ13Cmar] / [δ13Cterr – δ13Cmar]
Example: A sample of sediment recovered from an estuary has organic carbon with δ13C = -25.
Assuming that the terrestrial OC in this area has a δ13C of -27 and the marine OC has a δ13C of
-20, how much of the OC in this sample is terrestrial and how much is marine?
Fterr = [(-25) – (-20)] / [(-27) – (-20)] = 0.71 (71%)
Fmar = 1 – 0.71 = 0.29 (29%)
Exercise: A recent study by Zong et al. (2006) utilized the trend in δ13C along the axis of the
lower Pearl River estuary in southern China to interpret the record of river discharge preserved
in a 10 – m-long sediment core containing Holocene sediments. Their reconstruction relies on
the observation that the position of the freshwater/salt water interface in estuaries is governed
by the balance between river flow and tides. During times of increased river discharge,
terrestrial OC tends to accumulate further seaward in estuaries than during times of diminished
river flow.
The data below are based on the study by Zong et al. (2006), but have been modified
for the purpose of this exercise.
a) For each data point, use the carbon isotope mass balance equation (equation 3)
to calculate the fractions of terrestrial (δ13C = -26.5 ‰) and marine
(δ13C = -21.0 ‰) carbon.
b) What do these trends suggest about changes in river discharge and climate in
southern China during the Holocene?
c) What other types of data might you want to collect from the Pearl River estuary
core to test your interpretation?
d) In what other types of sedimentary deposits/sedimentary rocks might carbon
isotopes be used in a similar fashion for paleoenvironmental information?
Table 5 Core Data for Part III
Depth in core
(cm)
Approximate age of
core interval (years
before present,
based on 14C)
Measured δ13C of
organic matter
0
recent
-23.3
100
1470
-24.5
200
3350
-25.0
300
4400
-25.5
400
5476
-25.7
500
6050
-25.8
600
6800
-26.5
700
7200
-26.2
800
7800
-25.5
900
8155
-24.6
1000
8500
-23.5
Calculated %
terrestrial and %
marine OC
Summarize your results for each part and discuss what your results mean.

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