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Biogeochemical Dynamics
COORDINATORS: MICHAEL B. MCEE ROY AND
BERRIEN MOORE IIT
UNDERSTANDING THE ROLE OF BIOGEOCHEMICAL
DYNAMICS IN GLOBAL CHANGE
With hydrogen and oxygen, four elements carbon, nitrogen,
sulfur, and phosphorus are of particular interest in the study of
our planet. Through the active intervention of the biota, each of
these four elements follows a closed loop or cycle, passing through
molecular species of increasing energy content as the elements are
incorporated into living tissue, and then moving through decreasing
energy levels as the organic matter is returned to inorganic form.
These cycles significantly influence atmospheric and oceanic chem-
istry and the global energy balance. The varied dynamical patterns
reflected in different stages of these cycles are the consequences of a
myriad of biological, chemical, and physical processes that operate
across a wide spectrum of time scales.
For the TGBP, departures from biogeochemical "quasi-steady
state" are of greatest interest. From ice core records, we know that
atmospheric concentrations of carbon dioxide (CO2) and methane
(CH4) were substantially reduced during periods of peak glaciation
This paper is the result of discussion at a workshop (see the appendix to this paper)
and further discussions among members of the Committee on Global Change.
47
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48
relative to interglacial or recent "preindustrial values." What is fun-
clamentally different in the very recent record since industrialization
began is the rate of change: CO2 has increased at rates and to levels
for which we have no historical or natural analogue back through
the last two interglacial events. Moreover, CH4 is increasing in the
atmosphere at a rate more than twice as fast as CO2 and is now at a
concentration of almost 5 times that which was present during glacia-
tion. These recent perturbations are believed to be anthropogenicaDy
induced. It is of great importance to develop an understanding of the
factors responsible for the rapid rise in CO2 and CH4, as well as the
concurrent changes in the nitrogen, phosphorus, and sulfur cycles.
Understanding the biogeochem~cal cycles of carbon, nitrogen,
sulfur, and phosphorus and the interactions of these cycles is of
fundamental importance to the IGBP (National Research Council
1985, 1986~. Although biogeochemical cycling in the terrestrial en-
vironment, rivers, the ocean, and the atmosphere is intricately in-
terrelated, processes controlling the cycling are distinctly different
in each of these environments. Thus, in order to identify scientific
priorities for understanding biogeochemical cycles in the context of
global change, in this paper each of these environments is discussed
separately.
BIO GEO CHEMICAL CYCLING
IN TERRESTRIAL SYSTEMS
The accumulation and cycling of carbon, nitrogen, and sulfur
within terrestrial ecosystems are ultimately controlled by the interac-
tion of climate and the amount of phosphorus in the parent material.
However, nitrogen supply is often the proximate factor regulating
carbon fixation and storage, particularly in temperate, boreal, and
agricultural areas. Patterns of carbon, nitrogen, and phosphorus
turnover vary within and among biomes, responding to topography,
land use, herbivory, and hydrology. For example, in cold-dominated,
wet tundra ecosystems, carbon fixation exceeds decomposition; there
is net carbon and nitrogen storage in soil, and plant growth is limited
by nitrogen. In contrast, tropical forests on old infertile sails cycle
large amounts of carbon and nitrogen, but biomass accumulation is
limited by phosphorus.
At least three types of change are relevant to terrestrial bio-
geochemistry. First, changing climate will vary the balance between
carbon fixation and release partly because photosynthesis responds
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less to changes in temperature than does respiration, and partly
because water availability strongly influences storage and release of
carbon. Second, changing the supply of carbon, nitrogen, phospho-
rus, or sulfur can alter the storage and release of all-four elements.
Finally, human changes in land use, associated, for example, with
agriculture or pasture, can cause very rapid changes in carbon and
nitrogen storage in terrestrial ecosystems.
In many cases, we know the direct effects of changes in climate,
element supply, or land use on primary production: elevated temper-
atures increase growth of tundra plants, and elevated atmospheric
CO2 concentrations increase carbon fixation in most plants. How-
ever, the ramifications of such changes at the ecosystem level are less
clear.
.
For example, suppose elevated levels of CO2 increase carbon fix-
ation in a forest with Tow nitrogen in its soil. The added carbon
increases the C/N ratio in plant tissue. Such a change could affect
herbivores either positively or negatively, depending on the chemi-
cal form of the acIditional carbon. Eventually, plants or plant parts
return to the soil, where their elevated C/N ratio will affect rates
of decomposition and nitrogen release, probably negatively. Con-
sequently, ecosystem-level storage of carbon may be greater than
expected from the simple increase in plant carbon fixation due to ele-
vated atmospheric CO2 levels. The rate of plant litter decomposition
would ultimately be decreased. Trace gas production would also be
affected if changes in decomposition rates alter soil mineral-nutrient
dynamics. However, if the decomposition and nutrient release in the
soil are sufficiently delayed, nutrient limitation could become more
severe, decreasing rates of carbon fixation and hence decreasing car-
bon storage.
In the context of changes in biogeochemistry in terrestrial sys-
tems, five geographic areas are judged as critical foci for experimental
ecosystem studies; wet tundra, boreal forests, temperate forests in
areas receiving nitrogen deposition, tropical forests, and semiarid
ecosystems. These are selected for their potential sensitivity and
contribution to global change.
Tundra
Tundra is particularly important because of the large store of
organic carbon contained in soils as a consequence of slow deposition
caused by cold temperatures and waterlogging. Greenhouse warming
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is predicted to be most pronounced at high latitudes, and may be
expected to increase CO2 fixation by plants and to increase decompo-
sition and CO2 release. However, if the climate also becomes wetter,
the pool of organic soils in the tundra may increase, with the result
that there may be an increase in carbon accumulation but with an
associated increase in the release of CH4. Experimental studies are
needed to test these hypotheses.
The interactive effects of CO2 and climate can be addressed
through controlled studies. Replicated greenhouses with elevated
temperatures and/or CO2 have been established in tussock tundra,
and growth and net CO2 fixation have been measured for three grow-
ing seasons. This establisher! in part the basis for what is needed:
a large-scale effort (large greenhouses, year-round sampling, studies
on arctic coastal plain ant! subarctic mire as well as upland tundra,
investigations of interactions with the nitrogen cycle) to (1) elucidate
the net effects of cTimate-C02 interactions on carbon and nitrogen
storage and (2) clarify their probable feedbacks to the greenhouse
effect. The low physical stature of tundra ecosystems makes them
particularly suitable for such an experimental approach, although
their remoteness and the harshness of the environment pose a signif-
icant challenge. As a first step, Tow-stature temperate systems could
be used as candidates for enclosure experiments under climate and
CO2 and nutrient treatments.
In high latitudes, in situ and greenhouse experimental studies
should be supplemented en c} extended through measurement pro-
grams to obtain net fluxes of CO2 and CH4 during warm versus cold
periods over large spatial scales.
Boreal Forests
Many characteristics of boreal forests, upon which prediction of
responses to climate change couIcl be based, are inadequately un-
derstood. Biomass densities, rates of nutrient cycling, and chemical
characteristics of litter material would change as a consequence of
greenhouse warming. An example of the potential importance of
such change is that the temperature-moisture niche occupied by bo-
real forests no longer exists in many of the climate model projections
for a (loubled CO2. Even granting the imperfection of climate models
and the uncertainty of using temperature and moisture patterns as
statistical estimators of potential vegetation, it is likely that major
changes will occur in the high-latitude forests of the world.
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51
How might the distributions and composition of boreal systems
change? How might these changes feed back to the atmosphere and
climate? What is the fate of the stored nutrients in these systems?
Experiments to answer these and other questions need to focus upon
the linkages between processes within boreal forests and climatic
conditions. This is not done easily through enclosure experiments
or even manipulations. However, natural gradients of climate within
boreal systems could be used to gain much of the needed insights
on their dynamics. Such gradients should cross both areas of high
nitrogen deposition and unaffected regions.
Temperate Forests: Nitrogen Depositional Areas
Nitrogen emission from industrial activity (and consequent rede-
position) represents a large flux of nitrogen (ca. 50 x 10~2 g/yr) and
is globally significant relative to biological fixation. Moreover, the
deposition is concentrated in temperate areas (eastern North Amer-
ica, northern Europe), where (at least in the past) nitrogen probably
limits plant growth and carbon accumulation. Annual deposition in
eastern North America (ca. 10 kg/ha/yr) is significant in comparison
to annual nitrogen circulation in those forests (ca. 100 kg/ha/yr). In
some areas of Europe, deposition is as high as SO kg/ha/yr. How
much of the nitrogen deposited is re-emitted as N2, N20, and NOx or
leacher! as NO3-? How much additional carbon is fixed and stored
as a consequence of deposition?
These questions can be approached in a manner analogous to
that used to study effects of elevated CO2 in enclosure experiments.
Controlled ecosystem-level studies can be set up in which treatments
are applied including added nitrogen (Iow levels similar to deposition
rather than fertilization), elevated CO2, and changes in moisture and
temperature regimes. The nitrogen portion of these measurements
is under way at several sites. The interaction between elevated CO2,
altered temperature and moisture, and added nitrogen will require
controlled glasshouse studies and could be supported by atmospheric
boundary layer studies. Ecosystem-leve] modeling and measurement
programs across natural gradients will be needed to supplement en-
closure experiments.
Ecosystem-level enclosure experiments, as for the tundra, are
essential since the added nitrogen may decrease tissue C/N and
thereby increase decomposition/nitrogen release. While enclosure
experimental work may be difficult because of the increased stature,
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certain Tow-stature temperate systems do exist; furthermore, logisti-
cal difficulties, paramount in high latitudes, are less of a problem.
Tropical Forest
The direct ejects of increasing CO2, possible changes in precip-
itation, and anthropogenic changes in tropical systems could all be
glob ally significant. Tropical forests on young fertile soils are highly
productive and circulate more nitrogen and phosphorus than any
other terrestrial ecosystem. In contrast, infertile tropical soils (e.g.,
central Amazonia) remain relatively productive but are extraor(li-
nariTy low in phosphorus. Conceivably, elevated CO2 could cause
increased photosynthesis and possibly storage, but this may not be
the case in the more infertile areas. It would be useful to undertake
measurements to examine the question-of whether increased CO2
increases photosynthesis in a range of tropical forests. It would be
of particular interest to examine whether the increase in carbon fix-
ation could cause increased nitrogen fixation, given the abundance
of nodulated legumes in many tropical forests. This measurement
is difficult because of the stature of tropical forests, but first steps
could probably be acldressed without chambers.
The distribution of precipitation in the tropics is interesting be-
cause of the very sharp transitions in both space and time between
forest and savanna. Savanna is dominated by C4 grasses, stores less
carbon and cycles less nitrogen than forest, and burns more readily
(and often is present because of burning). The transition from forest
to savanna and back thus could involve differential changes in storage
of carbon and nitrogen and gas release (between or during fires). An
unclerstanding of the underlying mechanisms could indicate whether
(1) areas currently forested can invade savanna areas (because el-
evated CO2 favors C3 species) or (2) human activity can convert
additional forest to savanna. Such information would also be useful
for interpreting the paleorecord during the last glacial cycle when, as
many believe, much of what is now lowland forest was savanna.
Finally, current human population growth is concentrated in the
tropics and will remain so for the foreseeable future. Large-scale land
clearing is primarily a tropical phenomenon at present: the current
range on the estimate of the rate of conversion of closed canopy
tropical forest to agriculture is 70,000 to 100,000 km2/yr. Most
natural systems dominated by perennials rapidly lose large amounts
of carbon and nitrogen from soil upon conversion to agriculture:
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the carbon as CO2 (except where wetland rice is established), the
nitrogen as nitrate. Overall Toss is often 25 to 40 percent of the
amount in soil, and whereas this process takes 40 to 50 years in
temperate regions, it can occur 10 times faster in the tropics.
Large fluxes of carbon and nitrogen are released with tropical
land clearing. It is important to determine what systems are be-
ing converted, what the relevant standing stocks are, and what the
rate of conversion is. Space-based observations are perhaps most ap-
propriate for answering these questions. More difficult but equally
important, we need to understand what regulates the rate and path-
way of important loss of carbon and nitrogen following land clearing
or conversion, and to establish what regulates the quantity and qual-
ity of carbon and nitrogen pools upon recovery. Such an effort would
require determining the fraction of Toss that occurs as CO2 versus
CH4, and as NO3- versus N2 versus N20 versus NO=. Nitrogen is
particularly important because, while NO3- is the primary form of
nitrogen Toss in the temperate zone, N20 and NOT fluxes are much
greater in tropical forests than they are in temperate forests. Most
importantly, the mechanisms controlling pathways of Toss or gain
must be analyzed in order to extrapolate the fluxes over the range of
land uses/ecosystems that are being affected. CO2 is also interesting;
while elevated CO2 may not significantly affect forest carbon storage,
it could certainly affect the rate of recovery on fertile sites.
Semiarid Ecosystems
Humans depend very heavily on the livestock and agricultural
productivity of subhumid and semiarid ecosystems, particularly in
the tropics. Any changes in these areas, which are already marginal,
would have important ramifications for human society. Subtropical
areas may become drier with greenhouse-induced warming, which
could interact with human-caused desertification (overgrazing, irriga-
tion-induced saTinization, accumulation of toxic metals) to cause
large-scare changes in carbon and nitrogen storage and nitrogen gas
procluction. Opposing this effect would be an increase in the effi-
ciency of plant water use caused by elevated CO2. Nutrient inter-
actions will also occur since in most semiarid systems productivity
is jointly controlled by water and nitrogen. Herbivory is ubiquitous
in semiarid areas and greatly influences water, nitrogen, en c! CO2
dynamics, as well as other parameters controlling physical climatic
interactions. Controlled studies involving both CO2 and aridity in
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54
desert grassland, shrub-steppe, and dry tropical forest could deter-
mine whether the net effect would result in carbon storage or release.
In summary, there is a need for coordinated studies of selected
ecosystems, to define the diverse impacts of human activity associ-
ated with altered supplies of carbon, nitrogen, and sulfur, and the
sensitivity of paths for nutrient cycling to changes in climate. In-
tegrative, coordinated studies of a broad areal extent over natural
ecosystems are needed in an overall research strategy, as are exper-
imental modifications, including enclosure experiments, of systems
that can provide invaluable insights. Finally, a comprehensive strat-
egy must include a commitment to a Tong-term observing system
both from space and from the ground.
BIOGEOCHEMICAL CYCLING IN FLUVIAL SYSTEMS
Rivers provide an important means for transfer of materials
from the land to the ocean They supply a significant fraction of
the ocean's store of nitrogen nutrient and the bulk of its phosphate.
Changes in this input may have particularly important effects on
coastal ecosystems and in addition can affect oceanic productivity
during the transition in and out of glacial periods. Rivers offer an
excellent integration of biogeochemical processes operating in specific
watersheds. Consequently, studies of riverine chemistry can provide
an invaluable perspective on the significance of changes occurring
over large regions. For these reasons, in consort with directed studies
of specific terrestrial systems, they should play an important role in
the overall strategy of the IGBP.
Estuaries and coastal regions are of particular interest. They
may be expected to undergo especially rapid change due to the rising
level of the ocean, which is anticipated to occur as a consequence of
climatic warming over the next century or so. Sedimentary deposition
in estuaries of major rivers can represent an important intermediate
reservoir for phosphorus and nitrogen. As a result, processes that
affect sedimentation and resuspension may exert a major influence
on the flux of phosphorus and nitrogen to the ocean as well as having
a direct influence on estuarial and coastal ecosystems. Recent studies
suggest that inorganic processes in turbid estuaries may enhance the
dissolved phosphorus-flux by up to a factor of 2. It is important to
assess the size of the sedimentary reservoir as well as the factors that
influence its deposition and erosion, and to identify how it might
change in response to changes in climate.
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Measurements of the chemistry of primary nutrients in selected
major estuaries merit careful study. There is a need for a Tong-term
measurement program to define the flux of phosphorus and nitro-
gen to the ocean, with attention directed to the role of sediments in
estuaries and coastal regions in light of their potential importance
as temporary holding reservoirs. Complementary laboratory experi-
ments on sedimentary material wiD also be needed to clarify poorly
understood chemical processes.
The residence time of dissolved phosphorus in the ocean is ap-
proximately 100,000 years. The time scale for changing the phos-
phorus-concentration in the ocean is therefore similar to that for ma-
jor episodes of glaciation. The time scale for oceanic nitrogen is much
shorter, about 10,000 years. Fluctuations in the terrestrial fluxes on
nitrogen and phosphorus, due to variations in weathering and estu-
arine processes, and exchange with coastal sediments, could have an
effect on temporal variations in oceanic nutrients, and consequently
global climate, through changes in oceanic productivity. Thus pre-
vious models of the geochemical cycles of nitrogen and phosphorus
that assume steady state behavior may need to be modified to explore
implications of nonsteady state models for ocean nutrient cycles.
The supply of phosphorus to the worId's oceans is controlled
ultimately by the rate of continental weathering. Hence transport of
phosphorus can be directly affected by climate through its influence
on weathering rates. Studies of riverine chemistry can contribute to
a better understanding of this interaction. Partitioning of phospho-
rus between aqueous solutions and solid phases depends upon the
chemical conditions of the weathering environment. It is important
to understand the mechanics of weathering under various climatic
conditions in order to assess the chemical parameters that determine
the initial partitioning between phases.
After weathering there is an opportunity for additional chemical
alteration of solid phosphorus-bearing phases as phosphorus is in-
corporated in terrestrial ecosystems, and as it is transported in the
rivers. It has been suggested that modifications of the chemical form
of phosphorus can occur through surface interactions with colloidal
metal oxides, and through dissolution caused by changes in solu-
tion parameters and biological activity. These matters merit further
study.
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BIOGEOCHEMICAL CYCLING IN OCEAN SYSTEMS
The Carbon System and the Biological Pump
.
The oceans are by far the largest active reservoir of carbon.
Recent estimates of the total amount of dissolved inorganic carbon
in the sea establish its range as between 34,000 and 3S,000 x 10~5
g carbon. Only a small fraction is CO2 (mole fraction 0.5 percent);
the bicarbonate ion with a mole fraction of 90 percent and the
carbonate ion with a mole fraction of just under 10 percent are the
dominant forms of dissolved inorganic carbon. The dissolved organic
carbon pool has been reported to be similar in size to the pool of
terrestrial soil carbon, but recent data suggest that it may in fact be
considerable larger.
Although the oceans are the largest active reservoirs of carbon
and cover 70 percent of the globe, the total marine biomass is only
about 3 x 10~5 g C (though such estimates are uncertain at best),
or just over 0.5 percent of the carbon stored in vegetation. On
the other hand, the total primary production is 30 to 40 x 10~5
g C/yr, corresponding to 25 to 40 percent of the total primary
production of terrestrial ecosystems. A portion of this production
results in a sink for atmospheric CO2, primarily through the sinking
of particulate carbon. As a consequence of this "biological pump,"
the concentration of dissolved inorganic carbon is not uniform with
depth: the concentration in surface waters is 10 to 15 percent less
than that in deeper waters. There is a corresponding depletion of
phosphorus and nitrogen in surface waters, even in areas of intense
upwelling, as a result of biological uptake and loss of detrital material.
The fate of this material depends, in part, upon its chemical
characteristics. If it is in the form of organic tissue, then it is ox~-
dized at intermediate depths, which results in an oxygen minimum
and a carbon, nitrogen, and phosphorus maximum. If it is carbon-
ate, it dissolves below the Tysocline, raising both alkalinity and the
concentration of carbon, at depths where the high pressure increases
the solubility of calcium carbonate.
Thus the "biological pump" Towers the partial pressure of CO2
In surface waters and enhances the partial pressure in waters not
in contact with the atmosphere. The efficiency of the biological
pump depends on the supply of nutrients to surface waters, food
web dynamics, and sinking losses of particulates to the deep sea. It
may be expected to respond both to changes in the strength of the
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overall thermohaiine circulation and to variations in the abundance
of nutrients, primarily nitrogen and phosphorus.
A portion of the nutrient flux to the surface returns to the deep
sea unused by the biota, carried along by the return flow of waters
in downwelling systems at high latitude. A high concentration of
inorganic nutrients in downwelling systems would indicate that the
efficiency of the biological pump is low and would favor transfer of
CO2 from the deep sea to the atmosphere. It is important to define
the physical, chemical, and biological processes that regulate the
concentration of organic nutrients in descending water masses, the
flux of so-called preformed nutrients. The concentration of preformed
nutrients may be expected to reflect physical processes, and it can be
influenced also by biological activity to the extent that this activity
can result in packaging of carbon, nitrogen, and phosphate in fecal
material that can fall to the deep, providing a path for transfer of
nutrients from the surface to the deep independent of the physical
processes such as those responsible for the formation of deep water
in high latitudes.
There is a need for careful, coordinated studies of the processes
responsible for transfer of nutrients from the surface to the deep.
There is a particular need for studies of the relative role of physics
and biology in regulating transfer at high-latitude, where the transfer
mechanism may be influenced by seasonal variations in the extent
of sea ice. Measurements must extend over all seasons, posing con-
siderable difficulties in light of the logistical problems posed by the
need for measurements during the harsh conditions characteristic of
the high-latitude marine environment.
Internal Nitrogen Cycling in the Ocean
In the ocean today the process of nitrogen fixation provides less
than 1 percent of the nitrogen demand of the primary producers.
Global contributions from riverine discharge plus wet and dry atmo-
spheric deposition are thought to be similarly small. Nearly all of
the nitrogen requirement is met by recycling via heterotrophic pro-
cesses (ammonification and nitrification): ammonium, with lesser
quantities of nitrate and nitrite, provides most of the nitrogen re-
quirement for primary production in the sea. Organic nitrogen exists
at intermediate concentrations, but the bulk of this material is very
refractory, with turnover times of 102 to 103 years.
Some nitrogen is shunted out of this loop via permanent burial in
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sediments, but the major loss of nitrogen from the marine system oc-
curs because of denitrification, whereby nitrate is reduced to N2 and
N2O and lost to the atmosphere. This process is most active in the
ocean today in the eastern tropical Pacific, in the waters underlying
highly productive upwelling regions. In fact the best global estimates
for denitrification lead one to conclude that, currently, nitrogen is
being lost from the sea more rapidly than it is being gained.
Very little is known about the factors that regulate the dominant
input term, nitrogen fixation. The most abundant oceanic cyanobac-
terium known to be capable of fixation, Trichoclesmium, has never
been cultured. At best, isolates have been maintained in the labora-
tory for a few months. When this organism is successfully established
in laboratory culture, and optimal growth conditions defined, we win
be able to ascertain better the factors that currently limit nitrogen
fixation in the sea.
There is increasing evidence that eucaryotic phytoplankton, di-
atoms in particular, harbor intracellular inclusions of cyanobacteria
that may be significant in terms of global marine fixation of nitrogen.
Strategies involving monoclonal antibodies are now being suggested
as a new approach to identifying and quantifying the process of nitro-
gen fixation in the sea. Undoubtedly, there are other opportunities
yet to be explored that could bring the modern methods of molecu-
lar biology to bear on pressing issues related to the marine nitrogen
cycle. It is essential that we develop a more complete understand-
ing of the physical, chemical, and biological processes regulating the
complex life cycle of nutrients in the sea.
The Sedimentary Record
Certain geochemical and biological properties are recorded in
oceanic sediments and form the basis for our deductions about global
environmental changes. For example, we infer past temperatures of
the ocean from counts of the relative abundance of the fossils of
organisms preferring cold and warm ocean waters, or from measure-
meets of the oxygen isotope composition of the fossils.
While the empirical and theoretical justification for these infer-
ences is generally accepted, there is a distinct lack of direct gIobal-
scale documentation of the relationship between the sedimentation
and geochemistry of fossils and the physical and chemical proper-
ties of the modern ocean. Such studies are imperative if we are to
quantify the error limits to be placed on inferences concerning past
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climates and ocean chemistry. They are essential if we are to recog-
nize situations where our inferences may be misleading or in error. As
an example, consider the carbon isotope composition of pl~nktonic
foraminifera from high-latitudes. One class of theories for the Tow
glacial levels of atmospheric CO2 predicts that the [~3C in the high-
latitucle surface waters in glacial times should be shifted to reflect a
larger abundance of i3C relative to the deep sea. In principle, we ex-
pect that we should be able to monitor past changes in high-latitude
i3C using measurements of carbon in the shells of fossils that grew
in surface waters. But it is reported that high-latitude planktonic
fossils reveal a Tower abundance of i3C in glacial times than would
be indicated by theoretical expectations. Does this mean that the
theories are wrong, or does it mean that the evidence is mislead-
ing? Perhaps it means that the foraminifera do not accurately record
the i3C of the water they grow in, or perhaps that the sedimentary
foraminifera were formed in a season other than that crucial to the
theory.
Rather than reject either the theory or the oceanic evidence out
of han(l, a study of the global behavior of biological sedimentation,
through ocean flux measurements, provides the opportunity to make
a direct determination of the accuracy of foraminifera as recording
systems for high-latitude surface i3C and the extent to which sea-
sonal flux changes might bias the sedimentary record. With knowI-
edge gained from studies of the contemporary ocean we would hope
to be able to read the sedimentary record better and therefore de-
rive valuable information on past ocean circulation, chemistry, and
primary productivity.
Deep ocean circulation is one of the important controls on climate
and atmospheric CO2, due to its role in the global redistribution of
heat, salt, and biochemically important elements. In or(ler to predict
future climate, it is important to understand the potential variability
of deep ocean circulation. The study of past changes in ocean circula-
tion inferred from deep-sea cores will provide a Tong-term perspective
on the ongoing effort to develop an ocean climate model, in partic-
ular, with regard to past and future changes in atmospheric CO2,
as noted above. Ocean circulation modifies the effectiveness of the
"biological pump" in isolating the atmosphere from the deep ocean
and is a significant factor in controlling the alkalinity of the ocean
through its influence on the deep ocean concentration of CO3-- an
the lysocline.
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60
Data on geochemical tracers from fossils of bottom-dwelling or-
ganisms show that ocean circulation during the most recent glacial
maximum was drastically different. In particular, it appears that
North AtIantic deep water formation was significantly curtailed,
while intermediate-depth waters in the North AtIantic were sub-
stantially more nutrient-depleted. Nonetheless, i4C studies of deep
ocean fossils suggest that the overall ventilation rate of the deep
ocean has remained similar to that of the modern ocean.
Continued development of a global database documenting three-
dimensional changes in deep ocean circulation during the late Pleis-
tocene is needed. Such a database should include measurements of
carbon isotopes, cadmium, and i4C in benthic foraminifera. These
measurements should be coupled with documentation of changes in
the deep ocean carbonate system through studies of the preservation
and accumulation of calcium carbonate in deep ocean sediments. The
results of these studies should be coupled with biogeochemical mod-
els for the transfer of nutrients and carbon through the ocean. These
goals can be achieved through the continued study of archive sedi-
ment cores, but win also require continued efforts to obtain suitable
large-diameter cores in key parts of the ocean. Large-diameter cores
are needed to provide material sufficient to allow simultaneous mea-
surement of key properties as well as retention of archive material to
be used as new techniques are developed over the next decade. Cores
taken in regions of high sedimentation rates are needed to provide
information on rates of change that have occurred in the recent past.
Studies of the effects of rapid change, such as the Younger Dryas cold
interval about 10,000 years ago, on ocean circulation and chemistry
can provide valuable information on the response time of the global
climate system.
A global carbon isotope data base wiB also be of use in the
evaluation of the magnitude and rates of change of the continental
biomass. Carbon that is currently in the biomass was transferred to
inorganic form in the ocean during the last glacial maximum. The
magnitude of the associated transfer should be reflected in the {~3C
of benthic forams.
The magnitude and timing of past changes in the phosphorus
content of the ocean will be key to obtaining an understanding of
the global phosphorus cycle. Because of the Tong time constants
involved (approximately 105 years), variability in sources and sinks
of phosphorus are difficult to study directly on a global basis. But
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61
the consequences of past imbalances between input and output of
phosphorus in relation to climate change can be examined.
Most of the phosphorus in deep ocean sediments is detrital (i.e.,
it is what remains of the particulate phosphorus that fed through
the water without being released to dissolved form), but it is diffi-
cult to make a satisfactory estimate of the rate of Toss of dissolved
ocean phosphorus into sediments. Because of the biological and cli-
matological importance of the phosphorus budget of the ocean, it
is important that continued attempts be made to overcome this dif-
ficulty. There are indications that much of the Toss of phosphorus
may occur in limited regions of the ocean (such as in areas of high
biological productivity and/or Tow bottom water oxygen), and it is
particularly important to encourage the study of authigenic phospho-
rus sedimentation in these environments. The success of these efforts
will depend on significant breakthroughs in the methods of studying
phosphorus sedimentation. The importance of the phosphorus mass
balance justifies significant efforts in this direction.
Study of the past phosphorus content of the ocean is also a key
in testing some models of past changes in atmospheric CO2. The
phosphorus content of the deep ocean is one of the most significant
factors in setting the CO2 content of the atmosphere. Current ev-
idence based on studies of the carbon and phosphorus analogues,
i3C and cadmium, respectively, surest that the oceanic phosphorus
inventory has not changed as drastically in the past as suggested
by some models seeking to account for the observed reduction of
atmospheric CO2 (to 200 ppmv during glacial times). For example,
the cadmium content of the ocean, which is empirically correlated
with phosphorus concentration though the causal mechanisms are
not clear, does not appear to have changed by more than 20 percent
over the last 300,000 years. It is possible that the Cd/P content of
the ocean is not fixed for Tong geological times. Further constraints
can be placed by paired measurements of i3C and cadmium, since
the slope of the relationship between these two properties depencis on
the oceanic phosphorus content. Progress can be made then without
making any assumptions concerning the Cd/P ratio of the ocean.
In view of the importance of documenting changes in the phos-
phorus cycle of the ocean, extension of the database of paired i3C
and cadmium measurements from benthic foraminifera from the late
Pleistocene ocean, and exploration of the relationship between these
properties in the more distant geological past, are imperative.
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BIOGEOCHEMICAL CYCLING IN THE ATMOSPHERE
An understanding of the factors regulating the chemistry of the
atmosphere is essential to the success of the IGBP. The atmosphere
provi(les an early warning of changes in globally dispersed ecosys-
tems. Measurements of selected gases, CO2, CH4, N20, hydrocar-
bons, en c! dimethyisulfide for example, can help diagnose changes in
the metabolism of specific systems. In addition, we need a continuing
focus on the significance and nature of the changes taking place in
the troposphere and stratosphere.
The phenomenon of the antarctic ozone hole, its recent discov-
ery and belated investigation, clearly attests to the still fragmentary
nature of our understanding. We are just beginning to focus on the
changes taking place in tropospheric 03. There is growing evidence
that the abundance of tropospheric 03 iS increasing over large re-
gions, that the urban smog phenomenon is no longer confined to
cities. This has clear implications for productivity in impacted areas
and may be expected to significantly affect biogeochemical cycling
over extensive regions. The chemistry of tropospheric O3 assumes
additional importance in that the abundance of OH may be expected
to change in response to changes in lower atmospheric O3.
The radical OH is the ultimate cleansing agent for a wide range
of gases emitted to the atmosphere. It regulates oxidation of nitrogen
and sulfur compounds and oxidation of CO, triggers the initial steps
in oxidation of various hydrocarbons, and is responsible for removal
of a wide variety of industrial halocarbons.
The abundance of stratospheric O3 iS influenced by the input of
halogenated gases. Oxides of nitrogen, introduced to the stratosphere
by decomposition of N2O and by processes triggered by absorption
of cosmic rays and solar protons, play an important role in removal
of 03. The level of 03 iS expected to change in response to changes
in CO2, leading to stratospheric cooling compensating tropospheric
warming. An increase in CH4 can reduce the reservoir of chlorine
radicals by favoring conversion of C] to HCI. Changes in CH4 can
also lead to changes in the abundance of stratospheric H2O, with
important consequences for the chemistry of NO=, CI=, and 0~ and
potentially for climate. It is essential that we develop an understand-
ing of the factors resulting in changes in the abundance of all of the
stratospherically relevant species, with particular attention to CH4,
CO2, N2 O. and the halocarbons.
These objectives are being addressed in the stratospheric re-
search programs coordinated mainly by NASA. They must continue
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63
to receive vigorous attention. A significant role is played by NOR in
production of tropospheric O3. In the presence of elevated levels of
NO=, oxi(lation of hydrocarbons, both natural and anthropogenic, is
expected to lead to production of tropospheric 03.
The phenomenon has been studied extensively in cities and is
an important contributor to the formation of urban smog. There is
evidence that effects of pollution on tropospheric 03 are widespread.
Episodes of high 03 are observed over extensive spatial scales in
summer in the eastern Uniter! States and in Europe. Levels of O3 are
high enough to affect the productivity of agricultural crops and nat-
ural ecosystems. The interactions of evident changes in atmospheric
chemistry and climate with vegetation must be better quantified.
Preliminary results from the Atmospheric Boundary Layer Ex-
periment (ABLE) experiments in the Amazon Basin indicate that
removal of O3 from the atmosphere is correlated with uptake of CO2
by vegetation. Experimental strategies have been developed to in-
vestigate this interaction. They should be applied to a variety of
ecosystems if we are to unclerstand how the biosphere responds to
changes in atmospheric chemistry. We need to define the response of
the biota to this change and how the chemical environment might be
altered by the altered state of the vegetation.
Studies of experimentally manipulated systems would contribute
to a better understanding of the underlying synergisms. These stud-
ies should include investigations of the consequence of deposition,
both (fry and wet, of acid species, particularly oxides of nitrogen
and of sulfur. It is also important to study the response of natural
ecosystems to enhanced levels of ultraviolet radiation, particularly
so in light of recent evidence for a globally significant decline in the
level of stratospheric O3. Studies of tropospheric chemistry are less
mature than studies of the stratosphere, but equally important.
The Global Tropospheric Chemistry Program and its national
component (NRC, 1984; UCAR, 1986) are well formulate(l, but im-
plementation is so far slow. There is a clear need for resources to
be directed to these activities to stimulate the pace of research. The
objectives are to understand the processes regulating the composi-
tion of the troposphere with particular attention to oxidants and to
define paths for removal of biospherically formed gases.
The abundance of tropospheric O3 iS expected to depend on rates
of input of NOR and hydrocarbons. O3 and other oxidants in surface
air can interact with vegetation. We need to understand the factors
regulating this interaction, its impact on the biota, and the nature
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of the response of the biota as it might affect the emission of impor-
tant chemical elements. We need a better understanding of processes
regulating emission of NO, N2O, CH4, CO2, hydrocarbons, natural
halocarbons, and hydrocarbons. This wiD require intensive inves-
tigations of specific ecosystems, supported by appropriate chemical
investigations of the life cycles of these gases in the atmosphere. Our
understanding of processes must evolve such as to allow prediction
of the response of ecosystems to change. If our agenda is confined to
simply describing what happens now, we shall fait seriously to meet
our objectives. The current agenda for research in atmospheric chem-
istrv is directed toward understanding the atmo~r~her~ a.s it its ~.n`1 n.s
., ~ ~ , ~ . ~ . . ~ . _ . . . ~ . .
it may change in the immediate future. It recognizes the importance
of the atmosphere as an agent for transfer of chemical species from
one compartment of the biosphere to another KNOX and SO=` for
. ~
examples. ~t recognizes tnat emission of biogenic gases such as CO2,
CH4, N2O, and dimethyIsulfide can lead to effects on climate. It is
important to extend this perspective to the past. The information
contained in the paTeorecord wiB allow models to be developed for
the paleoatmosphere. These moclels in turn win play an essential
role in the interpretation of the paleorecord. For example, it should
be possible to estimate rates for production of CH4 in the past using
measurements of CH4 in ice cores in combination with data on NOX
en c! other relevant species.
Fortunately' ice cores offer a record closeIv related to conditions
i
~ 7 J
n the atmosphere. Air bubbles preserved in ice provide a rare oppor-
tunity to determine the past composition of the atmosphere. We can
see clearly recorded through time the changes in CO2 and CH4 since
the beginning of the industrial revolution. Changes in atmospheric
composition associates! with major changes in climate are also pre-
served. The available data provide a glimpse of conditions in our
atmosphere extending back to about 160,000 years before present
(B.P.~. It may be possible to expand this horizon even further, per-
haps as far back as 400,000 years B.P., using the planned Greenland
Ice Sheet Program IT core from Greenland.
Our knowledge of the changes in atmospheric composition that
have taken place since the inclustrial revolution is based almost ex-
clusively on the measurements from ice cores. We know that the
level of CO2 has risen from about 280 ppm to almost 350 ppm. The
ice has provided also a record of CH4 that indicates that CH4 abun-
ances have risen from about 0.7 ppm to a contemporary value near
1.7 ppm. Further, the ice core record has a limited overlap with
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65
modern analytical measurements in the atmosphere, which provides
an important test of the reliability of the data derived from the ice.
Studies of the isotopic composition of CO2 in ice allow us to dis-
criminate between sources of CO2 derived from biomass burning and
CO2 from fossil fuel. When taking up CO2 during photosynthesis,
vegetation discriminates against i3C and i4C. Consequently, vege-
tation, humus, and fossil fuels are depleted in i3C. Vegetation and
humus are similarly depleted in t4C, but because i4C iS not stable,
there is no i4C in fossil fuels. As a consequence, when one oxidizes
vegetation and humus versus fossil fuels, there are different dilution
factors operating. Thus, given the period of fossil fuel combustion, a
record of atmospheric isotopic ratios, the uptake of CO2 by oceans,
en c! estimates of the fractionation curing CO2 transfer from air to
sea and from air to terrestrial vegetation, it is possible to provide
solid checks on any mode! of the CO2 system.
Similarly, i3CH4 measurements are available and provide a strin-
gent test of moclels seeking to account for the recent rise in CH4 as
wed as providing invaluable clues as to the nature of the processes
responsible for the rise. There are indications that the preindustrial
source of CH4 was isotonically lighter, by about 2 percent. An ad-
equate model for CH4 must account for the isotopic composition of
the preindustrial source and for the enhanced recent production of
13CH4.
The ice cores also record anthropogenic disturbances in the cycles
of nitrogen and sulfur. Industrial sources of NO3- and SO4-- are
seen clearly in cores from Greenland. These data are especially useful,
in combination with general circulation models of the atmosphere, in
assessing the Tong-term impact of human activities. Measurements
of NO3- and SO4-- in mid-latitude and tropical latitude glacial
reservoirs can also be useful in this context.
The Tong-term record of change is equally illuminating. Studies
of gases trapped in polar ice cores have shown that the level of atmo-
spheric CO2 is Tow, about 200 ppm, in glacial times, rising to about
280 ppm during interglaciais. It is generally assumed that variability
in CO2 on such time scales must reflect changes in the function of the
ocean, since the quantity of carbon stored in the ocean vastly exceeds
that in the combined reservoirs represented by the atmosphere, soils,
and terrestrial biospheres. However, evidence that CH4 appears to
track climate is intriguing and puzzling. The concentration of CH4
reaches as Tow as 0.3 ppm at peak glacial conditions. Since terres-
trial systems are thought to play a dominant role in production of
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CH4- in contrast to the case of CO2, where exchange with the ocean
is important we expect that the new data on CH4, in combination
with pollen records allowing reconstruction of the geographic dis-
tribution of biomes, will permit valuable information to be drawn
concerning the past condition of the terrestrial biosphere.
Measurements of CH4 in combination with data on H202 and
NO3- should also provide clues to the changes that may have taken
place in the chemistry of the atmosphere in the past. In turn, such
studies will broaden the perspective of atmospheric chemistry, en-
hancing our ability to assess the present and hopefully predict the
future. Measurements of atmospheric species interpreted in this man-
ner can be used to monitor the metabolism of the global biosphere
and can provide a focus for a wide range of paleo-investigations.
SUMMARY OF RESEARCH OBJECTIVES
The detailed research needs to understand the biogeochemical
component of global change as described above can be summarized
in terms of the following general objectives:
To develop a better understanding of the current disposition
of the major biogeochemical elements. This requires better definition
of the quantities of carbon, nitrogen, phosphorus, and sulfur stored
· ~
in mayor ecosystems.
~ To develop a Tong-term database documenting changes in
environmental parameters that affect rates of nutrient cycling, in-
cluding a record of changes in the geographical distribution of major
ecosystems and their capacities as storage reservoirs for carbon, ni-
trogen, phosphorus, and sulfur.
~ To enhance understanding of processes regulating disposition
of nutrients in selected terrestrial ecosystems. This will require care-
fully crafted experimental strategies using a variety of approaches,
including passive observations of natural systems, selected manipu-
lation of natural systems, studies of large and small enclosures, and
selected laboratory investigations. Experimental strategies should be
designed to enhance understanding of how cycling of biogeochemical
elements in specific terrestrial ecosystems might respond to changes
in physical and chemical climate.
To define the changes in fluvial chemistry that might occur
as a consequence of changes in land use patterns. Riverine and lake
studies can provi(le an integrated record of the large-scale impact
of changes in watersheds. Such studies can also contribute to a
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better understanding of processes regulating transfer of nutrients to
estuaries, coastal ecosystems, and ultimately to the ocean.
· To improve understanding of the factors regulating fixation
and denitrification in the ocean. Process studies to address this
objective are needed.
~ To improve understanding of controls on marine phosphate
and to better define the influence of nutrient cycling in the ocean
on the level of atmospheric CO2. Processes at high latitudes merit
special attention in this respect.
~ To quantify sources and sinks of important greenhouse gases
such as CO2, CH4, and N2O and to define the response of the bio-
sphere to changes in atmospheric composition. Studies of atmo-
spheric chemistry in combination with ecosystem investigations are
needed, as are integrated studies of the troposphere and stratosphere.
· To use the archives of the paleoenvironment preserved in ice
and sediments to help develop and test models of the cycling of major
biogeochemical elements and the feedbacks and linkages.
REFERENCES
National Research Council. 1984. Global Tropospheric Chemistry. Washington, D.C.:
National Academy Press.
National Research Council. 1985. Goals and objectives for the global hydrologic cycle.
Chapter 6 in A Strategy for Earth Science from Space in the 1980's and 1990's
Part II: Atmosphere Ed Interactions with the Solid Earth, Oceans, and Biota.
Washington, D.C.: National Academy Press.
National Research Council. 1986. Global Change in the Geosphere-Biosphere: Initial
Priorities for an IGBP. Washington, D.C.: National Academy Press.
University Corporation for Atmospheric Research. 1986. Global Tropospheric Chem-
istry: Plans for the U.S. Research Effort. Once for Interdisciplinary Earth
Studies Report 3. Boulder, Colo.
APPENDS: WORKING GROUP
ON BIOGEOCHEMICAL DYNAMICS
February 20-21, 1988
Harvard University
Cambridge, Massachusetts
Michael B. McElroy, Harvard University, chairman
Fakhri A. Bazzaz, Harvard University
Edward Boyle, Massachusetts Institute of Technology
William C. Clark, Harvard University
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68
.
Margaret Davis, University of Minnesota
John Edmonds, Massachusetts Institute of Technology
Lewis Fox, Harvard University
James J. McCarthy, Harvard University
John Torrey, Harvard University
Peter Vitousek, Stanford University
Representative terms from entire chapter:
carbon fixation
