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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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49 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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50 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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52 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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53 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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55 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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56 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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57 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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58 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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59 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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62 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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64 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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66 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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67 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