Ocean, the North Pacific, and the equatorial. Pacific, where sediments are ... tivity tool met in June 1996 at a workshop in. Brest, France. .... studies of diatoms' life cycle will help us bet ter interpret the ... Building, Palisades, N Y. The failure to ...
Eos, Vol. 77, No. 49, December 3, 1996 sphere and will allow detection of sound waves produced by very small explosions anywhere in the oceans. These data also should provide new insights into submarine earthquakes, slumps, and volcanoes. The radiochemical network will be the first comprehensive international system to monitor atmospheric radionuclides, allow ing greatly improved global monitoring of nu clear particulates and gases, both natural and man-made.
"These various signals, along with the quarry blast, and earthquake signals re corded by the new seismic system, will com prise the 'background noise' against which w e are checking continuously to ensure that no explosion has been conducted," said Bratt. "But the high-fidelity recording of these processes will affect many research ar eas relevant to AGU." The International Monitoring System and its intended and potential benefits to science
Opal Studied as a Marker of Paleoproductivity PAGE 491 Biogenic silica, better known as opal, is one of the three biogenic components of pe lagic sediments, along with carbonate and or ganic matter, and it is a powerful tool for understanding the carbon cycle of the pre sent and past. Opal is formed in surface wa ters during the photosynthesis of siliceous phytoplankton such as diatoms. After cells die, they sink through the water column; the fraction that is not dissolved ultimately is bur ied in the underlying sediments. In the mod ern ocean, siliceous phytoplankton generates more than 50% of the biological pump of CO2. Like carbonate and organic carbon, opal can be used to reconstruct past surface ocean processes. This is especially true in coastal upwellings and in high-nutrient/lowchlorophyll regions such as the Southern Ocean, the North Pacific, and the equatorial Pacific, where sediments are high in opal content due to the abundance of diatoms. Widespread areas of the world ocean that are vitally important in understanding climate— for example, the Southern Ocean—contain almost no carbonate or residual organic mat ter. In these areas, opal provides a good alter native for paleoproductivity reconstructions. Fifty scientists specializing in the modern biogeochemical cycle of silica, paleoceanography, and the use of opal as a paleoproduc tivity tool met in June 1996 at a workshop in Brest, France. They examined the potential of opal as a paleoceanographic tracer that can be used with other proxies of export pro duction (for example, organic carbon, Ba, Pa/Th) and nutrient utilization (for example, d^N.d^Si.Cd/Ca). Discussion at the workshop emphasized that rigorous interpretation of the opal re cord requires complete knowledge of the modern processes that control the produc tion, sinking, and accumulation of biogenic silica in the sediments. This message also holds for the other tracers of paleoproductiv
ity that were discussed. The workshop pro vided the first opportunity to bring together paleoceanographers and modern biogeochemists, and it demonstrated the urgent need for closer collaboration between the two communities toward a common objec tive: a better understanding of the silica cycle in modern and past oceans. One cannot re construct the past before completely under standing the present.
O p a l a n d Paleoproductivity Reconstruction Several parameters related to opal tell us about the ocean's past. Studies of changes of species assemblages provide important infor mation on export productivity in coastal up wellings as well as on opal dissolution in the water column and the sediments. They also provide new insights into the dynamics of oceanic frontal structures. For example, in the equatorial Pacific, the frontal zone con vergence signal is recorded by different dia tom species than is the upwelling primary productivity signal. Indeed, another impor tant message articulated at the workshop is that micropaleontologists and biogeochemists have much to gain from working to gether. Another reason for using opal as a paleo ceanographic tool is that the distribution of opal accumulation rate along the equatorial Pacific US-JGOFS transect reflects the distri bution of surface primary productivity of the equatorial Pacific surface waters much better than the distribution of organic carbon does. To give another example, the distribution of diatom valves in the Arctic Ocean closely re flects surface silicic acid distribution and thus potential silica production.
N e e d for Multiproxy A p p r o a c h Opal, however, cannot be used alone to reconstruct past surface ocean productivity for two reasons. First, proxies of nutrient utili
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will be the focus of the AGU Fall Meeting ses sions entitled CTBT Verification Regime: Monitoring the Earth System (U71B, U72C, and U l 1A). Another set of sessions, Radioactive W a s t e H a z a r d s : L e g a c y of the C o l d W a r (U12C and U 2 1 B ) will ad dress other ways that geophysicists are helping society c o p e with 50 years of the nuclear arms race.—Michael Carlowicz, with assistance from Thorne Lay and Steven Bratt
zation or particle flux can be used to help re construct opal fluxes themselves. Second, given the decoupling between the cycles of carbon and silicon, opal can provide infor mation on paleoproductivity due to siliceous phytoplankton, but other proxies of export production (for example, organic carbon) are also required to provide information on the nonsiliceous fraction of the primary pro duction. Combining opal with these proxies in a multiproxy approach is the best way to re construct past surface ocean history. The multiproxy approach also reduces the uncertainties that arise when a single proxy is used. Each of the proxies discussed at the workshop is subject to uncertainties, mainly because w e lack a c o m p l e t e under standing of the processes that control their cycles in the modern o c e a n . T e m p o ral and regional variations in opal preser vation, d e p e n d e n c y of authigenic uranium accumulation rates on bottom water oxygen concentration, and differen tial affinity of mineral phases for Pa are examples of mechanisms that c o m p l i c a t e quantitative interpretations of sedimen tary records in terms of paleoproductiv ity. Comparison of several of these records at a given location should help provide a clearer picture of past changes in oceanic productivity. The overall goal of this research is to com bine several properly calibrated proxies whose cycles in the modern o c e a n are suf ficiently well understood, which will reduce uncertainties about how to interpret their re cords. A specific objective of the working group that met at the workshop was to exam ine how recent improvements in our knowl edge of the modern biogeochemical cycle of silica may help paleoceanographers deci pher the sedimentary record of opal accumu lation rates.
M o d e r n Silica Cycle During the last decade, substantial pro gress has been made toward understanding the processes that control the modern ma rine biogeochemical cycle of silicon. Realis tic regional and global models have been built. The development of Si stable and radio-
Eos, Vol. 77, No. 49, December 3, 1996 active isotope techniques to measure bio genic silica production and dissolution have given us a much better idea of where opal is being produced. The processes that control opal production are also etter understood when the role played by silicic acid availabil ity is considered. Indeed, silicic acid is often neglected as a potential limiting nutrient in pelagic environments, despite its key role in the control of phytoplankton succession and in the export of biogenic matter. Significant progress has been made re cently in the study of sediments, in particular by using flow-through dynamic techniques that quantify biogenic silica solubility and re activity. There are many important factors that should be incorporated in models de scribing the early diagenesis of biogenic opal. These include the quantification of the downcore decrease in silica reactivity due to surface aging or growth of surface coatings; the importance of aluminum in controlling silica solubility; the role of the opal rain rate and the importance of the detrital material in controlling asymptotic silicic acid concentra tion; and the normalization of opal flux to the known flux of , which is scavenged from the water to correct for sediment focusing or winnowing.
Despite substantial progress in these ar eas, w e still need a better understanding of several important aspects of the modern sili con cycle before opal can be used effectively as a proxy. The process of silicification needs to be described in detail if w e are to better un derstand the processes that control opal dis solution, and ecological and physiological studies of diatoms' life cycle will help us bet ter interpret the sedimentary record of sili ceous microfossils assemblages. In the water column, it is not yet clear whether or not graz ing acts as an accelerator of the biogenic par ticle flux or favors opal dissolution. Furthermore, high interannual variability, lat eral transport, and the importance of epi sodic events that produce radiolarian pulses or laminated diatom sediments are other ex amples of short timescale phenomena that may greatly influence the sediment record but are difficult to take into account. The need for further study of paleoproduc tivity proxies in the modern ocean led the participants to recommend that the meeting be used to launch a broad OPALEO project in which biogeochemists, micropaleontologists, and paleoceanographers will join ef forts through a series of seasonal cruises at particular sites that present a large range of
FORUM Tropospheric Chemistry: Global Change and Urba Neglect PAGE 490 During the past decade, major advances have been made in the field of tropospheric chemistry, especially with regard to ozone and associated species (reactive nitrogen, peroxides, and O H ) . These advances have been driven largely by a series of field measurement campaigns in locations ranging from Brazil and Hawaii to Green land and Nova Scotia. There is a major gap in the c o v e r a g e of the tropospheric re search program, h o w e v e r . O z o n e chemis try is not measured in urban and d o w n w i n d locations in the United States that are subject to violations of air quality standards. The key species for evaluating ozone chemistry (speciated reactive nitrogen and hydrogen peroxide) are measured exten sively at rural sites in the United States. Over
the past 5 years, more than 50 articles analyz ing measurements of ozone chemistry have appeared in [he Journal of Geophysical Re search and other peer-reviewed journals. Only two of these articles gave field measure ments for cities in the United States (Los An geles and Atlanta), and analyses of these measurements were far less extensive than those for the rural sites. The lack of attention to urban air exhib ited by the atmospheric research community is striking. For example, a recent field cam paign followed the evolution of the urban plume from the northeast corridor of the United States as it traveled across the Atlantic Ocean, but no field campaign has ever ana lyzed the chemistry of air within the north east corridor itself. It is almost as though highly polluted environments are outside the domain of interest of the atmospheric chemis try community.
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conditions for opal sedimentation. A followup meeting for the principal investigators of OPALEO is planned for early 1997. Sponsorship The workshop was organized under the umbrella of the European Commission, the North Atlantic Treaty Organization, the Ministere de l'Education Nationale, de l'Enseignement Superieur et de la recherche, the Centre National de la Recherche Scientifique (CNRS), the Conseil Regional de Bretagne, the University of Western Brittany, the Communaute Urbaine, and the town of Brest. All these partners are gratefully ac knowledged. The authors also wish to thank all the participants and the local organizing committee for participating in such a fasci nating and convivial meeting.—Olivier Ragueneau, Aude Leynaert, and Paul Treguer, Unite de Recherche Associee au CNRS 1513, Institut Universitaire Europeen de la Mer, Brest, France; David J. Demaster, Depart ment of Marine, Earth and Atmospheric Sci ences, North Carolina State University, Raleigh; and Robert F. Anderson, LamontDoherty Earth Observatory, Geochemistry Building, Palisades, N Y.
The failure to include urban and urbandownwind regions in field campaigns re flects the attitude among funding agencies that only rural and remote locations are wor thy of scientific investigation. The chemistry of ozone in urban areas is thought of as an af fair for regulatory policy rather than science. To the contrary, urban chemistry is every bit as challenging as the remote troposphere, and it is certainly worthy of attention from the best scientists in the field. Five years ago National Research Council recommended that a program of research into the chemistry of urban ozone be established and controlled independently of the U.S. Environmental Pro tection Agency. Despite this recommendation, the funding agencies that traditionally support atmospheric chemistry research continue to devote little effort to urban environments and instead focus on global-scale processes. At the AGU Fall Meeting, special sessions on the Southern Oxidants Study (A12E, A21D, and A 3 1 A ) measurement effort in Nash ville, Tenn., will illustrate the exciting science that can be done in urban environments. 1 invite all conference attendees to take the opportunity to learn about this frequently neglected sub ject.—Sanford Sillman, Department of Atmos pheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor
