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Tracers in the ocean
2902
Book Reviews
The extensive analyses of the inorganic and organic geochemistry of Messel oil shale make this volume a required reference and important reading for those involved in the diagenesis of inorganic and organic constituents in sediments. Messel oil shale is noteworthy because it is immature, containing a complex array of organic molecules as well as organically preserved plant and animal megafossils. While geochemistry and microscopy have been presented for one alga (Tet-
Tracers in the Ocean edited by H. Charnock, J. E. Lovelock, P. S. Liss, and M. Whitfield, The Royal Society, 1988, v t 236p., f48.00 (ISBN o-85403-350-5). THE ROYAL WCIET~ takes things at the flood. As the gigantic World Ocean Circulation Experiment prepares to slip its moorings, it is good to have a summary of the status of current thinking on the subject. In oceanographic parlance, tracers are physical or, more usually, chemical labels of individual water masses with particular histories. The ocean is ventilated by convective processes occurring in quite restricted locations and driven by the very delicate interplay of the local budgets of fresh water and heat. Water masses propagate away from their formation areas and into the interior of the oceans along quite well-defined trajectories. These were deduced in the 30’s and 40’s from large scale studies of the steady-state conservative tracers, temperature, and salinity, and a few nonconservative species such as dissolved oxygen and the essential nurtients. The general picture has held up for over 40 years while, of course, being refined in excruciating detail. However, since the tracers used are, or are assumed to be, at steady state this picture contains no time information. For that one needs soluble radioactive tracers with half lives comparable to the time scales of the processes being studied or else transient “dyes.” Nature gave us C- 14 and Ra-226 and the other natural series isotopes and considerately arranged their half lives to be what we needed. Man by ignorance, negligence, and routine foolishness produced the dyes. These include radioisotopes released by the weapons tests and, increasingly, by the nuclear fuel cycle, C-14, H-3, and Sr-90 in particular, and the various freons. Since the information content of a single seawater sample is vanishingly small, it has taken some marvellous developments in analytical chemistry to generate the vast amounts of data necessary to exploit these dye tracers. What makes tracer work especially interesting is that it requires close collaboration between chemists and physicists, something known well to be impossible. It should be pointed out to optimists that in a two-page review of the present volume in the first Eos of 1990, Willebrand and Wunsch make only a one-sentence reference to tracers. The physicists have come a long way from the “low, it moves” attitude to the natural world that gave them their start. They want to knowhow? Since a given parcel of water is subject to continuous advective and turbulent dissipation into its neighbours, any “age” assignment depends on a physical understanding of these processes. The reward for complete understanding of any natural system is a reliable estimate of its relevant time scales. Thus, the chemists are happy to know, for instance, by direct observation of Cs- I37 that it takes about ten years for the effluent from the Windscale reprocessing plant on the Irish Sea to transit to the region of deep convection east of Greenland and thence to propagate into the deep North Atlantic; the physicists worry that they can’t invert the concentration field to give the source function. It is this derangement of attitudes that gives the book its spice. It seems obligatory for volumes of this type to open with the discussion of yet another vast and inconclusive model of the carbon cycle. One of the things that prevent environmental scientists from having a good time is that about half the fossil CO, released simply vanishes without trace. Having no idea where it goes, embarrassment has turned to obsession. Sarmiento and his colleagues do a good job of outlining just how many things we have to know, and don’t, in order to model the oceanic carbon cycle. The discussion following is, as usual, full of sound and fury. There follows a review of tracer models and CO2 by Crane and then, preliminaries over, we launch into one of Jenkin’s long papers on the ventilation ofthe thermocline
ruedron), other papers in this volume illustrate the exceptionally preserved plants and animals that still are awaiting geochemical research. Florida Museum of Natural History Department of Natural Sciences University of Florida Gainesville, FL 32611. USA
David Dilcher
using tritium and He-3. This is profound stuff with direct bearing on productivity through upward mixing of nutrients into the photic zone. Roether and Fuchs who follow with a report of their own efforts in this area confine themselves to a short note that does not do their impressive data set justice. Then it’s back to CO2 again and a map of the gas-transfer coefficient for the world ocean based on observed and computed wind speeds. Then comes an oddly prevaricating critique of inverse models by Shepherd. This is followed by a rather tight-lipped discussion by Jenkins, Wunsch, and Minster. This section is concluded by a concise review of air-sea gas exchange by Liss. The more chemical section starts with a review of the REEs by Elderfield. These highly reactive elements are removed quite rapidly from the water column by scavenging onto sinking biogenic debris. Thus their distributions are sensitive to local sources and sinks. In this they are quite analogous to the non-steady state tracers. Burton then presents a general review of trace element geochemistry. With few exceptions the analyses of these species is still sufficiently cumbersome so that “chemical hydrography” remains a dream. Bacon then discusses the elegant work, largely his own, being done using the natural series radioisotopes to identify reactive sites in the ocean. Since much of the chemistry of the oceans is driven by organisms, living and dead, the three dimensional reactivity reflects the essentially two dimensional distribution of productivity in the surface waters and is surprisingly heterogeneous. Bacon provides several beautiful examples of this. Livingston then updates his spectacular exploitation of the Windscale effluents as tracers of the circulation of the surface and deep waters of the North Atlantic. As mentioned previously, he has shown that the communication time for dissolved material between the European shelf and the boundary current of the Northwest Atlantic is less than ten years. Over the last two hundred years huge amounts of other pollutants from the European mainland must also have followed this route, a realisation that gives one pause. In the concluding set of papers Bigg and Killworth use inverse modeling to tackle the central problem of tracer studies, especially the transients: where to sample and how often? Do we need one station per one degree square or per ten degree square? Should it have five samples in the vertical or a hundred? Should observations be repeated every year or every twenty years? Without objective answers to these questions it is hard to get funding agencies to break with the habits of mortgage companies and play the horses like they are supposed to. Watson and Ledwell report on their purposeful release of sulphur hexafluoride, gaseous teflon, as a tracer of local processes. The sampling of streaky and erratic distributions is carefully described. Wunsch has the last word. He shows that he can ventilate the thermocline by Eckman pumping and that the tracers don’t add anything in the way of constraints. He tries hard to find a use for transient tracers but can’t, really. So it goes. The discussion was active. To end on a note of puzzlement; where were the freons? They are the ultimate transient tracer, measurable on shipboard on small volumes of water as fast as the conventionally collected variables. Obsessive secrecy is a well-known feature of the He-3 business but there must be at least 100,000 unpublished freon measurements out there. Enough for another symposium, indeed. In the interim read this book as an excellent summary of our present understanding of the circulation of the oceans and as an illustration of the clash of two great cultures, physics and chemistry. Department of Earth, Atmospheric, and Planetary Sciences Massachusetts Institute of Technology Cambridge, MA 02139, USA
John M. Edmond
Book Reviews
The extensive analyses of the inorganic and organic geochemistry of Messel oil shale make this volume a required reference and important reading for those involved in the diagenesis of inorganic and organic constituents in sediments. Messel oil shale is noteworthy because it is immature, containing a complex array of organic molecules as well as organically preserved plant and animal megafossils. While geochemistry and microscopy have been presented for one alga (Tet-
Tracers in the Ocean edited by H. Charnock, J. E. Lovelock, P. S. Liss, and M. Whitfield, The Royal Society, 1988, v t 236p., f48.00 (ISBN o-85403-350-5). THE ROYAL WCIET~ takes things at the flood. As the gigantic World Ocean Circulation Experiment prepares to slip its moorings, it is good to have a summary of the status of current thinking on the subject. In oceanographic parlance, tracers are physical or, more usually, chemical labels of individual water masses with particular histories. The ocean is ventilated by convective processes occurring in quite restricted locations and driven by the very delicate interplay of the local budgets of fresh water and heat. Water masses propagate away from their formation areas and into the interior of the oceans along quite well-defined trajectories. These were deduced in the 30’s and 40’s from large scale studies of the steady-state conservative tracers, temperature, and salinity, and a few nonconservative species such as dissolved oxygen and the essential nurtients. The general picture has held up for over 40 years while, of course, being refined in excruciating detail. However, since the tracers used are, or are assumed to be, at steady state this picture contains no time information. For that one needs soluble radioactive tracers with half lives comparable to the time scales of the processes being studied or else transient “dyes.” Nature gave us C- 14 and Ra-226 and the other natural series isotopes and considerately arranged their half lives to be what we needed. Man by ignorance, negligence, and routine foolishness produced the dyes. These include radioisotopes released by the weapons tests and, increasingly, by the nuclear fuel cycle, C-14, H-3, and Sr-90 in particular, and the various freons. Since the information content of a single seawater sample is vanishingly small, it has taken some marvellous developments in analytical chemistry to generate the vast amounts of data necessary to exploit these dye tracers. What makes tracer work especially interesting is that it requires close collaboration between chemists and physicists, something known well to be impossible. It should be pointed out to optimists that in a two-page review of the present volume in the first Eos of 1990, Willebrand and Wunsch make only a one-sentence reference to tracers. The physicists have come a long way from the “low, it moves” attitude to the natural world that gave them their start. They want to knowhow? Since a given parcel of water is subject to continuous advective and turbulent dissipation into its neighbours, any “age” assignment depends on a physical understanding of these processes. The reward for complete understanding of any natural system is a reliable estimate of its relevant time scales. Thus, the chemists are happy to know, for instance, by direct observation of Cs- I37 that it takes about ten years for the effluent from the Windscale reprocessing plant on the Irish Sea to transit to the region of deep convection east of Greenland and thence to propagate into the deep North Atlantic; the physicists worry that they can’t invert the concentration field to give the source function. It is this derangement of attitudes that gives the book its spice. It seems obligatory for volumes of this type to open with the discussion of yet another vast and inconclusive model of the carbon cycle. One of the things that prevent environmental scientists from having a good time is that about half the fossil CO, released simply vanishes without trace. Having no idea where it goes, embarrassment has turned to obsession. Sarmiento and his colleagues do a good job of outlining just how many things we have to know, and don’t, in order to model the oceanic carbon cycle. The discussion following is, as usual, full of sound and fury. There follows a review of tracer models and CO2 by Crane and then, preliminaries over, we launch into one of Jenkin’s long papers on the ventilation ofthe thermocline
ruedron), other papers in this volume illustrate the exceptionally preserved plants and animals that still are awaiting geochemical research. Florida Museum of Natural History Department of Natural Sciences University of Florida Gainesville, FL 32611. USA
David Dilcher
using tritium and He-3. This is profound stuff with direct bearing on productivity through upward mixing of nutrients into the photic zone. Roether and Fuchs who follow with a report of their own efforts in this area confine themselves to a short note that does not do their impressive data set justice. Then it’s back to CO2 again and a map of the gas-transfer coefficient for the world ocean based on observed and computed wind speeds. Then comes an oddly prevaricating critique of inverse models by Shepherd. This is followed by a rather tight-lipped discussion by Jenkins, Wunsch, and Minster. This section is concluded by a concise review of air-sea gas exchange by Liss. The more chemical section starts with a review of the REEs by Elderfield. These highly reactive elements are removed quite rapidly from the water column by scavenging onto sinking biogenic debris. Thus their distributions are sensitive to local sources and sinks. In this they are quite analogous to the non-steady state tracers. Burton then presents a general review of trace element geochemistry. With few exceptions the analyses of these species is still sufficiently cumbersome so that “chemical hydrography” remains a dream. Bacon then discusses the elegant work, largely his own, being done using the natural series radioisotopes to identify reactive sites in the ocean. Since much of the chemistry of the oceans is driven by organisms, living and dead, the three dimensional reactivity reflects the essentially two dimensional distribution of productivity in the surface waters and is surprisingly heterogeneous. Bacon provides several beautiful examples of this. Livingston then updates his spectacular exploitation of the Windscale effluents as tracers of the circulation of the surface and deep waters of the North Atlantic. As mentioned previously, he has shown that the communication time for dissolved material between the European shelf and the boundary current of the Northwest Atlantic is less than ten years. Over the last two hundred years huge amounts of other pollutants from the European mainland must also have followed this route, a realisation that gives one pause. In the concluding set of papers Bigg and Killworth use inverse modeling to tackle the central problem of tracer studies, especially the transients: where to sample and how often? Do we need one station per one degree square or per ten degree square? Should it have five samples in the vertical or a hundred? Should observations be repeated every year or every twenty years? Without objective answers to these questions it is hard to get funding agencies to break with the habits of mortgage companies and play the horses like they are supposed to. Watson and Ledwell report on their purposeful release of sulphur hexafluoride, gaseous teflon, as a tracer of local processes. The sampling of streaky and erratic distributions is carefully described. Wunsch has the last word. He shows that he can ventilate the thermocline by Eckman pumping and that the tracers don’t add anything in the way of constraints. He tries hard to find a use for transient tracers but can’t, really. So it goes. The discussion was active. To end on a note of puzzlement; where were the freons? They are the ultimate transient tracer, measurable on shipboard on small volumes of water as fast as the conventionally collected variables. Obsessive secrecy is a well-known feature of the He-3 business but there must be at least 100,000 unpublished freon measurements out there. Enough for another symposium, indeed. In the interim read this book as an excellent summary of our present understanding of the circulation of the oceans and as an illustration of the clash of two great cultures, physics and chemistry. Department of Earth, Atmospheric, and Planetary Sciences Massachusetts Institute of Technology Cambridge, MA 02139, USA
John M. Edmond