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Why should we believe 210Pb sediment geochronologies?
Journal of Environmental Radioactivity 55 (2001) 121–123
Millennial Editorial Series We continue here the series of invited editorials by members of the journal’s board. The invitation was for each author to contribute a short article describing a topic of particular interest or giving a personal view on the state of radioecology. The articles are being published in order of their receipt by JER. They obviously reflect the personal opinions of the authors. We hope that the series is provoking thought and discussion amongst readers (who are encouraged to send in their own viewpoint articles).
Why should we believe 210Pb sediment geochronologies? John Norton Smith Bedford Institute of Oceanography, Fisheries and Oceans Canada, Dartmouth, N.S., Canada B2Y 4A2
The authors of the first paper using 210Pb to date marine sediments (Koide, Bruland & Goldberg, 1973) had the good sense to validate their method by performing their measurements in varved sediments from the Santa Barbara Basin, off California, for which time-stratigraphic horizons had already been well established. Robbins and Edgington (1975) subsequently pointed out that most sediment regimes undergo mixing or bioturbation in the upper 4 cm, mainly as the result of the foraging and physiological activities of benthic organisms. They showed that it was necessary to incorporate a simulation of this mixing process into their tracer flux model and then confirm the accuracy of their model using an independent time-stratigraphic marker, such as fallout 137Cs. Regrettably, since the early days marking the introduction of 210Pb as a geochronometer, these lessons have been frequently forgotten or ignored. It is now common to read articles published in very reputable international journals where the sedimentation rates are calculated from 210 Pb distributions with little regard for the possibility, or even probability, that the sediments have undergone mixing or that the assumptions for dating are met. In most cases there is little concern about validation using independent tracers. In some instances, 210Pb ‘‘dating’’ is treated with such casual indifference that the primary data are not even reported, with dates simply being affixed to the sediment depth axis. Many authors convince themselves that 210Pb distributions showing little nearsurface flattening represent exponentially decreasing concentrations with depth and E-mail address: [email protected] (J.N. Smith). 0265-931X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 5 - 9 3 1 X ( 0 0 ) 0 0 1 5 2 - 1
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J.N. Smith / J. Environ. Radioactivity 55 (2001) 121–123
therefore reflect sediment regimes that have undergone no post-depositional mixing. One only has to refer to Robbins’s (1978) seminal and prophetic (aptly anticipating the present day misapplications of 210Pb as a geochronometer) review article to appreciate how various combinations of single-particle sedimentation and post-depositional mixing can conspire to generate many different types of ‘‘exponential’’ 210Pb distributions. Indeed, it is unpleasant to contemplate the interpretation of contaminant profiles in the presence of mixing, because it then becomes impossible to establish a definitive geochronology for the sediments. Instead, one must rely on model simulations of the mixing/sedimentation process using trial contaminant input functions in order to reproduce the experimental results. One can then no longer arrange sediment deposition dates neatly along the sediment depth axis, thereby spoiling a convenient analogue to tree ring dating and a simple interpretation of the history of environmental contamination that is so readily accessible to managers and the public. Inadequate applications of the use of 210Pb in this context frequently go unnoticed, because the investigators invariably have one point right. The surface of the sediment always reflects the date of sampling if the core has been collected properly and, as a result, the vertical distribution of contaminants usually supports our intuitive sense of contaminant inputs during the 19th–20th century. Regardless of the rate or depth of mixing or how inappropriate the sedimentation model, the concentrations of contaminants (Hg, Pb, PAHs, etc.) will generally increase from background values at a sediment depth somewhere in the 19th or 20th century (according to the 210Pb interpretation), pass through a maximum and begin to decline near the top of the core. This general trend simply reflects increased inputs of contaminants with industrialisation at the turn of the century and the more recent cutbacks in pollutant releases triggered by a growing awareness and sensitivity to environmental health concerns beginning in the 1960 s. However, this superficial agreement with general trends of environmental contamination during the 20th century frequently masks some of the basic inadequacies of the 210Pb data and a misinterpretation of both relative rates of mixing and sediment accumulation and of the fundamental assumptions of the model. This almost invariably leads to the reporting of incorrect dates for anthropogenic impacts on the environment, misunderstandings regarding the fundamental processes that govern particle transport in lake and marine systems and exaggerated expectations concerning scientists’ ability to ‘‘date’’ modern sediment regimes. In order to correct this state of affairs, a single clear protocol should be established by research journals for the acceptance of papers that rely on 210Pb dating to establish a sediment core geochronology. The 210Pb geochronology must be validated using at least one independent tracer that separately provides an unambiguous timestratigraphic horizon. This should be considered as fundamental to the validation of 210 Pb sedimentation models inasmuch as the use of radioactive tracers and standards is to the quality control and verification of analytical methodologies. Independent validation of 210Pb geochronologies must become an integral part of the overall experimental methodology. If the validation is inconclusive, then either a more appropriate particle transport model must be formulated and applied to the
J.N. Smith / J. Environ. Radioactivity 55 (2001) 121–123
123
interpretation of the experimental results or the core must be considered to be undateable. Only once journals have adopted a protocol of this nature will the scientific community be justified in having confidence in the 210Pb geochronology literature. Note that acceptance of this protocol requires that (1) at the time of review all authors submitting manuscripts must be expected to provide figures of the data or a hard-copy appendix containing the data and a description of the methods of calculation, including verification, and (2) the editors publish the figures and/or this appendix in smaller type or provide an easily accessible electronic version for readers1.
References Koide, M., Bruland, K., & Goldberg, E. D. (1973). Th-228/Th-232 and Pb-210 geochronologies in marine and lake sediments. Geochimica et Cosmochimica Acta, 37, 1171–1187. Robbins, J. A., & Edgington, D. N. (1975). Determination of recent sedimentation rates in Lake Michigan using 210Pb and 137Cs. Geochimica et Cosmochimica Acta, 39, 285–304. Robbins, J. A. (1978). Geochemical and geophysical applications of radioactive lead isotopes. In J. O. Nriagu, Biochemistry of Lead (pp. 285–393). Amsterdam: Elsevier.
1
Editor’s Footnote: So reasonable and correct seems the above request that we will henceforth implement the requested protocol within JER’s review process. We encourage other journals to follow suit.
Millennial Editorial Series We continue here the series of invited editorials by members of the journal’s board. The invitation was for each author to contribute a short article describing a topic of particular interest or giving a personal view on the state of radioecology. The articles are being published in order of their receipt by JER. They obviously reflect the personal opinions of the authors. We hope that the series is provoking thought and discussion amongst readers (who are encouraged to send in their own viewpoint articles).
Why should we believe 210Pb sediment geochronologies? John Norton Smith Bedford Institute of Oceanography, Fisheries and Oceans Canada, Dartmouth, N.S., Canada B2Y 4A2
The authors of the first paper using 210Pb to date marine sediments (Koide, Bruland & Goldberg, 1973) had the good sense to validate their method by performing their measurements in varved sediments from the Santa Barbara Basin, off California, for which time-stratigraphic horizons had already been well established. Robbins and Edgington (1975) subsequently pointed out that most sediment regimes undergo mixing or bioturbation in the upper 4 cm, mainly as the result of the foraging and physiological activities of benthic organisms. They showed that it was necessary to incorporate a simulation of this mixing process into their tracer flux model and then confirm the accuracy of their model using an independent time-stratigraphic marker, such as fallout 137Cs. Regrettably, since the early days marking the introduction of 210Pb as a geochronometer, these lessons have been frequently forgotten or ignored. It is now common to read articles published in very reputable international journals where the sedimentation rates are calculated from 210 Pb distributions with little regard for the possibility, or even probability, that the sediments have undergone mixing or that the assumptions for dating are met. In most cases there is little concern about validation using independent tracers. In some instances, 210Pb ‘‘dating’’ is treated with such casual indifference that the primary data are not even reported, with dates simply being affixed to the sediment depth axis. Many authors convince themselves that 210Pb distributions showing little nearsurface flattening represent exponentially decreasing concentrations with depth and E-mail address: [email protected] (J.N. Smith). 0265-931X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 5 - 9 3 1 X ( 0 0 ) 0 0 1 5 2 - 1
122
J.N. Smith / J. Environ. Radioactivity 55 (2001) 121–123
therefore reflect sediment regimes that have undergone no post-depositional mixing. One only has to refer to Robbins’s (1978) seminal and prophetic (aptly anticipating the present day misapplications of 210Pb as a geochronometer) review article to appreciate how various combinations of single-particle sedimentation and post-depositional mixing can conspire to generate many different types of ‘‘exponential’’ 210Pb distributions. Indeed, it is unpleasant to contemplate the interpretation of contaminant profiles in the presence of mixing, because it then becomes impossible to establish a definitive geochronology for the sediments. Instead, one must rely on model simulations of the mixing/sedimentation process using trial contaminant input functions in order to reproduce the experimental results. One can then no longer arrange sediment deposition dates neatly along the sediment depth axis, thereby spoiling a convenient analogue to tree ring dating and a simple interpretation of the history of environmental contamination that is so readily accessible to managers and the public. Inadequate applications of the use of 210Pb in this context frequently go unnoticed, because the investigators invariably have one point right. The surface of the sediment always reflects the date of sampling if the core has been collected properly and, as a result, the vertical distribution of contaminants usually supports our intuitive sense of contaminant inputs during the 19th–20th century. Regardless of the rate or depth of mixing or how inappropriate the sedimentation model, the concentrations of contaminants (Hg, Pb, PAHs, etc.) will generally increase from background values at a sediment depth somewhere in the 19th or 20th century (according to the 210Pb interpretation), pass through a maximum and begin to decline near the top of the core. This general trend simply reflects increased inputs of contaminants with industrialisation at the turn of the century and the more recent cutbacks in pollutant releases triggered by a growing awareness and sensitivity to environmental health concerns beginning in the 1960 s. However, this superficial agreement with general trends of environmental contamination during the 20th century frequently masks some of the basic inadequacies of the 210Pb data and a misinterpretation of both relative rates of mixing and sediment accumulation and of the fundamental assumptions of the model. This almost invariably leads to the reporting of incorrect dates for anthropogenic impacts on the environment, misunderstandings regarding the fundamental processes that govern particle transport in lake and marine systems and exaggerated expectations concerning scientists’ ability to ‘‘date’’ modern sediment regimes. In order to correct this state of affairs, a single clear protocol should be established by research journals for the acceptance of papers that rely on 210Pb dating to establish a sediment core geochronology. The 210Pb geochronology must be validated using at least one independent tracer that separately provides an unambiguous timestratigraphic horizon. This should be considered as fundamental to the validation of 210 Pb sedimentation models inasmuch as the use of radioactive tracers and standards is to the quality control and verification of analytical methodologies. Independent validation of 210Pb geochronologies must become an integral part of the overall experimental methodology. If the validation is inconclusive, then either a more appropriate particle transport model must be formulated and applied to the
J.N. Smith / J. Environ. Radioactivity 55 (2001) 121–123
123
interpretation of the experimental results or the core must be considered to be undateable. Only once journals have adopted a protocol of this nature will the scientific community be justified in having confidence in the 210Pb geochronology literature. Note that acceptance of this protocol requires that (1) at the time of review all authors submitting manuscripts must be expected to provide figures of the data or a hard-copy appendix containing the data and a description of the methods of calculation, including verification, and (2) the editors publish the figures and/or this appendix in smaller type or provide an easily accessible electronic version for readers1.
References Koide, M., Bruland, K., & Goldberg, E. D. (1973). Th-228/Th-232 and Pb-210 geochronologies in marine and lake sediments. Geochimica et Cosmochimica Acta, 37, 1171–1187. Robbins, J. A., & Edgington, D. N. (1975). Determination of recent sedimentation rates in Lake Michigan using 210Pb and 137Cs. Geochimica et Cosmochimica Acta, 39, 285–304. Robbins, J. A. (1978). Geochemical and geophysical applications of radioactive lead isotopes. In J. O. Nriagu, Biochemistry of Lead (pp. 285–393). Amsterdam: Elsevier.
1
Editor’s Footnote: So reasonable and correct seems the above request that we will henceforth implement the requested protocol within JER’s review process. We encourage other journals to follow suit.