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Chapter 10 Summary
CHAPTER 10
SUMMARY
We have treated a wide-ranging variety of topics in this review. Perhaps the best way to summarize the basic information and conclusions we have reached here is to present an itemized account which follows the chronology of the study itself. (1)The integration of modern paleomagnetic, radiometric, and biostratigraphic studies has provided an accurate geochronological framework for the past ten million years -the Late Neogene. (2) Marine zones based on calcareous and siliceous planktonic organisms are recognized from the Subarctic to Subantarctic regions (60"N to 60"s) and their correlation to the paleomagnetic time-scale is feasible in some detail over the past 5 m.y. Zones in sediments older than 5 m.y. are primarily developed in tropical-temperate regions, and correlation with high-latitude regions awaits further research. (3) Late Neogene chronostratigraphic boundaries can be recognized by reinforcing multiple paleontologic criteria and the following ages are assigned to them: Pliocene/Pleistocene (base Calabrian): ca. 1.6-1.8 m.y. Zanclian/Piacenzian (Early/Late Pliocene): ca. 3.3 m.y. Miocene/Pliocene (Messinian/Zanclian): ca. 5.0 m.y. Tortonian/Messinian: ca. 6.6 m.y. Serravallian/Tortonian (Middle/Late Miocene): ca. 10.5-10.7 m.y.
(4)Discrepancies in the literature regarding the age and position of the Miocene/Pliocene and Pliocene/Pleistocene boundaries, as well as other boundaries lower in the chronostratigraphic hierarchy, are discussed and shown to be due primarily to different interpretations of paleontologic criteria, incorrect biostratigraphic correlation and calibration to inaccurate radiometric dates. (5) Correlation and calibration of Late Neogene continental mammal ages to marine stages is possible with a relatively high degree of accuracy. A sequence of major events in the marine plankton and continental mammals is shown for the past 15 m.y. (6) Late Neogene earth history must be viewed within the framework of a cooling climatic trend which accelerated during the Pleistocene. Although climatic changes characterize the Pleistocene history of the globe, they cannot be used in delineating the age of its lower boundary. Polar ice built
166
up great thicknesses in Antarctica much earlier than in the Arctic region. Initial ice-cap formation in Antarctica may be older than 40 m.y., and the presence of thick ice sheets is indicated throughout the Neogene. A great. advance of grounded shelf ice is linked to Late Miocene Antarctic cooling and the recession of this ice may be correlated to initial Pliocene inundation of the desiccated Mediterranean Basin. Floating ice, calved from sea-level glaciers first appeared in the North Atlantic and the North Pacific about 3.0 m.y. ago. The “long school” of glacial chronology which puts the continental glacial history of Europe and North America in a chronological framework in excess of 2 m.y. is based on false assumptions linking evidence for initial cooling and montane glacial advances with the initiation of major temperate-region continental ice sheets. The evidence of opinion is that Neogene cooling was progressive, with relatively rapid decrements of lower temperature approximately 1.5-1.2 m.y. ago and again from 0.9 to 0.5 m.y. There is no well-known calibrated evidence to link the base of the Calabrian (beginning of the Pleistocene) about 1.8 m.y. ago with a marked cooling event, and all indications point to the first temperate glaciation at a significantly younger age with the next general lowering of the average temperature. (7) A general correlation between North American and European paleoclimatological sequences, based on radiometric and paleomagnetic calibration of mammalian biostratigraphy, appears to be corroborated by the deepsea paleoclimatological record. A salient feature of this correlation is support for earlier research which suggested, contrary to conventional views, that the first two North American glaciations (Nebraskan and Kansan) are represented in Central European climatological history by cold intervals without major lowland glaciation (Donau, Gunz). Details of this correlation bring out the fact that far too many former correlations were naive in supposing that glaciation was independent of regional variations in topography, meteorology or latitude, or that the influence of changing climate on one set of evidence might show different apparent intensities than on another coeval set. In consequence of this it is emphasized that the first continental lowland glaciation in Central Europe, the Mindel, is equivalent to the most intense glacial conditions of the Pleistocene and to the Illinoian glaciation of North America. This interval of maximum cold climate extended from ca. 0.6 to 0.45 m.y. in deep-sea cores and is in agreement with dates on the Mindel and Illinoian continental deposits. (8) The principle that Northern Hemisphere climate cycles were, in general, synchronous on both continental masses, is upheld, but the assumption that correlated climate cycles had comparable absolute temperature levels is shown to be insufficient. (9) The integration of biostratigraphic data from the marine and continental rock record and its calibration to an ordinal time-scale provides the appropriate background for interpretive studies in the historical geological evolution of our earth - in this case over the past 10-15 million years.
SUMMARY
We have treated a wide-ranging variety of topics in this review. Perhaps the best way to summarize the basic information and conclusions we have reached here is to present an itemized account which follows the chronology of the study itself. (1)The integration of modern paleomagnetic, radiometric, and biostratigraphic studies has provided an accurate geochronological framework for the past ten million years -the Late Neogene. (2) Marine zones based on calcareous and siliceous planktonic organisms are recognized from the Subarctic to Subantarctic regions (60"N to 60"s) and their correlation to the paleomagnetic time-scale is feasible in some detail over the past 5 m.y. Zones in sediments older than 5 m.y. are primarily developed in tropical-temperate regions, and correlation with high-latitude regions awaits further research. (3) Late Neogene chronostratigraphic boundaries can be recognized by reinforcing multiple paleontologic criteria and the following ages are assigned to them: Pliocene/Pleistocene (base Calabrian): ca. 1.6-1.8 m.y. Zanclian/Piacenzian (Early/Late Pliocene): ca. 3.3 m.y. Miocene/Pliocene (Messinian/Zanclian): ca. 5.0 m.y. Tortonian/Messinian: ca. 6.6 m.y. Serravallian/Tortonian (Middle/Late Miocene): ca. 10.5-10.7 m.y.
(4)Discrepancies in the literature regarding the age and position of the Miocene/Pliocene and Pliocene/Pleistocene boundaries, as well as other boundaries lower in the chronostratigraphic hierarchy, are discussed and shown to be due primarily to different interpretations of paleontologic criteria, incorrect biostratigraphic correlation and calibration to inaccurate radiometric dates. (5) Correlation and calibration of Late Neogene continental mammal ages to marine stages is possible with a relatively high degree of accuracy. A sequence of major events in the marine plankton and continental mammals is shown for the past 15 m.y. (6) Late Neogene earth history must be viewed within the framework of a cooling climatic trend which accelerated during the Pleistocene. Although climatic changes characterize the Pleistocene history of the globe, they cannot be used in delineating the age of its lower boundary. Polar ice built
166
up great thicknesses in Antarctica much earlier than in the Arctic region. Initial ice-cap formation in Antarctica may be older than 40 m.y., and the presence of thick ice sheets is indicated throughout the Neogene. A great. advance of grounded shelf ice is linked to Late Miocene Antarctic cooling and the recession of this ice may be correlated to initial Pliocene inundation of the desiccated Mediterranean Basin. Floating ice, calved from sea-level glaciers first appeared in the North Atlantic and the North Pacific about 3.0 m.y. ago. The “long school” of glacial chronology which puts the continental glacial history of Europe and North America in a chronological framework in excess of 2 m.y. is based on false assumptions linking evidence for initial cooling and montane glacial advances with the initiation of major temperate-region continental ice sheets. The evidence of opinion is that Neogene cooling was progressive, with relatively rapid decrements of lower temperature approximately 1.5-1.2 m.y. ago and again from 0.9 to 0.5 m.y. There is no well-known calibrated evidence to link the base of the Calabrian (beginning of the Pleistocene) about 1.8 m.y. ago with a marked cooling event, and all indications point to the first temperate glaciation at a significantly younger age with the next general lowering of the average temperature. (7) A general correlation between North American and European paleoclimatological sequences, based on radiometric and paleomagnetic calibration of mammalian biostratigraphy, appears to be corroborated by the deepsea paleoclimatological record. A salient feature of this correlation is support for earlier research which suggested, contrary to conventional views, that the first two North American glaciations (Nebraskan and Kansan) are represented in Central European climatological history by cold intervals without major lowland glaciation (Donau, Gunz). Details of this correlation bring out the fact that far too many former correlations were naive in supposing that glaciation was independent of regional variations in topography, meteorology or latitude, or that the influence of changing climate on one set of evidence might show different apparent intensities than on another coeval set. In consequence of this it is emphasized that the first continental lowland glaciation in Central Europe, the Mindel, is equivalent to the most intense glacial conditions of the Pleistocene and to the Illinoian glaciation of North America. This interval of maximum cold climate extended from ca. 0.6 to 0.45 m.y. in deep-sea cores and is in agreement with dates on the Mindel and Illinoian continental deposits. (8) The principle that Northern Hemisphere climate cycles were, in general, synchronous on both continental masses, is upheld, but the assumption that correlated climate cycles had comparable absolute temperature levels is shown to be insufficient. (9) The integration of biostratigraphic data from the marine and continental rock record and its calibration to an ordinal time-scale provides the appropriate background for interpretive studies in the historical geological evolution of our earth - in this case over the past 10-15 million years.