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Climate modes of the phanerozoic
Geochimica
Pergamon
et Cosmochimica
Acta, Vol. 58, No. 9. p. 2 149-2 150, 1994 Convrizht -~.. - 0 1994 Elsevier Science Ltd Printed in the USA. All rights reserved 0016.7037/94 $6.00 + .OO
BOOK REVIEWS
Anorthosites by Lewis D. Ashwal. Springer-Verla& US $119.00
tracers. Flaws are few, and rarely vital. (A major work on gravity over the Nain anorthosite by Stephenson and Thomas is unfortunately amone the few works missed.1 Ashwal brines his own skillful synthesis to bear on the important features of anorthosite genesis. while giving due credit to previous work. Emplacement of massif anorthosites as mushes from Al-Fe magmas at the base of the crust is favored. The tectonic setting of these rocks is called the frontier of anorthosite research. Excellent tables of occurrences, and figures, adorn the text, and the final chapter is a critical and refreshing synthesis and summary of the whole. Among treatments of controversial topics that I found felicitous are the discussions of wet vs. dry magmas, and the short shrift (but good history) given to nutty theories of origin at page 74. One well-known author is caught out with six different analog origins of Archean anorthosite. The mere characterization of massif anorthosites that defeats most writers is graced with clever and competent decisions at pp. 82-83. The writing in places has the flavor of addressing the reader personally, as with Fielding inventing the novel in Tom Jones. My chief reaction on reading this book is how glad I am that it was Lew Ashwal who spent six years at this task and not I; the book is ever so much better as a result. Maybe it is the Tom Jones spirit in Lew himself that makes it so good.
1993, xix + 422 p.,
(ISBN O-387-55361-4).
THERE ARE SIX KINDS of the plagioclase-rich rock called anorthosite: Archean megacrystic, Proterozoic massif-type, those in layered mafic complexes, those of oceanic settings, inclusions in other igneous rocks, and extraterrestrial anorthosites. You should want to know about these widespread rocks, because they are of great interest to the evolution of planetary crusts, notably that of the Earth. Lew Ashwal visits and treats each of these types succinctly and very well in this book. Some of the best of a wide geographic selection of photos are his own; his footprints are all over the place. Readers will find his treatment of the rocks they know even-handed and highly competent. They will find his judgments quick, informed, and accurate: “This obviously incorrect idea . .” (p. 2); “This complex history is immediately apparent from the map .” (p. 38); “I am not sure it would be worth the effort” (p. 106). Oceanic varieties of anorthosite include dredge hauls and ophiolites; inclusions range from the Gardar dikes of Greenland to Hawaii to cognate inclusions in felsic intrusions. Lunar anorthosites receive the appropriate extended treatment, and a venture is made into the prospects for anorthosites on Mercury. Quite naturally, much of the book’s space is given to the Archean and massif anorthosites that have occupied much of Ashwal’s life in research. As expected, due attention is paid to geochronology, tectonic environment, magma types, phase equilibria, mineralogy, and isotopic
Department of’Geology and Geography University of Massachusetts at Amhevyt Amherst. MA 01003, CTSA
Climate Modes of the Phanerozoic by Lawrence A. Frakes, Jane E. Francis, and Jozef I. Syktus. Cambridge University Press 1992, 274 p., US $69.95 (ISBN O-52 l-36627-5 ).
S. A. Morse
Phanerozoic cycles (i.e.. four vs. three). This difference, however, is far from trivial. Insertion of a rather controversial cool mode in the middle of a balmy Mesozoic throws a monkey wrench in the major climatic cycles by shortening their periodicity from 300 Myr to 162 Myr, a figure that agrees quite well with the 150 Myr half Galactic cycle of G. Williams. The question of whether the agreement is just a felicitous coincidence, or is the result of a cause and effect relation is not answered by the authors. What are the criteria to establish the overall climate mode of a planet the size of Earth with permanent latitudinal climatic gradients, and what are the diagnostic markers of the cool and warm modes? According to these authors, presence/absence of ice-rafted debris indicating either permanent or seasonal ice at high latitudes dictates the climate mode of a planet. The wide arsenal of paleoclimatic indicators also include the classic Wegener-type clues such as coals, evaporites, shelf carbonates, eolian deposits, etc., that are supplemented by oxygen and carbon isotope records retrieved from stratigraphically continuous marine carbonate sediments. A cool mode is diagnosed on the basis of ( 1) considerable extent of high latitude land (obtained from paleomagnetic data), (2) long interval of cooling leading to high latitude glaciation, (3) marked increase in d “C, and (4) marked decrease in the abundance of evaporites. The advent of warm modes, indicated by the termination of ice rafting, was remarkably sudden and was accompanied by abrupt decreases in 613C. Whereas the markings of a cool mode are welldefined and assessed to be repetitive, the ones pertaining to the warm modes (i.e., rapid decrease in 6 ‘%, increasing deposition of organic matter, warming, and volcanism) are more diffuse, and each one of the four warm modes seems to have some exceptional characteristics. According to the authors, the variable rates of organic carbon accumulation acted as the principal toggle, switching the climate modes through a complicated circuitry whose components are tectonics, atmospheric CO2 levels, sea-level changes, nutrient supply, and ocean circulation. Consequently, the emphasis is on the 613C records that track variations in the partition of carbon between the inorganic
“SEEKING THE CAUSE of climate change is both an exciting activity and a difficult task, as borne out by the long history of enquiry and the scarcity of firm conclusions.” This sentence (p. 189) could serve as the rationale for this new endeavor by Frakes, Francis, and Syktus in paleoclimatology. Lawrence Frakes is an old hand in the business of deciphering the climate history of our planet. Many of us (including this author) found their motivation to engage in paleoclimatic research from reading his 1979 best-seller Climate Through Geologic Time. The more recent product of collaboration between Frakes, Francis, and Syktus differs in some important aspects from the previous solo contribution. To begin with, in the first ten chapters it updates and synthesizes a large body of paleoclimatic information collated over the past ten years. Equally important, it attempts to impose some order in the rather chaotic flood of data covering the history of climate for the past 600 Myr. The latter is achieved by establishing a framework of periodic Climate Modes during the Phanerozic, and then using it to compare and contrast the climates of the past, and to ease the task of recognition of some common causes of climate change. The authors are at their best in Chapter I 1, where they discuss the causes and chronologies of the climate changes. Unlike other recent contributions to geologic climates, Frakes et al. have recognized the importance of the Quaternary high frequency glacial-interglacial cycles as a “key to the past” and have provided a thorough review of the Quaternary climate history, including a well-rounded discussion of the causes and consequences of the still puzzling 11,000 year-old Younger Dryas climate oscillation. The organizational mold used by the authors consists of four alternating cool and warm modes that transcend the accepted geologictime boundaries. The major distinction between these modes and A. Fisher’s States of Icehouse-Greenhouse is in the number of identified
2149
2150
Book Reviews
(ocean-carbonates dominated) and organic (biota dominated) reservoirs. The organic-derived carbon, however, is not a generic entity, but the product of the contemporaneous biota. It is well established by now that biotic crises accompanied by extinction events at geologic boundaries were associated with sharp decreases in 6°C. By overlooking possible links between biotic turnovers and climatic changes, the authors have weakened their arguments favoring internal mechanisms as the cause of the climate changes. In search for generalizations, the authors have painted the carbon cycle with broad brush strokes. For example, the failure to detect an increase in 613Cduring a particularly stubborn cool mode is explained by a loss of 13Cin enhanced limestone deposition (p. 200 and Fig. 11.4). Such a mechanism is highly improbable, because 6’-‘C fractionation between seawater inorganic carbon and limestone (about 0 to 2%0) is insignificant by comparison with the organic carbon fractionation (about 20%0). It would be more reasonable to attribute the failed 13“C increase to diminished fractionation between the marine inorganic and organic carbon reservoirs, resulting from lower CO2 concentrations. Similar inconsistencies, resulting in deficient interpretations, are found in the sections dealing with the 6 ‘*O systematics. For example, embarrassingly light b”O values signifying either an ice-free planet or high temperatures (p. 2 17) are explained as the imprints of “0 consumption during oxidation of organic matter in the deep sea. This interpretation is incorrect on two grounds. First, the oxygen consumed during oxidation is the dissolved gaseous O2 phase, and not H20. Second, the O2 concentration in the oceanatmosphere system was at all times insignificant by comparison with the amount of oxygen residing in the water. Consequently, the purported isotope fractionation will have no practical effect on the oceanic 6i80 composition.
If, as emphasized by the authors on a number of occasions, planetary tectonism played such a crucial role in the carbon cycle through repeated release and consumption of CO* during uplift, weathering, and volcanism, then what better witness than marine 87Sr/86Srevolution could be brought to substantiate these inferences? Unfortunately, the authors have ignored the strontium isotope records and also the less potent but useful neodymium isotope records. Paleoclimate research has had a checkered history, but is now ready for a major revival, as a result of new developments in geochemistry that improved the precision of paleotemperature and absolute chronology, as well as emergent bold ideas challenging the old paradigms. Among these are Oberbeck’s reinterpretation of glacial tillites (the key to Paleozoic glaciations) as sedimentary products of bolide impacts, and the emerging evidence of tropical refrigerations during glacials. Unfortunately, none of these new exciting developments are in the book, either because they postdate the publication of the book, or because the authors have chosen a conservative path. In spite of the weaknesses that I see, the book is very knowledgeable and readable. It should be of interest to all geologists delving into the past records of the planet and climatologists and oceanographers interested in global climate cycles. It could also be used with caution as a supplemental reading in undergraduate historical geology classes. Would I buy the book if it was not offered for free as a reward for the preceding rumblings? You bet I would.
Department of Geology and Geophysics Louisiana State University Baton Rouge, LA 70803, USA
Paul Aharon
Pergamon
et Cosmochimica
Acta, Vol. 58, No. 9. p. 2 149-2 150, 1994 Convrizht -~.. - 0 1994 Elsevier Science Ltd Printed in the USA. All rights reserved 0016.7037/94 $6.00 + .OO
BOOK REVIEWS
Anorthosites by Lewis D. Ashwal. Springer-Verla& US $119.00
tracers. Flaws are few, and rarely vital. (A major work on gravity over the Nain anorthosite by Stephenson and Thomas is unfortunately amone the few works missed.1 Ashwal brines his own skillful synthesis to bear on the important features of anorthosite genesis. while giving due credit to previous work. Emplacement of massif anorthosites as mushes from Al-Fe magmas at the base of the crust is favored. The tectonic setting of these rocks is called the frontier of anorthosite research. Excellent tables of occurrences, and figures, adorn the text, and the final chapter is a critical and refreshing synthesis and summary of the whole. Among treatments of controversial topics that I found felicitous are the discussions of wet vs. dry magmas, and the short shrift (but good history) given to nutty theories of origin at page 74. One well-known author is caught out with six different analog origins of Archean anorthosite. The mere characterization of massif anorthosites that defeats most writers is graced with clever and competent decisions at pp. 82-83. The writing in places has the flavor of addressing the reader personally, as with Fielding inventing the novel in Tom Jones. My chief reaction on reading this book is how glad I am that it was Lew Ashwal who spent six years at this task and not I; the book is ever so much better as a result. Maybe it is the Tom Jones spirit in Lew himself that makes it so good.
1993, xix + 422 p.,
(ISBN O-387-55361-4).
THERE ARE SIX KINDS of the plagioclase-rich rock called anorthosite: Archean megacrystic, Proterozoic massif-type, those in layered mafic complexes, those of oceanic settings, inclusions in other igneous rocks, and extraterrestrial anorthosites. You should want to know about these widespread rocks, because they are of great interest to the evolution of planetary crusts, notably that of the Earth. Lew Ashwal visits and treats each of these types succinctly and very well in this book. Some of the best of a wide geographic selection of photos are his own; his footprints are all over the place. Readers will find his treatment of the rocks they know even-handed and highly competent. They will find his judgments quick, informed, and accurate: “This obviously incorrect idea . .” (p. 2); “This complex history is immediately apparent from the map .” (p. 38); “I am not sure it would be worth the effort” (p. 106). Oceanic varieties of anorthosite include dredge hauls and ophiolites; inclusions range from the Gardar dikes of Greenland to Hawaii to cognate inclusions in felsic intrusions. Lunar anorthosites receive the appropriate extended treatment, and a venture is made into the prospects for anorthosites on Mercury. Quite naturally, much of the book’s space is given to the Archean and massif anorthosites that have occupied much of Ashwal’s life in research. As expected, due attention is paid to geochronology, tectonic environment, magma types, phase equilibria, mineralogy, and isotopic
Department of’Geology and Geography University of Massachusetts at Amhevyt Amherst. MA 01003, CTSA
Climate Modes of the Phanerozoic by Lawrence A. Frakes, Jane E. Francis, and Jozef I. Syktus. Cambridge University Press 1992, 274 p., US $69.95 (ISBN O-52 l-36627-5 ).
S. A. Morse
Phanerozoic cycles (i.e.. four vs. three). This difference, however, is far from trivial. Insertion of a rather controversial cool mode in the middle of a balmy Mesozoic throws a monkey wrench in the major climatic cycles by shortening their periodicity from 300 Myr to 162 Myr, a figure that agrees quite well with the 150 Myr half Galactic cycle of G. Williams. The question of whether the agreement is just a felicitous coincidence, or is the result of a cause and effect relation is not answered by the authors. What are the criteria to establish the overall climate mode of a planet the size of Earth with permanent latitudinal climatic gradients, and what are the diagnostic markers of the cool and warm modes? According to these authors, presence/absence of ice-rafted debris indicating either permanent or seasonal ice at high latitudes dictates the climate mode of a planet. The wide arsenal of paleoclimatic indicators also include the classic Wegener-type clues such as coals, evaporites, shelf carbonates, eolian deposits, etc., that are supplemented by oxygen and carbon isotope records retrieved from stratigraphically continuous marine carbonate sediments. A cool mode is diagnosed on the basis of ( 1) considerable extent of high latitude land (obtained from paleomagnetic data), (2) long interval of cooling leading to high latitude glaciation, (3) marked increase in d “C, and (4) marked decrease in the abundance of evaporites. The advent of warm modes, indicated by the termination of ice rafting, was remarkably sudden and was accompanied by abrupt decreases in 613C. Whereas the markings of a cool mode are welldefined and assessed to be repetitive, the ones pertaining to the warm modes (i.e., rapid decrease in 6 ‘%, increasing deposition of organic matter, warming, and volcanism) are more diffuse, and each one of the four warm modes seems to have some exceptional characteristics. According to the authors, the variable rates of organic carbon accumulation acted as the principal toggle, switching the climate modes through a complicated circuitry whose components are tectonics, atmospheric CO2 levels, sea-level changes, nutrient supply, and ocean circulation. Consequently, the emphasis is on the 613C records that track variations in the partition of carbon between the inorganic
“SEEKING THE CAUSE of climate change is both an exciting activity and a difficult task, as borne out by the long history of enquiry and the scarcity of firm conclusions.” This sentence (p. 189) could serve as the rationale for this new endeavor by Frakes, Francis, and Syktus in paleoclimatology. Lawrence Frakes is an old hand in the business of deciphering the climate history of our planet. Many of us (including this author) found their motivation to engage in paleoclimatic research from reading his 1979 best-seller Climate Through Geologic Time. The more recent product of collaboration between Frakes, Francis, and Syktus differs in some important aspects from the previous solo contribution. To begin with, in the first ten chapters it updates and synthesizes a large body of paleoclimatic information collated over the past ten years. Equally important, it attempts to impose some order in the rather chaotic flood of data covering the history of climate for the past 600 Myr. The latter is achieved by establishing a framework of periodic Climate Modes during the Phanerozic, and then using it to compare and contrast the climates of the past, and to ease the task of recognition of some common causes of climate change. The authors are at their best in Chapter I 1, where they discuss the causes and chronologies of the climate changes. Unlike other recent contributions to geologic climates, Frakes et al. have recognized the importance of the Quaternary high frequency glacial-interglacial cycles as a “key to the past” and have provided a thorough review of the Quaternary climate history, including a well-rounded discussion of the causes and consequences of the still puzzling 11,000 year-old Younger Dryas climate oscillation. The organizational mold used by the authors consists of four alternating cool and warm modes that transcend the accepted geologictime boundaries. The major distinction between these modes and A. Fisher’s States of Icehouse-Greenhouse is in the number of identified
2149
2150
Book Reviews
(ocean-carbonates dominated) and organic (biota dominated) reservoirs. The organic-derived carbon, however, is not a generic entity, but the product of the contemporaneous biota. It is well established by now that biotic crises accompanied by extinction events at geologic boundaries were associated with sharp decreases in 6°C. By overlooking possible links between biotic turnovers and climatic changes, the authors have weakened their arguments favoring internal mechanisms as the cause of the climate changes. In search for generalizations, the authors have painted the carbon cycle with broad brush strokes. For example, the failure to detect an increase in 613Cduring a particularly stubborn cool mode is explained by a loss of 13Cin enhanced limestone deposition (p. 200 and Fig. 11.4). Such a mechanism is highly improbable, because 6’-‘C fractionation between seawater inorganic carbon and limestone (about 0 to 2%0) is insignificant by comparison with the organic carbon fractionation (about 20%0). It would be more reasonable to attribute the failed 13“C increase to diminished fractionation between the marine inorganic and organic carbon reservoirs, resulting from lower CO2 concentrations. Similar inconsistencies, resulting in deficient interpretations, are found in the sections dealing with the 6 ‘*O systematics. For example, embarrassingly light b”O values signifying either an ice-free planet or high temperatures (p. 2 17) are explained as the imprints of “0 consumption during oxidation of organic matter in the deep sea. This interpretation is incorrect on two grounds. First, the oxygen consumed during oxidation is the dissolved gaseous O2 phase, and not H20. Second, the O2 concentration in the oceanatmosphere system was at all times insignificant by comparison with the amount of oxygen residing in the water. Consequently, the purported isotope fractionation will have no practical effect on the oceanic 6i80 composition.
If, as emphasized by the authors on a number of occasions, planetary tectonism played such a crucial role in the carbon cycle through repeated release and consumption of CO* during uplift, weathering, and volcanism, then what better witness than marine 87Sr/86Srevolution could be brought to substantiate these inferences? Unfortunately, the authors have ignored the strontium isotope records and also the less potent but useful neodymium isotope records. Paleoclimate research has had a checkered history, but is now ready for a major revival, as a result of new developments in geochemistry that improved the precision of paleotemperature and absolute chronology, as well as emergent bold ideas challenging the old paradigms. Among these are Oberbeck’s reinterpretation of glacial tillites (the key to Paleozoic glaciations) as sedimentary products of bolide impacts, and the emerging evidence of tropical refrigerations during glacials. Unfortunately, none of these new exciting developments are in the book, either because they postdate the publication of the book, or because the authors have chosen a conservative path. In spite of the weaknesses that I see, the book is very knowledgeable and readable. It should be of interest to all geologists delving into the past records of the planet and climatologists and oceanographers interested in global climate cycles. It could also be used with caution as a supplemental reading in undergraduate historical geology classes. Would I buy the book if it was not offered for free as a reward for the preceding rumblings? You bet I would.
Department of Geology and Geophysics Louisiana State University Baton Rouge, LA 70803, USA
Paul Aharon