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Summarize the evidence captured in the geologic record supporting the interpretation of El Niño episodes over the past several million years. (See Essay 10.2)

Summarize the evidence captured in the geologic record supporting the interpretation of El Niño episodes over the past several million years. (See Essay 10.2)

Applications and Investigations in Earth Science (9th Edition)
Applications and Investigations in Earth Science (9th Edition)
9th Edition
ISBN:9780134746241
Author:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa
Publisher:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa
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Summarize the evidence captured in the geologic record supporting the interpretation of El Niño episodes over the past several million years. (See Essay 10.2)

Lengthy records of El Niño episodes of the past would provide a useful perspective on more recent events and perhaps help determine whether a connection exists between global warming and the frequency and intensity of El Niño. However, in most areas of the world, reliable instrument-based weather records that signal past El Niño events extend back only to about the mid-19th century. For information on earlier occurrences of El Niño, scientists must rely on proxy climatic data, that is, information inferred from documentary (such as logs and diaries), geological (such as ocean bottom sediment core and glacial ice cores), or biological (such as tree growth rings and tropical corals) indicators of climate. Suppose, for example, that written records from India and Southeast Asia list agricultural losses due to prolonged drought. Meanwhile, a sediment core extracted from a lake bottom contains evidence of heavy rainfall in normally arid coastal Peru during the same time interval. These simultaneous events are consistent with a strong El Niño. Geological evidence indicates that El Niño episodes were occurring at least as far back as late in the Pleistocene Ice Age (2.6 million to 11,700 years ago), and that permanent “El Niño type conditions" may have prevailed in the Pacific prior to that, during the early Pliocene (4.5 to 3.0 million years ago) when Earth's average temperature was higher than today. This essay provides some examples of research on past El Niño episodes. As one example, analysis of ancient corals from Sulawesi Island, Indonesia, indicated that El Niño was occurring at 3- to 7-year intervals about 124,000 years ago. This research was conducted using oxygen isotope analysis to distinguish between El Niño and neutral (normal) years. Living corals build their external skeletons of limestone (CaCO;) that incorporates two isotopes of O18. The lighter oxygen isotope evaporates more readily than does the heavier oxygen isotope. Hence, during relatively dry weather, evaporation rates are high, and the coral skeleton is enriched in O18 relative to O16. On the other hand, during rainy episodes, evaporation rates are low, and the coral skeleton is enriched in O!6 relative to O18. Corals build their skeletons in annual growth rings that can be dated so that oxygen isotope analysis permits a detailed chronology of general weather conditions that can be tied to El Niño. oxygen: O16 and Other researchers examined coral reef terraces in New Guinea dating back some 130,000 years. Isotopic and chemical analyses of these corals revealed that El Niño events were more intense over the past 100 years than at any time during the past 130,000 years. Furthermore, El Niño was about 50% weaker during the Ice Age and intensified during warm episodes, again spurring speculation that the recent upturn in El Niño intensity is linked to the global warming trend. Using corals on Christmas Island in Kiribati and tropical Pacific Ocean islands, scientists recently constructed a long-term record of ocean temperature, which suggests that the extreme warmth of recent El Niño events is influenced by global warming. The El Niño signal shows up in a 4000-year record of lake sediments extracted from Glacial Lake Hitchcock, which occupied the Connecticut River valley during the waning phase of the last Ice Age. Scientists analyzed time series of glacial varves, which are yearly sediment layers occurring in a glacial lake. These sediments layers chronicle short-term climate fluctuations spanning the period from about 17,500 to 13,500 years ago (during the final retreat of glacial ice from New England), and apparently resolve both intense and weak El Niño events (having periods of 2.5 to 5 years). As another example, researchers analyzed two 8-m (26-ft.) sediment cores extracted from the bottom of Laguna (Lake) Pallcacocha high in the Andes Mountains of southern Ecuador. Rainfall governs the amount of sediment delivered to the lake but rainfall associated with a weak El Niño is not likely to reach the 4200-m (13,800-ft.) high lake. Hence, the researchers interpreted anomalously thick layers of silt in the cores as indicating heavy rainfall associated with an intense El Niño. The lake sediment cores span the past 12,000 years and indicate that between 12,000 and 7000 years ago, strong El Niño episodes occurred five or fewer times per century. The frequency of intense El Niño episodes then increased and peaked during the 9th century CE when they occurred about every three years. The Laguna Pallcacocha study and other studies indicated the El Niño episodes were considerable less frequent in the early Holocene (beginning about 12,000 years ago) than the present. In contrast, oxygen isotope analysis of clam shells in coastal Peru locations indicates that ENSO may be as active in the early Holocone as it is in the present day. The long-term reconstructed climate record suggests that El Niño has occurred regularly since at least the late Pleistocene (1.7 million years ago to l10,000 years ago)– albeit with a possible, but now debated lull 12,000 to 7000 years ago. However, El Niño intensity has varied over thousands of years, that is the frequency of occurrence has remained roughly the same but the range of intensities has alternately increased and decreased. The current period appears to be characterized by particularly intense El Niño events.
Transcribed Image Text:Lengthy records of El Niño episodes of the past would provide a useful perspective on more recent events and perhaps help determine whether a connection exists between global warming and the frequency and intensity of El Niño. However, in most areas of the world, reliable instrument-based weather records that signal past El Niño events extend back only to about the mid-19th century. For information on earlier occurrences of El Niño, scientists must rely on proxy climatic data, that is, information inferred from documentary (such as logs and diaries), geological (such as ocean bottom sediment core and glacial ice cores), or biological (such as tree growth rings and tropical corals) indicators of climate. Suppose, for example, that written records from India and Southeast Asia list agricultural losses due to prolonged drought. Meanwhile, a sediment core extracted from a lake bottom contains evidence of heavy rainfall in normally arid coastal Peru during the same time interval. These simultaneous events are consistent with a strong El Niño. Geological evidence indicates that El Niño episodes were occurring at least as far back as late in the Pleistocene Ice Age (2.6 million to 11,700 years ago), and that permanent “El Niño type conditions" may have prevailed in the Pacific prior to that, during the early Pliocene (4.5 to 3.0 million years ago) when Earth's average temperature was higher than today. This essay provides some examples of research on past El Niño episodes. As one example, analysis of ancient corals from Sulawesi Island, Indonesia, indicated that El Niño was occurring at 3- to 7-year intervals about 124,000 years ago. This research was conducted using oxygen isotope analysis to distinguish between El Niño and neutral (normal) years. Living corals build their external skeletons of limestone (CaCO;) that incorporates two isotopes of O18. The lighter oxygen isotope evaporates more readily than does the heavier oxygen isotope. Hence, during relatively dry weather, evaporation rates are high, and the coral skeleton is enriched in O18 relative to O16. On the other hand, during rainy episodes, evaporation rates are low, and the coral skeleton is enriched in O!6 relative to O18. Corals build their skeletons in annual growth rings that can be dated so that oxygen isotope analysis permits a detailed chronology of general weather conditions that can be tied to El Niño. oxygen: O16 and Other researchers examined coral reef terraces in New Guinea dating back some 130,000 years. Isotopic and chemical analyses of these corals revealed that El Niño events were more intense over the past 100 years than at any time during the past 130,000 years. Furthermore, El Niño was about 50% weaker during the Ice Age and intensified during warm episodes, again spurring speculation that the recent upturn in El Niño intensity is linked to the global warming trend. Using corals on Christmas Island in Kiribati and tropical Pacific Ocean islands, scientists recently constructed a long-term record of ocean temperature, which suggests that the extreme warmth of recent El Niño events is influenced by global warming. The El Niño signal shows up in a 4000-year record of lake sediments extracted from Glacial Lake Hitchcock, which occupied the Connecticut River valley during the waning phase of the last Ice Age. Scientists analyzed time series of glacial varves, which are yearly sediment layers occurring in a glacial lake. These sediments layers chronicle short-term climate fluctuations spanning the period from about 17,500 to 13,500 years ago (during the final retreat of glacial ice from New England), and apparently resolve both intense and weak El Niño events (having periods of 2.5 to 5 years). As another example, researchers analyzed two 8-m (26-ft.) sediment cores extracted from the bottom of Laguna (Lake) Pallcacocha high in the Andes Mountains of southern Ecuador. Rainfall governs the amount of sediment delivered to the lake but rainfall associated with a weak El Niño is not likely to reach the 4200-m (13,800-ft.) high lake. Hence, the researchers interpreted anomalously thick layers of silt in the cores as indicating heavy rainfall associated with an intense El Niño. The lake sediment cores span the past 12,000 years and indicate that between 12,000 and 7000 years ago, strong El Niño episodes occurred five or fewer times per century. The frequency of intense El Niño episodes then increased and peaked during the 9th century CE when they occurred about every three years. The Laguna Pallcacocha study and other studies indicated the El Niño episodes were considerable less frequent in the early Holocene (beginning about 12,000 years ago) than the present. In contrast, oxygen isotope analysis of clam shells in coastal Peru locations indicates that ENSO may be as active in the early Holocone as it is in the present day. The long-term reconstructed climate record suggests that El Niño has occurred regularly since at least the late Pleistocene (1.7 million years ago to l10,000 years ago)– albeit with a possible, but now debated lull 12,000 to 7000 years ago. However, El Niño intensity has varied over thousands of years, that is the frequency of occurrence has remained roughly the same but the range of intensities has alternately increased and decreased. The current period appears to be characterized by particularly intense El Niño events.
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