Amber provides new insights into the evolution of the Earth’s atmosphere

<IMG SRC="/Images/22324740.jpg" WIDTH="350" HEIGHT="233" BORDER="0" ALT="The team analyzed amber samples from almost all well-known amber deposits worldwide. This amber originates from the Cretaceous period, an inclusion of foilage of the extinct conifer tree Parataxodium sp. from the Foremost Formation at Grassy Lake, Alberta, Canada. It is approximately 77 million years old. – Ryan C. McKellar”>
The team analyzed amber samples from almost all well-known amber deposits worldwide. This amber originates from the Cretaceous period, an inclusion of foilage of the extinct conifer tree Parataxodium sp. from the Foremost Formation at Grassy Lake, Alberta, Canada. It is approximately 77 million years old. – Ryan C. McKellar

Scientists encounter big challenges when reconstructing atmospheric compositions in the Earth’s geological past because of the lack of useable sample material. One of the few organic materials that may preserve reliable data of the Earth’s geological history over millions of years are fossil resins (e.g. amber). “Compared to other organic matter, amber has the advantage that it remains chemically and isotopically almost unchanged over long periods of geological time,” explains Ralf Tappert from the Institute of Mineralogy and Petrography at the University of Innsbruck. The mineralogist and his colleagues from the University of Alberta in Canada and universities in the USA and Spain have produced a comprehensive study of the chemical composition of the Earth’s atmosphere since the Triassic period. The study has been published in the journal Geochimica et Cosmochimica Acta. The interdisciplinary team, consisting of mineralogists, paleontologists and geochemists, use the preserving properties of plant resins, caused by polymerization, for their study. “During photosynthesis plants bind atmospheric carbon, whose isotopic composition is preserved in resins over millions of years, and from this, we can infer atmospheric oxygen concentrations,” explains Ralf Tappert. The information about oxygen concentration comes from the isotopic composition of carbon or rather from the ratio between the stable carbon isotopes 12C and 13C.

Atmospheric oxygen between 10 and 15 percent


The research team analyzed a total of 538 amber samples from from well-known amber deposits worldwide, with the oldest samples being approximately 220 million years old and recovered from the Dolomites in Italy. The team also compared fossil amber with modern resins to test the validity of the data. The results of this comprehensive study suggest that atmospheric oxygen during most of the past 220 million years was considerably lower than today’s 21 percent. “We suggest numbers between 10 and 15 percent,” says Tappert. These oxygen concentrations are not only lower than today but also considerably lower than the majority of previous investigations propose for the same time period. For the Cretaceous period (65 – 145 million years ago), for example, up to 30 percent atmospheric oxygen has been suggested previously.

Effects on climate and environment

The researchers also relate this low atmospheric oxygen to climatic developments in the Earth’s history. “We found that particularly low oxygen levels coincided with intervals of elevated global temperatures and high carbon dioxide concentrations,” explains Tappert. The mineralogist suggests that oxygen may influence carbon dioxide levels and, under certain circumstances, might even accelerate the influx of carbon dioxide into the atmosphere. “Basically, we are dealing here with simple oxidation reactions that are amplified particularly during intervals of high temperatures such as during the Cretaceous period.” The researchers, thus, conclude that an increase in carbon dioxide levels caused by extremely strong vulcanism was accompanied by a decrease of atmospheric oxygen. This becomes particularly apparent when looking at the last 50 million years of geological history. Following the results of this study, the comparably low temperatures of the more recent past (i.e. the Ice Ages) may be attributed to the absence of large scale vulcanism events and an increase in atmospheric oxygen.

Oxygen may not be the cause of gigantism


According to the results of the study, oxygen may indirectly influence the climate. This in turn may also affect the evolution of life on Earth. A well-known example are dinosaurs: Many theories about animal gigantism offer high levels of atmospheric oxygen as an explanation. Tappert now suggests to reconsider these theories: “We do not want to negate the influence of oxygen for the evolution of life in general with our study, but the gigantism of dinosaurs cannot be explained by those theories.” The research team highly recommends conducting further studies and intends to analyze even older plant resins.

Amber provides new insights into the evolution of the Earth’s atmosphere

<IMG SRC="/Images/22324740.jpg" WIDTH="350" HEIGHT="233" BORDER="0" ALT="The team analyzed amber samples from almost all well-known amber deposits worldwide. This amber originates from the Cretaceous period, an inclusion of foilage of the extinct conifer tree Parataxodium sp. from the Foremost Formation at Grassy Lake, Alberta, Canada. It is approximately 77 million years old. – Ryan C. McKellar”>
The team analyzed amber samples from almost all well-known amber deposits worldwide. This amber originates from the Cretaceous period, an inclusion of foilage of the extinct conifer tree Parataxodium sp. from the Foremost Formation at Grassy Lake, Alberta, Canada. It is approximately 77 million years old. – Ryan C. McKellar

Scientists encounter big challenges when reconstructing atmospheric compositions in the Earth’s geological past because of the lack of useable sample material. One of the few organic materials that may preserve reliable data of the Earth’s geological history over millions of years are fossil resins (e.g. amber). “Compared to other organic matter, amber has the advantage that it remains chemically and isotopically almost unchanged over long periods of geological time,” explains Ralf Tappert from the Institute of Mineralogy and Petrography at the University of Innsbruck. The mineralogist and his colleagues from the University of Alberta in Canada and universities in the USA and Spain have produced a comprehensive study of the chemical composition of the Earth’s atmosphere since the Triassic period. The study has been published in the journal Geochimica et Cosmochimica Acta. The interdisciplinary team, consisting of mineralogists, paleontologists and geochemists, use the preserving properties of plant resins, caused by polymerization, for their study. “During photosynthesis plants bind atmospheric carbon, whose isotopic composition is preserved in resins over millions of years, and from this, we can infer atmospheric oxygen concentrations,” explains Ralf Tappert. The information about oxygen concentration comes from the isotopic composition of carbon or rather from the ratio between the stable carbon isotopes 12C and 13C.

Atmospheric oxygen between 10 and 15 percent


The research team analyzed a total of 538 amber samples from from well-known amber deposits worldwide, with the oldest samples being approximately 220 million years old and recovered from the Dolomites in Italy. The team also compared fossil amber with modern resins to test the validity of the data. The results of this comprehensive study suggest that atmospheric oxygen during most of the past 220 million years was considerably lower than today’s 21 percent. “We suggest numbers between 10 and 15 percent,” says Tappert. These oxygen concentrations are not only lower than today but also considerably lower than the majority of previous investigations propose for the same time period. For the Cretaceous period (65 – 145 million years ago), for example, up to 30 percent atmospheric oxygen has been suggested previously.

Effects on climate and environment

The researchers also relate this low atmospheric oxygen to climatic developments in the Earth’s history. “We found that particularly low oxygen levels coincided with intervals of elevated global temperatures and high carbon dioxide concentrations,” explains Tappert. The mineralogist suggests that oxygen may influence carbon dioxide levels and, under certain circumstances, might even accelerate the influx of carbon dioxide into the atmosphere. “Basically, we are dealing here with simple oxidation reactions that are amplified particularly during intervals of high temperatures such as during the Cretaceous period.” The researchers, thus, conclude that an increase in carbon dioxide levels caused by extremely strong vulcanism was accompanied by a decrease of atmospheric oxygen. This becomes particularly apparent when looking at the last 50 million years of geological history. Following the results of this study, the comparably low temperatures of the more recent past (i.e. the Ice Ages) may be attributed to the absence of large scale vulcanism events and an increase in atmospheric oxygen.

Oxygen may not be the cause of gigantism


According to the results of the study, oxygen may indirectly influence the climate. This in turn may also affect the evolution of life on Earth. A well-known example are dinosaurs: Many theories about animal gigantism offer high levels of atmospheric oxygen as an explanation. Tappert now suggests to reconsider these theories: “We do not want to negate the influence of oxygen for the evolution of life in general with our study, but the gigantism of dinosaurs cannot be explained by those theories.” The research team highly recommends conducting further studies and intends to analyze even older plant resins.

Late Cretaceous Period was likely ice-free

In a new study, MacLeod found evidence that a continental ice sheet did not form during the Late Cretaceous Period more than 90 million years ago. This information could help scientists predict changes in earth's climate as our temperatures rise. -  University of Missouri
In a new study, MacLeod found evidence that a continental ice sheet did not form during the Late Cretaceous Period more than 90 million years ago. This information could help scientists predict changes in earth’s climate as our temperatures rise. – University of Missouri

For years, scientists have thought that a continental ice sheet formed during the Late Cretaceous Period more than 90 million years ago when the climate was much warmer than it is today. Now, a University of Missouri researcher has found evidence suggesting that no ice sheet formed at this time. This finding could help environmentalists and scientists predict what the earth’s climate will be as carbon dioxide levels continue to rise.

“Currently, carbon dioxide levels are just above 400 parts per million (ppm), up approximately 120 ppm in the last 150 years and rising about 2 ppm each year,” said Ken MacLeod, a professor of geological sciences at MU. “In our study, we found that during the Late Cretaceous Period, when carbon dioxide levels were around 1,000 ppm, there were no continental ice sheets on earth. So, if carbon dioxide levels continue to rise, the earth will be ice-free once the climate comes into balance with the higher levels.”

In his study, MacLeod analyzed the fossilized shells of 90 million-year-old planktic and benthic foraminifera, single-celled organisms about the size of a grain of salt. Measuring the ratios of different isotopes of oxygen and carbon in the fossils gives scientists information about past temperatures and other environmental conditions. The fossils, which were found in Tanzania, showed no evidence of cooling or changes in local water chemistry that would have been expected if a glacial event had occurred during that time period.

“We know that the carbon dioxide (CO2) levels are rising currently and are at the highest they have been in millions of years. We have records of how conditions have changed as CO2 levels have risen from 280 to 400 ppm, but I believe it also is important to know what could happen when those levels reach 600 to 1000 ppm,” MacLeod said. “At the rate that carbon dioxide levels are rising, we will reach 600 ppm around the end of this century. At that level of CO2, will ice sheets on Greenland and Antarctica be stable? If not, how will their melting affect the planet?”

Previously, many scientists have thought that doubling CO2 levels would cause earth’s temperature to increase as much as 3 degrees Celsius, or approximately 6 degrees Fahrenheit. However, the temperatures MacLeod believes existed in Tanzania 90 million years ago are more consistent with predictions that a doubling of CO2 levels would cause the earth’s temperature could rise an average of 6 degrees Celsius, or approximately 11 degrees Fahrenheit.

“While studying the past can help us predict the future, other challenges with modern warming still exist,” MacLeod said. “The Late Cretaceous climate was very warm, but the earth adjusted as changes occurred over millions of years. We’re seeing the same size changes, but they are happening over a couple of hundred years, maybe 10,000 times faster. How that affects the equation is a big and difficult question.”

MacLeod’s study was published in the October issue of the journal Geology.

Late Cretaceous Period was likely ice-free

In a new study, MacLeod found evidence that a continental ice sheet did not form during the Late Cretaceous Period more than 90 million years ago. This information could help scientists predict changes in earth's climate as our temperatures rise. -  University of Missouri
In a new study, MacLeod found evidence that a continental ice sheet did not form during the Late Cretaceous Period more than 90 million years ago. This information could help scientists predict changes in earth’s climate as our temperatures rise. – University of Missouri

For years, scientists have thought that a continental ice sheet formed during the Late Cretaceous Period more than 90 million years ago when the climate was much warmer than it is today. Now, a University of Missouri researcher has found evidence suggesting that no ice sheet formed at this time. This finding could help environmentalists and scientists predict what the earth’s climate will be as carbon dioxide levels continue to rise.

“Currently, carbon dioxide levels are just above 400 parts per million (ppm), up approximately 120 ppm in the last 150 years and rising about 2 ppm each year,” said Ken MacLeod, a professor of geological sciences at MU. “In our study, we found that during the Late Cretaceous Period, when carbon dioxide levels were around 1,000 ppm, there were no continental ice sheets on earth. So, if carbon dioxide levels continue to rise, the earth will be ice-free once the climate comes into balance with the higher levels.”

In his study, MacLeod analyzed the fossilized shells of 90 million-year-old planktic and benthic foraminifera, single-celled organisms about the size of a grain of salt. Measuring the ratios of different isotopes of oxygen and carbon in the fossils gives scientists information about past temperatures and other environmental conditions. The fossils, which were found in Tanzania, showed no evidence of cooling or changes in local water chemistry that would have been expected if a glacial event had occurred during that time period.

“We know that the carbon dioxide (CO2) levels are rising currently and are at the highest they have been in millions of years. We have records of how conditions have changed as CO2 levels have risen from 280 to 400 ppm, but I believe it also is important to know what could happen when those levels reach 600 to 1000 ppm,” MacLeod said. “At the rate that carbon dioxide levels are rising, we will reach 600 ppm around the end of this century. At that level of CO2, will ice sheets on Greenland and Antarctica be stable? If not, how will their melting affect the planet?”

Previously, many scientists have thought that doubling CO2 levels would cause earth’s temperature to increase as much as 3 degrees Celsius, or approximately 6 degrees Fahrenheit. However, the temperatures MacLeod believes existed in Tanzania 90 million years ago are more consistent with predictions that a doubling of CO2 levels would cause the earth’s temperature could rise an average of 6 degrees Celsius, or approximately 11 degrees Fahrenheit.

“While studying the past can help us predict the future, other challenges with modern warming still exist,” MacLeod said. “The Late Cretaceous climate was very warm, but the earth adjusted as changes occurred over millions of years. We’re seeing the same size changes, but they are happening over a couple of hundred years, maybe 10,000 times faster. How that affects the equation is a big and difficult question.”

MacLeod’s study was published in the October issue of the journal Geology.

Researchers say a comet killed the dinosaurs

Professors Mukul Sharma (left) and Jason Moore of the Department of Earth Sciences revisit the departure of the dinosaurs. -  Eli Burakian, Dartmouth College
Professors Mukul Sharma (left) and Jason Moore of the Department of Earth Sciences revisit the departure of the dinosaurs. – Eli Burakian, Dartmouth College

In a geological moment about 66 million years ago, something killed off almost all the dinosaurs and some 70 percent of all other species living on Earth. Only those dinosaurs related to birds appear to have survived. Most scientists agree that the culprit in this extinction was extraterrestrial, and the prevailing opinion has been that the party crasher was an asteroid.

Not so, say two Dartmouth researchers. Professors Jason Moore and Mukul Sharma of the Department of Earth Sciences favor another explanation, asserting that a high-velocity comet led to the demise of the dinosaurs.

Recently, asteroids have been in the headlines. On February 15, 2013, an asteroid exploded in the skies over Siberia. Later that day, another swept past the Earth in what some regard as a close call-just 17,000 miles away.

The asteroid impact theory of extinction began with discoveries by the late physicist and Nobel Laureate Luis Alvarez and his son, the geologist Walter Alvarez, a professor at the University of California, Berkeley. In 1980 they identified extremely high concentrations of the element iridium in a layer of rock known as the K-Pg (formerly called K-T) boundary. The layer marks the end of the Cretaceous period (abbreviated “K”), the epoch of the dinosaurs, and the beginning of the Paleogene period, with its notable absence of the large lizards.

While iridium is rare in the Earth’s crust, it is a common trace element in rocky space debris such as asteroids. Based on the elevated levels of iridium found worldwide in the boundary layer, the Alvarezes suggested that this signaled a major asteroid strike around the time of the K-Pg boundary-about 66 million years ago. Debate surrounded their theory until 2010, when a panel of 41 scientists published a report in support of the Alvarezes’ theory. The panel confirmed that a major asteroid impact had occurred at the K-Pg boundary and was responsible for mass extinctions.

The scientific community today looks to the deeply buried and partially submerged, 110-mile wide Chicxulub crater in Mexico’s Yucatán as the place where the death-dealing asteroid landed. The 66-million-year age of Chicxulub, discovered in 1990, coincides with the KT boundary, leading to the conclusion that what caused the crater also wiped out the dinosaurs.

Moore and Sharma do agree with fellow scientists that Chicxulub was the impact zone, but dispute the characterization of the object from space as an asteroid. In a paper presented to the 44th Lunar and Planetary Conference on March 22, 2013, they described their somewhat controversial findings.

Moore notes that in the past geochemists toiled away, isolated from their geophysicist colleagues, each focused on his or her particular area of expertise. “There hadn’t been a concerted synthesis of all the data from these two camps,” says Moore. “That’s what we’ve tried to do.”

The Dartmouth duo compiled all the published data on iridium from the K-Pg boundary. They also included the K-Pg data on osmium-another element common in space rock. In sifting through all this they found a wide range of variability, so consequently kept only the figures they demonstrated to be most reliable. “Because we are bringing a fresh set of eyes into this field, we feel our decisions are objective and unbiased,” says Sharma.

For example, they deleted data drawn from deep ocean cores where there were very high amounts of iridium. “We discovered that even then there was a huge variation. It was much worse in the oceans than on the continents,” Sharma said. “We figured out that the oceanic variations are likely caused by preferential concentration of iridium bearing minerals in marine sediments.”

In the final analysis, the overall trace element levels were much lower than those that scientists had been using for decades and being this low weakened the argument for an asteroid impact explanation. However, a comet explanation reconciles the conflicting evidence of a huge impact crater with the revised, lower iridium/osmium levels at the K-Pg boundary.

“We are proposing a comet because that conclusion hits a ‘sweet spot.’ Comets have a lower percentage of iridium and osmium than asteroids, relative to their mass, yet a high-velocity comet would have sufficient energy to create a 110-mile-wide crater,” says Moore. “Comets travel much faster than asteroids, so they have more energy on impact, which in combination with their being partially ice means they are not contributing as much iridium or osmium.”

Moore attributes much of the early resistance to a comet impact theory to a lack of knowledge about comets in general. “We weren’t certain whether they were dirty snowballs or icy dirt balls,” he says. “Today, we are inclined toward the icy dirt ball description.”

Comet composition and physical structure were unknown, but with the advent of NASA missions to comets like “Deep Impact” in 2010, a much larger database has been developed. “We now have a much better understanding of what a comet may be like and it is still consistent with the K-Pg boundary data we are seeing,” Moore adds.

Sharma says that, “In synthesizing the data generated by two very disparate fields of research-geochemistry and geophysics-we are now 99.9 percent sure that what we are dealing with is a 66-million-year-old comet impact-not an asteroid.”

Researchers say a comet killed the dinosaurs

Professors Mukul Sharma (left) and Jason Moore of the Department of Earth Sciences revisit the departure of the dinosaurs. -  Eli Burakian, Dartmouth College
Professors Mukul Sharma (left) and Jason Moore of the Department of Earth Sciences revisit the departure of the dinosaurs. – Eli Burakian, Dartmouth College

In a geological moment about 66 million years ago, something killed off almost all the dinosaurs and some 70 percent of all other species living on Earth. Only those dinosaurs related to birds appear to have survived. Most scientists agree that the culprit in this extinction was extraterrestrial, and the prevailing opinion has been that the party crasher was an asteroid.

Not so, say two Dartmouth researchers. Professors Jason Moore and Mukul Sharma of the Department of Earth Sciences favor another explanation, asserting that a high-velocity comet led to the demise of the dinosaurs.

Recently, asteroids have been in the headlines. On February 15, 2013, an asteroid exploded in the skies over Siberia. Later that day, another swept past the Earth in what some regard as a close call-just 17,000 miles away.

The asteroid impact theory of extinction began with discoveries by the late physicist and Nobel Laureate Luis Alvarez and his son, the geologist Walter Alvarez, a professor at the University of California, Berkeley. In 1980 they identified extremely high concentrations of the element iridium in a layer of rock known as the K-Pg (formerly called K-T) boundary. The layer marks the end of the Cretaceous period (abbreviated “K”), the epoch of the dinosaurs, and the beginning of the Paleogene period, with its notable absence of the large lizards.

While iridium is rare in the Earth’s crust, it is a common trace element in rocky space debris such as asteroids. Based on the elevated levels of iridium found worldwide in the boundary layer, the Alvarezes suggested that this signaled a major asteroid strike around the time of the K-Pg boundary-about 66 million years ago. Debate surrounded their theory until 2010, when a panel of 41 scientists published a report in support of the Alvarezes’ theory. The panel confirmed that a major asteroid impact had occurred at the K-Pg boundary and was responsible for mass extinctions.

The scientific community today looks to the deeply buried and partially submerged, 110-mile wide Chicxulub crater in Mexico’s Yucatán as the place where the death-dealing asteroid landed. The 66-million-year age of Chicxulub, discovered in 1990, coincides with the KT boundary, leading to the conclusion that what caused the crater also wiped out the dinosaurs.

Moore and Sharma do agree with fellow scientists that Chicxulub was the impact zone, but dispute the characterization of the object from space as an asteroid. In a paper presented to the 44th Lunar and Planetary Conference on March 22, 2013, they described their somewhat controversial findings.

Moore notes that in the past geochemists toiled away, isolated from their geophysicist colleagues, each focused on his or her particular area of expertise. “There hadn’t been a concerted synthesis of all the data from these two camps,” says Moore. “That’s what we’ve tried to do.”

The Dartmouth duo compiled all the published data on iridium from the K-Pg boundary. They also included the K-Pg data on osmium-another element common in space rock. In sifting through all this they found a wide range of variability, so consequently kept only the figures they demonstrated to be most reliable. “Because we are bringing a fresh set of eyes into this field, we feel our decisions are objective and unbiased,” says Sharma.

For example, they deleted data drawn from deep ocean cores where there were very high amounts of iridium. “We discovered that even then there was a huge variation. It was much worse in the oceans than on the continents,” Sharma said. “We figured out that the oceanic variations are likely caused by preferential concentration of iridium bearing minerals in marine sediments.”

In the final analysis, the overall trace element levels were much lower than those that scientists had been using for decades and being this low weakened the argument for an asteroid impact explanation. However, a comet explanation reconciles the conflicting evidence of a huge impact crater with the revised, lower iridium/osmium levels at the K-Pg boundary.

“We are proposing a comet because that conclusion hits a ‘sweet spot.’ Comets have a lower percentage of iridium and osmium than asteroids, relative to their mass, yet a high-velocity comet would have sufficient energy to create a 110-mile-wide crater,” says Moore. “Comets travel much faster than asteroids, so they have more energy on impact, which in combination with their being partially ice means they are not contributing as much iridium or osmium.”

Moore attributes much of the early resistance to a comet impact theory to a lack of knowledge about comets in general. “We weren’t certain whether they were dirty snowballs or icy dirt balls,” he says. “Today, we are inclined toward the icy dirt ball description.”

Comet composition and physical structure were unknown, but with the advent of NASA missions to comets like “Deep Impact” in 2010, a much larger database has been developed. “We now have a much better understanding of what a comet may be like and it is still consistent with the K-Pg boundary data we are seeing,” Moore adds.

Sharma says that, “In synthesizing the data generated by two very disparate fields of research-geochemistry and geophysics-we are now 99.9 percent sure that what we are dealing with is a 66-million-year-old comet impact-not an asteroid.”

Arctic climate under greenhouse conditions in the Late Cretaceous

Fossil diatom algae of Cretaceous age from the Alpha Ridge of the Arctic Ocean
Fossil diatom algae of Cretaceous age from the Alpha Ridge of the Arctic Ocean

New evidence for ice-free summers with intermittent winter sea ice in the Arctic Ocean during the Late Cretaceous – a period of greenhouse conditions – gives a glimpse of how the Arctic is likely to respond to future global warming.

Records of past environmental change in the Arctic should help predict its future behaviour. The Late Cretaceous, the period between 100 and 65 million years ago leading up to the extinction of the dinosaurs, is crucial in this regard because levels of carbon dioxide (CO2) were high, driving greenhouse conditions. But scientists have disagreed about the climate at this time, with some arguing for low Arctic late Cretaceous winter temperatures (when sunlight is absent during the Polar night) as against more recent suggestions of a somewhat milder 15°C mean annual temperature.

Writing in Nature, Dr Andrew Davies and Professor Alan Kemp of the University of Southampton’s School of Ocean and Earth Science based at the National Oceanography Centre, Southampton, along with Dr Jennifer Pike of Cardiff University take this debate a step forward by presenting the first seasonally resolved Cretaceous sedimentary record from the Alpha Ridge of the Arctic Ocean.

The scientists analysed the remains of diatoms – tiny free-floating plant-like organisms – preserved in late Cretaceous marine sediments. In modern oceans, diatoms play a dominant role in the ‘biological carbon pump’ by which carbon dioxide is drawn down from the atmosphere through photosynthesis and a proportion of it exported to the deep ocean. Unfortunately, the role of diatoms in the Cretaceous oceans has until now been unclear, in part because they are often poorly preserved in sediments.

But the researchers struck lucky. “With remarkable serendipity,” they explain, ” successive US and Canadian expeditions that occupied floating ice islands above the Alpha Ridge of the Arctic Ocean, recovered cores containing shallow buried upper Cretaceous diatom ooze with superbly preserved diatoms.” This has allowed them to conduct a detailed study of the diatom fossils using sophisticated electron microscopy techniques. In the modern ocean, scientists use floating sediment traps to collect and study settling material. These electron microscope techniques that have been pioneered by Professor Kemp’s group at Southampton have unlocked a ‘palaeo-sediment trap’ to reveal information about Late Cretaceous environmental conditions.

They find that the most informative sediment core samples display a regular alternation of microscopically thin layers composed of two distinctly different diatom assemblages, reflecting seasonal changes. Their analysis clearly demonstrates that seasonal blooming of diatoms was not related to the upwelling of nutrients, as has been previously suggested. Rather, production occurred within a stratified water column, indicative of ice-free summers. These summer blooms comprised specially adapted species resembling those of the modern North Pacific Subtropical Gyre, or preserved in relatively recent organically rich Mediterranean sediments called ‘sapropels’.

The sheer number of diatoms found in the Late Cretaceous sediment cores indicates exceptional abundances equalling modern values for the most productive areas of the Southern Ocean. “This Cretaceous production, dominated by diatoms adapted to stratified conditions of the polar summer may also be a pointer to future trends in the modern ocean,” say the researchers: “With increasing CO2 levels and global warming giving rise to increased ocean stratification, this style of (marine biological) production may become of increasing importance.”

However, thin accumulations of earthborn sediment within the diatom ooze are consistent with the presence of intermittent sea ice in the winter, a finding that supports “a wide body of evidence for low Arctic late Cretaceous winter temperatures rather than recent suggestions of a 15C mean annual temperature at this time.” The size distribution of clay and sand grains in the sediment points to the formation of sea ice in shallow coastal seas during autumn storms but suggests the absence of larger drop-stones suggests that the winters, although cold, were not cold enough to support thick glacial ice or large areas of anchored ice.

Commenting on the findings, Professor Kemp said: “Although seasonally-resolved records are rarely preserved, our research shows that they can provide a unique window into past Earth system behaviour on timescales immediately comparable and relevant to those of modern concern.”

Refining the date of dinosaur extinction





At Zumaia in the Basque country of northern Spain, sediments laid down around the end of the Cretaceous period show layers of light limestone and dark marl reflecting warm and cool periods, respectively, in Earth's climate. These alternating climatic periods are caused by 100,000-year and 405,000-year cycles in Earth's orbital eccentricity. Because Earth's orbit, and thus the relative ages of the sediment layers, can be precisely calculated, dating of the sediments by the argon-argon method provided a much-needed recalibration of the method and made it possible to pinpoint the Cretaceous/Tertiary boundary at 65.95 million years ago. (Image courtesy of Science)
At Zumaia in the Basque country of northern Spain, sediments laid down around the end of the Cretaceous period show layers of light limestone and dark marl reflecting warm and cool periods, respectively, in Earth’s climate. These alternating climatic periods are caused by 100,000-year and 405,000-year cycles in Earth’s orbital eccentricity. Because Earth’s orbit, and thus the relative ages of the sediment layers, can be precisely calculated, dating of the sediments by the argon-argon method provided a much-needed recalibration of the method and made it possible to pinpoint the Cretaceous/Tertiary boundary at 65.95 million years ago. (Image courtesy of Science)

Improved rock-dating method pinpoints dinosaur demise with unprecedented precision



Scientists at the University of California, Berkeley, and the Berkeley Geochronology Center have pinpointed the date of the dinosaurs’ extinction more precisely than ever thanks to refinements to a common technique for dating rocks and fossils.



The argon-argon dating method has been widely used to determine the age of rocks, whether they’re thousands or billions of years old. Nevertheless, the technique had systematic errors that produced dates with uncertainties of about 2.5 percent, according to Paul Renne, director of the Berkeley Geochronology Center and an adjunct professor of earth and planetary science at UC Berkeley.



Renne and his colleagues in Berkeley and in the Netherlands now have lowered this uncertainty to 0.25 percent and brought it into agreement with other isotopic methods of dating rocks, such as uranium/lead dating. As a result, argon-argon dating today can provide more precise absolute dates for many geologic events, ranging from volcanic eruptions and earthquakes to the extinction of the dinosaurs and many other creatures at the end of the Cretaceous period and the beginning of the Tertiary period. That boundary had previously been dated at 65.5 million years ago, give or take 300,000 years.



According to a paper by Renne’s team in the April 25 issue of Science, the best date for the Cretaceous-Tertiary, or K/T, boundary is now 65.95 million years, give or take 40,000 years.



“The importance of the argon-argon technique is that it is the only technique that has the dynamic range to cover nearly all of Earth’s history,” Renne said. “What this refinement means is that you can use different chronometers now and get the same answer, whereas, that wasn’t true before.”



Renne noted that the greater precision matters little for recent events, such as the emergence of human ancestors in Africa 6 million years ago, because the uncertainty is only a few tens of thousands of years.



“Where it really adds up is in dating events in the early solar system,” Renne said. “A 1 percent difference at 4.5 billion years is almost 50 million years.”


One major implication of the revision involves the formation of meteorites, planetessimals and planets in the early solar system, he said. Argon-argon dating was giving a lower date than other methods for the formation of meteorites, suggesting that they cooled slowly during the solar system’s infancy.



“The new result implies that many of these meteorites cooled very, very quickly, which is consistent with what is known or suggested from other studies using other isotopic systems,” he said. “The evolution of the early solar system – the accretion of planetessimals, the differentiation of bodies by gravity while still hot – happened very fast. Argon-argon dating is now no longer at odds with that evidence, but is very consistent with it.”



Renne has warned geologists for a decade of uncertainty in the argon-argon method and has been correcting his own data since 2000, but it took a collaboration that he initiated in 1998 with Jan R. Wijbrans of the Free University in the Netherlands to obtain convincing evidence. Wijbrans and his Dutch colleagues were studying a unique series of sediments from the Messinian Melilla-Nador Basin on the coast of Morocco that contain records of cycles in Earth’s climate that reflect changes in Earth’s orbit that can be precisely calculated.



Wijbrans’ colleague Frits Hilgen at the University of Utrecht, a coauthor of the study, has been one of the world’s leaders in translating the record of orbital cycles into a time scale for geologists, according to Renne. Renne’s group had proposed using the astronomical tuning approach to calibrate the argon-argon method as early as 1994, but lacked ideal sedimentary sequences to realize the full power of this approach. The collaboration brought together all the appropriate expertise to bring this approach to fruition, he said.



“The problem with astronomical dating of much older sediments, even when they contain clear records of astronomical cycles, is that you’re talking about a pattern that is not anchored anywhere,” Renne said. “You see a bunch of repetitions of features in sediments, but you don’t know where to start counting.”



Argon-argon dating of volcanic ash, or tephra, in these sediments provided that anchor, he said, synchronizing the methods and making each one more precise. The argon-argon analyses were conducted both in Berkeley and Amsterdam to eliminate interlaboratory bias.



Argon-argon dating, developed at UC Berkeley in the 1960s, is based on the fact that the naturally-occurring isotope potassium-40 decays to argon-40 with a 1.25-billion-year half-life. Single-grain rock samples are irradiated with neutrons to convert potassium-39 to argon-39, which is normally not present in nature. The ratio of argon-40 to argon-39 then provides a measurement of the age of the sample.



“This should be the last big revision of argon-argon dating,” Renne said. “We’ve finally narrowed it down to where we are talking about fractions-of-a-percent revisions in the future, at most.”



Klaudia Kuiper, the lead author of the Science paper, was a Ph.D. student in Amsterdam working with study coauthors Wijbrans, Hilgen and Wout Krijgsman when the study was initiated. She also conducted lab work with Renne and Alan Deino, a geochronologist with Renne at the Berkeley Geochronology Center who was also one of the study’s coauthors.



The work was funded by the U.S. and Dutch National Science Foundations and the Ann and Gordon Getty Foundation.

A Warming Climate Can Support Glacial Ice





Sea cliff at Tilleul Beach on the coast of Normandy, France are rich in microfossils and of the same age as the marine chalks used in the study to understand Earth's climate history.
Sea cliff at Tilleul Beach on the coast of Normandy, France are rich in microfossils and of the same age as the marine chalks used in the study to understand Earth’s climate history.

New research challenges the generally accepted belief that substantial ice sheets could not have existed on Earth during past super-warm climate events. The study by researchers at Scripps Institution of Oceanography at UC San Diego provides strong evidence that a glacial ice cap, about half the size of the modern day glacial ice sheet, existed 91 million years ago during a period of intense global warming. This study offers valuable insight into current day climate conditions and the environmental mechanisms for global sea level rise.



The new study in the Jan. 11 issue of the journal Science titled, “Isotopic Evidence for Glaciation During the Cretaceous Supergreenhouse,” examines geochemical and sea level data retrieved from marine microfossils deposited on the ocean floor 91 million years ago during the Cretaceous Thermal Maximum. This extreme warming event in Earth’s history raised tropical ocean temperatures to 35-37°C (95-98.6°F), about 10°C (50°F) warmer than today, thus creating an intense greenhouse climate.



Using two independent isotopic techniques, researchers at Scripps Oceanography studied the microfossils to gather geochemical data on the growth and eventual melting of large Cretaceous ice sheets. The researchers compared stable isotopes of oxygen molecules (d18O) in bottom-dwelling and near-surface marine microfossils, known as foraminifera, to show that changes in ocean chemistry were consistent with the growth of an ice sheet. The second method in which an ocean surface temperature record was subtracted from the stable isotope record of surface ocean microfossils yielded the same conclusion.






A micrograph of two types of foraminifera, M. sinuosa and W. baltica, and uses to study climate conditions during the Cretaceous Thermal Maximum, 91 million years ago.
A micrograph of two types of foraminifera, M. sinuosa and W. baltica, and uses to study climate conditions during the Cretaceous Thermal Maximum, 91 million years ago.

These independent methods provided Andre Bornemann, lead author of the study, with strong evidence to conclude that an ice sheet about 50-60 percent the size of the modern Antarctic ice cap existed for about 200,000 years. Bornemann conducted this study as a postdoctoral researcher at Scripps Oceanography and continues this research at Universitat Leipzig in Germany.



“Until now it was generally accepted that there were no large glaciers on the poles prior to the development of the Antarctic ice sheet about 33 million years ago,” said Richard Norris, professor of paleobiology at Scripps Oceanography and co-author of the study. “This study demonstrates that even the super-warm climates of the Cretaceous Thermal Maximum were not warm enough to prevent ice growth.”



Researchers are still unclear as to where such a large mass of ice could have existed in the Cretaceous or how ice growth could have started. The authors suggest that climate cycles may have favored ice growth during a few times in the Cretaceous when natural climate variations produced unusually cool summers. Likewise, high mountains under the modern Antarctic ice cap could have been potential sites for growth of large ice masses during the Cretaceous.





Graph depicts geochemical data collected from microfossils on the growth and eventual melting of ice sheets during the Cretaceous Period.
Graph depicts geochemical data collected from microfossils on the growth and eventual melting of ice sheets during the Cretaceous Period.

Ice sheets were much less common during the Cretaceous Thermal Maximum than during more recent “icehouse” climates. Paradoxically, past greenhouse climates may have aided ice growth by increasing the amount of moisture in the atmosphere and creating more winter snowfall at high elevations and high latitudes, according to the paper’s authors.



The results from the study are consistent with other studies from Russia and New Jersey that show sea level fell by about 25-40 m (82-131 ft) at the same time that the ice sheets were growing during the Cretaceous period. Sea level is known to fall as water is removed from the oceans to build continental ice sheets; conversely, sea level rises as ice melts and returns to the sea.



The presence or absence of sea ice has major environmental implications, specifically in terms of sea level rise and global circulation patterns. As humans continue to add large amounts of carbon dioxide and other greenhouse gases that accelerate the heating of the atmosphere and oceans, research on Earth’s past climate conditions is critical to predict what will happen as Earth’s climate continues to warm.



This research study was supported by the German Research Foundation and the National Science Foundation under the management of the Joint Oceanographic Institutions.