For millions of years after the formation of the earth, numerous ice ages have dominated the planet. They have come and gone over the years, leaving their marks on the land on which they have traversed. Proof of their existence rests in fossil records excavated from once ice-covered locations, the markings found on certain rock formations and the evidence recorded in the ice themselves. One interesting notion found in these records is the fact that glacial cycles seem to exist. There are evidences that the ice sheets of long ago had waxed and waned for certain period of time. The cause of these waxing and waning have thrown scientists into debates, especially when one interesting transition came about.
Paleoclimatology, the study of the earths ancient climate, has uncovered one event that involved glacial cycles. Scientists have discovered that in the late Pliocene and up to the early Pleistocene, the glacial cycles lasted about 40,000 years. But during the late Pleistocene period, the cycle became a 100,00 year period. This transition intrigued scientists what caused this change in pattern Many hypotheses have been put forth, each one trying to explain this phenomenon.
Mysterious Markings
In the Northern Hemisphere, great mountain ranges dominate the landscape. These have been formed by powerful geological forces that have carved and shaped the entire planet. Some of these mountains are capped with snow due to their height.
One example of a great mountain range is the Alps, a natural feature of Switzerland. Down in the slopes of this mountain range, and even further below in the valleys of Switzerland, strange markings can be found on the landscape. Certain grooving and rounding in rocks have caused scientists to wonder what caused these markings on the rocks What kind of force was strong enough to carve visible and lasting indentions in the rocks
In 1837, one scientist offered an explanation for this phenomenon. Louis Agassiz, now known as the Father of Glaciology, proposed that vast ice sheets once extended down from the higher Alps and had covered the entire valley of northwestern Switzerland and had also reached the southern slopes of Jura. He also said that Switzerland once possessed the same glacial features that Greenland has today. The question then was how was that possible How could great ice sheets flow down from the top of the mountains
The Ice Ages
Not long after Agassiz suggestion of a vast ice sheet covering the European and North American continents, the idea that glacial cold periods have started to take hold in the scientific community. Later on, it would be discovered that the earth experienced a number of ice ages scattered in the millions of years before Agassiz time.
In the study of the ancient climate of the earth, it has been found that these glacial cold periods or ice ages were separated by periods of interglacial climate. This means that the great ice sheets seem to flow downward, covering almost everything in its path, and then eventually retreat to where its origins are. This waxing and waning gave rise to debates among scientists on what caused such an event to happen.
The Emergence of the Hypotheses
There were a number of hypotheses that tried to explain the transition from glacial to interglacial periods. They used parameters such as eccentricity, obliquity, precession and insolation as determinants for the glacial-interglacial transition. This paper would be discussing four of these proposed explanations.
Five years after Agassiz theory about glaciation, one Joseph Adhemar put forth the idea that glaciation was connected with how the earth orbits the sun. The changes in how the planet revolves around the sun affects its climate. He proposed that glaciation happens when winters are strangely long. This occurs when wintertime coincides with the period when the earth is farthest from the sun, or in scientific terms, is in its aphelion.
In the 1860s, James Croll suggested that these ice ages occur due to the weak solar radiation or insolation at the time of the long aphelion-related winter. He said that the fact that winter is unusually long does not mean that glaciation is occurring.
In the 1930s, a new hypothesis surfaced. This was proposed by Milutin Milankovitch. According to him, glaciation happens when the intensity of insolation is weak at high northern latitudes during summer. This event occurs when both the earths spin axis is less tilted with respect to the orbital plane and aphelion coincides with summer (not winter) in the Northern Hemisphere.(Raymo and Huyber, 2008) Snow and ice remain throughout the year, eventually forming ice sheets, due to the low insolation during the summer. These ice sheets can get so vast, thus forming great glaciers.
Recently, new approaches to the aforementioned hypotheses have been made. New arguments have evolved from the hypotheses presented by Adhemar, Croll and Milankovitch. Some of them follow one, two, or all of the theories. One researcher, Peter Huybers, argued that glaciation does occur due to the insolation in summer, but that this solar radiation is not controlled by precession. Rather it is obliquity that drives insolation. He explained that, following Keplers second law, the duration of the summer is inversely proportional to Earths distance from the Sun.(Huybers, 2009)
Each hypothesis provide their own explanation on how glaciation occurs, and why there are cycles of ice ages. But one intriguing notion that baffles scientists up to this day is the transition of glacial cycles from a 40,000 year period to a 100,000 one. This change was seen in late Pleistocene period. What caused this to happen
Evidence from the past
In order to analyze the cause of the glacial cycles change from a 40ky cycle to a 100ky cycle, scientists use drill core samples of ocean sediment and ice. Each drill core contains different bands which represent a certain era or period in the earths geologic timescale. Using this technique, scientists were able to prove (or even disprove) some of the hypotheses presented earlier.
For example, in 1976, three scientists, namely James Hays, John Imbrie and Nicholas Shackleton, used core samples that helped strengthen the orbital hypothesis for glaciation. They applied the newly developed geomagnetic timescale to a deep-sae sediment core, and found that long-term changes in oxygen isotope ratios were concetrated at the frequencies predicted by the aforementioned hypothesis. This was what they found recorded in the fossil remains of foraminifera. The ratio of oxygen-18 to oxygen-16 in the ocean was known to increase with glaciation, because oxygen-16 evaporates preferentially and is concentrated in ice sheets. Hays showed that the change in O18 varied with cycles of 41,000 years, the period associated with changes in the tilt of Earths spin axis (or obliquity), and around 21,000 years, the period associated with the location of aphelion with respect to the seasons (also known as climatic precession or the precession of equinoxes).(Raymo and Huybers, 2008)
On a different test, ice core record indicated that greenhouse gases co-varied with antarctic temperature over glacial-interglacial cycles, suggesting a close link between natural atmospheric greenhouse gas variations and temperature. Variations in CO2 over the last 420 kyr broadly followed antarctic temperature, typically by several centuries to a millennium(Mudelsee, 2001).(Jansen and Overpeck, N.D.) This just means that the levels of atmospheric gases also contributed to the glacial-interglacial transitions that the earth had experienced.
Complications
Even if proxy ice records like benthic O18 were used to study the change in transition of the glacial cycles, there are still many complications that remain to be solved by the scientists. One example of this is the weak signal of the 23 kyr before 1 MA. This is because Earths orbital precession is out of phase between hemispheres, 23 kyr changes in ice volume in each hemisphere cancel globally integrated proxies such as ocean O18 or sea level leaving the in-phase obliquity (41 kyr) component of insolation to dominate the record.(Lisiecki, Nisancioglu, Tziperman, and Kawamura, N.D.) Also, there is not enough established links between eccentricity, obliquity or insolation and the 40kyr-100kyr transition, or even with the 100 kyr cycle itself. For example, eccentricity has one of the most issues in the glaciation cycle. The following is from a research condicted which investigated eccentricity as part of its research
The simple model developed for the linearly forced precession and tilt frequency bands cannot be applied directly to the eccentricity (100 ky) band. Some of the issues
In a recent paper, Shackleton has compared 18O and CO2 in trapped air in the Vostok ice core with deep-sea 18O records. By independently tuning the ice and oceanic records to the Milankovitch tilt and precession cycles, he concludes that in the100 ky band, maximum NH summer radiation, deep water temperature, antarctic ice surface temperature (based on DH ratio), and atmospheric CO2 are in phase (with 95 confidence), whereas ice volume lags by about 15 ky (Figure GC-11). This result throws no extra light on the link between eccentricity and the correlated proxy changes, however.
Figure GC-11. Amplitude, coherence, and phase spectra for deep-sea temperature (A), and ice volume (B) from deep sea sediments and for DH (C) and CO2 (D) from the Vostok ice core relative to an eccentricity(e)-tilt(t)-precession(p) record that assigns roughly equal variance to each component and with the phase of midsummer Northern Hemisphere insolation. The solid line is the amplitude spectrum, the shaded area the coherence spectrum, and the phase spectrum (with 95 confidence limits) uses the convention that a positive angle is a phase lag.
One of the conclusions drawn by in the research was that linear forcing by variations in incoming radiation due to eccentricity is not possible.
The samples used themselves can prove to cause some complications. There have been core samples that resemble ocean and ice records, but they have been found to differ greatly from the SPECMAP and other tuned chronologies. Another is that drill core samples may be rejected if they do not meet the specifications given by the Milankovitch hypothesis, thus contributing to the computational error of the acquired data.
There are some researchers who have developed explanations for the 40ky-100ky transition. Maureen Raymo and her colleagues put forward a model in which Northern Hemisphere ice sheets wax and wane at precession periods, driven by strongly nonlinear response of ice ablation to summer insolation intensity.(Raymo and Huybers, 2008) Another researcher, Peter Huybers, said that a cooling climate during the Pleistocene may have permitted ice volume to build up over multiple forcing cycles, allowing sensitivity to increase until an increase in summer energy triggers glacial termination.(Huybers, 2009)
What is the conclusion
Presently, what is accepted in the scientific community is the theory proposed by Milankovitch, and this appears to be the most used main basis of emerging theories today. But there have been some records that seem to defy the arguments of the presented hypothesis, even that of Milankovitch. As what was mentioned before, there are issues concerning the records used themselves. Another is that the methods used in reading ice core records still need thorough development, and even the way they are interpreted need to be refined. Because of this, further study is needed to fully grasp the reason behind the glacial-interglacial cycles, let alone the transition from 40ky cycles to 100ky cycles. Even the researchers Raymo and Huybers suggest that a more thorough investigation of proxy records is needed to be done so as to be able to interpret more properly the data incorporated in them. Armed with additional knowledge, we would be able to finally decipher the mysteries hidden in the ice. We would finally see clearly the history kept within the great ice sheets of our planet. And maybe, we would be able to predict what would happen to the earths glaciers in the future
Paleoclimatology, the study of the earths ancient climate, has uncovered one event that involved glacial cycles. Scientists have discovered that in the late Pliocene and up to the early Pleistocene, the glacial cycles lasted about 40,000 years. But during the late Pleistocene period, the cycle became a 100,00 year period. This transition intrigued scientists what caused this change in pattern Many hypotheses have been put forth, each one trying to explain this phenomenon.
Mysterious Markings
In the Northern Hemisphere, great mountain ranges dominate the landscape. These have been formed by powerful geological forces that have carved and shaped the entire planet. Some of these mountains are capped with snow due to their height.
One example of a great mountain range is the Alps, a natural feature of Switzerland. Down in the slopes of this mountain range, and even further below in the valleys of Switzerland, strange markings can be found on the landscape. Certain grooving and rounding in rocks have caused scientists to wonder what caused these markings on the rocks What kind of force was strong enough to carve visible and lasting indentions in the rocks
In 1837, one scientist offered an explanation for this phenomenon. Louis Agassiz, now known as the Father of Glaciology, proposed that vast ice sheets once extended down from the higher Alps and had covered the entire valley of northwestern Switzerland and had also reached the southern slopes of Jura. He also said that Switzerland once possessed the same glacial features that Greenland has today. The question then was how was that possible How could great ice sheets flow down from the top of the mountains
The Ice Ages
Not long after Agassiz suggestion of a vast ice sheet covering the European and North American continents, the idea that glacial cold periods have started to take hold in the scientific community. Later on, it would be discovered that the earth experienced a number of ice ages scattered in the millions of years before Agassiz time.
In the study of the ancient climate of the earth, it has been found that these glacial cold periods or ice ages were separated by periods of interglacial climate. This means that the great ice sheets seem to flow downward, covering almost everything in its path, and then eventually retreat to where its origins are. This waxing and waning gave rise to debates among scientists on what caused such an event to happen.
The Emergence of the Hypotheses
There were a number of hypotheses that tried to explain the transition from glacial to interglacial periods. They used parameters such as eccentricity, obliquity, precession and insolation as determinants for the glacial-interglacial transition. This paper would be discussing four of these proposed explanations.
Five years after Agassiz theory about glaciation, one Joseph Adhemar put forth the idea that glaciation was connected with how the earth orbits the sun. The changes in how the planet revolves around the sun affects its climate. He proposed that glaciation happens when winters are strangely long. This occurs when wintertime coincides with the period when the earth is farthest from the sun, or in scientific terms, is in its aphelion.
In the 1860s, James Croll suggested that these ice ages occur due to the weak solar radiation or insolation at the time of the long aphelion-related winter. He said that the fact that winter is unusually long does not mean that glaciation is occurring.
In the 1930s, a new hypothesis surfaced. This was proposed by Milutin Milankovitch. According to him, glaciation happens when the intensity of insolation is weak at high northern latitudes during summer. This event occurs when both the earths spin axis is less tilted with respect to the orbital plane and aphelion coincides with summer (not winter) in the Northern Hemisphere.(Raymo and Huyber, 2008) Snow and ice remain throughout the year, eventually forming ice sheets, due to the low insolation during the summer. These ice sheets can get so vast, thus forming great glaciers.
Recently, new approaches to the aforementioned hypotheses have been made. New arguments have evolved from the hypotheses presented by Adhemar, Croll and Milankovitch. Some of them follow one, two, or all of the theories. One researcher, Peter Huybers, argued that glaciation does occur due to the insolation in summer, but that this solar radiation is not controlled by precession. Rather it is obliquity that drives insolation. He explained that, following Keplers second law, the duration of the summer is inversely proportional to Earths distance from the Sun.(Huybers, 2009)
Each hypothesis provide their own explanation on how glaciation occurs, and why there are cycles of ice ages. But one intriguing notion that baffles scientists up to this day is the transition of glacial cycles from a 40,000 year period to a 100,000 one. This change was seen in late Pleistocene period. What caused this to happen
Evidence from the past
In order to analyze the cause of the glacial cycles change from a 40ky cycle to a 100ky cycle, scientists use drill core samples of ocean sediment and ice. Each drill core contains different bands which represent a certain era or period in the earths geologic timescale. Using this technique, scientists were able to prove (or even disprove) some of the hypotheses presented earlier.
For example, in 1976, three scientists, namely James Hays, John Imbrie and Nicholas Shackleton, used core samples that helped strengthen the orbital hypothesis for glaciation. They applied the newly developed geomagnetic timescale to a deep-sae sediment core, and found that long-term changes in oxygen isotope ratios were concetrated at the frequencies predicted by the aforementioned hypothesis. This was what they found recorded in the fossil remains of foraminifera. The ratio of oxygen-18 to oxygen-16 in the ocean was known to increase with glaciation, because oxygen-16 evaporates preferentially and is concentrated in ice sheets. Hays showed that the change in O18 varied with cycles of 41,000 years, the period associated with changes in the tilt of Earths spin axis (or obliquity), and around 21,000 years, the period associated with the location of aphelion with respect to the seasons (also known as climatic precession or the precession of equinoxes).(Raymo and Huybers, 2008)
On a different test, ice core record indicated that greenhouse gases co-varied with antarctic temperature over glacial-interglacial cycles, suggesting a close link between natural atmospheric greenhouse gas variations and temperature. Variations in CO2 over the last 420 kyr broadly followed antarctic temperature, typically by several centuries to a millennium(Mudelsee, 2001).(Jansen and Overpeck, N.D.) This just means that the levels of atmospheric gases also contributed to the glacial-interglacial transitions that the earth had experienced.
Complications
Even if proxy ice records like benthic O18 were used to study the change in transition of the glacial cycles, there are still many complications that remain to be solved by the scientists. One example of this is the weak signal of the 23 kyr before 1 MA. This is because Earths orbital precession is out of phase between hemispheres, 23 kyr changes in ice volume in each hemisphere cancel globally integrated proxies such as ocean O18 or sea level leaving the in-phase obliquity (41 kyr) component of insolation to dominate the record.(Lisiecki, Nisancioglu, Tziperman, and Kawamura, N.D.) Also, there is not enough established links between eccentricity, obliquity or insolation and the 40kyr-100kyr transition, or even with the 100 kyr cycle itself. For example, eccentricity has one of the most issues in the glaciation cycle. The following is from a research condicted which investigated eccentricity as part of its research
The simple model developed for the linearly forced precession and tilt frequency bands cannot be applied directly to the eccentricity (100 ky) band. Some of the issues
In a recent paper, Shackleton has compared 18O and CO2 in trapped air in the Vostok ice core with deep-sea 18O records. By independently tuning the ice and oceanic records to the Milankovitch tilt and precession cycles, he concludes that in the100 ky band, maximum NH summer radiation, deep water temperature, antarctic ice surface temperature (based on DH ratio), and atmospheric CO2 are in phase (with 95 confidence), whereas ice volume lags by about 15 ky (Figure GC-11). This result throws no extra light on the link between eccentricity and the correlated proxy changes, however.
Figure GC-11. Amplitude, coherence, and phase spectra for deep-sea temperature (A), and ice volume (B) from deep sea sediments and for DH (C) and CO2 (D) from the Vostok ice core relative to an eccentricity(e)-tilt(t)-precession(p) record that assigns roughly equal variance to each component and with the phase of midsummer Northern Hemisphere insolation. The solid line is the amplitude spectrum, the shaded area the coherence spectrum, and the phase spectrum (with 95 confidence limits) uses the convention that a positive angle is a phase lag.
One of the conclusions drawn by in the research was that linear forcing by variations in incoming radiation due to eccentricity is not possible.
The samples used themselves can prove to cause some complications. There have been core samples that resemble ocean and ice records, but they have been found to differ greatly from the SPECMAP and other tuned chronologies. Another is that drill core samples may be rejected if they do not meet the specifications given by the Milankovitch hypothesis, thus contributing to the computational error of the acquired data.
There are some researchers who have developed explanations for the 40ky-100ky transition. Maureen Raymo and her colleagues put forward a model in which Northern Hemisphere ice sheets wax and wane at precession periods, driven by strongly nonlinear response of ice ablation to summer insolation intensity.(Raymo and Huybers, 2008) Another researcher, Peter Huybers, said that a cooling climate during the Pleistocene may have permitted ice volume to build up over multiple forcing cycles, allowing sensitivity to increase until an increase in summer energy triggers glacial termination.(Huybers, 2009)
What is the conclusion
Presently, what is accepted in the scientific community is the theory proposed by Milankovitch, and this appears to be the most used main basis of emerging theories today. But there have been some records that seem to defy the arguments of the presented hypothesis, even that of Milankovitch. As what was mentioned before, there are issues concerning the records used themselves. Another is that the methods used in reading ice core records still need thorough development, and even the way they are interpreted need to be refined. Because of this, further study is needed to fully grasp the reason behind the glacial-interglacial cycles, let alone the transition from 40ky cycles to 100ky cycles. Even the researchers Raymo and Huybers suggest that a more thorough investigation of proxy records is needed to be done so as to be able to interpret more properly the data incorporated in them. Armed with additional knowledge, we would be able to finally decipher the mysteries hidden in the ice. We would finally see clearly the history kept within the great ice sheets of our planet. And maybe, we would be able to predict what would happen to the earths glaciers in the future
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