I am in Group 3 and we are presenting this week for part 3 chapters 8,9,10. Each member is responsible for 5 sections, however I read all the sections and took notes to make sure I understood the content. Notes below are the things I highlighted in my book.

8.1 Earth’s Tilted Axis of rotation and the season

• Earth rotates around an axis that passes through its poles
  • tilted at an angle of 23.58, called Earth’s “obliquity or tilt
• Earth has a once-a-year revolution around the Sun.
  • seasonal differences culminate at the summer and winter solstices, which mark the longest and shortest days of the year
• The changing position of the tilted Earth with respect to the Sun affect many things
• Earth maintains a constant angle of tilt (23.58) and a constant direction of this tilt in space.
  • Tilt towards the sun = summer and away form the sun = winter
• the lengths of night and day become equal in each hemisphere at the March and September equinoxes

8.2 Earth’s Eccentric Orbit: Distance Between Earth and Sun

• Earth’s orbit is not a perfect circle: it has a slightly eccentric or elliptical shape.
  • result of the gravitational pull of other planets on Earth as it moves through space
• the Sun lies at one of the two focal points in its elliptical orbit, as required by the physical laws of gravitation.
• Earth’s distance from the Sun changes according to its position in this elliptical orbit
  • changes in Earth-Sun distance affect the amount of solar radiation Earth receives, especially at two extreme positions in the orbit.
  • The position in which Earth is closest to the Sun is called perihelion
    • January 3 and causes winter radiation in the Northern Hemisphere and summer radiation in the Southern Hemisphere to be slightly stronger than they would be in a perfectly circular orbit.
  • The position farthest from the Sun is called aphelion.
    • July 4 causes summer radiation in the Northern Hemisphere and winter radiation in the Southern Hemisphere slightly weaker than they would be in a circular orbit
  • The effect of Earth’s elliptical orbit on its seasons is small, by just a few percent.
  • the time intervals between the two equinoxes are not equal: there are seven more days in the long part of the orbit than in the short part of the orbit.
    • The greater length of the interval tends to compensate for the fact that Earth is farther from the Sun in this part of the orbit and is thus receiving less solar radiation.

8.3 Changes in Earth’s Axial Tilt Through Time

• Without any tilt, no seasonal changes occur in the amount of solar radiation received at any latitude.
  • solstices and equinoxes do not even exist
• with a maximum tilt of 90 degrees. Solar radiation is directed straight at the summer-season pole, while the winter-season pole lies in complete darkness. Would flip in six months
• The angle of Earth’s tilt has varied through time in a narrow range, between values as small as 22.28 and as large as almost 24.58
• Cyclic changes in tilt angle occur mainly at a period of 41,000 years, the interval between successive peaks or successive valleys. The 41,000-year cycle is fairly regular, both in period and in amplitude.
• Changes in tilt amplify or suppress the strength of the seasons
  • Larger tilt angles turn the poles more directly toward the Sun in the summer and increase the amount of solar radiation received. Also away from the Sun further in the opposite hemisphere
  • Decreases in tilt have the opposite effect: they diminish the amplitude of seasonal differences. Smaller tilt angles put the Earth slightly closer to the configuration.

8.4 Changes in Earth’s Eccentric Orbit Through Time

• The shape of Earth’s orbit around the Sun has also varied in the past, at times becoming more circular and at other times more elliptical than today.
• The shape of an ellipse can be described by reference to its two main axes: the “major” axis and the “minor” axis. The degree of departure from a perfectly circular orbit can be described by:
  • ε = square root of a^2-b^2 divided by a  
  • where ε is the eccentricity of the ellipse and a and b are half of the lengths of the major and minor axes.
• The eccentricity of the elliptical orbit increases as these two axes become more unequal in length.
• Eccentricity (ε) has varied over time between values of 0.005 and 0.0607. The present value (0.0167) lies well toward the lower (more circular) end of the range.
• Changes in orbital eccentricity are concentrated mainly at two periods.
• One eccentricity cycle shows up as variations at intervals near 100,000 years.
  • consists of four cycles of nearly equal strength and periods ranging between 95,000 and 131,000 years
• The second eccentricity cycle has a wavelength of 413,000 years.
  • alternations of the 100,000-year cycles between larger and smaller peak values. Larger amplitudes can be seen ,200,000, ,600,000, ,1,000,000, and ,1,400,000 years ago
• A third eccentricity cycle has a wavelength of 2,100,000 years, but this cycle is much weaker in amplitude.

8.5 Precession of the Solstices

• The positions of the solstices and equinoxes in relation to the eccentric orbit have not always been fixed at their present locations.
  • they have slowly shifted through time with respect to the eccentric orbit and the perihelion (close pass) and aphelion (distant pass) positions.
• The cause of these changes lies in a long-term wobbling motion similar to that of a spinning top.
• The wobbling motion referred to here is not caused by changes in the amount of lean of the top, but rather by changes in the direction in which it leans
• Earth’s wobbling motion, called axial precession, is caused by the gravitational pull of the Sun and Moon on the slight bulge in Earth’s diameter at the equator.
  • Earth wobbles very slowly; it revolves 25,700 times around the Sun and rotates almost 10 million times on its axis during the time it takes to complete just a single wobble (25,700 x 365 = 9,380,000).
• A second kind of precessional motion is known as precession of the ellipse. This motion is even slower than the wobbling motion of axial precession
• The combined effects of these two precessional motions cause the solstices and equinoxes to move around Earth’s orbit, with one full orbit around the Sun completed approximately every 22,000 years
• we make use of two basic geometric characteristics of precessional motion to help understand precession of the equinoxes.
  • The first characteristic has to do with the angular form of Earth’s motion with respect to the Sun.
  • The second aspect of Earth’s orbital motion to be considered is eccentricity.
•  These gradual movements of precession bring the solstices and equinoxes into orbital positions that vary in distance from the Sun.
•  Eccentricity plays an important role in the effect of precession on the amount of solar radiation received on Earth.
• Long-term variations in the precessional index have two major characteristics.
  •  they occur at a cycle with a period near 23,000 years because of the regular angular motion of precession at that cycle.
  • individual cycles vary widely in amplitude because changes in eccentricity modulate the 23,000-year signal. At times, the 23,000-year cycle swings back and forth between extreme maxima and minima, while at other times the amplitude of the changes is small.
•  With greater eccentricity, the differences in distance between a close pass and a distant pass are magnified. With a nearly circular orbit, differences in distance almost vanish.
• The modulation of the εsinω signal by eccentricity is not a real cycle,
  • You have learned that eccentricity varies at cycles of 100,000 and 413,000 years, and you can see that the upper and lower envelopes of the εsinω signal vary at these periods. But the offsetting effects of the upper and lower envelopes cancel each other out

8.6 Insolation Changes by Month and Season

• The long-term trends of tilt and εsinω contain the information needed to calculate the amount of insolation arriving at any latitude and season.
• June and December insolation values over the last 300,000 years show a strong dominance of the 23,000- year precession cycle at lower and middle latitudes, and also at higher latitudes during the summer season.
• The 41,000-year cycle of tilt is not evident at lower latitudes but is visible in the low amplitude variations of winter-season insolation at higher mid-latitudes
• Changes in annual mean insolation at the 41,000- year tilt signal at high latitudes have the same sign as the summer insolation anomalies but are lower in amplitude.
• very small variations in received insolation do occur in connection with Earth’s eccentric orbit around the Sun, but these appear only as changes in the total energy received by the entire Earth, not as seasonal variations. These changes are governed by the term (1 – ε^2 ) 1/2. We have already seen that ε varies through time between 0.005 and 0.0607. Compared to changes in seasonal insolation of 10% or more at the tilt and precession cycles, these annual eccentricity changes are negligible
• The patterns of insolation changes for tilt and precession can be compared by season and by hemisphere.
• Insolation variations at high latitudes caused by changes in tilt are in phase between the hemispheres from a seasonal perspective.
• At high tilt angles, summer insolation maxima in the Northern Hemisphere occur at the same time in the 41,000- year cycle as summer insolation maxima in the Southern Hemisphere on the opposite side of the orbit.
  • Higher tilt produces more insolation at both poles in their respective summers because both poles are turned more directly toward the Sun.
  • more pronounced insolation minima also occur at both poles in winter as a result of a higher tilt: the two winter poles are tilted away from the Sun during the same orbit.
•  If we compare the North Pole with the South Pole during a particular month in the orbit, however, the two hemispheres are exactly out of phase
  • tilt causes opposite insolation effects at the North and South poles for a given point in the orbit
•  For precession, the relative sense of phasing between seasons and hemispheres differs from that of tilt
  • Because Earth-Sun distance is the major control on these changes in insolation, a position close to the Sun produces higher insolation than normal over all of Earth’s surface. A precessional cycle insolation maximum occurring at June 21 (or December 21) will be simultaneous everywhere on Earth. Distant pass positions (at aphelion) will simultaneously diminish insolation everywhere on Earth.
•  An important fact to remember about precession is that the seasons are reversed across the equator.
  • As a result of the seasonal reversal at the equator, insolation signals considered in terms of the season of the year are out of phase between the hemispheres for precession.
•  Another way of looking at the relative phasing of precessional insolation is to track changes between seasons in a single hemisphere.
  • precessional variations in insolation at any one location always move in opposite directions for the summer versus winter seasons
•  Precessional changes in insolation have an additional characteristic not found in changes caused by tilt: an entire family of insolation curves exists for each season and month (and even day) of the year
• As a result, each season and month experiences the same 23,000-year cycle of increasing and decreasing insolation values relative to the long-term mean, but the anomalies are offset in time from those of the preceding month or season. These offsets produce a family of monthly (and seasonal) insolation curves (Figure 8-18). Each successive month passes through perihelion (or aphelion) roughly 1,916 years later than the previous month did (1/12 x 23,000 = 1,916).

8.7 Insolation changes by Caloric Seasons

• Calculations of monthly insolation are complicated by an additional factor related to the eccentricity of Earth’s orbit.
  • its rate of angular motion in space is not constant.
    • Earth speeds up as it nears the extreme perihelion position and slows down near aphelion.
    • the solstices move slowly around the eccentric orbit, they gradually pass through regions of faster or slower movement in space
  • These changes in speed cause changes in the lengths of the months and seasons in relation to a year as determined by “calendar time” (day of the year).
    • changes in the amplitude of insolation variations in the monthly signals tend to be canceled by opposing changes in the lengths of the seasons
  • One way of minimizing these complications is to calculate the changes in insolation received on Earth in the framework of caloric seasons.
    • The summer caloric half-year is defined as the 182 days of the year when the incoming insolation exceeds the amount received during the other 182 days.
    • Caloric seasons are not fixed in relation to the calendar because the insolation variations caused by orbital changes are added to or subtracted from different parts of the calendar year
  • Changes in insolation viewed in reference to the half-year caloric seasons put a somewhat different emphasis on the relative importance of tilt and precession.
  • low-latitude insolation anomalies are still dominated in both seasons by the 23,000-year precession signal, the 41,000-year tilt rhythm is much more obvious in high-latitude anomalies. Another aspect of caloric season calculations is that the insolation values vary by a maximum of only ,5% around the mean, compared to variations as large as 12% for the monthly insolation changes.

8.8 Time Series Analyses

• To simplify analyses of cyclic variations in climate changes, scientists use time series analysis.
  • The term “time series” refers to records plotted against age. These techniques extract rhythmic cycles embedded in records of climate.
• The first step in time series analysis is to convert climatic records to a time framework.
  • After individual measurements of a climatic indicator have been made, all available sources of dating are used to define the ages of particular levels in the sediment sequence. A complete time scale for the entire sequence is then created by interpolating the ages of sediment depths between dated levels. 
• One technique is spectral analysis.
  • imagine taking a climate record plotted on a time axis and gradually sliding a series of sine waves of different periods across it.
  • As this is done, the correlation between each sine wave and the full climatic signal is measured at each step in the sliding process.
• A spectral analysis run will extract the three component (orbital) cycles, which can be displayed on a plot called a power spectrum.
  • The horizontal axis shows a range of periods plotted on a log scale, with the shorter periods to the right. The vertical axis represents the amplitude of the cycles, also known as their “power.” The height of the lines plotted on the power spectrum is related to the square of the amplitude of the cycle at that period.
• In actual studies of climate, however, power spectra are never this simple.
  • orbital cycles such as the tilt changes are not perfect sine waves, errors in dating records of climate change or in measuring their amplitude and  random noise.
•  A second useful time series analysis technique is called filtering.
  • This technique extracts individual cycles at a specific period from the complexity of the total signal. This process is often referred to as “bandpass filtering”
  •  Filters are constructed directly from well-defined peaks in power spectra. The highest point on the spectral peak defines the central period of the filter, and the sloping sides of the spectral peak define the shape of the rest of the filter
• We can create filters for these three cycles based on the peaks in the power spectrum
• In effect, the filtering operation extracts the varying shapes of individual cycles in specific bands of time embedded in the complexities of actual climate records.

8.9 Effect of under sampling climate records

• The technique of spectral analysis can be used only for a specific range of cycles in any climate record.
  • Confident identification of a cycle by time series analysis requires that the cycle be repeated at least four times in the original record.
  • At the other extreme, at least two samples per cycle are needed to verify that a given cycle is present, although many more are needed to define its amplitude accurately.
    • With fewer than two samples per cycle, time series analysis runs into the problem of aliasing, a term that refers to false trends generated by under sampling the true complexity in a signal.
  • under sampling is a common problem in climate records.

8.10 Tectonic-scale changes in earth’s orbit

• Over time scales of hundreds of millions of years, some of Earth’s orbital characteristics slowly evolved, as shown by evidence in ancient corals.
  • Corals are made of banded CaCO3 layers caused by changes in environmental conditions.
  • The primary annual banding reflects seasonal changes in sunlight and water temperature.
  • A secondary banding follows the tidal cycles created by the Moon and Sun.
    • The tidal cycles also affect water depth and other factors in the reef environment that influence coral growth.
  • Corals from 440 million years ago show 11% more tidal cycles per year than modern corals do, implying that Earth spun on its rotational axis 11% more times per year than at present.
    • each year had 11% more days.
  • Gradually over the last 440 million years, the spin rate and number of days decreased to their current levels.
    • gradual slowing in Earth’s rate of rotation was caused by the frictional effect of the tides
  • Other changes in Earth’s orbit that can be inferred from this kind of information, such as changes in Earth-Moon distance, are thought to have affected the wavelengths of tilt and precession over tectonic scale intervals.

Chapter 9 Insolation Control of Monsoons

9-1 Orbital Scale Control of Summer Monsoons

• The idea that changes in insolation control the strength of monsoons over orbital time scales was proposed by the atmospheric scientist John Kutzbach in the early 1980s
  • This concept is called the orbital monsoon hypothesis
• Because seasonal monsoon circulations are driven in the modern world by changes in solar radiation, orbital-scale changes in summer and winter insolation should have produced a similar response.
  • When summer insolation was higher in the past than today, the summer monsoon circulation should have been stronger (increased heating, moist air, and rainfall)
  • Conversely, summer insolation levels lower than those today should have driven a weaker summer monsoon in the past.
• The same kind of reasoning applies to the winter monsoon. Winter insolation minima weaker than the one occurring today should have enhanced the cooling of the land surface
• more intense summer insolation maxima and deeper winter insolation minima occur together at any one location. As a result, stronger in-and-up monsoonal flows in summer should occur at the same times in the past as stronger down-and-out monsoonal flows in winter.
• At first, it might seem that the climatic effects of these opposed insolation trends in the two seasons might cancel each other, but this is not the case for annual precipitation.
  • Monsoonal winters are always dry, regardless of the amount of insolation, because the air descending from higher in the atmosphere holds very little moisture.
  • orbital scale changes in winter insolation have little or no effect on annual rainfall.
  • summer monsoon winds coming in from the ocean carry abundant moisture, orbital-scale changes in summer insolation have the dominant impact on annual rainfall.
• This imbalance is an example of a nonlinear response of the climate system to insolation: the amount of rainfall is highly sensitive to insolation change in one season (summer) but largely insensitive to changes in the other (winter).
• As a result, the annual response of the system has a strong summer signature even though the equal-but-opposite insolation trends in the two seasons might have been expected to cancel each other

9-2 Stinky Muds in the Mediterranean

• The water that fills the Mediterranean Sea today has a high oxygen content.
• This sinking motion is a result of two factors: salt and temperature
  
  • As a result of this large-scale flow, the floor of the present Mediterranean Sea is covered by sediments typical of well-oxygenated ocean basins: light tan silty mud containing shells of plankton that once lived at the sea surface and benthic foraminifera that once lived on the seafloor
• Mediterranean sediments also contain distinct layers of black organic-rich muds, called sapropels.
  • Their high organic carbon content indicates that they formed at times when the waters at the seafloor were anoxic. The lack of oxygen led to stagnation of the deep waters and deposition of iron sulfides, giving the sediments a “stinky” odor and kept benthic foraminifera and other creatures from living on the seafloor.
• Sinking and oxidation of this organic carbon continually depleted the oxygen levels in the deep Mediterranean and produced the stinky muds on the seafloor
• The most recent sapropel in the eastern Mediterranean dates from 10,000 to 8,000 years ago, an interval when summer insolation levels were higher than today, the African summer monsoon was stronger, and African lakes were at higher levels.
• The Mediterranean Sea lies at high subtropical latitudes (30-40N), beyond the greatest northward expansions of past summer monsoons indicated by lake-level evidence across North Africa.
• If climate in the confines of the Mediterranean region never became truly monsoonal, how could the stinky muds deposited in that basin be a response to the North African monsoon?
• The critical link turned out to be the Nile River receive summer rains during the relatively weak tropical monsoon and delivers the water to the Mediterranean Sea far to the north.
  • At times when summer insolation was much stronger than it is today, the strengthened summer monsoon expanded northward and eastward, bringing much heavier rainfall to these high-elevation regions.
  • In effect, rainfall in the North African tropics exerts a remote control on the salinity of the subtropical Mediterranean Sea via the Nile River connection.
• Satellite sensors have detected the buried remnants of streams and rivers that once flowed across Sudan but are now covered by sheets of sand blowing across the hyper arid southeastern Sahara Desert.
• The fact that these streams once flowed eastward and joined the Nile River indicates that lower-elevation regions also contributed to the Nile’s stronger flow during major monsoons.

9-3 Freshwater Diatoms in the Tropical Atlantic

• Evidence that North African lakes fluctuate at the 23,000-year tempo of orbital precession can also be found in sediment cores from the north tropical Atlantic Ocean.
  • These sediments contain layers with high concentrations of the opaline (SiO2·H2O) shells of the freshwater diatom Aulacoseira granulata.
  • Diatoms couldn’t have lived in the ocean wind transport them to the ocean.
    • In arid and semiarid regions, winds scoop out sediment from the beds of dry lakes and blow the fine debris far away, some of it to the nearby oceans.
  • The intervals in the Atlantic cores containing freshwater diatoms mark times in the past when North African lakes were drying out and their muddy lakebeds were becoming exposed to, and eroded by, strong winter trade winds.
  • Records from the Atlantic sediment cores show that lake diatoms were delivered in distinct pulses separated by 23,000 years similar to the Mediterranean sapropels, this 23,000-year
    • each diatom pulse occurs later than the summer insolation maxima by as much as 5,000 to 6,000 years (Figure 9-9).
•  This delay makes sense if seen as a part of the sequence of likely events during a typical monsoon cycle.
  • Lakes filled to max size during the sum insolation maxima resulting in strong monsoons.
  • These high lake levels deposited lakebeds rich in diatoms.
  • decrease toward the next insolation minimum resulted in the lake levels dropping.
  • The fall in lake levels exposed the diatom-bearing silts and clays to winter winds, which scooped them up and blew them out to the ocean.
•  Another indication of a link to the North African summer monsoon comes from the amplitude of the diatom peaks.
  • Each 23,000-year diatom pulse has the same relative strength as the immediately preceding summer insolation maximum.
  • This pattern is consistent with a scenario in which stronger insolation maxima drove stronger summer monsoon maxima, which created bigger lakes, which provided larger sources of diatom-bearing sediments for subsequent transport to the ocean.

9-4 Upwelling in the Equatorial Atlantic

• Atlantic Ocean sediments contain additional evidence consistent with the hypothesis that the North African monsoon fluctuates at the 23,000-year tempo of orbital precession.
  • Cores in the eastern Atlantic just south of the equator show that the structure of the upper water layers has varied with a prominent 23,000-year rhythm.
    • the North African summer monsoon imposes an atmospheric circulation pattern that overrides the local circulation.
•  When the North African summer monsoon is relatively weak (as it is today), trade winds along the equator have a strong east-to-west flow.
  • The strongest trade winds occur in southern hemisphere winter and blow from the South Atlantic toward the equator.
  • strong trade winds at times like today blow mainly toward the west and drive warm surface waters southward away from the equator.
    • As the thermocline shallows, cooler waters rich in nutrients rise toward the sea surface just south of the equator
• In contrast, at times when summer insolation was higher than it is today, such as 11,000 years ago, the stronger summer monsoon flow overrode this circulation pattern.
• A much larger portion of the southern trade-wind flow crossed the equator, turned to the northeast, and was drawn into North Africa in the monsoonal circulation.
• This strengthening of the monsoon flow into North Africa weakened the westward trade-wind flow along the equator, and the weaker trade winds reduced the upwelling of cold waters, leaving the surface waters poorer in nutrients from below
• Changes between these two circulation patterns over time can be measured by examining variations in the relative amounts of planktic organisms that inhabit near-surface waters and leave shells in the sediments below.
• Sediment cores from the Atlantic Ocean just south of the equator show 23,000-year cycles of alternating abundances in these two types of plankton, still another indication of the effects of the North African summer monsoon.

9-5 The Phasing of Summer Monsoons

• The idealized monsoon model presented so far in this chapter has suggested that peak development of past summer monsoons at the 23,000-year cycle occurred as a direct response to strong insolation forcing at the timing of the June 21 summer solstice. however, the strongest monsoons have occurred about 2,000 years later.
• This offset has been interpreted in two ways
• Earth’s precessional motion produces a family of monthly insolation curves, each offset from the preceding month by one-twelfth of a 23,000-year cycle, or slightly less than 2,000 years.
• One interpretation starts with the assumption that June 21 is the best choice because the summer solstice is the time of peak insolation forcing.
  • In this view, some kind of physical process must have retarded full development of the summer monsoon by 2,000 years.
  • an ice sheet cooled northern hemisphere climate enough to retard full development of the summer monsoon for 2,000 years.
• Another view is that the June 21 summer solstice is not the correct choice for the time of critical insolation forcing.
  • the extra insolation forcing at the 23,000-year cycle could be more effective if it occurs during the time of peak development of the summer monsoon, which occurs in late July to early August.
•  In this view, insolation changes at the 23,000-year precession cycle that are aligned with this most intense midsummer heating should have the greatest impact in boosting continental temperatures still further and driving even stronger monsoons.
• Choosing late July or early August as the time of critical insolation forcing (rather than June 21) would eliminate the apparent 2,000-year lag in monsoon responses.

9-6 Monsoons on Pangaea 200 million years ago and 9-7 Joint Tectonic and Orbital Control of Monsoons were my section in the presentation so that information is in the PowerPoint 

Chapter 10 Insolation Control of Monsoons

What Controls the size of ice sheets?

10-1 Orbital Scale Control of Ice Sheets: The Milankovitch Theory, 10-2 Insolation control of ice sheets size, and 10-3 Ice Sheets Lab Behind Summer Insolation Forcing were my section in the presentation so that information is in the PowerPoint 

10-4 Delayed Bedrock response beneath ice sheets

• As ice sheets grow, so does the pressure of their weight on the underlying bedrock
• a 3.3-km ice sheet would eventually depress the underlying bedrock by 1 km.
  • 1 km of elevation change is equivalent to a 6.5C change in temperature at Earth’s mean prevailing lapse rate.
  • these large changes in bedrock elevation can translate into significant effects on temperature and mass balance at the surface of the overlying ice sheet.
• Bedrock responds to the ice load in two phases. The initial reaction is a quick sagging beneath the weight of the ice. This elastic response represents about 30% of the total vertical change in the bedrock. Then, over the next several thousand years, the bedrock continues to sink in a much slower viscous response caused by the extremely slow flow of rock in a relatively “soft” layer of the upper mantle at a depth of between 100 and 350 km.
• This viscous response slows progressively as the bedrock adjustment moves toward a final state of equilibrium. Viscous behavior has a response time of about 3,000 years: that is, about half of the remaining response needed to reach final equilibrium is achieved every 3,000 years
  • After six response times of 3,000 years each (totaling 18,000 years), only a tiny fraction (-1-2%) of the eventual bedrock depression remains unrealized.
• Bedrock behavior works in the same sense but in the opposite direction if the ice load is abruptly removed. The rock surface will rebound toward the level that is in equilibrium with the absence of an ice load.
• The initial rapid elastic rebound will be followed by a slow viscous rebound lasting many thousands of years. Today, parts of Canada and Scandinavia are still undergoing a slow viscous rebound in response to ice melting that occurred more than 10,000 years ago.
•  Actual ice sheets grow and melt much more slowly than these idealized (instantaneous) examples. When ice begins to accumulate on the land, the immediate (elastic) sagging of the bedrock depresses the land and promotes ice sheet melting.
  • But the slower (and larger) viscous bedrock response keeps the growing ice sheet at higher elevations, where temperatures are colder, ablation is slower, and the ice mass balance is more positive. Overall, the delay in bedrock sinking provides positive feedback to the growing ice sheet
•  Bedrock plays the same overall positive feedback role when the ice is. The weight of a large ice sheet that exists for thousands of years creates a deep depression in the underlying bedrock. As the ice begins to melt, the (smaller) elastic part of the rebound quickly lifts the bedrock and eliminates part of the depression.
  • But the (larger) viscous part of the rebound leaves the ice sheet at lower elevations in the depression it created and the warmer air at these levels causes further ice melting

10-5 A Full Cycle of Ice Growth and Decay

• we explore the interactions of insolation, ice volume, and bedrock responses during a typical cycle of ice sheet growth and decay. Because of the long lags inherent in these responses, these factors interact in intricate ways to create and destroy ice sheets.
• As summer insolation begins to decrease from a previous maximum, the equilibrium line shifts to the south and the climate point gradually moves onto the land. Some snow survives summer ablation on the far northern part of the continent, and a small ice sheet begins to form.
• As this ice sheet slowly grows, it reaches higher, colder elevations where accumulation dominates over ablation. The ice also advances southward because the equilibrium line is moving S and partly because flow from the area of ice accumulation in the N carries ice to the S. The thickening ice sheet slowly begins to weigh down the bedrock, but most of the bedrock depression lags several thousand years behind ice accumulation.
  • This delay in bedrock sagging helps to keep the surface of the ice sheet at higher and colder elevations where accumulation exceeds ablation
• The highest rate of ice accumulation occurs when summer insolation reaches a minimum value, and the equilibrium line is displaced farthest south.
  • the ice sheet has not yet reached maximum size because of the lag of ice volume behind the insolation driver. The rapid growth of new ice continues to weigh down the bedrock even more, with a lag of thousands of years for each new increment of ice.
• Summer insolation then begins to increase and shift the equilibrium line slowly back to the north, but the ice sheet continues to grow to its maximum size for several thousand more years.
  • Ice growth continues because insolation levels are still relatively low and because most of the surface of the ice sheet lies above the equilibrium line, protected from the slowly increasing levels of ablation.
• At some point, the combined effects of the ongoing northward shift of the equilibrium line along with the increasing amount of bedrock depression bring the southern end of the ice sheet below the equilibrium line.
  • With a larger surface area of the ice sheet now undergoing ablation, the overall mass balance turns negative, and ice volume begins to decrease. Melting is aided by the delayed rebound of bedrock from the weight of the earlier ice sheet. In effect, the southern edge of the ice sheet has now sagged into its own bedrock hole, where temperatures are relatively warm.
• Eventually, rising summer insolation drives the equilibrium line far enough north to move the climate point back over the Arctic Ocean.
  • Most of the remaining ice now lies in an ablation regime, and the last remnants may disappear several thousand years later. But if a small amount of ice survives intact through a summer insolation maximum, it can serve as a nucleus from which the next ice sheet can grow.

10-6 Ice Slipping and Calving

• Ice is transferred in the body of ice sheets by slow flow from colder, higher regions of net accumulation to lower, warmer regions of net ablation.
  • In two-dimensional ice models, this flow is usually represented in a simplified way as a slow diffusion (spreading) of ice from higher to lower elevations.
• Several other types of ice behavior that are usually omitted from these models because they are inherently less predictable may also be important.
• One such process is basal slip.
  • Slipping occurs because meltwater at the base of the ice sheet saturates soft sediments and creates a lubricated layer across which the ice can slide.
  • This process is usually not included in models because of the difficulty in predicting when and where it will occur.
• Iceberg calving, which occurs along the ocean margins of ice sheets, is another unpredictable process.
  • The ice sheets that existed 20,000 years ago in North America and Scandinavia had large borders along the Atlantic Ocean.
  • These margins lost a substantial fraction of their mass by calving icebergs to the sea, but this loss is also difficult to quantify in models because it is unpredictable
• One method for modeling the long-term evolution of ice sheets is to couple a two-dimensional (altitude/ latitude) ice sheet model to a simplified two-dimensional physical model of the atmosphere and ocean.
• Similar to the ice sheet models, the 2-D atmosphere-ocean models have one vertical dimension (altitude in the atmosphere, depth in the ocean) and one horizontal dimension (latitude), with changes in the other horizontal dimension (longitude) omitted.
• The goal of these coupled models is to simulate the linked changes in ice sheets and the atmosphere-ocean system.

Northern Hemisphere Ice Sheet history

• History of glaciation in the NH has been reconstructed during the last 4 decades from the ocean

10-7 Ice Sheet History

• At first thought, it might seem that the best records of past glaciations would be found on the continents where the ice sheets existed.
  • Unfortunately, these deposits are of little use in reconstructing long-term glacial history because each successive glaciation erodes and destroys most of the sediment left by the previous ones.
• Continuous records of glacial history come from ocean basins where sediment deposition is uninterrupted.
• Ocean sediments contain two key indicators of past glaciations: ice-rafted debris, a mixture of course and fine sediments delivered to the ocean by melting icebergs that calve from ice sheet margins and δ18O records from the shells of foraminifera, which provide a quantitative measure of the combined effects of changes in ice volume and in the temperature of ocean water.
• marine scientists Cesare Emiliani and Nick Shackleton pioneered the use of oxygen isotope ratios recorded in the shells of marine foraminifera to study past climates.
• In 1976, James Hays and John Imbrie joined with Shackleton on a landmark paper that conclusively linked changes in δ18O to changes in orbital insolation.
  • They found that orbital periods were clearly present in δ18O changes over the last 300,000 years and that the δ18O changes lagged behind changes in summer insolation forcing by several thousand years, consistent with the lag of ice volume behind summer insolation that Milankovitch had predicted.
• This long δ18O record shows two trends:
  • a very gradual drift toward more positive values, and numerous cyclic looking oscillations between positive and negative values.
• Both of these features reflect some combination of changes in temperature and fluctuations in ice volume.
  • Changes toward more positive δ18O values indicate more ice on the land and/or cooler deep-ocean temperatures.
  • More negative δ18O values indicate smaller ice sheets and/or warmer deep-ocean temperatures.
• This record also contained debris eroded from the continents by the ice sheets and carried to the ocean on ice “rafts.”
• Prior to 2.75 million years ago, the δ18O values were relatively negative and no ice-rafted debris was present. The smaller variations in δ18O during this interval probably reflect temperature changes in the deep waters or changes in the size of the Antarctic ice sheet.
• Beginning 2.75 million years ago, significant amounts of ice-rafted debris appeared in the record, an indication that ice sheets of considerable size were now present at least sporadically.
• This regime of 41,000-year cycles persisted for the first two-thirds of the interval of northern hemisphere glaciation from 2.75 to 0.9 million years ago.
  
  • The slow background shift of the δ18O signal toward more positive values during this interval also indicates a gradual underlying drift into a colder world.
• Beginning near 0.9 million years ago and becoming more obvious after 0.6 million years ago, the character of the δ18O record changes.
  • Maximum δ18O values reach larger amplitudes but are spaced farther apart, indicating that ice sheets persisted for longer intervals of time and grew larger in a colder world.
  • These glacial intervals come to an end during abrupt δ18O decreases that indicate rapid ice melting and ocean warming. Over the last 0.6 million years, there have been six of these large δ18O maxima, each followed by an abrupt deglaciation at an average spacing near 100,000 years.
• Almost hidden in the highly compressed record are smaller 41,000-year and 23,000-year δ18O oscillations that persist during the last 0.9 million years as secondary cycles superimposed on the larger oscillations near 100,000 years.
  
  • Near 130,000 years ago, an abrupt shift occurred into an interglacial interval that lasted until 120,000 years ago. Like the modern interglaciation, this interval had no ice-rafted debris in North Atlantic sediments, because northern ice sheets were not present except in Greenland.
• Between 125,000 and 80,000 years ago, the δ18O signal oscillated several times between values that indicate more or less ice and colder or warmer temperatures.
• The spacing of these oscillations at approximately 23,000 years confirms the presence of the orbital precession signal in this record. The two later glacial maxima near 63,000 and 21,000 years ago are separated by about 42,000 years, an indication that the 41,000-year orbital tilt signal is also present in this record.
• The rapid transition between 17,000 and 10,000 years ago marks another abrupt deglaciation, the first one since 130,000 years ago. These terminations are the most prominent marker of the longer-period oscillations near a period of 100,000 years.

10-8 Confirming Ice volume changes, choral reefs and sea level

• The oceans provide another measure of ice volume the fossil remains of coral reefs.
• Reefs have strong structural frameworks that remain intact long after individual coral organisms have died and that preserve records of past sea level positions.
• As sea level rises and falls, coral reefs follow by migrating upslope and downslope. Fluctuations in sea level result mainly from changes in the amount of water extracted from the ocean and stored in ice sheets on land. As a result, the sea level histories recorded by coral reef dipsticks are also a record of ice volume.
• Old coral reefs can be dated by radiometric decay methods. Their skeletons contain small amounts of 234U, which slowly decays to 230Th.
• Ocean islands in tectonically stable regions like Bermuda have a prominent fossil coral reef that lies about 6 m above modern sea level and dates to 125,000 years ago.
  • These reefs are the only indication of a sea level higher than today during the last 150,000 years. This evidence agrees with the marine δ18O record.
  • this interval was the only time in the last 150,000 years when the amount of ice on Earth was as small as it is now.
• All other coral reefs that grew during the last 150,000 years formed when sea level was below its modern position because of the greater amount of seawater tied up in ice sheets.
• The two most intensively studied of these islands are Barbados in the eastern Caribbean Sea, and New Guinea in the western Pacific Ocean.
  • These reefs record higher sea levels caused by ice melting, but the melting was not as complete as today and sea levels were not as high.
• Drilling of deeply submerged reefs has shown that the lowest global sea level reached during the last 125,000 years was close to 120 meters below the modern level near 20,000 years ago. The timing of this sea level minimum correlates with the largest δ18O maximum.
• The sea level minimum from 20,000 years ago and the 16-meter maximum level from 125,000 years ago serve as anchor points for removing the effect of uplift on these tectonically active islands and calculating changes in sea level during the intervening interval.
• This method reveals that the reefs at 82,000 and 104,000 years ago were formed when sea level was lower than today by an estimated 17 meters. This estimate falls matches the proportional change in δ18O
• This agreement provides even more confirmation that δ18O is a good index of ice volume, despite the temperature overprint known to be present. Each 10-meter change in global sea level results in an isotopic (δ18O) change of 0.8–1.1%.