Pleistocene Glaciations

Transcription

1 Chapter 14 Pleistocene Glaciations I. Geologic evidence 1. glacial deposits, etc. Pleistocene Glaciations 2. The Oxygen Isotope Record (1970s) II. Explanation of the glacial-interglacial periods The Milankovitch Theory (1920s) III. Glacial climate feedbacks Theories for the inconsistency i between The Milankovitch it Theory and the Oxygen Isotope Record Pleistocene Epoch: 1.8 million to 10,000 years ago Holocene Epoch: since 10,000 years ago The Quaternary Period includes the Pleistocene and Holocene ice sheets have grown and shrunk many times since 2.5mya Definitions (confusing): Glacial Period: When discussing climate change over millions of years, periods during which the Northern Hemisphere had ice sheets. Very few such periods. Includes the Pleistocene and Holocene, including current Glacial Interval / Ice Age: time during the Pleistocene when more than just Greenland/Antarctica have ice sheets Interglacial Period/Interval: time during the Pleistocene when only Greenland/Antarctica have ice sheets (the Holocence, includes current) Maximum extent of the Pleistocene ice sheet Geologic Evidence of Pleistocene Glaciations (discussed previously in this class): 1. Glacial striations 2. Glacial moraines 3. Till 4. Loess There have been many glaciations, i but evidence of previous ice ages gets destroyed d when the ice advances over the same region. There have been four ice ages known since the 19 th century, in order from oldest to most recent: Gunz, Mindel, Riss, and Wurm (Europe) Nebraskan, Kansan, Illinoian, and Wisconsin (N. America) 1

2 In Central Park In Central Park Rouches Moutonnees rock hills shaped by the passage of ice to give a smooth up-ice side and a rough, plucked and cliffed surface on the down-ice side. The upstream surface is often marked with striations Glacial Erratic boulder transported and deposited by a glacier having a lithology different than the bedrock upon which it is sitting In Central Park 1 Blue sky over Umpire Rock 2 Erratic 3 Folds 4 Groove 5 Groove 6 Groove 7 Meeting Place 8 Outcrop or boulder? 9 Outcrop or boulder? 10 Students 11 Students 12 Striations 13 Talus or erratic? 14 Talus or erratic? 15 Erratic 16 Asymmetric hill 17 Dike in raised relief 18 X-dike: How geologists argue - Explanation of Ice Ages Milankovitch (1920s) Serbian mathematician Milankovitch theorized that the waxing/waning of ice sheets should be related to the amount, and seasonal timing, of insolation reaching higher latitudes. He calculated, based on the physics of gravitational attraction between earth, other planets, the sun, and moon, that characteristics of the earth s orbit, and therefore insolation and variations in the amount of ice cover on earth, varies over thousands of years, with three primary cyclicities: thousand year cycle of precession (combined effects of precession 25.7 kyr cycle, precession of perihelion, and influence of Venus and Jupiter) 41 thousand year cycle of obliquity 100 thousand year cycle of eccentricity With not much solid geological evidence to back it up, this theory went nowhere 2

3 - Explanation of Ice Ages Milankovitch (1920s) Obliquity and contrast in insolation between the seasons Precession of Perihelion less contrast between seasons Milankovictch Cycles: periodical changes in the Earth s climate system due to (1) changes in the orbit around the Sun; 100,00 years period, (2) changes in the inclination of the rotation axis; 41,000 years period, and (3) changes in the orientation of the rotation axis with respect of the orbit; 25,700 years period. more contrast between seasons Glacial-Interglacial as Equilibrium States Combination of various orbital forcing causes earth s climate to oscillate between two states: glacial and interglacial. Equilibrium states can be represented as below, with the glacial state (longer periods) in a deeper valley than the interglacial state. Orbital forcing periodically and continually send the system from one valley to the other, back and forth. Glaciations Sun is in one of the foci of the ellipse, at perihelion earth is closer to the sun than at aphelion more energy at perihelion than aphelion perihelion when Northern Hemisphere (NH) is in the middle of winter earth moves faster in its orbit around the sun at perihelion than at aphelion therefore NH winters are milder and shorter than Southern Hemisphere (SH) winters and NH summers tend to be longer and milder than milder than SH summers Obliquity creates contrast between the seasons, no net change in the amount of sunlight received by earth the eccentricity of earth s orbit causes this contrast to slightly greater in the SH than in the NH Precession (related to direction of the spin axis) modifies the relationship between the seasons and the distance from the sun every half precession cycle the hemisphere h with the greatest t degree of seasonal contrast t switches between north and south: when the SH has mild summers and winters, the NH has hot summers and cold winters and vice versa. Glaciations are promoted with northern summer at aphelion and thus low seasonal contrast (as today). In Summary: when eccentricity is large, NH glaciation is especially favored when precession causes the NH summer to occur at aphelion. 3

4 The Oxygen Isotope ( 18 O) Record (1970s) 1. The skeletons of marine organisms record the 18 O content in the water (sediments) 2. Marine organisms incorporate 18 O preferentially in cold water 3. Ice sheets preferentially incorporate water with 16 O, leaving the water remaining in the ocean enriched in 18 O 4. So, local climate variations (water Temp), and the global scale advance and retreat of ice sheets, are recorded in the 18 O/ 16 O ratios in ocean sediments As a response to climate changes ice sheets advance over the continents and these changes are recorded in the chemical changes that take place in the ocean. O 2 isotopes analysis is used to determine the record of glacial-interglacial oscillations. Net transfer of 16 O from oceans to ice sheets, leaving behind ocean water rich in 18 O The Oxygen Isotope ( 18 O) Record (1970s) What does this record tell you about climate variations over the past few million years? Deep-sea sediment record of the δ 18 O of seawater during the Pleistocene analysis on two genera, sediments from mid-latitude North Atlantic. The Oxygen Isotope ( 18 O) Record (1970s) Fourier Analysis: mathematical procedure designed to identify the strength of different cycles in a time series. Fourier analysis of the oxygen isotope record shows the same periods predicted by Milankovitch! The Oxygen Isotope ( 18 O) Record (1970s) How is the precession effect modulated by eccentricity? Why is the 100K yr cycle stronger in the climate than in the forcing? 4

5 Glacial Climate Feedbacks Milankovitch prediction and observed δ 18 O record Why is the 100k-year cycle so pronounced in the actual climate, when the solar forcing is so muted? There must be one or more positive feedbacks to amplify the initial change. 1. Ice-Albedo Feedbacks 2. Evidence for Feedbacks affecting atmospheric CO 2 on Glacial Time Scales possible amplifying mechanisms (a, b, c) Vostok Ice Core link between global climate change and variations of GHGs amount in the atmosphere 3. Cloud-Albedo Feedbacks 4. Changes in Terrestrial Biomass (negative feedback!) 1. Ice albedo feedbacks: definitely happens, but can not explain global nature of the results, must be other mechanisms at work 2. Evidence for feedbacks affecting atmospheric CO 2 on glacial time scales THE VOSTOK ICE-CORE DATA C02, CH4, and temperature time series for the last 400,000 years How are they derived: isotope analysis (O and H) These co-varying changes are interpreted as evidence of system-like behavior 2a. Possible amplifying mechanism THE BIOLOGICAL PUMP Why should the biological pump respond dto glacial lchanges? 5

6 2a. Possible amplifying mechanism THE BIOLOGICAL PUMP Why should the biological pump respond dto glacial lchanges? We know that sea level drops during glacial periods, could this lead to phosphorus fertilization? 2a. Possible amplifying mechanism THE BIOLOGICAL PUMP phosphorus fertilization: not consistent with other evidence 2b. Possible amplifying mechanism IRON FERTILIZATION Glacial periods are believed to have been dry and dustier, as shown in ice cores (more dust in the core). 2c. Possible amplifying mechanism CORAL REEF GROWTH/DECAY We know that sea level drops during glacial lperiods, could ldthis lead to a coral reef feedback? 6

7 2c. Possible amplifying mechanism CORAL REEF GROWTH/DECAY We know that sea level drops during glacial lperiods, could ldthis lead to a coral reef feedback? 3. Possible amplifying mechanism not CO2 related Cloud albedo feedback / ocean sulfur cycle The Production of DMS by phytoplankton lead to cooling of climate via 2 mechanisms sulfate aerosol and cloud seeding MSA content of Antarctic ice (bottom) and relative local temperature (top) from hydrogen isotopic composition of the ice. 3. Possible amplifying mechanism not CO2 related Cloud albedo feedback / ocean sulfur cycle The Production of DMS by phytoplankton lead to cooling of climate via 2 mechanisms sulfate aerosol and cloud seeding 4. Possible negative feedback: change in forest cover, related to atmospheric CO2 Is this link + or -? Difference in vegetation between the last glacial maximum and present day potential: (ignoring human impact on forests). 7

8 4. Possible negative feedback: change in forest cover, related to atmospheric CO2 8