biogeo lec 7 glaciation and the pleistocene

GLACIATION AND GLACIATION AND BIOGEOGRAPHIC DYNAMICS BIOGEOGRAPHIC DYNAMICS

OF THE PLEISTOCENEOF THE PLEISTOCENE

Extent and Causes of Glaciation:Extent and Causes of Glaciation:• Glacial events often associated with the

positioning of large landmasses over or near the poles.

Extent and Causes of GlaciationExtent and Causes of Glaciation

• Global climate relatively warm during the Mesozoic and and early Cenozoic

Extent and Causes of Glaciation:Extent and Causes of Glaciation:

Extent and Causes of Glaciation:Extent and Causes of Glaciation:• Glacial events often associated with the

positioning of large landmasses over or near the poles.

• Many glaciation events throughout the Phanerozoic

Extent and Causes of Glaciation:Extent and Causes of Glaciation:

• several occurred during the Pleistocene

• > massive continental ice sheets

• > often 2 to 3 km thick• > at maximum, 1/3 of

earth’s land mass

Extent and Causes of Glaciation:Extent and Causes of Glaciation:• > during glacial maxima

• 1) unglaciated temperate regions were cooler and wetter than those of today => referred to as Glacio-pluvial (ice-rain) periods in now arid areas

• 2) Tropical regions tended to be drier during glacial maxima

Extent and Causes of Glaciation:Extent and Causes of Glaciation:• most recent glacial period,

• ice sheets in the Northern Hemisphere covered to the 45 o latitude

• North America and Central Asia

• 80% of ice sheets during Pleistocene occurred in Northern Hemisphere

Extent and Causes of Glaciation:Extent and Causes of Glaciation:• > in Southern Hemisphere (except for

Antarctica), glaciation was mostly confined to high latitudes

Milankovitch CyclesMilankovitch Cycles• Changes in interception and absorption of

solar radiation by the earth’s surface resulted from changes in its orbit => Milankovitch Cycles

Milankovitch CyclesMilankovitch Cycles

• Changes in interception and absorption of solar radiation by the earth’s surface resulted from changes in its orbit => Milankovitch Cycles

• 1) Earth’s orbit varies in ellipticity with a period of 100,000 years


• Changes in interception and absorption of solar radiation by the earth’s surface resulted from changes in its orbit => Milankovitch Cycles

• 1) Earth’s orbit varies in ellipticity with a period of 100,000 years

• 2) Tilt of the earth (obliquity) varies from 22.1 to 24.5 o with a 41,000 year period


• 3) Earth’s orientation or Precession wanders with the axis of the North Pole shifting from one “North Star” (presently Polaris of Ursa Minor) to another (Vega of Lyra) with a periodicity of 22,000 years


• > combined effects result in significant changes in the amount of solar energy striking the earth

Glacial and Interglacial PeriodsGlacial and Interglacial Periods

• Transitions between glacial and interglacial periods were influenced by remarkable feedback effects



• 1) Initial stages of glaciation resulted in increased reflectance (from snow and ice) over large parts of the earth -> reduced effective solar heating and further increased cooling rate



• 1) Intial stages of glaciation resulted in increased reflectance (from snow and ice) over large parts of the earth -> reduced effective solar heating and further increased cooling rate

• 2) Deglaciation was increased by buildup of greenhouse gases (carbon dioxide and methane)

Effects on Nonglaciated Areas:Effects on Nonglaciated Areas:

• Latitudinal gradients were much less pronounced during most of the Phanerozoic


• In the Miocene, global climates began to cool and become drier

• > oceanic and atmospheric circulation during the mid-Miocene (15 million years B.P.) established a strong latitudinal thermal gradient that intensified during interglacial periods


• > average air temperatures were 4 to 8 o C cooler during glacial maxima than during interglacial periods

• -> cooling appears to have more pronounced effect at lower latitudes


• > in mountainous regions, snow lines shifted by as much as 1000 m in elevation between glacial and interglacial periods


• Resulted in geographic shifts in climatic zones -> glacial winters were less severe and glacial summers were cooler and less subject to heat waves


• > Jet stream of North America changed during the Wisconsin

Effects on Nonglaciated Areas:Effects on Nonglaciated Areas:• -> jet stream diverged around the glacier • -> an anticyclonic (clockwise) circulation

pattern was established

Effects on Nonglaciated Areas:Effects on Nonglaciated Areas:• -> resulted in elevated water levels in lakes

of the American Southwest

Biogeographic Responses to Biogeographic Responses to GlaciationGlaciation

• Biogeographic Responses of Terrestrial Biotas

Biogeographic Responses of Biogeographic Responses of Terrestrial BiotasTerrestrial Biotas

Sea Level Changes in the Sea Level Changes in the PleistocenePleistocene

Sea Level Changes in the PleistoceneSea Level Changes in the Pleistocene

Sea Level Changes in the PleistoceneSea Level Changes in the Pleistocene

• Pygmy Mammoth• Mammuthus exilis• Pleistocene Epoch (20,000 years ago)• Channel Islands of California• Remains of Dwarfed proboscideans have been

discovered on islands in the Mediterranean Sea, in the Arctic Ocean off the coast of Russia, and the Pacific Ocean off the coast of California. The California species is a descendent of the 12-foot-tall Columbian Mammoth. When fully grown, the dwarf mammoth stood only 3.5 to 6 feet tall. Scientists speculate that the full-sized ancestors of this tiny mammoth reached the islands during the period of low sea level of the last glacial period. With sea level lowered as much as 300 feet, the northern Channel Islands (Santa Rosa, Santa Cruz, and San Miguel) were all part of a single larger off shore island.

• The Unique ecological aspects of islands, such as small land area, limited resources, and absence of large predators, result in interesting evolutionary products most commonly occurs with large-bodied herbivores such as elephants and hippos, while gigantism most commonly occurs with small-bodied herbivores such as rodents.

• The cast of a nearly complete pygmy mammoth skeleton shows the actual arrangement of the bones as they were found on the ground. The original specimen was discovered on Santa Rosa Island.


Evolutionary Responses and Pleistocene Refugia

• Neotropical Pleistocene Refugia • Nunataks: Glacial Refugia

Neotropical Refugia Neotropical Refugia

Refugia and Tropical DiversityRefugia and Tropical Diversity

• Haffer’s Hypothesis – Speciation Pump Model

• Possible explanation for tropical diversity


• Probably not the case for several reasons


• 1) Not old enough– Species older than refugia

• 2) Little overlap of areas of endemism

• 3) Cooling uniform across Amazonia – NOT as fragmented as hypothesized

• 4) Tropics more heterogeneous than previously thought


• Climate driven fragmentation may still have been important Instead of savannah areas fragmenting tropical forest

• Sea level increases (as much as 100 meters) may have resulted in fragmented mountaintop islands

Nunataks: Glacial RefugiaNunataks: Glacial Refugia

• 3 large Refugia during Wisconsin– Beringia– Southern– Eastern (Nova Scotia)

• Pacific Coast– Mountainous regions

BeringiaBeringia

Proposed Zoogeographic History Proposed Zoogeographic History of the Bog Turtleof the Bog Turtle

• Fig. 2: Proposed ancestral distribution and dispersal routes of bog turtles.

Clemmys muhlenbergii

Nunataks: Glacial RefugiaNunataks: Glacial Refugia• Important opportunities for genetic

divergence

• High levels of endemism

• Why?


– Aquatic Systems: Postglacial and Pluvial Lakes

• Glacial (Cryogenic) Lakes • Pluvial Lakes in Arid Regions

Glacial (Cryogenic) LakesGlacial (Cryogenic) Lakes

Biogeographic Responses Biogeographic Responses to Glaciationto Glaciation

• Pluvial Lakes in Arid Regions– Resulted from low evaporation rates and high

precipitation– Basin-and–range topography of Nevada

Glacial Cycles and ExtinctionsGlacial Cycles and Extinctions


• The Overkill Hypothesis

Pleistocene ExtinctionsPleistocene Extinctions

The Overkill Hypothesis

Glacial Cycles and ExtinctionsGlacial Cycles and ExtinctionsAlternative Explanations for Pleistocene Extinctions


• Pleistocene extinctions were not synchronous

• Climate changes were synchronous globally

• Does this support the Overkill Hypothesis?

Australian ExtinctionsAustralian Extinctions

Steller’s Sea CowMoa from NZ

Ancient DNA from giant extinct lemurs confirms single origin of Ancient DNA from giant extinct lemurs confirms single origin of Malagasy primates Malagasy primates

K. Praveen Karanth K. Praveen Karanth , Thomas Delefosse , Berthe , Thomas Delefosse , Berthe Rakotosamimanana ¶, Thomas J. Parsons ||, and Anne D. Yoder Rakotosamimanana ¶, Thomas J. Parsons ||, and Anne D. Yoder , ,

biogeo lec 7 glaciation and the pleistocene

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