Download Geological time scale is hierarchical

Historical Patterns of Global
Change
Geological time scale is
hierarchical
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Eon
Era
Period
Epoch
Index fossils
• Wide-ranging species
• Not restricted to microhabitats
• Often marine organisms
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planktonic, calcareous foraminiferans
siliceous radiolarians (Protozoa) of the Cenozoic era
chitinous colonial graptolites (Hemichordata)
floating animals (Ordovician and Silurian)
swimming ammonoid cephalopods (Mollusca) of
Mesozoic
– vascular plants of Carboniferous period
Theory of Continental Drift
• has had the largest impact on biogeography
• evidence is conclusive
Development of Plate Tectonics
Theory
• Lyell (mid 1800’s)
• Antonio Snider-Pelligrini (1858)
• F. B. Taylor (1908 - 1910)
– American geologist
• Alfred L. Wegener
– meteoroligist who developed ideas of continental drift
independently of Taylor while noting congruence of
opposite coastlines on Atlantic Ocean;
– working hypothesis presented in 1912; published in
1915
Main points of Wegener’s theory
• continental rocks fundamentally different
than those of ocean
– floor, and are lighter & thus float on the mantel
• major landmasses once united into a
supercontinent, Pangaea
– Pangaea broke into small continental plates,
which moved apart
Main points of Wegener’s theory
• breakup of Pangaea began as a rift valley,
which widened into an ocean
– midoceanic ridges mark where continents were
joined
– distribution of major earthquake and volcanic
regions, and mountain building regions are
related to continental movement
Main points of Wegener’s theory
• continental blocks have retained their initial
outlines, except in mountain orogeny regions
– joining the margins of continents
– lines up stratigraphic zones, fossils, and reconstructed
paleoclimates
• rates of movement range between 0.3 and 36 m
per year
• radioactive heating in the mantle may be the
primary cause of continental movement
– forces of movement are gradual
Opposition to the theory
• Involved shifting paradigm
– New ideas challenged in science
• Contained many factual errors
• Testing model difficult
– would have required much more geological and
biogeographical evidence than presented
• Lacked a plausible mechanism
Acceptance of theory took time
• Five decades after Wegener’s proposals, sufficient
evidence was gathered by geologists,
paleontologists and biogeographers
• S. W. Carey (1955, 1958); Bullard et al (1965)
demonstrated the fit of continents into Pangaea
using submarine contours of continental shelves.
Evidence for plate tectonics
• Stratigraphic
– Topographic features align along Wegener’s proposed
connections of fragmented Gondwanaland
• Paleoclimatic
– Southern Hemisphere glacial scars aligned only if
Gondwanaland reconstructed
• Paleontological
– The late Paleozoic glacial deposits of southern
continents are covered with Permian rocks bearing the
Glossopteris flora (arborescent gymnosperms of
temperate climates) - circumscribes Gondwonaland
– vertebrate fossil assemblages with similar distribution
Evidence from Marine geology
• WWII stimulated charting of ocean floor
topography
– discovery that submarine mountain ranges bisecting the
oceans are segments of a continuous global system
65,000 km long
• interpretation that midoceanic ridges are zones of seafloor
spreading
– trenches located far from ridges
• up to 10 km deep
• earth’s crust is very thin at the trenches
• hypothesis that the crust is pulled downward into trenches and
material is reincorporated into the mantle
Evidence from paleomagnetism
• Paleomagnetism
– orientation of magnetized crystals, titanium and iron
oxides, at the time of mineral formation when molten
rock solidifies
– by measuring the direction and declination of remnant
magnetism in cooled lavas, it’s possible to determine
the relationship of any landmass to the magnetic poles
at time of formation
– reversals of magnetic field occurs every 10,000 to
million years
Significance of magnetic stripes
for continental drift theory
• Basaltic rocks at the mid-oceanic ridges have
normal field (present-day) magnetic properties
• The widths of alternating magnetic stripes on the
opposite sides of a ridge are often roughly
symmetrical, and the stripes are generally parallel
to the long axis of the ridge.
• The banding pattern of any one ocean closely
matches that of the others, and the ocean patterns
correspond approximately to reversal timetables
from terrestrial lava flows.
Model of seafloor spreading
• Proposed by Herman Hess (1960, 1962); R.S.
Dietz (1961)
• oceans formed by addition of material and
spreading at mid-oceanic ridges
– moving away from the ridges, basalt along the ocean
floor increases in age and is marked by magnetic stripes
• records polarity of prevailing magnetic field
• differences in widths of stripes suggest rate of spreading varies
over time
• not uniform across oceans or parts of the same ocean
Finally: a plausible mechanism!
• Current model suggests that three forces may be
responsible for crustal movements
– ridge push
– mantle drag
– slab pull
• 16 major plates are currently recognized
– strong correlation with Wallace’s map of biogeographic
regions
So, what are biogeographic
regions?
• Biogeographic regions are defined by biotas
and portions of the earth’s crust that share
both evolutionary and tectonic histories
– representative assemblages on each plate
evolved in isolation from those on other plates.
Plate boundaries classified into
three types of zones
• Spreading
– midoceanic ridges and rift zones on continents
• Collision
– if plates are of roughly equal density, collision of plates
will cause uplift and formation of mountains along
plate boundary
– if ocean plate is denser than the other, will sink below
lighter continental plate to form subduction trench
• Transform
– plates of roughly equal density may slide and grind
against each other, without subduction or mountain
uplift
Tectonic phenomena (earthquakes,
volcanism, mountain formation, island arcs)
closely associated with interactions among
plates
Changing face of the planet
• Pangea
– probably a temporary supercontinent during
late Paleozoic and early Mesozoic
– North: Laurasia
– South: Gondwanaland
Changing face of the planet
• Gondwanaland
– includes foundations of present-day South
America, Africa, Madagascar, Arabia, India,
Australia, Tasmania, New Guinea, New
Zealand, New Caledonia, and Antarctica
– most ancient of the Pangean land masses
– great antiquity accounts for similarity of
isolated biotas of southern hemisphere
Changing face of the planet
• Laurasia
– Pre-Laurasian landmasses were isolated until
early Devonian (400 mybp), when precursors of
Northern Europe drifted northward from
subantarctic latitudes to collide with precursors
of North American and northern part of Siberia
to form the “Sandstone Continent”
Breakup of Pangaea
• Initiated in early Jurassic (180 mybp).
• Evolutionary radiations of most terrestrial
families and many orders occurred after
breakup of Pangaea and Panthalassa
– resulted in rafting biotas, reduced gene flow,
altered selective pressure and rapid speciation
Selected Histories of Different
Biogeographic Regions - Central
America and the Antilles
• Precursors of North and South America remained
connected until about 160 mybp
– connection severed by late Jurassic (ca 150 mybp)
• chain of volcanic islands along the eastern edge of
the Caribbean Plate
– drifted eastward to form Proto Antilles archipelago
• by Eocene
– core of Greater Antilles achieved present position
Selected Histories of Different
Biogeographic Regions - Central
America and the Antilles
• Central American landbridge
– first emerged during late Cretaceous (80 to 65 mybp) as
chain of islands far east of current position
– By late Miocene (10 to 5 mybp), Central American
archipelago provided stepping stone route
– 3.5 mypb, archipelago fused to form current landbridge.
Selected Histories of Different
Biogeographic Regions - Marine basins
and island chains
• Epeiric or epicontinental seas
– form when sea levels rise and oceans flood
continental plates
• Mediterranean and Red Seas
– Mediterranean started out as the Tethyan
Seaway
– Red Sea formed by rifting which began ca 35
mypb
Selected Histories of Different
Biogeographic Regions - Marine basins
and island chains
• Pacific Ocean
– No central midoceanic ridge as seen in Atlantic ocean
– Island chains form over hot spot
• Triple junctions
– where three plates rest against each other to form a
complex of trenches
• Pacific basin had six ancient plates
– Phoenix; Kula; Pacific; Farallon; Cocos; Caribbean
Freshwater systems
• River system drainages have also shifted
over time
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Amazon
Mississippi
St. Lawrence
Great Basin of North America
Climatic and Biogeographic
Consequences of Plate Tectonics
• Mountain ranges form when plates collide
– subduction & volcanic activity
– accretion
– plates with similar densities
• Biotic exchange, extinction, adaptive radiation,
rapid speciation
– follows history of contact and breakup of continental
plates
– diversity increases with area; diversity and similarity
among biotas decrease with increasing isolation
Climatic and Biogeographic
Consequences of Plate Tectonics
• Climatic shifts correlated with continental
movements
– Affects precipitation patterns, paleocurrents, wind,
temperature
– Position of land masses relative to poles and equator
affect local climate
– Landmasses over poles trigger episodes of glaciation
and changes in sea level
• Mass extinctions associated with tectonic events
Glaciation and Biogeographic
Dynamics of Pleistocene
Glaciation and Biogeographic
Dynamics of Pleistocene
• Pleistocene and Holocene (past 2 million
years)
– events during this period had big impact on
current distributions of taxa
– but have noterased the patterns resulting from
older periods
– Recency of this period of time allows for
sources of paleoecological data not available
for older time periods
Glaciation
• Global glacial events associated with position of
large landmasses relative to the poles
• Glaciation during the Pleistocene was not result of
land mass position relative to poles.
– Climatic changes resulted from changes in the
interceptions and absorption of solar radiation by
earth’s surface due to changes in its orbit
Milankovitch cycles
• eccentricity of orbit (shape of ellipse) varies
on 100,000 yr cycle
• obliquity (tilt) varies from 22.1˚to 24.5˚ on
a 41,000 yr cycle
• precession (axis orientation) wanders on a
22,000 yr cycle
Milankovitch cycles
• combined effects cause fluctuations in the amount
of solar energy striking the earth
• transitions between glacial and interglacial periods
influenced by feedback effects
• temperatures were much warmer during most of
last 600 my with less of a gradient from equator to
poles
– During the Miocene, global climates gradually became
cooler and drier
Pleistocene glaciation
• several glacial-interglacial cycles with
advance and retreat of continental glaciers
– often 2 to 3 km thick
– Massive enough to deform underlying crust
– at maximum covered up to 1/3 of earth’s
surface
– unglaciated regions had very different climates
Estimation of Paleotemperatures
• Can measure oxygen isotope ratios in marine
fossils and ice cores
• Two common isotopes of oxygen in water are O16
and O18
– The lighter isotope (O16) evaporates faster than the
heavier (O18), especially in warmer periods
• marine exoskeletons: calcite oxygen ratios
• ice cores: direct isotope measurements
• Reconstruction of temperatures suggest that 10
major glacial periods occurred over past 2 million
years
The effect on global climate
• Declines in air temperatures ranging 4° to 8°C
• Climatic zones shifted toward the equator during
glaciation and shifted northward in the
interglacials
– position of land masses, glaciers and major bodies of
water complicated the patterns
• Glacial winters less severe and glacial summers
cooler
The effect on global climate
• Tropical regions tended to be cold and dry
• SW North America was wetter
• There was a strong association between
glacial periods and monsoonal circulation
and precipitation
Take home message:
What we see in current climatic regimes is not
what was typical over the past 2 mybp
Sea Level Changes during
Pleistocene
• Global changes resulting from freezing or melting
of sea ice (Eustatic changes)
• Isostatic changes reflect sea level changes when
portions of the earth’s crust rise or sink into the
athenosphere
• During Wisconsin glacial maximum, nearly 1/3 of
land in northern hemisphere covered in ice and
there were huge fields of sea ice.
Sea Level Changes during
Pleistocene
• Sea level dropped by 100 m
– Other glacial periods saw drop in sea level up to 160 m
– Sea levels have been up to 70 m higher than at present
– Total changes of 230 m through global history
• Land bridges form between biogeographic regions
when sea level drops by exposing the continental
shelves
– e.g., Wallace’s line coincides with the division between
landbridges of the Sunda Shelf (north) and the
Australia-New Guinea-Tasmania land bridges.
Glacial melt:
• takes centuries for crust to rebound
• with rising sea levels, flooding of interior
regions caused shallow seas
– forms corridors for dispersal of shoreline flora
and marine and estuary species
Biogeographic responses to
glaciation
• Environmental changes:
– Changes in the location, extent and configuration of
habitats
– Changes in the nature of climatic and environmental
zones
– Formation and dissolution of dispersal routes
• Responses by species:
– Migration - moving with optimal habitat
– Adaptation to local changes
– Range reduction and extinction
Biogeographic responses to
glaciation
• Biomes shifted 10° to 20° in latitude (toward
equator)
– Zonal vegetation belts compressed during glacial
maxima, but relative positions were unchanged
– North-South rivers and mountain systems in North
America served as corridors for migration
– E-W mountains and other barriers in Europe blocked
migration
• Upward migration of vegetation zones in
mountainous regions since glacial retreat
Biogeographic responses to
glaciation
• Shifts of vegetation zone occurred in Southern
Hemisphere as well as in north
• Biomes and communities often disintegrated
rather than migrating as units
– Responses to climatic change varied among species
– Animals migrated more quickly than plants, but were
influenced by vegetative
Aquatic systems
• Glacial activity of Pleistocene had major impact
on lake formation
• Many glacial lakes formed by ice damming
– drainage of lakes spectacular and devastating when ice
dams broke
• e.g., Columbia River Gorge.
• Other types of lakes from glacial activity
– kettle lakes and glacial plunge pool lake
– basins formed by scouring action of glaciers
– dams formed by glacial moraines
Pluvial lakes
• Southwestern US has basin-and-range topography
• low, flat basins filled with rainwater
– slow rate of evaporation from cooler temperatures and
high amounts of rainfall
• Dried pluvial lakes
– saline lakes and dry lake beds
• Biogeographic consequences of disappearance of
pluvial lakes
– wholesale extinction of many plants and animals that
lived in or around the lakes
– dissection of large lakes resulted in vicariant speciation
of survivors (e.g., pup fishes of SW NA)
Biotic Exchange
• Mountains:
– shift to lower elevation during glacial maxima
allowed for corridors to form between
mountain ranges
• remixing of previously isolated taxa
– shift to more arid climates isolates peaks from
one another during interglacials
• formation of “sky islands”
Biotic Exchange
• Marine organisms experienced similar types
of phenomena
• Landbridges exposed during lowered sea
levels functioned as corridors between
continents, between continents and islands,
or among islands
• Landbridges fragmented marine biotas
Biotic Exchange
• Dispersal in terrestrial and marine biomes
was assymetrical
– More spp migrating from larger to smaller areas
• Great American Interchange
– a dominating transfer of northern species to the
southern hemisphere
Pleistocene Refugia
• Neotropics:
– lowland tropical rainforests may have contracted in size
within past 40,000 years
– Amazonia has ca. 6 centers of endemism and high
species diversity
– Insular nature of these areas of species richness and
endemism led to the idea that they were Pleistocene
refugia
• islands of lowland rain forest that persisted during glacial
maxima.
Pleistocene Refugia
• Cyclical vicariance or speciation pump model:
– Populations became isolated as their prime habitat became
fragmented
– As glacier receded, refugia expanded to eventually reconnect to
each other
– Range expansion of taxa brought relatives back into contact
• Objections to the model:
– forests may not have contracted nearly as much as model suggests
– molecular and phenotypic data indicate that many endemics are
much older than the hypothesized refugia
– there is little overlap in centers of endemism among different taxa
– Haffer’s hypothesis not the most parsimonious explanation
Pleistocene Refugia
• Nunataks
– Glacial refugia that persisted within or adjacent to the
ice sheets (continental margins, pockets in mountain
ranges, mountain tops, steep bluffs of coastline,
between ice sheets)
• Land south of ice also served as refugium.
• Driftless area in southern Wisconsin and adjacent
Illinois and Iowa
– elliptical area bypassed by Laurentide glacial front
Pleistocene Refugia
• Three large coastal refugia in North
America during Wisconsin glacier
– Beringia
– coastal regions of Pacific Northwest
– Nova Scotia
• Refugia notable for high amount of
divergence in animals and plants, and high
degree of endemism.
Beringia
• First proposed by E. Hultén in 1937
• Evidence to support hypothesis:
– fossil pollen records show that mesic tundra was
widespread in northern and central portions of the
landbridge
– strong evidence for the occurrence of deciduous trees
and larch in Yukon at this time
– mammalian fossils from 40,000 ybp show high
diversity of large ungulates (woolly mammoth, woolly
rhino, muskox, moutain sheep, steppe antelope,
reindeer, horse, camel).
Pleistocene extinctions
• plants unable to disperse in advance of glaciers
went extinct
• mass extinctions of marine invertebrates during
late Neogene associated with glacial cycles
• terrestrial vertebrate extinction pattern very
different from plants and marine invertebrates
• extinctions did not occur until glaciers retreated
Overkill Hypothesis
• humans responsible for the extinctions
• evidence to support hypothesis:
– fossil evidence shows that prehistoric humans and large
mammals coexisted and that humans hunted them
(arrow points in carcasses)
– extinctions were nonrandom
• large animals became extinct at a higher rate than small
mammals
– Animals adapted to human predation (e.g., caribou,
moose, deer, bighorn sheep) fared much better in terms
of extinction
– extinctions began in north and proceeded southward
– Definite correlation of extinction patterns and human
migrationpatterns