Cenozoic Geologic History: The Pleistocene and Holocene Epochs

Chapter 17 Cenozoic Geologic History: The Pleistocene and Holocene Epochs

Jungfrau Firn, Switzerland • The Jungfrau Firn mergers with two other valley glaciers • to form the Aletsch Glacier • Valley glaciers are present on all continents • except Australia, • but during the Pleistocene Epoch (the Ice Age) • they were more numerous and much larger than they are now

The Pleistocene Epoch • The Pleistocene Epoch and • Holocene or Recent Epoch • are the designations for the most recent • 1.8 million years of geologic time • In the past, the Quaternary Period • encompassed the Pleistocene and Holocene • but more recently the Quaternary • is a subperiod within the Neogene

The Pleistocene Epoch • The chronostratigraphic status • of the Quaternary • has not been resolved, • and we will not reference it • Accordingly, the Pleistocene and Holoceone epochs • are the last two epochs • of the Neogene Period

Cenozoic Time Scale • The geologic time scale • for the Cenozoic Era • The Pleistocene Epoch • from 1.8 million to 10,000 years ago • takes up more time than the Holocene

Pleistocene: 38 Seconds • Recall our analogy of all geologic time • represented by a 24-hour clock • In this context, the Pleistocene is only 38 seconds long, • but they are certainly important seconds, • because during this time our species evolved • Homo sapiens • and it was one of the few times in Earth history • when vast glaciers were present

Geologic Time in 24-hours • The Pleistocene • is only 38 seconds long • at this scale

Pleistocene Glaciation • A glacier • is a body of ice on land • that moves as a result of plastic flow • internal deformation in response to pressure • and by basal slip • sliding over its underlying surface • Continental glaciers cover at least 50,000 km2 and are unconfined by topography • Ice caps are similar, • but cover less than 50,000 km2 • while valley glaciers are long tongues of ice • confined to mountain valleys

Continental Glacier • The West and East Antarctic Ice sheets merge to form a nearly continuous ice cover • that averages 2160 m thick.

Ice Cap • The Penny Ice Cap on Baffin Island, Canada, • covers about 6000 km2

Valley Glacier • A valley glacier such as this one in Alaska • is a long, narrow tongue of moving ice • confined to a mountain valley

Biblical Deluge Versus Glaciers • In hindsight, it is difficult to believe • that many scientists of the 1830s • refused to accept the evidence • indicating that widespread glaciers • were present on the Northern hemisphere continents during the recent geologic past • Many invoked the biblical deluge • to explain the large boulders throughout Europe • far from their source; • whereas others thought the boulders • were rafted by ice during vast floods

Louis Agassiz • By 1837, • Swiss naturalist Louis Agassiz • argued convincingly • that the large displaced boulders • as well as polished and striated bedrock and U-shaped valleys • in many areas • resulted from huge masses of ice • moving over the land

Glacial Features • Features seen in areas once covered by glaciers • glacial polish • the sheen • striations • scratches • These features are convincing evidence that • a glacier moved over these rocks • in Devil’s Postpile National Monument, California

Fluctuating Climate • We now know that the Pleistocene Epoch, • more popularly known as the Ice Age, • was a time of several major episodes of • glacial advances • separated by warmer interglacial intervals • In addition, during times of glacial expansion • more precipitation fell in regions now arid, • such as the Sahara Desert of North Africa • and Death Valley in California • both of which supported streams, lakes, and lush vegetation

Unresolved Questions • Indeed, cultures existed • in what is now the Sahara Desert • as recently as 4500 years ago • Is the Ice Age is truly over? • Or are we in an interglacial period • that will be followed by renewed glaciation?

Pleistocene Glaciation • We focus on Pleistocene glaciers • because they had such a profound impact • on the continents • but remember that even at their maximum extent • glaciers covered only about 30% of Earth’s land surface. • Of course, the climatic conditions that led to glaciation • had world-wide effects, • but other processes were operating as usual • in the nonglaciated areas

Systems Approach • From the systems approach, • glaciers are part of the hydrosphere, • although some geologists • prefer the term cryosphere • for all of Earth’s frozen water, • which includes glaciers, sea ice, snow, • and even permafrost • permanently frozen ground

Pleistocene and Holocene Tectonism and Volcanism • The Pleistocene is best known for vast glaciers and their effects, • but it was also a time • of tectonism and volcanism, • processes that continued through the Holocene to the present • Today plates diverge and converge, • and, in places, slide past one another • at transform plate boundaries • As a consequence, orgoenic activity is ongoing • as is seismic activity and volcanic eruptions

Tectonism • These areas of orogenic activity continue unabated • the continent-continent collision • between India and Asia • and the convergence of the Pacific plate • with South America that formed the Andes • As do those • in the Aleutian Islands, • the Philippines, • and elsewhere

Uplift and Deformation • Interactions between • the North American and Pacific plates • along the San Andreas transform plate boundary • produced folding, faulting, • and a number of basins and uplifts • Marine terraces • covered with Pleistocene sediments • attest to periodic uplift along the US Pacific Coast

Marine Terraces • Marine terraces on the west side of San Clemente Island, California • Each terrace represents a period when that area was at sea level • The highest terrace is now about 400 m above sea level

Deformed Sedimentary Rocks • These deformed sedimentary rocks are only a few hundred meters • from the San Andreas Fault in southern California

Volcanism • Ongoing subduction of remnants • of the Farallon plate • beneath Central America and the Pacific Northwest • accounts for volcanism in these two areas • The Cascade Range • of California, Oregon, Washington, and British Columbia • has a history dating back to the Oligocene, • but the large volcanoes and Lassen Peak, • a large lava dome, • formed mostly during the Pleistocene and Holocene • Lassen Peak (California) and Mount St. Helens (Washington) • erupted during the 1900s, • and Mount St. Helens showed renewed activity in 2004

Mount Bachelor • Mount Bachelor at 11,000 to 15,000 years old is the youngest volcano in the range

Other Volcanism • Volcanism also occurred in several other areas • in the western United States including • Arizona, Idaho, and California • Following colossal eruptions, huge calderas formed • in the area of Yellowstone National Park, WY • Vast eruptions took place 2.0 and 1.3 million years ago • and again 600,000 years ago • that left a composite caldera • Since its huge eruption, part of the area has risen, • presumably from magma below the surface, • forming a resurgent dome • And finally, between 150,000 and 75,000 years ago, • the Yellowstone Tuff was erupted • and partially filled the caldera

High Hole Crater • This 115-m-high cinder cone lies on the flank of a huge shield volcano in northern California • The aa lava flow in the foreground was erupted 1100 years ago

McCloud River Lower Falls • The Lower Falls of the McCloud River in California • plunges 3.5 m over a precipice • in a Pleistocene lava flow

Other Volcanism • Elsewhere, volcanoes erupted in • South American, the Philippines • Japan, the East Indies, • as well as in Iceland, Spitzbergen, and the Azores • Even though the amount of heat • generated within Earth • has decreased through time, • volcanism and other processes • driven by internal heat • remain significant processes

Pleistocene Stratigraphy • Although geologists continue to debate • which rocks should serve as the Pleistocene stratotype • Recall that a stratotype is a section of rocks where a named stratigraphic unit such as a system or series was defined • they agree that the Pleistocene Epoch began 1.8 million years ago

Pleistocene–Holocene Boundary • The Pleistocene-Holocene boundary • at 10,000 years ago, • is based on climatic change • from cold to warmer conditions • concurrent with the melting • of the most recent ice sheets • Changes in vegetation • as well as oxygen isotope ratios • determined from shells of marine organisms • provide ample evidence for this climatic change

Terrestrial Stratigraphy • Soon after Louis Agassiz proposed his theory for glaciation, • research focused on deciphering the history of the Ice Age • This work involved recognizing and mapping • terrestrial glacial features • and placing them in a stratigraphic sequence

Glaciers Three km Thick • From glacial features such as • moraines, • erratic boulders, • and glacial striations, • geologists have determined that • Pleistocene glaciers at their greatest extent • up to 3 km thick • covered about three times • as much of Earth's surface • as they do now • or about 45,000,000,000 km2

Glaciers in North America • Centers of ice accumulation • and maximum extent • of Pleistocene glaciers • in North America

Glaciers in Europe • Centers of ice accumulation • and maximum extent • of Pleistocene glaciers • in Europe

Mapping • Detailed mapping of glacial features • reveals that several glacial advances and retreats occurred • By mapping the distribution glacial deposits, • geologists have determined • that North America • has had at least four major episodes • of Pleistocene glaciation

Four Glacial Stages • Each glacial advance • was followed by retreating glaciers • and warmer climates • The four glacial stages, • the Wisconsinan, • Illinoian, • Kansan, • and Nebraskan, • are named for the states • where the southernmost glacial deposits • are well exposed

Three Interglacial Stages • The three interglacial stages, • the Sangamon, Yarmouth, and Aftonian, • are named for localities • of well exposed interglacial soil and other deposits • Recent detailed studies of glacial deposits • indicate, however, that there were • an as yet undetermined number • of pre-Illinoian glacial events • and that the history of glacial advances and retreats • in North America • is more complex than previously thought

Traditional Pleistocene Terminology • Traditional terminology for Pleistocene glacial and interglacial stages in North America

Succession of Deposits • Idealized succession of deposits and soils • developed during the glacial and interglacial stages

Advances in Europe • Six or seven major glacial advances and retreats • are recognized in Europe, • and at least 20 major warm–cold cycles • have been detected in deep-sea cores • Why isn't there better correlation • among the different areas • if glaciation was such a widespread event? • Part of the problem is that • glacial deposits are typically chaotic mixtures • of coarse materials that are difficult to correlate

Minor Fluctuations • Furthermore, glacial advances and retreats • usually destroy the sediment left by the previous advances, • obscuring older evidence • Even within a single major glacial advance, • several minor advances and retreats may have occurred • For example, careful study of deposits • from the Wisconsinan glacial stage • reveals at least four distinct fluctuations • of the ice margin during the last 70,000 years • in Wisconsin and Illinois

Deep-Sea Stratigraphy • Until the 1960s, the traditional view • of Pleistocene chronology • was based on sequences of glacial sediments on land • However, new evidence • from ocean sediment samples • indicate numerous climatic fluctuations • during the Pleistocene

Evidence for Climatic Fluctuations • Evidence for these climatic fluctuations • comes from changes in surface ocean temperature • recorded in the shells of planktonic foraminifera, • which after they die sink to the seafloor • and accumulate as sediment • One way to determine past changes • in ocean surface temperatures • is to resolve whether planktonic foraminifera • were warm- or cold-water species

Response to Temperature • Many planktonic foraminifera are sensitive to variations in temperature • and migrate to different latitudes • when the surface water temperature changes • For example, the tropical species • Globorotalia menardii • during period of cooler climate • is found only near the equator, • whereas during times of warming • its range extends into the higher latitudes

Coiling Direction • Some planktonic foraminifera species • change the direction they coil during growth • in response to temperature fluctuations • The Pleistocene species • Globorotalia truncatulinoides coils predominantly • to the right in water temperatures above 10°C • but coils mostly to the left in water below 8°-10°C • On the basis of changing coiling ratios, • geologists have constructed detailed climatic curves • for the Pleistocene and earlier epochs

Oxygen Isotope Ratio • Changes in the O18-to-O16 ratio • in the shells of planktonic foraminifera • also provide data about climate • The abundance of these two oxygen isotopes • in the calcareous (CaCO3) shells • of foraminifera • is a function of the oxygen isotope ratio in water molecules • and water temperature when the shell forms • The ratio of these isotopes • reflects the amount of ocean water stored • in glacial ice

Lighter Isotopes in Glacial Ice • Seawater has a higher O18-to-O16 ratio • than glacial ice • because water containing the lighter O16 isotope • is more easily evaporated • than water containing the O18 isotope • Therefore, Pleistocene glacial ice • was enriched in O16 relative to O18, • while the heavier O18 isotope • was concentrated in seawater

Climate Change from Isotopes • The declining percentage of O16 • and consequent rise of O18 in seawater • during times of glaciation • is preserved in the shells of planktonic foraminifera • Consequently, oxygen isotope fluctuations • indicate surface water temperature changes • and thus climatic changes

Ocean Surface Temperature • O18-to-O16 isotope ratios • from a sediment core in the western Pacific Ocean • indicate that ocean surface temperatures • changed during the last 56 million years • A change from warm to colder conditions • took place 32 million years ago

Cenozoic Geologic History: The Pleistocene and Holocene Epochs