A SHORT HISTORY AND LONG FUTURE FOR PALEOPEDOLOGY
GREGORY J. RETALLACK
Department of Geological Sciences, University of Oregon, Eugene, Oregon 97302, USA
e-mail: [email protected]
ABSTRACT: The concept of paleosols dates back to the eighteenth century discovery of buried soils, geological unconformities, and fossil forests, but the
term paleopedology was first coined by Boris B. Polynov in 1927. During the mid-twentieth century in the United States, paleopedology became mired in
debates about recognition of Quaternary paleosols, and in controversy over the red-bed problem. By the 1980s, a new generation of researchers envisaged
red beds as sequences of paleosols and as important archives of paleoenvironmental change. At about the same time, Precambrian geochemists began
sophisticated analyses of paleosols at major unconformities as a guide to the long history of atmospheric oxidation. It is now widely acknowledged that
evidence from paleosols can inform studies of stratigraphy, sedimentology, paleoclimate, paleoecology, global change, and astrobiology. For the future,
there is much additional potential for what is here termed ‘‘nomopedology,’’ using pedotransfer functions derived from past behavior of soils to predict
global and local change in the future. Past greenhouse crises have been of varied magnitude, and paleosols reveal both levels of atmospheric CO2 and
degree of concomitant paleoclimatic change. Another future development is ‘‘astropedology’’, completing a history of soils on early Earth, on other
planetary bodies such as the Moon and Mars, and within meteorites formed on planetismals during the origin of the solar system.
KEY WORDS:
paleosol, history, Precambrian, soil, Mars
INTRODUCTION
On the opening page of his famous novel Lolita, Vladmir Nabokov
(1955) mentions paleopedology as an obscure scientific interest of his
antihero, Humbert Humbert. Paleopedology developed late in the
history of science, because it is a composite science like ecology, which
followed the development of biology (Osterkamp and Hupp 1996).
Paleopedology had to wait for pedology (soil science), which is also
remarkable for ‘‘the brevity of its history and the length of its
prehistory’’ (Krupenikov 1992). Both pedology and paleopedology,
like Nabokov, were born in St Petersburg, Russia, through the efforts of
V.V. Dokuchaev (1883), B.B. Polynov (1927), and C.C. Nikiforoff
(1943, 1959). Interest in soils and elementary understanding of
paleosols, however, long predate these Russian origins.
Paleopedology is the study of ancient soils, and it is derived from
ancient Greek words for old (pakaio1), ground (pedov), and word
(koco1). It has nothing to do with pedestrians (from Latin pes, pedis) or
pediatricians (from Greek pais, paidos). Paleosol (fossil soil) is thus a
Greek–Latin hybrid, from Latin for soil (solum, soli). Soils of the past,
either buried within sedimentary sequences or persisting under
changed surface conditions, are the subject matter of paleopedology.
My predictions for the future of paleopedology are inevitably personal
ones, areas that currently excite my interest. A comprehensive history
and prehistory of paleopedology have yet to be written: This account is
the longest yet attempted and focuses on the pioneering studies of
scientists now deceased.
OBSERVATIONS IN ANTIQUITY
Religious views of buried and surface soils may have been the first
human appreciation for this indispensable source of life (Winiwarter
and Blum 2006). Civilizations of alluvial, cumulic, and buried soils
share myths about human origin from soil. The oldest written myth of
this kind is a cuneiform text dating to 2000 BC from the Sumerian city
of Nippur in present-day Iraq. The myth describes a feast of the gods,
presided over by Ninmah (mother earth) and Enki (god of the waters).
The minor gods had complained about the work of digging irrigation
channels, and pressed for help. Inspired after a banquet of much food
and wine, Ninmah created six different kinds of humans out of
floodplain clay, while Enki decreed their work and gave them bread to
eat (Kramer 1944). An ancient Egyptian bas-relief from the Luxor birth
room (King and Hall 1906), dating to about 1400 BC, depicts the
future pharaoh fashioned from clay by Khnum, a ram-headed god of
Nile River waters (Fig. 1). Such beliefs are understandable in lands
such as Egypt and Iraq, where frogs and other creatures emerge from
the soil after increments of flood alluvium. Similar views are found
also in the original (Yahwist) portion of the Judaic biblical account of
Genesis, dating from 900 to 500 BC. The name Adam means ‘‘red
clay’’ in Hebrew. The name Eve is the Hebrew verb ‘‘to be.’’ The
Genesis account thus implies that Adam and Eve were living soil
(Hillel 1991).
Sacred views of soil vary between cultures. Soil is inviolable, and
tilling or digging is sacrilege to the pastoral Maasai of arid savannas on
Aridisols of the Kenyan Rift Valley. The Maasai god (Engai) lives in
the sky, and they do not believe in life after death or in souls, knowing
that the body and its vitality return to the soil (Bentsen 1989). In
contrast, agrarian Kikuyu of the fertile humid uplands around Nairobi,
above the central Kenyan Rift, live by intensive market gardening on
Oxisols and Alfisols. The Kikuyu god (Ngai) lives alone on nearby
Mount Kenya, and some specific old trees are sacred as homes of
spirits and dead souls (Routledge and Routledge 1910). The consistent
nature of local soil resources around the temples to particular deities of
ancient Greece suggests that similarly diverse tribal views from
different lifestyles, such as herding, cropping, and fishing, were
amalgamated into ancient Greek polytheism (Retallack 2008).
Reverence for soil in the past extended to libation, animal sacrifice,
and heirogamy (Frazer 1922). Today, soil reverence persists in
Christian holy sacraments of bread and wine, both soil produce, and
Chinese offerings of sorghum root on soil altars (Winiwarter and Blum
2006).
Practical scientific observations of the kinds of soils, soil geography,
and suitable crops abound in the literature of classical Greece and
Rome, particularly in the writings of Xenophon (430–340 BC),
Theophrastus (372–287 BC), Cato (234–149 BC), Varro (116–27 BC),
Virgil (70–19 BC), Strabo (64 BC–24 AD), and Columella (4–70 AD)
(Krupenikov 1992, Borras 1999, Winiwarter 2006). Such soil expertise
was widespread in agrarian cultures, but it takes archeological and
textural ingenuity to reconstruct in ancient societies without large
literatures (Sandor et al. 2006, Williams 2006).
New Frontiers in Paleopedology and Terrestrial Paleoclimatology j DOI: 10.2110/sepmsp.104.06
SEPM Special Publication No. 104, Copyright Ó 2013
SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-322-7, p. 5–16.
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GREGORY J. RETALLACK
FIG. 1.—Ancient Egyptian god of the Nile, Khnum (‘‘Lord of the Dark
Waters’’), creating the future pharaoh Amenhotep III and his Ka
(‘‘vital force’’), on a potters table, from a bas-relief in the Birth
Room at the Temple of Luxor, Egypt (redrawn from King and Hall
1906).
Some classical authors had a concept of buried and relict soils,
which are two kinds of paleosols, as implied in the following two
quotations. ‘‘Egyptian soil is black and friable, which suggests it was
once mud and silt carried down from Ethiopia by the river’’ (Herodotus
[484–425 BC], Histories II:12:1; translation of Waterfield and Dewald
1998). Soils were thus buried during floods in the valley of the Nile
River. ‘‘In its former extent, at an earlier period, [a high terrace from
Mt. Hymettos around the Athenian Parthenon] went down towards the
Eridanus and the Ilissus [Rivers], and embraced within it the Pynx
[hill], and had the Lycabettus [hill] as its boundary over against the
Pynx, and it was all rich in soil, and save for a small space [of bedrock
hills], level on the top’’ (Plato [428–347 BC], Critias 112A; translation
of Bury 1912). The terrace remnants around the Pynx are a relict of a
much more widespread high terrace soil of the past that surrounded the
Acropolis (Hurwit 1999). These examples show that processes in the
formation of paleosols were understood, but not appreciated as
paleosols.
EIGHTEENTH-NINETEENTH CENTURY
PALEOSOLS DISCOVERED
The oldest published observation of a Quaternary paleosol was by
Luigi Ferdinando Marsigli (Fig. 2) in his monumental work on the
Danube River in Hungary (Marsigli 1726). His cross section of the
Danube River bank shows four distinct layers, from the bottom up: (1)
sand bank, (2) yellow ash with carbonate, (3) black fertile soil, and (4)
black fertile carbonate soil. Units 2 and 3 were a buried calcareous
Mollisol, a paleosol like the surface soil (unit 4: Markovic et al. 2009).
Marsigli (1658–1730) was a count from Bologna, Italy, and pursued
this work from 1696 to 1700 as a military engineer for Leopold I,
Habsburg Emperor (Stoye 1994). During the late nineteenth century,
many buried soils were recognized within surficial deposits of loess
and till. These ‘‘weathered zones,’’ ‘‘forest zones,’’ and ‘‘soils,’’ as they
were variously termed, were found in the Russian Plain by Feofilatkov
(in the 1870s as recounted by Polynov 1927), in the midcontinental
United States by McGee (1878), and in New Zealand by Hardcastle
(1889). By the turn of the century, such observations had been used for
stratigraphic subdivision of glacial deposits (Chamberlain 1895).
Paleosols were implied in the following explanation by James
Hutton (Fig. 2) of the first major geological unconformity discovered
along the River Jed and at Siccar Point (Fig. 3A), southeast Scotland:
‘‘From this it will appear, that the schistus mountains or vertical strata
of indurated bodies had been formed, and had been wasted and worn in
natural operations of the globe, before horizontal strata were begun to
be deposited in these places...’’ (Hutton 1795, v. 1, p. 438). Arguments
of past causes from processes that can be observed today (uniformitarianism) and indications of geological ages much greater than biblical
accounts (deep time) mark the origin of modern geological thought,
and these insights overshadowed this first indication of a paleosol at a
major geological unconformity. James Hutton (1726–1797) studied
law and medicine before inventing a new process and manufacturing
business making ammonia salts from soot, which afforded him the time
and money for independent research affiliated with the Royal Society
of Edinburgh (Baxter 2004). Hutton thus lacked the simplifying
influence of teaching students, and his writing style was prolix. His
work and ideas were better advanced by other Scotsmen: John
Playfair’s (1802) concise and clear Illustrations of the Huttonian
Theory of the Earth, and then Charles Lyell’s many editions and
volumes of Principles of Geology (Lyell 1830). John Playfair (1748–
1819) was a scientist, mathematician, and professor at the University of
Edinburgh (Dean 1983). Charles Lyell (1797–1875) studied law and
attended lectures of William Buckland at Oxford University. Because
of deteriorating eyesight, he turned from law to geology as a
profession, and became one of the most influential geologists of his
day through his books, travels, and offices of the Geological Society of
London (Bailey 1989). Studies of paleosols at major unconformities
continued, especially in Precambrian rocks (Sharp 1940, Sidorenko
1963).
The oldest record of buried soils within consolidated sedimentary
rocks were the ‘‘dirt beds’’ (Fig. 3C) and permineralized stumps and
cycadeoids reported in latest Jurassic limestones (Purbeck Formation;
Francis 1986) of the Dorset coast by Webster (1826). Thomas Webster
(1773–1844) was born in the Scottish Orkney Islands, and studied at
the University of Aberdeen. He was house-secretary and curator of the
Geological Society of London from 1826, and foundation professor of
geology at University College London from 1841 to 1842 (Challinor
1964). The Purbeck paleosols were popularized by William Buckland
(Fig. 2) in his Bridgewater Treatise (Buckland 1837), which reiterated
his controversial earlier conclusions that Pleistocene bones and
coprolites in Kirkdale Cave (Yorkshire) were remains of hyena dens,
and not evidence of the biblical flood. Together with numerous other
observations, this influential work of popular science extended the
uniformitarian approach of James Hutton from geology to paleontology and paleopedology. William Buckland (1784–1856) was the son of
a Devonshire rector, and also became ordained as a minister, but he
spent most of his career at Corpus Christi and Christ Church Colleges
of Oxford University (Rudwick 2008). Other pioneering studies of
paleosols in sedimentary rocks include those of Barrell (1913), Allen
(1947), Thorp and Reed (1949), and Schultz et al. (1955).
Buried agricultural and archeological soils are illustrated and
discussed by Charles Darwin in a paper (Darwin 1837) and book on
worms (Darwin 1881). Near Maer Hall, Staffordshire, England
(childhood home of his wife Emma), Darwin indicated in an illustrated
section ‘‘original black peaty soil’’ buried by ‘‘fragments of burnt marl,
of cinders and a few quartz pebbles’’ beneath turf of a modern soil
profile. He also noted that ruins of an old Roman villa at Abinger in
Surrey were covered in soil created by worm castings. Charles Darwin
(1809–1882) was trained in medicine and theology at Edinburgh and
Cambridge, respectively, but he had independent means for scientific
research and is better known for his theory of evolution by natural
selection, which is widely regarded as the birth of modern biology
(Browne 1995, 2002). Archeological ‘‘occupation levels’’ were
subsequently popularized by Heinrich Schliemann’s (1874) excavations at Troy in western Turkey, and Arthur Evans’ (1921) excavations
of Knossos on Crete.
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A SHORT HISTORY AND LONG FUTURE FOR PALEOPEDOLOGY
7
FIG. 2.—Pioneers of paleopedology: clockwise from top left: Luigi F. Marsigli (from commons.wikimedia.org), James Hutton (oil on canvas by
Henry Raeburn, courtesy of National Galleries of Scotland PG2686), William Buckland (oil on canvas by Thomas Phillips, courtesy of UK
National Portrait Gallery NPG1275), Albert C. Seward (retouched from photo of Wilding 2005), Dan Yaalon (courtesy of Dan Yaalon), Kirk
Bryan (retouched from photo of Osterkamp 1989), C.C. Nikiforoff (photo from Cady and Flach 1997), and Boris B. Polynov (retouched photo
from Krupenikov 1992)
‘‘Fossil forests’’ of preserved stumps and logs were widely
discovered in the nineteenth century and preserved as tourist attractions
(Thomas 2005). The stumps and associated fossil plants were
described, but little was made of their substrates as fossil soils.
Examples include the Eocene Sequoia forests of Yellowstone National
Park, USA, and Carboniferous stumps of tree-lycopsids at Clayton
(Yorkshire, England), Victoria Park (Glasgow, Scotland; Fig. 3B), and
at Joggins (Nova Scotia, Canada). A summary of these early
discoveries of pre-Quaternary fossil soils is available in introductory
chapters of the first volume of A.C. Seward’s (Fig. 2) monumental
work Fossil Plants (Seward 1898), and again reiterated in his popular
book Plant Life through the Ages (Seward 1931). Albert Charles
Seward (1863–1941) was a professor of Botany at Downing College,
Cambridge (Andrews 1980). Seward appreciated fossil forests as
evocative evidence of past plant life, but was better known for studies
of Gondwanan fossil plants as evidence of continental drift and
profound climate change (Wilding 2005). Study of the paleosols
themselves awaited development of soil science.
The origin of paleopedology as a discrete field of inquiry required the
late nineteenth century development of soil science and its key concepts
of soil profiles and soil geography (Tandarich and Sprecher 1994). Soils
had been studied from the point of view of plant nutrition since classical
times. It was not until 1862 (Fallou 1862) that the Saxon scientist
Fredrich A. Fallou (1795–1877) first published the term ‘‘pedologie’’ for
the study of soil science, as opposed to what he termed ‘‘agrologie,’’ or
practical agricultural science. The foundations of modern soil science
were laid by Vasilii V. Dokuchaev (1846–1903) with a detailed account
of the dark, grassland soils of the Russian Plain (Dokuchaev 1883). This
book made clear the soils could be described, mapped, and classified in a
scientific fashion, and were not merely epiphenomena of geochemical
and physical weathering processes. Furthermore, their various features
could be related to environmental constraints, of which climate and
vegetation were considered especially important. By the early part of the
twentieth century, there was an established scientific tradition of
research on soil geography, classification, and genesis in Russia
(Krupenikov 1992), as summarized in the influential general works of
K.D. Glinka (1927). Russian pedology established soils as objects of
natural history worthy of careful study, although soils remain difficult to
define (Hole 1981). Perhaps the most inspiring definition from
Nikiforoff (1959, p. 186) is soil ‘‘as an excited skin of the subaerial
part of the Earths crust.’’ Put more prosaically, soil is material forming
the surface of a planet or similar body and altered in place from its parent
material by physical, chemical, or biological processes (Retallack
2001).
PALEOPEDOLOGY BORN 1927
In the course of early Russian soil mapping, certain soils were found
to be anomalous because their various features did not fit the general
relationship between soil profile type and their climate and vegetation.
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8
GREGORY J. RETALLACK
FIG. 3.—Notable pre-Quaternary paleosols recognized by the pioneers of geology: A) Angular unconformity between Early Silurian (430 Ma)
Queensberry Grits Rocks and Late Devonian (360 Ma) Upper Old Red Sandstone (Greig 1971) at Siccar Point, Scotland (55.9323238N,
2.2999338W): B) fossil stumps of Stigmaria ficoides in mid-Carboniferous (Namurian) Limestone Coal Group (Bluck 1973), at Fossil Grove
in Victoria Park, Glasgow (55.8767558N, 4.3382638W): C) ‘‘Dirt beds’’ (paleosols) including the ‘‘Mammal bed’’ (below) and ‘‘Great Dirt
bed’’ (at hammer) in a stratigraphic section through the latest Jurassic (Tithonian, 150 Ma) Purbeck Formation (Francis 1986), on Durlston
Head, near Swanage, England (50.5961858N, 1.9520528W). All photographs are by author, June 1980.
It had long been suspected that these were very old soils, perhaps
products of past environments. In 1927, Boris B. Polynov (Fig. 2)
summarized Soviet observations of this kind for an All-Union Soviet
Soil Conference in Leningrad, in preparation for the International
Congress of Soil Science in Washington, DC, later that year. His short
paper, which introduced the term paleopedology, can be considered the
foundation of this branch of science. Polynov included the study of four
kinds of materials within paleopedology. ‘‘Secondary soils’’ encompass
those formed by two successive weathering regimes, such as grassland
soils degraded by the advance of forests after retreat of glacial ice.
‘‘Two-stage soils’’ were recognized to have an upper horizon of recent
origin, but deeper horizons of more ancient vintage. ‘‘Fossil soils’’ were
defined as soil profiles developed on a surface and subsequently
buried. Polynov’s final category of ‘‘ancient weathering products’’
included the redeposited remnants of soils such as laterites and china
clays. Boris Borisovich Polynov (1877–1952) was a student of K.D.
Glinka at the Novalexandrovsk Institute of Agriculture in St.
Petersburg before the Russian Revolution, and his wide experience
came from K.D. Glinka’s ambitious program of soil mapping and
characterization. Polynov worked on soils in Siberia, Mongolia, Don
Basin, Black Sea, Caucasus coast, northern Ukraine, and lower Volga
River, and was chairman of pedology at Leningrad State University
(Kovda and Dobrovolsky 1974, Krupenikov 1992).
Modern soil science in North America can be traced back to Eugene
W. Hilgard’s (1892) monograph on the relationship between soil and
climate. This, and the first large-scale mapping and classification of
North American soils by Milton Whitney (1909) were largely
independent of comparable research done by Russian soil scientists.
Their impact was diminished by personal rivalry and scientific
disagreement: Hilgard at Berkeley emphasized soil chemistry, and
Whitney in Washington, DC, emphasized soil structure (Jenny 1961).
Soviet influence first appeared in America with the work of Curtis
Marbut, particularly, his monumental soil survey of the United States
(Marbut 1935). Paleopedology also was introduced into North America
through a Soviet connection. Constantin C. (‘‘Niki’’) Nikiforoff (1867–
1979; Fig. 2) completed doctoral studies in 1912 under N.M. Sibirtsev
at the Novoalexandrovsk Institute, and then joined the staff of the
Russian Department of Agriculture and Land Improvement. During the
Russian Revolution, he fought with the White Army, before defecting
to New York. At first, he drove a taxi in New York City, and was a farm
laborer in northern Minnesota in 1927, when his expertise was
discovered by a visiting soil survey. After a short stint at the University
of Minnesota, he was a soil scientist for the U.S. Department of
Agriculture from 1931 until retirement in 1957 (Stelly 1979). He was
best known for advancing the concept of soil equilibrium, along with
his close friend, the geomorphologist John Hack (Osterkamp and Hupp
1996). Nikiforoff (1943) published a short essay outlining the role and
scope of paleopedology. A supporting study of paleosols in the same
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A SHORT HISTORY AND LONG FUTURE FOR PALEOPEDOLOGY
journal in 1943 by Kirk Bryan and Claude Albritton made clear the
practical application of such studies. Kirk Bryan (1888–1950; Fig. 2)
was born in Cody, Wyoming, and received an undergraduate education
at the University of New Mexico and PhD from Yale. As successor to
William Morris Davis in the Department of Physical Geography at
Harvard in 1925, Bryan was among the most influential geomorphologists of the twentieth century (Whittlesey 1951, Holliday 2006).
ORGANIZATIONS, MEETINGS, AND TEXTS
SINCE 1965
The first organization for this young science was the Commission on
Paleopedology established in 1965 by Dan H. Yaalon (Fig. 2) and J.
Hans V. van Baren, at the 7th Congress of the International Association
for Quaternary Research in Denver, USA. An early result of the
commission’s activities was the publication of recommendations for
recognizing and classifying paleosols in a volume of research papers
edited by Yaalon (1971). A second international meeting was
organized for September 1987 by Gregory J. Retallack and Patricia
McDowell (1988) as a Penrose Conference at Kah-nee-tah resort in
central Oregon under the auspices of the Geological Society of
America (Fig. 4). The theme of this meeting was ‘‘paleoenvironmental
interpretation of paleosols,’’ and the scope broadened to include preQuaternary paleosols, with an excursion to Eocene and Oligocene
paleosols of central Oregon. It was organized as an open discussion,
with only 20 minutes of talks followed by an hour of discussion. A
third international meeting on paleopedology was organized at
Allerton House, near Urbana, Illinois, in August 1993 by Leon
Follmer (1998). With excursions to the Sangamon Geosol, this meeting
emphasized Quaternary paleosols and was notable for its working
groups on a variety of contentious issues of definitions, classification,
stratigraphy, methods, diagenesis, and field description. The fourth
international meeting on paleopedology was held in September 2010 as
an SEPM (Society for Sedimentary Geology) Research Conference
and Workshop at Petrified Forest National Park, Arizona, by Steven
Driese, Lee Nordt, Stacy Atchley, Steve Dworkin, Dan Peppe, Cindy
Stiles, Curtis Monger, and Eugene Kelly. The emphasis this time was
on pre-Quaternary paleosols and their surface system analogs, again
with working groups producing recommendations designed specifically to improve research funding for paleopedology.
Useful early collections of papers on paleopedology were edited by
Yaalon (1971), Wright (1986), Reinhardt and Sigleo (1988), Martini
and Chesworth (1992) and Thiry and Simon-Coinçon (1999). There
also were some useful early monographic works on paleosols (Ruhe
1969, Retallack 1983). For a textbook of paleopedology, the first useful
book was by Birkeland (1974), because of its emphasis on
fundamentals of soil science, Quaternary paleosols, and the soil factor
approach of Jenny (1941). Pre-Quaternary paleosols and problems of
diagenesis were added in subsequent textbooks of paleopedology
designed for university courses (Retallack 1990, 2001) and for
interested professionals of other earth sciences (Retallack 1997).
Australian authors have provided textbooks on relict paleosols, though
using the ambiguous terminology of ‘‘regolith’’ (Ollier and Pain 1996,
Taylor and Eggleton 2001).
Meetings and texts were needed, because paleopedology in the midtwentieth century was mired in controversy: ‘‘Many people have had
the experience in field conferences that one man’s [buried] soil
becomes another’s geologic deposit’’ (Birkeland 1974, p. 24). A poem
penned in 1958 by Luna Leopold for his field buddies Charlie Denny,
Charlie Hunt, Reds Wolman, John Goodlett, Niki Nikiforoff, and
Johnny Hack, captures this well (Osterkamp 1989, p. 288).
We drive along the road, we stop at every cut,
We’re boys who must be showed, we won’t be in a rut,
Don’t like your fossil soil, nor even buried ones,
9
From your A2 we recoil, we’re skeptic sons-of-guns.
One solution to the debate has been to ignore it by using terms such as
regolith and gradation. Merrill (1897, p. 299) introduced the term
regolith in the following way, ‘‘In places this covering is made up of
material originating through rock-weathering or plant growth in situ. In
other instances it is of fragmental and more or less decomposed matter
drifted by wind, water or ice from other sources. This entire mantle of
unconsolidated material, whatever its nature or origin, it is proposed to
call the regolith.’’ Gradation is a counterpart term for all the
sedimentary, pedogenic, and other processes that form regolith
(Greeley 1994). The vagueness of pedolith and gradation can be
useful in exploration of remote regions (Ollier and Pain 1996, Taylor
and Eggleton 2001) and other planetary bodies (Greeley 1994). This is
essentially a geological or engineering approach, whereas pedology
and paleopedology are concerned with disentangling soil-forming
from sedimentary and other processes.
The problem of paleosol recognition was only partly palliated by the
mantra of (1) root traces, (2) soil horizons, and (3) soil structures, which
are foreign concepts to many geologists (Retallack 1990). The real
problem in many cases was diagenesis, erosion, or alteration of paleosols
soon after burial. Burial decomposition, for example, can strip a paleosol
of organic matter, changing its color from dark brown to light yellow
(Retallack 1991). In Quaternary Palouse Loess of Washington State,
fossil Aridisols were easily recognized from their prominent calcareous
nodules (Busacca 1989) and cicada burrows (O’Geen et al. 2002), but
the Mollisols that form interbeds at Milankovitch temporal frequencies
were not recognized until detailed studies of soil micromorphology (Tate
1998) and phytoliths (Blinnikov et al. 2002). The mollic crumb structure
and fine root traces are still there (Retallack 2004), but not the dark
organic matter, which would make these grassland paleosols more
obvious. Erosion of paleosols immediately before burial can compromise some aspects of their interpretation, but is not evident in many
eolian and volcaniclastic sequences of paleosols like the Palouse Loess,
in which radiocarbon dating and tephrostratigraphy demonstrate
continuous aggradation (Busacca 1989, Blinnikov et al. 2002).
In the rock record, paleosol recognition was confounded by the ‘‘redbed problem’’ (Turner 1980). Petrographic observations showed some
red transported grains indicating redeposition. Some wisps of red stain
crossing grains indicating a diagenetic origin and which were assumed
have formed after burial. Diagenesis also includes soil formation and
other alterations after deposition, not just after burial. Most red beds
turn out to be paleosols or sequences of them (Retallack 1997), but
here again diagenetic dehydration of ferric hydroxides and local burial
gleization of organic matter can change the appearance of subtle gray–
brown soils to gaudy caricatures of green–red mottled paleosols, unlike
any modern soil (Retallack 1991).
The problem of classification and interpretation of paleosols also
became contentious, despite efforts to skirt the problem with nongenetic
naming systems. Early studies labeled them with names such as
Yarmouth soil, intended as a stratigraphic marker beds (Ruhe 1969). For
these purposes, the term geosol is best, for example, Yarmouth Geosol,
as ratified by the North American Commission on Stratigraphic
Nomenclature (1982). Geosols are not suited to paleoenvironmental
interpretation, because they change form along strike in response to
drainage and other soil-forming factors (Follmer 1978). For this
purpose, a different unit, the pedotype, has been proposed (Retallack
1994) based on a type pedon. These can be mapped laterally like soil
series across paleolandscapes, and also in geological successions of
paleosols, for example, the Gleska pedotype, Eocene, Badlands
National Park, South Dakota (1983). Pedotypes were established so
that paleosols could be described objectively in a systematic fashion
before they were classified and otherwise interpreted.
Controversy does not stem from the formalism of geosols or
pedotypes, but rather from moving directly to interpretation, including
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10
GREGORY J. RETALLACK
FIG. 4.—Penrose Conference on paleoenvironmental interpretation of paleosols at Kah-nee-tah Lodge, Oregon, September 11–17, 1987.
classification within systems devised for modern soils. Classifications
such as those of Soil Taxonomy (Soil Survey Staff 2010) and the Food
and Agriculture Organization (1974, 1988) were not primarily
designed for paleosols, and for this reason, classifications specifically
for paleosols were devised (Mack et al. 1993; Nettleton et al. 1998,
2000). The main point of contention in applying modern classifications
to paleosols is their reliance on climatic measurements or chemical
indices such as base saturation or pH not preserved in paleosols.
Nevertheless, proxies such as depth to carbonate as a climatic indicator
(Retallack 2005) and geochemical transfer functions for pH (Nordt and
Driese 2010a) have been used to identify paleosols, enabling
comparison of paleosols within the large literature of soil geography
and genesis (Retallack 1983). Studies of stable isotopes (primarily
d13C and d18O) from paleosol carbonate have provided proxies of
unexpected soil-forming variables, including proxies for atmospheric
CO2 (Cerling 1991, Breeker et al. 2010), secondary productivity
(Retallack 2009a), and rates of evapotranspiration (Ufnar et al. 2008).
Furthermore, soils as living entities have evolved with changing
conditions over geological time. Extinct soil orders can also be
identified, such as ‘‘Green Clays’’ (Viridisols), which are deeply
weathered soils with soil creep and corestones formed under Archean
atmospheres with unusually low levels of oxygen (Retallack 2001).
Such interpretations of former soil type, vegetation, or drainage
character are widely made from paleosols, and are useful adjuncts to
evidence from sedimentology, fossils, geochemistry, and stratigraphy
in forming ever more detailed impressions of past landscapes and
biota.
NOMOPEDOLOGY
A promising future direction for paleopedology is nomopedology,
from the Greek molo1 (law), here defined as a branch of soil science
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A SHORT HISTORY AND LONG FUTURE FOR PALEOPEDOLOGY
devoted to discovering general rules of soil formation. Nomopedology
is not new, and its essence is well captured in the seminal text, Factors
of Soil Formation by Hans Jenny (1941), which outlined a variety of
mathematical relationships between soil features and environmental
variables. He proposed the lawyerly formalism of basing climofunctions (relationships between soil features and climate) on climosequences (a collection of soils for which all factors other than climate
are more or less equal). Jenny also proposed biofunctions, topofunctions, lithofunctions, and chronofunctions for the main soilforming factors. Improving pedotransfer functions (Bouma 1989) is an
ongoing preoccupation of pedologists, as well as paleopedologists,
because rules gleaned from modern soils can be invaluable in the
quantitative interpretation of paleosols (Driese et al. 2005, Nordt et al.
2006, Sheldon and Tabor 2009, Nordt and Driese 2010b). Not all these
rules need necessarily be derived only from surface soils. The wellknown mineral stability series during chemical weathering devised by
Samuel S. Goldich (1938) was based on a study of mid-Cretaceous
(Cenomanian) paleosols developed on granite in southern Minnesota.
In this case, it saved him a trip to the tropics to see comparable modern
deep weathering of granite. In other cases, the paleosol record reveals
soil-forming conditions no longer found on Earth, such as the early
evolution of vascular land plants (Elick et al. 1998, Driese et al. 2000),
and the low O2 atmosphere of the Precambrian (Rye and Holland 1998,
Driese and Medaris 2008).
Other factors unique to the paleosol record are CO2-greenhouse
crises of the geological past, some of which, such as the Late Permian
one, were so extreme that they were occasions of mass extinction
(Retallack and Jahren 2008). Levels of atmospheric CO2 in the past can
be inferred from isotopic study of pedogenic carbonate (Retallack
2009a) and from stomatal index of fossil Ginkgo leaves (Retallack
2009b). Within long time series of paleoclimatic proxies, such as depth
to carbonate nodules, now available from paleosols, these times of
greenhouse crisis stand out as transient times of much deeper calcic
horizons. Such a record of arid-land paleoclimate from fossil Aridisols
of Utah and Montana was log-transformed (Fig. 5) because it is lognormal in distribution due to ubiquitous Milankovitch period climate
variation, with more precession and obliquity, than eccentricity and
FIG. 5.—Running mean (64 Ma) by million-year increments and 2
standard deviation envelope of log-transformed depth to calcic
horizon in paleosols of Utah and Montana. The 40 peaks (gray
spikes) representing unusually humid paleoclimate in excess of 2
standard deviations (black error envelope) are marked with
geological ages. Calcic depths at the other extreme, below 2
standard deviations represent unusually arid paleoclimate or highly
eroded paleosols (from Retallack 2009b).
11
longer period variation. Anomalous peaks can thus be defined as those
more than two standard deviations above than a running mean. These
spikes in calcareous nodule depth in paleosols are coeval with spikes
from proxy CO2 records, and these can be used to derive power laws
predicting climatic change in the past for different levels of
atmospheric CO2. The paleoclimatic parameters of mean annual
temperature and mean annual precipitation come from geochemical
climofunctions of modern soils applied to the paleosols (Sheldon et al.
2002) and from depth of calcic horizons of modern soils applied after
compaction correction to the paleosols (Retallack 2005). These power
laws (Fig. 6A–C) thus quantify predicted warming and precipitation
increases with rising anthropogenic CO2. These power laws are
empirical and based on past Earth system performance, not models.
Models are needed to estimate future CO2 emissions, and using a
scenario of business as usual without strongly centralized intervention
(model A2 of Solomon et al. 2007), the power laws predict a rise in
temperature by the year 2100 of þ1.28 C. This is low but close to global
model predictions (Solomon et al. 2007; þ2.48 C, with range from 1.4–
FIG. 6.—Empirical power laws relating past climate and atmospheric
CO2. A–C) Atmospheric CO2 for 52 adequately sampled intervals
from stomatal index of Ginkgo and related taxa as independent
variable. Open symbols are for a largely ice-free world (Late
Permian to Eocene), filled symbols are for world with a large
Antarctic ice cap (Oligocene to Pliocene and Early to Middle
Permian): A) mean annual temperature (MAT) calculated from
chemical alkali index of paleosols; B, C) mean annual precipitation
(MAP) calculated from compaction-corrected depth to calcic
horizon in paleosols (B) and from potash-free chemical index of
alteration (C); D–F) predicted climate change by the year 2100
using power laws of A–C, and concentrations of CO2 (gray curves)
from ISAM model of emission scenario A2. Error envelopes in D–F
include other emission scenarios (from Retallack 2009b).
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12
GREGORY J. RETALLACK
FIG. 7.—Reconstructed soil profile at the Viking 1 site on Chryse
Planitia, Mars (from Retallack 2001).
3.88 C). Local temperature sensitivity (mean annual temperature
change with CO2 doubling) using these empirical power laws also is
low (0.88 C) compared with global sensitivity estimates (1.5–6.28 C)
from theoretical models of past and present climate (Royer et al. 2006).
Precipitation sensitivity (mean annual precipitation increase for CO2
doubling) is 89 mm for calcic horizon estimates and 128 mm for
geochemical estimates. Paleosols have much to offer regarding changes
in climate, vegetation, and agriculture during coming global change.
ASTROPEDOLOGY
Another promising future direction is astropedology, from the
ancient Greek arsgq (star), and here defined as a branch of soil
science concerned with soils of the distant geological past and of other
planetary bodies. In these respects, its aims and scope are similar to
astrobiology, in short, to understand our place in the universe
(Morrison 2004). For those adhering to the view that life defines soil,
and thus pedology (Hole 1981), astropedology is at best an oxymoron,
and such planetary surface materials are better termed regolith (Greeley
1994). Astropedology follows a more general definition of soils
formed by alteration in place by physical, chemical, or biological
processes, and so does not beg the question of life on other planetary
bodies (Retallack 2001). Soils of the Moon have already supported life
in the form of astronauts, and it seems unlikely that all forms of life
have been sterilized from Martian robotic landers. Paleosols offer
useful information on past and future behavior of Earth’s soil and
atmosphere (Fig. 6), and could be similarly useful for understanding
and exploiting other planetary bodies.
The clayey and salty soils of Mars are similar to those of Earth,
particularly those of the Dry Valleys of Antarctica (Campbell and
Claridge 1987). Their parent material is mafic lavas of basalt and
komatiite, which have been weathered in acidic aqueous solution to
smectite clay, and salts such as gypsum (Amundson et al. 2008, Bishop
et al. 2008). Calcareous soils may be present in the Nili Fossae region,
but they are only known by remote sensing (Ehlmann et al. 2008).
Gypsic soils examined robotically in Chryse Planitia are thin, and
retrorockets on the Viking landers blew away soil to reveal the salt crust
(gypsic horizon) beneath the gray surface and thin red oxidation rind
(Fig. 7). Such field relationships are evidence against hydrolysis only
during burial diagenesis, as proposed by McLennan et al. (2005). The
thin, dark red, surface rind may be due to photo-oxidation in solar wind
(‘‘space weathering’’ of Hiroi et al. 2005). Chemical analysis of Viking
1 soil (McSween and Keil 2000) shows clear weathering trends of base
depletion and clay formation. Soils revealed by the Viking landers are
in a terrane dated by crater counting at 3.2 þ0.6/2 Ga (Hartmann et al.
FIG. 8.—An interpretation of the most prominent paleosols in the Apollo 15 deep core at Palus Putredinus near Hadley Rille on the Moon. Soil and
paleosol maturity indices include mean grain size, percent agglutinates, and relative ferromagnetic resonance. Evidence for impact comes from
large non-mare to mare rock fragment ratios (from Retallack 2001).
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A SHORT HISTORY AND LONG FUTURE FOR PALEOPEDOLOGY
13
away from the sun (Wasson 1985). By the soil-formation hypothesis,
however, the carbonaceous and volatile-rich chondrites could represent
the upper horizons of soils, whereas chondrule-rich and metamorphic
chondrites represent lower horizons. Such paleosol interpretations and
their relevance for planetary volatile inventories and the origin of life
remain little explored (Retallack 2001).
CONCLUSIONS
FIG. 9.—Petrographic thin section of Allende carbonaceous chondrite,
viewed under crossed nicols, from thin section 818 at the Center for
Meteorite Studies at Arizona State University. The fine-grained
matrix to the left has birefringence fabric (insepic plasmic fabric),
and the large barred chondrules of olivine show thick opaque
weathering rinds (sesquans)(from Retallack 1997).
1981), and there is some doubt whether even the modest amount of
clay formation observed could have been accomplished on frigid Mars
today (Chevrier et al. 2007). Outwash channels are evidence for water
flow at the surface before 2 Ga, and clayey surface soils could be
paleosols remaining from that early time of hydrous soil formation
(Tosca and Knoll 2009). Recent missions to Mars have paid little
attention to soil-forming processes (Squyres et al. 2004), but this will
change if the planet is terraformed for human habitation (Fogg 1998).
Other planetary bodies, such as our Moon, stretch the very definition
of soil formation. Without effective water or atmosphere, the principal
soil-forming process on the Moon is a continuing rain of micrometeoroids, which create silt-to-sand-sized grains of impact glass,
meteoritic metal, and grains cemented by impact glass (agglutinates;
Lindsay 1976). These modify coarser impact debris of larger impacts,
as can clearly be seen in the Apollo 15 deep core (Fig. 8). This core
demonstrated at least seven horizons of agglutinate enrichment, which
also had a high ferromagnetic resonance from added meteoritic metal
(Heiken et al. 1973, 1976). These dark ferromagnetic bands are
paleosols, formed between the larger impacts, which introduced rock
fragments from the shattered highlands (non-mare regions) to the
basaltic plains (mare regions). The underlying mare basalt has been
dated at 3.3 Ga (Taylor 1982), so that each of these lunar paleosols was
hundreds of millions of years in the making. The thinness of lunar soils
and paleosols will be an important constraint on their evaluation and
exploitation for raw materials at lunar bases (Horton et al. 2003).
Carbonaceous chondrites also have been interpreted as pieces of
former soils. With ages of 4.6 Ga (Baker et al. 2005), these meteorities
represent the oldest paleosols of planetismals formed early in the
evolution of the solar system (Bunch and Chang 1980). Their spectral
similarity with the surfaces of asteroids (Hiroi et al. 2005) suggests that
some of these paleosols may have persisted after the volatile degassing
of their parent bodies had ceased. Evidence of hydrolytic alteration is
abundantly visible in thin section of carbonaceous chondrites, and
includes fine-grained smectite and organic compounds with birefringence fabrics, and thick opaque weathering rinds around hightemperature grains (chondrules; Fig. 9). An alternative explanation
of these features is creation in cometary orbits of bodies that accreted
cold in different parts of the protoplanetary disk, and by this view, the
chondrites with higher volatile and clay contents condensed further
Paleosols may have been appreciated in classical antiquity, but they
were first recognized in scientific treatises of Marsigli’s (1726) tomes
on the Danube River terraces of Hungary. Paleopedology as a
subdivision of pedology became formalized as a discipline with Boris
Polynov’s (1927) article outlining its aims and scope, but it was slow to
grow. Paleopedology remained a small field, mired in controversy
during the mid-twentieth century until the appearance of international
meetings (Yaalon 1971, Follmer 1998, Retallack and McDowell 1988)
and organizing texts (Birkeland 1974; Retallack 1990, 1997, 2001).
Growth in paleopedology came from other disciplines for which
information from fossil soils was useful: sedimentology (Barrell 1913),
Precambrian geology (Collins 1925), paleobotany (Retallack 1977a,
1977b), and vertebrate paleontology (Bown and Kraus 1981). Now a
first generation of scientists trained specifically in paleopedology is
finding jobs in academia and government laboratories. For the future,
paleosols will continue to inform allied disciplines, especially
pedology, to which it owes many fundamental concepts and an arcane
terminology (Retallack 1997). There is also the prospect of paleosols
on other planets such as Mars (Retallack 2001) and a role for studying
soils of the past to predict their behavior in the face of future global
climate change (Retallack 2009b).
ACKNOWLEDGMENTS
Steven Driese deserves special thanks for spearheading the
paleopedology meeting at Petrified Forest National Park, Arizona,
and for inviting this history and prospectus. Dan Yaalon, Douglas
Helms, Phil Hunt, and Bernard Horrocks provided useful historical
photographs. For helpful discussions, thanks are due to Fio Ugolini,
John Tandarich, Don Johnson, and Leon Follmer.
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