Odekon M. Encyclopedia of paleoclimatology and ancient environments

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Figure C58 Vertical cross sections showing typical stratigraphic expressions of coastal deposits. ( a) Prograding, wave-dominated shoreline, formed

during a time of “greenhouse” climate. Note the episodic vertical offset of sandy beach-shoreface deposits, caused by small-scale sea-level

changes. Iles Formation (Cretaceous), northwestern Colorado, United States (after Crabaugh, 2001, figure 2.8). (b) Coast-parallel section, Delaware,

United States. The valleys cut during the last glacial maximum have been filled by a transgressive succession of fluvial, estuarine /lagoonal, and

barrier-island deposits. All deposits above the –6 m level will be removed by landward migration of the shoreface, which will generate a

ravinement surface (after Kraft et al., 1987, figure 3). (c) Coast-normal section perpendicular to b, showing the stacking and amalgamation of

coastal deposits of five sea-level highstands during the Pleistocene. Each lowstand generated a through-going unconformity termed a sequence

boundary. Each sequence (numbered oldest to youngest) contains a transgressive stacking of back-barrier mud and barrier-island sand. Note the

erosional truncation of barrier sands in more down-dip areas. Most of this erosion is caused by transgressive shoreface ravinement. The greater

degree of stratigraphic complexity relative to a is a result of the high-amplitude sea-level changes that characterized the “icehouse” conditions of

the Pleistocene (after Demarest and Kraft, 1987, figure 4).

COASTAL ENVIRONMENTS 185

which eolian dunes may be important. The deposits generated by

coastal progradation consist of a succession that coarsens upward

from offshore mud into shoreface sand (Figure C58a). Such suc-

cessions are abundant in the stratigraphic record and may be

stacked on each other, reflecting repeated transgression and

regression of the paleo-shoreline. Each upward-coarsening suc-

cession, termed a parasequence (Van Wagoner et al., 1988), is

separated from the next one by a flooding surface, across which

offshore shale rests abruptly on more nearshore deposits.

Transgressive, wave-dominated conditions are very com-

mon along modern coasts. Where the hinterland is low lying,

as would be the case in most ancient sedimentary basins, the

coastline is fronted by barrier islands that are separated by tidal

inlets (Figure C57d). Drowned river-valley estuaries and

lagoons lie behind these sandy islands. In such back-barrier

areas, tidal action is commonly the dominant physical process.

Tidal channels radiate from the tidal inlets, cutting through the

flood-tidal deltas that lie immediately landward of inlets. Tidal

flats rim the lagoons and estuaries; muddy deposits accumulate

in the deeper-water areas. Bayhead deltas, which typically

exhibit a river-dominated morphology, may be developed at

river mouths. Such wave-dominated estuaries display a charac-

teristic tripartite facies zonation, from sandy sediments at the

barrier, through muddy lagoonal deposits, to more sandy sedi-

ments in the bayhead area, and are a common component of

valley-fill successions (Figure C58b). The more seaward por-

tions of the estuary, including the sandy barrier and related

environments, are rarely preserved, however, because they are

eroded by the landward passage of the shoreface, which gener-

ates a ravinement surface (Figure C58b).

Tide-dominated coastal environments most commonly occur

in macrotidal areas (i.e., tidal ranges >4 m). At locations where

there is a significant coast-normal tidal flux, a network of

tidal channels is generally developed (Figure C57b,c). The

migration of these channels produces an architecturally com-

plex succession of lateral-accretion deposits. These deposits

are commonly very heterolithic because of the high sus-

pended-sediment concentrations. Tide-dominated sedimenta-

tion is favored in transgressive settings, because the flooding

of an irregular topographic surface (i.e., a sequence boundary)

generates many embayments that are sheltered from wave

action and enhance the influence of tidal currents. However,

large tidal ranges and tidal dominance can occur at any point

in a relative sea-level cycle because tidal resonance is depen-

dent on instantaneous, regional-scale basin morphology.

Geologic setting

The geologic setting creates the general context for the coastal

zone. Coastal zones in tectonically active areas (e.g., coastlines

on subducting or strike-slip continental margins, or in young

rift basins) tend to be bordered by rugged hinterlands, espe-

cially if the location of the coastline is determined by an active

fault. As a result, they are typically fed by small rivers that sup-

ply relatively coarse-grained sediment. The extent of estuarine

environments is minimized because the steep fluvial gradient

limits the intrusion of salt water and tidal action. Coastlines

along passive continental margins or in foreland or intracra-

tonic basins generally have lower gradients and may be fed

by a smaller number of large rivers (Wright, 1985). The sedi-

ment reaching the river mouth is typically relatively fine-

grained. Slow subsidence in tectonically stable areas causes

transgressive coastal erosion to have a more profound influence

than in areas of high subsidence rates. Thus, coastal deposits

will tend to be truncated to a greater degree, leading to a more

fragmentary record of sea-level variations than in more rapidly

subsiding areas (Figure C58c).

Influence of climate

Climate also influences the general context in which coastal envir-

onments exist. For example, the frequency and intensity of storms,

which determine the intensity of wave action, is a function of

the climatic belt. Mid-latitude coasts are subject to frequent

storms, which favors the occurrence of high-energy, wave-domi-

nated shorelines. By comparison, areas approximately 30



north

and south of the equator tend to have low average wind speeds,

with infrequent but intense hurricanes.

Humid climates have significant river flow, which delivers

large amounts of sediment and fresh water to river mouths. This

favors the development of deltas and prograding coastal environ-

ments, as well as the widespread occurrence of brackish-water

conditions. Coastal vegetation is generally abundant and vege-

tated salt marshes are widespread in the upper intertidal zone

(Luternauer et al., 1995). In tropical to sub-tropical settings,

mangroves grow into the intertidal zone. Arid climates with high

evaporation rates promote the development of hypersaline condi-

tions in areas with limited tidal circulation (e.g., coastal lagoons

and estuaries). Terrestrial vegetation is sparse and evaporite

minerals such as gypsum and halite can grow in the surficial

sediments of the intertidal zone. Caliche (also called calcrete)

is commonly present in paleosols formed in areas with a semi-

arid climate.

Organically produced carbonate sediment is abundant in areas

with limited freshwater input and a consequent scarcity of terrige-

nous-clastic sediment. The nature of these carbonate sediments is

strongly controlled by water temperature, as well as the availabil-

ity of nutrients (James, 1997). In tropical and subtropical areas

where water temperatures are higher than ca. 20



C, the fauna

today is dominated by corals and calcareous algae that produce

reefs. These reefs, together with the associated shallow carbonate

platform, damp wave action and allow tidal flats to develop along

many shorelines (Figure C57c). The repeated progradation and

transgression of these flats produces stacked successions of

upward-shallowing packages, typically 1–3 m thick, separated by

flooding surfaces (Pratt et al., 1992). In temperate and polar areas,

by contrast, reef-producing organisms do not thrive. Sediment-

production rates are low and gently sloping offshore ramps occur

instead of flat-topped platforms. Because of this, wave energy

can reach the shoreline more readily, causing beaches to be more

widespread than in tropical areas.

Polar areas experience seasonal or permanent growth of sea

ice that limits the impact of wave energy. As a result, coastlines

tend to be of lower energy and muddier than might be the case

otherwise. Soft-sediment deformation caused by the grounding

of ice pushed onshore by winds may be significant (Dionne,

1985). Permafrost is also an important constraint on coastal

morphology.

Climate change at Milankovitch frequencies has had a pro-

found influence on modern coastal areas because of the fre-

quent, high-amplitude, sea-level cycles that have occurred

throughout the “icehouse” conditions of the Quaternary. Each

of the last few interglacial highstands has reached approxi-

mately the same elevation (Chappell et al., 1996), leading

to the amalgamation and superposition of coastal deposits of

significantly different ages but similar facies (Figure C58c).

Significant time gaps (i.e., sequence boundaries) occur between

these packages because of non-deposition and/or erosion

186 COASTAL ENVIRONMENTS

during the sea-level lowstands. For example, along much of

the east and southern coasts of the United States, Holocene

barrier-lagoon complexes lie only a short distance seaward of

similar deposits of late Pleistocene age (Figure C57d; Demarest

and Kraft, 1987). This leads to enormous stratigraphic com-

plexity, especially in areas of low sediment supply. Sea-level

oscillations with a similar period but much reduced amplitude

probably occurred during “greenhouse” times such as the

Cretaceous. Coastal deposits formed under these conditions

may not show as much complexity because a smaller number

of significant unconformities will be present (Figure C58a).

Summary

The coastal zone is arguably the most complex environment on

earth because of the many independent factors that influence its

characteristics. Pronounced spatial gradients in the intensity of

the main physical processes (river currents, waves, and tidal

currents) create complex assemblages of facies, which are then

stacked in complex stratigraphic successions because of the

migration of the shoreline in response to sea-level change and

sediment addition or removal. The geologic setting and climate

have less direct, but nevertheless important, influences on the

nature of coastal environments. As a result of the interplay of

these many factors, coastal deposits are a rich source of infor-

mation about past environmental conditions.

Robert W. Dalrymple and Kyungsik Choi

Bibliography

Bird, E., 2000. Coastal Geomorphology: An Introduction. New York:

Wiley.

Boyd, R., Dalrymple, R.W., and Zaitlin, B.A., 1992. Classification of

coastal sedimentary environments. Sediment. Geol., 80, 139–150.

Chappell, J., Omura, A., Esat, T., McCulloch, M., Pandolfi, J., Ota, Y.,

Pillans, B., 1996. Reconciliation of late Quaternary sea levels derived

from coral terraces at Huon Peninsula with deep sea oxygen isotope

records. Earth Planet. Sci. Lett., 141, 227–236.

Crabaugh, J.P., 2001. Nature and Growth of Nonmarine-to-Marine Clastic

Wedges: Examples from the Upper Cretaceous Iles Formation, Western

Interior Basin (Colorado) and the Lower Paleogene Wilcox Group of

the Gulf of Mexico Basin (Texas). Ph.D. Thesis, Laramie, Wyoming:

University of Wyoming.

Dalrymple, R.W., Zaitlin, B.A., and Boyd, R., 1992. Estuarine facies

models: Conceptual basis and stratigraphic implications. J. Sediment.

Petrol., 62, 1130–1146.

Demarest, J.M., II, and Kraft, J.C., 1987. Stratigraphic record of Quatern-

ary sea levels: Implications for more ancient strata. In Nummedal, D.,

Pilkey, O.H., and Howard, J.D. (eds.), Sea-Level Fluctuation and

Coastal Evolution. Tulsa, OK: SEPM, Society of Economic Paleontol-

ogists and Mineralogists, Special Publication 41, pp. 223–239.

Dionne, J.-C., 1985. Formes, figures et faciès sédimentaires glaciels des

estrans vaseux des régions froides. Palaeogeogr, Palaeoclimatol,

Palaeoecol., 51, 415–451.

Hayes, M.O., 1975. Morphology of sand accumulations in estuaries: An

introduction to the symposium. In Cronin, L.E. (ed.), Estuarine

Research, vol. II. New York: Academic Press, pp. 3–22.

James, N.P., 1997. The cool-water carbonate realm. In James, N.P.,

and Clarke, A.D. (eds.), Cool-Water Carbonates. Tulsa, OK: SEPM,

SEPM (Society for Sedimentary Geology), Special Publication 56,

pp. 1–20.

Kraft, J.C., Chrzastowski, M.J., Belknap, D.F., Toscano, M.A., and

Fletcher, C.H., III, 1987. The transgressive barrier-lagoon coast of

Delaware: Morphostratigraphy, sedimentary sequences and responses

to relative rise in sea level. In Nummedal, D., Pilkey, O.H., and

Howard, J.D. (eds.), Sea-level Fluctuation and Coastal Evolution.

Tulsa, OK: SEPM, Society of Economic Paleontologists and Minera-

logists, Special Publication 41, pp. 129–143.

Luternauer, J.L., Atkins, R.J., Moody, A.I., Williams, H.F.L., and Gibson,

J.W., 1995. Salt marshes. In Perillo, G.M.E. (ed.), Geomorphology

and Sedimentology of Estuaries. Developments in Sedimentology,

vol. 53, Amsterdam: Elsevier. pp. 307–332.

Posamentier, H.W., Allen, G.P., James, D.P., and Tesson, M., 1992. Forced

regressions in a sequence stratigraphic framework: concepts, examples,

and exploration significance. Am. Assoc. Pet. Geol. Bull., 76,

1687–1709.

Pratt, B.R., James, N.P., and Cowan, C.A., 1992. Peritidal carbonates.

In Walker, R.G., and James, N.P. (eds.), Facies Models—Response to

Sea-level Change. St. John’s: Geological Society of Canada,

pp. 303–322.

Pritchard, D.W., 1967. What is an estuary? Physical viewpoint. In Lauff, G.H.

(ed.), Estuaries. Washington, DC: American Association for

the Advancement of Science. AAAS Publication 83, pp. 3–5.

Reading, H.G., and Collinson, J.D., 1996. Clastic coasts. In Reading, H.G.

(ed.), Sedimentary Environments: Processes, Facies and Stratigraphy.

Oxford: Blackwell Science, pp. 154–231.

Van Wagoner, J.C., Posamentier, H.W., Mitchum, H.W., Vail, P.R., Sarg, J.F.,

Loutit, T.S., and Hardenbol, J., 1988. An overview of sequence stra-

tigraphy and key definitions. In Wilgus, C.K., Hastings, B.S., Kendall,

C.G. St. C., Posamentier, H., Ross, C.A., and Van Wagoner, J. (eds.),

Sea-level Changes – an Integrated Approach. Tulsa, OK: SEPM,

Society of Economic Paleontologists and Mineralogists, Special Publi-

cation 42, pp. 39–45.

Woodroffe, C.D., 2002. Coasts: Form, Process and Evolution. Cambridge,

UK: Cambridge University Press.

Wright, L.D., 1985. River deltas. In Davis, R.A., Jr. (ed.), Coastal Sedi-

mentary Environments, 2nd edn. New York: Springer, pp. 1–76.

Cross-references

Astronomical Theory of Climate Change

Carbonates, Cool Water

Carbonates, Warm Water

Cretaceous Warm Climates

Deltaic Sediments, Climate Records

Glacial Eustasy

Greenhouse (warm) Climates

Icehouse (cold) Climates

Sea Level Change, Last 250 Million Years

Sea Level Change, Post-Glacial

Sea Level Change, Quaternary

Sea Level Indicators

Sedimentary Indicators of Climate Change

COCCOLITHS

Coccoliths are an important group of microfossils much used in

paleoceanographic studies. They are minute (typically 1–10 µ)

calcite platelets that are produced by unicellular planktonic

algae, coccolithophores. In life, the individual coccolithophore

cell is typically almost entirely covered by coccoliths, forming

an exoskeleton or coccosphere. In the fossil record, coccoliths

are often accompanied by nannoliths, i.e., similar sized calcite

fossils of uncertain affinities (e.g., discoasters, nannoconids).

Geologists study coccoliths and nannoliths together and refer

to them as calcareous nannofossils.

Coccolithophores belong to the algal division (phylum)

Haptophyta, which also includes many non-calcifying taxa

(e.g., Phaeocystis, Prymnesium and Pavlova). Haptophytes

are distinguished from other algae by thei r possession of

two smooth flagella and a unique third flagellum-like organelle,

the haptonema. Molecular genetic research has confirmed that

the Haptophyta are a discrete clade that probably diverged

from other protists in the Early Paleozoic and subsequently

COCCOLITHS 187

acquired chloroplasts by endosymbiosis, possibly in the Early

Mesozoic. Non-calcifying haptophytes, especially Phaeocystis,

are important in shelf environments but coccolithophores are

predominant in oceanic environments and the only group with

a fossil record. The coccolithophore Emiliania huxleyi pro-

duces spectacular blooms with very high abundances of loose

liths, which are readily identified by satellite (Figure C59).

Haptophytes typically have a life-cycle with discrete hap-

loid and diploid phases, both of which are capable of indefinite

asexual reproduction (Billard, 1994). In coccolithophores, the

two phases produce different coccolith types. The haploid

phase is usually motile and if it calcifies, it produces coccoliths

formed of numerous euhedral crystallites; these “holococco-

liths” are very small and rarely preserved. The diploid

phase is often non-motile (i.e., flagella and haptonema are not

developed) and produces more robust coccoliths formed of

a few radial cycles of elaborate-shaped interlocking crystal

units –“heterococcoliths” (Young et al., 1999). Both life-cycle

stages are fully planktonic and there is no cyst-stage in the life

cycle. The alternation between two life-cycle strategies may, how-

ever, aid adaptation to the open-marine environment – the haploid

holococcolith-bearing phase typically occurs in shallower and

more oligotrophic environments than the diploid heterococcolith-

bearing phase. The haploid phase may also be mixotrophic. In

any case, it is clear that coccolithophores are particularly well-

adapted to oligotrophic open-ocean conditions and they often

dominate the phytoplankton in such areas. In more eutrophic

areas, coccolithophore numbers may increase but they become

subordinate to the much more abundant diatoms. Similarly, cocco-

lithophore blooms usually occur later in the seasonal succession

than diatom blooms (Winter and Siesser , 1994).

In the fossil record, coccoliths are very important as open

ocean sediment formers. They are usually the dominant cal-

careous component of pelagic deposits, including both Late

Cretaceous chalks and modern deep sea oozes. They have pro-

ven invaluable as biostratigraphic marker fossils in such sedi-

ments and are the prime means of dating sediments recovered

by the DSDP and ODP projects (Bown, 1998). Despite the

minute size of coccoliths, such studies are usually carried out

by light microscope, since the high birefringence of calcite

means they are readily identified in cross-polarized light.

Electron microscopy is only used for special studies. Numerous

studies of nannofloral change have elucidated responses of

coccoliths to oceanographic change and they can provide

key constraints to hypotheses of change in the phytoplankton

based on in direct geoch emical evidence – e.g., an infer-

ence of productivity change based on carbon isotopes can be

tested by determining if concomitant change occurs in the

phytoplankton. However, reliable quantitative nannofloral pro-

xies and transfer functions are still at a relatively early state of

development. The most important coccolithophore-derived

geochemical proxy is the uk37 alkenone paleothermometer.

There has also been much recent study of stable isotope and

Sr/Ca records from coccoliths.

Jeremy R. Young

Bibliography

Billard, C., 1994. Life cycles. In Green, J.C., and Leadbeater, B.S.C. (eds.),

The Haptophyta Algae. Oxford: Clarendon Press, 51, pp. 167–186.

Bown, P.R., 1998. Calcareous Nannofossil Biostratigraphy. British Micro-

palaeontological Society Publication Series, London: Chapman & Hall,

pp. 315.

Young, J.R., Davis, S.A., Bown, P.R., and Mann, S., 1999. Coccolith

ultrastructure and biomineralisation. J. Struct. Biol., 126, 195–215.

Winter, A., and Siesser, W.G. (eds.), 1994. Coccolithophores. Cambridge,

UK: Cambridge University Press, pp. 242.

Cross-references

Alkenones

Dating, Biostratigraphic Methods

Deep Sea Drilling Project (DSDP)

Diatoms

Marine Biogenic Sediments

Ocean Drilling Program (ODP)

COHMAP

Introduction

Climate change is global in scope, multivariate in character,

and varies both temporally and spatially. Single investigator

studies of sediment cores can provide individual data points

for studying past climate, but as CLIMAP (Climate: Mapping

and Prediction) demonstrated, it takes internationally and

inter-institutionally organized groups of interdisciplinary inves-

tigators to compile the global data sets and models needed

to construct a global view. Patterned after CLIMAP (1981),

COHMAP (Cooperative Holocene Mapping Project) featured

researchers who assembled continental-to-global data sets pri-

marily of radiocarbon-dated pollen and lake-level data and ana-

lyzed these data to produce time series of maps of estimated

climate variables that show the changing synoptic patterns of

climate change during the late Quaternary. During its 18 years

of existence from 1977 to 1995, COHMAP evolved from four

principal investigators focusing on mapping late Quaternary

climates in eastern North America to over 30 cooperating

scientists who together were mapping and modeling the tem-

poral changes in spatial patterns of climate from all continents

but Antarctica, and from the Atlantic, Pacific, and Indian

Oceans (Wright et al., 1993). Starting with the Last Glacial

Maximum (LGM) at 21,000 calendar years ago (or 18,000

radiocarbon y

BP), COHMAP members twice made a sequence

of model runs that showed the simulated impact of the changing

boundary conditions on late Quaternary climates (Kutzbach et al.,

1993, 1998) and then thoroughly checked the model results

against the data (COHMAP, 1988; Wright et al., 1993; Webb,

Figure C59 Left: Coccosphere of Emiliania huxleyi heterococcoliths.

Right: Several Helicosphaera carteri holococcoliths.

188 COHMAP

1998). A key finding was the direct impact of orbitally forced

changes in seasonal insolation on monsoon climates in Africa

and southern Asia (Kutzbach, 1981; Kutzbach and Street-Perrott,

1985), which demonstrated how the climate forcing postulated

by Adhemar, Croll, and Milankovitch (Imbrie and Imbrie, 1979)

affected tropical climates without ice sheets being involved. By

formulatinga mechanism involving the enhancement and weaken-

ing of monsoons and then testing its plausibility with models

and data, COHMAP investigators discovered one way for orbital

forcing to affect global climates throughout Earth history. Geolo-

gical evidence showing periodic frequencies on orbital time scales

is abundant from the Permian, Triassic, Cretaceous, and other geo-

logical periods (DeBoer, 2004).

History, investigators, and meetings

COHMAP was a group of active researchers interacting and col-

laborating in productive ways (Wright and Bartlein, 1993). The

group began as the brainchild of H. E. Wright and E. J. Cushing,

established Quaternary geologists and paleoecologists, and

J. E. Kutzbach and T. Webb III, meteorologically trained paleo-

climatologists, who were both students of R. A. Bryson from

whom they gained training in climate history and climate

dynamics. In initial discussions at the 1974 AMQUA (American

Quaternary Association) meetings in Madison, WI, these

researchers identified a major opportunity to compile radiocar-

bon-dated pollen data into continental data sets for mapping

and climatic reconstructions (Webb and Bryson, 1972). By

1977, when they were first funded by the National Science Foun-

dation (NSF), the project had the potential to check the results of

general circulation models (Peterson et al., 1979). Within a year,

they began collaborating with F. A. Street-Perrott, who was

developing a global lake level database and had good dated cov-

erage in Africa and other tropical areas where pollen data were

sparse (Street and Grove, 1979). Street-Perrott soon became a

member of the executive committee along with paleoceanogra-

phers W. Ruddiman and W. Prell, who provided a CLIMAP per-

spective and SST estimates from the Atlantic, Pacific, and Indian

Oceans, where sedimentation rates were high enough to allow

resolution within the Holocene. In the early 1980s, P. J. Bartlein

added his climatic, statistical, and cartographic expertise to the

leadership group. By then, Kutzbach (1980, 1981) had devel-

oped methods for estimating precipitation from lakes levels and

had begun using general circulation models (GCMs) to simulate

the impact of orbital forcing of Holocene climates. The modeling

studies evolved into a time series of runs in 3,000 year intervals

from 18,000 radiocarbon years ago to present (Kutzbach et al.,

1993). With a goal of global mapping of pollen and lake level

data for data/model comparisons, the COHMAP executive com-

mittee recruited regional experts for acquisition, quality control,

and interpretation of the pollen and lake-level data (COHMAP,

1988; Wright et al., 1993). A policy of subsidizing the radiocar-

bon dating of pollen data from critical areas encouraged many

North American palynologists to contribute their data to the

North American database.

Key to research planning and coordination was regular meet-

ings involving either just the executive committee members or

all cooperating scientists. At these meetings, investigators shared

their expertise and educated each other across their diverse disci-

plines. They gained a common understanding of the global con-

trols on the climate system and its spatio-temporal nature of

variation. With a sophisticated view of the climate system, they

used climate-modeling results to simulate possible changes in

circulation patterns and teleconnections that helped explain

why the timing and magnitude of changes in the data differed

among regions. They also used their continental-to-global data

sets to test the accuracy of the model simulations. The meetings

allowed both the modelers and the data compilers to learn from

each other and to formulate productive plans for mutual

research. Root and Schneider (1995) cited COHMAP as a model

for “cooperative SCS (strategic cycle scaling)-like research”

because of its integration of modeling and data analysis and its

“interdisciplinary multi-institutional research” and education.

Key methods, findings and new concepts

In the beginning, COHMAP researchers aimed to compile and

add to sets of modern and fossil pollen data for climate calibra-

tion and mapping in eastern North America. The goal was to

map geographic patterns of vegetation and climate variables that

show how and perhaps even why past climates had changed.

Fieldwork was central to the project in order to fill gaps in the

data networks, especially in critical regions. The Radiocarbon

Laboratory directed by Margaret Bender at the Center for Cli-

matic Research in Madison, Wisconsin, provided many dates to

improve chronologies within sites with fossil pollen data. Two

sets of pollen data were compiled. One was a calibration data

set of over 900 sites with modern pollen data collected from sur-

face sediments. When combined with the modern climate data

for each site, these data were used to calculate transfer functions

and later response surfaces that provided estimates of climate

variables from the fossil pollen data (Bartlein et al., 1984,

1986). The fossil pollen data were in the second data set, which

included site information, the pollen counts, and depth-date files

for constructing chronologies. These data sets are now archived

Figure C60 Changes in key climate controls during the last 21,000

years including (1) percentage coverage by terrestrial ice sheets,

(2) concentration of atmospheric carbon dioxide, and (3) orbitally

controlled insolation at the top of the atmosphere for the northern

hemisphere for summer (June, July, and August) and winter (December,

January, and February) (modified from Kutzbach et al., 1998).

COHMAP 189

Figure C61 Continued.

190 COHMAP

and in active use at the NOAA World Data Base (http://www.

ngdc.noaa.gov/paleo/napd.html).

With the addition of climate modeling, lake level data, and

pollen data from other regions, the COHMAP effort expanded

to a global program with a focus on comparing maps and time

series of the model results with the data (Wright et al., 1993).

CLIMAP’s modeling of the LGM at 18,000 radiocarbon years

ago or 21,000 calendar years ago was expanded by Kutzbach et al.

(1993, 1998), who ran a global climate model to equilibrium with

the boundary conditions at each 3,000-radiocarbon- or 5,000-

calendar-year-interval between the LGM and present day

(Figure C60). The climate model allowed COHMAP researchers

to show the regional climatic impact of the broad-scale controls,

such as ice sheet size, seasonal insolation, and greenhouse gas

concentrations. The modeling results aided development of con-

ceptual models for late Quaternary climate changes by illustrating

mechanisms and circulation changes, and provided results that

could be compared and thus tested by the data (Figure C61).

Because the successful testing of the model results was key to

their use in helping to explain late Quaternary climate changes,

COHMAP researchers developed several methods for com-

paring the data to the model results. For each method, regional

to continental-wide data sets were essential because of the

500-by-700-km gridding for the model results, which meant that

only the broadest scale patterns could be compared.

In one approach, calibration data sets were used to calculate

multiple regression equations and response surfaces that turned

fossil pollen data into temperature and moisture estimates that,

when mapped, could be compared to comparable maps of the

model simulations. Hydrological models were also used to

obtain moisture-balance estimates from lake-level data. In a

second approach, response surfaces were used to turn model-

simulated temperature and moisture values into maps of pollen

or P-PE (precipitation minus potential evaporation) values that

then could be compared to the pollen and lake-level data. In a

final, more heuristic approach, cartoon-like maps illustrated the

main features of the data and model simulations and allowed

these visual representations to be compared (Figure C61). Such

visualizations combined information from many diverse sets of

data on the maps and brought focus to regions where the cli-

mate estimates from the data were robust.

COHMAP (1988) illustrated the first results of data-model

comparisons in the context of time-series model simulations,

as described in Wright et al. (1993). For the LGM over North

America, these show that, in the model simulations, the high

Laurentide Ice Sheet forced the winter jetstream southward

over California, where winter storms brought moisture to fill

the pluvial lakes (Figure C61). At the same time, the simulated

glacial anticyclone produced anomalous easterly winds that

created drier-than-present conditions in the Pacific Northwest

just south of the ice sheet. As the ice sheet retreated, the jet

and glacial anticyclone shifted in position, which altered the

pattern of moisture conditions across the west. In the east,

much colder-than-present temperatures at the LGM favored

spruce trees growing where oaks grow today. Then, the retreat-

ing ice sheet delayed the timing of peak warm temperatures

during the Holocene in northeastern North American com-

pared with the northwest and MacKenzie Delta areas. For the

monsoonal regions of Africa and southern Asia, the climate

model simulations showed an immediate (vs. delayed) response

to orbital forcing and a much enhanced monsoonal circulation

from 12,000 to 6,000 radiocarbon years ago when the contrast

in seasonal insolation was greatest. Raised lake levels and

steppe and savannah vegetation from western Africa into

southern Asia fit well with the pattern of changes there.

These initial modeling results used version 0 of the NCAR

Community Climate Model (CCM0), for which sea-surface tem-

peratures needed to be prescribed. COHMAP researchers then

updated the results using CCM1 with a mixed-layer ocean for

a second set of time series runs (Kutzbach et al., 1998). They

also used calendar-year calibrations for the radiocarbon dates

and an ice sheet reconstruction that estimated a lower height

for the Laurentide Ice Sheet at the LGM. The new simulations

showed no split jet over North America in winter for the LGM,

but showed most of the other main results (described above) to

be robust and in good agreement with the data (Webb, 1998).

A global biome model provided a new approach for data-model

comparisons, and several papers illustrate its use to good effect.

The simulated surface energy balance was also analyzed in detail

and helped COHMAP researchers to separate dynamic from

thermodynamic controls of regional climates (Bartlein et al.,

1998). The data and model results showed clearly that climate

changes vary spatially in magnitude as well as in timing and that

phenomena like the “hypsithermal” were not globally uniform,

either in magnitude or timing. With the patterns in moisture var-

iations and seasonality varying along with temperature, no sim-

ple description of late Quaternary climates is possible.

Conclusions

Besides the key synthesis work (COHMAP, 1988; Wright et al.,

1993; Webb, 1998), COHMAP results appear in over 200 journal

articles and in edited books and journal issues by Wright (1983),

Webb and Dubois (1985), Ruddiman and Wright (1987), and

Huntley and Webb ( 1988 ). COHMAP research showed the broad

impact of orbital forcing on late Quaternary climates and pro-

vided a background of forced changes onto which the many mil-

lennial-scale changes can be superimposed. Subsequent work

has led to the mapping and modeling of Younger Dryas age cli-

mates; a study of possible feedbacks that affect the magnitude,

pattern, and timing of late Quaternary climates; and an under-

standing of how climate patterns unlike any today have resulted

in vegetation assemblages unlike any today.

Thompson Webb III

Figure C61 Changes in the atmosphere, geosphere, and biosphere that accompanied the transition from glacial to interglacial conditions during

the past 18,000 years (18 ka), as illustrated by geological and paleoecological evidence (left panels) and features of the climate-model simulations

(right panels). The maps of the data show the extent of ice sheets and of year-round and winter-only sea ice. For 18 ka, sea level lowering resulted

in a broadened land area. The distributions of oak and spruce are inferred from pollen data, and the status of effective moisture (relative to

present) in western North America is inferred from lake level records. The present region where annual precipitation is less than 300 mm is shown

on the map for Present. The maps for the model simulations show the distribution of ice sheets and sea ice that served as boundary conditions for

the simulations. Thin black arrows show the surface winds and thick black arrows show the jet stream position. The broken line for 18 ka over Alaska

indicates a weakened jet (modified from color version in COHMAP, 1988).

COHMAP 191

Bibliography

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change in the northern Midwest: Pollen-derived estimates. Quaternary

Res., 22, 361–374.

Bartlein, P.J., Prentice, I.C., and Webb, T. III., 1986. Climatic response

surfaces based on pollen from some eastern North America taxa.

J. Biogeogr., 13,35–57.

Bartlein, P.J., Anderson, P.M., Anderson, K.H., Edwards, M.E., Thompson,

R.S., Webb, R.S., Webb, T. III., and Whitlock, C., 1998. Paleoclimate

simulations for North America for the past 21,000 years: Features of

the simulated climate and comparisons with paleoenvironmental data.

Quaternary Sci. Rev., 17, 549–585.

CLIMAP, 1981. Seasonal Reconstruction of the Earth’s Surface at the

Last Glacial Maximum. Boulder, CO: Geological Society of America.

Geological Society of America Map Chart Series, MC-36.

COHMAP Members, 1988. Climatic changes of the last 18,000 years:

Observations and model simulations. Science, 241, 1043–1052.

DeBoer, P.L. (ed.), 2004. Orbital Forcing and Cyclic Sequences. Oxford,

UK: Blackwell Science, 576pp.

Huntley, B., and Webb, T. III. (eds.), 1988. Vegetation History. Hand-

book of Vegetation Science, vol. 7. Dordrecht, The Netherlands:

Kluwer, 803pp.

Imbrie, J., and Imbrie, K., 1979. Ice Ages Solving the Mystery. Hillside, NJ:

Enslow Publishers.

Kutzbach, J.E., 1980. Estimates of past climate at paleolake Chad, North

Africa, based on a hydrological and energy budget model. Quaternary

Res., 14, 210–223.

Kutzbach, J.E., 1981. Monsoon climate of the early Holocene: Climatic

experiment using the earth’s orbital parameters for 9000 years ago.

Science, 214,59–61.

Kutzbach, J.E., and Street-Perrott, F.A., 1985. Milankovitch forcing of

fluctuations in the level of tropical lakes from 18 to 0 kyr

BP. Nature,

317, 130–134.

Kutzbach, J.E., Guetter, P.J., Behling, P., and Selin, R., 1993. Simulated

climatic changes: Results of the COHMAP climate-model experiments.

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Street-Perrott, F.A., and Bartlein, P.J. (eds.), Global Climates Since

the Last Glacial Maximum. Minneapolis, MN: University of Minnesota

Press, pp. 24–93.

Kutzbach, J.E., Gallimore, R., Harrison, S.P., Behling, P., Selin, R., and

Laarif, F., 1998. Climate and biome simulations for the past 21,000

years. Quaternary Science Rev., 17, 473–506.

Peterson, G.M., Webb, T. III., Kutzbach, J.E., van der Hammen, T.,

Wijmstra, T.A. and Street, F.A., 1979. The continental record of

environmental conditions at 18,000

B.P.: An initial evaluation. Quater-

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Root, T.L., and Schneider, S.H., 1995. Ecology and climate: Research

strategies and implications. Science, 269, 334–341.

Ruddiman, W.F., and Wright, H.E., Jr. (eds.), 1987. Nort h America

and Adjacent Oceans during the Last Deglaciation, Decade

of North American Geology. Boulder, CO: Geological Society

of America.

Street, F.A., and Grove, A.T., 1979. Global maps of lake-level fluctuations

since 30,000 yr

B.P. Quaternary Res., 12,83–118.

Webb, T. III. (ed.), 1998. Late Quaternary Climates: Data Syntheses and

Model Experiments. Quaternary Sci. Rev., 17, 465–688.

Webb, T. III., and Bryson, R.A., 1972. Late- and post-glacial climatic

change in the northern Midwest, U.S.A.: Quantitative estimates derived

from fossil pollen spectra by multivariate statistical analysis. Quater-

nary Res., 2,70–115.

Webb, T. III., and Dubois, J.M. (eds.), 1985. Tendances climatiques

l’Holocene en Amerique du Nord. Gèographie physique et Quaternaire,

39, 113–226.

Wright, H.E., Jr. (ed.), 1983. Late-Quaternary Environments of the

United States, vol. 2: The Holocene, Minneapolis, MN: University of

Minnesota Press.

Wright, H.E., Jr., and Bartlein, P.J., 1993. Reflections on COHMAP.

The Holocene, 3,89–92.

Wright, H.E., Jr., Kutzbach, J.E., Webb, T. III., Ruddiman, W.F., Street-

Perrott, F.A., and Bartlein, P.J. (eds.), 1993. Global Climates Since

the Last Glacial Maximum. Minneapolis: University of Minnesota

Press, 569pp.

Cross-references

Astronomical Theory of Climate Change

Atmospheric Circulation During the Last Glacial Maximum

Carbon Dioxide and Methane, Quaternary Variations

CLIMAP

Climate Change, Causes

Dating, Radiometric Methods

Foraminifera

Heat Transports, Oceanic and Atmospheric

Holocene Climates

Hypsithermal

Ice Cores, Antarctica and Greenland

Interstadials

Lake Level Fluctuations

Last Glacial Maximum

Last Glacial Termination

Late Quaternary-Holocene Vegetation Modeling

Laurentide Ice Sheet

Monsoons, Quaternary

North Atlantic Deep Water and Climate Change

Obliquity

Ocean Paleotemperatures

Oxygen Isotopes

PAGES

Paleobotany

Paleoclimate Modeling, Quaternary

Paleoceanography

Paleolimnology

Paleo-Precipitation Indicators

Paleotemperatures, Proxy Reconstructions

Pollen Analysis

Precession, Climatic

Quaternary Vegetation Distribution

Radiocarbon Dating

Sea Level Change, Post-Glacial

SPECMAP

Transfer Functions

Wisconsinan (Weichselian, Würm) Glaciation

CONTINENTAL SEDIMENTS

Introduction

Continental sediments can be very sensitive paleoclimate indica-

tors because they are commonly deposited either in direct contact

with the atmosphere or in close proximity to it. Relative to marine

sediments, however, terrestrial sedimentary records of paleocli-

mate can be more difficult to interpret, due to incomplete preser-

vation, fluctuating rates of sedimentation that commonly include

significant periods of erosion, and post-depositional modifica-

tion by bioturbation or diagenesis. This chapter first focuses on

the broad association of various continental depositional systems

(glacial, lacustrine, fluvial, and eolian) with specific climatologic

settings. Following, several important paleoclimatologic indica-

tors common to continental sediments are discussed.

Continental depositional systems and paleoclimate

Glacial systems

The preservation of pre-Quaternary and Quaternary continental

glacial deposits indicates that the Earth’s climate has oscilla-

ted between an “icehouse” (i.e., glacial) and a “greenhouse”

(i.e., interglacial) mode. Observations of modern glacial systems,

192 CONTINENTAL SEDIMENTS

theoretical studies of glacial dynamics, and documentation of

Quaternary glacial sediments form the basis for development of

glacial system depositional models. However, because of the diffi-

culty in examining modern subglacial processes and the fact that

glacial environments are not at present extensive, the detailed

environmental interpretation of ancient continental glacial sedi-

ments is not always straightforward. What is clear is that wide-

spread deposition of glacial sediments on the continents,

particularly in low latitude situations, indicates periods of signifi-

cant global cooling.

Terrestrial glacial systems range in scale from local, alpine

glaciers to continent-scale ice sheets. Glacial systems of all

scales contain a variety of subenvironments that may be recog-

nizable in the ancient sedimentary record. Among the most

important are subglacial settings (under the glacier) and pro-

glacial settings (in front of or adjacent to the glacier). Both

environments may involve ice contact. Supraglacial settings

(on top of the glacier) and englacial settings (within the glacier)

have low preservation potential and will not be discussed here.

Subglacial ice contact deposits typically include some sort

of glacial till – characterized by its very poor sorting and abun-

dance of angular clasts, some of which may be glacially

striated. Three till types are commonly recognized in ancient

continental glacial deposits. Lodgement till typically forms by

basal slip of the glacier against the sediment substrate. It is

typically rather compacted and characterized by weak clast

imbrication. More common is deformation till that forms

through subglacial sediment shearing and is characterized by

extremely poor sorting, random clast orientation, and evidence

of soft-sediment deformation. Also common is melt-out till

that usually contains evidence of subglacial water-reworking

of sediment.

Although their sedimentology is not well documented, subgla-

cial lakes of significant volume and longevity may be present

beneath continental glaciers because of the insulation effects

of the ice and internal heat of the underlying crust. For example,

glacial Lake Vostok is estimated to be 70 km long and to have

existed for up to 20 million years under the Antarctic Ice Sheet

(Siegert et al., 2001). Although not a universally accepted idea,

some workers have speculated that large subglacial lakes may

have contributed significantly to glacial outwash flows, particu-

larly during periods of deglaciation (Shaw et al., 1999).

Proglacial lakes can provide some of the best evidence for

ancient glaciation because they commonly contain a variety of

easily recognizable sedimentary structures. Perhaps most impor-

tant are glacial-lacustrine rhythmites. Many (though not all) such

rhythmites result from annual seasonal changes in sediment flux.

Typically, glacial varves are characterized by a sharp base, corre-

sponding to the onset of spring melt-out, overlain by relatively

coarse-grained sediment deposited during spring and summer, fol-

lowed by decreasing sediment sizes during the return to winter

conditions. Sedimentation rates in proglacial lakes in which varves

are being formed can be extremely high – commonly cm yr

1

(Ashley, 1975). In addition to rhythmites resulting from seasonal

changes, proglacial lake environments are commonly character-

ized by widespread sediment gravity flow deposition, including

overflows, interflows, and underflows (Gustavson, 1975).

These result mainly through introduction of sediment by subgla-

cial currents as well as through remobilization of sediments from

lake margins. Ice-rafted debris (IRD), another common attri-

bute of proglacial lake systems, consists of randomly-oriented

coarse clasts (pebbles, cobbles, boulders) that are encased in

deeper-water glacial lake sediments (Levish, 1997). In the absence

of association with other glacial sediments, IRD may be difficult to

distinguish from poorly-sorted sediment associated with other

sedimentary environments (e.g., some mud-rich debris flows).

In addition to lakes, proglacial settings can contain a spatially

complicated suite of environments that may include bedload-

dominated streams (glacial outwash) and eolian environments.

Bedload-dominated streams associated with glacial outwash are

difficult to distinguish from those not associated with glacial set-

tings and thus are not by themselves good indicators of glaciation.

Widespread eolian deposits in proglacial settings are commonly

produced by adiabatic winds flowing off continental ice sheets.

Proglacial eolian deposits can include both sand dunes and

silt-sized loess,commonly occurring in paired dune-loess systems.

Loess deposits typically consist of tabular beds of well-sorted

silt that may contain included ventifacts and well-developed

paleosols.

Lake systems

Lakes can contain some of the highest resolution sedimentary

records of paleoclimate preserved in continental settings, because

many of the physical, chemical, and biological processes that

directly control sediment type and flux in lakes are driven by sea-

sonal changes. Importantly, the preservation potential of lakes

is generally high because lakes commonly form in areas where

tectonic subsidence is significant.

Rates of lake basin infill by sediment and water are a func-

tion both of climate (e.g., effective precipitation rates) and tec-

tonics (e.g., rate of subsidence of the lake basin bottom). This

important relationship led Carroll and Bohacs (1999) to classify

modern and ancient lakes as underfilled, balanced-filled, and

overfilled. This is a useful classification because it can be read-

ily applied to ancient lacustrine basins without the need for

paleogeomorphic reconstructions. Underfilled lake basins typi-

cally reflect the inability of the delivery rate of sediment

plus water to keep up with tectonic subsidence. Such lakes

are typically closed and commonly retain some evidence of

water column stratification, reduced bottom oxygen levels,

and increased salinity levels. Overfilled lakes are usually open

and reflect the inability of the subsidence rate to keep up with

the delivery rate of sediment plus water. Such lakes are usually

well-oxygenated and are characterized by progradation of del-

tas into the lake and a non-stratified water column. As the name

implies, balanced-filled lakes occur in settings where subsi-

dence and the rate of lake-basin infill by sediment and water

are co-equal.

Climate records in lake sediments can be reflected by a mul-

titude of sedimentologic, biologic, and geochemical character-

istics that change over timescales ranging from 10

to 10

years. Annual seasonality may be faithfully recorded in lake

sediments in the form of varve laminations that are controlled

by seasonal environmental changes, such as variations in sedi-

mentation flux, lake chemistry, or biology (Nuhfer et al., 1993).

At longer time scales, oscillations in climate have been inter-

preted from a variety of sedimentologic characteristics in which

some element of cyclicity is preserved. For example, Olsen

(1990) described a series of upward-shoaling cycles within lake

sediments from the Triassic rift basin of the eastern US. Apply-

ing spectral analysis to the temporal variations in relative

lake depth, Olsen (1990) demonstrated that the frequency of

water depth variation was consistent with that expected from

astronomically-driven changes in the Earth’s orbital parameters

CONTINENTAL SEDIMENTS 193

(i.e., Milankovitch cyclicity). In a second example, Trauth et al.

(2001) studied variations in sedimentology, diatom assemblages,

and geochronology from East African lakes and concluded that

the lakes alternated between deep freshwater conditions and

shallow highly alkaline conditions on a 23-kyr-frequency band

that is consistent with Milankovitch-driven cyclicity.

In addition to interpretation of orbitally-forced Milankovitch

cyclicity in ancient lake deposits, lake sediments have proven

useful in revealing the presence of other cyclic frequencies that

may reflect atmospheric or coupled oceanic-atmospheric pro-

cesses that are as yet poorly understood. For example, Munoz

et al. (2002) studied laminated lake sediments from Spain and

concluded that cyclic sedimentation reflected climate variations

due to the El Niño Southern Oscillation (ENSO) and the North

American Oscillation (NAO).

Three types of end-member lake types have been recognized

as having particular paleoclimatologic significance. These are

glacial lakes, playa lakes, and organic-rich lakes. Glacial lakes

are discussed above. Playa lakes are shallow, closed lakes formed

in areas of low effective precipitation and high rates of evapora-

tion. Playa lakes usually have alkaline or saline water chemistries.

Common characteristics include: (a) deposition of evaporites or

alkaline minerals such as dolomite, various zeolites, and Mg-rich

clays; (b) organic-rich laminated mudrock in the central part

of the lake basin; and (c) abundant mudcracks – particularly in

lake margin facies. The margins of saline or saline, alkaline lakes

commonly interfinger with alluvial fan sediments, indicating

the important role of tectonics not only in creating the lake basin,

but also in the production of orographic climate effects (e.g., rain

shadows) in some cases. Organic-rich lakes occur in a variety

of tectonic and climatologic settings, so it is typically necessary

to supplement climate interpretations based on sedimentary char-

acter with those from other data sets such as organic geochemistry

or the composition of diatom assemblages. Organic-rich bottom

sediments are commonly deposited in lakes with permanent ther-

mal or salinity stratification, and anoxic bottom conditions

develop in tropical climates with low seasonality and high humid-

ity. For example, organic-rich lakes are common in equatorial

Africa (Talbot, 1988) – an area characterized by little seasonality

but abundant rainfall. In contrast, organic-rich lakes also are

common in temperate North America (between 42



N and 55



latitude) where seasonality is strong but annual productivity is

high (Anderson et al., 1985).

River and alluvial fan systems

River system dynamics are controlled by a large number of

variables that may be influenced by climate but may also

reflect other external controls such as rates of subsidence and

sedimentation. Unfortunately, the geomorphology of ancient

fluvial systems is rarely preserved in the rock record. As a

result, paleoclimatologic interpretations of ancient fluvial sys-

tems typically have focused on documenting the nature and

geometry of sand bodies deposited by riverine processes and

on determining whether ancient streams were ephemeral (i.e.,

not flowing continuously) or perennial (flowing continuously).

Use of fluvial sand body geometries to infer paleoclimate,

though qualitative, usually focuses on distinguishing between

mud-dominated streams that are inferred to be meandering,

and bedload-dominated streams that are inferred to be more

braided. Although both stream types can occur in a wide variety

of climate settings and both are controlled by numerous factors

other than climate – tectonics in particular – the implication is

that mud-dominated streams usually represent a more humid cli-

mate than bedload-dominated streams. Meandering stream chan-

nel sands tend to be isolated, with well-preserved channel forms

and relatively few internal scours, a relatively fine overall grain

size, and common preservation of lateral accretion surfaces

within channel deposits. Paleocurrent indicator directions tend

to be widely dispersed. Overbank regions are characterized by

abundant evidence of plant colonization and, commonly, the

occurrence of coal. Braided river deposits are usually character-

ized by sand bodies that are multistoried and multilateral, an

abundance of cut and fill scour surfaces, relatively coarser and

less well sorted sediment, and common planar and trough

cross-bedding. Individual channels are rarely preserved, the ratio

of sand to mud is higher than meandering streams, and evidence

of plant colonization is less common. Overbank regions com-

monly contain well-developed calcisol (caliche) horizons (Miall,

1992, 1996).

Distinguishing between ancient ephemeral and perennial

stream flow may be a somewhat more reliable means of interpret-

ing paleoclimate than determining whether ancient streams were

mud- or bedload-dominated, although both forms of interpretation

are qualitative assessments of paleoclimate. Generally, ephemeral

streams are more likely to represent dry climates than perennial

streams. Common features of ancient ephemeral streams include:

(a) relatively small sandstone bodies with upper flow regime struc-

tures produced by short but powerful flash floods; (b) abundant

reworked sand and mud intraclasts, suggesting short duration

flows; (c) common desiccated mud drapes and/or interbedded

eolian deposits; (d) abundant root traces within the channel sys-

tem,and (e) well-developed calcisols in overbank regions (Parrish,

1998). In contrast, perennial streams are typically characterized

by larger sandstone bodies and low flow regime sedimentary struc-

tures, such as subaqueous dunes and current ripples, which are

interpreted to reflect lower velocity and more sustained flows.

Alluvial fans are generally not regarded as good indicators

of paleoclimate, although specific climatologic indicators (e.g.,

evaporites, coal) can be associated with alluvial fan systems. Allu-

vial fans are generally more useful as evidence of tectonic activity,

because steep topographic gradients are required for alluvial fan

formation. Nevertheless, climate can be an important influence

on fan sedimentation. For example, Blair and McPherson (1994)

described alluvial fans in terms of two end-members: debris-flow

dominated fans, generally associated with more arid climate

regimes; and sheet flood dominated fans, generally associated

with more humid climate regimes. In many ancient cases, alluvial

fans are indistinguishable from braided river deposits. Although

this similarity might imply that alluvial fans are mostly associated

with arid environments, fans are also common in well-watered

environments as, for example, proglacial outwash fans, which

commonly occur downstream from alpine glaciers.

Deserts and eolian systems

Deserts are generally defined as areas characterized by low rates

of precipitation and high rates of evaporation. More specifically,

the definition depends on the ratio of actual precipitation (P)

to evaporation and evapo-transpiration (Et

). Deserts are usually

characterized by P/Et

ratios of 0.2 (Parrish, 1998). Modern

deserts generally occur in three different settings, each of which

is characterized by air masses that retain moisture as they heat

up, thereby inhibiting atmospheric condensation and result-

ing precipitation. The largest modern deserts (e.g., the Sahara)

occur at lower middle latitudes (30



Nand30



S) where dry,

194 CONTINENTAL SEDIMENTS