Odekon M. Encyclopedia of paleoclimatology and ancient environments

Подождите немного. Документ загружается.

and/or cementation of pre-existing soil, sediment, bedrock, or

weathered material. They can vary in hardness from powdery

to highly indurated, and may reach thicknesses of >50 m

although 1–10 m is more common (Goudie, 1973). Calcrete

or caliche (cemented by calcium carbonate) is the most wide-

spread form and is estimated to underlie >13% of the Earth’s

land surface (Wright and Tucker, 1991). Iron- and aluminum-

rich ferricretes and alcretes are the next most common, and occur

over large areas of west, central and east Africa, Australia,

and India. Silica-rich silcrete is less widespread but is locally

important in southern Africa, Australia, and Europe (Watson

and Nash, 1997). Other accumulations such as calcium sulfate-

cemented gypcrete are sometimes defined as duricrusts but are

less likely to persist within the landscape. In contrast, durable

duricrust varieties such as ferricrete and silcrete can act as impor-

tant controls upon landscape evolution once exposed since they

may form resistant caprocks (Figure D43 ).

There is no single model to explain the formation of all dur-

icrusts. Ferricretes and alcretes commonly form by the relative

accumulation of iron and aluminum oxides and hydroxides

in seasonally-wet humid to sub-humid tropical environments

as other more mobile elements are leached from weathering

profiles. Absolute accumulation may also occur where iron

and aluminum are transported downslope mechanically or

in solution, or are mobilized within the weathering mantle.

Silcretes and calcretes usually develop through the absolute

accumulation of silica and calcium carbonate derived from

rock weathering, dust, rainfall, surface- and groundwater, or

biogenic sources. The translocation of solid or dissolved mate-

rials to sites of accumulation can be achieved via lateral move-

ment of solutions through fluvial, lacustrine, or groundwater

systems, and/or by vertical transfer mechanisms such as perco-

lation and capillary rise (often in association with pedogenesis).

Once at a site of accumulation, silica and calcium carbonate

precipitation can be triggered by evaporation, pH changes,

reactions with other cations, and organic activity.

Duricrusts may act as important paleoenvironmental indica-

tors, particularly when found beyond their zone of contempor-

ary formation or within the geological record. However, it is

vital that the factors controlling formation are understood prior

to their use as such indicators. Low relative relief appears to be

a requirement for the development of all varieties since the rate

of formation must exceed that of removal of material by ero-

sion. Topography may also influence the ability of a host mate-

rial to retain moisture or permit leaching. The availability of

a source material is critical in determining the degree and

rate of formation. There is less agreement about the climatic

requirements for duricrust formation. Calcretes are known to

be forming today in dryland regions with rainfall in the range

200–600 mm (Goudie, 1973), and may, therefore, be a useful

indicator of semi-arid climates. The environment of silcrete

formation is more complex, with pedogenic varieties thought

to develop under humid tropical climates and non-pedogenic

silcretes under arid tropical conditions. However, as silcretes

are only forming in one location today (through fixation by

cyanobacteria in the highly alkaline Sua Pan, Botswana), it is

difficult to identify representative modern analogs. The require-

ment of intensive leaching for ferricrete and alcrete develop-

ment makes them a potentially useful indicator of humid

tropical climates, with alcretes more prevalent where contem-

porary rainfall levels are particularly high.

David J. Nash

Bibliography

Goudie, A.S., 1973. Duricrusts in Tropical and Subtropical Landscapes.

Oxford, UK: Clarendon Press, 174pp.

Watson, A., and Nash, D.J., 1997. Desert crusts and varnishes. In

Thomas, D.S.G. (ed.), Arid Zone Geomorphology. Chichester, UK:

Wiley, pp. 69–107.

Wright, V.P., and Tucker, M.E. (eds.), 1991. Calcretes. Oxford, UK: Black-

well Scientific, 352pp.

Cross-references

Arid Climates and Indicators

Laterite

Paleosols, Pre-Quaternary

Paleosols – Quaternary

Sedimentary Indicators of Climate Change

Weathering and Climate

Figure D43 A 10 m thick exposure of silcrete and calcrete in the flanks of the Auob Valley at Kalkheuval, Namibia (photograph: David J. Nash).

DURICRUSTS 285

DUST TRANSPORT, QUATERNARY

The atmospheric cycle of soil-derived dust aerosol is expected to

respond to changes in global climate conditions, such as the over-

all intensity of winds that entrain dust, and changes of the hydro-

logical cycle that can affect the extent and dryness of source areas

as well as the length of time that dust remains in the atmosphere.

At the same time, dust can potentially impact global and regional

climate conditions in several ways, by impacting the radiative

balance of the atmosphere or by providing micronutrient fertili-

zers to marine and terrestrial plants, thereby modulating atmo-

spheric CO

concentrations. Major dust source regions today

are the Saharan desert, Arabia, the Asian deserts, Australia, and

North America. The majority of atmospheric dust comes out of

North Africa (50–70%) and Asia (10–25%).

Dust is particularly important within the context of the

Quaternary because large fluctuations and overall increases in

atmospheric dust concentrations coincided with the onset of

Northern Hemisphere glacial cycles, approximately 2.8 Ma.

The longest land records suggest that relatively continuous

atmospheric dust deposition occurred on the Chinese Loess

Plateau since about 7 Ma (Ding et al., 1999). These records,

along with marine records from the North Pacific (Rea et al.,

1998), suggest that dust deposition was enhanced following a

geologically more recent phase of uplift of the Tibetan Plateau

(approximately 3.6 Ma). This uplift blocked Indian moisture

sources from reaching East Asia and created conditions favor-

able for the development of the East Asian monsoon, which

modulates the southward flow of cold dry air masses across

the East Asian continent in winter. As glacial cycles intensified,

they enhanced conditions of stronger winds and higher aridity

that favor dust emission during cold periods.

Marine sediment records of dust from around Africa also sug-

gest a shift from a predominantly monsoon-driven climate before

2.8 Ma towards more arid conditions modulated by the periodicity

of Northern Hemisphere glaciations after 2.8 Ma (deMenocal,

1995). The intensification of Northern Hemisphere glaciations

resulted in cooler North Atlantic surface ocean temperatures that

inhibited the expansion of the North African summer monsoon

into the African continent, thereby increasing Saharan and

Sahelian aridity.

Dust deposited over land, ocean or ice surfaces can be pre-

served in the geologic record and serve as archives of periods

of past dustiness. Determining exact changes in deposition

rates between different time periods is limited by the many dif-

ferent techniques used to measure dust, as well as the limited

number of precise dates generated on these different sediment

types. Nevertheless, significant efforts have been made to quantify

changes in dust deposition through time, and the best-documented

period of intensified dustiness in the Quaternary is the Last Glacial

Maximum (approximately 17–23 ka). From these records, we

know that dust was about 2–3 times increased globally during gla-

cial periods compared with interglacial periods (Figure D44). Dur-

ing the last glacial period, North Africa remained the most

important source of dust, but both data and models suggest an

additional expansion of Asian, Australian, and South American

dust sources during this time.

Loess deposits

During the last glacial period, loess deposition was greatly

expanded, mostly in periglacial regions such as North America,

the Eurasian continent, and the South American Pampas region.

The source of dust forming loess deposits is largely material that

has been ground by glaciers and subsequently becomes available

following fluvial transport to arid, non-vegetated source regions.

Loess deposits are indicators of changes in both the extent

and activity of dust sources, but also strongly depend on local

conditions like terrain forms. For example, flat basins are rela-

tively stable sites of continuous deposition. River terraces very

near sources frequently show amplified deposition rates and

are more likely to contain erosional discontinuities. Dust parti-

cles deposited in loess regions consist of relatively coarse par-

ticles (10–63 mm size) that have not been transported over

large distances. As a result, deposition rates in loess deposits

have been measured to be as high as 20,000 g m

2

1

in the

mid-continental USA (Roberts et al., 2003), while much of the

loess deposited on the Chinese Loess Plateau falls in the range

of 300–500 g m

2

1

Patterns of thickness and loess grain size can provide informa-

tion about surface transport trajectories of loess. For example, in

the mid-continental USA, thickness trends in the last glacial loess

indicate that predominant surface winds came from the west and

northwest (Figure D45). Loess thicknesses are greatest near dust

sources along rivers, at the southern edge of the North American

ice sheet, and directly downwind of the Rocky Mountains, and

rapidly decrease to the east of these source regions.

Marine sediments

In order to examine dust records in marine sediments, one must

first isolate the terrigenous material within marine sediments,

and then avoid contamination by non-eolian terrigenous material

such as turbidites, river plumes, ice bergs, and deep currents ice-

rafted detritus. Once these complications are circumvented, mar-

ine sediment records can be good indicators for intermediate and

long-distance transport of dust particles. Mean grain sizes in mar-

ine sediments are usually much less than 10 mm, although larger

grains are found in sediments close to continental source areas.

Accumulation rates in marine sediments can be as high as

100 g m

2

1

, but are usually in the order of 1–10 g m

2

1

Overall, marine sediments suggest about a 2-fold increase in

glacial compared with interglacial dust deposition in the tropics

and mid-latitudes, where the records are reliable. Different

regions experienced different changes in dust deposition during

glacial-interglacial cycles. Marine sediments in the North Pacific

region suggest a 1–3-fold increase in dust deposition at the LGM

compared to the late Holocene period, deposition off Africa

increased by approximate 2–5 times, and deposition to the south-

east of Australia shows 2–9-fold increases in dust deposition

during glacial periods. In contrast, although the absolute magni-

tude of dust entering the Arabian Sea is high, increases in glacial

dust deposition over the Arabian Sea are relatively small (60–

80% increase at the LGM compared to today). In general, the

spatial patterns of where dust was deposited did not undergo

major changes, suggesting that the actual transport pathways of

dust did not shift significantly between glacial and interglacial

periods for most parts of the world.

Ice cores

Ice cores provide a record of long-distance transport of dust

that reflects combined changes in source strengths, transport,

and deposition through time. Grain sizes of dust found in ice

cores are almost always smaller than 5 mm in diameter. Glacial

increases in dust fluxes are highest in the Antarctic ice cores,

with an approximately 10-fold increase in the Vostok Ice Core

286 DUST TRANSPORT, QUATERNARY

(Petit et al., 1999). While the changes in dust fluxes at these

locations are very high, the actual fluxes are low (on the order

of 0.0001–0.01 g m

2

1

Mineralogical and isotope studies of the dust from ice cores

in both Greenland and Antarctica have been used to constrain

the polar transport and origins of dust. Current studies indicate

that glacial dust arriving at Antarctica originates in southern

South America (which is the dominant source), with lesser

amounts possibly derived from northern New Zealand and the

Antarctic dry valleys (Delmonte et al., 2002). Dust arriving in

Greenland originates in the Asian deserts. Ice-core dust concen-

trations during interglacial periods are generally so low that

their source regions cannot be pinpointed. However, studies

on snow pits near the Greenland ice cores suggest that modern

dust is still derived from Asian deserts (specifically the Takla

Makan and Inner Mongolian deserts of northern China (Bory

et al., 2002).

Ice core data from Vostok have also revealed the relative

timing of changes in dust fluxes compared with other environ-

mental parameters such as temperature and CO

(Figure D46).

In these records, dust increased well after the onset of glacial

cooling and lowering of atmospheric CO

, with peak dust

fluxes occurring only during peak glacial periods. This timing

may provide insights into the causes of increased dustiness,

as well as feedbacks of dust on climate parameters such as tem-

perature, precipitation, and radiative balance.

Causes of increased glacial dustiness

Possible causes for increased dust emissions in glacial periods

are stronger surface wind speeds that could lift higher amounts

of dust aerosols from surfaces compared to modern conditions;

increased aridity, which would expand the areas with drier soils

and reduced vegetation where soil deflation by wind is possi-

ble; and increased availability of fine sediments that could

easily be picked up by surface winds. Atmospheric loading of

dust particles could also be increased under cold climate condi-

tions as a consequence of the weakened hydrological cycle.

(This can be deduced from the Clausius-Clapeyron relation-

ship, which predicts lower atmospheric water content at lower

temperatures.) The decrease in atmospheric water content leads

Figure D44 Simulated and observed glacial-interglacial changes in dust deposition between the late Holocene (0–6 ka) and the last glacial period

(17–23 ka). Geological data are represented by circles taken from the Dust Indicators and Records of Terrestrial and Marine Paleoenvironments

(DIRTMAP) database (Kohfeld and Harrison, 2001) superimposed on model results of Werner et al. (2003).

DUST TRANSPORT, QUATERNARY 287

to decreased precipitation, reducing the washout of atmospheric

dust particles and thus leading to longer atmospheric lifetimes.

The Last Glacial Maximum (LGM) has been a major focus

of dust cycle modeling (e.g., Joussaume, 1993; Andersen et al.,

1998; Mahowald et al., 1999; Reader et al., 1999; Lunt and

Valdes, 2002; Werner et al., 2003), partly because dust deposi-

tion rates were high and well-documented for comparison

(e.g., DIRTMAP, Kohfeld and Harrison, 2001), and partly

because the changes in boundary conditions (e.g., ice sheet

extent, atmospheric composition) are relatively well known.

Global dust models can only reproduce the increase in glacial

dust deposition observed at high latitudes when the combined

impact of increased wind speeds, reduced intensity of the hydro-

logical cycle, and vegetation changes are included together.

Neither increase in surface wind speeds and decrease in soil

moisture alone (Joussaume, 1993; Andersen et al., 1998; Reader

et al., 1999), nor additional increase in dust source areas caused

by exposure of continental shelves can explain the observed

increased high latitude dust (Andersen et al., 1998; Reader et al.,

1999). Incorporating increases in dust source areas from changes

in vegetation cover (Mahowald et al., 1999; Lunt and Valdes,

2002) produces a 20-fold increase in atmospheric dust loads at

high latitudes, which is consistent with the high dust fluxes

observed in polar ice cores. An LGM simulation using a full sea-

sonal cycle of vegetation cover as a mask for dust emissions, as

well as dried lake-beds as preferential dust emission regions,

shows a more moderate increase in dust emissions, of a factor

of 2–3 during the LGM, in agreement with the available marine

sediment and ice core data (Werner et al., 2003). The potentially

large effect of dust supplied by glacial outwash, and increase

in dry lake-beds at the LGM has not yet been evaluated by such

global dust cycle models.

Effects of increased glacial dust

Dust may impact the climate by its direct radiative effect, i.e.,

it may either decrease or increase surface temperatures and

modify atmospheric circulation depending on its optical prop-

erties and the albedo of the surface underlying the dust cloud.

Overpeck et al. (1996) hypothesized that absorption of solar

radiation over bright, ice covered surfaces in the Northern

Hemisphere could cause warmer temperatures compared with

those that would have occurred without increased dustiness at

the LGM. In contrast, Claquin et al. (2003) showed that warm-

ing by dust would have a negligible effect at high latitudes

when compared to the cooling impact of increased albedo due

to the tremendous ice sheet expansion at the LGM. On the

other hand, Claquin et al. ( 2003 ) estimated that the albedo feed-

back due to increased glacial atmospheric dust could reduce the

incoming radiation in the tropics by as much as 2 W m

2

which is the same order of magnitude as the decrease in green-

house forcing due to reduced atmospheric CO

. This decrease

in solar radiation at the surface might have caused addi-

tional cooling at low latitudes during the LGM. So far, no esti-

mates regarding the impact of increased glacial dust on cloud

Figure D45 Thickness of last glacial loess from the midcontinental USA (Kohfeld and Harrison, 2001).

288 DUST TRANSPORT, QUATERNARY

properties exist, but it is suspected that the presence of dust

particles could have a significant impact on the brightness of

clouds and precipitation formation (Rosenfeld et al., 2001).

In addition to the radiative effects of dust, the addition of iron

from the increased dust deposited over the ocean could have fer-

tilized the oceanic biosphere and subsequently enhanced the

drawdown of atmospheric CO

. This effect has been estimated

to lie between 10 and 40 ppm of the observed glacial-interglacial

atmospheric CO

change (Archer et al., 2000; Watson et al.,

2000; Bopp et al., 2003). However, these estimates still incorpo-

rate large uncertainties, ranging from the estimation of Fe solubi-

lity to the exact role of Fe within marine ecosystems.

Karen E. Kohfeld and Ina Tegen

Bibliography

Andersen, K.K., Armengaud, A., and Genthon, C., 1998. Atmospheric dust

under glacial and interglacial conditions. Geophys. Res. Lett., 25,

2281–2284.

Archer, D., Winguth, A., Lea, D., and Mahowald, N., 2000. What caused

the glacial /interglacial atmospheric pCO

cycles? Rev. Geophys., 38,

159–189.

Bopp, L., Kohfeld, K.E., Le Quéré, C., and Aumont, O., 2003. Dust impact

on marine biota and atmospheric CO

during glacial periods. Paleocea-

nography, 18, doi: 10.1029/ 2002PA000810.

Bory, A.J.-M., Biscaye, P.E., Svensson, A., and Grousset, F.E., 2002.

Seasonal variability in the origin of recent atmospheric mineral dust at

NorthGRIP, Greenland. Earth Planet. Sci. Lett., 196, 123–134.

Claquin, T., Roelandt, C., Kohfeld, K.E., Harrison, S.P., Tegen, I.,

Prentice, I.C., Balkanski, Y., Bergametti, G., Hansson, M., Mahowald,

N., Rodhe, H., and Schulz, M., 2003. Radiative forcing of climate by

ice-age atmospheric dust. Climate Dyn., 20, 193–202, doi: 10.1007/

s00382-002-0269-1.

Delmonte, B., Petit, J.R., and Maggi, V., 2002. Glacial to Holocene impli-

cations of the new 27000-year dust record from the EPICA Dome C

(East Antarctica) ice core. Climate Dyn., 18, 647–660.

deMenocal, P.B., 1995. Plio-Pleistocene African climate. Science, 270,

53–59.

Ding, Z.L., Xiong, S.F., Sun, J.M., Yang, S.L., Gu, Z.Y., and Liu, T.S.,

1999. Pedostratigraphy and paleomagnetism of a similar to 7.0 Ma

eolian loess-red clay sequence at Lingtai, Loess Plateau, north-central

China and the implications for paleomonsoon evolution. Palaeogeogr.

Palaeoclimatol. Palaeoecol., 152,49–66.

Joussaume, S., 1993. Paleoclimate Tracers: An investigation using an atmo-

spheric general circulation model under ice age conditions 1. Desert

dust. J. Geophys. Res., 98, 2767–2805.

Kohfeld, K.E., and Harrison, S.P., 2001. DIRTMAP: The geologic record

of dust. Earth Sci. Rev., 54,81–114.

Lunt, D.J., and Valdes, P.J., 2002. Dust deposition and provenance at the

last glacial maximum and present day. Geophys. Res. Lett., 29, 2085,

doi: 10.1029 /2002GL015656.

Mahowald, N., Kohfeld, K.E., Hansson, M., Balkanski, Y., Harrison, S.P.,

Prentice, I.C., Schulz, M., and Rodhe, H., 1999. Dust sources and

deposition during the last glacial maximum and current climate: A com-

parison of model results with palaeodata from ice cores and marine

sediments. J. Geophys. Res., 104, 15,895–15,916.

Overpeck, J., Rind, D., Lacis, A., and Healy, R., 1996. Possible role of

dust-induced regional warming in abrupt climate change during the last

glacial period. Nature, 384, 447–449.

Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.M., Basile, I.,

Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte, M.,

Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C., Pepin, L.,

Ritz, C., Saltzman, E., and Stievenard, M., 1999. Climate and atmo-

spheric history of the past 420,000 years from the Vostok ice core,

Antarctica. Nature, 399

, 439–436.

Figure D46 Records of temperature, carbon dioxide, and dust concentrations taken from the Vostok Ice Core, Vostok, Antarctica (Petit et al., 1999).

DUST TRANSPORT, QUATERNARY 289

Rea, D.K., Snoeckx, H., and Joseph, L.H., 1998. Late Cenozoic

eolian deposition in the North Pacific: Asian drying, Tibetan uplift,

and cooling of the northern hemisphere. Paleoceanography, 13,

215–224.

Reader, M.C., Fung, I., and McFarlane, N., 1999. The mineral dust aerosol

cycle during the Last Glacial Maximum. J. Geophys. Res. Atmos., 104,

9381–9398.

Roberts, H.M., Muhs,D.R., Wintle, A.G., Duller, G.A.T., and Bettis, III, E.A.,

2003. Unprecedented last-glacial mass accumulation rates determined by

luminescence dating of loess from western Nebraska. Quat. Res., 59,

411–419.

Rosenfeld, D., Rudich, Y., and Lahav, R., 2001. Desert dust suppressing

precipitation: A possible desertification feedback loop. Proceedings of

the National Academy of Sciences U.S.A., 98, 5975–5980.

Watson, A.J., Bakker, D.C.E., Ridgwell, A.J., Boyd, P.W., and Law, C.S.,

2000. Effect of iron supply on Southern Ocean CO

uptake and impli-

cations for glacial atmospheric CO

. Nature, 407, 730–734.

Werner, M., Tegen, I., Harrison, S.P., Kohfeld, K.E., Prentice, I.C., Balkanski,

Y., Rodhe, H., and Roelandt, C., 2003. Seasonal and interannual variability

of the mineral dust cycle under present and glacial climate conditions.

J. Geophys. Res., 108, doi: 10.1029/2002JD002365.

Cross-references

Aerosol (Mineral)

Atmospheric Circulation During the Last Glacial Maximum

Eolian Dust, Marine Sediments

Eolian Sediments and Processes

Ice Cores, Antarctica and Greenland

Last Glacial Maximum

Loess Deposits

Monsoons, Quaternary

Mountain Uplift and Climate Change

Quaternary Climate Transitions and Cycles

290 DUST TRANSPORT, QUATERNARY

EARLY PALEOZOIC CLIMATES

(CAMBRIAN-DEVONIAN)

Introduction

Early Paleozoic climates obviously cannot be studied directly.

Proxies for climate must be employed as a substitute for direct

observations. These proxies include lithological indicators, such

as evaporites, calcretes, tillites, coals, kaolins, and bauxites. Eva-

porites, marine and nonmarine, are evidence of climatic conditions

adequate to permit precipitation of salts from concentrated sea-

water or freshwater, i.e., arid climates. Marine evaporites are

formed only in shoreline regions and not in the open ocean or in

estuarine, lagoonal, or other situations where seawater is subject

to enough evaporation to permit precipitation of its load of dis-

solved salts. This is also the case with nonmarine evaporites

formed in permanent (lakes) or temporary (playa) environments.

The evaporate minerals commonly preserved in the geological

record are calcium carbonate (most commonly calcite), sodium

chloride (halite), and calcium sulfate (anhydrite [anhydrous] or

gypsum [hydrated]). Evaporites are overwhelmingly representa-

tive of arid, desert type environments. Calcretes are lumps

(nodules) or layers (limestones) of calcium carbonate precipitated

in the lower layers of ancient soil profiles under semiarid condi-

tions. Tillites are lithified glacial deposits, and may also include

glaciomarine deposits. Coals are chiefly lithified higher land plant

materials, representing overall humid conditions, warm to cold,

that first appeared in the Late Devonian when woody plants

became widely preserved in what were peats in swamp or bog

environments. Kaolins are clay deposits dominated by the

hydrated aluminum silicate kaolin, or transported grains of the

mineral derived from nonmarine clay deposits, formed under at

least annually warm, humid weathering conditions. Bauxite is a

lateritic product formed under tropical-subtropical, annually

humid weathering conditions. Note that all of these materials are

basically nonmarine in origin, although their salts are of marine

origin. “Marine” evaporites occur in the shoreline region, although

they commonly mix with nonmarine evaporitic materials in

the landward direction. An extended discussion of these items is

provided by Boucot et al. (in press). Other potential climatic

proxies, such as oolitic sedimentary iron ores and sedimentary

phosphate deposits, are still too poorly understood in this context

to be employed.

Additionally, there is an important suite of sedimentary

rocks and associated cool/cold climate faunas in the Cambrian

through the early half of the Middle Devonian that provide evi-

dence about very high southern latitude conditions, the so-

called Malvinokaffric and Atlantic realms (see Boucot and

Blodgett, 2001, for an example).

Geographically, it is important to understand that the present

distribution of these climatic proxy materials commonly does not

correspond to their original geographic position owing to subse-

quent movements of varied crustal blocks following the Early

Paleozoic, as well movements that occurred during the lengthy

Cambrian-Devonian. With this in mind, the Cambrian-Devonian

climatic proxy distributions may, in principle, be used to estimate

the changing positions of Cambrian-Devonian climatic belts. How-

ever , owing to the somewhat limited known record of Cambrian-

Devonian climatic proxy materials, there are limitations imposed

on these climatic reconstruction possibilities. For example, there

are no known Silurian bauxites, with very few in the Ordovician

and none in the Cambrian, although a few that are present in the

very late Precambrian permit estimation of where bauxite-forming

environments might have been located in the Cambrian.

It has been concluded that there were several significant

changes in the global climatic gradient during the Cambrian-

Devonian (i.e., the Equator to polar temperature gradients,

which are high during times of polar glaciation and low during

times of polar warm temperate conditions), from moderately

high in the Cambrian through earlier Middle Ordovician, very

high during the later Middle to Late Ordovician, back to mod-

erately high in the Silurian through earlier Middle Devonian,

followed by a globally lower later Middle Devonian, a rela-

tively low Late Devonian, and terminating in a relatively high

gradient in the very latest Late Devonian.

Atmospheric and oceanic composition

There is no direct information about Cambrian-Devonian atmo-

spheric or oceanic composition(s). Boucot and Gray (2001)

summarized current information about atmospheric carbon

dioxide tenor that discounts earlier modeling attempts by

others. Figure E1 provides a summary of changing Cambrian-

Devonian global climatic gradients. If atmospheric carbon

dioxide concentrations are a direct function of global climatic

gradient (i.e., global mean temperature with carbon dioxide

assumed to be the major greenhouse gas), the Cambrian

through earlier Middle Devonian concentrations were probably

not much higher than today, but were significantly higher for

the much lower global climatic gradients of the later Middle

Devonian through much of the Late Devonian.

Oceanic composition, i.e., major and trace element ionic

composition and concentration, cannot be directly measured.

However, when one the physiological requirements of marine

organisms, chiefly megascopic, are considered, it can be con-

cluded that it is unlikely that oceanic composition has changed

significantly from the Cambrian to the present unless there

have been varied evolutionary changes affecting their physiol-

ogies, which is unsupported by available data.

Global climatic gradients

Boucot and Gray ( 2001) summarized Phanerozoic global cli-

matic gradients (Figure E1). It is clear from the evidence that

average global mean annual temperatures have changed back

and forth over time. High global climatic gradients featuring

continental glaciation are present only in the later Ordovician

(part of the Caradocian plus the Ashgillian epochs) where a

large Southern Hemisphere region, including at least Africa,

the southern two-thirds of South America, and glaciomarine

conditions extending into Central Europe, are involved. The

later Devonian is characterized by a far lower global climatic

gradient than at present.

Cambrian climatic evidence

The Cambrian features the beginning of high latitude Southern

Hemisphere cool/cold regions that include Africa; South

America, except for the Argentine Precordillera, where evapor-

ites and calcrete are recognized; parts of easternmost North

America; and most of Europe, except for Sardinia, where local

evaporites are known. Arid conditions are indicated for a wide-

spread, lower latitude Southern Hemisphere belt to the north of

the cool/cold climate belt, which includes North America, the

Near East and Arabia, northwestern peninsular India, North

and South China, southern Siberia, and Australia. There is no

evidence for a tropical-subtropical belt owing to the nonrecog-

nition of bauxites, but such a belt is inferred by extrapolating

from the prior late Precambrian and subsequent Ordovician

situations.

Early Ordovician

The known Early Ordovician climatic distribution data are simi-

lar to that for the Cambrian. Cool/cold, high southern latitude

rocks and faunas are known from parts of South America and

North Africa as well as most of Southern and Central Europe.

To the north of these, there is widespread evidence of arid con-

ditions at lower latitudes, with widespread evaporites in North

America, the Argentine Precordillera, North China, Australia,

southern parts of the Siberian Platform, and southern Thailand

(the Shan-Thai Terrane), together with an occurrence in Northern

India,. Again, there are no known bauxites but evidence extra-

polated from the overlying Middle Ordovician suggests the

presence of a warm, humid tropical-subtropi cal belt. Climatic

evidence is lacking for Antarctica, Northern Europe, and

Central Asia.

Figure E1 Periods when warm temperature indicators are present at high latitudes are taken as representative of intervals of low climatic gradient.

Periods of temperate indicators at high latitudes display moderate climatic gradients, while times of cool to cold climate indicators near the

poles suggest freezing writers, indicating “moderately high” climatic gradients. Continental glaciation at one pole represents an interval of high

climatic gradient. Continental glaciation at both poles (i.e., the Quaternary), represents a period of “very high” climatic gradient (based on data

from Boucot et al., in press).

292 EARLY PALEOZOIC CLIMATES (CAMBRIAN-DEVONIAN)

Middle through end Ordovician

The middle to end Ordovician features later Ordovician evi-

dence of global heightening of the climatic gradient, culminat-

ing in widespread latest Ordovician, Southern Hemisphere,

high latitude, continental glaciation. Prior to this glaciation,

there is evidence for cooler climates occurring earlier in the

later-Ordovician, indicated by dropstones in North Africa and

tillites in South Africa that are earlier in time than the latest

Ordovician continental glaciation. Evidence of the latest Ordo-

vician includes widespread sites in North Africa and Arabia,

scattered localities in South America from southern Peru south,

and widespread dropstones in central and Southern Europe, all

of which probably reflect freezing winters at the very least, if

not actual glaciation. Throughout this interval in the Southern

Hemisphere, there is faunal and lithological evidence for

cool/cold water conditions at higher latitudes. At lower south-

ern latitudes, there is widespread evidence of an arid belt with

widespread evaporites and calcretes in North America, and eva-

porites in Northern Europe (Baltic region and northern Russia),

North and South China, Australia, and southern Siberia. Evi-

dence for a humid tropical-subtropical belt is provided by Mid-

dle Ordovician bauxites in southeastern Kazakhstan and on part

of the Siberian Platform. A Northern Hemisphere arid belt is

indicated by the presence of evaporites in southern Siberia.

The presence of kaolins in the same region as evaporites, in

North America for example, is most easily explained by at least

annually humid intervals at higher elevations with overall arid

conditions widespread at lower elevations.

The presence of later Ordovician bauxites in northwestern

Sudan is supportive of a brief incursion of warm climate condi-

tions during the later Ordovician featuring warm water fauna

and limestone in Central and Southern Europe and North

Africa.

Silurian

There is widespread, high southern latitude lithological and

faunal evidence for cool/cold climate conditions, with asso-

ciated tillites, in the earlier Silurian at scattered Brazilian

localities and dropstones at several North African localities.

At lower southern latitudes, there is widespread evidence,

including both calcretes and evaporites, from North America,

Europe, Iran, northern India, and Australia that is indicative of

a Southern Hemisphere arid belt. Evaporites in southern Siberia

are consistent with the presence of a Northern Hemisphere arid

belt. No bauxites are presently recognized in the Silurian, but a

tropical-subtropical belt is extrapolated from later Ordovician

and Devonian data.

Early Devonian

Climatically useful data for the Early Devonian indicates the pre-

sence of a high southern latitude cool/cold climate region, con-

sistent with both faunal and lithological evidence. A lower

latitude Southern Hemisphere arid belt is indicated by North

American evaporates; calcretes and evaporites in Europe; eva-

porites in Iran, Libya, and northwestern Arabia; calcretes and

evaporites in North and South China and Tarim; and evaporites

in Australia. A Northern Hemisphere evaporate belt is suggested

by widespread evaporites in Southern Siberia. Tropical-subtropical

belt evidence is restricted to bauxite occurrences in the northern

part of the eastern Urals and the Salair.

Middle Devonian

The Middle Devonian features the presence of the same cool/

cold climate high southern latitude region indicated both faun-

ally and lithologically for the earlier half of the Middle Devo-

nian, followed by a lowering of the global climatic gradient

consistent with the disappearance of this cool/cold climate

region in the latter half of the interval. A warm temperate,

mid southern latitude belt is consistent with North African coal

at a single locality, more widespread kaolins in northern South

America and North Africa, and coal in Libya. At still lower

southern latitudes, there are widespread evaporites and cal-

cretes in North America and Europe, evaporites in Australia,

and calcrete in the later Middle Devonian of Antarctica. Eva-

porites in southern Siberia are consistent with a Northern

Hemisphere arid belt. A tropical-subtropical belt is consistent

with bauxites in southern Siberia, southeastern Kazakhstan,

and northeastern Russia as well as coal from Central Asian

and South China. In the later Middle Devonian, coals overlying

earlier Middle Devonian evaporites in Arctic North America

indicate movement into the tropical-subtropical belt.

Upper Devonian

The Upper Devonian features a latest Devonian restricted region

of continental glaciation in the region from Bolivia through the

Amazon and Parnaibá Basins, plus possible occurrences in

Chad. A lower latitude Southern Hemisphere arid belt is repre-

sented by widespread North American calcretes and evaporites

(except for northernmost coals), European and Australian

evaporites and calcretes, plus evaporates and calcretes in local-

ities in northern India, Asiatic Russia, Kazakhstan, Iran, and

Turkey , and calcretes in Vietnam. Northern Hemisphere arid

conditions are represented by Siberian Platform evaporites. The

tropical-subtropical belt is represented by coals in South China,

Bear Island and Svalbard; and bauxites in South China, the

Salair and southern Siberia, the Urals, southeastern Kazakhstan,

and the Russian Tien Shan and Timan.

Arthur J. Boucot

Bibliography

Boucot, A.J., and Blodgett, R.B., 2001. Silurian-Devonian biogeography.

In Brunton, C.H., Cocks, L.R.M., and Long, S.L. (eds.), Brachiopods

Past and Present. The systematics Association Special Vol. series 63,

London: Taylor & Francis, pp. 335–344.

Boucot, A.J., and Gray, J., 2001. A critique of Phanerozoic climatic models

involving changes in the CO

content of the atmosphere. Earth-Sci.

Rev., 56,1–159.

Boucot, A.J., Chen, X., and Scotese, C.R., Preliminary compilation of

Cambrian through Miocene climatically sensitive deposits (In press).

Tulsa, OK: Society of Sedimentary Geology.

Cross-references

Arid Climates and Indicators

Coal Beds, Origin and Climate

Duricrusts

Evaporites

Glacial Sediments

Glaciomarine Sediments

Laterite

Mineral Indicators of Past Climates

Paleosols, Pre-Quaternary

Sedimentary Indicators of Climate Change

Tills and Tillites

EARLY PALEOZOIC CLIMATES (CAMBRIAN-DEVONIAN) 293

EARTH LAWS AND PALEOCLIMATOLOGY

Introdu ction

Paleoclimatology, as one of the Earth Sciences, is subject to the

laws of physics, chemistry and biology. Specifically, five

“Earth laws” apply to this planet. Four of these, with limita-

tions, also apply to other celestial bodies in the Solar System

(Shirley and Fairbridge, 1997). Paleoclimatology falls within

the scope of all five Earth laws.

The five laws, proposed here, are modified from earlier writ-

ings (Fairbridge, 1980). They are summarized as follows but

discussed in some more detail (with selected references) later

in this entry.

 First Earth law: Stellar evolution and history of the Sun.

The Solar System has a finite existence that depends upon

the evolution of the Sun, which behaves as a “main-

sequence ” star, burning hydrogen into helium, with nuclear

fusion as its primary source of energy. The Sun’ s energy is

in part gravitational, relating to the inertial effects of its

angular momentum developed by its rotation (axial spin),

and to its orbital motions, which are accelerated by those

of the planets (and cumulative). In addition, there are two

types of solar emissions: electromagnetic radiation (domi-

nated by optical wavelengths, e.g., 400 –750 nm) and

ionized, magnetic particles in the solar wind. Of importance

to Earth history and climate is the long-term stability of the

Solar System due to the Sun’ s estimated 10 billion-year life-

span, thus enabling evolution to progress to more advanced

forms of life.

 Second Earth law: Galactic cycles. As components of the

Milky Way Galaxy, the Sun and Solar System are subject

to its motions and history, following an inertial orbit from

perigalacticum (closest approach to the center, a “black hole” )

to apogalacticum , the farthest, with the cycle lasting about

240 Myr. On Earth, the ice ages, long-term geomagnetic rever-

sals, and organic evolution may be modulated by it.

 Third Earth law: Physical evolution of the Earth and

Moon . The physical evolution of the planet Earth, through

4.6 billion years, involves the gravitational ordering of its

successive spheres – core, mantle, lithosphere, hydrosphere

and atmosphere – a process known as differentiation, which

occurred early in Earth history. This evolution has subse-

quently been modulated by the motions and tides of the

Earth ’ s only natural satellite, the Moon, which has a revolu-

tion closely locked into its own 365.25 d period. Earth ’ s

history and climate have also been influenced by secular

changes in the obliquity of its spin axis.

Changes in the state of the Earth over time are recorded in

the rock strata. Cutler (2003), in a recent biography of Nicolaus

Steno (1638–1686), a Danish priest and early geologist, points

out Steno’ s fundamental observation that sedimentary rocks

were laid down in water sequentially, establishing the geologi-

cal “law of superposition.” It was based on exhaustive field

work and proved conclusively the principle of physical evolu-

tion. It also provided evidence for the immensity of geological

time. This latter inference remained to be demonstrated some

two centuries later when applied to biological evolution.

 Fourth Earth law: The organic or biological evolution of the

Earth. This has proceeded without interruption following

the first appearance of self-reproducing organisms at a very

early stage in its history (“Hadean”). A certain degree of pla-

netary stability is implied by the continual existence of life

on Earth for over 3 billion years. During this time, the Earth

has not undergone a runaway greenhouse effect, as on Venus

(see Atmospheric evolution, Venus, t hi s v ol um e ) , n or h a s i t

frozen entirely (placing constraints on the concept of

“Snowball Earth,” Hyde et al., 2000; Hoffman and Schrag,

2000; Snowball Earth hypothesis – this volume). Increases

in species and speciation (as shown by stratigraphic paleontol-

ogy) have accelerated through time, in spite of periodic mass

extinctions that appear to be due to asteroid impacts, volcan-

ism, ice ages and plate-tectonically forced isolation. Each

mass extinction creates an ecologic “vacuum” that leads to

an abruptly accelerated new speciation. Classical biological

evolution per se through mutations (such as shown by embry-

ology or DNA) or natural selection are of secondary signifi-

cance compared with the dramatic speciation demonstrated

by the paleontological/stratigraphic sequence following such

extinction events. The “biosphere” (habitable region of the

planet) interacts closely with the atmosphere, hydrosphere

and superficial lithosphere (“pedosphere”), with mainly bio-

geochemical changes through time.

 Fifth Earth law: Principle of Homeostasis. The fifth law

refers to dynamic equilibrium, a multi-feedback mechanism

that tends to keep all natural systems in relative balance. On

the planetary scale, that balance is represented by the “invari-

able plane” of the Solar System – the ecliptic. Passage

through it affects the physics of every planet and their satel-

lites. The 88-day highly elliptical orbit of Mercury (reflected

in the Sun’s radiation spectrum) contrasts with the ecliptic-

crossing orbits of Neptune and Pluto (with a revolution of

248 years). The Earth’s lunar orbit crosses this plane in an

orbit of about 25,500 years (1,370 18.6134 year, the Moon’s

nodal cycle). During ice ages, Earth’s spin rate increases

with near-axial ice loading, and the lunar orbit becomes shorter.

During deglaciation the spin and orbit changes are opposite,

butwithdistinctlags.

This law also applies to such terrestrial processes as isostasy,

which determines how a crustal uplift or downwarp will be

compensated in various ways (exhibited by gravitational

“anomalies”). “Glacio-isostasy” applies to ice loading or melt-

ing on land and “hydro-isostasy” concerns the effect of adding

glacial meltwater to the ocean, both processes affect global sea

level. Another aspect of the equilibrium law is shown in oro-

geny, where mountain-building uplift leads to accelerated ero-

sion, transport and sedimentation. This erosion in turn leads

to renewed uplift due to isostatic rebound. The “peneplain”

represents a long pause in crustal movements and is the theore-

tical end-phase of this equilibrium.

Earth laws: discussion and some climatological

consequences

External influences on climate

The Sun

The Sun is a spinning body, tilted at about 5



from the mean

plane of the planetary orbits. Its internal spin rate is about

25 days, while the photosphere’s rotation is slower, about

26–28 days, varying with solar latitude and time. The Sun also

undergoes an inertial orbital revolution around the barycenter

of the entire solar system. These patterns repeat sequentially

but unevenly over about 178 years (Fairbridge and Sanders,

294 EARTH LAWS AND PALEOCLIMATOLOGY