Odekon M. Encyclopedia of paleoclimatology and ancient environments

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concentrations of microspherules and noble gas ratios in fullerenes

suggestive of an extra-terrestrial source, but the case for a bolide

impact is not as strong as at the Cretaceous-Tertiary boundary.

Terrestrial plants provide evidence for mass mortality at the

Permo-Triassic boundary, in particular anomalous abundances of

fungal spores indicative of increased rates of decomposition.

Storms

Catastrophic climatic cooling by a global aerosol cloud would cre-

ate a strong temperature inversion and generally stable atmo-

sphere. However, oceanic impact of a large bolide could create a

hypercane, an intense cyclonic storm that forms over the impact

site when water temperature exceeds 50



C. Pressures in a hyper-

cane can be as low as 500–600 mbar, wind speed can exceed

Mach 1, and the storm top can extend into the middle stratosphere.

A hypercane injects large quantities of water into the strato-

sphere. This would destroy significant amounts of ozone, increase

the flux of UV radiation, and cause the death of terrestrial and

shallow-water organisms (Emanuel, 2003).

Acid rain

Acid rain changes the pH of water and soils. The most widely

proposed mechanism for the formation of acid rain is a bolide

impact. Nitric acid forms after the passage of a bolide at high

speed through the atmosphere, which creates nitrogen oxides

through intense heat and pressure. Sulfuric acid forms after

the impact vaporization of gypsum and anhydrite. The amount

of acid created by impact depends on the size and velocity of

the bolide, the mass of vaporized sulfate, and the extent

to which basic minerals such as larnite form during impact

(Maruoka and Koeberl, 2003; Toon et al., 1997).

Volcanism is a second proposed mechanism for the forma-

tion of acid rain. Rapid extrusion of large masses of lava, such

as those found in many flood basalts, would inject large quan-

tities of sulfur dioxide into the atmosphere, which forms sul-

furic acid (Wignall, 2001). This could lead to the formation

of acid rain with a sufficiently high rate of influx.

Empirical evidence for the formation of acid rain at mass

extinction boundaries is limited. Disproportionate extinction

of calcareous organisms at the Cretaceous-Tertiary boundary

is consistent with ocean acidification, but high survival of

freshwater aquatic organisms is inconsistent with acid rain

(Hallam and Wignall, 1997).

Long-term global warming

Long-term global warming results from changes in the levels of

greenhouse gases, most notably carbon dioxide (CO

) and

methane (CH

). Carbon dioxide enters the atmosphere by mantle

outgassing at spreading ridges and volcanoes. It is removed from

the atmosphere by silicate weathering and incorporation into

organic matter and carbonate rock. Methane is produced by bac-

terial reduction of CO

under anaerobic aquatic conditions. It is

oxidized in the atmosphere to form CO

. Organic carbon is

strongly depleted in

C relative to atmospheric and carbonate

because of enzymatic fractionation: carbon fixed by C

photo-

synthesis has d

C ranging from –22 to –32 per mil, while bio-

genic methane has d

C ranging from –55 to –85 per mil.

Changes in the rate of transfer of carbon between different reser-

voirs can be detected by changes in the d

C of the atmosphere,

which is recorded in the d

C of organic matter and carbonate.

Major perturbations to the global carbon cycle are indicted

by excursions in the stable isotopic record. Simultaneous

negative excursions in d

C for organic matter and carbonate

indicate an increased rate of oxidation of organic carbon and/

or a decrease in its rate of burial (Kump and Arthur, 1999),

which indicates possible global warming through an increase

in atmospheric pCO

and/or methane. Simultaneous positive

excursions in d

C for organic matter and carbonate indicate

increases in the rate of burial for organic carbon and possible

global cooling through a decrease in atmospheric pCO

. Flood

basalts from large igneous provinces can inject significant

quantities of CO

into the atmosphere if they are extruded at

a rapid rate (Wignall, 2001), but changes in the d

C of the

atmosphere are probably small. Proxies such as stomatal index

in plants corroborate trends based on isotopes and provide

independent estimates of p CO

(Royer et al., 2001).

Negative excursions in carbon isotopes and large igneous

provinces occur at the Cretaceous-Tertiary, Triassic-Jurassic,

and Permo-Triassic boundaries. Isotopic evidence for long-term

warming at the Cretaceous-Tertiary boundary is corroborated

by direct temperature proxies such as leaf physiognomy and

oxygen isotopes, direct estimators of atmospheric pCO

such

as stomatal index, and carbon cycle models (Beerling et al.,

2002; Kaiho et al., 1999; Wolfe, 1990). Post-impact levels of

pCO

may have exceeded 2,300 ppm. Simulation studies indi-

cate that rapid extrusion of Deccan Intertrappan flood basalts is

insufficient to explain the increase in pCO

and that impact

vaporization of carbonate is needed (Beerling et al., 2002).

The carbon isotopic excursion starts at the Cretaceous-Tertiary

boundary and continues well into vegetation recovery, indicat-

ing that it is controlled by marine processes rather than biomass

burning (Beerling et al., 2001).

A negative carbon isotope excursion occurs at the Triassic-

Jurassic boundary and lasts for approximately 600,000

years (Hesselbo et al., 2002). Increases in pCO

are indicated

by changes in stomatal index, and warming is indicated

by major increases in Classopollis, a thermophilic plant. The

first extrusions of the Central Atlantic Magmatic Province

began 20 kyr after the Triassic-Jurassic boundary and contin-

ued for 2 Myr.

The Permo-Triassic extinction is known as the “mother of

mass extinctions” because of its extremely high mortality

(Erwin, 1993). Not surprisingly, the carbon isotopic anomaly

at the Permo-Triassic boundary is the largest negative excur-

sion associated with a mass extinction event. Hypotheses to

account for this excursion include massive release of CO

from

an anoxic ocean, sudden release of CH

from ocean sediments,

mass mortality of the biota, volcanic release of CO

from the

Siberian traps, and reduced oxygen levels. Evaluation of these

hypotheses with a global carbon cycle model (Berner, 2002)

shows that methane release was probably the most important

contributor to the isotopic excursion. This is interesting in light

of Ryskin’s suggestion that mass mortality at the Permo-

Triassic boundary was caused by the combustion of a

methane-rich atmosphere (Ryskin, 2003). Carbon cycle model-

ing indicates a rise in pCO

> 1,000 ppm, while combined C-S

cycle modeling and sulfur isotope analysis indicate a sudden

drop in atmospheric O

from 30 to 15% at the Permo-Triassic

boundary. These results corroborate suggestions that anoxia

was an important contributor to the Permo-Triassic extinction

(Berner, 2002; Berner et al., 2003).

Summary and conclusions

The cause(s) of mass extinction is still open to debate. Environ-

mental change during mass extinctions appears to be more pro-

nounced than environmental change at other times, but whether

MASS EXTINCTION: ROLE OF CLIMATE 549

all mass extinctions have a single underlying cause is unclear.

Temperature change appears to be one common theme to

the five major mass extinctions of the Phanerozoic, but the

driving mechanism(s) can differ between extinctions. The Late

Ordovician extinction, for example, is clearly related to a gra-

dual decline in global temperatures, while the Cretaceous-

Tertiary boundary extinctions are related (at least in part) to

catastrophic temperature decline resulting from bolide impact.

Each mass extinction event probably has its own unique set

of environmental changes.

Further understanding of mass extinction will require an

interdisciplinary effort involving geologists, paleontologists,

geochemists, and geophysicists.

Garland R. Upchurch, Jr.

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Beerling, D.J., Lomax, B.H., Royer, D.L., Upchurch, G.R., and

Kump, L.R., 2002. An atmospheric pCO

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Berner, R.A., 2002. Examination of hypotheses for the Permo-Triassic

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Erwin, D.H., 1993. The Great Paleozoic Crisis: Life and Death in the

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Hesselbo, S.P., Robinson, S.A., Surlyk, F., and Piasecki, S., 2002.

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Cross-references

Bolide Impacts and Climate

Carbon Cycle

Carbon Isotope Variations Over Geologic Time

Cretaceous/ Tertiary (K-T) Boundary Impact, Climate Effects

Flood Basalts: Climatic Implications

Late Paleozoic Paleoclimates (Carboniferous-Permian)

Methane Hydrates, Carbon Cycling, and Environmental Change

Paleobotany

Volcanic Eruptions and Climate Change

MAUNDER MINIMUM

The Maunder Minimum refers to the period from about 1645 to

1715 during which very few sunspots were observed. Follow-

ing Galileo’s refinement of the telescope around 1610, observa-

tions of sunspots showed that they appeared to be a fairly

regular phenomenon. One of the main features was that their

abundances waxed and waned approximately every 11 years,

which became known as the solar cycle or sunspot cycle. Dur-

ing the Maunder Minimum, however, relatively very few sun-

spots were seen for roughly seventy years, and the 11-year

cycle was dramatically reduced (Eddy, J.A., 1976). After 1715,

the cycle built up again, and such a dramatic decrease in sun-

spots has not been seen since. When the sun is more active there

are a greater number of sunspots. Though these are dark patches

that emit less radiation than normal, their abundance is correlated

with overall greater solar output since bright spots (faculae) are

more abundant at the same times. The Maunder Minimum, with

an extremely quiescent sun, is therefore thought to have been a

low point for solar output. This may have contributed to colder

climate conditions over many regions of the Earth during this

550 MAUNDER MINIMUM

period (see Little Ice Age). The name for the minimum honors

the work of E. W. Maunder.

Other evidence agrees with that of the early astronomers.

Records of two independent cosmogenic isotopes (

Be,

C),

chemical variants formed only from cosmic rays, have been

obtained from ice cores and tree rings. Since the intensity of

cosmic rays reaching the Earth is modulated by the Sun, these

isotopes serve as a proxy for solar output. The abundance of

these isotopes varies in concert with the sunspot record over

the past several centuries during which they overlap. The isoto-

pic records corroborate the astronomical evidence for reduced

solar output during the Maunder Minimum. Based on both

the sunspot and isotope records, several other periods of

reduced solar activity have also been identified. The exact

size of the change in solar output that took place during the

Maunder Minimum (or during other historical times) is still

uncertain. Most estimates place it in the range of 0.2–0.5 watts

per meter squared less irradiance reaching the Earth’s surface.

Drew T. Shindell

Bibliography

Eddy, J.A., 1976. The Maunder Minimum. Science, 1182–1202.

Cross-references

Beryllium-10

Climate Variability and Change, Last 1000 Years

Cosmogenic Radionuclides

Little Ice Age

Sun-Climate Connections

MEDIEVAL WARM PERIOD

Histories in western Europe have long recognized a “post-

Carolingian” climatic amelioration, approximately since the

death of Charlemagne (

AD 814), the first of the “Holy Roman

Emperors” of the early Middle Ages. The Medieval Warm Per-

iod (MWP) was also called the “Early Medieval Warm Epoch”

(Lamb, 1977, 1982) and the “Neo-Atlantic” by some palynolo-

gists. The expression “Little Climatic Optimum” (LCO) has

sometimes been employed to contrast it with the dramatic

warming of the “Atlantic” phase of the late Mesolithic to Neo-

lithic ages (about 6700–4200

BC, (sidereal year)). The Atlantic

phase was long known in Scandinavia as the mid-Holocene

“Climatic Optimum” (also termed the “Hypsithermal”), when

local temperatures were 2–3



C above present (Roberts, 1998).

Lamb (op cit. p. 35) identified a broad long-term “summer

wetness index” (rising) and compared it with a “winter severity

index” (falling), based on 50-year means. Both display a sys-

tematic lengthening of the wavelength of upper westerlies

across northern Europe, reaching its peak around

AD 1100.

A comparable warming swing also marked the twentieth century.

However, analysis of this period is complicated by anthropo-

genic greenhouse warming. The two most recent warm periods

were separated by the “Little Ice Age” (LIA) from approximately

AD 1300 to 1750. The warmest interval of the MWP in Europe

was

AD 1150–1300. A study of Canadian borehole temperatures

(mentioned by Lamb, op .cit., p. 104–105) suggested that the

mean MWP was up to 1.6



C warmer than the average for the

last millennium. In the Sargasso Sea area of the North Atlantic

(see Figure M15), studies of planktonicbenthic foraminifera sug-

gest that the SST was about 1



Cwarmerthantoday1,000years

ago and above average for the last 3 millennia, about 2



Chigher

than during the LIA (Keigwin, 1996).

Human responses to the MWP have been outlined by many

writers (see LeRoy Ladurie, 1971 ; Bryson and Murray, 1977;

Bryson and Swain, 1981). The dramatic start of the LIA has

been eloquently described by Lamb (op. cit.), but less attention

has been paid to the MWP as a whole. Biological proxies

are very interesting. In central Europe history reports, incidents

of locust (Locusta migratoria) plagues in the drought summers

occurred as follows: nineteenth century, 8 seasons; tenth,

eleventh, twelfth century, none or next to none; fourteenth

century, 15 seasons; fifteenth century, none; sixteenth century,

6; seventeenth century, one only; eighteenth century, 6; nine-

teenth century, 4 summers. From recent data in North Africa,

Landscheidt (1987) suggested there was a correlation between

locust plagues and solar radiation.

Sea-surface temperatures in the North Atlantic were higher

during the MWP, (by 5



C or more), and sea-ice coverage

was appreciably reduced (Mayewski et al., 1994). Wind and

wave action were likewise reduced. In the Greenland ice cores,

a marked lowering of sodium (from sea-salt) has been

observed. These climatic factors played an important role in

the occupation of Iceland by Viking (Norse) colonists in the

nineteenth century (

AD 860). In the North Atlantic, there were

Figure M15 Variability in two proxies of sea-surface temperatures in the

Sargasso Sea over the last three millennia, showing a marked contrast

between the MWP and LIA (modified from Keigwin, 1996). (a)(top) d

O(%)

in the surface-dwelling foraminifera Globigerinoides ruber (white variety),

Bermuda Rise, northern Sargasso Sea; (b)(bottom)weight%CaCO

MEDIEVAL WARM PERIOD 551

frequent summers with calm seas and easterly winds that

greatly favored their particular type of ships (shallow draft

and square sails). Eventually, explorations were carried out by

Eric the Red and Leif Ericsson, southern Greenland was colo-

nized by the Norsemen (Dansgaard et al., 1975), and they

settled briefly (

AD 986) at Anse aux Meadows in Newfound-

land (“Vinland”), the first European foothold in North America.

During the same MWP Arctic warming came a widespread

development of Eskimo cultures, reaching as far north as

Ellesmere Land (

AD 900). From the Bering Sea, they spread

out to northern Alaska, and in Siberia, they reached as far north

as the New Siberian Islands. In the eleventh century a new

wave of migrants brought the “Thule Culture” that spread from

Alaska to northern Greenland.

In North America, native peoples spread northwards up the

valleys of the Mississippi and Missouri, bringing an agricul-

tural economy into Wisconsin and Minnesota, where pollen

studies have shown there were reliable summer rains (Griffin,

1961). Eventually they reached the northern Rockies and even

Utah. The Hohokam people were meanwhile developing agri-

culturally oriented communities in New Mexico and Arizona.

In Asia, the behavior of the Caspian Sea provides hydro-

graphic evidence of the MWP climate. From the nineth to four-

teenth Centuries, the rise of the Caspian Sea by 18 m flooded

vast areas, notably in the Volga Delta. This reflected a general

increase in precipitation over much of northern Russia. Clearly,

this also reflects increased evaporation over the warming Gulf

Stream and North Atlantic in general. The populations of cen-

tral Asia showed a general expansion, but this only reached a

crescendo in the early thirteenth century, which launched the

celebrated conquests of Genghis Khan and his “Mongol

Hordes” (

AD 1205–1225).

It was the exploration of central Asia in the first decades of

the twentieth century that provided the data base for Ellsworth

Huntington’s concept of climatic determinism (Huntington,

1924), which caused a great deal of controversy at a time when

climate was assumed (by the conservative “Establishment ” )to

be a changeless aspect of the environment. A long sediment

core obtained from Lake Saki (Crimea) illustrates the higher

precipitation levels of the MWP that also occurred in southern

Russia. The thickness of clay varves suggests a heavier rainfall

at this time (Lamb, 1977, p. 408; Xanthakis et al., 1995).

The MWP warm cycle was also seen in Scandinavia and the

Alps, by glacier retreats (Röthlisberger, 1986; Karlén et al.,

1995). Particularly in Sweden such glacier melts have been

dated precisely by sedimentation measurements in the corre-

sponding glacial lakes. Worldwide response was also marked

by eustatic sea-level indicators including higher-than-normal

beach ridges in many different regions, ranging from the Arctic

coast of Alaska to the coasts of the Gulf of California and

southwestern Florida (Fairbridge, 1992). The first radiocarbon

dates of this higher sea-level stage were obtained in Western

Australia, on Rottnest Island, and this therefore became known

as the “ROTTNEST STAGE” (Fairbridge, 1961). In Scandinavia,

it is often referred to as the “Viking Stage” due to the abundant

relics produced from the isostatic emergence around Stockholm

and Uppsala. In northern Germany, the Netherlands and Belgium

(Bennema, 1954), evidence for the higher sea level is partly

drowned by tectonic subsidence but, after correction, its level

was about 0.5 m above present MSL.

Inundation of low-lying coastal areas completely changed

the geography in some areas. As sea level rose, there was

frantic dike building in northern Germany, the Netherlands,

Belgium, northern France and in parts of eastern England.

The critical moment when man-made dikes were overtopped

in particular storms is recorded by historic disasters when tens

of thousands of people and livestock were drowned and farms

and whole villages were totally destroyed (Bakker, 1957). In the

lower Rhine Valley, river floods amplified the oceanic factors

(Berendsen, 1995). Outstanding events included the flooding of

the lower Ijssel valley in the Netherlands, with the creation

of the Zuider Zee (1250–1251, 1287), and in Britain, with the

creation of the Norfolk Broads (Lamb, 1977, p. 433). Other

areas were affected including western France, the Rhone and

Po Deltas.

The MWP (“Rottnest”) rise of sea level had an important

demographic effect inasmuch as it drove coastal dwellers inland,

creating endless conflict with the people (mostly farmers and

pastoralists) already occupying those areas. In the southern Baltic

(modern Poland and northeast Germany), southern Denmark

and the Frisian Belt in northwest Germany, post-glacial tectonic

subsidence amplified the eustatic rise of the MWP.

An economic side effect of the MWP sea-level rise occurred

in the salt industry (see a recent book, “Salt” by Mark

Kurlansky). During periods of low sea level, as during much

of the Roman Era, there were coastal salt pans with sea-water

ponds that were organized in series to permit progressive eva-

poration. This was a major industry because common salt

(NaCl) is an essential dietary condiment, as well as being in

widespread demand for food preservation, leather tanning,

etc. With the MWP rise, the salt pans were repeatedly inun-

dated and in many cases could not be expanded inland because

of low bluffs, dunes or cliffs. Industrial disaster was alleviated

by a wholesale shift inland to “fossil” salt deposits, mostly of

Triassic or Permian age, which had been long exploited since

the Bronze Age or earlier. These deposits are known to occur

in Britain, Spain, France, Germany, Austria, Poland and Russia

(Moores and Fairbridge, 1997

) (Many geographic names in

central Europe incorporate the roots Salz, Sal, Sel, etc., e.g.,

Salzburg, Salzkammergut, Salzach).

Metallic ores were extensively mined (mainly by Saxon

miners) in mountainous areas of Europe, where the Bronze Age

had developed after the glaciers had receded and exposed dry

outcrop areas. These expanded during the glacial recessions of

the MWP. With the ensuing Little Ice Age (after 1300), there

were neo-glacial advances and flooding of mines, leading to clo-

sures of the high passes in the Alps (Lamb, 1977 , p. 273–274).

Lamb (1967, 1977, p. 276 ff) was fascinated by the history

of grape vines and viticulture in Medieval Britain, a country

not particularly celebrated today for its home-grown wines.

South of lat. 53



N, this study disclosed widespread vineyards

there in Medieval times, some 500 km north of present-day

limits in France and Germany. To quote from William of

Malmesbury writing in 1150 (tr. from Latin) of the vale

of Gloucester “here you may behold highways and public

roads full of fruit-trees, not planted, but growing naturally...

No county in England has so many or so good vineyards as

this, either for fertility or for sweetness of the grape...” The

export of British wines to France and Germany was a major

factor in international trade, and even led to diplomatic friction

(when the British undersold the French). It should be recalled

that in 1066 William of Normandy had led an invasion of

Saxon Britain, at that time considered a remarkable prize. How-

ever, the Little Ice Age began and soon after 1300, the entire

wine industry collapsed within 10 years, the vines surviving

only in rare cases under the protection of walled gardens.

552 MEDIEVAL WARM PERIOD

The abruptness of the LIA onset was a major aspect of the

termination of the MWP, because it gave little leeway for

organic adaptation. In human terms, this cultural shock is well

dramatized by Tuchman (1978). Less well known is the effect

on coral reefs that had grown vigorously to a low-tide limit

about 60 cm higher than present. The fall by about 1 m created

a small cliff on reef shores, with no gradual transition.

The eastern parts of North America and Greenland seem to

have shared the European experience of the MWP equally

(though detailed histories are lacking), as did New Zealand

and some other parts of the Southern Hemisphere. However,

in the North Pacific region, China, Japan and the western part

of North America experienced the exact climatic opposite.

In Japan, the mean blooming date of the cherry trees of the

imperial gardens in Kyoto has long been monitored as the

harbinger of spring and the occasion of joyful festivals. For

the MWP, however, the mean blossoming date was a full 10 days

later than the millennial average, or 11 days when comparing the

mean for the twelfth century with that for the fifteenth century. To

Lamb (1977, p. 400) “this suggests an eccentric position of the

circumpolar vortex over the Northern Hemisphere in Europe’s

High Middle Ages, with the climatic zones persistently displaced

north over the Atlantic sector and south over the whole Pacific

sector and the Far East.”

Archaeomagnetic studies employing the paleomagnetic

orientation of baked bricks or fireplace clays indicate that the

North Magnetic Pole at the time of the MWP lay in the eastern

hemisphere, i.e., north of Siberia. A similar antiphase relation-

ship is observed during Europe’ s cold phases (Bucha, 1984,

1988), but is not always consistent.

As noted earlier, at Mediterranean latitudes, there were

commonly heavy summer rains during the MWP. In central

America, these had an important role in bringing about the

downfall of the Mayan civilization in Yucatan and elsewhere,

as the maize (sweet corn) economy requires less humid sea-

sons for ripening (Brooks, 1949); the maize culture required

rainy and dry seasons. Repeated multiyear droughts between

ca.

AD 800 and AD 900 may have contributed to the collapse

of the Classic Mayan civilization (Hodell et all, 2001; Haug

et al., 2003).

The same delicate adjustment of agriculture to seasonal

cycles may also help to explain the wonderful surge in culture

and religious buildings at Angkor in Cambodia, which reached

a maximum around

AD 1000. During the Little Ice Age, these

structures were seasonally inundated by the rise of the Mekong

and back-filling of Tonle-Sap that eventually buried the entire

complex in 10 m or more of sediment.

Northern Hemisph ere synthesis

Basing his estimates mainly on botanical proxies (palynology),

Lamb (1977, p. 404) presented a comparison between the

major climatic intervals for Britain of the last 10,000 years.

The MWP discloses the warmest annual mean temperatures

since the Atlantic stage 6,000 years ago, 0.8



C warmer than

the warmest decades of the twentieth century. The MWP was

also wetter, except for the two summer months July and

August, which were characteristically dry.

A series of Northern Hemisphere oriented maps is presented

by Lamb (1977, p. 444–5) for selected centuries, depicting

troughs of the upper westerlies across Europe and the typical

frontal depressions. For summer months during the MWP (

1000–1099), the mean polar jet swung from southern Green-

land across northern Scandinavia to the Arctic shore of Siberia.

Meanwhile the major frontal depressions radiated from northern

Greenland to east coast North America, from near Spitsbergen

to Sicily, from near Lake Baikal to western China, and from

Alaska to around Wake Island. For the same areas and season

during the LIA (1550–1599), the polar jet ran across Scotland

and southern Sweden, while the principal European frontal

depressions ran from western Spain across the British Isles

and the west of Norway.

For the winter seasons, the MWP polar jet crossed Quebec

and swung north of Iceland to the Barents Sea while its south-

ern branch crossed Spain and the Mediterranean to Syria.

The latter brought plentiful winter rains and prosperity to the

kingdom of Palmyra (Syria) and the peoples of Palestine and

Mesopotamia. Five centuries later in the LIA, the winter jet

unified and crossed Iceland to head for southern Russia, and

to bring desertification to parts of Syria, Mesopotamia, and

the Indian Peninsula (Bryson and Murray, 1977).

These mean jet trajectories help explain the antiphase rela-

tionships between climate proxies in Western Europe and the

Far East. The inferred positions of the polar vortex swing to

and fro, to eastern or western hemisphere; this appears to be

analogous, on a greatly amplified (century) scale, with the

“North Atlantic Oscillation” (NAO), which operates on a

roughly decadal basis.

[Editor’s note: More recent paleoclimate reconstructions

over the last millennium provide additional support for the

existence of a warmer period roughly 1,000 years ago, at least

over the Northern Hemisphere. Less can be said about the

Southern Hemisphere, inasmuch as reliable data are still fairly

scarce. Based on long tree-ring records from 14 sites in North

America and Eurasia, the warming of the MWP peaked

between 950 and 1045, with temperatures comparable to those

of the twentieth century prior to 1990 (Esper et al., 2002).

A composite curve derived from multi-proxy data, including

tree rings, boreholes, marine and lake sediments, and spe-

leothems, also shows a period of higher temperatures around

AD 1000 to AD 1100, followed by below average temperatures,

with a minimum around

AD 1600 (Moberg et al., 2005).]

Summary

1. The MWP (or Little Climatic Optimum) is a roughly 450-

year climatic interval, at around

AD 850–1300. Based

on historical documentation, this was a warming cycle in

Europe and eastern North America, but cooling in the

north Pacific.

2. Based on (a) CO

and temperature indicators in the Green-

land ice cores, and (b) C-14 flux (inverse) levels in the

dendrochronology, this interval is confirmed as a warming

cycle in the Atlantic realm, with temperatures at least 1



above millennial means, on average.

3. Based on astrochronology (using the 45.4-year resonance

interval of the three major planets, Jupiter, Saturn and

Uranus) there were three warm peaks: “Post-Carolingian”

(

AD 852), “Dunkerque-3” (944), and “Viking/Rottnest”

(1125). Two cool peaks (marked by extended interruptions

caused by low sunspots and reduced solar emissions)

occurred at the “Normanian” (peak:

AD 1034) and the

“Ottoman” (

AD 1307).

4. In general, precipitation rose in step with the mean tem-

perature oscillations, but most glaciers tended to retreat.

In exceptional areas, glaciers showed temporary advances.

5. Mean sea level rose and fell eustatically and caused coastal

retreat matching eustatic rise in more or less stable crustal

MEDIEVAL WARM PERIOD 553

areas, and vice versa. However, in areas of crustal subsi-

dence or rapid compaction, coasted retreat increased.

6. During much of the MWP, the general atmospheric circula-

tion (prevailing westerlies, trade winds, etc.) slackened, but

in the subarctic latitudes, the 45-year summer storminess

cycle persisted. With weakening trade winds, El Niño

cycles became more important (but detailed monitoring is

not available). However, the associated warm coastal cur-

rents are known to have reached farther south in Peru,

and farther north in California.

7. The influence of the Gulf Stream and Kuroshio Currents

were shifted northwards, together with their characteristic

ameliorating effects in maritime areas (increased oceani-

city), contrasted by reduced continentality.

8. Sea ice coverage in the North Atlantic and Barents Sea

decreased. Due to the longer ice-free seasons in areas of

abundant sediment supply, the beach ridge buildup was

anomalously high in places.

9. In subarctic lakes and swamp deposits, the level of pine

(or conifer) pollen rose in contrast to that of tundra spe-

cies. Farther south, the mixed forest boundaries moved

northwards.

10. Equatorial lakes displayed localized change in level, as did

rainfall, while savanna (monsoon) boundaries tended to

shift poleward by some hundreds of km. The Southern

Hemisphere was broadly in phase with the Northern Hemi-

sphere, but out-of-phase for short cycles.

Rhodes W. Fairbridge

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Cross-references

Climate Variability and Change, Last 1,000 Years

Dendroclimatology

Hypsithermal

Little Ice Age

Maritime Climate

North Atlantic Oscillation (NAO) Records

Paleoclimate Proxies, an Introduction

MESOZOIC CLIMATES

Introduction

The Mesozoic Era (250–65.5 million years ago) consists of the

Triassic, Jurassic and Cretaceous Periods. Each of these periods

is divided into many epochs and stages. The name Mesozoic

comes from the Greek words “meso” meaning middle and “zoe”

meaning life. The first to use the term Mesozoic was John Phillips

in 1840. It has been considered to represent a time of greenhouse

climates (e.g., Hallam, 1985), with elevated atmospheric CO

levels and higher mean global surface temperatures and is there-

fore considered as a possible analog for future climates.

Only sparse and often equivocal evidence for Mesozoic glacial

sediments is seen, whereas abundant evidence for Precambrian,

Paleozoic and late Cenozoic glaciations are recognized

(Figure M16). Changes in continental configuration throughout

the Mesozoic Era had important consequences for global climate.

554 MESOZOIC CLIMATES

At the end of the Paleozoic, as a result of the final Variscan sutur-

ing of all of the continent components, the supercontinent Pangea

(meaning “all-earth”) was formed. Very few continental fragments

escaped incorporation into Pangea and because of its sheer size it

was relatively unstable from its beginning. Hence, parts of the

supercontinent began to break away, whilst collision was occur-

ring in other areas. The break-up of the continent continued

throughout the Mesozoic and into the Cenozoic.

The proportion of greenhouse gases, and in particular CO

,in

the atmosphere of the Mesozoic has had a major control on global

climate. CO

transmits incoming short-wave solar radiation, but

greenhouse gases absorb outgoing long-wave radiation and the

trapped radiation warms the lower part of the Earth’s atmosphere.

The atmospheric CO

curve shows low values during the Carboni-

ferous, which then rise in the Permian, throughout the Triassic

and Jurassic, followed by a gradual decline through both the late

Mesozoic and Cenozoic (Berner and Kothavala, 2001). The

long-term carbon cycle reflects atmospheric CO

exchange

between the atmosphere and carbon stored in rocks. Loss of CO

from the atmosphere is accomplished by photosynthesis and burial

of organic matter in sediments and by the reaction of atmospheric

with Ca and Mg silicates during continental weathering.

The release of CO

to the atmosphere takes place by means of

oxidative weathering of old organic matter and by the thermal

breakdown of buried carbonates and organic matter (via diagen-

esis, metamorphism and volcanism) resulting in degassing to the

Earth’s surface (Berner and Kothavala, 2001). The long-term glo-

bal (eustatic) sea-level trend shows a gradual fall, from the end of

the Paleozoic to the early Mesozoic, changing in the latest Triassic

to a rising trend that continued into the middle Cretaceous. Sea

levels only begin to fall again in the latest Cretaceous and into

the Cenozoic (Figure M16). Although atmospheric levels of CO

are commonly assumed to be a main driver of global climate,

clearly longer-term changes in sea level are also strongly influ-

enced by other (tectonic) processes, the most important of which

is sea-floor spreading, which changes the volume and elevation

of ocean ridges. A first order correlation is seen between atmo-

spheric CO

and global temperature estimates.

Early Mesozoic Pangean climates

The dawn of the Triassic saw the geography of the world look-

ing quite similar to how it did during the Permian. Data suggest

that sea level rose a little from its Permian low. The combina-

tion of a huge land mass and lower global sea level (related

to the reduced rates of tectonic activity) should have resulted

in extremely arid continental climatic conditions. Sedimentary

deposits in Brazil and South Africa give us a window into the

Triassic world. Many of the sedimentary rocks deposited in

Figure M16 Phanerozoic climatic indicators and reconstructed pCO

levels. The upper set of curves describes the reconstructed histories of the

past pCO

variations (GEOCARB III) by Berner and Kothavala (2001). The shaded region depicts the uncertainties quoted within the GEOCARB

III reconstructions. Sea level data is from Vail et al. (1977) and temperature data from Merrits et al. (1998). The Phanerozoic distribution of glaciations is

modified from Hambrey (1994). A semi-quantitative measure of the extent of the Earth’s surface affected is indicated by the heights of the peaks.

MESOZOIC CLIMATES 555

Triassic terrestrial environments are dune-deposited sandstones

or oxidized fluvial sandstones and mudstones (red beds). Addi-

tionally, evaporite deposits are more extensive in the Triassic that

at any other time during the Phanerozoic. These deposits attest to

the hot and dry conditions within the interior of Pangea. Model-

ing results suggest that the global average temperatures were up

to 7



C warmer than today. Extreme temperature ranges >45



are predicted for the lowland interior of Pangea. Precipitation in

large areas of the interior was less than 0–2mmd

1

, compar-

able with modern arid and semi arid deserts.

Temperatures of the equatorial oceans may have been 5–10



warmer than those seen today, but data are scarce and are

not derived from the most reliable sources. Nevertheless, warm

conditions are thought to have extended to the poles. For example,

a combination of roots, logs, and leaves of woody plants has

been taken to be indicative of temperate climatic conditions in

Antarctica during the middle Triassic. The wood samples of

Antarctica exhibit distinct growthrings, which have also been ana-

lyzed for climate signals. Such analyses reveal that the climate was

strongly seasonal and that light may have been a limiting factor in

the growth of these foresttrees. The distributions of fossil reefs and

carbonate build-ups has also been interpreted as reflecting the pre-

sence of ancient warm water seaways analogous in terms of their

distributions to those of the present day. However, the com-

munities of organisms that constructed many ancient build-ups

were often dissimilar to the coral-rich ecosystems represented by

modern reefs. During the Late Triassic, carbonate build-ups

became both large and widespread throughout the Tethyan mar-

gins, particularly along the western and northern borders. These

were formed by corals, calcisponges, hydrozoans and algae.

The assumption that the climatic history of the Triassic was

characterized by uninterrupted hot and arid conditions has, how-

ever, been questioned. Humid zones occurred close to the coast

and at high latitudes, as shown by coals and bauxite deposits in

these areas. A number of General Circulation Model simulations

of the Triassic (e.g., Kutzbach, 1994; Wilson et al., 1994) predict

that the geography of the supercontinent Pangea was conducive

to the establishment of a “megamonsoonal” circulation (Parrish,

1993). Particularly in the late Triassic, the climate of the Colorado

Plateau region became relatively wet, though still seasonal.

Furthermore, Parrish (1993) suggests that evidence for monsoonal

conditions is apparent at higher latitude locations such as Australia

(beyond the normal influence of a monsoonal circulation), during

the Late Triassic. The strength of the monsoon must therefore have

been at its greatest at this time.

A few studies have also provided possible evidence of glacial

conditions during the late Triassic. Breccia-like conglomerates

of Triassic age containing large angular rock fragments from Japan

and alleged tills from southern South America have also been

recorded (see Price, 1999 for a summary). A glacial origin for

these deposits has, however, been strongly questioned. The possi-

ble existence of sizeable ice cover during the middle and late

Triassic, based upon evidence for high frequency Milankovitch-

influenced sea level oscillations, has been recorded in platform

carbonates from Europe. In as much as tectonic movements can-

not operate on a sufficiently rapid timescale to account for the

observed high frequency sea level changes, only glacio-eustatic

mechanisms likely controlled such sea level change. If these

sea level changes were glacio-eustatically controlled, this implies

the occurrence of a significant number of sufficiently large gla-

ciations affected global sea level during the Triassic. However,

if ice caps were either absent or negligible during the Mesozoic,

alternative mechanisms to account for the observed changes in

sea level are required, such as possibly regional tectonic events.

However, tectonic movements have generally been considered

to operate neither at a sufficiently rapid timescale, nor on a global

scale. An alternative mechanism is that changes in plate density

associated with tectonic-induced stresses could account for many

of the transgressions and regressions traceable over areas larger

than individual sedimentary basins. Other possibilities to account

for short-term global changes in sea level include the thermal

expansion and contraction of ocean water and desiccation of iso-

lated ocean basins. The General Circulation Model simulations

for the Triassic (e.g., Kutzbach, 1994; Wilson et al., 1994) using

elevated CO

concentrations (ranging from 1,000 to 1,650 ppm)

and relief reaching a maximum of 2 km also provide results con-

sistent with extreme temperature variability at polar latitudes. For

example, the model of Kutzbach (1994) simulated a mean annual

temperature of 5



C, with winter temperatures of 30



C and

summer temperatures of 25



C over parts of Antarctica. The

simulation of Wilson et al. (1994) produced summer tempera-

tures of 20



C and winter temperatures dropping to a minimum

of 50



C over Antarctica and solid permafrost zone. Although

a tract of sea-ice was simulated all year round, Wilson et al.

(1994) considered the results to be inconsistent with polar ice

sheet development.

The climate of disintegrating Pangea

Latest Triassic to Jurassic times saw the progressive break-up of

Pangea. Ocean floor was present in the central Atlantic region by

middle Jurassic times, heralding the separation of Gondwana and

Laurasia. This rifting removedthe major forcing factor of the mon-

soon, the distinctive Pangean geography (Parrish, 1993). The pos-

tulated warmth that characterized the Triassic appears to have

continued into the Jurassic (Figure M16). Global sea level began

to rise, possibly associated with the increased sea-floor spreading

as Pangea began to break up. Based on the global distributions

of lithological climate indicators such as coals, as well as evapor-

ites and eolian sands, a general symmetry of climate zones about

the paleoequator appeared throughout the Jurassic. Using litholo-

gical and floral data, Rees et al. (2000) suggested that the Jurassic

world was one in which low latitudes were seasonally dry, suc-

ceeded polewards in both hemispheres by desert, seasonally dry,

warm temperate and cool temperate biomes. Modeling studies

(e.g., Moore et al., 1992) have implied that the tropical regions

reached temperatures as high as 40



C during parts of the summer

months. Precipitation in these regions during the summer months

locally exceeded 16 mm per day. Siberian floras were assigned

to a cool temperate biome, characterized by cold winters and warm

summers, implying polar conditions considerably warmer than

today. Rees et al. (2000) did not, however, identify any geological

evidence for tropical year-round wet or, at the other extreme, tun-

dra or glacial biomes. The occurrence of a theropod dinosaur of

Jurassic age from Antarctica has likewise been used as evidence

for a warm climate in polar regions.

One of the most common methods of establishing quantita-

tive values for past ocean temperatures has been the use of oxy-

gen isotopes derived from well- preserved biogenic carbonate

material. Studies assessing Jurassic ocean temperature variation

at high southern latitudes, using isotopic ratios from belem-

nites, certainly support the notion that high latitudes were sig-

nificantly warmer than today and ice-free. Paleotemperature

estimates for the European area range from 12 to 30



C, again

considerably warmer than today. The Lower and Upper parts

556 MESOZOIC CLIMATES

of the Jurassic appear to have been the hottest periods. Such a

trend possibly mirrors the CO

curve (Figure M16). A cooler

middle Jurassic is not inconsistent with the findings of Frakes

et al. (1992 ). A great deal of information regarding the tem-

perature of Earth during the Jurassic has been gained from fau-

nal data and in particular the analysis of the patterns and degree

of provinciality. It has been widely considered that, in particu-

lar, the early Jurassic period was characterized by low faunal

provinciality, possibly due to equable climates and the lack of

polar ice caps. Faunal provinciality became more marked from

Middle Jurassic times onwards and two major faunal provinces

existed in the Northern Hemisphere, the northern Boreal realm

and the lower latitude Tethyan realm. A Southern Hemisphere

Austral realm corresponding to the northern Boreal realm has

also been recognized. The possible existence of Boreal and

Austral realms, delineated by temperature, during parts of the

Mesozoic certainly does lend support to the existence of cold

conditions at the poles for certain times.

In a similar fashion to the Triassic, General Circulation Model

simulations of the Jurassic climate (e.g., Moore et al., 1992)reveal

also that high latitude regions may have developed cold temp-

eratures providing adequate leeway to argue for the presence or

absence of high latitude ice. Siberian Jurassic tillites stretching

fromtheYenisey River eastwards to the Oloy Ridgehave also been

described (see Price, 1999, for a review). The principal character-

istics of these deposits are non-bedded, silty and pebbly claystones

that have been considered to be glacio-marine in origin or formed

by seasonal ice of mountain rivers. A recent study by Chumakov

and Frakes (1997) re-investigating some of these Jurassic deposits

suggested,however,thatsedimentspreviously attributedto iceraft-

ing display instead characteristics of mass movement, including

contorted bedding and evidence of turbidity currents.

The late Jurassic–early Cretaceous was characterized in many

areas of the world by a sea-level lowstand (Figure M16) possibly

initiated by low spreading rates, resulting in large semi-restricted

epicontinental seas and widespread deposition of non-marine

sediments. The progressive weakening of the monsoonal circula-

tion is also evident resulting from the increasing dispersion of

continental components and increasing asymmetrical continen-

tal configuration. The latest Jurassic Purbeck group of the UK,

which contains evidence of evaporite deposition, may be the local

expression of the spread of an equatorial arid zone across Europe

during these times. Clay mineral assemblages also reflect the

expansion of the arid zone northwards during the middle to late

Jurassic within the East European Platform.

The Cretaceous Period was a time of considerable tectonic

activity. The supercontinent Gondwanaland began to break up

in the early Cretaceous: India began to move northwards and

the North Atlantic began to open. Early Cretaceous freshwater

sediments and plants of Europe point to a change to warm and

humid conditions. A warm and wet climate may be a result of

the continents becoming significantly more dispersed and ris-

ing sea levels. By mid-Cretaceous times, the Tethys Ocean

began closing. Evidence points towards an unusually high rate

of volcanic activity, especially at mid-oceanic ridges. Magma

erupted at oceanic ridges and created new oceanic crust,

thereby forcing the continental landmasses apart. Rates of con-

tinental drift (i.e., sea-floor spreading rate) were then about

three times as great as they are now. During the mid-Cretaceous

(90 million years ago), sea levels rose dramatically, possibly

up to 200 m higher than present levels, flooding up to 40%

of the continents and resulting in the widespread deposition

of chalk-dominated facies. The Western Interior Seaway of

the USA was at this time at its maximum. Much of central

and western Europe was under water. Hence, mid- Cretaceous

indicators of aridity, such as evaporites, are generally restricted

to low latitudes and occur within northern Africa, South Amer-

ica, the Gulf of Mexico and also in the middle East. By late

Cretaceous times, the evaporate-producing arid zones had con-

tracted to a Mesozoic minimum (Hallam, 1985). These areas

generally correlate with areas of climate model-predicted

warmth, coupled with low soil moisture. Year-round moist-

wet conditions prevailed at higher latitudes. Extensive coal

deposits of mid-Cretaceous age are found in Northern Eurasia

and Eastern Europe. Abundant coals occur also in the USA,

particularly adjacent to the Western Interior Seaway.

In addition, during the mid-Cretaceous, vast outpourings of

lava created a succession of immense undersea plateaus beneath

the Pacific Ocean between 135 and 115 million years ago. One

of these – the Ontong Java Plateau in the southwest Pacific –

may be the largest flood basalt province in the world (roughly

the size of Alaska) and certainly led to an huge increase in hydro-

thermal activity and an increase in CO

outgassing, which was

ultimately released to the atmosphere. Extensive outgassing of

would have led to climatic warming, due to the greenhouse

effect. In addition to this increase in outgassing of CO

,the

reduced continental area (due to the sea level maximum) would

presumably also have resulted in a decrease in continental weath-

ering and a reduction in the downdraw of CO

. Therefore, the glo-

bal climate at this time should have been one of the warmest in

Earth’s history . Abundant isotopically-derived foraminiferal

paleotemperature data, gathered particularly from equatorial lati-

tudes, suggest that globally averaged ocean surface temperatures

in the Cretaceous were as much as 6–12



C higher than at present.

Although changes in equatorial regions may not have been so

marked, these data support the hypothesized “Cretaceous green-

house” and a thermal maximum occurring at 90 Ma. However,

Figure M16 shows a mismatch between peak Cretaceous warmth

(and sea level) and peak CO

production. A pulse of volcanic

activity and crustal production and hence CO

outgassing appears

absent or has yet to be identified at this time (Wilson et al., 2002).

The prospect of generally warmer tropics poses important

implications for hurricane genesis. Frequency of hurricanes

greatly increases during times when an extensive ocean area

has surface temperatures in excess of 27



C. As broad areas

of the Cretaceous oceans were probably above this threshold

temperature, in all likelihood hurricanes should have been sub-

stantially more frequent in the mid-Cretaceous than at present.

Particularly compelling evidence for polar warmth is also pro-

vided by the analysis of fossil wood and by the physiognomic

analysis of middle and late Cretaceous floras from Alaska and

northeastern Asia (e.g., Herman and Spicer, 1997). Data from

this latter study suggests that an Arctic Ocean with a winter

surface temperature of 6



C maintained Northern Alaska

coastal temperatures above freezing. Rich fern, gymnosperm

and angiosperm floras are also known from the Cretaceous of

Greenland, Siberia and Antarctica. Such warmer polar condi-

tions would have been maintained by increased atmospheric

and oceanic poleward heat transport. Significantly warmer tro-

pical oceans also provided appropriate environmental condi-

tions for enhanced evaporation and hence the production of

warm saline bottom waters. Today, oceanic deep waters form

in higher latitudes, where sea-ice formation increases the sali-

nity and therefore the density of surface water.

MESOZOIC CLIMATES 557

It is possible that the extensive outgassing of CO

would have

had other effects upon the oceans. Oxidation of reduced material

in hydrothermal effluents and the stimulation of primary produc-

tivity owing to the injection of hydrothermal Fe into surface waters

are both potential O

reducing mechanisms. Major episodes of

organic carbon rich black shale deposition within the Cretaceous

corresponded to Oceanic Anoxic Events. These have been inter-

preted in terms of increased burial of organic carbon, attributed

to enhanced preservation under reduced O

conditions, or driven

by changes in surface water productivity (that delivered more

organic carbon to the sea floor). A loss of CO

from the atmo-

sphere may well have resulted from enhanced organic-matter pro-

duction and burial in the oceans, sequestering CO

and eventually

leading to a reduction in paleotemperatures. Such trends may not

necessarily be resolvable on the atmospheric CO

curves of Berner

and Kothavala (2001).

Paleoclimate data for the early Cretaceous are derived largely

from mid-low latitudes and may not necessarily reflect ambient

climatic conditions at the poles. At least seasonally cold ocean

temperatures and more controversially limited polar ice caps have

been postulated to have existed during the early Cretaceous. Stu-

dies have revealed distinct faunal belts throughout parts of the

early Cretaceous resulting from considerable temperature gradi-

ents from north to south. These paleobiogeographic patterns sup-

port the idea of a cooler phase for parts of the early Cretaceous

and hence disagree with the suggestion of a globally warm equable

climate throughout the Cretaceous. Many studies have recorded

large and globally synchronous Milankovitch-influenced oscilla-

tions of Cretaceous sea level and fluctuating polar ice volumes

have sometimes been invoked as a primary mechanism to account

for such changes. Furthermore, Frakes et al. (1992)described

tillites and dropstones, providing at least equivocal evidence for

the presence of ice sheets. However, such data are poorly distribu-

ted in the Cretaceous. Short-term climatic cycles, related to varia-

tions in the geometry and mechanics of the Earth-Sun system (i.e.,

Milankovitch cycles), have been recognized throughout the

Mesozoic. (The Milankovitch cyclic variations include eccentri-

city of the Earth’s orbit with a periodicity of 100 kyr, precession

with a periodicity of 21 kyr, and obliquity of the axis of rota-

tion with a periodicity of 41 kyr). Small-scale rhythms, often

marl-limestone couplets within some Mesozoic sedimentary

successions, have also been attributed to orbitally-modulated

(Milankovitch) climatic change. Changes in productivity and

temperature, for example, provide mechanisms for the transla-

tion of orbital variations into cyclic sedimentation during non-

glacial times.

Towards the end of the late Cretaceous there was a decrease

of temperature and a fall in global sea level (Figure M16), reducing

the area of shallow marine environments. The close of the Cretac-

eous witnessed a major extinction event that saw the disappear-

ance of at least 75% of the species on Earth, both in the oceans

and on the continents. The most famous of these were the dino-

saurs, although they represent only a small fraction of the plants

and animals that disappeared. Although most agree that the

Cretaceous-Tertiary extinction event was caused by a catastrophic

bolide impact, climate change, continental reconfigurations and

sea level change have also been considered as complementary

mechanisms to account for the extinction events.

Summary

The Mesozoic Era has been considered to represent a time

of greenhouse climates, with elevated atmospheric CO

levels and higher mean global surface temperatures. However,

empirical observations and modeling studies do not conclu-

sively suggest high latitude ice-free environments during the

whole of the Mesozoic. The combination of the massive size

of the Pangean landmass and lower global sea level resulted

in extremely arid continental climates, especially during the

Triassic period. More humid zones did occur close to the coast

and at high latitudes, as shown by coals and bauxite deposits

in these areas. Associated with the progressive break-up of

Pangea, global sea level began to rise. Aridity was slowly

replaced by climates that were more humid during the late Meso-

zoic, although they were still punctuated by periods of extreme

aridity. This increase in continental humidity certainly appears

to have been related to an increasing maritime influence. Global

temperatures and sea levels rose, reaching a maximum at 90

Ma. The atmospheric CO

curve shows increasing values

throughout the Triassic, with a maximum in the early Cretac-

eous, whereas temperatures and sea levels were at their highest

somewhat later. Thus, mismatch between peak Cretaceous

warmth (and sea level) and peak CO

production is observed.

Gregory D. Price

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