Odekon M. Encyclopedia of paleoclimatology and ancient environments

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Cross-references

Astronomical Theory of Climate Change

Carbon Cycle

Carbon Dioxide, Dissolved (Ocean)

Carbonate Compensation Depth

Coccoliths

Diatoms

Foraminifera

Iron and Climate Change

Paleo-Ocean pH

Phosphorus Cycle

Weathering and Climate

MARINE CLAY MINERALS

Clay minerals found in the marine environment may be arbitra-

rily divided into two genetic groups: those forming in and those

being transported into marine waters and sediments. Clay

minerals forming in seawater and sediments link directly to

diagenetic processes and are not examined in detail here. Detri-

tal clay minerals can be compared to source terrain or prove-

nance and hinterland climate, so are described here.

Clay minerals forming in the marine environment com-

monly display neoformational features, as they are often sec-

ondary to some precursor. Although this neoformation can

occur in the water column or within surficial sediments, this

is patently a diagenetic process with links to adjacent sea and

land surface conditions (such as paleoclimate) that are too ten-

uous for discussion here. Such links do exist; an example is

given by Worden et al. (2000). Early (near-surface) diagenetic

clay minerals include the group of clay minerals termed glauco-

nite, which may replace shells (e.g. foraminifera) or clastic

grains (e.g. feldspars). Deep burial diagenetic marine clay

minerals are beyond the scope of this description.

Clay minerals being transported into the marine environ-

ment are detrital in origin, being transported by water, ice,

air, or volcanic eruption. The types of clay minerals are thus

variable, depending on the source terrain lithology, weathering

characteristics during erosion and transport, transport time and

method, and final depositional environment. Thus, a granitic

source terrain will contain abundant feldspars; these may be

weathered in humid conditions to form kaolinite which will

be deposited close to shore as it flocculates and forms large

clay crystals. However, at any point in this story, our assump-

tions may be incorrect! In an arid environment, kaolinite pro-

duction will be diminished, even with the correct source

lithology (granite). Clays may be deposited as floodplain soils

where they become neoformed as successor clays during

temporary storage prior to a second phase of erosion and

marine deposition that will not reflect the hinterland litho-

logy. The moral of this story is that caution is required during

interpretation of both Recent and modern marine clay mineral

assemblages, as well as the more problematic ancient materials.

The mineral structure of clays is problematic (Parker and Rae,

1998). Since clays are layer silicates, often around 2 mm in size,

we are dependent on the Scanning Electron Microscope, Trans-

mission Electron Microscope, and X-ray diffractometer in per-

forming mineralogical analysis. This article follows the United

States Geological Survey convention (http:/ /pubs.usgs.gov/of/

of01-041/htmldocs/clay.htm) of listing “groups” of minerals

where appropriate, roughly by overall abundance, examining

the interpretation possibilities and problems in each case.

The abundant clay minerals of recent and ancient

marine sediments (in no particular order)

Chlorite group

The chlorites are Mg, Al, Fe aluminosilicates of the brittle mica

suite. They originate as hydrothermal minerals, but are most abun-

dant in chlorite schists and greenschist metamorphic terrains (their

most common provenance). Their occurrence in marine sediments

as detrital components reflects this primary source terrain as the

ultimate point of their origin and a clear association with physical

weathering (as opposed to chemical weathering and thus mineral

degradation). Rapid erosionby glaciers and rivers of mountain ter-

rains are the most common “first pass” interpretation proxy for the

chlorite group. The problems with chlorites stem from their wide-

spread occurrence and robust nature. Thus, they may survive more

than one cycle or erosion, being eroded out of their primary source

terrain, and subsequently deposited and stored in a secondary sedi-

mentary rock, before being re-eroded and deposited out of context

from their original source. Chlorites may also form during burial

diagenesis, thus potentially confusing any studies of paleo-

weathering or paleoclimates.

Illite. Illite comprises a K, Al alumino silicate of the mica

group. They originate as the weathering product of feldspars

and other clay minerals, especially the degradation of muscovite.

Illites may form hydrothermally in alkaline conditions rich in K,

often as an inter-layered mineral structure with smectites (‘mixed

layer illite-smectites’). Thus, they have a similar, if lower, meta-

morphic grade origin to chlorites. A very common component of

detrital marine sediments, illite is notable usually only by its

absence or abundance. A common interpretation strategy is to

consider illite as the ever-present “clay mineral rain” of the seas,

to be used as a ratio against smectite or kaolinite in paleoweather-

ing studies. The same interpretation problems pertain as with the

chlorite group: illite is robust and may survive many sediment

cycles; thus, its original provenance is rarely clear. The crystalli-

nity of illite is often maintained during erosion, reflecting the

metamorphic grade of the original hinterland.

Kaolinite group

The kaolinites comprise hydrous alumino silicates of the ser-

pentine-kaolin group. Their primary origin is associated with

the hydrothermal alteration of feldspars and other silicates.

Humid weathering also promotes similar degradation and the

kaolinization of feldspars is common under such climatic con-

ditions and is accelerated by warm, humid conditions. Volcanic

ashes often contain kaolinite. The kaolinites form one of the

most abundant and useful paleoclimate proxy materials as their

detrital presence always suggests humid conditions at some

point in their origin. Interpretation problems are also abundant

of course! A non-feldspar-bearing hinterland terrain may be

humid and warm, yet produce no kaolinite. As mentioned

above, kaolinite is also abundant and thus its presence is not

always enough to determine changing paleoclimatic conditions.

Ratios of kaolinite with other minerals have commonly been

employed in determining horizons of paleoclimate change.

Changing ratios, when compared favorably to other paleoclimate

proxies (stable isotopes, palynomorphs) form a useful method of

identifying episodes of past climate change (Chamley, 1989).

The tendency of kaolinite to flocculate and settle close to

shore thus means that changing ratios may also reflect variable

MARINE CLAY MINERALS 539

sea-levels, not changes in paleoweathering of the hinterland.

Finally, kaolinite is a common shallow burial diagenetic mineral.

Smectite group

The smectites are variable composition Ca, Na, Al, Mg Fe alu-

mino silicates for which montmorillonite is a practical syno-

nym. Smectites originate from the weathering of basic and

intermediate igneous and meta-igneous rocks, as well as being

a common constituent in volcanic ashes (bentonites, fuller’s

earths). As a result of weathering, soils forming in temperate

and humid climates, especially where seasonally wet, com-

monly contain smectite. Thus, their proxy interpretation is

similar to that of the kaolinites, if not quite as clear. Problems

in interpreting the presence and changing abundance of smec-

tites in rock successions include the inverse problem to kaoli-

nite: commonly being a small crystal, smectite tends to stay

buoyant for longer than other clays and thus is deposited off-

shore. For this reason, it has been associated with deep marine

and transgressive conditions – the paleoclimate “signal” in its

abundance may be masked by relative sea-level changes. An

even greater hazard to smectite interpretation is its volcanic ori-

gin, as ash-falls can appear randomly in the sedimentary

record, masking any paleoclimatic control. Detailed petro-

graphic investigation of the presence of volcanic glass shards

or ash-fall textures may resolve this interpretation hazard.

Mixed-layer clays. Many of the clay mineral crystal groups

listed above and below can become interstratified. Most com-

mon are the illite-smectites and chlorite-smectites. Such clays

often have a hydrothermal origin. In soils, they form when con-

ditions are episodic, such as wetting and drying cycles. Thus,

the presence of mixed-layer illite-smectites has been related

to seasonal climates. Interpretation is hazardous, however, as

these minerals are difficult to characterize.

The less abundant clay minerals of recent and ancient

marine sediments (in no order)

For the sake of brevity, I have selected two of the many tens of

rarer clay minerals to demonstrate their use in paleoclimatic

interpretations.

Sepiolite and palygorskite

Sepiolite is part of the palygorskite group and possesses a Mg-

bearing, hydroxide chain-like silicate structure. Palygorskite is

chemically and structurally more complex. Both are of use to

paleoclimatologists, especially palygorskite as it forms in

near-surface arid conditions. If proven to be detrital and found

in marine sediments, an arid hinterland can be assumed. The

problem is its rarity and actual provenance as wind-blown dust

may be transported in the stratosphere from arid zones to those

of a humid nature, confusing the interpretation.

Alastair Ruffell

Bibliography

Chamley, H., 1989. Clay Sedimentology. Berlin: Springer, 349pp.

Parker, A., Rae, J.E. (eds.), 1998. Environmental Interactions of Clays:

Clays and the Environment. Berlin: Springer/ GmbH & Co. KG; 210pp.

Worden, R.H., Ruffell, A.H., and Cornford, C., 2000. Surface temperature,

sequence stratigraphy and deep burial diagenesis. Clay Miner., 35,

13–23.

Cross-reference

Mineral Indicators of Past Climates

MARS: WATER AND PAST CLIMATES

Mars has long been the focus of a great deal of attention,

largely due to the presence of water and the potential for har-

boring life, either now or in the past. Until recently the question

of life on Mars has focused on whether Mars was once “ warm

and wet” and Earthlike or “cold and dry” and barren like the

Moon. The recent orbiter and rover missions have revealed that

Mars is a remarkably diverse place and these simple labels no

longer fit. Mars has experienced a striking diversity of environ-

ments from extremely arid deserts, to standing water in lakes,

to a planet blanketed by snow and ice. The key questions in

the search for life have therefore become how warm, how

wet, and most importantly, for how long?

The evidence for these past environments is contained in

the rocks, minerals, and landforms of Mars. Minerals in parti-

cular contain the record of geologic processes and ancient

environments in which they formed; certain minerals only

form in water, whereas others are rapidly destroyed in wet

environments.

Mars is a volcanic planet

The first close-up images of Mars revealed a surface dotted

with massive volcanoes and provided the first hints of a planet

dominated by volcanic processes (Figure M9). However, the

definitive information on the composition of Martian rocks

has come from the visible, infrared, neutron, and gamma ray

spectrometers that have been used to map mineral and elemen-

tal compositions. Spectral data from Earth-based telescopes

and early spacecraft missions provided hints of composi-

tional differences and the presence of iron-bearing minerals

(e.g., Soderblom, 1992). The first global maps of the minerals,

rocks, and ice on the surface, as well as the dust and conden-

sates in the atmosphere of Mars were made using data from

the Thermal Emission Spectrometer (TES) infrared spectro-

meter aboard the Mars Global Surveyor (MGS) spacecraft

(e.g., Christensen et al., 2001). These observations have been

followed up by near-IR spectra from the Mars Express

OMEGA experiment (Mustard et al., 2005).

TES infrared spectra have shown that the rocks and sands are

composed almost entirely of volcanic minerals characteristic of

basaltic rocks (e.g., Christensen et al., 2001). These common

minerals– feldspar, pyroxene, and olivine – are found in the basal-

tic lavas that also cover much of the Earth, from the ocean floor

to the volcanic islands of Hawaii and Iceland. OMEGA has con-

firmed the widespread occurrence of olivine and pyroxene

over much of the surface (Mustard et al., 2005). The rocks in the

ancient cratered terrains are basaltic in composition, whereas

the younger rocks of northern lowlands have more glass, more

silica-rich minerals, and fewer iron-bearing pyroxene and olivine

minerals (Christensen, 2001; Mustard, 2005). Volcanic rocks

high in silica typically form by complex processes that include

remelting of existing basalts, fractional crystallization, or melting

of a water-rich source region. The possible existence of higher-

silica rocks on Mars may imply more water in the mantle than

on Earth, or differing temperatures or pressures at which melting

occurred. Alternatively, the high-silica components may have

been produced by minor aqueous weathering of basalt (e.g., Wyatt

et al., 2004). In either case, the vast majority of the Martian rocks

is volcanic in origin.

540 MARS: WATER AND PAST CLIMATES

Early fluvial activity

Many of the ancient, heavily-cratered terrains of Mars contain

channels that were eroded by water. Analysis of the global distri-

bution of these channels has shown that extensive fluvial activity

occurred over portions of the ancient highlands (Figure M10).

These data indicate a period of erosion early in Mars history that

produced extensive infilling of craters, the development of local

and regionally integrated drainage networks, and the formation

of deltas and alluvial fan deposits (e.g., Gulick and Baker, 1989;

Malin and Edgett, 2003;Howardetal.,2005). The development

of these drainage networks and depositional fans is suggestive of

runoff from precipitation or rapid melting of snow, possibly aided

by groundwater sapping. It has also been suggested that Mars

had extensive oceans on the basis of images and topographic

data that suggest shorelines, smooth ocean floors, and glacial

eatures (Baker et al., 1991).

The prevailing environment during the early period of flu-

vial erosion may have been one of a cold, thin atmosphere with

very limited mechanical erosion, punctuated by short periods of

thicker atmosphere and greater erosion. These thicker atmo-

spheres may have been produced by periodic climate changes

(e.g., Pollack and Toon, 1982 ), or generated in the impact cra-

tering process and lasting hundreds to thousands of years

(Segura et al., 2002). Fluvial erosion appears to have reached

a climax in the late Noachian period (3–3.5 Gyr) (Howard

et al., 2005), with only limited activity since that time that

is primarily associated with large catastrophic flood events, iso-

lated channel formation, and minor incision of small gullies

(Malin and Edgett, 2000).

A cold, dry Mars

Although there is strong evidence for early fluvial activity in iso-

lated locations, the global mineral mapping shows an ancient,

cratered surface whose original volcanic minerals are still largely

preserved, implying very little alteration by water. Olivine and

pyroxene form at very high temperatures (>800



C), but are

unstable in the presence of water at the surface as on Earth

today. However, ancient pyroxene- and olivine-rich basalts

are common; they are exposed more than 4.5 km below the sur-

face in the walls of Valles Marineris and throughout the equa-

torial plains (Hoefen et al., 2003; Hamilton and Christensen,

2005; Mustard et al., 2005). Near the summit of Syrtis Major

are a series of old, volcanic cones and flows composed of

glassy lavas (Christensen et al., 2005) that are also highly sus-

ceptible to chemical erosion. The occurrence of these unstable

minerals suggests that neither sub-surface hydrothermal activ-

ity nor surface aqueous weathering has significantly affected

these very old rocks since they were emplaced.

Figure M9 Apollinaris Patera, a 200 km wide ancient volcano, is seen in this mosaic of THEMIS infrared images. Apollinaris erupted huge volumes

of ash that have been carved into channels by water.

MARS: WATER AND PAST CLIMATES 541

The Spirit rover at Gusev Crater has provided additional

mineralogical evidence for limited aqueous weathering. The

rocks on the crater floor are volcanic and extremely old, per-

haps as much as 3 billion years, yet the Mini-TES, Mössbauer,

and Alpha Particle X-ray spectrometer (APXS) spectrometers

and cameras on the rover clearly show the presence of oli-

vine and pyroxene that have been very little altered since

these lavas first cooled (McSween et al., 2004). These pristine

volcanic rocks provide evidence that these plains have not

been exposed to any significant amount of water since these

lavas were erupted. They may have been “wetted, ” as evi-

denced by thin coatings on some rocks, but they have not been

“soaked.”

What is not found

In assessing the climate history of Mars, it is important to

consider what rock types are not present. On Earth, vast deposits

of carbonate rocks, composed of CO

2–

with calcium, magnesium,

or iron, have precipitated from warm oceans through interactions

between CO

gas, water, and mineral grains. If Mars ever had a

warm ocean, it has been suggested that thick deposits

of carbonates would have also formed through interactions

with its CO

atmosphere. Spectra from TES and the Mini-

TES on the Mars Exploration Rovers have shown that small

amounts of carbonate are present in the ubiquitous Martian dust

(Bandfield et al., 2003). These minerals most likely formed by

direct interaction with water vapor in the atmosphere. However,

no evidence of extensive carbonate rock layers has been found

by the TES, THEMIS, or OMEGA instruments (Christensen

et al., 2001; Bibring et al., 2005), suggesting that if Mars ever

had oceans, they were either too cold, too short-lived, were

covered by ice that limited interactions with the atmosphere,

or had unique chemical conditions that prevented carbonates

from forming. Clay minerals, which form primarily by aqueous

weathering and are common on Earth, have also been shown

by OMEGA and TES to be rare on Mars (Christensen et al.,

Figure M10 Evidence of fluvial processes early in Martian history is illustrated in the drainage network of Warrego Valles. This image is a mosaic

of THEMIS daytime infrared images covering an area 200 km wide centered near 42



S, 266



542 MARS: WATER AND PAST CLIMATES

2001; Bibring et al., 2005), again suggesting that Mars has

experienced little aqueous alteration.

While lakes and drainage networks clearly point to fluvial

processes, the lack of extensive carbonates and clays, together

with the presence of original olivine, pyroxene, and glass,

argue that these processes must have been relatively short-

lived. Thus, on a global scale, aqueous chemical weathering

appears to have been a relatively minor process throughout

much of Martian history.

Meridiani – a locally wet Mars

The Meridiani Planum site explored by the Opportunity rover

was chosen based on spectral evidence from the TES for

high abundances of crystalline hematite, an iron oxide that

forms by several processes, most of which involve water

(e.g., Christensen and Ruff, 2004)(Figure M11). Hematite

can form by precipitation from fluids circulating through

saturated sediments or by deposition of precursor water-bearing

iron minerals that are dehydrated over time. The mineralogic

evidence for water at Meridiani is corroborated by the unique

appearance of the hematite-rich surface in Meridiani indicative

of water deposition: the Meridiani rocks are finely layered and

easily eroded, suggesting a sedimentary deposit; they sit atop

an older, heavily cratered surface, implying a different process

of formation; and they embay pre-existing channels, suggesting

that these rocks were deposited in water rather than cover-

ing the entire landscape as volcanic ash or windblown dust

(Christensen and Ruff, 2004).

Upon landing, Opportunity’s spectrometers and cameras

confirmed the importance of water at Meridiani (Squyres et al.,

2004). Outcrops of layered sedimentary rocks were observed

by Opportunity’s cameras, and the Mössbauer, APXS, and

Mini-TES spectrometers found that these rocks contained high

(30–40%) abundances of sulfates most likely formed by eva-

poration of sulfur-rich water. The hematite occurs as 1–5mm

diameter spheres that are embedded within the rock layers,

and were likely formed by iron-rich fluids circulating through

the lake sediments (e.g., Squyres et al., 2004). The margins

of this body of water may not have extended much beyond

the current deposits of hematite seen from orbit, making this

a large lake (Christensen and Ruff, 2004), rather than part of

a global ocean. Several nearby craters also have hematite-rich

layered rocks, suggesting they may have held separate lakes

(Christensen and Ruff, 2004).

The coarsely-layered rocks exposed at the base of Burns

Cliff in Endurance Crater appear to have been formed by

migrating dunes, whereas the layers at the top of the cliff are

made of sulfate-rich sediments deposited in water (Grotzinger

Figure M11 The distribution of crystalline hematite discovered by TES in Meridiani Planum. The TES hematite abundances are shown

superimposed over a mosaic of THEMIS daytime infrared images illustrating the complex morphology of the Meridiani region.

MARS: WATER AND PAST CLIMATES 543

et al., 2005) (Figure M12). These layers may record variations

in climate from arid eolian dunes, to wet evaporating playas

where the sulfate salts were formed, to a groundwater system

that formed the hematite spheres (Grotzinger et al., 2005). This

system may have persisted for thousands of years.

Evidence for standing water is also seen at Aram Chaos, a

300-km diameter crater basin that is filled with layered rocks

that contain both hematite and sulfate and show evidence for

a lake created by the release of massive amounts of sub-surface

water (Glotch and Christensen, 2005; Gendrin et al., 2005).

Finely-layered erodable rocks that contain hematite and sulfate

are also found in the troughs of Valles Marineris, suggesting

that they also were deposited in standing water.

These discoveries provide strong evidence that water was

stable in isolated regions for brief periods of time. The factors

that caused water to accumulate and allowed it to remain at

these sites (perhaps a combination of heat from sub-surface

volcanic activity, high salt concentrations that lowered the

freezing temperature, or a protective covering of ice over water

released by catastrophic floods) are currently unknown.

Modern snow and ice

There is growing evidence that Mars has a large inventory of

water, currently in the form of ice at mid- to high-latitudes. Both

poles have deposits of ice or ice-rich sediments that are up to three

km thick and cover morethan500,000km

. Temperature measure-

ments have demonstrated that the perennial north polar cap was

water ice, whereas a layer of CO

ice remains over much of the

south polar cap even through the summer (Kieffer, 1979). Recent

measurements by the MOC, THEMIS, and OMEGA instruments

have shown that windows of water ice are exposed within the

south polar cap, indicating that water ice is present there beneath

a thin layer of overlying CO

ice (Byrne and Ingersoll, 2003;Titus

et al., 2003; Langevin et al., 2004).

A substantial inventory of water ice was discovered using

data from the Gamma Ray Spectrometer (GRS) and the

High Energy Neutron Detector (HEND) instruments on Mars

Odyssey (Feldman et al., 2002). The regions poleward of

60



in both hemispheres are made up of more than 80%

water ice (Figure M13). Such high ice abundances could not

have formed by diffusion of water vapor from the atmosphere

into the pores of the soil. Ice must have been deposited as a

layer of snow or frost.

These ice deposits are reservoirs of water that can migrate

as the climate changes. The Martian climate is predicted to

change dramatically as the tilt of the Martian spin axis oscil-

lates by as much as 20



on timescales of 10

–10

years (Ward,

1974). These changes likely produce a significant mobilization

of polar water, redistributing it to lower latitudes as snow and

ice (e.g., Jakosky and Carr, 1985). When the axis is oriented

more perpendicular to the Sun (low obliquity), the poles are

the coldest places on the planet and water migrates to them.

As the axis tilts toward the Sun (high obliquity) the poles

warm, the mid-latitudes cool, and water, in the form of snow

and ice, migrates to the mid-latitudes. Remnants of these ice

deposits can be found in unusual landforms throughout the

mid-latitudes. One type is a “basketball ” textured terrain that

only occurs between 30



and 50



latitude in both hemispheres

and was suggested to have formed as soils cemented by ice

deposited during the last “ice age’’ lost their ice and eroded

as the climate warmed (Mustard et al., 2001).

A second type of deposit occurs preferentially on cold, pole-

facing slopes (Carr, 2001). These “ pasted on” slope mantles are

up to 10 m thick, often occur in hollows, and can have features

suggestive of flow (Figure M14). These characteristics suggest

an ice-rich layer that was once more extensive but has been

removed from all but the coldest, pole-facing slopes.

One of the more remarkable discoveries made by the MGS

MOC camera is the small young gullies that also occur

between 30



and 50



latitude in both hemispheres (Malin

and Edgett, 2000)(Figure M14). These gullies have been pro-

posed to form from discharge of liquid water from sub-surface

aquifers, the rapid release of liquid CO

, dry landslides, the

melting of ground ice that percolates to an impermeable

layer, the melting of pore ice that diffused inward from the

atmosphere during periods of colder temperatures, or through

Figure M12 Burns Cliff, Endurance Crater. After descending into

Endurance Crater, the Spirit rover drove to Burns Cliff, a spectacular

rock outcrop 7 m high made of finely-layered rock. These layers are

composed of basaltic sands and evaporite minerals that were

deposited in a varying environment of wind and water.

544 MARS: WATER AND PAST CLIMATES

melting of a snow layer deposited during periods of higher

obliquity when surface ice was stable at these latitudes (see

recent review by Heldmann and Mellon, 2004).

The proposed melting of pore ice does not account for the

fact that as the surface and sub-surface temperatures warm,

the upper soil layer will become desiccated before significant

liquid water can be produced (Mellon and Phillips, 2001).

Water release from sub-aquifers can explain gully morphology,

latitudinal distribution, and slope position (Heldmann and

Mellon, 2004). The primary arguments against this model are

the presence of gullies on isolated knobs and dunes with no

obvious aquifer source. An aquifer source is a survival and

recharge mechanism that would allow these aquifers to persist to

the present, their formation only at latitudes poleward of 30



and the ability of water to remain liquid long enough to carve gul-

lies up to 2 km long under current atmospheric conditions.

In the snowmelt model (Lee et al., 2001; Christensen, 2003),

the snow acts as a greenhouse, absorbing sunlight beneath the sur-

face, but preventing the heat from being re-radiated (Clow, 1987).

This layer of snow also acts as an insulating layer that limits

evaporation of the melted water. As a result, snow can melt

10–20 cm beneath the surface and remain liquid, even when the

surface temperatures are far below freezing (Clow, 1987). In this

model (Christensen, 2003), a layer of snow is deposited at mid-

latitudes during periods of high obliquity. As the obliquity

decreases and the mid-latitude climate warms, the snow melts

and erodes gullies in the snow and underlying surface. Eventually

most of the snow completely melts, revealing gullies carved in the

hillsides. Patches of snow remain today as “pasted on” deposits

where they are protected against sublimation by a layer of desic-

cated dust/sediment. Melting could be occurring at the present

time in favorable locations in these snowpacks, and these patches

may provide fascinating locales in which to search for life, if it

exists (Christensen, 2003).

Summary – and where to look next

The new tools that have permitted a more detailed view of

Mars from orbit and the surface have shown that Mars is

remarkably diverse. The role of water has varied tremendously

at different times and in different places on Mars. There is no

doubt that Mars experienced periods of channel formation early

in its history, yet those ancient rocks contain minerals that

quickly break down in a wet environment, and also lack exten-

sive clays and carbonates. The current climate is very dry and

cold, yet the Opportunity rover landed on the floor of an

ancient body of water. Finally, liquid water is not stable at

the surface today, yet recent gullies emerge from modern aqui-

fers or beneath modern snowpacks.

How can these apparently conflicting views be reconciled?

One possibility is that the erosional periods were too brief for

extensive chemical weathering to have occurred – mechanical

erosion of channels can proceed much more rapidly than

Figure M13 The distribution of sub-surface water ice. This global map was made using data from the Neutron Spectrometer that is part of

the Odyssey Gamma Ray Spectrometer suite. Hydrogen in the form of water ice occurs in abundances >80% poleward of 60



in both

hemispheres (data are adapted from Feldman et al., 2002).

MARS: WATER AND PAST CLIMATES 545

chemical weathering. The erosional periods may have been

caused by large impacts that produced very short-lived, thick

atmospheres that allowed liquid water to persist (Segura et al.,

2002). Thus, while Mars possesses a large inventory of water, it

is likely that this water has remained frozen throughout much

of its history, resulting in very little chemical weathering. Spor-

adically this water is released to form channels and lakes.

While neither typical nor common, the locations where liquid

water may have been present for long periods of time – ancient

lakes and modern snows – nonetheless hold the greatest promise

for future exploration. In the modern gully systems, liquid

water likely occurs very near the surface, is stable for extended

periods of time, and is regenerated on the relatively short time-

scales associated with climate oscillations. Repeated access to

near-surface liquid water could provide a means for life to

have survived over extended periods of Martian history, and could

provide favorable sites for past or present life. Finally, the great

diversity of surface environments from place to place and through

time on Mars is a sign of a complex climate history. It is also is

one of the most hopeful indicators for the possibility of Martian

life, providing a rich suite of environments where life may have

taken hold.

Phillip R. Christensen

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Cross-references

Astronomical Theory of Climate Change

Atmospheric Evolution, Mars

Mineral Indicators of Past Climates

Obliquity

Sedimentary Indicators of Climate Change

MASS EXTINCTION: ROLE OF CLIMATE

Introduction

Mass extinction is a defining feature of life’shistory.Over

short periods of geologic time, geographically widespread and

ecologically diverse groups of organisms disappear from the fossil

record. The five largest extinctions of the Phanerozoic occurred

during the Late Ordovician, Late Devonian, and at the Permo-

Triassic, Triassic-Jurassic, and Cretaceous-Tertiary boundaries,

when over 40% of genera and 60% of species became extinct.

Some mass extinctions completely restructured many components

of the world’s marine ecosystems, while others caused major loss

of species but little ecological restructuring.

Major changes in the Earth’s climate system have been

proposed to explain mass extinctions and their associated geo-

chemical anomalies. Current debate focuses on the nature of

climatic change during mass extinction and whether it results from

extra-terrestrial perturbations of the Earth system or from internal

processes. Additional debate centers on whether changes in the

physical climate system or chemistry of the atmosphereand oceans

are most important. The search for the environmental cause(s) of

mass extinction is a dynamic field with a voluminous literature.

Outlined below are climatic mechanisms proposed for the five lar-

gest Phanerozoic extinctions and a few important references.

MASS EXTINCTION: ROLE OF CLIMATE 547

Gradual climatic cooling

Gradual climatic deterioration has long been cited as an agent of

mass extinction (Stanley, 1984). Extinction is caused through

the gradual contraction of ranges for tropical and subtropical spe-

cies, which causes loss of diversity through loss of habitat.Gradual

climatic cooling is commonly associated with major regression

of the world’s oceans, which shrinks habitat area for organisms

of the continental shelf. Temperature decline and regression are

causally linked through an inverse relationship between global

sea level and the volume of glacial ice, and through a direct

relationship between global sea level, plate spreading rates, and

atmospheric pCO

Gradual climatic cooling was initially proposed as the domi-

nant agent of extinction for the end-Cretaceous, based on

oxygen isotopic evidence for a gradual decline in temperature

during the Late Cretaceous and evidence for long-term cooling

at the Cretaceous-Tertiary boundary (Clemens, 1982). How-

ever, gradual cooling is contradicted by evidence for significant

temperature fluctuations during the last few million years of

the Cretaceous, warming at the Cretaceous-Tertiary boundary,

rapid extinction of organisms coincident with a bolide impact,

and mass mortality of the terrestrial biota (see below).

Stronger evidence exists for mass extinction by gradual cli-

matic cooling during the Late Ordovician (Hirnantian). The Late

Ordovician extinction occurred in two major pulses. The first

pulse occurred at the onset of global cooling and lowering of sea

level associated with the growth of continental glaciers on Gond-

wana, while the second occurred during the rise of global sea level

associated with melting of glacial ice (Sheehan, 2001). Glaciation

occurred under significantly increased levels of atmospheric

pCO

, perhaps 10-fold pre-industrial levels or greater. Glacial ice

was able to develop during a short-term (<1Myr)fluctuation

in atmospheric pCO

because of reduced solar luminosity, low

seasonality on Gondwana, and the absence of trees and shrubs

(Crowley, 2000; Gibbs et al., 2000).

Catastrophic climatic cooling

Catastrophic cooling consists of a sharp drop in temperature

that is caused by the rapid production of an optically dense and

globally widespread aerosol cloud. The lowest temperatures occur

in continental interiors, which are removed from the thermal

buffering of the ocean, and at high latitudes in the summer hemi-

sphere. The severity and duration of the low-temperature excur-

sion depends on the type and quantity of materials present in the

cloud, the optical properties of these materials, and the rapidity

with which dust, soot, and aerosols are removed from the atmo-

sphere by settling and precipitation (Covey et al., 1994; Toon

et al., 1997).

“Volcanic winter” results from the production of a global

aerosol cloud by massive volcanic eruption, in particular strato-

spheric sulfate aerosols, which cause cooling that can last for

up to a decade (Rampino et al., 1988; Wignall, 2001). Volcanic

ash enhances the radiative effects of sulfate aerosols. However,

it is removed from the atmosphere within a few months.

Historic volcanic eruptions have had minor effects on global

temperature (<1



C) but greater effects on regional temperature

at high latitudes. Large explosive volcanic eruptions of the geo-

logic past produced aerosol loading of the stratosphere at

least one order of magnitude greater than that of recent

eruptions. The potential short-term cooling effects of fissure eru-

ptions, which produce flood basalts and large igneous pro-

vinces, are not well understood (Wignall, 2001).

“Impact winter” results from the vaporization and fragmen-

tation of the bolide and its target rock (Alvarez et al., 1980).

Dust is produced in large quantities but remains in the atmo-

sphere for only a few months because coagulation of dust

particles promotes rapid sedimentation from the atmosphere.

Ignition of vegetation and formation of a smoke cloud is

caused by the impact fireball and a broiler effect resulting from

the reentry of rock fragments into the atmosphere. If the bolide

strikes gypsum and anhydrite rocks, then the resulting impact

vaporization will create stratospheric sulfate aerosols that

persist for a decade or more and cause a period of “global

twilight” following the attenuation of the dust cloud (Toon

et al., 1997). (see Bolide impacts and climate; Cretaceous/Ter-

tiary (K-T) boundary impact, climate effects; this volume).

Blockage of sunlight can produce freezing temperatures,

which kill cold-sensitive organisms. Photosynthesis ceases in

the absence of light, which causes starvation of animals and

animal-like protists. Survivorship would be highest for organ-

isms with well-developed dormancy mechanisms (e.g., seeds,

resting spores) and lowest for organisms with no dormancy mec-

hanisms. Tropical ecosystems would be highly vulnerable to the

effects of cold temperatures and the cessation of photosynthesis,

while temperate and boreal ecosystems would be less vulnerable.

Surface waters of the open ocean would be highly vulnerable to

the cessation of photosynthesis because photosynthesis is the

major source of energy for consumer organisms. Nearshore mar-

ine and deep ocean ecosystems would be less vulnerable because

of their dependence on detritus as a major source of energy.

Freshwater ecosystems would be least vulnerable because of

the high influx of detritus and widespread dormancy mechan-

isms in freshwater algae and invertebrates (Sheehan and Han-

sen, 1986; Upchurch, 1989).

Catastrophic climatic cooling caused by a bolide impact

is best supported for the Cretaceous-Tertiary boundary, where

extinction of many organisms occurred at the base of a bound-

ary clay that contains elevated levels of iridium, shock-meta-

morphosed minerals, glassy spherules, and other evidence of

impact. Terrestrial plants indicate mass mortality at the

Cretaceous-Tertiary boundary through an anomalous abun-

dance of fern spores. The Chicxulub impact structure dates to

the Cretaceous-Tertiary boundary, and remnants of the impact-

ing body have been found at the Cretaceous-Tertiary boundary

in the Pacific, North America, and Europe. Some authors pro-

pose that three impacts occurred across the Cretaceous-Tertiary

boundary based on the stratigraphic distribution of spherule-

containing beds (Keller et al., 2003).

Catastrophic climatic cooling from multiple bolide impacts

probably occurred during the Late Devonian mass extinction,

but the relationship between impact and extinction is not fully

resolved. Evidence for Late Devonian impacts includes iridium

anomalies, impact spherules, and at least two impact craters

(McGhee, 1996).

Catastrophic cooling probably occurred at or near the

Triassic-Jurassic boundary, based on evidence for a bolide

impact (Olsen et al., 2002). Evidence includes a small iridium

anomaly associated with an anomalous abundance of fern

spores. Disagreement exists over whether the impact was

linked to major turnover in vertebrates and plants. Massive

flood volcanism started near the Triassic-Jurassic boundary

(Wignall, 2001) and might have produced significant sulfate

aerosols and short-term cooling events.

Catastrophic cooling also may have taken place at the

Permo-Triassic boundary. Evidence for impact includes high

548 MASS EXTINCTION: ROLE OF CLIMATE