Odekon M. Encyclopedia of paleoclimatology and ancient environments

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Figure S18 (a) Glacial varves with dropstones, Proterozoic Gowganda Formation near Ontario (thickness shown is about 40 cm). (b) Coal bed (dark)

in West Virginia overlying an aluminous, blocky, claystone with abundant root casts interpreted as an ultisol. The coal bed is overlain by fluvial

sandstone. Pennsylvanian Kanawha Formation, West Virginia. Man is about 1.75 m tall. (c) Eolian cross-bedding formed by migration of large dunes

in a sand sea. The Jurassic Navajo Sandstone in Utah. Section shown is about 40 m high. (d) Mudstone with abundant root casts, carbonate

nodules (light blebs) and curved slickensided planes (arrows) interpreted as a vertisol. Silurian Bloomsburg Formation, Pennsylvania. Hammer is

about 35 cm long. (e) Gypsum crystals (dark) that grew upward from the sediment floor in a standing brine, now replaced by halite

(pseudomorphs). Pennsylvanian Paradox Formation, Utah. Section shown is about 14 cm thick. (f) Basal view of pseudomorphs after ikaite

(CaCO

–6H

O) now composed of calcite. Pleistocene deposits of Lake Lahontan, Nevada. Section shown is about 30 cm wide.

904 SEDIMENTARY INDICATORS OF CLIMATE CHANGE

resolution to the climate record. Problems arise where layering is

not annual, where periods of erosion or non-deposition introduce

gaps, or where non-climatic phenomena such as tides or autocyclic

mechanisms introduce patterns of change. The rise and fall of sea

level in response to the growth or melting of polar ice caps strongly

affects sedimentation patterns of marine and continental settings

(for instance, Perlmutter and Mathews, 1989; Cecil and Edgar,

2003) and represents the most noted record of climate change in

the sedimentary record.

Changes in precipitation are reflected in the fluvial record

as shifts in the scale of discharge or its frequency. However,

application of this idea to the sedimentary record has proven

difficult due to the complex response among factors such as

drainage area, provenance, and slope (see Nanson and Tooth,

1999; Blum and Torquist, 2000). Given some constancy of

the former variables, one should be able to recognize end mem-

bers of sedimentation styles that reflect frequent large floods

versus long periods of low discharge or even channel desicca-

tion. The floodplain has more potential for recording relative

climate change, expressed as shifts in vegetation and soil types.

The non-uniform distribution of indicators and the overlap of

causative mechanisms make fluvial records only suitable for

coarse climate change recognition.

Lakes, ponds, and playas are important locations for sedi-

mentary records of precipitation change because they are accu-

mulation sites dependent upon inflow. The most sensitive

record of precipitation shift is found in lakes with no outlet

(closed basins) whose depth is controlled by the balance of

inflow and desiccation. Within this setting, changes in indica-

tors of wave strength or frequency, changes in mineralogy, or

changes in the frequency or duration of desiccation can be

attributed to shifts in this balance (Figure S20 a and b). Lakes

whose size and depth are controlled by a spill level tend to

be less sensitive to variations in precipitation or at least show

less dramatic sedimentary changes. Small lakes and ponds are

more likely to be influenced by local phenomena, playas may

have long periods of non-deposition or erosion, and desiccation

features or saline minerals may be superimposed on the records

of earlier climates. Similar arguments do not apply to marine

deposits, because sea level is influenced more by ice cap size

than by precipitation. Coastal ponds and salinas may contain

a precipitation-evaporation record, but separation from eustatic

sea level controls may be difficult.

Relative changes in temperature are less easily recognized or

applied to the sedimentary record. In marine or lacustrine set-

tings, carbonate minerals are more soluble in colder water,

and changes in the biotic material deposited as sediment may

be extremely sensitive to temperature change. The difference,

however, may also be due to changes of inflow, currents, or

sediment influx that may have either no relation with or only

a complex relation with temperature. The introduction of drop-

stones or glacial flour (clay-sized ground rock) would seem to

be a good indicator of cooling, although some cases have been

documented where warming has allowed the transport of these

materials to other settings.

Ice cap growth and retreat is strongly reflected in coastal and

shelf depositional environments. Stratigraphic successions indi-

cative of this shift consist of deeper water facies grading into

wave-influenced deposits and/or subaerial exposure features

(Figure S20 c and d) or deeper water facies overlying shallow-

water or subaerial facies. Large-scale seismic profiles indicate

these sea-level shifts by onlap and offlap of nearshore facies

and by erosion when base level is significantly lowered, as

initially pointed out in the classic paper by Vail and others

(1977). The conceptual model of sequence stratigraphy pro-

duced by transgressive-regressive deposits currently dominates

marine sedimentation studies. The shifting of marine base level

is also reflected in river deposits inland of oceans through

changes in the stream gradient (for instance, Blum and

Tornquist, 2000). In deep marine basinal settings, the sedimen-

tary evidence of ocean depth is generally less easily identified

although biological evidence may be preserved within the sedi-

ments. However, some features indicating precipitation change

may also be produced by sea-level change. For instance,

increased abundance of desiccation features or saline minerals

would occur in shallower basins rather than deeper ones, inde-

pendent of aridity. Likewise, sediment discharge in fluvial sys-

tems could increase as channels incise into unconsolidated

deposits as sea level drops.

Conclusion

Abundant evidence exists in the sedimentary record showing

changes in climate. The recognition and interpretation of the

climatic controls, however, are difficult or ambiguous in many

settings. The clear impact of climate change on Quaternary to

Figure S19 Core segment of laminated sediment from Pyramid Lake,

Nevada. Dark laminae (c) are comprised of alternations of diatoms

(darker) and calcite (lighter), whereas lighter laminae (w) are comprised

of aragonite (white) and diatoms (darker). The former (c) represent

periods of cold, fresh water whereas the latter (w) represent periods of

warm, more saline water. Segment shown is about 9 cm thick.

SEDIMENTARY INDICATORS OF CLIMATE CHANGE 905

Recent sedimentary settings suggests that a climatic effect on

sedimentation should be expected. The significance of this

sedimentary impact depends upon the depositional setting and

the reliability of direct or relative indicators of climate change.

Joseph P. Smoot

Bibliography

Blum, M.D., and Tornquist, T.E., 2000. Fluvial responses to climate and

sea-level change: A review. Sedimentology, 47(Supplement), 2–48.

Cecil, C.B., and Edgar, N.T. (eds.) 2003. Climate Controls on Stratigraphy.

Tulsa, OK: SEPM (Society for Sedimentary Geology). SEPM Special

Publication No. 77, 275pp.

Fischer, A.G., 1986. Climatic rhythms recorded in strata. Annu. Rev. Earth

Planet. Sci., 14, 351–376.

Frakes, L.A., 1979. Climates Through Geologic Time. New York: Elsevier,

310pp.

James, N.P., and Clarke, A.D. (eds.) 1997. Cool Water Carbonates. Tulsa,

OK: SEPM (Society for Sedimentary Geology). SEPM Special Publica-

tion 56, 440pp.

Nairn, A.E.M., 1961. Descriptive Paleoclimatology. New York: Inter-

sciences Publishers, 380pp.

Figure S20 (a) Outcrop of cyclic lacustrine mudstone. The base of four cycles (marked by arrows) consists of laminated gray shale that grades

upward into unbedded mudstone with abundant desiccation cracks. Triassic Lockatong Formation, New Jersey. Thickness shown is about 15 m.

(b) Detail of lake cycle showing black laminated shale (dark), representing a deep lake sharply overlying a massive mudstone. Gradual upward

transition to massive mudstone (light) includes increasing frequency of desiccation features, vertebrate tracks, and root structures that indicate

shallowing of water. Triassic Bull Run Formation, Virginia. Thickness shown is about 4 m. (c) Large scale exposure of repeated alternations of

wave-dominated shoreline sandstone (light ledges) and deep water marine shales (vegetated slopes) attributed to rise and fall of sea level.

Cretaceous Book Cliffs Formation, Utah. View shown is about 50 m high. (d) Outcrop of deepwater marine shale (dark) grading upward to

shallow-water sandstone with tidal influence (light) interpreted as an indication of sea level change. Devonian Mahantango Formation,

Pennsylvania. Outcrop is about 18 m high.

906 SEDIMENTARY INDICATORS OF CLIMATE CHANGE

Nairn, A.E.M. (ed.) 1963. Problems in Palaeoclimatology. London: Inter-

science Publishers, 705pp.

Nanson, G.E., and Tooth, S., 1999. Arid-zone rivers as indicators of

climate change. In Singhvi, A.K., and Derbyshire, E. (eds.), Paleoenvir-

onmental Reconstructions in Arid Lands. Rotterdam: A.A. Balkema,

pp. 175–216.

Parrish, J.T., 1998. Interpreting Pre-Quaternary Climate from the Geologic

Record. New York: Columbia University Press, 338pp.

Perlmutter, M.A., and Mathews, M.D., 1989. Global cyclostratigraphy, a

model. In Cross, T. (ed.), Quantitative Dynamic Stratigraphy. Engle-

wood Cliffs, NJ: Prentice Hall, pp. 233–260.

Retallack, G.J., 2001. Soils of the Past. Oxford: Blackwell Science, 404pp.

Schwarzbach, M., 1963. Climates of the Past. London: D. Van Nostrand

Company, 328pp.

Vail, P.R., Mitchum, R.M., and Thompson, S. III., 1977. Seismic stratigra-

phy and global changes of sea-level, part 4: global cycles of relative

changes in sea-level. In Payton, C.E. (ed.), Seismic Stratigraphy –

Applications to Hydrocarbon Exploration. Tulsa, OK: American Asso-

ciation of Petroleum Geologists. AAPG Memoir 26, pp. 83–97.

Washburn, A.L., 1980. Geocryology. New York: Wiley, 406pp.

Cross-references

Arid Climates and Indicators

Carbonates, Cool Water

Carbonates, Warm Water

Coal Beds, Origin and Climate

Coastal Environments

Continental Sediments

Cyclic Sedimentation (cyclothems)

Eolian Sediments and Processes

Evaporites

Glacial Sediments

Glendonite/ Ikaite

Mineral Indicators of Past Climates

Paleosols, Pre-Quaternary

Paleosols, Quaternary

Periglacial Geomorphology

SNOWBALL EARTH HYPOTHESIS

The hypothesis

The snowball Earth hypothesis (SEH) suggests that the Earth

experienced surface temperatures so low that virtually its entire

surface was covered by glaciers and/or thick sea ice periodi-

cally during its early history. Such a condition has been

hypothesized for parts of the Neoproterozoic Era from about

750 million years ago (Ma) to about 600 Ma (Figure S21).

A similar frozen state has been proposed during the early part

of the Paleoproterozoic Era at about 2,300 Ma. There is little

evidence of glaciation in the long intervening period but

recently Williams (2005) presented evidence for glaciation in

the Kimberley region of Western Australia at around 1,800 Ma.

The evidence

Support for this hypothesis comes primarily from field evi-

dence, including the widespread preservation of diamictites,

which are conglomerates containing large rock fragments sepa-

rated by abundant fine grained matrix materials in which they

appear to “float.” Included rock fragments commonly have

scratched or striated surfaces. Rock fragments with planar sur-

faces are said to be “faceted.” The presence of many faceted

and striated clasts in a diamictite suggests a glacial origin.

Another glacial indicator is the preservation of large scale

striated or furrowed rock surfaces that are thought to have

formed by grinding action as rock-charged glaciers moved over

older rocks or penecontemporaneously deposited sediments.

Glacial activity is also indicated by the presence of finely

bedded (laminated) sedimentary rocks that contain isolated

large rock fragments. These fragments commonly show evi-

dence of vertical emplacement in the form of downward pene-

tration of the clast into enclosing sedimentary layers or as

“splash up” structures – upfolding of the finely laminated sedi-

mentary rocks on the sides of the fragment. Such rocks are

called “dropstones” and are thought to have been released from

melting icebergs.

These kinds of evidence have been found in Neoproterozoic

rocks on all of Earth’s continents. Rocks with similar glacio-

genic characteristics have also been described from older

(2,300 Ma) successions on several continents. The older gla-

cial deposits are best known from North America, where they

occur in the Huronian Supergroup on the north shore of Lake

Huron in Ontario, Canada. Similar rocks occur in Michigan

and bordering states, in SE Wyoming, on the west side of

Hudson Bay and in the Chibougamau area of Quebec. Paleo-

proterozoic glaciogenic rocks also occur in NW Europe, South

Africa and Western Australia. Although preserved on four con-

tinents, evidence of Paleoproterozoic glaciation is much less

widespread than for the younger Neoproterozoic occurrences.

It is not known whether this reflects an originally restricted dis-

tribution or whether much of the older rock record has been

lost as a result of subsequent erosion.

Discovery of the wide distribution of Neoproterozoic glacio-

genic rocks led Mawson (1949) and Harland (1964) to suggest

Figure S21 Distribution of glaciations throughout geologic time. Note

the large gap between the Paleo- and Neoproterozoic glaciations.

Question mark indicates uncertainty concerning the number of

Neoproterozoic glacial episodes.

SNOWBALL EARTH HYPOTHESIS 907

that the surface of the entire planet may have been frozen dur-

ing parts of the Neoproterozoic. This concept was crystallized

in the phrase “snowball Earth” by Kirschvink (1992), who pro-

posed a genetic relationship among Neoproterozoic glaciogenic

rocks, banded iron-formations (BIFs), and the snowball Earth

state. Banded iron formations are thought to be chemically pre-

cipitated sediments that typically consist of finely interbedded

(or “banded”) layers of siliceous material (chert and jasper)

and iron oxides and other iron-rich minerals. Banded iron for-

mations are common in Late Archean to Palaeoproterozoic suc-

cessions (2,500 Ma to 1,800 Ma) but are rare-to-absent from

most sedimentary successions deposited in the next billion

years. Deposition of the Paleoproterozoic BIF has been attribu-

ted to the onset of the oxygenation of Earth’s atmosphere,

beginning at about 2,300 Ma. Extensive bodies of younger

Fe-rich sedimentary rocks are unlikely because, following oxy-

genation of the Earth’s atmosphere, large amounts of Fe could

not have accumulated in the world’s oceans, since Fe (as Fe

)

is virtually insoluble under oxidizing conditions. Alternatively,

the deep oceans were not oxygenated during the period from

about 2.3 Ga to 800 Ma but rather were rich in sulfide, which

would have reacted with any dissolved iron to precipitate iron

sulfides (Anbar and Knoll, 2002). Both scenarios predict a

dearth of Fe-oxide-rich chemical sedimentary rocks. The fact

that some widespread BIFs, such as those in the Hamersley

Basin of Western Australia, appear to pre-date the oxygenation

of the atmosphere as indicated by various parameters in the

Huronian Supergroup, suggests that deposition of these iron

formations was also controlled by other (tectonic?) factors.

Kirschvink (1992), however, linked the Neoproterozoic BIFs

to the snowball Earth hypothesis. He considered that isolation

of the atmosphere (oxygenated) and oceans by ice would have

led to the buildup of dissolved Fe in the oceans as the result of

on-going hydrothermal activity at subaqueous mid-ocean ridges

and other volcanic centers. Once oxygen was re-introduced to

the oceans at the termination of the world-encircling glaciation,

iron would have precipitated.

Another line of evidence, proposed by Hoffman et al.

(1998), involves low d

C values from carbonate rocks both

below and above Neoproterozoic glaciogenic diamictites in

Namibia. They attributed these unusually low d

C values to

the near-cessation of photosynthetic activity in the world

oceans, due to the inferred widespread climatic deterioration

and development of world-encircling ice sheets and sea ice.

Carbonate rocks precipitated from the oceans inherit many

of their stable isotopic characteristics from sea water. Many

photosynthetic micro-organisms preferentially secrete the

lighter isotope of carbon (

C) so that during periods of high

biological productivity the oceans are correspondingly enriched

in the heavier isotope (

C). Thus, d

C values of carbonates

formed in such organic-rich periods would be relatively high,

and conversely these values would be low if photosynthetic

activity significantly decreased, as might be expected during a

global glaciation. Because little evidence exists to support a

significant decrease in photosynthetic activity in the Neoproter-

ozoic, Hoffman and Schrag (2002) subsequently suggested that

the low d

C values commonly found in post-glacial carbonate

rocks (so-called “cap carbonates”) are due to rapid introduction

of large amounts of CO

to the oceans from the atmosphere,

rather than due to a biological catastrophe. The ultimate source

of this CO

may have been dissociation of oceanic methane

hydrate during deglaciation, followed by oxidation to CO

(Jiang et al., 2003).

Low latitude glaciation

The angle of inclination of the Earth’s magnetic field to the pla-

net’s surface varies systematically with latitude. The field is stee-

ply inclined in polar regions whereas it is nearly parallel to the

Earth’s surface at low latitudes. Thus, if the magnetic field direc-

tion is “frozen” into sedimentary rocks at or close to the time of

their deposition then the paleolatitude can readily be determined.

There are several reports of low (tropical to subtropical) paleola-

titudes from both Neoproterozoic and Paleoproterozoic glacio-

genic rocks. In some cases, it has been shown that these low

latitude glacial deposits were laid down close to sea level. This

is very important as some glaciers occur in tropical latitudes

(e.g., on Mt. Kiliminjaro) at present but they can exist only above

the snowline – at very high altitudes. The widespread nature of

low paleolatitude magnetic signals from Neoproterozoic glacial

deposits in SE Australia has been demonstrated. This condition

was probably long-lived, for it has been shown that it spanned

times of magnetic reversals (periods when the Earth’s magnetic

field spontaneously reverses direction, so that the north pole

becomes the south). Similar low paleolatitudes are also claimed

for some Paleoproterozoic glaciogenic rocks in North America

and South Africa. These indications of ice at sea level in tropical

latitudes have been considered to provide support for the idea of

global glaciation – the snowball Earth hypothesis. Even before

such paleomagnetic evidence was widely available, Williams

(1975) suggested that the unusual rock associations in Neopro-

terozoic sedimentary successions might be explained as a result

of differences in the tilt angle of the Earth’ s spin axis. He pro-

posed that there was a significant increase in the obliquity of

the Earth’s ecliptic in the Precambrian. He noted that, with a high

obliquity (greater than 54



), tropical regions would receive less

insolation than polar regions in a given year. If the Earth under-

went cooling, the effect would have been greater at low latitudes

so that glaciation would have occurred preferentially near the

equator. Such a high obliquity would also tend to accentuate

seasonal differences throughout the globe and might provide an

explanation for evidence of seasonality in some glaciogenic

rocks that apparently formed at low latitude (see following

section).

Evidence of seasonality at low paleolatitudes

At present, strong seasonality is not observed at low latitudes. In

both Paleo- and Neoproterozoic glaciogenic successions, how-

ever, there is evidence of seasonality in sedimentary rocks that,

according to paleomagnetic results, formed in tropical regions.

This is manifested by the preservation of laminated sedimentary

rocks that closely resemble varves in younger (Pleistocene) gla-

cial sediments formed at high-to-intermediate latitudes. These

sediments are organized into couplets, each of which comprises

a relatively coarse and fine layer. The fine laminae are thought

to represent cold winter conditions when little water or sediment

entered the depositional basin and fine material in suspension

settled from the water column. The coarse layers formed during

summer months when there was abundant sediment-charged

meltwater. Many Pleistocene varves contain isolated dropstones.

Such clasts are also common in Proterozoic laminated sequences

and lend credence to their interpretation as annual deposits

formed by significant seasonal temperature changes.

Evidence of strong seasonality in Proterozoic glacial depos-

its is also provided by large structures that, in plan view, are

polygonal and in sectional view take the form of downward-

tapering wedges. These structures are typically filled with

908 SNOWBALL EARTH HYPOTHESIS

sand that exhibits laminations sub-parallel to the edges of

the structure. These structures have been described from a

number of Neoproterozoic glacial successions, including some

that formed at low paleolatitudes. They are interpreted as fossil

ice-wedge structures, which are typical of permafrost regions

today. They form as a result of thermal contraction and expan-

sion cycles and indicate strong seasonal fluctuations in surface

temperature over long periods of time. Although attempts have

been made to explain these structures as a result of other

phenomena (such as rapid advance-retreat cycles of surging

glaciers), the modern and Pleistocene permafrost examples pro-

vide the best analog.

Proble ms with the snowba ll Eart h hypoth esis

Lack of precis e age control

The ages of Proterozoic glaciations are largely unknown. There

is stratigraphic evidence (i.e., one glaciogenic formation occur-

ring on top of another) of several glacial episodes in some areas

in both the Paleo- and Neoproterozoic, but the number of gla-

ciations is still under debate. Contemporaneous glaciation of

huge areas of the planet is a sine qua non for the snowball

Earth hypothesis but it has yet to be demonstrated. Part of the

problem lies in the fact that most sedimentary rocks (including

glaciogenic deposits) cannot be dated using currently available

geochronological techniques although some dates are available

from contemporaneous volcanic rocks.

Incon sistent geochem ical trends

Many diamictite-bearing glaciogenic successions are overlain

by ca rbonate rocks (common ly dolostones). A cco r ding to

Hoffman et al., ( 1998), the cap carbonates are thought to be

the products of extreme alkalization of the oceans brought

about by reaction between the abundant CO

(that caused

destruction of the global ice cover) and fine grained material

(rock flour) produced by the grinding of rocks in the glaciers.

If this theory is correct then any sediment formed from the

residue of the putative extreme alteration should carry chemical

evidence of very strong weathering, followed by an upward

decrease in weathering intensity as the CO

content of the

atmosphere diminished. A method of quantifying the degree

of weathering of sediments and sedimentary rocks, known as

the Chemical Index of Alteration (CIA) was used by Nesbitt

and Young, ( 1982) to study post-glacial sedimentary rocks of

the Paleoproterozoic Gowganda Formation in Ontario. This

study showed the opposite trend to that predicted by the

SEH, i.e., upward-increasing CIA values following the end

of glaciation. Similar studies have not been carried out on

post-glacial siliciclastic sediments of Neoproterozoic age, but

in many cases there could be problems in interpreting such data

because of the high proportion of recycled materials (older

sedimentary rocks that have been through a previous weather-

ing cycle) and carbonate rocks (which affect the CIA).

Great stratigra phic thickn esses of Prot erozoic glacio genic

depos its

Under the snowball Earth hypothesis, it is difficult to envisage

accumulation of a thick succession of glaciogenic sedimentary

rocks, especially at low latitudes. For glaciation to reach low

latitudes, the SEH predicts a totally frozen condition, preceded

by rapid onset and followed by abrupt disappearance of glacial

ice (Figure S22). On an Earth whose surface was completely

frozen, the hydrologic cycle would virtually stop, so that the

great ice masses and large-scale ice movements required for

production of thick sedimentary successions are unlikely. Many

glaciogenic successions, in both the Paleo- and Neoproterozoic,

are several km in thickness. It has been suggested by propo-

nents of the SEH that ablation of ice at low paleolatitudes

might provide sufficient hydrologic activity but without access

to a huge reservoir of water (such as the world oceans). The

production of great volumes of glacial sediments remains pro-

blematic. It is further hypothesized that much of the continental

lithosphere was situated at low paleolatitudes so that in tropical

regions, where most ablation would have occurred, there

should only have been a thin ice cover and therefore limited

ablation (as opposed to having the entire ocean as a moisture

source). However, a low paleolatitudinal distribution of conti-

nents may not have existed for all snowball Earth glaciations.

Rapid onset and demi se of snowbal l glaciatio ns

Onset of the “snowball Earth” condition has been explained as

the result of a “runaway albedo ” feedback. If glaciation initiates

at the poles and gradually descends to lower latitudes, a thresh-

old occurs beyond which there is rapid lowering of surface

temperatures and the entire surface of the planet becomes fro-

zen, due to a positive feedback mechanism. Ice reflects solar

energy more efficiently than land or water; therefore, as more

of the Earth’ s surface becomes ice-covered, it becomes pro-

gressively cooler until ice spreads rapidly to cover even low

latitude regions of the planet (Hoffman, 2000). Once the entire

planetary surface is frozen, the problem becomes one of finding

a mechanism to release it from its frozen state. It was proposed

(Kirschvink, 1992; Hoffman et al., 1998) that the Earth’s sur-

face was re-warmed through buildup of atmospheric CO

from

ongoing volcanic eruptions. This would have been accentuated

by the absence of normal CO

drawdown from rock weather-

ing, which would have ceased during the snowball state

because of the widespread ice cover and low temperatures.

Thus, the SEH invokes sudden (a few thousand years) onset

and demise of glaciations, separated by much longer (several

millions of years) glacial periods (Figure S22).

Figure S22 Proposed relationships between geologic time and surface

temperature during a snowball Earth period (modified from Hoffman,

2000). Note the rapid growth and destruction of ice sheets.

SNOWBALL EARTH HYPOTHESIS 909

In the Paleoproterozoic Huronian Supergroup, however,

there is sedimentological and geochemical evidence that the

formations both below and above widespread glaciogenic rocks

of the Gowganda Formation testify to a very gradual deteriora-

tion of climate before the glaciation and slow amelioration at

the end. Likewise, in the Neoproterozoic of the Adelaide geo-

syncline in South Australia, Sturtian glaciogenic successions

are overlain by thick (up to 1 km) accumulations of mudstone

with abundant dropstones (Young and Gostin, 1989). Such

occurrences testify to a long-lived, gradual amelioration of cli-

mate, with abundant floating glacial ice, in contradiction to the

rapid amelioration implicit in the SEH. A less-than-catastrophic

end to glaciation is also supported by recent evidence for multi-

ple geomagnetic reversals within a cap carbonate, over a span

of at least hundreds of thousands of years.

Origin of banded iron-formations associated wit h

Proterozoic glaciogenic deposits

As noted above, the association of banded iron formations

(BIFs) with some Neoproterozoic glaciogenic deposits led

Kirschvink (1992) and others to propose that there was a

genetic link with the snowball Earth condition, with BIFs

depositing once deglaciation began. This relationship has also

been proposed for the Paleoproterozoic but in the best-known

basins, such as the Huronian of Ontario, glaciogenic rocks

and Superior-type BIF are separated by about 300 Ma. Like-

wise, in the Hamersley basin of Western Australia, glacial

deposits formed after deposition of major BIF, and not before,

as predicted by the SEH. In contrast, there is a much more inti-

mate association between BIF and glacial deposits in the Neo-

proterozoic. For example, ice-rafted dropstones are present in

some laminated iron-formations, clearly attesting to the con-

temporaneity of glaciation and deposition of iron-formation.

This relationship was attributed by some to precipitation of

Fe concentrated in the world’s oceans as a result of isolation

of the atmosphere from the oceans during a snowball Earth per-

iod. There are, however, problems with this interpretation. The

distribution of Neoproterozoic BIF is much more sporadic than

that of the glaciogenic rocks. If deposition of the BIF were the

result of global processes in the oceans, they should be much

more common. In some cases (e.g., in the Rapitan Group of

the northern Cordilleran region of North America), huge depos-

its of BIF occur below rather than above the glaciogenic diamic-

tites. This is significant because the theory predicts that Fe

should be released after the destruction of the oceans’ ice cover.

Many of the Neoproterozoic BIF occurrences formed in rift

basins. This is shown by the presence of strong facies and thick-

ness changes (Yeo, 1981; Young and Gostin, 1989) in the glacio-

genic successions and the association, in some basins, with

volcanic rocks. Furthermore, the geochemistry of the BIF sup-

ports a hydrothermal origin, which is compatible with the gla-

ciated rift hypothesis for their origin (Yeo, 1981). This theory

proposes that the iron was produced by hydrothermal circulation

of seawater in restricted Red Sea-type rift basins. Iron-charged

brines were moved and mixed with “normal” seawater and melt-

water as a result of overturn caused by water movements related

to release of meltwaters from glaciers that debouched into the

basin from surrounding rift shoulders. Experimental studies

show that such dilution and oxidation could lead to precipitation

of dissolved metals such as Fe. This mechanism provides expla-

nations for the hydrothermal nature of the BIF, their relatively

restricted occurrence, fault-related facies and thickness changes

and the intimate relationship with glaciogenic deposits.

Conclusions

Glaciogenic rocks are widely preserved in supracrustal succes-

sions of early Paleoproterozoic and late Neoproterozoic age

and sparse in the long (1,500 Ma) intervening interval of geo-

logic time. This evidence suggests that the Earth went through

significant climatic perturbations near to the beginning and end

of the Proterozoic Eon. These perturbations in the Earth’s sur-

ficial environments may have provided the impetus for signifi-

cant evolutionary changes in the biosphere, especially near the

end of the Proterozoic Eon when the first animals emerged.

Whether the Earth’s surface achieved a totally frozen condition

(the snowball Earth hypothesis) during the great Proterozoic ice

ages remains a possibility but currently available evidence is by

no means conclusive.

Grant M. Young

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Nesbitt, H.W., and Young, G.M., 1982. Early Proterozoic climates and

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Cross-references

Albedo Feedbacks

Atmospheric Evolution, Earth

Banded Iron Formations and the Early Atmosphere

Carbon Isotopes, Stable

Carbonates, Cool Water

Diamicton

Glacial Sediments

Glaciations, Pre-Quaternary

Ice-Rafted Debris (IRD)

Obliquity

Periglacial Geomorphology

Proterozoic Climates

Varved Sediments

Weathering and Climate

910 SNOWBALL EARTH HYPOTHESIS

SPECMAP

SPECMAP – ma jor goals and achievements

In the 1980s, the international SPECMAP project (SPECtral

MAping Project) was launched with the aim of producing

continuous time series of Ice Age climate recorded in deep-

sea sediments, and studying their spectral properties. One of

SPECMAP’s first, and most referenced, achievements was to

publish an astronomical time-scale for the upper Pleistocene

(last 780 ka) based on an oxygen isotope reference curve con-

structed from stacking five planktonic foraminifer isotopic

records from low- and mid-latitudes (Imbrie et al., 1984). This

stacked curve, phase-locked (tuned) to obliquity and precession

orbital oscillations, provided a continuous geological time scale

for the upper Pleistocene, accurate to within 5 kyr. A few years

later, Martinson et al. (1987) developed a 0–300 ka astronom-

ical time scale transferred to a high-resolution oxygen stable

isotope reference curve constructed from stacking benthic fora-

minifer records (Pisias et al., 1984). Four different tuning proce-

dure approaches were followed, each of which was based upon

different assumptions concerning the response of the orbital sig-

nal recorded in the data. The final chronology has an average

error of 5 kyr.

The SPECMAP astronomical time scale based on stable

oxygen reference curves provided a powerful stratigraphic tool

to tie deep-sea climatic records within a common temporal

framework with the precision needed to analyze the response

of the Earth external system to orbital cyclic modulation of

solar radiative forcing. For the first time, amplitude and phase

of cycles in different climatic proxy records could be compared

at different sites with the geographic pattern of insolation

changes. This information provided insight into processes

responsible for climate change on glacial-interglacial time

scales (i.e., Imbrie et al., 1989).

Background: th e astronomical theory of Ice Age

About 2.6 million years ago, Earth climate entered the so-

called Ice Age, with its large amplitude glacial-interglacial

cycles characterized by the quasi-periodical waxing and wan-

ing of large continental ice sheets from the Northern Hemi-

sphere. In the beginning of the nineteenth century, the first

geological evidence of these major Quaternary glacial episodes

came from observations by Louis Agassiz of sediment deposits

(moraine) and erosion features attributed to ancient glaciers in

the Jura mountains of western Europe. By the middle of the

nineteenth century, the French mathematician Joseph Adhémar

and the Scottish geologist James Croll had suggested that past

climates might have responded to cyclic changes in the orbital

parameters of the Earth. In the 1920s and 1930s, the Serbian

mathematician Milutin Milankovich produced time-series of

the amount of solar energy received by the Earth at different

latitudes and different seasons. Based on these data, he pro-

posed an orbital theory of Ice Age in which changes in the

intensity of the seasons in the Northern Hemisphere controlled

the waxing and waning of northern high latitude ice sheets. In

his astronomical theory of climate, the Northern Hemisphere

high latitude summer temperatures hold the key to the onset

of glaciations.

Within a few decades, however, the astronomical theory

of climate was largely disputed, with discussion based on

fragmentary geological records supported by incomplete and

frequently incorrect radiometric age data. From the 1960s

onwards, access to continuous and longer oceanic records

revealed many more than the four glacial periods recognized

from the terrestrial record in Europe, providing more support

for the astronomical theory of climate. In 1976, Hays and co-

authors published a landmark paper in which they convincingly

demonstrated that all the orbital frequencies predicted by the

astronomical theory of climate could be found through spectral

analysis of climate sensitive indicators from deep-sea records

(Hays et al., 1976).

The SPECMAP astronomically derived timescale

In order to analyze the geographical propagation and time-

dependence (lead/lag) of various climatic responses to seaso-

nal and latitudinal insolation changes, age-models developed

from a few, radiometrically-controlled stratigraphic tie-points

are not precise enough. With the convincing validation of the

astronomical theory of climate by Hays et al. in 1976, Earth

sciences were in need of continuous timescales that would

allow cross-correlation and aging of deep-sea sedimentary

records to be performed with a precision better than a frac-

tion of Earth’s orbital periods. To achieve such a precision, var-

ious authors suggested that a timescale could be developed

from deep-sea sedimentary records by using orbitally-related

oscillations of paleoclimatic proxies as internal “pacemakers”

(i.e., Morley and Hays, 1981). The so-called astronomical (or

orbital) “tuning” approach for developing timescales rests upon

the phase-locking of those orbitally-related oscillations in sedi-

mentary records to primary orbital oscillations, namely the

climatic precession (with periods of 19–23 kyr) and the obli-

quity of the Earth’s axis (with a main period of 41 kyr).

To get a global, marine stratigraphic framework on which to

develop an upper Pleistocene astronomical time scale, Imbrie

et al. (1984) selected five planktonic foraminifer oxygen isoto-

pic records from low- and mid-latitudes in which globally per-

sistent isotopic excursions could be graphically correlated.

Over the Ice Age glacial-interglacial cycles, global sea water

oxygen isotopic excursions recorded in the foraminifer calcite

reflect the waxing and waning of continental ice sheets, enriched

in the light

O isotope relative to seawater. The general circula-

tion of the world ocean (the so-called Great Conveyor Belt

depicted by Broecker in 1987) insures that isotopic changes

related to variations in ice-sheet volume are spread globally over

the world’s ocean water masses within a few hundred years.

An initial, radiometrically-controlled age-model was devel-

oped for each of the five isotopic records, using few strati-

graphic levels: core tops (with a zero age); two isotopic

events in stage 2 with

C ages of 18 and 21 ka; the 6.0 isotopic

state boundary, taken as 127 ka; and the Brunhes-Matuyama

magnetic reversal, taken as 730 ka. The astronomical tuning

procedure applied by Imbrie et al. (1984) iteratively modified

the initial depth-age models to increase the coherency and

phase lock between filtered 19–23 kyr

1

and 41 kyr

1

oscilla-

tions of the oxygen isotopic records on the one hand, and the

precession and obliquity components of the Earth’s orbital var-

iations on the other. Such an orbital tuning approach assumes

constant phase lags at the primary Milankovitch frequencies

between the marine oxygen isotopic variations and Earth’s

orbital changes. Those phase lags had been determined in an

earlier paper (Imbrie and Imbrie, 1980) by adjusting the para-

meterization of a nonlinear (exponential) climatic model pre-

dicting ice volume variations, so that the model output would

match a radiometrically-dated 130 kyr long isotopic record.

SPECMAP 911

The five astronomically-dated isotopic records were finally

normalized and stacked, and the resulting curve smoothed with

a 9-point Gaussian filter to produce a reference stratigraphic

record (Figure S23). The rationale for the stacking approach

is that global isotopic changes related to ice sheet waxing/wan-

ing would be enhanced in the resulting composite curve,

whereas local or regional isotopic variations would tend to

cancel out. Figure S24 shows the comparison of filtered

Figure S23 (a) Normalized variations in d

O as a function of astronomically-tuned ages in the five deep-sea cores used to develop the SPECMAP

reference curve. (b) Resulting oxygen isotopic reference curve obtained after stacking the 5 records and smoothing the resulting record

using a 9-point Gaussian filter (figure modified from Imbrie et al., 1984).

Figure S24 Solid lines show variations in obliquity (a) and precession (b) and the corresponding frequency components extracted by band-filtering

the d

O stack record (dashed line) over the past 780 kyr (figure modified from Imbrie et al., 1984).

912 SPECMAP

precession- and obliquity-related oscillations from the stacked

isotopic record with obliquity (upper panel) and precession

(lower panel) orbital oscillations.

The geological timescale obtained by Imbrie and co-workers

for the upper Pleistocène is accurate to within 5 kyr (Imbrie

et al., 1984). Since its publication, the orbitally-dated isotopic

stacked record has been used in hundreds of paleoceanographic

studies as a reference curve for developing age-models and

tying marine sedimentary records into a common stratigraphic

framework.

Three years after the landmark paper by Imbrie et al. (1984),

Martinson et al. (1987) developed an orbital timescale

for the last 300 ka that they transferred to the stacked,

benthic oxygen-isotope stratigraphy from Pisias et al. (1984;

Figure S25). Martinson et al. (1987) followed four different

tuning strategies. This made it possible to test the robustness

and accuracy of astronomical tuning for developing an upper

Pleistocene time scale. The error measured by the standard

deviation about the average of their four chronologies has an

average magnitude of 2.5 kyr. Transferring the final chronology

to the stacked isotopic record of Pisias et al. (1984) leads to

additional errors. The final chronology has an average error

of 5 kyr. Comparison of the younger portion of the chronol-

ogy of Imbrie et al. and the 300-kyr chronology of Martinson

et al. shows that ages of isotopic events agree to within the

error bars.

Following the initial SPECMAP effort, astronomical time-

scales were developed for the Middle and Early Pleistocene

periods (Raymo et al., 1989; Ruddiman et al., 1989). It also

became increasingly clear that orbital control of climate was

not restricted to the Ice Age. This allowed the astronomical tun-

ing approach to be applied to older periods of the Earth history,

reaching back to the Neogene (Shackleton et al., 1999).

Limits of the astronomical calibration approach

Orbital tuning provides a global reference standard for dating

long, continuous sediment records. However, the approach

is not without its problems. Subsequent revisions of the

0–780 ka SPECMAP timescale showed, for instance, that sev-

eral oscillations predicted by the astronomical theory of climate

(in isotopic stages 17 and 18) were apparently missing in the

stacked record (Bassinot et al., 1994). Their absence is related

(a) to the incompleteness of some of the isotopic records that

reached those stages, and (b) to an erroneous Brunhes-

Matuyama (BM) magnetic reversal age assignment in the

age-model developed at the first step of the Imbrie et al.,

1984 tuning approach.

Ar/

Ar radiometric dating of the

Brunhes-Matuyama magnetic boundary gives an age of about

780 ka (Baksi et al., 1992), significantly older than the K/Ar

age (730 ka) available in the early 1980s, and used by Imbrie

et al. (1984) to constrain their initial age-model. Although the

time constraint at the B/M boundary was removed midway

in the orbital tuning approach of Imbrie et al., it is quite clear

that the final age-model suffered from this erroneous tie-point.

Applying astronomical tuning strategies with a higher de-

gree of freedom (i.e., no initial age assignment to the B/M

boundary) on carefully selected, high-resolution isotopic re-

cords, Shackleton et al. (1990) and Bassinot et al. (1994)

obtained astronomical ages for the Brunhes-Matuyama mag-

netic reversal that are in good agreement with

Ar/

Ar radio-

metric ages. This example illustrates the fact that theoretical

precision of astronomical time scales (usually a few thousand

years) may actually mask potential, larger uncertainties related

to the tuning strategy and/or to incompleteness of paleocli-

matic records used to provide the stratigraphic framework on

which the astronomical tuning is conducted.

Analyzing the Ice Age record within the framework

of the SPECMA P time scale

Using their astronomically dated oxygen isotopic stratigraphy,

the SPECMAP group produced a research effort to determine

how orbital signals are transferred into climatic responses dur-

ing the upper Pleistocene part of the Ice Age. The rigorous and

robust strategy they developed was based upon partitioning the

climatic response at different locations of the world ocean into

its frequency components (Imbrie et al., 1989). This strategy

allows analysis of three major aspects of the Earth’s climatic

response to orbital changes in insolation forcing:

1. Examination of climate spectra makes it possible to separate

between linear and nonlinear responses to radiative forcing.

Many paleoclimatic records indicate that there exist direct,

linear responses of Earth’s climatic sub-systems to insola-

tion forcing in the precession and obliquity bands (Imbrie

Figure S25 The 0–300 ka time scale developed by Martinson et al. (1987) and transferred to the stacked, benthic oxygen isotopic reference

curve of Pisias et al. (1984). Isotopic stages are indicated on the bottom of the figure. Major isotopic events are shown along the isotopic curve,

following definition by Pisias et al. (1984) (figure modified from Martinson et al., 1987).

SPECMAP 913