Odekon M. Encyclopedia of paleoclimatology and ancient environments

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culprit. More importantly, it is difficult to say where all the

carbon from those gases has gone if it has been locked up in

the crustal reservoirs and withdrawn from the atmosphere.

There are limited coal deposits of middle-late Eocene age, but

none on the scale of the Carboniferous coals that locked up

the greenhouse gases and shifted the late Carboniferous world

from greenhouse to icehouse. The volume of carbonate precipi-

tation in the middle Eocene is insufficient to serve as the chief

reservoir, either. Some have suggested that these gases are

locked up in methane hydrates on the seafloor (as discussed

above), but so far there is no way to test this hypothesis.

The Oligocene

The Eocene-Oligocene boundary is formally recognized by the

extinction of the hantkeninid foraminifera, although no other

major climatic or extinction events occurred at this time.

This invalidates the old idea from the 1970s and 1980s that a

“Terminal Eocene Event,” comparable to the event that ended

the Cretaceous, marked the end of the Eocene.

The most significant climatic event occurred in the earliest

Oligocene (as currently defined, using the hantkeninid criter-

ion), about 33 million years ago (now known as the “Oi1

event”). In the marine record from both benthic and planktonic

foraminifera, the oxygen isotopes show about a 1.3% increase.

Miller (1992) calculated that about 0.3–0.4% of the change

was due to a major expansion of Antarctic ice sheets, which

lowered global sea level by at least 30 m. The remaining

0.9–1.0% is explained by global cooling of about 5–6



which lowered global mean temperature from as high as



C in the early Eocene and 7



C in the latest Eocene to

values just a few degrees above freezing (as a global average –

the poles were well below freezing for the first time, while

the tropics remained relatively unchanged).

Abundant evidence suggests that this global cooling event

was largely due to the growth of the first major Antarctic ice

sheet since the Permian, over 250 million years ago. Drilling

on the margin of the Antarctic continent and in oceanic

plateaus in the Southern Ocean (such as Maud Rise and the

Kerguelen Plateau) has produced abundant evidence of the

growth of the Antarctic ice sheet. Not only do the isotopic

records show its effect, many of the sediments drilled from

the Antarctic margin are glacial in origin, and even well out

into the Southern Ocean where there are sediments dropped

by melting icebergs.

What caused this global cooling and the extinctions in the

early Oligocene? A few geologists have suggested that the late

Eocene impact events or major volcanic eruptions in the Ethio-

pian Plateau might have had an effect, but these ideas are con-

tradicted by the geological record. As noted above, the impacts

occurred in the middle of the late Eocene, about 2 million

years before the early Oligocene cooling, and 2 million years

after the end-middle Eocene cooling. Likewise, the Ethiopian

traps are now dated in the late Oligocene, when no signifi-

cant extinctions are recorded. As discussed above, the primary

culprit has been identified as the development of the Circum-

Antarctic Current.

So what triggered the growth of this current? The most

obvious factor is plate tectonics. In the late Cretaceous, Australia

and South America were still attached to Antarctica as rem-

nants of the ancient Gondwana supercontinent. As noted above,

this caused the tropical currents to mix with polar currents

and moderated global temperatures, since there were barriers

between the Atlantic, Pacific, and Indian Oceans, and there

was not yet a global Southern Ocean. Geophysical evidence

shows that these three southern continents began their separa-

tion in the latest Cretaceous, but they were not far apart enough

to allow ocean currents to pass through until the latest Eocene

or early Oligocene (Exon et al., 2002). By the early Oligocene,

deep-sea cores south of New Zealand reveal that a blast of cold

water was passing between Australia and East Antarctica. As

the Oligocene progressed, the separation grew wider, and larger

and more powerful cold-water currents developed. Originally,

geologists thought that the separation between the tip of South

America and the Antarctic Peninsula did not occur until the end

of the Oligocene, but recent evidence (Diester-Haass and Zahn,

1996) has suggested that it was also occurring in the early Oli-

gocene, so the entire Circum-Antarctic Current developed in a

short period of time.

In addition to these important currents, it is also thought that

another body of water, the North Atlantic Deep Water, which

flows out of the Arctic Ocean past Greenland into the bottom

of the North Atlantic, originated sometime in the Oligocene

(Davies et al., 2001). Thus, the global “icehouse” conditions

of the Oligocene can be largely attributed to the development

of modern oceanic stratification and circulation patterns.

The effects of these global changes in oceanic temperatures

are critical, not only to marine climates and organisms but

also to land climates. The most complete record comes from

the North American continent, where paleobotanical records

(Wolfe, 1978, 1994) show that mean annual terrestrial tem-

peratures dropped 7–11



C in the earliest Oligocene. This is

true of floral records all the way from Alaska and the Pacific

Northwest to the Gulf Coast. In addition to this rapid cooling,

the record of ancient plants and soils also suggests that the

continent underwent significant drying, with much more seaso-

nal, drought-prone climates. In the Big Badlands of South

Dakota, late Eocene paleosols suggest over a meter of annual

rainfall, supporting a dense forest (Retallack, 1983). By con-

trast, in the early Oligocene, mean annual rainfall was less

than half a meter and the vegetation was patchy scrubland with

limited riparian forests. The land snails from the Badlands

also change, from late Eocene forms like those found today

in tropical Central America, to early Oligocene forms, which

are smaller and more drought-tolerant, and found today in Baja

California. In addition, the late Eocene reptilian fauna that was

dominated by crocodilians and pond turtles was replaced by

dry land tortoises in the early Oligocene.

Once the early Oligocene climatic deterioration was com-

pleted, the earth remained in this “icehouse” mode through

the rest of the Oligocene. The other significant event was sev-

eral pulses of glaciation during the middle part of the Oligo-

cene, about 30 million years ago (the “Oi2” event). Thick,

extensive mid-Oligocene glacial deposits are found throughout

the Antarctic region, and benthic foraminiferal oxygen isotopes

shifted by 1.6%, suggesting another increase in ice volume and

drop in global temperatures. As these ice sheets grew, they

pulled water out of the oceans, resulting in the largest drop of

sea level in the past 100 million years. Originally, it was sug-

gested that sea level dropped almost 150 m, although more

recent estimates suggest it was only about half that much.

Whatever its magnitude, it had a major effect on the shallow

marine realm, causing the continental shelves to be deeply

incised once they were exposed to the aerial erosion, and pro-

ducing huge mid-Oligocene unconformities in most marine

rocks around the world.

PALEOGENE CLIMATES 731

The effect of the middle Oligocene cooling and regression

on land climates was less obvious. The sensitive tropical floral

elements were already gone by the mid-Oligocene, so the land

plant record shows only minor cooling effects. The record of

ancient soils from the Big Badlands of South Dakota shows

that, as the climate became cooler and much drier, sand dune

deposits became common in the mid-western United States in

the late Oligocene (Retallack, 1983). These same soils suggest

that vegetation was a mixture of scrublands and grasses, with

few trees, by the late Oligocene.

As the climatic deterioration of the Eocene and Oligocene

began, the total diversity of land mammals or marine organisms

decreased significantly from Eocene levels, reaching an all-

time low in the late Oligocene. The forests and jungles of the

early Eocene were rapidly disappearing by the late Eocene, so

that by the Oligocene most of the temperate latitudes were cov-

ered by a mixture of forest and scrubland vegetation. Along

with the change in vegetation triggered by this cooling and dry-

ing was a major change in many of the land organisms. In this

realm lived a land mammal fauna dominated by primitive

members of mostly living families. These included three-toed

horses (which began to radiate into multiple lineages by the late

Oligocene), three different lineages of rhinos, early camels,

deer, and peccaries, as well as a handful of archaic groups left

over from the Eocene. Numerous modern carnivoran groups

(especially early dogs, and the catlike nimravids, as well as pri-

mitive members of the bear, weasel, and raccoon families)

became the dominant predators as the last of the archaic carni-

vorous mammals straggled on. On all the northern continents

and Africa, rodents and rabbits both underwent a huge diversi-

fication as the niches for ground-dwelling seed-eaters

increased, and the habitat for squirrel-like nut and fruit eaters

diminished.

In Eurasia, many of the same trends were apparent. In the

early Oligocene, the Turgai Strait across the Obik Sea between

Europe and Asia opened up, allowing Asian mammals (such as

rhinos and ruminants) to immigrate to Europe and drive many

of the endemic natives to extinction. This early Oligocene event

is known as the Grande Coupure. However, there was only lim-

ited migration between Asia and North America via the Bering

Strait. In Eurasia, the Oligocene saw a similar diversification of

rhinos (including one group, the giant indricotheres of Mongolia

and Pakistan, which reached 6 m at the shoulder and weighed

20 tonnes), plus some of the earliest members of the deer, giraffe,

pig, and cattle families. Tree-dwelling mammals became much

less common and vanished from many continents. For example,

primates once dominated all the northern continents during the

early and middle Eocene, but became restricted to Africa and

South America, where they evolved into Old World and New

World monkeys, respectively. The rest of the African fauna was

also endemic to this island continent, which was not connected

to Eurasia at the Arabian Peninsula until the early Miocene.

Instead, Africa had a fauna populated by archaic mastodonts, a

wide diversity of hyraxes, and other peculiar endemic forms,

such as the horned arsinoitheres. South America and Australia

were also island continents, unconnected to the rest of the world,

and each developed their own endemic faunas.

In the marine realm, the early Oligocene extinctions triggered

by global cooling were severe, causing major extinction in the

planktonic and benthic foraminifera, and even in the planktonic

algae such as diatoms and coccolithophores. In the Gulf Coast

of the United States, 97% of the marine snail species and 89%

of the clam species found in the late Eocene did not survive into

the late early Oligocene, and over 50% of the sea urchins and

sand dollars went extinct as well. However, the overall taxo-

nomic composition of the marine fauna remained essentially

the same, with new species of clams, snails, and sea urchins

replacing the extinct species (but at lower diversity), and making

up the bulk of the fossilizable organisms in the Oligocene. By the

end of the early Oligocene, diversity was at an all-time low. Mar-

ine faunas were dominated by groups tolerant of the cooler

waters that began in the Oligocene. This is true especially in

the mollusks of the Pacific Rim, which are mostly cold-water tol-

erant forms that migrated south to California from Alaska and

Siberia during the Oligocene. Planktonic organisms were not

only low in diversity, but occupied relatively few, simple biogeo-

graphic realms (since the area of the tropics had decreased), and

evolved relatively slowly during the Oligocene.

Summary and conclusions

The Paleogene (from 65 to 23 million years ago) markedthe global

transition from “greenhouse” climates that had persisted from the

Mesozoic to the modern “icehouse” world with glaciers on the

poles. “Greenhouse” conditions persisted from 65 to 50 million

years ago, with a “super-greenhouse” short-term warming event

at 55.6 million years ago. The causes of this global warming are

still controversial, although some combination of greenhouse

gases, such as carbon dioxide and methane, are usually implicated.

In addition, the circulation and mixing of polarwaters with tropical

waters prevented the poles from freezing, and allowed alligators

and temperate plants to live above the Arctic Circle. This “green-

house” world began a slow cooling from 49 to 33 million years

ago, with the first signs of Antarctic ice appearing at 49 million

years ago. At 33 million years ago, the Antarctic ice cap appeared,

possibly due to the separation of Antarctica from Australia and

South America to form the Circum-Antarctic Current, which gen-

erated cold bottom waters and began modern oceanic circulation

patterns. Since that time, the Antarctic ice cap has waxed and

waned, but the Earth continues in the “icehouse” state that began

33 million years ago.

Donald R. Prothero

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Cross-references

Animal Proxies, Invertebrates

Animal Proxies, Vertebrates

Antarctic Bottom Water and Climate Change

Antarctic Glaciation History

Carbon Isotope Variations over Geologic Time

Carbon Isotopes, Stable

Cenozoic Climate Change

Deep Sea Drilling Project (DSDP)

Evolution and Climate Change

Foraminifera

Glaciations, Pre-Quaternary

“Greenhouse” (warm) Climates

Ice-Rafted Debris (IRD)

“Icehouse” (cold) Climates

Methane Hydrates, Carbon Cycling, and Environmental Change

North Atlantic Deep Water and Climate Change

Obliquity

Ocean Drilling Program (ODP)

Ocean Paleocirculation

Oxygen Isotopes

Paleobotany

Paleocean Modeling

Paleocene-Eocene Thermal Maximum

Paleoclimate Modeling, Pre-Quaternary

Paleoclimate Proxies, an Introduction

Plate Tectonics and Climate Change

PALEOHYDROLOGY

Introduction

Ecosystems, human life, economic activities, ... , all life on the

continents depends on the availability of liquid fresh water.

Today, the hydrosphere (about 1,400  10

) contains

2.5% of fresh water, of which 2/3 is stored as ice and perma-

nent snow, 1/3 as groundwater, and only 0.3% is found in lake,

wetland, soil, and river systems (Shiklomanov, 1998). The

“global hydrological cycle”–the overturning of water from

the Earth’s surface to the sky and back – recycles an amount

of water equivalent to the world’s ocean in 3,000 years. With

highly variable delays, precipitation falling on land surface

(about 0.111  10 km

1

) returns to the sea through over-

land, subterranean, and partly atmospheric paths.

Earth’s water distribution varies through space and time.

The dramatic floods of August 2002 in central Europe and

the 1970–1980s Sahel drought caused substantial damage but

hydrological changes of much larger amplitude and duration

have occurred in the past. From 10 to 5 kyr ago, the Sahara

was a verdant landscape with numerous lakes, supporting Neo-

lithic populations; and 21 kyr ago, the sea level was 120 m

lower and the Northern Hemisphere continental ice volume

was 20 times greater than today. Hydrological changes occur

on all timescales (Figure P20).

Paleohydrology aims at reconstructing the timing, fre-

quency, and magnitude of changes in water storage and quality,

in moisture sources and trajectories, and understanding their

causes and mechanisms. To represent the full range of natural

variability, it has to go back in time far beyond the instrumental

period, i.e., at most the past 200 years. Practically, paleohy-

drology studies the land-based portion of the hydrological

cycle and focuses on its liquid phase; other components

of the hydrosphere (atmosphere, vegetation, ocean, ice) are

PALEOHYDROLOGY 733

considered because of their relationships with liquid inland

water bodies. The present dating techniques mainly allow cov-

erage of the late Quaternary period.

Major concepts

Inland water systems

The irregular distribution of inland water bodies results from

the global climate zonal belts, the regional climate, geological

patterns, and topography. The latter provides a natural parti-

tioning of land into drainage basins or catchment areas, also

called “watersheds.” In an “exoreic” basin, water falling within

its divides gathers to the same outlet toward the sea; an

“endoreic” basin is a land-locked, inland drainage system.

Hydrological systems range from small ponds < 0.1 km

the largest water bodies, e.g., the Caspian Sea (371,000 km

;

78,200 km

). They also differ in lifetime, from a few weeks

for ephemeral floodplains to millions of years for subsident tec-

tonic lake basins or deep aquifers.

The time of complete water renewal, or residence time,

ranges in average from tens of days in rivers, to tens of years

in lakes and 10

years in groundwater reservoirs and mountain

glaciers. It may reach up to 10

years in deep groundwater sys-

tems, especially in endoreic basins of arid zones.

Hydrological variability

The major cause of hydrological fluctuations is climate varia-

bility. Human activities and geological events (e.g., landslides,

lava flow dams) can also modify hydrological systems locally

or regionally.

The variation of the amount of stored water in a time inter-

val, ▵S, is the difference between total inflow and outflow,

which can further break down as:

DS ¼ P þ R

þ G

ðÞE þ R

þ G

ðÞ ð1Þ

where P is precipitation, R

and G

are the surface and ground

water inflows, respectively, E is evaporation + evapotranspira-

tion, and R

and G

the surface and ground water outflows,

respectively, during that interval. The terms P and E of this

water-balance equation depend on climate, while R, G, and part

of E are related to watershed specific factors, i.e., hydraulic

properties, river bed geometry, soil and vegetation cover, and

man-controlled water diversion.

Since Equation (1) expresses the conservation of matter, it is

equally applicable to solutes, isotopes, dissolved substances,

and solid particles.

Processes that relate hydrological variations to changes in

climate and human activities need further research, e.g., how

are the different surface and subsurface compartments physi-

cally and chemically interconnected? How do they respond to

natural or human-induced disturbances? What is their indivi-

dual response time? Although many paleohydrogical records

are still qualitative, emphasis is now placed on exchange pro-

cesses and rates between water pools, on quantification of past

hydrological conditions, and on periodicities and probability

distributions of hydrological events. Crucial to any paleohy-

drology study is the establishment of a reliable and accurate

chronology.

History

Paleohydrology sprang from several branches with their own

techniques and methods, e.g., fluvial paleohydrology, paleolim-

nology. It emerged as an earth science of its own over the past

decades (Schumm, 1965; Gregory et al., 1995). Integration of

groundwaters as paleohydroclimatic archives is even more

recent. This emergence was triggered by the urgent issue of

predicting water resource evolution at global, regional, and

local scales, in response to climatic and societal stress. It was

favored by the development of new dating techniques, new

methods for inferring past hydrological conditions, and com-

puting facilities for data storage and modeling approaches.

Methods and techniques

Paleohydrological analysis of water systems usually begins at

the watershed level, with a field observations and measure-

ments, material collection and laboratory analyses (Stage 1).

Dating is a major step (Stage 2). Then, primary data are con-

verted into time series of hydrological parameters (Stage 3).

The last stage analyzes the observed and inferred data through

statistics and/or modeling (Stage 4), putting the findings into

broader perspective, and eventually linking them with known

geological, climatic, or human facts.

Archives and proxies (Stage 1)

Documentary records provide direct information over many

centuries in Europe or China, for example on major floods or

droughts, advance and retreat of mountain glaciers, or land-

use changes.

Beyond instrumental and historical data, paleohydrology

resorts to indirect indicators of past conditions, the proxies,

Figure P20 Hydrological variability on different timescales: lake-level

changes in East Africa, reflecting changes in the Precipitation minus

Evaporation balance in the lake catchment are; (a) after Trauth et al.

(2003); (b) after Gasse (2000); (c) after Verschuren et al. (2000).

734 PALEOHYDROLOGY

archived in the geomorphic features and sediments of ancient

water bodies (Figure P21), and in fossil water trapped in aqui-

fers and glaciers.

In river systems, the past stream regime and discharge are

primarily inferred from geomorphic and hydraulic properties

of the drainage network, the channel geometry (terraces,

bankfull width, cross-sections, depth, slope, meander wave-

length, ...), the characteristics of channel sediments (particle

sizes), botanic evidence, and indicators of past flood levels,

e.g., slack-water or bouldery flood-bar deposits. Dead paleo-

channels can be detected through remote sensing and field

observations.

Changes in lake and swamp water area, depth, and volume,

as well as chemistry and biology, are reconstructed from geo-

morphology (ancient shorelines, ancient tributaries or outflow,

spatial sediment distribution), and from the analysis of sedi-

ments collected from drillings and outcrops. The most widely

used proxies derived from sediments are stratigraphic features,

mineralogy and chemistry of the allochtonous (detrital) and

endogenic fractions, stable isotopes of organic matter, primary

carbonates, biogenic silica ..., and biological remains (e.g.,

diatoms, ostracods, insects, pollen macrofossils ...).

With speleothems (cave deposits), the annual growth layers

and the carbonate stable isotope composition can inform on

past precipitation and temperature.

The use of groundwater as a paleohydrological archive re-

quires knowledge of the hydraulic conductivity, porosity, and

geological structure of the system. Aquifer water is sampled

from wells and boreholes, or springs in the discharge zones.

The water isotopic composition, atmospheric noble gases,

nitrates, chloride contents, and bromide/chloride ratio are the

major indicators on input conditions (isotopic composition of

paleoprecipitation, ground temperature, vegetation cover).

Dating (Stage 2)

Besides instrumental and historical records, a few archives deli-

ver calendar ages intrinsically through annual layer counting:

varved sediments, glacier ice, and tree rings.

C dating is most

widely used today when aerial plant remains are found; it is also

applied to groundwater-dissolved carbon. It covers the last 40

kyr. Other radiometric methods are now available for different

time-scales, which cover up to 10

years: e.g.,

210

Pb (0–200 yr)

in lake sediments,

230

Th/

234

U in carbonates and organic matter,

Be to date geomorphic surfaces (fluvial terraces, lake shore-

lines) or sediments, and K-Ar and

Ar/

Ar in volcanic inter-

bedded materials. Dating methods also include luminescence,

tephrochronology, and paleomagnetism for long continuous stra-

tigraphic profiles.

The time resolution depends on the archive and on the dat-

ing method. Laminated deposits can provide annual resolution

in best cases. Conversely, uncertainty on groundwater chronol-

ogy can reach as much as thousands of years, due to the inher-

ent characteristics of subsurface flow and the long residence

time. However, groundwaters register the average magnitude

of long-term climate change since the signal of local effects

or short-term extremes is smoothed out.

Proxy calibration (Stage 3)

Data derived from the initial proxy analyses translate into

hydro-climatic parameters, assuming that the relations between

a proxy and its environment are similar to present. Calibration

functions, also called transfer functions, can be established

through statistical techniques, using a training set including

modern observations on the proxies and measured parameters,

e.g., diatom-inferred lake water salinity, or pollen-inferred rain-

fall amount and seasonality. They can also result from the

study of the physical processes that control, retain, and modify

the hydrological signal archiving, e.g., precipitation inferred

from oxygen isotopes in speleothem calcite; these require

quantitative adjustment against measured parameters. Because

of uncertainties in all methods, multiple proxies and techni-

ques are desirable for cross-checking. Calibrated data provide

a record of hydrological variations through time at a particular

location. Their synthesis can be conducted at a watershed-scale

or wider, provided more local factors are identified.

Time-series analys es, hydrological modeling (Stage 4)

Spatial patterns of paleohydrological estimates through time

can sometimes be transformed into statistical syntheses.

Periodicities can be identified through spectral analyses. The

distribution of discrete events, e.g., flood-frequency dis-

tribution, can be assessed through a variety of distribution

functions.

Figure P21 Examples of proxy sources. Upper panel: fluvial terraces

(T1, T2) abandoned during phases of river entrenchment (Dang

River, central China). Lower panel: early Holocene sediments of

Paleolake Abhe

(6,000 km

, 160 m deep; see Figure P20), Afar desert

(Ethiopia).

PALEOHYDROLOGY 735

Understanding the dynamics of hydrological changes often

requires modeling of the investigated system, and calls for data

beyond those derived from paleohydrological archives. For

example, in a paleolake of known ▵S (Equation (1)), why

and how have the water balance terms fluctuated? Evaporation

and runoff rates are affected by solar radiation, wind speed,

temperature, vegetation cover, etc. The evaluation of such fac-

tors comes from astronomical data, botanic evidence, or from

outputs of Global Circulation Model (GCM) simulations,

although the low GCM spatial resolution is usually inappropri-

ate to the regional or local problem at hand. Models depend on

the proxy source, the variable estimated, and the spatial scale

under consideration. They greatly differ in complexity and

accuracy. Once calibrated and validated on instrumental data,

models can simulate past hydrological parameters, e.g., P, E or

R (Equation (1)) using lake water, water-energy, water-isotope,

or water-salt balance models. They also can predict hydrological

changes in response to varying climate or land-use.

Time-series analyses and modeling represent the upper level

of paleohydrological reconstruction and provide the greatest

insight into the mechanisms and potential causes of hydrologi-

cal changes.

Applications

Paleohydrology and paleoclimatology

Hydrological fluctuations are intrinsically linked to changes in

climatic forcing factors; in turn, they have act on climate

through varying evaporative fluxes from the continental sur-

faces, and energy exchanges as water undergoes phase

changes. While GCM simulations of past climatic conditions

help understand the mechanisms of observed hydrological

changes, conversely paleohydrological data are used to validate

the climatic models: quantitative estimates of hydrological vari-

ables are then essential.

At the orbital time-scale, records primarily reflect the gla-

cial/interglacial cycles and the smooth, long-term variations

of the Earth’s orbit. For example, over the past 70 kyr, changes

in level of Lake Lisan (the precursor of the Dead Sea which

extend along the Dead Sea-Jordan Valley Dead) correlates with

temperature changes inferred from isotopic data in polar ice

cores: high/low levels generally coincide with colder/warmer

global climate intervals, respectively (Figure P22). Tropical

hydrological changes are initially driven by the impact of the

orbital precession cycle on monsoon strength; the spectacular

early-mid Holocene wetting in the northern tropics (e.g., Figure

P20b; Gasse and Roberts, 2004) is an example, although orbital

forcing was modified by feedbacks from sea surface tempera-

tures, vegetation, and moisture recycling from soils and large

land waterbodies.

At shorter time-scales, subtle disturbances in the climate

systems generate, abrupt hydrological changes. Sudden injec-

tions of large amounts of continental freshwater in the North

Atlantic Ocean have generated remote rainfall anomalies, such

as catastrophic droughts in the eastern Mediterranean region

coeval with Heinrich events (Figure P22), as shown by abrupt

decreases in the Lake Lisan level (Figure P22); the outbreak

of Lake Agassiz in the Laurentide at 8.2 kyr BP may have

had a widespread impact on rainfall. Holocene abrupt changes

are commonly attributed to shift in total solar irradiance or

injections of volcanic aerosols in the atmosphere. The Little

Ice Age, a cold event in northern high-mid latitudes, is

expressed as a wet episode in equatorial East African lakes

(Fig. P20C) but as a dry period in tropical East African and

South American records, suggesting persistent El Niño-like

conditions (Brown and Johnson, 2006). Despite significant

progress in paleohydrological reconstructions during the past

decades, further work is needed to quantify hydrological

changes, to fill geographical gaps in data coverage, and to ana-

lyse changes in seasonality, in order to turn paleohydrology

into a more accurate paleoclimatic tool.

Paleohydrology and landscape evolution

Changes in water routing induced by geological events or cli-

mate changes may drastically affect the regional geographic

features. For instance, 90 kyr ago, an ice sheet over northern

Eurasia dammed the large rivers flowing north to the Arctic

Ocean; and huge lakes developed between the sheet and the

water divides to the South, and overflowed towards the Aral,

Caspian, and Black Seas, and ultimately the Mediterranean,

draining large, now endoreic, areas to the sea. The drainage

direction of major catchments in Europe and western Siberia

was reversed, with considerable impact on both continental

and sea hydrology (Mangerud et al., 2001).

Paleohydrology allows quantification of such processes as

erosion or river incision rate under varying climatic conditions.

For instance, over the past 12,000 years, the northern alluvial

piedmont of the Tian Shan in central Asia has been incised as

much as 300 m by the rivers flowing from the mountain.

Changes in hydrological regime were inferred from the geo-

morphic survey of one of them and from a model of alluvial

stream erosion based on a transport-limit erosion law (Poisson

and Avouac, 2004). The estimated incision rate increased mark-

edly at the transition from the generally wet early Holocene

conditions to a rather arid climate with enhanced seasonal con-

trasts around 6,000 years ago.

Paleohydrology and human societies in the past

Several studies suggest relationships between past natural

hydrological variability and societal changes. A striking exam-

ples is that of the Holocene occupation in the Sahara controlled

by changes in water availablitiy (Kuper and Kröpelin, 2006),

as illustrated in Figure P23. The history of droughts evi-

dent in the 1,000-year hydrological reconstruction in Lake

Naivasha, Kenya (Figure P20c, is in line with the cultural his-

tory of the region; prosperity during the wettest episode (the

Little Ice Age).

Paleohydrology can also detect human impacts on hydrolo-

gical systems. Increased erosion rate induced by forest clear-

ance is fingerprinted in lake sediment magnetic properties.

Atmospheric pollution (e.g., from lead in Roman times or acid

rains in recent times) and eutrophication has been recorded in

lake sediment chemistry and lake-water pH.

Water resource implications and hydrological

risk-assessment

Groundwater is a crucial source of freshwater throughout the

world. Paleohydrology shows that, in many areas, exploited

groundwater is “fossil” and dates from periods of greater moist-

ure availability thousands of years ago or more. Groundwater is

often being mined at a rate vastly exceeding that of possible

recharge under current climatic conditions, leading to a swift

water resource decline.

Ice melting water is the prime water resource and hydroelec-

tric power supply in several areas, e.g., the Andean countries of

736 PALEOHYDROLOGY

South America. The current global warming induced a rapid

decay of mountain glaciers, especially in the tropics. Has such

a decay an equivalent in the past? This question is approached

through study of the glacier history and the dependent river and

lake systems.

The risk of extreme floods and droughts is of critical con-

cern for societies. It must be assessed for selecting the location

of equipment, such as dams. Streamflow-gauging records are

too short to provide realistic probabilities of the heavy flow

magnitude and frequency distribution. Paleoflood data can be

incorporated into flood-frequency analyses to extend the re-

cords, as exemplified by the regional Holocene paleoflood

approach performed in northwestern Colorado (Jarrett and

Thomlison, 2000). This study could estimate the peak dis-

charge at recurrence intervals ranging from 10 yr to 10 kyr.

The suitability of potential repositories of chemical and

radioactive waste also refers to paleohydrology. Estimating

groundwater flow paths and fluxes through rocks of low per-

meability under past and current climates is necessary to assess

future risk.

Conclusions

Paleohydrological research unveils an enormous natural

variability in inland water quality and availability, primarily

driven by climate change, at least over the Quaternary period.

Superimposed on the natural variability, human activities affect

the global climate and individual hydrological systems. Knowl-

edge of hydrological changes in the absence of any significant

human impact is key to disentangle natural and anthropogenic

factors.

Paleohydrology is concerned with very practical applica-

tions, from changes in the hydrological cycle at a continental

scale to local water resource management.

The anticipated global warming is expected to cause the

hydrological cycle to churn more vigorously, putting more

moisture into the atmosphere, generating a stormier climate

in some regions and drier conditions elsewhere, and higher

frequency and amplitude of extreme hydrological events

(IPCC, 2007). However, even the most sophisticated numerical

climate models do not yet provide a realistic representation of

Figure P22 Correlations between levels of Lake Lisan and oxygen isotope records from GISP 2 (The Greenland Ice Sheet Project 2, 1997). After

Bartov et al., 2003.

PALEOHYDROLOGY 737

precipitation patterns or amounts at regional scales. Better doc-

umenting and understanding of the full range of climate driven

hydrological variability is a most urgent and significant tasks in

paleoscience.

Françoise Gasse

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episodes in the Eastern Mediterranean linked with the North Atlantic

Heinrich events. Geology, 31(5), 439–442.

Brown, E.T., and Johnson, T.C., 2005. Coherence between tropical East

African and South American records of the Little Ice Age. Geochemis-

try, Geophysics, Geosystems, 6 (12) doi:10.1029/2005GC000959.

Gasse, 2000. Hydrological changes in the African tropics since the Last

Glacial Maximum. Quaternary Sci. Rev., 19, 189–211.

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the arid and semiarid belt of northern Africa. Implications for past

atmospheric circulation. In “The Hadley Circulation: Present, Past

and Future.” (Diaz H.F., and Bradley R.S., Eds.), Kluwer Ac. Pub.,

Dordrecht. pp. 313–345.

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Panel on Climate Change), 2007. Fourth Assessment report, WMO,

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Jarrett, R.D., and Tomlison, E.M., 2000. Regional interdisciplinary paleo-

flood to assess extreme flood potential. Water Resour. Res., 36(10),

2957–2984.

Kuper, R., and Kröpelin, S., 2006. Climate controlled Holocene occupation

in the Sahara. Science, 313, 803–807.

Mangerud, J., Astakhov, V., Jakobson, M., and Svendsen, J.I., 2001. Huge

Ice-age lakes in Russia. J. Quaternary Sci., 16(8), 773–777.

Poisson, B., and Avouac, J.P., 2004. Holocene hydrological changes

inferred from alluvial stream entrenchment in North Tian Shan

(Northwestern China). J. Geol. 112, 231–249.

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Fret, D.G. (eds.), The Quaternary of the United States. Princeton: Prin-

ceton University Press, 783–794.

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National Snow and Ice Data Center, University of Colorado at Boulder,

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cal Data Center, Boulder, Colorado.

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403, 410–414.

Cross-references

Continental Sediments

Dating, Dendrochronology

Dating, Magnetostratigraphy

Dating, Radiometric Methods

Glacial Megalakes

Lacustrine Sediments

Lake Level Fluctuations

Little Ice Age

Millennial Climate Variability

Mountain Glaciers

Ostracodes

Paleoclimate Proxies, an Introduction

Paleotemperatures and Proxy Reconstructions

Paleolimnology

Speleothems

Stable Isotope Analysis

Time-Series Analysis of Paleoclimate Data

Transfer Functions

PALEOLIMNOLOGY

Paleolimnology is the multidisciplinary science that studies past

environmental conditions from the evidence preserved in lake

sediments. It addresses a very broad range of issues relating to for-

mer ecosystem composition and development, lake ontogeny, the

variability of catchment and land use processes, climate change,

water quality trends, pollution studies, and even the hydrocarbon

potential of ancient lake sediments. Such diversity has led to the

emergence of at least three major traditions within the subject.

The first deals with trends in extant lakes, often investigating rela-

tively short time scales (10–200 years) and allied to work by the

limnological community. A second strand studies ancient lake

basins, an area merging with the sub-discipline of limnogeology,

and usually the realm of sedimentology. The third tradition consid-

ers the evidence of climate changepreserved by lakes, usually with

a recent or Quaternary perspective, using tools that bridge the

aforementioned areas of paleolimnology. A convergence of meth-

odologies, shared principles, and assumptions unifies paleolim-

nology as a discipline and these will form the initial part of

this discussion. Given the focus of this encyclopedia, emphasis

will then be placed on the application of paleolimnology to studies

of past climates and climate change.

Principles, assumptions, and chronologies

Lakes are temporary stores of sediment in the drainage basin

and have been described as an ecosystem’s memory (Smol,

2002). The length of the memory depends on the age of the

lake and on the sedimentation rate. Since lakes accumulate

Figure P23 Late Neolithic rock engraving in central Sahara (Temet, Aı

Northern Niger).

738 PALEOLIMNOLOGY

sediments faster than equivalent marine environments, they are

often well-equipped to furnish high-resolution records. Some

lakes are very young, for example those in recent volcanic cra-

ters or man-made reservoirs; others in formerly glaciated

regions were formed after the retreat of the last ice sheet that

covered the region and are typically less than 16,000 years

old. A further group of lakes, including the large rift lakes of

East Africa and Lake Baikal, are extremely old and have been

accumulating sediment for millions of years (Cohen, 2003).

The diversity of lakes matches the range of studies underta-

ken by paleolimnologists, but commonalities in approach and

methods are evident. Paleolimnological studies exploit the fact

that sediment accumulates in lake basins from different

sources, sometimes arriving from great distances, as is the case

in airborne deposition of fly ash, although more immediate and

important sources are the lake catchment and materials pro-

duced within the lake itself (Figure P24). Allochthonous mate-

rials are those introduced from catchments including sediments

derived from eroding soils, weathered materials, and vegeta-

tion. Some allochthonous components have origins outside

the immediate catchment, for example long distance transport

of pollen, volcanic ash, and airborne industrial pollutants. Iden-

tifying the source and flux of allochthonous materials is a

major goal of many paleolimnological investigations as these

can give clues about changing catchment conditions and regio-

nal processes. Autochthonous materials, or those living or pro-

duced within the lake, constitute the other major components of

the sediments. The remains of algae, higher plants, fish, inver-

tebrate animals, etc. that once lived within the lake, together

with chemical precipitates including salt deposits are all termed

autochthonous. Diagenetic minerals that form in situ from pre-

viously deposited sediments would also contribute to this auto-

chthonous group.

One key assumption of the paleolimnologist is the law of

superposition. In collecting material from lakes it is usually rea-

sonable to assume that more recent sediments overlie older

materials, making them especially suited to temporal studies,

and giving lakes an advantage in this respect over other sedi-

ment stores. However, there are instances when this assumption

does not hold and the stratigraphy is disturbed or even inverted.

All lakes experience some sediment focusing, a process that is

a function of their bathymetry, catchment topography, river

inputs, and internal water movements, creating uneven sedi-

mentation patterns within a basin. Even in simple lakes some

fining of deposits takes place, so that the representation of a

particular indicator may be skewed, and therefore not all sam-

pling sites are equally representative. Problems frequently

occur in nearshore areas where the sedimentation may be erra-

tic, leading to slumps or hiatuses, or in deltaic areas where

variability in discharge may be important. Once deposited,

sediments can be ruptured and faulted by tectonic movements,

and localized slumps are common on slopes with even

relatively shallow angles. In deep lakes, turbidity currents are

common and these can intersperse the stratigraphy with homo-

genous turbidite layers. To offset the problems posed by post-

depositional deformation, a number of paleolimnologists have

used seismic profiling to identify undisturbed coring sites.

Excellent examples of this approach come from the large lakes

Figure P24 Principles of paleolimnology. The key sources of water and sediment to a typical lake are indicated. The core location is important to

sample the most complete sequence of sediments. Major stages in a palaeolimnological investigation are listed.

PALEOLIMNOLOGY 739

of East Africa where deep-drilling operations need to be sure of

collecting undisturbed sediment cores (Cohen, 2003).

In general, paleolimnology is more successful in small, deep

lakes within sheltered catchments such as volcanic craters,

rather than large shallow lakes that are more prone to wind

stress and sediment mixing from wind-induced turbulence.

Eddies can reach the sediment, causing remobilization of

recently deposited sediment and smoothing of the stratigraphic

record. A problem common to all paleolimnological studies is

that of bioturbation. Burrowing benthic organisms tunnel into

the sediments, once again smoothing the stratigraphy and limit-

ing the temporal resolution possible. Extreme examples of bio-

turbation include fish mounds and even mixing by mammals

using the lake as a watering hole. Bioturbation is less of a

problem in anoxic deepwater sediments, out of the range of

wading animals and often with a limited benthic fauna. The

discussion above demonstrates the unique nature of lakes.

While it is possible to generalize, each basin has to be treated

differently. In extant lakes, the paleolimnologist can rigorously

apply uniformitarian principles. By fully understanding present-

day limnology, sedimentation, patterns, and responses of the lake

to external forcing, interpretation of the sediment record becomes

comprehensible.

Sampling the sediments of extant lakes usually requires the

collection of sediment cores from a boat or other platform. The

method selected depends on the length of core required and

the nature of the study. If the aim is to investigate recent

high-resolution changes, the collection of the mud-water inter-

face is vital and sampling devices such as the Glue corer or

Hongve sampler should be used to get short, <50 cm, gravity

cores (see Smol, 2002). Both work by lowering a tube into

the sediment and triggering a bung that seals the tube and

enables it to be raised. A better, but more complicated system

for taking high resolution short cores is freeze-coring, where

dry ice or liquid nitrogen freezes the sediments and avoids

deformation during the coring process. It has even been suc-

cessfully deployed in warm tropical waters. Freeze-coring is

particularly important in the recovery of sediment structures

such as laminations.

Probably the most common coring system used by paleolim-

nologists is the 1 m Mackereth corer, which consists of a col-

umn of fixed height, containing a piston and a sampling tube,

mounted on a large drum. During its operation, the Mackereth

corer is lowered to the bottom of the lake, taking care to pre-

serve verticality. The sampling tube is then forced into the sedi-

ments using compressed air, which when fully extended allows

the air to fill a drum; this creates buoyancy to recover the

corer. This system works well in many lakes, providing that

sediments are sufficiently cohesive to remain within the tube,

neither too sandy nor excessively consolidated. Mackereth

corers are available in a range of fixed lengths up to 12 m.

A more flexible system is the Livingstone corer and its mod-

ified version, the Wright corer. These work by fixed rods that

are used to collect 1–1.5 m of material at a time. The corer con-

sists of a barrel, a piston, and sometimes a plastic inner tube

that can be used to store the core. The corer is lowered in

closed position and then the piston is raised using a cable

before it is forced into the sediments. The operation is repeated

for consecutive sections until an impenetrable barrier is reached

or all the rods are deployed. Typical cores gathered using this

method are 10–15 m long, but much longer (50–60 m) cores

have also been taken. Locating the core hole after each drive

is problematic in all but very shallow water and a core-hole

liner is often used both to facilitate relocating the hole and pre-

venting materials falling into it. Good practice involves taking

parallel cores with one offset to cover gaps between drives. The

mud-water interface is rarely recovered intact with a Living-

stone-type corer and has to be taken separately with a short

coring system. Some recent work on large lakes has been under-

taken using the GLAD800 coring rig, a system capable of reach-

ing combined water-sediment depths of 800 m (Cohen, 2003).

The technology involved here is broadly analogous to that used

in the offshore oil and gas industries.

Not all paleolimnological work is from extant lakes. Some

studies have used modified versions of the Wright and Living-

stone corers to obtain cores from former lake beds or season-

ally drained lakes without the need for a boat. While these

cores will not be continuous, the extra stability gained by work-

ing in a relatively dry environment can be advantageous in

ensuring precision. Other workers have sampled exposures of

lake deposits, giving them the advantage of unlimited material

to study, ready inspection of deformation features, and no dis-

tortion due to coring. Such exposed deposits are rarely as con-

tinuous in time as cored sediments and can be more prone to

sub-aerial weathering. The study of fossil shorelines is fre-

quently allied to core studies, as these give precise indications

of former lake level and hence lake volume and surface area

(important in paleoclimate climate modeling), but these only

mark highstands and do not offer continuous sequences to

investigate rates or directions of change, or information about

lower lake stages.

All paleolimnological studies need to establish a chronology to

understand the rate of change or the timing of an event. A precise

method available in some lakes is the counting of annual varves

(Lamoureux,2001). Avarve, in de Geers’s original usage, is a cou-

plet of sediments of contrasting size fractions caused by seasonal

melting of glacier ice. Not all laminations are of glacial origin; in

temperate regions, the regular seasonal cycle of diatom blooms

in spring (and less strongly in autumn) can create a lightly colored

layer contrasting with darker organic material deposited during

the productive summer months. By counting the couplets, an abso-

lute chronology can be established, as long as the mud-water inter-

face is known or some other clearly identified dating horizon is

present, such as the tephra from a known volcanic eruption. An

exceptional series of varves has formed in Baldeggersee, an alpine

lake showing evidence of recent eutrophication (Lotter, 1998).

These have been used to tie trophic changes inferred using paleo-

limnological methods to a series of instrumental measurements.

Sites like these help test the assumptions implicit in paleolimnol-

ogy and the strength, or otherwise, of the methods. In areas of

strong seasonality, such as parts of the tropics, the precipitation

of salts during dry seasons often can create a distinct horizon,

or a characteristic bloom of algae may occur on a seasonal basis.

According to independent bracketing dates, it has been shown that

laminations are not always annual, and a range of cyclical climate

forcing mechanisms has been proposed to explain their origin.

Some laminations are too fine to be easily visible and are difficult

to enumerate without magnification or improved contrast. In these

cases, x-ray images can help as they can be digitally counted using

image analysis software. Similarly, gray-scale imaging can some-

times help decipher laminations, although difficulties can persist

(Dean et al., 2002).

Unfortunately, annually laminated sediments are rare, and

most paleolimnologists have to use either marker horizons of

known date, or more often radiometric methods, to obtain a

chronology. The most commonly used markers are volcanic

740 PALEOLIMNOLOGY