Earthworm Activity- A Source of Potential Disturbance of Archaeological Sediments

7/26/2019 Earthworm Activity- A Source of Potential Disturbance of Archaeological Sediments

1/14

Society for American Archaeology

Earthworm Activity: A Source of Potential Disturbance of Archaeological SedimentsAuthor(s): Julie K. SteinReviewed work(s):Source: American Antiquity, Vol. 48, No. 2 (Apr., 1983), pp. 277-289Published by: Society for American ArchaeologyStable URL: http://www.jstor.org/stable/280451.

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2/14

EARTHWORM ACTIVITY:

A SOURCEOF POTENTIALDISTURBANCE

OF

ARCHAEOLOGICALSEDIMENTS

Julie

K. Stein

Conspicuous

disturbances

in

archaeological

sites

are

readily

detected

during

excavation.

However,

one

animal

whose

destructive

effects

are

not

often

recognized

is the earthworm. Work at the Cariston

Annis

mound

n

Kentucky,

an Archaic shell

midden,

has

resulted

in

the

identificationof

areas

of

extensive

earth-

wormdisturbance.

Archaeological

ites

most

readily

affected

are those

with

the

appropriate

vegetation

cover,

moisture and

temperature

conditions,

and available chemical elements. The

type

of

disturbance

a site

will

undergo

depends

on

the

species

of

earthworms

present. Subsurface-casting pecies

mix

matrix

only

below the

surface while surface-castingspecies bringthe fine-grainedmatrix to the surface, thus concentrating arger

objects

below

ground.

If

earthwormcasts

are

identified

n

a

profile,

one

should

proceed

cautiously

with

inter-

pretations

concerning

soil matrix.

POSTOCCUPATIONALDISTURBANCE

of

archaeological deposits

is

an

expected

phenomenon

for most

archaeologists.

Rodents

move

through

strata

accumulating

their

backdirt

at

the surface.

Tree

roots

penetrating

downward

can

be

ripped

up

if

the

tree

is blown

over.

Differential

freezing

and

thawing transports

large

objects

upward,

while

ants and some

earthworms

are

capable

of

transporting

the same

material

to

lower levels.

Wind, water,

and

gravity

scatter

material

in

every

direction,

while

catastrophic

events such as

earthquakes

can

completely

obliterate

any

original

order.

Human

activity

(plowing,

excavating)

is often the most

powerful

force

of

disturbance.

Wood

and

Johnson

(1978)

have described nine

types

of

disturbance that

affect

archaeological

deposits.

One

type

of

disturbance,

faunalturbation-the

mixing

of

sediment

by

animals

(Thorp

1949)-is

commonly

observed

in

archaeological profiles

as

networks of

abandoned

animal burrows.

A

bur-

row,

after

abandonment,

fills with

material from another soil horizon.

Differences in soil texture

and

color

allow the

feature to be

easily recognized.

But

some

animal burrows

are

so

small that

they

are

not

easily

detected;

ants,

earthworms,

spiders,

and

crickets are a few

of

the

small

animals

that disturb the

soil

(Kiihnelt

1955).

Of

all

these small

creatures,

the

earthworm

is

the

most

easily

overlooked

although

it

is often the

most

destructive.

Earthworm burrows

are

so

small

they

may

go

undetected.

Instead

of

filling

with

material

from

another

soil

horizon,

these burrows can fill

with

the

excreta

of the

earthworm.

The

excrement

is

sometimes

very

similar

macroscopically

to

the

sediment

surrounding

the burrow.

The

role of

the

earthworm

as a

maor

source

of

disturbance

in

archaeological deposits

is

here

examined. The discussion begins with a description of the life cycle of earthworms together with

the

optimal

conditions

for the

animals'

survival. The

disruptive

effects of

earthworms

are

described

and

illustrated

by

results

of

investigations

at a

shellmound

in

western

Kentucky.

Sites

most

vulnerable to

earthworm

activity

are

identified,

and

the

means for

determining

earthworm

habitation

are

presented.

The

discussion

concludes with

cautionary

notes on

the

types

of

analyses

that

are

precluded

if

earthworms

have been

churning

the

debris.

Julie

K.

Stein,

Department of

Anthropology,

DH-05,

University

of

Washington,

Seattle,

WA

98195

Copyright

?

1983 by the Society for American Archaeology

0002-7316/83/020277-1

3$1

.80/1

277

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3/14

AMERICAN

NTIQUITY

EARTHWORMS

The

earthworm

is

said

to

be the most

important

macroanimal

of

soils. Its

significance

was

recognized

by

Darwin

(1890)

as

early

as 1837 when he

began

his observations

on

the

abundance

of earthworms and their effects on soil. In 1878 the Danish soil scientist P. E. Muller (Cruickshank

1972)

identified earthworm

activity

as a crucial element

involved

in

the

genesis

of

forest soils. He

described an

A horizon

as a

layer

of

thoroughly

mixed

mineral material and well

decomposed

humus.

Both

the

mixing

of

the

two

components

and the

decomposition

of

the

organic

material is

enhanced

by

the

activities

of

earthworms

(Pitty

1978;

Wilde

1958).

Although

earthworm

species

differ

in

size and

behavior,

the

general

activities

of all

species

are similar

(Edwards

and

Lofty 1972). They

move

through

the

soil,

displacing

and

ingesting

mineral and

organic

matter. Earthworms

especially

favor

dung,

succulent

herbage

(grass),

and

tree leaves.

The leaves

of

ash,

hickory, tulip

tree,

dogwood,

and basswood are

among

the

most

favored;

foliage

of

oaks

and conifers are least favored

(Satchell

1967;

Satchell and

Lowe

1967;

Bocock

and Gilbert

1957;

Gilbert and

Bocock

1960;

Barley

1959;

Thorp 1949).

After the material has passed through their digestive tracts, earthworms eject it, as castings,

on the surface

or

in

soil crevices. Casts have far

greater stability

and

water-storage capacity

than

the soil

surrounding

the casts.

In

fact,

with continuous

activity,

earthworms can alter

a

soil's

structure

from its

original

form to

a

granular

structure

composed totally

of

castings.

Forest

soils

typically

have

a

crumb

structure

produced

from

abundant

organic

material

that has

been

pro-

cessed

by

earthworms.

The

granular-cast

structure

in

some soils

is so

resistant

that it

was able

to

withstand

rigorous

water

erosion and

air-drying

tests conducted

by

Guild

(1955).

There is a

variety

of

explanations

for

the

way

in

which

the stable

granules

are

formed.

First,

the

casts

are

simply

reinforced

mechanically

by

filaments

of

vascular

bundles

from

ingested

plant

remains.

Second,

soil

particles

are cemented

together

by

calcium humate

formed

in

the

worm's

in-

testines

by

the

calciferous

glands.

Third,

bacterial

populations,

present

either in

the

dung,

the

gut,

or

the

casts,

are

responsible

for

gluing

the soil

particles together

with

bacterial

gums. Fourth,

the development of fungal hyphae after excretion stabilizes wormcasts. And fifth, the presence of

calcium stabilizes

clay.

Earthworms

work

the soil to create extensive

burrow

networks

that are more or less vertical

for

most of

their

depth,

but branch

near the surface.

Under unfavorable

surface

conditions

(e.g.,

soil

temperature greater

than

10?C,

or

mean annual

precipitation

of less than 560

mm

[Buntley

and

Papendick

1960]),

earthworms

will

excavate

to

depths

of 6

m

(Scully

1942:504;

Edwards

and

Lofty 1972:118).

In winter,

one

or more

individuals

curl

up

at the bottom

of a burrow

and

hiber-

nate.

An

earthworm's

average

life

span

is

usually

less than

two

years

(Satchell

1967).

They

are eaten

by

toads,

frogs,

snakes,

turtles, birds,

moles,

and

shrews.

Earthworms

are

preyed

upon

by

humans.

The

giant

Australian

earthworms

(Megascolides

australisj,

which

attain

lengths

of

more

than

3 m,

are

supposedly

hunted

by

aborigines,

who

consider

them

a

great

delicacy

(Schaller

1968:61).

If

the soil becomes

saturated

with

water,

earthworms

are forced

to the surface

for

ox-

ygen

to avoid

drowning.

However,

extended

exposure

to ultraviolet

light

on

the

surface

is

also

deadly.

Of

the

more than

1,800

species

of

earthworms

in the

world,

Lumbricus

terrestris,

reddish

in

col-

or,

and

Aporrectodea

trapezoides

(formerly

called

Allolobophora

caliginosa

[Gates

1972]),

which

is

pale pink,

are

the most

common

species

both

in

Europe

and

in the

eastern

and central

United

States.

Neither

species

is

native

to the United States.

L.

terrestris,

the common

nightcrawler,

rapidly

replaced

native

species

as

forests and

prairies

were

cultivated.

Olson

(1928) reports

that

L.

terrestris became

widely

distributed

over Ohio

during

the

1920s;

and

according

to

Smith

(1929),

this

species

was

noticed near

Champaign,

Illinois

in

1896.

Most earthworm

studies

in soil

zoology

have

been

conducted

in

England

on

European species

(Darwin 1890; Edwards and Lofty 1972; Evans 1948a, 1948b; Evans and Guild 1947, 1948a,

1948b;

Guild

1948, 1951,

1955;

Nelson and

Satchell

1962;

Satchell

1955, 1958,

1967;

Satchell

and

Lowe

1967).

The

ecological requirements

discussed

here are

derived

from those studies

and

may

278

[Vol. 48,

No.

2,1983



4/14

EARTHWORM ACTIVITY

AND

DISTURBANCE

not reflect

accurately

the

requirements

of

similar North American

species.

Until

research is

con-

ducted that

specifically

defines the behavior

of this continent's

species,

we

are forced to

apply

the

European

data.

L. terrestris is the most thoroughly studied species. It draws leaves and other materials into the

mouth

of

its

burrow,

and

produces

casts that

are

excreted

only

on the

surface around the

burrow

opening.

Thus this

species transports

great

quantities

of

sediment

to

the surface and can

conse-

quently bury

Roman walls

(Darwin

1890),

patios

(Wood

and

Johnson

1978),

and artifacts

(Atkinson

1957;

Rolfsen

1980)

within

a

few

years.

In

contrast,

A.

trapezoides,

also introduced from

Europe,

feeds below the surface

on dead

her-

bage,

bacteria,

and

fungi, excreting

casts in

nearby

soil crevices.

The results of this behavior

are

not

as

noticeable as

those of

the

common

nightcrawler

because casts

are

not

visible

on

the

sur-

face.

The

animal

ingests

and excretes

as much sediment as does L.

terrestris,

but

subsurface

casting

precludes

burial

of

objects.

Native

North

American

species

are not as

adaptable

environmentally

as these two

imported

individuals.

OPTIMAL

CONDITIONS FOR

EARTHWORMS

Earthworms can

inhabit

a wide

variety

of

environments,

but certain

conditions

are considered

optimal.

Texture, moisture,

temperature,

available food

source,

agricultural

practices,

and soil

acidity comprise

the most

important

characteristics

of

the

animal's

environment.

Texture

Soils

with

medium textures

create the best

habitats

(Evans

1948a;

Guild

1948,

1951).

Too much

sand

promotes drainage

and

lowers the

amount

of

moisture

and

organic

matter the

soil

can

re-

tain,

while

excessive

clay

restricts

burrowing

because

of

increased soil

hardness.

According

to

Guild

(1948:184),

a

light-loam (silt-loam

or

loam

[Buol

et

al.

1973])

texture is

optimal

for earth-

worms.

Moisture

Water

constitutes

75-90% of

the

earthworm's

body weight (Grant

1955)

and

to

maintain this

percentage,

moisture must

be

available all

year.

Yet too

much

moisture

will

cause

drowning.

Soil

moisture is

affected

by evapo-transpiration

(i.e.,

plant

factors such as

the

rooting, drought

tolerance, and stage and rate of plant growth; climatic variables including air temperature and

humidity,

and wind

velocity

and

turbulence;

and soil

characteristics

such

as

texture,

soil

stratification,

and

moisture suction

relations).

Most

earthworms do not

inhabit

soils

in

regions

with

mean

annual

precipitation

less

than 560 mm

(Buntley

and

Papendick

1960),

and

they

will not

occupy water-logged

locations. To

a

large

degree,

soil

porosity

and

landscape

relief control soil

moisture.

Proper drainage,

which

aerates

the

soil,

helps

keep

moisture

at a level

appropriate

for

earthworms.

Temperature

Temperature

influences

the

fecundity

of

certain earthworm

species

(Evans

and

Guild

1948a).

The

optimum

soil

temperature

is

10?C.

Earthworms are

rarely

seen

in

soils

where the

mean an-

nual air temperature is colder than 7 ?C. Earthworms can tolerate extreme seasonal

temperatures;

they respond

by moving

deeper

into the soil.

Gerard

(1967)

believes

that this

migra-

tion

occurs

when

the soil

temperature

falls below 5

?C.

Inactivity

will

also ensue

when

279

Stein]



5/14

AMERICAN ANTIQUITY

temperatures

rise

significantly.

Gates

(1961)

reports

that earthworm

species

in

the

tropics

become inactive

(little

movement or

feeding) except

during

the monsoon

season,

although

the

movement

may

be related

to

changes

in

moisture.

Food

Source

To

thrive,

earthworms

require

an

abundant food

source;

the

type

of

food consumed varies

with

species.

Surface-feeding species

usually

prefer

tree

leaves,

grass,

and

dung (Bocock

and Gilbert

1957;

Barley

1959;

Satchell

1967;

Satchell and

Lowe

1967;

Thorp

1949).

Subsurface-feeders

prefer decomposed plant

material,

subsurface

vegetation

(roots),

humic

material,

and

microorganisms

(bacteria

and

fungi) (Barley

1959;

Tracey

1951;

Waters

1955).

The abundant

organic

content

of

archaeological

debris serves as

an excellent food

source,

especially

for

subsur-

face feeders.

Agricultural Practices

While some

agricultural techniques promote

earthworm

activity,

others are detrimental.

In re-

cent

years many

forests,

especially

on

floodplains,

have

been cleared

for

cultivation.

With

the

loss

of

leaf litter as a

food

source,

earthworm

populations

declined

sharply.

According

to

Satchell

(1958:214),

earthworm

populations

decreased

by

70%

when

grasslands

were

plowed.

If

an area

has

been cultivated in recent

years,

the size of the earthworm

population depends

not

only

on

available

food

but

also

on

the

types

of

additives

the

farmer

has

applied.

If

fertilizer

(food

source)

or lime

(a

calcium

source,

see

below)

is added

to

a

field,

the

earthworm

population

will

usually

ex-

pand (Barley

1961;

Tischler

1955).

Even some herbicides

will increase

the

population

by

providing

more

dead

herbage

on

which

earthworms

can

feed.

Pesticides,

on

the other

hand,

often

eliminate

earthworms.

Depending

on the

type, they

can

either

kill

outright

or

accumulate

in

the

earthworm's

tissues until a

toxic

level is reached.

Soil

Acidity

The

pH

of a

soil also

influences the earthworm's condition.

Tolerance

to

changes

in

acidity

varies

widely depending

on the

species

involved. Most

species

cannot

tolerate

pH

values below

4.5

(Edwards

and

Lofty 1972)

and

prefer

neutral

conditions,

a

pH

around

7.

It

is

uncertain

whether

pH

condition

is itself the factor

limiting

earthworm

survival

(Satchell

1955;

Guild

1951).

Guild

(1955)

has

suggested

that it is the

lack

of

calcium

in

acid

soils that inhibits earthworm

metabolism,

rather than the

pH.

Calcium

is

required

for

the

function of the

calciferous

gland,

an

organ

believed

by

some

to

affect

digestion

of

food

(Guild

1955).

EFFECTSOF EARTHWORMS ON ARCHAEOLOGICAL ITES

Earthworms

disrupt

archaeological

sites

in the

following ways.

1.

They

obliterate

stratification

within the midden

matrix

by

mixing

sediment.

Earthworms,

especially

subsurface-casting species,

can

mix material

from

adjacent

strata,

blending

the

colors

and

textures into

hybrid

materials

with intermediate

properties.

Archaeologists

differentiate

site

profiles by noting

differences

in color and

texture,

but earthworm

activity

can

produce

over-

lapping parameters,

thus

inhibiting

the

archaeologist's

ability

to delineate

strata.

2.

Surface-casting species

can

bury objects

by systematically

bringing fine-grained

matrix

to

the

surface.

Rolfsen

(1980)

reports

that in an

experiment

conducted

with

marked

chert and

ceramic

fragments,

earthworms

buried

the material

to

depths

as

much

as 45 cm below

the

original position

after

five

years.

3. Earthworms obscure boundaries of soil and archaeological horizons. The animals will

especially

disrupt

the boundaries

of

archaeological

features

such

as

burial

pits

and

hearth

outlines.

This

phenomenon

is

particularly

noticeable

in

areas

of

contact

between cultural

and

280

[Vol.

48,

No.

2,1983



6/14

EARTHWORM

ACTIVITY

AND DISTURBANCE

noncultural

deposits.

If

such contact

is

located

within

the

upper

meter

of

sediment,

earthworms

will

effectively

blur

the

boundary.

4.

Earthworms

may

alter the botanical

assemblage

preserved

in

a site. Because

earthworms

can ingest anything smaller than the diameter of their mouths (approximately 2 mm), any small

carbonized

plant

remains

(i.e., seeds)

that

were

incorporated

at the site could

have been

digested

and

decomposed.

It is

unfortunate

that,

because

some

cultigens

and

wild

plants

are

usually

iden-

tified

by

the

presence

of their

seeds and

not

by

larger

plant parts,

earthworms can

selectively

remove

the evidence

for

the

presence

of such

plants.

Large

volumes

of

sediment must

be

pro-

cessed

to

insure

recovery

of

small botanical

remains.

5. Earthworms

alter the

chemistry

of

soils. Worm

casts and soil

samples

taken

adjacent

to

the

casts have

been

compared

chemically (Lunt

and

Jacobson

1944).

The results

indicate that casts

have

higher

soluble

concentrations

of

almost

all

soil

elements.

Such

properties

as

pH,

total and

exchangeable

calcium,

exchangeable

potassium

and

manganese,

available

phosphorus,

total ex-

changeable

bases,

and

organic

matter all

appear

at

higher

concentrations

in

casts than

in

the sur-

rounding

soil.

If an archaeologist is

using

the

chemistry

of soils to locate

archaeological

sites, then earthworm

activity

is not

a

significant

problem.

But if

small

chemical differences are

being mapped

within

a

signature

of

prehistoric

activity.

EARTHWORM ACTIVITY

AT

THE

CARLSTON ANNIS MOUND

The

Carlston

Annis

mound is

an Archaic shell

mound

located

on

the

Green

River,

in

west-

central

Kentucky.

The site

was

occupied

between

5149

+

300 B.P.

and 4349

+

300 B.P.

(Watson

and

Marquardt

1979).

Archaic inhabitants

subsisted

on

mussels

and fish

from

the

river,

as well

as various

nuts,

plants,

and animals

from

the

surrounding floodplain

and

uplands.

These

people

discarded

debris and so created

the

mound

that

today

rises

2

m

above the

surrounding plain,

with

dimensions of 80 by 100 m (Figure 1). Besides shells, animal bones, charred plant remains, and

abundant rocks and

sediments,

the

site

contains numerous

human

burials

(Marquardt

and

Wat-

son

1976).

At

the

Carlston Annis

mound the

dominant

earthworm now

present

is the

European species

Aporrectodea

trapezoides

(identified by

William

Fender,

personal

communication

1981).

Native

North

American

species

identified thus

far are

Diplocardia

ornata

(Gates 1942)

and D.

varivesicula

(Murchie

1966);

both

are

subsurface-casting species.

The abundance of

subsurface

casts

suggests

that these

animals

have been

active

for

some

time.

Certain

characteristics of

the

mound are

ecologically

favorable for

earthworms

in

general

and

especially

for

A.

trapezoides.

The

shell midden

has

a silt-loam to

loam texture

(mean grain

size is

.006

mm).

This

texture,

combined with

the

pore

space

created

by

the

presence

of

shells and sand-

stone

fragments, creates an ideal environment for a subsurface-casting earthworm.

The

moisture

content of

the soil is

also

ideal.

Kentucky's

mean annual

precipitation

of

1,220

mm

(Schwendeman

1958)

maintains

soil

moisture

throughout

the

year.

The

porosity

and

relief

of

the

mound

provide

the

necessary drainage

to

aerate the wet

soil,

keeping

the soil

moisture at about

30%.

Historically

recorded

temperature

extremes between

43 ?C

and

-29

?C

rarely

occur

with

a

suddenness

that

exceeds the

earthworm's

ability

to

burrow. In the

winter the

maximum frost

penetration

is

approximately

37

cm

(Wood

and

Johnson

1978:336),

leaving

sufficient room for

the

earthworms

to

hibernate.

Before

modern

vegetation

clearance,

the

food source of the

earthworm

population

at the

Carlston

Annis

mound was

most

likely

derived from the

tall

canopied

forest

and

understory plants

that

dominated the

area

(Wagner

1979),

as well

as

decomposed

plant

material,

subsurface

vegetation

(roots),

humic

residues,

and

microorganisms

(bacteria

and

fungi).

The

high

organic

content of the occupation debris was a nutritious supplement to the diet. In recent years, as the

natural

floodplain

vegetation

was

removed and

insecticides

applied,

the earthworm

population

has

probably

declined.

281

tein]



7/14

15

Bt 5

CARLSTON

NNIS ITE

BUTLER

COUNTY,

KY

CONTOUR INTERVAL 0.25 METERS

0 10

20

30 40

50

Mag.

+

0,0

SITE

DATUM

TOP

Of

WELL

ASSUMED

TO

BE 102 00

M

Figure

1.

The

topography

of

the

Carlston

Annis

mound with

the

surrounding

plain

and

the

Green

River

(dra



8/14

EARTHWORM ACTIVITYAND DISTURBANCE

The

chemistry

of the mound

deposits

also

represents

optimal

conditions for the animals. In the

shell

midden,

the

pH

ranges

from 7.6 to 8.3 with an

average

of 7.8. This

slightly

alkaline

pH

results

from the abundant

calcium

released

from

shells.

At the

mound,

earthworms benefit from

both ideal pH conditions and abundant available calcium, as well as from the presence of other

nutrients such as

nitrogen, phosphorus,

and

carbon.

Calculations

of

Earthworm

Activity

Close examination

of

a

profile

at

the

mound

reveals abundant earthworm casts. The

casts,

found

principally

in

large

concentrations

in

soil

crevices,

indicate

the

activity

of subsurface

casters.

The

casts

are

continuously

distributed

from

near the surface

to

the lowest

midden

deposits

2 m below the

surface,

with few cast-free

deposits

observable. Because

casts are

ex-

tremely

durable,

their

presence

does

not

only

indicate modern earthworm

activity;

like

the mid-

den,

they

could have been

accumulating

during

the

years

of

site

deposition,

or

they may

have been

deposited

in

the

4,000

years

since

deposition

ceased.

European scientists have calculated the number of earthworms that inhabit certain soils as

well

as the amount

of

material

they

rework.

Although

comparing

such

calculations

to

a

site

in

Kentucky

is

highly

speculative,

it

does serve a

purpose.

The

calculations

indicate

just

how

poten-

tially disruptive

earthworms

can

be should

an

archaeological

site be located

in

a

setting

that

is

optimal

for

the

animals'

survival. The

Carlston

Annis site

represents

such

a

site with

extraor-

dinarily

beneficial conditions.

Although

these calculations are

not

applicable

to

all forested

sites

located

in

open floodplains, they

are

presented

here

to

warn

archaeologists

of

the

potential

ef-

fects

of

earthworms.

The

number

of

earthworms

necessary

to

rework the entire

5,848

m3 of

mound material

in

4,000

years

can

be

roughly

estimated

(Table 1).

Of the total midden

volume, 1,345

m3

is

matrix. Studies

in

England estimating

the number

of

earthworms

in

soil

(reviewed

in

Satchell

[1967:273])

indicate

that the

weight

of

earthworms

in

the

top

17

cm

of

the soil was 148-162

g/m2

on

land

with

mixed

woodland

forests,

a volume of between 871 and 953

grams-of-earthworm/m3.

The

Carlston

Annis

mound was surrounded

by

mixed-woodland forests

for

most

of

its

history

(Wagner

1979).

Even

Table 1. Estimate

of

Earthworm

Population

n the

Carlston

Annis Mound.

Earthworm

Calculations

Number

of

worms

5,848

m3

(total

volume

of

mound)

23 %

1,345 m3 (volumeof matrix in mound)

871-953

g/m3 (g

of

earthworms in

1

m3,

[Satchell

1967])

1,345

m

1,172,000

g (g

of

earthworm

n

matrix)

or

2,724,000

(No.

of

whole

earthworms

n

matrix)

Rate

soil

is

ingested

4,423-43,411

g/m3/year

(weight

of soil

ingested

by

a

large

earthworm

population

n one

year,

from

Evans

[1948b],

Satchell

[1967],

and Guild

[1955])

1

m3

=

979,000

g

(weight

of

midden)

1

m3

=

225,000

g

(weight

of

matrix

only)

Time

required

to

rework matrix

equals

51

years

Time

site

exposed

to

matrix

reworking

equals

4,000

years

Stein]

283



9/14

AMERICAN ANTIQUITY

though

the

mound

affords

the

food

and soil conditions

preferred

by

earthworms,

the

lower

estimate

of

871

g/m3

is

adopted

for this

study.

The maximum biomass

associated

with

earthworms

for the site's

1,345

m3

of matrix

is

1,172,000 g or 2,724,000 whole earthworms. This population estimate represents the number of

worms the mound's

matrix could

theoretically

support

if

each

17-cm

depth

offered

conditions

similar

to

those

found

in

English

soils.

Obviously,

more worms must be located

in

the

upper

17 cm

of the

mound than

in the lower

2 m.

But as

the

midden

material was

accumulating,

earthworms

were

present

at

each surface.

Therefore,

it

is

probable

that

throughout

its

history

the

matrix

could

support

a

total of

almost

3,000,000

earthworms.

The

weight

of soil that

earthworms

ingest

has been calculated

(Evans

1948b;

Guild

1955;

Satch-

ell

1967)

by

collecting

and

weighing

casts added

to

the

surface

in

one

year.

Although

the

weights

are based

on

casts

from

species

that excrete

on the

surface,

it

is

assumed

by

Satchell

(1958:210)

that

the

casting weights

of

species

that

void

below

the

surface must have

similar values.

The

weight

of

soil

ingested by

a

large population

in

one

year

ranges

from 752 to

7,380

g/m2

(4,423

to

43,411

g/m3).

In the

mound,

1

m3 of midden

weighs

979,000

g

(Stein

1980).

In the

midden,

1

m3

has

an

average

of 23% matrix, which therefore

weighs

225,000 g.

The results

of

these calculations

indicate

that it would take

almost

3

million earthworms

only

51

years

to

ingest

and

disrupt

all

the

matrix

in the

Carlston

Annis

site.

Obviously

this calculation

is

not a

precise

measurement

of the rate of earthworm

disturbance.

It

assumes

no

negative

feed-

back mechanisms

limiting

the

reproduction

or

ingestion

rates

of

the

earthworms. The

presence

of

moles,

birds,

and

other

predators presently

at the site

suggests

that

depletion

of

the

population

is

occurring.

However,

it

does

demonstrate

how

easily

earthworms

could

modify

the entire

matrix

of

the

midden.

Effect

of

Earthworms

at

the

Carlston

Annis

Mound

At the

Carlston

Annis

mound,

earthworm disturbance

has

been so

great

that four of the

five

disrupting effects are evident.

1. The texture

of the midden matrix

shows

a

mixing

of

two sources

of

sediment,

sandy-silt

river

deposits

and

a

clayey-silt

lake

deposit.

Figure

2

illustrates

the

grain-size

distribution

of the

two

sources

(noncultural

deposits)

and

the

resulting

distribution

of the midden

(cultural

deposits)

after

earthworms have

mixed

the matrix.

Although

the

frequency

diagrams

do

not

prove

that

earth-

worms

were the

mechanism,

they

do

suggest

that the material

has

been

mixed.

2. Boundaries

in the mound

deposits

are almost

entirely

absent.

Burials have

no

discernible

pit

boundaries.

The sediments

below the mound

grade

subtly

into the

midden

matrix.

Even

the

plow

zone is

undistinguishable

unless

sprayed

with a fine mist

of

water.

If one examines

the areas

where

boundaries

should

be

evident,

only

earthworm

casts

are

detected.

3.

In

the

project's

collection

of charred botanical

remains

we

have found

very

few seeds

(Wagner 1979). But because the site has been totally reworked by earthworms, the scarcity of

seeds need

not

imply

that

the

inhabitants

of the

mound did

not

rely

on these

plants.

However,

because

a

large

volume

of sediments

have

been

processed

by

flotation

and water

separation

(over

8,000

1),

one would

expect

to find

more seeds and

seed

fragments

than

are

now

available.

4. At

the

Carlston

Annis

mound

the

chemistry

of

the

archaeological profiles

shows little

soil

horizonation.

Profiles

commonly

contain

horizons with

well

defined soil

zones,

(e.g.,

A, B,

and

C

horizons),

but chemical

elements

in the shell

midden

are

not distributed

throughout

the archaeo-

logical profile

in

the

traditional manner.

Their distribution

reflects

the

results

of

mixing

and

dis-

rupting processes.

Many

factors can

interrupt

the

development

of soil horizons

in

a

profile

(e.g.,

intrusion

of

burials,

ground

water

fluctuations,

and

additional

sedimentation),

but

at

the Carlston

Annis mound,

which

is an

open-air

forested

landscape

composed

of

fine-grained

sediments,

the

source

of

disruption

is most

likely

earthworms.

Forest

soils

usually

have

well

defined

surface

zones (A horizons) composed of mineral and humus components mixed by earthworms. In the

mound,

earthworms

have extended

the

mixing beyond

the surface

zone

to

a

depth

of 2

m,

altering

284

[Vol. 48,

No.

2,1983



10/14

EARTHWORMCTIVITY ND DISTURBANCE

Figure

2. The

upper

diagram

illustrates

the

grain-size

frequency

curves constructed for the noncultural

deposits

near

the

Carlston Annis

mound. These two sediment

types

are

easily

distinguished

from

each

other: the lake sediment

(L)

has more

clay

and less sand than

the

samples

collected from the river

deposits

(R).

The lower

diagram

illustrates the

grain-size

frequency

curves constructed

for

the cultural

deposits.

Their

similarity

indicates

a

random

mixing

of the two noncultural

deposits

(river

and

lake), probably pro-

duced

by

earthworm

activity. Samples

from

the shell midden

(SM),

the shell-free midden

(SfM),

and

the col-

luvium

(C)

all have

comparable proportions

of

sand, silt,

and

clay. Samples

were

analyzed

following

Folk

(1974),

including

treatment

with

H202

to remove

organics.

285

Stein]



11/14

AMERICAN

ANTIQUITY

the

distribution

of

chemicals

within

the

profile.

Any

archaeological

interpretations

based

on

the

chemical

analysis

of

samples

from

the

matrix

of

this

mound

must

be considered

suspect.

Although

these

observations do

not

prove

that earthworms were the

dominant mechanism

disturbing the midden, the facts that the midden is rife with casts, the matrix is unstratified, the

chemical

elements

in

the soil are

mixed,

and the soil

conditions are ideal

for

earthworm

habitation,

all

strongly

suggest

that

these

creatures have indeed

played

a

major

role

in

disturbing

the

deposits.

CONCLUSIONS

The Most

Vulnerable

Kind

of

Archaeological

Sites

The

types

of

archaeological

sites most

vulnerable

to

earthworm

disturbance are

those

that

display

the

previously

described characteristics

of

food,

soil

texture,

and

soil

moisture favorable

to

the animals. These characteristics

are

most

frequently

found

in

forested

regions

in

open

set-

tings.

Favorable habitats associated

with

rivers are

levees,

dry

portions

of

the

floodplains,

and

any colluvial fans located on the margins of the floodplain. Glacial terrain with moraines, kettle

lakes

and

loess-covered

slopes

would also

provide

numerous favorable

habitats,

although

out-

wash and

gravel

features are unfavorable. Sites located

in

favorable

geomorphic

situations are

likely

to have witnessed earthworm

activity.

Sites located in areas with unusual

physical

characteristics will be

exempt

from

earthworm

disturbance.

Extremely sandy

conditions

such

as those found on some beaches and

in

dune

fields

will not

support populations.

Caves,

whether

wet or

dry, rarely

provide

suitable

habitats.

Rockshelters

are

generally

too

dry,

as are

sites in

arid

or semiarid

regions.

Sites

presently

under-

water or

waterlogged

are also

protected, although they may

be disturbed

by

other

organisms.

Sites that

do not

presently support

earthworm

populations may

have

supported

them in the

past. During

the last few

millennia,

environmental

changes

have occurred

in

many

areas,

either

on

a

regional

or

local scale.

Mean annual air

temperature

and

precipitation

has

fluctuated

since

the end

of

the

Pleistocene,

and

forest

composition

has

changed dramatically (Davis

1976;

Delcourt

and

Delcourt

1979;

Wright

1976,

1981).

Also,

the effects

of

forest

clearance,

drainage

modifica-

tions,

highway

construction,

and

expanding

urbanization have altered

local environments

significantly.

When

looking

for

evidence

of

earthworm

disturbance one

must

consider

the

possibility

of climatic and

environmental

change

for

the area

in which the site is located.

How

to Detect the Presence

of

Earthworms

To

detect

the

presence

of

earthworms

in

a site

one must

examine

the soil structure

closely.

Macroscopically,

the

presence

of

0.5-mm-sized

granules

of

matrix

or burrows

approximately

10

mm in

diameter are

indicative of earthworm

activity.

But

an

effective

study

would

have to

include

the use

of

micromorphological

techniques (Brewer

1964;

Goldberg

1979;

Rutherford

1972)

to

iden-

tify

how the

earthworms

have

altered the

archaeological deposits. By making

thin-sections

of the

soil

structure and

examining

the features

microscopically,

one

could

ascertain

the

degree

to

which the

soil

has

been altered. One should also

collect and

identify any

earthworms encountered

during

excavation.

The

species

of

earthworm

inhabiting

the

site

may

differ in the

way they

affect

the

soil,

either

casting

on the surface or in soil crevices. The difference

in

casting

will

determine

how

much

disturbance has

occurred.

Although

the earthworm

has often been

overlooked,

its

disruptive

force

is

obviously great.

Ex-

cavators should

be aware that earthworms can

be

reworking

the matrix

of

a site.

They

may

not

necessarily

affect the

position

of

larger

objects,

but

will

definitely

disturb

material

less than

2

mm

in

diameter.

If

earthworm

casts

are identified

in a

profile,

one should

proceed

cautiously

with in-

terpretations concerning

soil

chemistry.

Because

the earthworm

aerates

the soil and

processes

organic

material,

it

has

long

been

recog-

nized as a

friend

of the farmer.

But,

as

I

hope

I

have demonstrated

in

this

paper,

the earthworm

may

be

an

unsuspected

nemesis for the

archaeologist.

286

[Vol.

48,

No.

2,1983



12/14

EARTHWORMACTIVITY

AND DISTURBANCE

Acknowledgments.

This research was a

part

of

the Shell Mound

Archaeological

Project

directed

by

Patty

Jo

Watson and William

H.

Marquardt.

It

was

supported by

a National Science Foundation Grant BNS

7808

916,

and a

University

of Minnesota Dissertation

Fellowship.

I

would like

to

thank

Stanley

E.

Chernicoff,

Herbert

E.

Wright, Jr.,

Patty

Jo

Watson,

Janet

E.

Levy,

and Karl

Butzer for their

assistance

in the

editing

of

the

manuscript, and William H. Marquardt for his contribution in drafting. Also special thanks must go to the peo-

ple

of

Logansport, Kentucky,

especially

John

L.

Thomas,

Waldemar Annis and

Ethie

Annis,

for

their

assistance

in

the collection of

earthworms

from

their

land,

and their

general

cooperation

and

enthusiasm.

REFERENCES CITED

Atkinson,

R.

J.

C.

1957 Worms and

Weathering.

Antiquity

31:219-233.

Barley,

K. P.

1959 The Influence of

Earthworms

on

Soil

Fertility,

II.

Consumption

of

Soil and

Organic

Matter

by

the

Earthworm

Allolobophora caliginosa.

Australian

Journal

of

Agricultural

Resources

10:171-185.

1961 The

Abundance

of

Earthworms

in

Agricultural

Land and Their

Possible

Significance

in

Agriculture.

Advances

in

Agronomy

13:249-268.

Bocock,

K.

L., and 0. J. W. Gilbert

1957 The

Disappearance

of

Leaf Litter under Different

Woodland

Conditions. Plant and

Soil 9:179-185.

Brewer,

R.

1964

Fabric and

Mineral

Analysis

of

Soils.

Wiley,

New

York.

Buntley,

G.

J.,

and

R. I.

Papendick

1960

Worm-worked Soils of

Eastern South

Dakota,

Their

Morphology

and

Classification.

Soil

Science

Society

of

America,

Proceedings

24:128-132.

Buol,

S.

W.,

F. D.

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Earthworm Activity- A Source of Potential Disturbance of Archaeological Sediments

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