USGS Open-File Report 02–192

Tiny Windows to Environmental Change:
the Fascinating World of Microfossils
ENVIRONMENTAL QUALITY AND PRESERVATION—

Though small in size, calcareous (calcium
carbonate) microfossils reveal volumes
about geologic time, events, processes,
and past climate. The U.S. Geological
Survey (USGS) has the capability to
provide signifi cant scientific information
based on microfossils that can be used
to help resolve issues raised in many
other disciplines. A source for pioneering
investigations and new discoveries little
recognized outside the fi eld, microfossils
and the stories they tell lie preserved in
the geologic record.

Microfossils are everywhere (Fig.
1). Generally the size of small sand
grains, they are the fossilized remains
of marine or terrestrial organisms. There
are many kinds of microfossils ranging
from terrestrial pollen spores to conodonts
(toothlike fossil elements in form
but not in function, produced by marine
worms) to shells of calcareous marine
organisms such as mollusks, pelecypods,
ostracods, and foraminifera (forams).
Nannofossils are extremely small calcareous
marine microfossils not visible
to the naked eye and include radiolarians
and diatoms. Marine microfossils
are important tools used to determine
various aspects of the geologic and climatic
record. In the modern world, some
living faunas that become microfossils
upon death are also used to measure
changing marine conditions, including
pollution. Pollution is generally considered
a result of human activities,
but other types of change such as variable
salinity (salt content) or temperature
of the water are more diffi cult to
defi ne as strictly natural or manmade. To
help ascertain causes of these types of
change, we must examine the geologic
record, a record of natural processes and
change.

One of the most common microfossil
groups used in environmental
studies is foraminifera. Forams are protozoans
with external chambered shells.
There are two types, planktic and benthic
(Plate 1A, B). Both are distributed
globally. Planktic (short for planktonic)
forams are pelagic organisms that float
as plankton in the open oceans. They
live at various depths and construct
their shells from elements dissolved in
seawater. Species vary depending upon
water depth, temperature, and latitude.
An abundance of fossil pelagic species
in deep-sea sediment indicates the overlying
water was warm and nutrient-rich.
Planktic species are relatively shortlived
(a few million to tens of millions
of years old) and are used to determine
precise geologic age of the sediment or
rock in which they are found, called
host rock. Planktic forams reveal more
than benthic forams about geologic time,
processes, past environments and climates.
Benthic forams are bottom dwellers.
They live on or within the top
layers of sediment from the shoreline
to abyssal depths. Most also build their
shells from elements dissolved in seawater,
but some species use sand grains or
other materials from their surroundings
to construct their shells. Benthic forams
are extremely long ranging (hundreds of
millions of years old) and are seldom
used for precise age dating. Instead,
assemblages consisting of specific spe-
cies are used to indicate the general setting
in which they lived (i.e., estuary,
inner shelf, outer shelf, slope, and
abyss), which implies a general water
depth. In the geologic record of a tectonically
stable area where there are
no mountain-building processes, general
water depth has implications for past
position of sea level.

The Field of Micropaleontology
Micropaleontology is the study of
microfossils. A micropaleontologist usually
specializes in a single type of
microfossil but is able to recognize
organisms from other groups. A foraminiferal
micropaleontologist uses forams
along with other types of data from
a sample site (for example, type of
host rock or sediment, geographic setting,
and seismic-survey data) to determine
various attributes of the sample
or group of samples. The attributes are
then placed into the context of local geologic
history—what happened at the site
and when did it happen. Once 'what'
and 'when' are resolved, the data can
often be extrapolated into a larger picture
—the regional or global record. For
example, pollen spores from sediment
cores are used to trace the presence
and location of terrestrial plants and
the movement of tree lines with changing
climate through time. Conodonts
become darker with age and thermal
pressure through time and are used in
the oil and gas industry as indicators
of conditions favorable for hydrocarbon
formation. When conodonts recovered
from a certain subsurface depth within
an exploratory well have attained a specific
dark color, they verify that organics
at that depth have reached the maturation
stage to form oil and gas. Assuming
that foram specimens are found in place,
i.e., where they lived in the case of
benthic species or where they fell to
the ocean floor after death in the case
of planktic species, the species are
used to develop part of the local geologic
record. Where fl oods, rivers, debris
flows, and other processes have
imported microfossils from somewhere
else, the displaced or reworked faunas
are used to tell something about the
local site as well as possible distant
sources and the types of processes that
transported them. Regardless of specialty,
given only the microfossils and
little other solid evidence, a micropaleonologist
is a forensic geologist who
applies scientific knowledge and logical
reasoning to interpret and reconstruct
events of the past.

Biostratigraphy
Stratigraphy is the study of sedimentary
or rock layers (strata) in the
geologic (stratigraphic) record (Fig. 2).
Layered strata are often observed in road
cuts. A technique called seismic surveying
is used to map subsurface strata.
Seismic records or profiles show vertical
slices through the ground that reveal the
different layers. Biostratigraphy is the
differentiation of rock layers based on
distribution of the fossils they contain.
The identifying fossils within each bed
are to be found only in that particular
portion of time when the bed was deposited
(Winchester, 2001). Stratigraphic
order represents the order through time
in which the beds accumulated. Whether
working with microfossils or dinosaurs,
biostratigraphy is the primary method
for determining ages of geologic events.
Deep-sea biostratigraphy is the study of
sediments and pelagic forams and nannofossils
deposited in a deep-water environment.
Identification of forams and
nannofossils is also a key component
in oil and gas exploration. Rock fragments
containing microfossils are recovered
from various intervals during drilling
of a well. Fossils are identified and
ages determined, and the depth interval
of the sample is compared to seismic
profiles and other types of data obtained
in the area. Drilling continues until the
fossils indicate that rock layers of an
age known to contain hydrocarbons in
the region are reached. Whether the new
well will contain oil or gas depends on
the structure of the hydrocarbon-bearing
rocks and whether there is an impermeable
layer or structural trap above them
that would confine the oil or gas.
Deep-sea biostratigraphy is also
used in settings that are no longer in
deep water, such as at a shallow carbonate-
platform margin to ascertain how
and when the platform built seaward and
the duration of expansion. The Great
Bahama Bank in the Bahamas is an
example of a carbonate platform. During
high stands of sea level, carbonate
platforms generally build seaward by
shedding of shallow-water debris produced
on the bank top. The process
is called margin progradation. In tectonically
active zones, biostratigraphy is
used to tell the age of pelagic sediments
now above sea level. Knowing the age
of uplifted pelagic sediments can help
determine the timing of processes, such
as when tectonics occurred in the area
or when volcanoes erupted. For instance,
uplift took place after deposition of
Cretaceous, Paleogene, and early
Neogene (Fig. 2) pelagic forams and
marls that now form mountains in
northern Cuba. Volcanic-ash beds on
St. Croix in the U.S. Virgin Islands
are sandwiched between pelagic chalk
layers that contain late Miocene foram
species. If there are deep-sea marine
sediments onshore in a tectonically
stable area, age of the sediments as
determined by the microfossils will
denote a time when sea level was higher
than today. Biostratigraphy of sediments
from multiple cores in an area also
enables correlation of the strata within
the cores from which a cross section of
the subsurface structure of the land can
be developed.


Figure 1. (top) Open-water fl oating plankton consists
of organisms of all sizes. (center) Extruded-protoplasm
bubbles around long thin spines on a planktic foram
entrap other types of plankton for food. (bottom) Reef-building
branching coral Acropora palmata and certain
species of bottom-dwelling forams require similar conditions
for survival.

Figure 2. Geologic time scale showing Cenozoic epochs.
Stratigraphic order occurs when sediments in a rock
section progressively range from oldest at the bottom to
youngest at the top, such as is observed in a core of
undisturbed deep-sea sediments. Ma = million years.

Figure 3. Excavated site of mudball (arrow) and turbidite (dashed lines) encased within fi ne-grained pelagic limestone
on St. Croix. Turbidite layer is ~0.5 m thick. Visible dimensions of mudball are 15 x 25 cm. Rock hammer for
scale. m = meters. cm = centimeters.

Figure 4. Simplifi ed structural models and generalized events characteristic of Cuban geology. The events could have produced the richly fossiliferous submarine
source of pelagic foraminifera that were imported to the western margin of the Bahama Bank. Letters indicate epochs (stratigraphic ages) as shown in Figure 2.
K = Cretaceous. km = kilometers. m = meters.



Planktic Forams: Indicators of
Paleoclimates and Paleoenvironments
Living planktic forams deposit in
their shells different concentrations of
carbon and oxygen isotopes and other
minerals dissolved in seawater depending
on water temperature and latitude.
Rapid changes in carbon-isotope concentrations
in fossil planktic forams have
been linked to sudden releases of frozen
methane from the ocean floor, events
that are thought to have caused periods
of rapid global warming in the past.
Though methane oxidizes to carbon
dioxide, a greenhouse gas, atmospheric
methane has 20 times the heat-trapping
power of carbon dioxide. Variations in
oxygen-isotope concentrations in fossil
species have also established ocean
temperatures and temperature variations
through time. The paleotemperature
curve correlates with other data on
climate change, such as glacial and
interglacial intervals and corresponding
data on sea-level changes. Planktic
forams can develop abnormal shells
as a response to environmental stress.
Deformed shells of species living in the
water column on the shelf off Newport
Beach in Southern California have been
linked to presence of offshore sewage
outfalls. Many abnormal shells of fossilized
species developed toward the end
of the Miocene in waters over what is
now St. Croix, probably in response to
increasing Antarctic glaciation and associated
cooling of the world's oceans.
The youngest pelagic forams on the
island, early Pliocene species, indicate
the island emerged from the sea shortly
after ~5 Ma (Fig. 2).

Benthic Forams: Indicators of Stressed
Environments and Pollution
Living benthic forams are especially
useful in detecting environmental perturbations.
They have relatively short
life cycles and specific habitats; thus,
they tend to respond quickly to changes
in their environments. They are diverse,
abundant, easily sampled, and provide a
cost-effective "bioindicator" that occurs
in statistically significant numbers.
Benthic foram assemblages are becoming
increasingly important as we learn
more about the particular species that
inhabit pristine waters, marginally polluted
waters, or survive in heavily
polluted areas. Some assemblages are
known to be tolerant of variable and
extreme conditions such as high or low
salinity. In the fossil record, such assemblages
could reflect a restricted shallow
nearshore environment that was periodically
impacted by high influx of
freshwater runoff. Some species contain
living algae when alive, as do species of
the stony or reef-building corals. Stress
symptoms in these species and in corals
and coral reef communities are similar.
Both types of organisms require clear
ambient oceanic conditions. Both are
now being affected in coral reef areas
worldwide and are considered bioindicators
of changing conditions. Like planktic
forams, benthic forams also produce
abnormal shells under stress. Pollution
is a type of stress and often includes
presence of heavy metals. Heavy metals
can be incorporated into benthic foram
shells as they grow. The type and
concentration of metals in normal and
abnormal parts of the shell can be
detected through x-ray analysis. The
USGS in cooperation with the National
Park Service is currently conducting
investigations of the effects of pollutants
on benthic forams in Biscayne Bay,
Florida. The bay is the marine com-
ponent of Biscayne National Park.
Abnormal benthic species found offshore
may indicate that pollutants are
reaching offshore waters. Benthic
forams and geochemical analyses are
complementary tools used for monitoring
water quality in estuarine or coral
reef settings.

Significance of Reworked
Microfossils: What Can They Reveal?

The geologic record contains the only evidence
anywhere for determining cause
and effect of natural change. The microfossil
part of the record can contain the
potential and capacity to provide far more
information than may at first be evident on
the surface of a study.

St. Croix, U.S. Virgin Islands
The elongate island of St. Croix
consists of volcanic rock at both ends
separated by limestone that was deposited
as layers of pelagic sediment in an
ancient seaway prior to tectonic uplift.
Depositional strata are revealed in road
cuts throughout the seaway. In 1980,
a specific hillside outcrop behind a private
residence was free of vegetation
and revealed a coarse-grained limestone
layer encased in massive chalky deepwater
limestone (Fig. 3). The coarse-grained
layer, called a turbidite, had
been deposited by a submarine debris
flow. The turbidite contained a mudball
inclusion consisting of homogenous
clay. When analyzed for microfossils,
the mudball yielded 39 species of
reworked planktic forams (Fig. 2);
23 species had not previously been
described from the island. The turbidite
contained younger middle Miocene species.
Though the mudball clay and the
older fossils were the same ages, the
mudball entity was middle Miocene
because it was a part of the turbidite.
The source of the mudball could not
be determined, but the mudball demonstrates
the existence relative to the
submerged seaway floor of a steep
submarine slope, probably of tectonic
origin, that was composed of clay and
was undergoing pelagic sedimentation
between ~57.8 and 15.2 Ma. The mudball
consistency and species composition
also show that subsea reworking
and mixing of fine-grained sediments
were occurring at the mudball source
at about 15.2 Ma. The turbidite-mudball
package further documents submarine
processes of large-scale sediment transport
and admixing of dissimilar sedimentary
bodies. Until the mudball was
found, the oldest pelagic sediments identified
on the island belonged to the early
Miocene (~23.0 Ma, Fig. 2). To this
day, the mudball forams remain the only
record known of earlier pelagic sedimentation
in the area of St. Croix. The
reworked microfossils had expanded the
timeframe and depositional record of
island-history knowledge by nearly 35
million years.

Western Margin of the Great Bahama
Bank, Bahamas
Limestones in two long drill cores
from the western margin of the Great
Bahama Bank contain a particularly rich
and enlightening record. The cores were
obtained to verify ages of the host rock,
inferred from seismic data to represent a
complex depositional setting. The planktic
microfossils revealed the magnitude
of complexity. The setting itself posed a
challenge for the foram/nannofossil part
of the study: the cores were taken in
shallow-water sediments in a shallow-margin
setting that would normally preclude
presence of deep-water faunas.
But they were there, in abundance,
especially in thin, condensed layers of
strictly pelagic sediments. Presence of
the pelagic microfossils and sediment
layers demonstrated several facts. Water
depth at the platform edge was once
deep enough that planktic faunas lived
there, sediment transport off the bank
was intermittent, and the top of the
platform was once farther inland and
smaller than it is today. Critical first
and last occurrences of marker species
(those that indicate age) of both types
of microfossils were out of stratigraphic
order because of mixing by shallow-water
debris shed from the platform.
The host rock was eventually determined
to range from ~7.5 to 1.9 Ma
(Fig. 2), but the planktic forams yielded
far more information about the bank
margin and the regional and global
record than was ever imagined at the
beginning of the study.
Besides host-rock faunas, the limestone
contained abundant specimens of
imported planktic forams that ranged
from ~66.5 to 15.2 Ma (Fig. 2). The
82 species identified were also uniquely
deposited in the host rock. Paleocene,
Eocene, Oligocene and early Miocene
species were present in the bottoms of
the cores. The middle parts of the cores
contained Eocene, Oligocene and early
Miocene faunas. Only Oligocene and
early Miocene species were found near
the core tops. In other words, the oldest
displaced species were progressively and
completely eliminated in successively
younger intervals of host rock. Such an
orderly distribution of reworked forams
is unique to the geologic record in the
Bahamas. Because of low-lying topography
on the bank and very young host
rock relative to ages of the reworked
specimens, the displaced forams could
not have been derived from elsewhere
on the bank. This discovery led to a new
direction of research, to determine their
source.

The new direction required application
of ancient regional geographic constraints,
speed and direction of ancient
currents, paleoclimatic data, and sea-level
changes. The investigation required
correlation of the original (source) and
secondary (bank) depositional records
of the reworked species with regional
and global proxy data from studies by
others. The reworked species represent
pelagic deposition over 51 million years
at their source. Their abundance indicates
that the sediments in which they
were originally deposited were easily
eroded (i.e., not rock) millions of years
later. Their unique redistribution in the
cores implies tectonic forces at the
source that produced a certain type
of stratigraphically ordered submarine
structure, then much later erosion of that
structure by marine bottom currents over
a period of ~6 million years (duration of
accumulation in the host rock). Although
Florida is built of pelagic limestone of
the same ages as the reworked species,
the direction and velocity of the Gulf
Stream, which was well established by
the time of secondary deposition, eliminates
Florida as the source. The only
area in the Caribbean that meets all criteria
for type and site of the source
is located somewhere along the tectonically
unstable northern coast of Cuba,
some 400 to 700 km away from the
western bank margin. The Cretaceous,
Paleogene, and early Neogene pelagic
forams now found in uplifted mountains
along that coast are evidence. The only
type of structure that could have been
the source is a submarine wall with
erodible strata either in stratigraphic or
reverse stratigraphic order.
There are three possible models, all
common tectonic structures. An eroded
overturned anticline can exhibit layers in
reversed stratigraphic order (Fig. 4A-C).
Stratigraphically ordered layers can be
found in a thrust fault hanging wall or
a horst-and-graben system (Fig. 4D-I).
Systematic redistribution of the forams
at the bank margin represents systematic
removal of layers containing the oldest
species from the erosional window at
the source, either by burial (submarine
deposition) or by emergence of an
eroded overturned anticline above sea
level. Processes that occurred at the
source include pelagic deposition, faulting,
changing sea level, subsidence,
erosion, and uplift and gradual emergence
of Cuba. Lastly, using foraminiferal
settling rates, size range of the
reworked shells, and velocity, turbulence
and direction of currents in channels
between Cuba and the bank, remarkably
short transit times were calculated for
the imported specimens to reach the
bank. Depending upon precise distance
between the source on Cuba and the
western bank margin, the reworked fossils
could have made the trip in as many
as 27 to 48 days or as few as 5 to 8
days.

The western margin of the Great
Bahama Bank has long been believed
to have built seaward solely through progradation.
Yet through pelagic foraminifera,
a new dimension and new depositional
component are identified. The link
between the bank and Cuba implies that
the bank margin has advanced seaward
primarily but not solely through progradation.
Sediments from Cuba had to
have been rafted along with the microfossils
and therefore have helped build
the margin. If it had not been for the
microfossils, we would not have known
that the young (~7.5-1.9 Ma) host rock
represents an approximately 6-million-year-
long time capsule. Through the
window opened by the reworked species,
we have a glimpse into, and a
better understanding of, regional geologic
processes and events and the
global climatic record spanning nearly
66.5 million years of time.

Summary
The application of research involving
living benthic forams is an increasingly
important part of assessing changing
conditions in the nearshore marine
environment. The problem lies in determining
whether the changes are natural,
manmade, or both. The geologic record
contains the only evidence anywhere
for determining cause and effect of natural
change. Earth-science studies that
incorporate microfossils have a very
high potential for leading to pioneering
investigations and new discoveries. The
study of microfossils can touch on many
important questions and can benefit
many other disciplines. Pelagic forams
in particular are excellent and precise
tools for a wide range of studies.
Different types of microfossil data have
contributed to understanding past processes
and climatic events that produced
fluctuations in global sea level. The
more we know about past climate and
the causes that led to changes, the better
we can predict what may trigger climate
and sea-level change in the future.
The planktic foraminiferal record
has unveiled remarkable evidence for
heretofore-unknown sedimentary components
and depositional complexity in
a large carbonate bank. The reworked
forams of the Great Bahama Bank
are evidence of paleoceanographic, tectonic,
and sea-level control on regional
Cenozoic history. The quantity and
diversity of the imported fossil faunas
indicate that the global deep-sea record
may be far more prevalent in uncommon
settings or regions where it has not
been sought than is presently realized.
Microfossils provide fascinating information
in ways that no other part of the
geologic record can.

Additional Reading:
Ausich, W.I., 2000, Bones in the ivory tower: Geotimes, October, p. 18-21.
Cockey, E., Hallock, P., and Lidz, B., 1996, Decadal-scale changes in benthic foraminiferal
assemblages off Key Largo, Florida: Coral Reefs, v. 15, p. 237-248.
Farley, M.B., and Armentrout, J.M., 2000, Fossils in the oil patch: Geotimes, October, p.
14-17.
Lidz, B.H., 1982, Biostratigraphy and paleoenvironment of Miocene-Pliocene hemipelagic
limestone: Kingshill Seaway, St. Croix, U.S. Virgin Islands: Journal of Foraminiferal
Research, v. 12, no. 3, p. 205-233.
Lidz, B.H., 1984, Oldest (early Tertiary) subsurface carbonate rocks of St. Croix, USVI,
revealed in a turbidite-mudball: Journal of Foraminiferal Research, v. 14, no. 3, p.
213-227.
Lidz, B.H., 1984, Neogene sea-level change and emergence, St. Croix, Virgin Islands:
Evidence from basinal carbonate accumulations: Geological Society of America Bulletin,
v. 95, p. 1268-1279.
Lidz, B.H., and McNeill, D.F., 1998, New allocyclic dimensions in a prograding carbonate
bank: Evidence for eustatic, tectonic, and paleoceanographic control (Late Neogene,
Bahamas): Journal of Sedimentary Research, v. 68, no. 2, p. 269-282.
Martin, R., 2000, The environmental stories of microfossils: A new research path for
micropaleontology: Geotimes, January, p. 19-21.
Scott, D.B., and Lipps, J.H., 1995, eds., Environmental applications of foraminiferal studies:
Journal of Foraminiferal Research, v. 25, no. 3, p. 189-286.
Winchester, S., 2001, The Map That Changed the World: William Smith and the Birth of
Modern Geology: HarperCollins Publishers Inc., New York, 330 p.

For more information,
please contact:
Barbara H. Lidz
U. S. Geological Survey
600 4th Street South
St. Petersburg, FL 33701
phone: 727-803-8747 x3031
fax: 727-803-2030
blidz@usgs.gov

This report is preliminary and has not been
reviewed for conformity with U.S. Geological
Survey editorial standards or with the North
American Stratigraphic Code. Reference therein
to any specifi c commercial product, process, or
service by trade name, trademark, manufacturer,
or otherwise does not necessarily constitute
or imply its endorsement, recommendation, or
favoring by the United States Government or
any agency thereof.




Plate 1. (A) Examples of planktic foraminifera. Shell shapes in many planktic species might be said to resemble popped
popcorn. Shells of some species are essentially spherical. Others have elongate chambers and some have a single thick
spine on each chamber. Other shells are conical, or flat like discs and have a peripheral keel. Shell shape, thickness, and
other morphologic characteristics vary according to the water depth range in which the species primarily lives. In planktic
species, shell morphology is linked to flotation.

Plate 1. (B) Examples of benthic foraminifera. Shell shapes include flat, angular, tubular, branching, elongate, oval, and round.
Some benthic species also have a single thick spine on each chamber. Depending on orientation, some species look like fans,
tri-cornered hats, or single, double, or triple-scoop ice cream cones. Some species are encrusting and attach themselves
to hard objects on the sea floor. Some burrow beneath the sediment surface. Some that live on grass blades are "mobile" to
the degree that they move up the blades toward sunlight. In benthic species, shell morphology is linked to seafloor setting
and depth below sea level.