USGS Open-File Report 00–046

Bedrock Beneath Reefs: the Importance of Geology in
Understanding Biological Decline in a Modern Ecosystem
ENVIRONMENTAL QUALITY AND PRESERVATION--

Introduction
At issue is the health of the entire
coral reef ecosystem that lines the outer
shelf off the Florida Keys. In recent
years, nearly all biological components
have declined markedly. In particular,
loss of coral species that are the building
blocks of a solid reef framework has
significant negative implications for economic
vitality of the region. What are the
reasons for this decline? Is it due to natural
change, or are human activities (recreational
diving, ship grounding, farmland
runoff, nutrient influx, air-borne contaminants,
groundwater pollutants) a contributing
factor and if so, to what extent? To
answer these questions, one must be able
to distinguish between natural change
and human impact. Logically, then, the
place to begin is to establish the geologic
framework and evolutionary history of
the ecosystem, before the advent of
man. By so doing, accurate scientific
information becomes available for proper
authorities and Marine Sanctuary management
to understand natural geologic
and biologic change. Thus they are able
to assess and lessen potential human
impact more effectively.

The Importance of a
Healthy Reef Ecosystem
The importance of a healthy reef ecosystem
is essential to the quality of human
life, whether in Florida or in areas worldwide
where other reef systems are also
deteriorating. The hard rock of reef framework
serves as a protective offshore barrier
to catastrophic wave action and storm
surges generated by tropical storms and
hurricanes. Without this protection,
homes, marinas, and infrastructure on the
Florida Keys that are designed to capture
a lucrative tourism industry and inland
agricultural areas of South Florida that
feed much of the Nation are more susceptible
to destruction by these forces.
Moreover, biologic resources of the reefs
and shoreline mangroves that include habitats
for endangered species, productive
marine and wetland nurseries, and economic
fisheries are also at risk. The irony
is that the very riches of this ecosystem
attract the residential, recreational, and
commercial activities that may be contributing
to its decline.

By examining the geologic record,
in this case the record of ancient reef
growth, we can understand what controls
where and why modern reefs grow. Corals
require narrow conditions: relatively shallow
depths and stable salinity, clear nutrient-
poor water, and warm temperatures. In
tropical areas, the primary control of these
elements (in other words, reef growth) is
sea level, whereas the primary control of
reef distribution is bedrock topography,
that is, the shape of the rock surface under
the modern reefs (Plate 1). In Florida,
ancient reefs form much of the bedrock
surface. Mapping surface contours of the
pre-existing reef rock is the first step
toward reconstruction of the geologic history
and processes at work. Geophysical
data, collected on seismic profiles with
high-resolution sound-wave instruments
that see through the overlying sediments,
provide the bedrock-surface depth (elevation)
below sea level.

Figure 1. Schematic cross section (not to
scale) showing the six general bedrock
provinces along the South Florida shelf
and terminology used in text. The
Pleistocene ranges from ~1.77 Ma to 10
ka. The Holocene ranges from 10 ka to
the present. Sea-level positions and timing
shown in years before present. If sea level
were to rise from its present position by
5 to 7 m, it is very likely that frameworkbuilding
corals would once again become
established on those areas of the Florida
Keys that are basically sediment free.

Figure 2. Seismic profile showing a "slice"  or
cross section through geologic features. This cross
section displays the shelf margin at left and multiple
prominent, parallel outlier reefs off Sand Key Reef,
located at the edge of the shelf margin southwest
of Key West (Plate 1, insert). The deep 'V'-shaped
line called a multiple, at the bottom of the profile,
mimics the sediment surface between the two
largest outlier reefs. Outlier reefs are so called
because they are disconnected from the adjacent
shelf margin.

The U.S. Geological Survey (USGS)
Coastal and Marine Geology Program, in
cooperation with the National Oceanic and
Atmospheric Administration's (NOAA)
National Marine Sanctuary Program, continues
investigations of factors that may
be affecting Florida's reefs. The USGS
studies described here identify location
and degree of development of major geologic
features during the last 125,000
years (125 ka) and are based primarily on
seismic geophysical data.
Environmental Signatures
in the Geologic Record
Bedrock Surface
Analysis of the bedrock surface
reveals facts on natural processes of past
reef growth and demise, rising and falling
sea level, flooding of the bedrock surfaces,
sediment movement and accumulation,
and changing water depth, circulation,
water chemistry, and wave energy.
Supplemental knowledge obtained from
samples taken by coring through reefs
documents various responses by corals
and reefs to rising sea level, such as coral
species zonation and landward reef and
mangrove-shoreline migration. Uraniumthorium
and carbon-14 (C14) dating of
corals tells us when they began to grow,
how long they lived, and when they
died. C14 dating of mangrove peat, which
forms at the shoreline and is known to
exist beneath offshore reefs, reveals when
shorelines existed farther seaward. The
mapped bedrock surface delineates areas
of different elevations (Plate 1) and shows
where new coral growth would have
first become established. Because coral
larvae generally settle on sites where the
bottom is hard and free of sediment, reefs
began growing on elevated bedrock. Sand,
which prevents reef establishment, filled
the depressions. The mapped surface also
shows pathways the water would have
taken inland as the sea flooded the shelf
and thus positions and configurations of
ancient shorelines.
The Florida Keys
The upper and middle Florida Keys
are themselves composed of the emergent
skeletal remains of a discontinuous coral
reef, called the Key Largo Limestone, that
grew long before the patch reefs and outer-
shelf reefs one sees today (Fig. 1). The
lower Keys were once sandbars actively
moving back and forth with the tides.
Uranium-thorium dates from corals in the
fossil reef give an age of ~125 ka, identifying
a time when sea level was higher
than today. The dominant coral species
comprising the emergent reef are massive
framework-building head corals that live
in low-energy (deeper than 5 m) conditions
(m = meters). They indicate that sea
level was too high for a surf zone to have
existed where they grew. It is commonly
believed that sea level at that time was 7.6
m higher than at present. It has not been
as high since then.
The Shelf Margin
The same species of massive head
corals that constructed the Key Largo
Limestone also built an immense, younger
reef that is now the submerged shelf
margin (Figs. 1-3). Uranium-thorium
dates from these corals yield an age of 80
ka, marking yet another period when sea
level was high but not as high as when
the Keys were formed. The massiveness
(up to 30 m of relief) and regional extent
of the margin reef indicate that conditions
were once optimal for coral growth nearly
everywhere in the area. No younger reefs
can compare in either relief or extent
with the 80 ka shelf-margin reef, denoting
that environmental factors for reef growth
since then have never been as good.
Outlier Reefs
Because of the optimal environment
for reef growth at 80 ka, of particular
interest is the evolution of massive linear
reefs that lie seaward of the margin reef
off the lower and middle Keys. Called
outlier reefs, these reefs are located on
an eroded upper-slope surface (terrace)
thought to have formed ~200 ka (Figs.
2, 3). Although the outlier reefs contain
the same massive head-coral species and
are the same age as the reef at the shelf
margin, they differ from the shelf-margin
reef in regional extent, and they exhibit
varied stages of evolution. The outliers
are mature off the lower Keys (28 m of
relief), moderately well developed off the
middle Keys (less relief), and appear in
seismic profiles off the upper Keys as
low-evolution beach-dune features buried
by lime sand. The diverse evolution of
the outlier reefs is believed to have
resulted in part from differences in sanddune
development and the angle at which
prevailing east southeasterly winds and
waves impinged upon the arc-shaped shelf
margin (Plate 1). Off the upper Keys,
onshore tides during a period of lower sea
level washed sands over the dunes, burying
them. Off the lower Keys, longshore
wind and wave energy moved sand parallel
to the margin, keeping the higher-elevation
incipient reefs sediment free (Fig.
4A-E). Through time and with recurring
rises and falls in sea level, the outlier
reefs off the lower Keys were repeatedly
colonized by corals that eventually constructed
tall rock ridges as wide as 0.5
km. Today, the outlier reefs remain separated
from the margin by comparatively
sediment-free backreef troughs (Figs. 2,
3). Only the top 6 m of the largest outlier
have been cored and dated. The corals are
80 ka. Although the age of the base of the
outliers is not known, it is likely that it is
many tens of thousands of years older.

Figure 4. Proposed model for evolution of lower Keys shelf margin (sea level fluctuated many
times but positions correspond to events shown). (A) Formation and cementation of sand dunes
on the erosional terrace surface during a sea-level lowstand. (B) Colonization of sediment-free
shelf-margin reef and dune ridges by corals. (C,D) Upward building of margin reef and outlier
reefs over time and through numerous rises and falls of sea level. (E) Final demise of Pleistocene
corals on outliers at ~80 ka and later migration of younger (~7- to 2-ka) outer-shelf reefs with
shelf-wide flooding. Essentially no corals are alive on the outliers today. However, if sea level
were to fall by some 10 m or so, it is very likely that framework-building corals would once again
become established on the hard rock surface of the outlier reefs.


Outer-Shelf Reefs
The most recent rise in sea level
began at ~10 ka as a result of warming
temperatures and melting glaciers. C14
dates on corals from fossil outer-shelf
reefs landward of the margin reef indicate
that they first began growing at about 7
to 6 ka. Thus, what is now the modern
reef tract was land until flooding of the
shelf began about that time. The coral
species in the outer-shelf reefs (Fig. 1)
are different from the massive head corals
that built the older reefs. Frameworkbuilding
branching corals that prefer highenergy
conditions constructed the outershelf
reefs and thus verify presence of
the surf zone when the reefs were alive.
Branching corals are known to grow only
in water depths of up to 5 m, which indicates
another relative position of sea level.
The extent and ages of the outer-shelf
reefs show they too thrived and lived from
about 7 to 2 ka. Although discontinuous
and not as massive as the older Key Largo
Limestone reef, the shelf-margin reef, and
the outlier reefs, thickness of the outershelf
reefs relative to thickness of modern
reefs indicates that conditions for coral
growth from 7 to 2 ka were nonetheless
considerably better than they are today.
Much of what is now under water was
still land until about 2 ka, when flooding
of the large shallow bedrock depressions
that became Florida and Biscayne Bays
occurred. Creation of the bays contributed
to demise of the reefs by allowing tidal
exchange of turbid lagoonal waters onto
the reef tract. The lagoonal bays were
filled when sea level was about 0.5 m
lower than today. Prior to about 40 years
ago, modern reefs consisted of healthy
branching corals. Today, very few of these
corals are alive and those that are, are in
very poor condition.

Cores taken from various locations
through the outer-shelf reefs give us
insights into reef growth during rising
sea level. Location of reefs (distribution),
types of corals (head or branching), landward
reef migration (known as backstepping),
and zones of changing coral species
are some of the corals’ responses to
suitable site (elevated-bedrock availabiliity),
environmental conditions (protected
or open to surf), and rising sea level.
An increase in water depth alters circulation
and wave energy, which are
also controlled by local bedrock topography.
Increasing water depth changes water
chemistry and temperature through tidal
flow of murky, nutrient-rich bay water
and cold Gulf of Mexico water into the
clear warm water surrounding the reefs.
Human influence on these deleterious elements
can severely impact an already naturally
weakened reef ecosystem.

Figure 3. Interpreted seismic profile shown in
Figure 2. Largest outlier reef developed at seaward
edge of broad erosional surface (Pleistocene
terrace). Note terrace surface is not visible beneath
the reefs. Coral reefs and reef rubble or debris
characteristically obscure sound-wave reflectivity of
rock surfaces beneath them. A core drilled into the
top of the main outlier recovered 6.09 m of coral
rock. Uranium-thorium dating yielded ages of 83 to
80 ka for corals in the core. Tick marks at bottom of
interpreted profile indicate time (24-hr scale).



Plate 1. Index map showing tip of South Florida and Cape Sable,
the Florida Keys, the Marquesas Keys in the Gulf of Mexico, and
the bedrock surface along the shelf and margin. The upper and
middle Keys are formed of the 125-ka coral reef; the lower Keys
are formed of cemented tidal bars of the same age. Colors on the
map represent different bedrock elevations ranging from highest
(pale blues) to lower (dark blues) to lowest (shades of green).
Contacts between colors can be interpreted as representing positions
and configurations of ancient shorelines as sea level rose.
Outlier reefs are in shades of brown (light=high relief, dark=low
relief or buried). The 80-ka shelf-margin reef coincides with a
dark blue line just landward of the outlier reefs. Note elevated
rectangular platform on which the Marquesas Keys are located.
Deep water on three sides and a current-swept, sediment-free
channel on the east indicate that sediments on the platform are
formed in place. In general, the map shows bedrock elevations
are several meters higher to the northeast than southwest, meaning
the shelf flooded sooner and shelf-margin reefs began growth
sooner to the southwest. The least healthy modern reefs occur off
the middle Keys, where wide, deep tidal passes between the Keys
allow turbid bay and cold gulf waters onto the reef tract.

In Florida, vitality of the fragile coral reef ecosystem depends on a critical
equilibrium between physical processes and landforms. The primary process
is rising sea level. The landform is the bedrock surface and its shape and
elevation above and beneath the sediments. Coral reefs and associated
organisms flourished along the outer shelf until the sea fl ooded shallow
inland bedrock depressions. As bay waters circulated seaward, water quality
and chemistry around the reefs changed, forcing a shift in biotic communities.
Reef-building corals are no longer alive. Understanding the natural
connection between geologic parameters and the well being of biologic
resources is an imperative fi rst step toward resource-wise restoration, preservation
and management of the ecosystem.

The USGS serves the nation by providing basic,
objective, and informative scientific knowledge on
Earth history and processes, and on human impact
on resources. Two of the most effective ways
are through descriptive and interpretive maps and
through the USGS marine homepage on the world
wide web at:
http://marine.usgs.gov.


Summary
The corals that form the middle and
upper Florida Keys verify the previous
existence of an inshore reef ecosystem.
Mangrove peats found beneath offshore
reefs record the locations of earlier, seaward,
shorelines. Zonation of coral species
within a reef is evidence of increasing
water depth and thus changing conditions,
from quiet water protected from
waves to high-energy surf to water too
deep for a surf zone. Sea level has
risen and fallen many times before,
and the massive limestone outlier reefs
were accordingly colonized, killed during
exposure, and re-colonized. These signatures
in the geologic record indicate that
reefs and shorelines have come and gone
in the past. Could the fragile modern ecosystem
be undergoing yet another natural
decline, or are human activities responsible
or, at the very least, an added component?
We cannot know without first
knowing what controlled evolution and
development of the ancient reefs, i.e.,
the bedrock characteristics, the processes
that produced them, and the responses
the ancient reefs had to environmental
change. The bedrock is far more important
than simply being the surface beneath
modern reefs. The environmental signatures
it contains provide the baseline
information from which understanding
decline of the modern reef ecosystem
must be derived.
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 specific 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.

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