Gulf of Mexico Integrated Science - Tampa Bay Study

Historical and Prehistorical Record of Tampa Bay Environments

Introduction

To study how Tampa Bay, Florida, 

has changed over time, the prehistorical conditions and natural variations 

in the bay environment are being 

evaluated. These variations can be 

tracked by examining the sediments 

that have accumulated in and around 

the bay. The prehistorical record, 

which pre-dates settlersʼ arrival in 

the Tampa Bay area around 1850, 

provides a baseline with which to 

compare and evaluate the magnitude 

and effects of sea-level, climate, biological, geochemical, and man-made 

changes. These data also are valuable 

for planning and conducting projects 

aimed at restoring wetlands and other 

estuarine habitats to their original 

state. In addition, the data provide a 

basis for judging efforts to improve 

the health of the bay.

Approach

Sediment record

The primary technique for identifying man-made and natural variations in the bay environment is to 

examine sediment that has been 

collected from beneath the bottom 

of the bay. Scientists push core barrels into the sea floor and recover the 

sediment trapped inside. Research 

partners from the U.S. Geological 

Survey (USGS), University of South 

Florida (USF), Eckerd College, and 

Florida Department of Environmental Protection (FDEP) have collected 

74 cores throughout Tampa Bay. Six 

have been selected for detailed examination, which includes analyzing the 

sediment and fossils contained in the 

cores. Geochemical analyses reveal 

the chemical elements and isotopes 

characteristic of the environment in 

which the organisms lived, such as 

water salinity and temperature. Climate variations affect the bay-water 

components and are determined by 

analyzing pollen from ancient vegetation and from geochemistry of the 

sediments in the bottom of the cores. 

The results of the detailed analyses 

on these six cores will be correlated 

with similar data from cores located 

throughout the rest of the bay to trace 

layers of different sediment types 

among the cores.

Contaminants in clams

A short-term record of contaminants in Tampa Bay can be determined from a recently developed 

chemical analysis that measures trace 

metals in clam shells from season to 

season. Large southern quahog clams, 

or Mercenaria campechiensis, live up 

to 30 years and form growth increments similar to annual growth rings 

in trees. When alive, it is possible to 

determine how old they are by counting the growth increments, and variations in trace metal concentrations 

found in the shell can be dated.

Local historic climate change

Mangroves are very sensitive to 

weather and cannot tolerate freezing 

temperatures for very long. Sediment cores taken from coastal lakes 

located near mangrove trees contain 

their pollen. Mangrove pollen does 

not travel far, indicating that the trees 

have been located next to the lakes 

for a long time. Initial results show 

that temperature in the Tampa Bay 

area has not changed significantly in 

the past 2,000 years.

Sea-level change

During the last ice age, water that 

formed the glaciers was drawn from 

 Open-File Report 2005-1016

January 2005

U. S. Department of the Interior

U. S. Geological Survey

Locations of 74 sediment cores taken for the 

Tampa Bay Study. All the cores show the 

nature of the sediment accumulation in the bay, 

but only six will be analyzed in detail to evaluate effects of anthropogenic change.

Indian midden (see text) inland from the water’s 

edge in Terra Ceia where sea level is thought to have 

encroached inland, resulting from a vertical rise in sea 

level of approximately 76 cm about 1500 years ago. 
Terry Edgar, Task Leader

U.S. Geological Survey, Geologic Discipline

600 4th Street South, St. Petersburg, FL 33701

Email: tedgar@usgs.gov

Kimberly Yates, Project Chief 

U.S. Geological Survey, Geologic Discipline

600 4th Street South, St. Petersburg, FL 33701

Email: kyates@usgs.gov

For more information, please contact:

http://gulfsci.usgs.gov

Gregg Brooks, Eckerd College

Tom Cronin, USGS/Geologic Discipline

Dave Duncan, Eckerd College

Ben Flower, University of South Florida

Jason Greenwood, University of South Florida

David Hastings, Eckerd College

Al Hine, University of South Florida

Contributing Scientists:

David Hollander, University of South Florida

Becka Larson, Eckerd College

Stan Locker, University of South Florida

Randy Runnels, Florida Dept. of Environmental Protection

Peter Swarzenski, USGS/Geologic Discipline

Deb Willard, USGS/Geologic Discipline

the oceans, which lowered sea-level 

about 125 m. During the time of 

lower sea level, the west coast of 

Florida would have been about 200 

km west of Tampa Bay, and the bay 

at that time would have been dry land 

and located in central Florida. At the 

end of the last glacial period 21,000 

years ago, the ice began to melt rapidly and sea level rose swiftly. During 

the last 5,000 years, sea level has 

remained fairly constant with only 

minor fluctuations. 

A preliminary review of Indian 

middens (trash piles of shells that 

we believe were deposited near old 

shorelines) in southwestern Florida 

and Tampa Bay indicates that sea 

level rose about 76 cm approximately 

1,500 years ago. A sea-level rise in 

Tampa Bay has been identified by 

the presence of middens inland from 

the present coast. As the water later 

retreated, it left a “bathtub ring” of 

these middens around the bay marking the ancient coastline. The ring has 

disappeared in places due to urbanization, but scientists continue to map 

the “bathtub ring,” using the middens 

as a guide. 

Links to other project 

research

Sediment cores will assist in defining the historic trends and current 

status of the health of Tampa Bay by 

establishing a baseline that dates back 

to before human times in the bay area. 

This baseline can be used to compare 

prehistoric and current levels of 

contaminants in sediment and water. 

In addition to helping evaluate the 

health of the bay, the prehistoric data 

also provide a better understanding of 

how the bay was formed during the 

last sea-level rise, and how it changed 

from a freshwater environment to that 

of an estuary. 

Variations in porewater salinity 

(water trapped in the spaces between 

sediment particles) have been 

recorded in some of the cores. These 

variations may indicate fossil waters 

trapped during sedimentation or 

waters from different sources entering the sediment after the layers were 

deposited (such as seepage of fresh 

water from aquifers into the overlying sediment). 

Geologists are conducting seismic 

studies of the bay. Seismic profiles 

record different sediment layers below 

the bay floor. Seismic data are used to 

identify suitable locations to take cores 

and to correlate sediment layers found 

in the various cores. Meanwhile, the 

core and seismic data are correlated 

to identify the types of sediment in the 

layers beneath the bay floor.

Sea-floor

Seismic lines showing layers of sediment beneath the bay floor, and locations of cores that penetrated and recovered sediment from these layers. The blue layer near the surface and at depth is 

marine sediment, the green layer is sediment deposited at the bottom of an ancient lake, and the 

yellow layer is sediment derived from streams and rivers when sea level was low.

Retrieving a sediment core from Tampa Bay.

Water Level

Restricted Marine

Marine

Lake

Non-Marine