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Home | Tampa Bay Study | Data | Task 3: History and Prehistory | Sea-level Changes

This page is archived and is no longer being maintained. Content was last updated in 2015. For current research, visit http://coastal.er.usgs.gov/.
Data > Task 3: History and Prehistory > Sea-level Changes
Project Lead: Thomas M. Cronin
Drs. Terry Edgar (USGS) and Gregg Brooks (Eckerd College) discuss the evidence of both marine and lacustrine (fresh-water) sediments found in a core collected from Tampa Bay.
Drs. Terry Edgar (USGS) and Gregg Brooks (Eckerd College) discuss the evidence of both marine and lacustrine (fresh-water) sediments found in a core collected from Tampa Bay.

The rate of global sea-level rise has increased from about 2 millimeters per year during the past century to about 3 millimeters per year during the past few decades due mainly to warming ocean temperatures, melting ice, possible decadal climate variability or a combination of factors. This totals between six inches and a foot over 100 years.

Estimates for future sea-level rise by the end of the 21st century range from about 0.2 to 0.5 meters, but these are subject to large uncertainties due to many factors. Many researchers however now believe these rates underestimate future sea-level rise because glaciers and small ice caps are rapidly melting and complex dynamical changes in the margins of the Greenland and Antarctic ice sheets might also lead to additional melting.

It is imperative that researchers gain a better understanding of patterns, rates and causes of sea-level change based on the geological record. Global sea level rose 120 to 125 meters since the last ice age (called the last glacial maximum) about 22,000 to 26,000 years ago. At this time, the Tampa Bay region was an emergent part of the Florida peninsula, consisting of land with lakes and wetlands and a very different climate [see Climate History part of Tampa Bay Study Website]. Since the glacial maximum, as ice sheets melted, sea level rose sometimes as rapidly as 40 to 50 millimeters per year equivalent to 70 to 75 feet in 500 years.

The USGS, in cooperation with Eckerd College and the University of South Florida, have studied the nature of the flooding of Tampa Bay by this post-glacial sea-level rise using sediment cores as part of the Tampa Bay Study. The cores were collected from ships using either a vibracore or push-core system and then brought back into the lab for analysis. Fossil shells and pieces of wood are dated by radiocarbon dating to provide accurate chronology. Small fossil shells of single-celled organisms called foraminifers and crustaceans called ostracodes are analyzed to reconstruct the environment in which the sediment was deposited. These core records reveal that a rapid environmental transition from lakes and freshwater wetlands to estuary and bay environments occurred about 7,000 years ago. Thus modern Tampa Bay was formed by this rising sea level. Continued analysis of the impacts of sea-level rise on Tampa Bay biota and coastlines are useful in understanding potential impacts of future sea-level rise.

Core is cut in half with circular saw. Cores are collected from a vibracore system mounted on a boat. The cores are brought back into the lab where they are split in half length-wise. One half is stored, the other half is sampled and analyzed in the lab to generate information on the core stratigraphy and to date the material. Geologist interpret this information to describe the palaeo-ecological development of the location where the core was collected. Half of core being sampled
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