New Geologic Explanation for the Florida Middle Ground in the Gulf of Mexico
The Florida Middle Ground is a series of underwater ridges on the otherwise gently sloping continental shelf off the west coast of Florida. The origin of this anomalous feature has long intrigued scientists, who developed hypotheses based on the shapes of the ridges, samples of surface sediment, and images of subseafloor layers. In 2010 and 2011, U.S. Geological Survey (USGS) scientists drilled into some of the ridges and discovered evidence of an origin no one had yet proposed: the ridges likely formed as a series of shore-parallel sediment bars eventually capped and preserved by vermetid gastropods, or “worm snails.” The results were reported in 2013 in the Journal of Coastal Research ("The Role of Vermetid Gastropods in the Development of the Florida Middle Ground, Northeast Gulf of Mexico").
The Florida Middle Ground covers an area of about 1,200 square kilometers (460 square miles) in the northeastern Gulf of Mexico, approximately 180 kilometers (110 miles) northwest of Tampa Bay. Running parallel to the shelf edge in a north-northwest direction, the ridges are as much as 60 kilometers (40 miles) long and 15 kilometers (10 miles) wide. Water depths range from about 25 meters (80 feet) on the ridge crests to more than 45 meters (150 feet) in the troughs. Individual banks have vertical relief of 10 to 15 meters (30 to 50 feet), about as high as 3- to 5-story buildings.
Near-vertical bank edges consisting of overhangs and caverns plus a supply of nutrients and warm tropical waters from the Loop Current have created environments rich in tropical and subtropical fish and invertebrate animals. The invertebrates include numerous coral species that give the Florida Middle Ground the distinction of being the northernmost living coral reef in the conterminous United States.
For more than a century, this unusual feature challenged those seeking to understand its origin. From its discovery in the late 1800s by fishermen from Tampa Bay and Apalachicola Bay, through the first reconnaissance survey in 1914 by the U.S. Coast and Geodetic Survey, to modern geophysical surveys, the dramatic bathymetric relief of the Florida Middle Ground inspired many hypotheses about its geologic composition and history. One of the first proposals, because of the shape and orientation of the ridges, was that the Florida Middle Ground was a drowned river valley, with the two main ridges being riverbanks that were drowned by rising seas and then cemented and lithified (turned into rock). The sediment that formed the riverbanks was thought to have come from either the Apalachicola or the Suwannee River about 10,000 years ago, when sea level was 40 meters (130 feet) lower than it is today. The most prevalent and persistent hypothesis was that the Florida Middle Ground was a buildup of coral reefs. The coral-reef hypothesis was first proposed in the early 1960s and survived until USGS scientists succeeded in collecting samples from inside the ridges.
In the spring of 2010, Chris Reich, Don Hickey, Keith Ludwig, and colleagues at the USGS St. Petersburg Coastal and Marine Science Center (SPCMSC) in St. Petersburg, Florida, took on the challenge of organizing a team of drillers and experienced scuba divers to be the first group to collect a “long” core from the Florida Middle Ground. Theirs would not be the first core—in a 1972 Master’s thesis for Florida State University, Robert M. Back mentioned a 2.1-meter (6.9 foot) core collected by another researcher from an unknown location in the Florida Middle Ground and described simply as “carbonate rock.” On the basis of this and other information available to them in 2010, the USGS scientists expected that the cores they planned to collect would confirm and add detail to the coral-reef hypothesis for the origin of the ridges.
Though the team members were highly experienced divers, many were novices when it came to drilling. Reich and Hickey tried to pair the experienced drillers with those that didn’t have much or any background in drilling. The dive team consisted of Lee Bodkin (USGS, Texas Water Science Center, Shenandoah), Kayla Gibbs (USGS, Texas Water Science Center, Ft. Worth), Don Hickey (USGS SPCMSC), Paul Knorr (USGS SPCMSC), Keith Ludwig (USGS SPCMSC), Justin McInnis (USGS, Texas Water Science Center, Austin), Chris Reich (USGS SPCMSC), BJ Reynolds (USGS SPCMSC), Jordan Sanford (USGS SPCMSC), Adam Brame (National Oceanic and Atmospheric Administration’s National Marine Fisheries Service, NOAA-NMFS), Anastasio Stathakopoulos (National Coral Reef Institute at Nova Southeastern University), and Libby Carnahan (Florida Sea Grant). Marc Blouin (USGS Bureau Dive Safety Program Manager) served as the dive safety officer (DSO). Gene Shinn (USGS, retired), Al Hine (University of South Florida [USF]), and Stan Locker (USF) were extremely helpful in identifying a coring site on the basis of previously collected high-resolution seismic profiles—images of subseafloor layers produced from acoustic (sound) data. Another factor in site selection was water depth; to avoid the need for decompression after their scuba dives, the team elected to spend no more than 45 minutes at depths of 86 feet or less.
The organizers chartered the merchant vessel (M/V) Spree, which became the team’s living quarters and center for diving operations. They towed the USGS research vessel (R/V) Halimeda for use as the drilling platform. The first round of coring took place from August 1 to 7, 2010, at a water depth of 26 meters (86 feet). The seas were calm for the entirety of the coring operations, which the team members more than welcomed and to which they give partial credit for the trip’s success. In order to verify that their findings were not unique to the northern part of the ridge drilled in 2010, six of the scientists made a separate drilling trip on the USGS R/V Gilbert in September 2011 and took a single 0.6-meter (2 foot)-long core. The September 2011 core was collected approximately 10 kilometers (6 miles) south of the 2010 coring site in approximately 29 meters (95 feet) of water.
The 2010 core samples were collected with the USGS rotary hydraulic corer, a versatile tool developed decades ago by the USGS (for example, see photographs in “Passing the Torch, Take 2—Barbara Lidz Steps Down as Sound Waves Contributing Editor at USGS Center in St. Petersburg, Florida,” Sound Waves, November/December 2013). The corer uses a wireline system, a high-speed/low-torque hydraulic motor, and a gear pump that circulates seawater as drilling fluid. The wireline system contains an inner barrel that retains a 2-inch-diameter core, which is extracted at 5-foot intervals. Once extracted, the barrel and core were sent up to the R/V Halimeda, where the core was removed from the barrel, placed in a box, and cataloged. Collectively, over the 5 days of diving and coring, the team logged 65 dives with a total bottom time of 101 hours. They drilled four holes that ranged in depth from 0.6 to 17 meters (2 feet to 57 feet) below the seafloor. Core recovery was very poor (0 to 20 percent) in all four holes, but the team collected enough material to complete a suite of analyses and make several definitive conclusions regarding the geologic history of the Florida Middle Ground.
So what did the scientists find? One thing they didn’t find in any of the recovered material was remnants of coral. The Florida Middle Ground was not a coral reef buildup as many had previously presumed it to be. Nor was it a drowned river valley. The scientists were all a bit dumbfounded to discover that the ridges consisted of unconsolidated marine calcareous muddy sand, about 12 meters (40 feet) thick, overlying a weathered, fossiliferous limestone of Miocene age (between 5 and 22 million years old) and capped by a carbonate rock composed primarily of the sessile vermetid gastropod Petaloconchus sp. (a marine snail that cements its tubular shell to a hard surface, such as a rock or another shell; see photograph). Their observations suggest that the Florida Middle Ground is the remnant of a series of shore-parallel bars that formed as sea level was rising in the early Holocene, approximately 10,000 years ago. Subsequently the bars were capped by a 3.6-meter (12 foot)-thick unit of vermetid gastropods. Accelerator mass spectrometry radiocarbon ages (uncalibrated) on the vermetid cap rock indicate that it developed during a sea-level stillstand in the early Holocene, about 9,000 to 8,200 years ago. The stillstand lasted nearly 1,000 years, providing ideal growing conditions for the vermetid gastropods. Waters during this period were shallow and had enough turbidity to provide material for binding the shells together, resulting in the development of a thick layer of vermetid shells. When sea-level rise accelerated, starting about 8,200 years ago, the vermetids could not adapt to the quickly changing environmental conditions and died.
Although the scientists now know that the ridges are armored with a vermetid gastropod cap rock, uncertainty remains regarding how and when the underlying, thick sequence of muddy sand was deposited. The drilling fluids excluded any of the sediment beneath the vermetid cap rock from being recovered in the inner barrel, but fortunately the divers were able to obtain some of this material in a short (approximately 20-centimeter [8 inch]-long) push core, collected by pushing a tube into the sediment. Where did all of the mud and sand come from, and are the researchers’ assumptions about its depositional history correct? The only way to test this theory is to make a return visit to sample the entire sediment sequence with push cores or vibracores (watch video of long cores collected by vibrating a core barrel into the sediment).
The complete citation for the article about this research is: Reich, C.D., Poore, R.Z., and Hickey, T.D., 2013, The role of vermetid gastropods in the development of the Florida Middle Ground, northeast Gulf of Mexico: Journal of Coastal Research, special issue 63, p. 46–57, doi:10.2112/SI63-005.1.
in this issue:
New Geologic Explanation for the Florida Middle Ground