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Coastal & Marine Geology Program > Center for Coastal & Watershed Studies > Professional Paper 1751

Systematic Mapping of Bedrock and Habitats along the Florida Reef Tract—Central Key Largo to Halfmoon Shoal (Gulf of Mexico)

USGS Professional Paper 1751

by Barbara H. Lidz, Christopher D. Reich, and Eugene A. Shinn

Introduction:
Table of Contents
Project Overview
Project Objective
Geologic Setting
Primary Datasets
Primary Products - Overview Maps & Evolution Overview:
Bedrock Surface map.
Introduction
Depth to Pleistocene Bedrock Surface
Reef & Sediment Thickness
Benthic Ecosystems & Environments
Sedimentary Grains in 1989
Summary Illustration Index Map
Evolution Overview
Tile-by-Tile Analysis
Satellite image of the Florida Keys showing location of tiles.
Organization of Report
Tiles: 1, 2, 3, 4,
5, 6, 7/8, 9/10,
11
Summary
Acknowledg-
ments
References
Disclaimer
Related
Publications

Tile 3

Sedimentary Petrology: Sedimentary petrology is the study of sediment grains. In a carbonate ecosystem, sand grains and sediments are biogenic—that is, derived from skeletons of the organisms that live there. Each skeletal grain has a specific crystal structure characteristic of a specific organism group, such as calcified (coralline) red or green algae, corals, molluscs, sea urchins, and so forth, even microscopic shelled protozoans such as foraminifera.

To detect this structure and identify the group of origin, one must examine thin slices of each grain. Sediment in a sample is first mixed to homogenize grains of different size. The mixed sediment is then impregnated with epoxy resin, allowed to harden and then sliced. The slice is mounted on a glass slide and ground down, making a thin section of the sediment. Thin sections are examined under a petrographic microscope that transmits polarized light through the glass slide and the grains, illuminating the diagnostic crystal structures. Figure 12 shows examples of skeletal carbonate grains in thin section.

The three dominant types of sand grains in the Florida-Caribbean region belong to molluscs, species of a calcified green alga (Halimeda), and corals (e.g., Thorp, 1935; Illing, 1954; Ginsburg, 1956). Recent studies have shown that, relative to molluscan and algal grains, the proportion of coral grains increased significantly in Florida sands between the 1950s and 1990s (Lidz et al., 1985; Lidz and Hallock, 2000).

The relative proportions of various types of sand grains in surface sediments provide information about the organisms in the local biologic community. When used in conjunction with other types of data obtained from the sample or sample site, sand grains are thus indicators of the type of environment in which those organisms are known to live. Where molluscan grains dominate the sand, live molluscs dominate the community (e.g., Hallock, 1988). The same is said for Halimeda grains or for grains of other organisms. Coral is the exception. Coral grains dominate areas where dead or dying corals are undergoing bioerosion. High percentages of coral grains are thus biologic (source) and geologic (sand) indicators of poor coral health.

The USGS has mapped sand-grain percentages throughout the Florida Keys reef tract, providing a shelf-wide baseline spatial dataset for use in monitoring future changes in the structure/environment of live biological communities (see Sedimentary Grains in 1989 map and Open-File Report 97-453). Temporal changes in community structure at a specific site can be similarly determined by examining thin sections of the biogenic grains sampled at the surface over time or at intervals down a sediment core.

The most comprehensive petrologic time-series dataset from surface sediments along the reef tract is from Tile 3 and an adjacent portion of Tile 4 (south Key Largo to Vaca Key). Three sets of samples were collected in cross-shelf transects over a period of 37 years (Fig. 59). The early samples were collected in 1952 (Ginsburg, 1956) and 1963 (Swinchatt, 1965) along selected but isolated transects. The 1989 sampling provided the most spatially extensive dataset (Fig. 60A, 60B, 60C; Lidz and Hallock, 2000). At that time, grains of Halimeda and molluscs dominated sediments off the upper Keys, while Halimeda and coral grains dominated off the middle and lower Keys (Fig. 61).

Index map of south Florida shows location of major geographic features, major coral reefs along the shelf margin (blue depth contour, in meters), and colored transects of three studies (in small box) that examined skeletal grains in surface sedimentsFigure 59. Index map of south Florida shows location of major geographic features, major coral reefs along the shelf margin (blue depth contour, in meters), and colored transects of three studies (in small box) that examined skeletal grains in surface sediments (from Lidz and Hallock, 2000). Dashed lines delineate the upper, middle, and lower Keys. Note locations and widths of tidal passes in the middle Keys compared to those in the upper and lower Keys. Inset under North arrow shows paths of major hurricanes in the area between sample years 1952 and 1989. Offshore arrows indicate northwestward Gulf Stream axis. Boxes A-C are enlarged in Figure 60A, 60B, and 60C. Area sampled in 1989 (red dots = sample sites of Lidz and Hallock, 2000) measures roughly 600 nmi2. [larger version]

Enlarged insets A-C (from Figure 59) show numbered 1989 sample sites (red dots of Lidz and Hallock, 2000), relative to the keys (dashed lines), tidal passes, and the shelf margin (blue depth contour, in meters).Figure 60. Enlarged insets A-C (from Fig. 59) show numbered 1989 sample sites (red dots of Lidz and Hallock, 2000), relative to the keys (dashed lines), tidal passes, and the shelf margin (blue depth contour, in meters). Note the parallelogram representing the Looe Key National Marine Sanctuary at the margin in (C). Sediment grains illustrated in thin section (Fig. 12) came from the Looe Key National Marine Sanctuary. [larger version]

Contour map shows areas where each of the three primary grain types (Halimeda, mollusc, and coral) was dominant in 1989.Figure 61. (A) Contour map shows areas where each of the three primary grain types (Halimeda, mollusc, and coral) was dominant in 1989 (from Lidz and Hallock, 2000). Black dots indicate 1989 sample sites. (B) Distribution and percentages of coral grains in 1989. (C) Generalized distribution of healthy, declining, and senescent reefs based on field observations. Correlation of observed reef vitality (C) with coral-grain proportions (D) shows that coral-grain percentages are a reliable method to delineate areas of healthy, declining, and senescent (senile) reefs quantitatively. Parallel vertical lines align areas of different reef vitalities with corresponding coral-grain percentages. Lidz and Hallock (2000) proposed a quantification method to determine reef vitality based on coral-grain percentages: where corals are present yet coral grains form <10% of sediment components, reefs are healthiest. Where coral grains compose 10% to 29% of the sediment, reefs are declining. Where coral grains constitute >30% of the sand, reef framework is rapidly deteriorating. In the middle and lower Keys in 1989, coral grains exceeded 50% and in some places 60% of all skeletal fragments along the margin. Latitude and longitude tick marks apply to (A). [larger version]

Comparisons of distributions and relative proportions of the three dominant grain types could be made in two areas over two periods of time: from Davis to Sombrero Keys (Tiles 2-4) over 26 years (1963-1989), and from Tennessee Reef to Sombrero Key (Tiles 3-4) over 37 years (1952-1989) (Figs. 62, 63, 64). The comparative petrologic data provide information that essentially answers the questions of what, where, how, why, and when changing sand constituents occurred. The data also demonstrate the correlation between changes in coral-sand proportions and coral reef health as observed in the field.

  • What: Coral-sand production accelerates in direct proportion to the amount of coral skeleton available for breakdown by boring and grazing organisms (bioerosion).

  • Where: Particulate or fragmented coral is the dominant grain in sands where reefs are observed to be declining or senescent (essentially dead). Observed reef vitality is poorer and the number of coral grains is greater off tidal passes than off protective islands (Fig. 61). Elongate islands prevent nearshore bay waters unsuitable for coral growth from reaching offshore reefs.

  • How: Weakened or dead coral skeletons are bioeroded more quickly than healthy corals.

  • Why: Adverse environmental factors are causing increasing incidences and types of coral diseases and increasing algal cover. Coral diseases often result in coral mortality. Because coral larvae settle preferentially on elevated hard surfaces, proliferating algae decrease the area accessible for new coral recruitment. These and other factors result in increasing skeletal-coral availability for bioerosion.

  • When: Percentages and distribution of coral grains have increased substantially over a 37-year period (1952-1989), mostly along the outer shelf (Fig. 61). This time frame coincides with the period of reef decline observed in the field.

Contour maps show 1952 and 1963 grain-type data between Sombrero Key and Tennessee Reefs.Figure 62. (A-D) Contour maps show 1952 and 1963 grain-type data between Sombrero Key and Tennessee Reefs (distance ~18 km; Ginsburg, 1956) and Sombrero Key and Davis Reefs (distance ~35 km; Swinchatt, 1965) in the middle Keys area (dashed lines delineate middle and upper Keys; from Lidz and Hallock, 2000). The 1952 maps, based on sparse samples (16 sites along two traverses), cannot be compared closely to the later maps but are useful for a sense of sediment composition in the area in 1952. The 1963 maps, more detailed and covering a broader area, are more comparable to those for 1989. The maps indicate a significant increase in the coral component of the sediments. [larger version]

Contour maps show spatial and temporal trends in coral-grain proportions in the middle Keys from 1952 to 1989. Figure 63. Contour maps show spatial and temporal trends in coral-grain proportions in the middle Keys from 1952 to 1989. [larger version]

Summary histogram shows historic differences in percentages of the three dominant types of sand grains in sample years 1952, 1963, and 1989. Figure 64. Summary histogram shows historic differences in percentages of the three dominant types of sand grains in sample years 1952, 1963, and 1989. [larger version]

Constituent sand-grain percentages can be placed into the context of a conceptual model that shows present status of the reef ecosystem in the upper and middle Florida Keys (Fig. 65A, 65B). The model predicts that increasing nutrients (or other adverse factors) in the environment will cause decreasing quantities of live endosymbiont-bearing hosts, that is, reef-building corals and larger foraminifera. These two groups are physiologically similar in that both require symbiotic algae for growth, color, and photosynthesis, and both show similar responses to environmental stress. Along with decreasing symbiont-bearing hosts, the model also predicts that an increase in percent grains of calcareous algae and bioeroded coral and molluscs will occur at intermediate stages of nutrification. Results of the 1989 study (Lidz and Hallock, 2000) are summarized (Fig. 66).

(A) Summary of petrologic data averaged by sample year for upper, middle, and lower Keys. (B) Conceptual nutrient-effect model predicts effects of nutrification (increasing nutrients) on skeletal sedimentary components in a reef ecosystem.
Figure 65. (A) Summary of petrologic data averaged by sample year for upper, middle, and lower Keys (from Lidz and Hallock, 2000). Note data sources. (B) Conceptual nutrient-effect model predicts effects of nutrification (increasing nutrients) on skeletal sedimentary components in a reef ecosystem (adapted from Hallock, 1988, and Hallock et al., 1988). Shaded area represents present conditions in the Florida Keys. Short arrows show postulated shifts in upper and middle Keys bioerosional components (i.e., a decline in live algae-bearing corals and larger algae-bearing foraminifera with a commensurate increase in grains of calcareous algae, molluscs, and corals) between 1963 and 1989. [larger version]

Summary: results from the petrologic study of Lidz and Hallock (2000) point to a proposed objective petrographic measure to quantify reef vitality (Figure 61D). Nutrient-effect model mentioned in conclusions 2 and 4 is shown in Figure 65B.
Figure 66. Summary: results from the petrologic study of Lidz and Hallock (2000) point to a proposed objective petrographic measure to quantify reef vitality (Fig. 61D). Nutrient-effect model mentioned in conclusions 2 and 4 is shown in Figure 65B.

Coastal & Marine Geology Program > Center for Coastal & Watershed Studies > Professional Paper 1751

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