Barbara H. Lidz

Sclerochronology: A tool for interpreting past environments

X-radiographs of stony coral slabs reveal two types of annual density bands. Detailed studies of these bands in relation to known variations in air temperatures indicate that sclerochronology is a valid tool for documenting time sequences and changing environmental conditions on a coral reef.

Foraminifera as Bioindicators in Coral Reef Assessment and Monitoring: The FORAM Index

Coral reef communities are threatened worldwide. Resource managers urgently need indicators of the biological condition of reef environments that can relate data acquired through remote-sensing, water-quality and benthic-community monitoring to stress responses in reef organisms. The “FORAM” (Foraminifera in Reef Assessment and Monitoring) Index (FI) is based on 30 years of research on reef sediments and reef-dwelling larger foraminifers. These shelled protists are ideal indicator organisms because:• Foraminifers are widely used as environmental and paleoenvironmental indicators in many contexts;• Reef-building, zooxanthellate corals and foraminifers with algal symbionts have similar water-quality requirements;• The relatively short life spans of foraminifers as compared with long-lived colonial corals facilitate differentiation between long-term water-quality decline and episodic stress events;• Foraminifers are relatively small and abundant, permitting statistically significant sample sizes to be collected quickly and relatively inexpensively, ideally as a component of comprehensive monitoring programs; and• Collection of foraminifers has minimal impact on reef resources.USEPA guidelines for ecological indicators are used to evaluate the FI. Data required are foraminiferal assemblages from surface sediments of reef-associated environments. The FI provides resource managers with a simple procedure for determining the suitability of benthic environments for communities dominated by algal symbiotic organisms. The FI can be applied independently, or incorporated into existing or planned monitoring efforts. The simple calculations require limited computer capabilities and therefore can be applied readily to reef-associated environments worldwide. In addition, the foraminiferal shells collected can be subjected to morphometric and geochemical analyses in areas of suspected heavy-metal pollution, and the data sets for the index can be used with other monitoring data in detailed multidimensional assessments.

Whitings, a sedimentologic dilemma

Whitings, drifting clouds of water, milky because of suspended carbonate, have been claimed to originate from either the action of bottom-feeding fish or direct precipitation of calcium carbonate. Five cruises during different seasons were made to the Great Bahama Bank to collect data pertinent to the controversy. Measurements of particulate concentrations average 10 mg/liter with a maximum of 20 mg/liter of carbonate sediment suspended in whiting water, compared with an average of 1.5 mg/liter for clear water outside the whitings. The particles are dominantly acicular aragonite, but Mg calcite composes as much as 20 percent of some whitings. Sedimentation rates, measured with fixed and drifting sediment traps, were as great as 34 g/m 2 /hr. Sediment suspended in whitings aggregated into silt- and sand-size fioccules and settled to the bottom of settling tanks within six hours, even on a rocking ship. Sediment in artificial whitings, created by stirring sediment from the bottom with a shrimp trawl, settled to the bottom in about the same time. Natural whitings, on the other hand, were never observed to dissipate. Because sedimentation from whitings occurs at rates sufficient to cause dissipation of the whitings within six hours, we conclude that the natural whitings are continually replenished by direct precipitation. The search for fish in whitings utilized sidescan sonar and fathometer imaging, shrimp trawls, rotenone, remote video, and direct scuba observation. These methods and 25 years of casual observations leading to this study indicate that fish are not involved in the formation of most Bahamian whitings. Several whitings were found over rocky or sandy bottoms where there was no mud available for fish to suspend. The distance of these whitings from areas of muddy bottom precluded their having been made elsewhere by fish. Stable carbon- and oxygen-isotopic analyses and Delta 14 C activity are interpreted to indicate that the suspended sediment in whitings contains some precipitated calcium carbonate and is not merely bottom sediment stirred into suspension. Estimates indicate that the amount of new carbonate produced in whitings on the Great Bahama Bank is substantially higher than that arising from algal production. Consequently, the amount of sediment transported to deep water may be much greater than previously thought.

Decadal-scale changes in benthic foraminiferal assemblages off Key Largo, Florida

Assemblages of foraminiferal tests in sediments sampled off Key Largo, Florida, in 1982, 1991, and 1992 were significantly different from assemblages sampled along the same traverses in 1959–1961. Larger, algal symbiont-bearing taxa, primarily Soritidae, comprised 50–80% of the specimens in samples collected in 1959–1961, whereas Miliolidae and Rotaliidae comprised 65-90% of the specimens collected in 1991 and 1992. Test abundance in 1992 samples ranged from 1.0 × 102/g to 8.1 × 104/g; tests were least abundant in coarse, well-sorted sediments. The lack of test-density data for the 1959-1961 samples prevented assessment of whether densities of smaller foraminifera have increased, symbiotic foraminifera have decreased, or both. Between 1982 and 1992, densities of smaller foraminifera appear to have increased. Although the causes of these changes in foraminiferal assemblages are not known, possible factors include nutrient loading inshore, winnowing and transport of tests by storm activity, and disease. The shift in dominance from long-lived, algal symbiont-bearing taxa in 1959--1961 to small, fast-growing, heterotrophic taxa in 1992 is consistent with predictions of community response to gradually increasing nutrient flux into south Floridas coastal waters. This study indicates that published accounts of foraminiferal assemblages from sediments collected 30 or more years ago can be valuable resources in efforts to determine if biotic changes have occurred in coastal ecosystems. This study also indicates that family-level identifications may be sufficient to detect decadal-scale changes in foraminiferal assemblages in reef-tract sediments.

Limestone compaction: An enigma

Compression of an undisturbed carbonate sediment core under a pressure of 556 kg/cm2 produced a “rock” with sedimentary structures similar to typical ancient fine-grained limestones. Surprisingly, shells, foraminifera, and other fossils were not noticeably crushed, which indicates that absence of crushed fossils in ancient limestones can no longer be considered evidence that limestones do not compact.

Distribution of Heavy Metals and Foraminiferal Assemblages in Sediments of Biscayne Bay, Florida, USA

Abstract Heavy-metal pollution is an issue of concern in estuaries influenced by agriculture, urban, and harbor activities. Foraminiferal assemblages have been shown to be effective indicators of pollution. Sediment samples (n = 110) from Biscayne Bay were analyzed for heavy metals, foraminiferal assemblages, and grain-size distribution. Highest Cu, Zn, Cr, Hg, Pb, and Ni concentrations were found closest to Miami and near the mouths of several canals along the western margin of the bay. Few samples exceeded limits of possible biological effects as defined by previous studies. Ammonia and Cribroelphidium, two known stress-tolerant genera, correlated positively with Cu, Zn, Hg, and Ni (r ≥ 0.43). Symbiont-bearing foraminifers, Archaias, Laevipeneroplis, and Androsina, correlated negatively with Cu, Zn, Hg, and Ni (r ≤ −0.26).

Foraminiferal assemblages in Biscayne Bay, Florida, USA: responses to urban and agricultural influence in a subtropical estuary.

This study assessed foraminiferal assemblages in Biscayne Bay, Florida, a heavily utilized estuary, interpreting changes over the past 65 years and providing a baseline for future comparisons. Analyses of foraminiferal data at the genus level revealed three distinct biotopes. The assemblage from the northern bay was characterized by stress-tolerant taxa, especially Ammonia, present in low abundances ( approximately 2.0 x 10(3) foraminifers/gram) though relatively high diversity ( approximately 19 genera/sample). The southwestern margin of the bay was dominated by Ammonia and Quinqueloculina, an assemblage characterized by the lowest diversities ( approximately 12 genera/sample) and highest abundances ( approximately 1.1 x 10(4) foraminifers/gram), influenced by both reduced salinity and elevated organic-carbon concentrations. A diverse assemblage of smaller miliolids and rotaliids ( approximately 26 genera/sample) characterized the open-bay assemblage, which also had a significant component ( approximately 10%) of taxa that host algal endosymbionts. In the past 65 years, populations of symbiont-bearing taxa, which are indicators of normal-marine conditions, have decreased while stress-tolerant taxa, especially Ammonia spp., have increased in predominance.

Multiple outer-reef tracts along the south Florida bank margin: Outlier reefs, a new windward-margin model

High-resolution seismic-reflection profiles off the lower Florida Keys reveal a multiple outlier-reef tract system {approximately}0.5 to 1.5 km seaward of the bank margin. The system is characterized by a massive, outer main reef tract of high (28 m) unburied relief that parallels the margin and at least two narrower, discontinuous reef tracts of lower relief between the main tract and the shallow bank-margin reefs. The outer tract is {approximately}0.5 to 1 km wide and extends a distance of {approximately}57 km. A single pass divides the outer tract into two main reefs. The outlier reefs developed an antecedent, low-gradient to horizontal offbank surfaces, interpreted to be Pleistocene beaches that formed terracelike features. Radiocarbon dates of a coral core from the outer tract confirm a pre-Holocene age. These multiple outlier reefs represent a new windward-margin model that presents a significant, unique mechanism for progradation of carbonate platforms during periods of sea-level fluctuation. Infilling of the back-reef terrace basins would create new terraced promontories and would extend or step the platform seaward for hundreds of meters. Subsequent outlier-reef development would produce laterally accumulating sequences.

Blackened Limestone Pebbles: Fire at Subaerial Unconformities

Irregularly shaped blackened limestone pebbles mixed with similar but unblackened material at unconformities have long presented a mystery to geologists examining Tertiary and Holocene limestones in the Caribbean. How can irregularly shaped limestone pebbles showing no signs of lateral transport be mixed together, especially when a distant or underlying source is invariably absent?

New Maps, New Information: Coral Reefs of the Florida Keys

Abstract A highly detailed digitized map depicts 22 benthic habitats in 3140.5 km2 of the Florida Keys National Marine Sanctuary. Dominant are a seagrass/lime-mud zone (map area 27.5%) throughout Hawk Channel and seagrass/carbonate-sand (18.7%) and bare carbonate-sand (17.3%) zones on the outer shelf and in The Quicksands. A lime-mud/seagrass–covered muddy carbonate-sand zone (9.6%) abuts the keys. Hardbottom communities (13.2%) consist of bare Pleistocene coralline and oolitic limestone, coral rubble, and senile coral reefs. Smaller terrestrial (4.0%) and marine habitats, including those of live coral (patch reefs, 0.7%), account for the rest (13.7%) of the area. Derived from aerial photomosaics, the seabed dataset fits precisely when transposed onto a newly developed National Geophysical Data Center hydrographic-bathymetry map. Combined, the maps point to new information on unstudied seabed morphologies, among them an erosional nearshore rock ledge bordering the seaward side of the Florida Keys and thousands of patch-reef clusters aligned in mid–Hawk Channel. Preliminary indications are that the ledge may represent the seaward extent of the 125-ka Key Largo and Miami Limestone that form the keys, and the patch reefs colonized landward edges of two noncoralline, non-dune-ridge topographic troughs. The troughs, their substrate, and inner-shelf location along the seaward side of the Hawk Channel bedrock depression are the first of that type of nuclei to be recognized in the Florida reef record. Together, the map datasets establish the efficacy and accuracy of using aerial photographs to define in extraordinary detail the seabed features and habitats in a shallow-reef setting.