Eberhard Gischler

An Early Carboniferous seep community and hydrocarbon-derived carbonates from the Harz Mountains, Germany

Early Carboniferous (latest Visean) seep deposits occur on top of the drowned Middle Devonian–Late Devonian Iberg atoll reef, Harz Mountains, Germany. These deposits include limestone with a low-diversity but high-abundance fauna of rhynchonellid brachiopods and rare solemyid bivalves, as well as microbial limestone. Rhynchonellids form dense, autochthonous shell accumulations and are generally articulated. They are closely associated with hydrocarbon-derived carbonates. The carbonates exhibit δ 13 C values as low as −32‰, relative to the Peedee belemnite standard, revealing that they are predominantly hydrocarbon derived. The fauna, carbonate fabrics, and isotope signatures provide unequivocal evidence for a seep origin of the Visean deposit. The occurrence of solemyid bivalves supports this interpretation as members of this family (1) are well known for their relationship with chemoautotrophic bacteria and (2) have been reported from ancient and modern seeps. Possible hydrocarbon sources are thermogenic methane derived from the volcanic base of the Iberg reef or methane from a petroleum reservoir.

Holocene cemented beach deposits in Belize

Abstract Two types of cemented beach deposits occur on reef islands off the coast of Belize. These are (1) intertidal beachrock that is dominantly cemented by marine aragonite and high-magnesium-calcite cements, and (2) supratidal cayrock that is cemented mainly by vadose low-magnesium-calcite cements. Besides differences in position relative to present sea level and resulting early diagenesic features, beachrock and cayrock can be distinguished on the basis of differences in composition, texture, geographical position, and age. Whereas the composition of beachrock is similar to that of the adjacent marginal reef sediments, cayrock is enriched in benthic foraminifera. Intertidal beachrock is moderately to well sorted and well cemented, while supratidal cayrock is very well sorted, poorly cemented and friable. Beachrock occurs preferentially on windward beaches of sand-shingle Gays on the middle and southern barrier reefs and on the isolated platforms Glovers and Lighthouse Reefs. Cayrock only occurs on larger mangrove-sand Gays of the isolated platforms Turneffe Islands, Lighthouse Reef, and the northern barrier reef. 14C-dating of ten whole-rock and mollusk shell samples produced calibrated dates between AD 345 and AD 1435 for beachrock and between BC 1085 and AD 1190 for cayrock. The large-scale distribution of beachrock in Belize supports the contention that physical processes such as water agitation rather than biological processes control beachrock formation and distribution. Only on windward sides of cays that are close to the reef crest, where large amounts of seawater flush the beaches, considerable amounts of cements can be precipitated to produce beachrock. Cayrock forms due to cementation in the vadose zone and is only preserved on larger, stable mangrove-sand cays.

Holocene lagoonal development in the isolated carbonate platforms off Belize

Abstract Thirty-one vibracores were taken in interior lagoons of Glovers Reef, Lighthouse Reef, and Turneffe Islands—three isolated carbonate platforms offshore Belize, Central America. Holocene facies successions overlying the Pleistocene limestone bedrock begin with soils, followed by mangrove peats, and marine carbonate sediments of lagoonal origin. The soils formed on top of subaerially exposed Pleistocene limestone before the Holocene transgression. Mangrove peats developed during initial flooding of the platforms (Glovers ca. 8.5 ky, Lighthouse ca. 7 ky, Turneffe ca. 6 ky BP). As water depths increased, reefs colonized platform margins, lagoonal circulation improved thereby promoting carbonate production. The basal lagoonal carbonate sediments are characterized by shell beds and/or Halimeda packstones–grainstones. Mollusk-dominated wackestones and packstones follow upsection in Glovers and Lighthouse Reefs. At present, open circulation prevails in Glovers and Lighthouse lagoons. In contrast, organic-rich Halimeda wackestones and packstones dominate the Turneffe Islands Holocene succession. The main lagoon area of Turneffe is enclosed by mangroves, and restricted circulation prevails. Factors that explain the differences in geomorphology, circulation, and facies are variations in depth of antecedent topography and degree of exposure to waves and currents. The thickness of Holocene lagoon sediments may exceed the maximum core length of 6 m in all atolls. Holocene sedimentation rates average 0.6 m/ky, with highest rates in Turneffe (0.82 m/ky), followed by Lighthouse (0.53 m/ky), and Glovers (0.46 m/ky). Like in many other isolated carbonate platforms and atolls, lagoon floor sedimentation did not keep pace with rising sea level, leading to unfilled accommodation space. At present, Glovers has an 18 m deep lagoon, while Lighthouse and the main Turneffe lagoon are 8 m deep. It is unlikely that the lagoons will be completely filled during the Holocene sea level highstand cycle. This observation should be kept in mind when using cycle thickness as a proxy for eustatic sea level change in fossil carbonate platforms.

Key Largo Limestone revisited: Pleistocene shelf-edge facies, Florida Keys, USA

SummaryNew dates and analysis of 12 deep and 57 shallow cores allow a more detailed interpretation of the Pleistocene shelf edge of the Florida Platform as found in various facies of the Key Largo Limestone beneath the Florida Keys. In this study a three-phase evolution of the Quaternary units (Q1–Q5) of the Key Largo is presented with new subdivision of the Q5. (1) In the first phase, the Q1 and Q2 (perhaps deposited during oxygen-isotope stage 11) deep-water quartz-rich environment evolved into a shallow carbonate phase, (2) Subsequently, a Q3 (presumably corresponding to oxygen-isotope stage 9) flourishing reef and productive high-platform sediment phase developed. (3) Finally, a Q4 and Q5 (corresponding to oxygen-isotope stages 7 and 5) stabilization phase occurred with reefs and leeward productive lagoons, followed by lower sea levels presenting a sequence of younger (isotope substages 5c, 5a) shelf-margin wedges, sediment veneers and outlier reefs. The Key largo Limestone provides an accessible model of a carbonate shelf edge with fluctuating water depth, bordering a deep seaward basin for a period of at least 300 ka. During this time, at least four onlaps/offlaps, often separated by periods of karst development with associated diagenetic alterations, took place. The story presented by this limestone not only allows a better understanding of the history of south Florida but also aids in the interpretation of similar persistent shelf-edge sites bordering deep basins in other areas.

Holocene development of three isolated carbonate platforms, Belize, Central America

Abstract Locally operating factors such as topography of the reef basement and exposure to waves and currents rather than regionally effective factors such as the post-glacial sea level rise in the western Atlantic explain the different Holocene developments of the three isolated carbonate platforms Glovers Reef, Lighthouse Reef, and Turneffe Islands offshore Belize. A series of NNE-striking tilted fault-blocks at the passive continental margin forms the deep basement of the Belize reefs. Glovers and Lighthouse Reefs are located on the same fault-block, while Turneffe Islands is situated west of Lighthouse Reef on an adjacent fault-block. The three platforms are surrounded by deep water and have surfacebreaking reef rims. Significant differences exist between platform interiors. Glovers Reef has only 0.2% of land and an 18 m deep, well-circulated lagoon with over 800 patch reefs. Lighthouse Reef has 3% of land and a well-circulated lagoon area. Patch reefs are aligned along a NNE-striking trend that separates a shallow western (3 m) and a deeper eastern (8 m) lagoon. Turneffe Islands has 22% of land that is mainly red mangrove. Interior lagoons are up to 8 m deep and most have restricted circulation and no patch reefs. Surface sediments are rich in organic matter. In contrast, the northernmost part of Turneffe Islands has no extensive mangrove development and the well-circulated lagoon area has abundant patch reefs. Holocene reef development was investigated by means of 9 rotary core holes that all reached Pleistocene reef limestones, and by radiometric dating of corals. Maximal Holocene reef thickness reaches 11.7 m on Glovers Reef, 7.9 m on Lighthouse Reef, and 3.8 m on Turneffe Islands. Factors that controlled Holocene reef development include the following. (1) Holocene sea level. The margin of Glovers Reef was flooded by the rising Holocene sea ca. 7500 YBP, that of Lighthouse Reef ca. 6500 YBP, and that of Turneffe Islands between 5400 and 4750 YBP. All investigated Holocene reefs belong to the keep-up type, even though the three platforms were flooded successively and, hence, the reefs had to keep pace with different rates of sea level rise. (2) Pre-Holocene topography. Pleistocene elevation and relief are different on the three platforms. This is the consequence of both tectonics and karst. Different elevations caused successive reef initiation and they also resulted in differences in lagoon depths. Variations in Pleistocene topography also explain the different facies distribution patterns on the windward platforms that are located on the same fault-block. On Lighthouse Reef tectonic structures are clearly visible such as the linear patch reef trend that is aligned along a Pleistocene fault. On Glovers Reef only short linear trends of patch reefs can be detected because the Pleistocene tectonic structures are presumably masked by the higher Holocene thickness. The lower Pleistocene elevation on Glovers Reef is probably a consequence of both a southward tectonic tilt, and stronger karstification towards the south related to higher rainfall. (3) Exposure to waves and currents. Glovers Reef, Lighthouse Reef, and the northernmost part of Turneffe Islands receive the maximum wave force as they are open to the Caribbean Sea. Adjacent lagoons are well-circulated and have luxuriant patch reef growth and no extensive mangrove development. By contrast, most of Turneffe Islands is protected from the open Caribbean Sea by Lighthouse Reef to the east and is only exposed to reduced wave forces, allowing extensive mangrove growth in these protected areas.

A 1500-Year Holocene Caribbean Climate Archive from the Blue Hole, Lighthouse Reef, Belize

Abstract Sediment cores (up to 6 m in length) from the bottom of the Blue Hole, a 125 m deep Pleistocene sinkhole located in the lagoon of Lighthouse Reef Atoll, Belize, consist of undisturbed, annually layered biogenic carbonate muds and silts with intercalated coarser grained storm beds. The sedimentation rate of the layered sections is 2.5 mm/y on average, and the long cores span the past 1500 years. Oxygen isotopes of laminated sediment provide a late Holocene climate proxy: A high-resolution δ18O time series traces the final Migration Period Pessimum, the Medieval Warm Period, the Little Ice Age, and the subsequent temperature rise. Carbon isotopes (δ13C) decrease up core and show the impacts of the decline of the Mayan culture and the Suess effect. Time series analyses of δ18O and δ13C content reveal 88-, 60-, 52-, and 32-year cyclicities, and suggest solar forcing. Storm event beds are most common during AD 650–850, around AD 1000, during AD 1200–1300, and AD 1450–1550. Major storm beds are rare during the past 500 years BP.

Characterization of Depositional Environments in Isolated Carbonate Platforms Based on Benthic Foraminifera, Belize, Central America

Abstract Tests of benthic foraminifera were analyzed quantitatively in 43 surface sediment samples collected along E–W traverses across the modern isolated carbonate platforms of Glovers Reef (N=14), Lighthouse Reef (N=14), and Turneffe Islands (N=15), offshore Belize, Central America. Water depths of sample stations range from 0 m (beach) to 40 m (fore reef). Each sample was sieved in the size classes 2–1, 1–0.5, 0.5–0.25, and 0.25–0.125 mm (ϕ=1–0, 0–1, 1–2, and 2–3, respectively) and 300 foraminiferal identifications were attempted for each sieve fraction (i.e., 1,200 individuals per sample, totaling 46,003 specimens). Cluster analyses of samples characterized platform environments and distinguished four benthic foraminifera associations. Fore reef samples were characterized by abundant Amphistegina gibbosa and the occurrence of Asterigerina carinata, both rotaliids. In high-energy, marginal-reef areas the encrusting rotaliid Homotrema rubrum predominated. Platform-interior environments were marked by a variety of common taxa, the most common of which include Miliolina such as Archaias angulatus, Quinqueloculina sp., and Triloculina sp., as well as the rotaliid Cribroelphidium poeyanum, the latter indicating low-energy or deep-lagoonal regimes. Diversity, expressed as numbers of identified taxa, appears to be a function of relative degree of circulation. The highest number (147) of taxa was found in the shallow, high-energy platform of Lighthouse Reef, followed by Glovers Reef (78), both of which are open to the Caribbean Sea. The lowest number of taxa (66) occurred in Turneffe Islands where large parts of the platform are characterized by the restricted circulation imposed by dense mangrove growth behind platform margins. The occurrence of tests of common foraminiferal species outside their original habitats, coupled with trends towards smaller grain-sizes and better sorting of tests away from original habitats along parts of the sample traverses, indicates sediment transport across platform margins. In the great majority of samples, however, sediment redeposition did not significantly alter characteristic taxonomic compositions. This result has important implications for the use of benthic foraminifera as facies indicators in fossil carbonate platforms.

Last interglacial reef growth beneath Belize barrier and isolated platform reefs

We report the first radiometric dates (thermal-ionization mass spectrometry) from late Pleistocene reef deposits from offshore Belize, the location of the largest modern reef complex in the Atlantic Ocean. The results presented here can be used to explain significant differences in bathymetry, sedimentary facies, and reef development of this major reef area, and the results are significant because they contribute to the knowledge of the regional geology of the eastern Yucatan. The previously held concept of a neotectonically stable eastern Yucatan is challenged. The dates indicate that Pleistocene reefs and shallow-water limestones, which form the basement of modern reefs in the area, accumulated ca. 125–130 ka. Significant differences in elevation of the samples relative to present sea level (>10 m) have several possible causes. Differential subsidence along a series of continental margin fault blocks in combination with variation in karstification are probably the prime causes. Differential subsidence is presumably related to initial extension and later left-lateral movements along the adjacent active boundary between the North American and Caribbean plates. Increasing dissolution toward the south during Pleistocene sea-level lowstands is probably a consequence of higher precipitation rates in mountainous southern Belize.

Historical Climate Variation in Belize (Central America) as Recorded in Scleractinian Coral Skeletons

Abstract Three cores from the scleractinian coral Montastraea faveolata and one core from the scleractinian coral Siderastrea siderea from the Belize barrier and atoll reef complex, Central America, were analyzed with regard to sclerochronology (skeletal extension rates) and stable isotope geochemistry (δ18O and δ13C). The core material covers the time span from 1815 to 2000. The four coral time series were compared with available instrumental climate data, such as sea surface temperatures (GISST), cloud cover (GHCN), and precipitation (COADS). Skeletal extension rates measured in the cores average 8.5–14 mm/year in M. faveolata and 8 mm/year in S. siderea. No systematic correlations between extension rates and instrumental climate data were detected. Annual variation in oxygen isotopes is 0.6– 0.8‰, which accounts for monthly averages of sea-surface water temperature fluctuations of 3–4°C. In three cores from the Belize shelf and barrier reef, negative correlations of δ18O with the GISST 2.3b data were observed. Time series analyses of the oxygen isotope data in these three cores revealed a decadal periodicity (10–15 years), which are attributed here to the influence of the Atlantic sea surface temperature (SST) dipole variation. Three- to six-year periodicities, indicative of the El Niño Southern Oscillation (ENSO), in the same data are only weakly developed. Carbon isotopes of shallow-water corals also exhibit negative correlations with the GISST data. It is speculated that warmer years were characterized by increased cloud cover leading to reduced photosynthesis rates in the corals. Indeed, there is a negative correlation between δ13C and historic cloud-cover data. Carbon isotopes in the Belize shelf and barrier reef cores further exhibit general trends towards lighter values in time indicating the uptake of fossil fuel CO2 in the coral skeletons. A proxy data time series from a core from the restricted Turneffe Atoll lagoon differs from those of the Belize shelf and barrier reef cores, and there are fewer systematic correlations with historical climate and proxy data. This is probably a consequence of the strong influence of local environmental factors, which obliterate broader scale environmental parameters. First analyses of oxygen isotopes in fossil (Holocene) coral cores of M. faveolata indicate higher SSTs around 7,000 ybp in Belize as compared to older, younger, and modern samples.

IBERGIRHYNCHIA CONTRARIA (F. A. ROEMER, 1850), AN EARLY CARBONIFEROUS SEEP–RELATED RHYNCHONELLIDE BRACHIOPOD FROM THE HARZ MOUNTAINS, GERMANY—A POSSIBLE SUCCESSOR TO DZIEDUSZYCKIA?

Abstract A new genus Ibergirhynchia, a member of the rhynchonellide superfamily Dimerelloidea, is described for the species Terebratula contraria Roemer, 1850, from Early Carboniferous deposits of the Harz Mountains, Germany. Ibergirhynchia contraria is from a monospecific brachiopod limestone that formed on top of the drowned Devonian Iberg Reef which persisted as a seamount during Famennian and Early Carboniferous times. Ibergirhynchia contraria is considered a cold seep-related brachiopod based on this locality. Such seep associations have been observed for Mesozoic representatives of the rhynchonellide superfamily Dimerelloidea. Ibergirhynchia is considered the first Paleozoic representative of the family Rhynchonellinidae. Ibergirhynchia resembles Dzieduszyckia externally and may be derived from this dimerelloid.