ABSTRACT Many types of carbonate platforms have been described, from homoclinal ramps to rimmed shelves and a full spectrum of variations in between; the distinction between these different types can ...be problematic. Nevertheless, classification of carbonate platforms is not just a semantic or academic issue. For example, it is clearly important for the accurate interpretation of seismic images of facies geometry and for assessing the potential of stratigraphic traps. Even though predictive efficiency of conceptual models depends on the degree of comprehension of the genetic factors controlling depositional profiles and the distribution of facies belts, current models for classification of carbonate platforms are basically descriptive and mainly based on depositional profile, size, and attachment to or detachment from a landmass.
A genetic approach considers the variability of depositional profiles among carbonate platforms as a function of the type of sediment that was produced (basically grain size), the locus of sediment production, and the hydraulic energy. Three groups of carbonate‐producing biota may be distinguished according to their dependence upon light: (1) euphotic (good light) in shallow, wave‐agitated areas; (2) oligophotic (poor light) in deeper, commonly non‐wave‐agitated areas; and (3) photo‐independent biota in all water‐depth ranges.
Several platform types in wave‐dominated seas can be considered in relation to genetic factors, even when simplifying the many possible scenarios. Euphotic framework‐producing biota create rimmed shelves similar to modern reef platforms. Soft‐substrate‐dwelling biota, which produce gravel‐sized carbonate in the shallow euphotic zone, create flat‐topped open shelves. Oligophotic gravel‐producing biota, such as some larger foraminifera and red algae, generate distally steepened ramps. Mud‐dominated carbonate production, in either euphotic or oligophotic zones, generate homoclinal ramps. Carbonate production dominated by photo‐independent biota (crinoids, sponges, bryozoans, etc.) above wave base give rise to open shelves or ramps, depending upon grain size, but may produce mounds if carbonate production occurs below the base of wave/current sweeping.
Describing, characterizing and interpreting the nearly infinite variety of carbonate rocks are conundrums – intricate and difficult problems having only conjectural answers – that have occupied ...geologists for more than two centuries. Depositional features including components, rock textures, lithofacies, platform types and architecture, all vary in space and time, as do the results of diagenetic processes on those primary features. Approaches to the study of carbonate rocks have become progressively more analytical. One focus has evolved from efforts to build reference models for specific Phanerozoic windows to scrutinize the effect of climate and long-term oscillations of the ocean–atmosphere system in influencing the mineralogy of carbonate components.
This paper adds to the ongoing lively debates by attempting to understand changes in the predominant types of carbonate-producing organisms during the Mesozoic–Cenozoic, while striving to minimize the uniformitarian bias. Our approach integrates estimates of changes in Ca
2+ concentration in seawater and atmospheric CO
2, with biological evolution and ecological requirements of characteristic carbonate-producing marine communities. The underlying rationale for our approach is the fact that CO
2 is basic to both carbonates and organic matter, and that photosynthesis is a fundamental biological process responsible for both primary production of organic matter and providing chemical environments that promote calcification. Gross photosynthesis and hypercalcification are dependent largely upon sunlight, while net primary production and, e.g., subsequent burial of organic matter typically requires sources of new nutrients (N, P and trace elements). Our approach plausibly explains the changing character of carbonate production as an evolving response to changing environmental conditions driven by the geotectonic cycle, while identifying uncertainties that deserve further research.
With metazoan consumer diversity reduced by the end-Permian extinctions, excess photosynthesis by phytoplankton and microbial assemblages in surface waters, induced by moderately high CO
2 and temperature during the Early Mesozoic, supported proliferation of non-tissular metazoans (e.g., sponges) and heterotrophic bacteria at the sea floor. Metabolic activity by those microbes, especially sulfate reduction, resulted in abundant biologically-induced geochemical carbonate precipitation on and within the sea floor. For example, with the opening of Tethyan seaways during the Triassic, massive sponge/microbe boundstones (the benthic automicrite factory) formed steep, massive and thick progradational slopes and, locally, mud-mounds. As tectonic processes created shallow epicontinental seas, photosynthesis drove lime-mud precipitation in the illuminated zone of the water column. The resulting neritic lime-mud component of the shallow-water carbonate factory became predominant during the Jurassic, paralleling the increase in atmospheric
pCO
2, while the decreasing importance of the benthic automicrite factory parallels the diversification of calcifying metazoans, phytoplankton and zooplankton.
With atmospheric
pCO
2 declining through the Cretaceous, the potential habitats for neritic lime-mud precipitation declined. At the same time, peak oceanic Ca
2+ concentrations promoted biotically-controlled calcification by the skeletal factory. With changes produced by extinctions and turnovers at the Cretaceous–Tertiary boundary, adaptations to decreasing Ca
2+ and
pCO
2, coupled with increasing global temperature gradients (i.e., high-latitude and deep-water cooling), and strategies that efficiently linked photosynthesis and calcification, promoted successive changes of the dominant skeletal factory through the Cenozoic: larger benthic foraminifers (protist–protist symbiosis) during the Paleogene, red algae during the Miocene and modern coral reefs (metazoan–protist symbiosis) since Late Miocene.
New applications in the realms of terahertz (THz) technology require versatile adaptive optics and powerful modulation techniques. Semiconductors have proven to provide fast all-optical terahertz ...wave modulation over a wide frequency band. We show that the attenuation and modulation depth in optically driven silicon modulators can be significantly enhanced by deposition of graphene on silicon (GOS). We observed a wide-band tunability of the THz transmission in a frequency range from 0.2 to 2 THz and a maximum modulation depth of 99%. The maximum difference between the transmission through silicon and GOS is Δt = 0.18 at a low photodoping power of 40 mW. At higher modulation power, the enhancement decreased due to charge carrier saturation. We developed a semianalytical band structure model of the graphene–silicon interface to describe the observed attenuation and modulation depth in GOS.
Internal waves occur nearly ubiquitously in lakes and oceans yet their sedimentary records remain largely unrecognized. Waves propagate at the interface between fluids of different densities. Surface ...waves propagate at the interface between air and water, which is a strong density gradient. Internal waves propagate along weaker gradients (pycnoclines) within density-stratified fluids, behaving similarly to surface waves but typically at lower frequencies and larger amplitudes. Internal waves that occur at tidal frequencies are called internal tides; they are very common on the outer continental shelf and slope, and are generated as the surface tides move stratified water up and down a sloping surface. Large internal solitary waves known as solitons are ubiquitous wherever strong currents and stratification occur in the vicinity of irregular topography. These waves can force short-period, strong bottom-current pulses and may trigger upslope-surging vortex cores of dense fluid (boluses) that can induce mobilization of bottom sediments.
Internal-wave deposits (internalites) are highly variable and definitive criteria for recognition are still to be developed. In terrigenous-clastic systems and shallow-water settings, internalites can be seen as “out-of-context” tempestites, detached from shore-related deposits and lacking thickening/coarsening upward sequences. In contrast to surface storm waves, the impact of internal waves is usually strongest in mid-outer-shelf regions and weaker in shallow water. Internal waves also provide a plausible mechanism to explain the origin of hummocky cross-stratification, especially their occurrence in different depositional environments. In deeper settings (continental slopes and canyons), internalites may have sedimentary structures indicating tidal currents and may coexist with turbidites. In carbonate systems, internal waves influence both sediment remobilization and the carbonate-producing biota. Differentiation between internal waves and surface storm waves is more reliable because many skeletal constituents have specific bathymetric distributions. Moreover, internal waves influence nutrient, plankton and larval distributions while inducing thermal variations by vertical displacements of the thermocline. The sharp gradient in nutrients and the chlorophyll-maximum zone typically correspond with the base of the seasonal pycnocline, which is commonly in the lower part of the photic zone. Suspension-feeding metazoans can thrive near the pycnocline, which explains the common occurrence of Phanerozoic metazoan buildups at mid- and outer ramp settings. During paleoceanographic changes that have influenced ocean stratification, internal waves may also have been a mechanism influencing diversification and extinction of these mid- and outer-ramp benthic communities.
PDF
