Reef building and carbonate production modes in the west-central Tethys during the Cenozoic

Abstract

Please cite this article as: Pomar, L., Baceta, J.I., Hallock, P., Mateu-Vicens, G., Basso, D., Reef building and carbonate production modes in the west-central Tethys during the Cenozoic, Marine and Petroleum Geology (2017), ABSTRACTChanging components, rock textures, lithofacies, platform types and architecture throughout time are unique characteristics of carbonate rocks. Characterizing these attributes has been approached by 1) building reference models for specific Phanerozoic intervals, 2) recognizing the climatic impact in modulating carbonate production, and 3) analyzing the influence of changing bio-geochemical conditions. The reference-model approach is mostly based on biological evolution, the climaticimpact approach emphasizes temperature, and the bio-geochemical approach considers the changes in Mg/Ca ratios and Ca ++ concentrations in the oceans. To date, however, an analysis integrating all of these factors is still missing. The analysis presented here includes all these factors but also CO 2 , which is fundamental for both photosynthesis and CaCO 3 precipitation.Here we analyze the waxing and waning of Cenozoic reef limestones from the central Tethys region through several steps: 1) on the basis of rock volume, rock textures, associated sediments and light-dependent skeletal components, as records of light penetration and wave energy (depth); 2) on global environmental conditions (δ 13 C, δ 18 O, pCO 2 , temperature); and 3) on the basis of functionality, nutritional requirements and available resources.Through the Cenozoic, water motion, whether induced by surface or internal waves or by currents, increased as the thermal gradients strengthened, both with depth and with latitude. Active water motion is essential for plankton catchers such as corals, but less so for many larger benthic foraminifers (LBF). Pycnoclines in the meso-oligophotic zone would then favor the benthic plankton catchers such as corals, but would be detrimental for many LBF. Warm temperatures favored LBF. The Eocene LBF families predominated during lowering of atmospheric pCO 2 by using respiratory CO 2 to enhance the symbiont production of photosynthates under oligotrophic conditions and limited turbulence, whereas the Miocene families had to adapt to a progressive increase in turbulence. The eurythermal coralline red algae, however, became preponderant producers in the mesophotic zone during times when the δ 13 C was relatively high. This explains two apparent paradoxes: 1) corals thrive best when the Earth's high latitudes cool, and 2) the dominance of corals and LBF is inversely correlated, despite they both require tropical conditions and have similar trophic strategies (mixotrophy).

Development of transient carbonate ramps in an evolving foreland basin

“…The LBF indicates a middle to lower photic zone and moderate water energy, which suggests a middle‐ramp environment (cf. Geel, ; Brandano et al ., ; Bassi et al ., ; Sarkar, ; Tomassetti et al ., ; Pomar et al ., ). Acervulinids are opportunistic foraminifers favouring conditions similar to those in which coralline algae flourish, and replacing the latter when the light intensity is reduced by increased water depth or turbidity (Bassi et al ., ; Varrone & D'Atri, ; Bassi & Nebelsick, ).…”

“…Biota can be a useful criterion for distinguishing palaeobathymetric zones with a predictive utility in palaeoenvironmental reconstructions, butas pointed out by Buxton & Pedley (1989) and Pomar (2001b) relatively little of such research has been conducted on the Cenozoic Tethyan carbonate ramps, particularly within the Alpine foreland basins. Case studies are few, including Scheibner et al (2007), Scheibner & Speijer (2008), Zamagni et al (2008Zamagni et al ( , 2012 and Pomar et al (2017).…”

Origin of facies zonation in microbial carbonate platform slopes: Clues from trace element and stable isotope geochemistry (Middle Triassic, Dolomites, Italy)

“…Pomar et al . () did not discuss microbial carbonate slopes, but pointed out that coral reefs occupied relatively deeper positions than present on most Cenozoic carbonate ramps before the late Miocene, and suggested a relationship with the depth to the pycnocline where nutrients and organic matter accumulated. Cenozoic coral reefs would have formed preferentially near well‐developed pycnoclines, because of strong internal waves that mobilized particulate organic matter.…”

The seagrass skeletal assemblage from modern to fossil and from tropical to temperate: Insight from Maldivian and Mediterranean examples

“…Despite this possibility, in the considered Eocene seagrasses, the skeletal assemblage is still clearly identifiable with the green algae percentage sufficiently conspicuous enough ( ca 10%) to characterize the skeletal assemblage. Taking into consideration also that large benthic foraminifera were the dominant sediment‐producing biota in the carbonate platforms during the early and middle Eocene (Buxton & Pedley, ; Loucks et al ., ; Racey et al ., ; Beavington‐Penney & Racey, ; Jorry et al ., ; Mateu‐Vicens et al ., ; Pomar et al ., ), their abundance depends not necessarily on the alteration and dissolution of aragonitic green algae but on a dilution effect within the dominant grains. Contrarily, the RA‐foralgal assemblage is dominated by calcitic skeletal grains.…”