Bryozoan beds in northern Italy as a shallow-water expression of environmental changes during the Oligocene isotope event 1

Abstract

Shifts in carbonate-producing biotic communities in the geological record provide evidence of past environmental changes in the neritic realm. The shallow-marine Calcare di Nago Formation exposed in the San Valentino section (northern Italy) covers the Late Eocene and Earliest Oligocene. The succession is characterized by the occurrence of light-dependent biota such as coralline algae and larger benthic foraminifera. In the uppermost part of the section, however,\ the fossil association is dominated by bryozoans, which are filter-feeder organisms. This ca. 12 m thick interval locally contains up to 86% bryozoans, while coralline algae as well as larger benthic foraminifera are absent. Coralline algae and nummulitid foraminifera recover in the upper part of the bryozoan beds, whereas orthophragminids do not recover. The gradual disappearance of larger foraminifera and coralline algae within the bryozoan-dominated deposits is coeval with a pronounced positive shift in δ 13 C. Based on its biostratigraphic position, this positive shift is interpreted to be linked to the positive shift in δ 13 C recognized in deepsea records shortly above the Eocene-Oligocene boundary, which in turn is associated to the positive shift in δ 18 O leading to the Oi-1 (Oligocene isotope event 1) cooling phase. Total phosphorus content increases in the bryozoan beds, suggesting enhanced nutrient supply to the neritic ecosystem. This phosphorous peak is coeval with the globally recognized increment in ocean productivity around the Oi-1 and δ 13 C positive shift. Thus, disappearance of light-dependent biota and the dominance of bryozoans in the platform carbonates studied are interpreted to result not necessarily from a deepening of the depositional environment but from the combination of lower seasurface temperatures and the deterioration of underwater light conditions on account of elevated turbidity in surface waters, resulting from enhanced primary productivity. As bryozoan beds occur in several Italian localities around the Eocene-Oligocene boundary, they are interpreted to represent the regional expression of neritic carbonate depositional systems to global environmental changes occurring at the dawn of an ice-house Earth.

Heterozoan carbonates: When, where and why? A synthesis on parameters controlling carbonate production and occurrences

“…bryozoans and molluscs) against seagrass and red algal carbonate systems (cf. (Brandano, 2017;Halfar et al, 2006;Hallock, 2001;Jaramillo-Vogel et al, 2016)). The latter factory nevertheless might be more resilient against such disturbance (Halfar & Mutti, 2005).…”

Cretaceous Evolution of the Central Asian Proto‐Paratethys Sea: Tectonic, Eustatic, and Climatic Controls

“…Heterozoan bryomol and echinofor grain association types are abundant in nontropical, mostly cool and cold water environments on high‐energy open shelves and ramps, whereas foramol type of grain associations are indicative of nontropical, warm temperate environments (Flügel, 2004). Modern‐day calcifying reef‐building and carbonate platform phototrophs (i.e., corals and larger benthic foraminifera) are found in waters with winter surface temperatures above 20°C, whereas bryozoan‐rich heterotrophic communities become dominant in cool waters (<20°C) or in warm settings below the photic zone (Jaramillo‐Vogel et al, 2016, and references therein). In terms of trophic conditions, phototrophs, with the exception of red algae, thrive in oligotrophic to slightly mesotrophic conditions while heterotrophs generally dominate in nutrient‐rich environments (Jaramillo‐Vogel et al, 2016, and references therein).…”

“…1,325 m, heterozoan bryozoa‐rich biota were replaced by photozoan reefal biota (bearing corals, rudists, and red algae), indicating a recovery in the carbonate factory. Species level determination of the bryozoan microfossils could further constrain interpretation of seawater temperature changes through recognition of cool or warm water species (Jaramillo‐Vogel et al, 2016; Moissette, 2000). Unfortunately, we were not able to perform taxonomic analysis at the species level on bryozoan microfossils due to their fragmented appearance in thin sections.…”

“…However, only the clastic input could be evaluated, and the influence of other nutrient sources remains unresolved. Future work revealing nutrient fluctuations, for example, evolution of the total phosphorus (Jaramillo‐Vogel et al, 2016), may elucidate the definitive role of nutrients, irrespective of their source.…”

The Eocene–Oligocene transition in the C-isotope record of the carbonate successions in the Central Mediterranean

“…This perturbation is associated with a major change in trophic resources as indicated in the Lessini Shelf by a rise in the total phosphorus content, which triggered an increased primary productivity and thus, a shallowing of the photic zone (Jaramillo-Vogel et al, 2016). Decreased water transparency would have limited light-dependent biota and left bryozoans as dominant benthic calcifying organisms.…”