Morphological patterns of Aptian Lithocodium-Bacinella geobodies: relation to environment and scale

Abstract

Lithocodium aggregatum and Bacinella irregularis are now extinct, shallow marine life forms of unknown taxonomic origin. Forming part of the tropical platform biota during much of the Mesozoic, these organisms experienced bloom periods and temporarily replaced rudist–coral assemblages during parts of the Early Aptian. Within the limitations of time resolution, this ‘out‐of‐balance’ facies is coeval with the Oceanic Anoxic Event 1a‐related black shale deposition in oceanic basins but the triggering factors remain poorly understood. Here, a platform‐wide comparison of Lithocodium–Bacinella geobodies and morphotypes from the Sultanate of Oman is presented and placed in its environmental, bathymetric and physiographic context. Lithocodium–Bacinella geobodies reach from kilometre‐scale ‘superstructures’ to delicate centimetre‐sized growth forms. Clearly, scale matters and care must be taken when drawing conclusions based on spatially limited observational data. Whilst the factors that cause Lithocodium–Bacinella expansion should probably be considered in a global context, regional to local factors affected growth patterns in a more predictable manner. Here, the unresolved taxonomic relationship remains the main obstacle in any attempt to unravel the response of Lithocodium–Bacinella to specific or interlinked environmental parameters as different organisms respond differently to changing environment. Acknowledging these limitations, the following tentative patterns are observed: (i) Lithocodium–Bacinella tolerated a wide range of hydrodynamic levels and responded to differences in energy level or physiographic settings (margin, intrashelf basin, inner platform) by obtaining characteristic growth forms. (ii) Lithocodium–Bacinella favoured low‐sediment input but had the ability to react to higher sedimentation rates by enhanced upward growth; a feature perhaps pointing to a phototrophic metabolism. Circumstantial evidence for continuous growth within the upper‐sediment column is debated. (iii) The availability of accommodation space had a direct influence on the maximum size of geobodies formed. (iv) Fluctuating nutrient levels and sea water alkalinity may have affected the growth potential of Lithocodium–Bacinella. Understanding the relationship between Lithocodium–Bacinella morphogenesis on a wide range of scales and local environmental parameters allows for better prediction of the spatial distribution of reservoir properties and also results in an improved interpretation of palaeoenvironments. This study might represent a useful first step in this direction.

Shallow water carbonate platforms (Late Aptian–Early Albian, Southern Apennines) in the context of supraregional to global changes: re-appraisal of palaeoecological events as reflectors of carbonate factory response

“…Also, mesotrophic conditions alone, testified by the diffused microbial colonization of the shallow water environments (cf. Whalen et al, 2002;Rameil et al, 2010;Schroeder et al, 2010;see below), were insufficient for the development and blooming of M. texana and M. parva.…”

“…increased pCO 2 and increased nutrient levels (Weissert and Erba, 2004;Kuroda et al, 2011;Najarro et al, 2011), may have influenced the sensitive carbonate factory of the Southern Apenninic platform, leading to the settlement of other peculiar shallow-water facies along the studied sections. It is possible, for example, that progressive enhancement of the continental weathering, due to the acceleration of the global water cycle, caused an increase of dissolved Ca 2+ and HCO − 3 in the ocean (Kump et al, 2000) which may potentially have facilitated the microbial colonization in shallow water environments (Kaźmierczak et al, 1996;Kaźmierczak and Iryu, 1999;Whalen et al, 2002;Rameil et al, 2010). Microbes are, in fact, important contributors in carbonate systems during times of environmental change and biotic crises (Whalen et al, 2002;Graziano, 2003; see also Huck et al, 2010) and develop well under high-trophic levels (Hallock, 1988;Miller et al, 1989;Mutti and Hallock, 2003).…”

Environmental consequences of Ontong Java Plateau and Kerguelen Plateau volcanism

“…Aptian carbonate platforms are punctuated by episodic demise and major changes in benthic communities (Föllmi et al, 1994;Vahrenkamp, 1996Vahrenkamp, , 2010Grötsch et al, 1998;Jenkyns and Wilson, 1999;Steuber, 2002;Wissler et al, 2003;Immenhauser et al, 2004Immenhauser et al, , 2005Burla et al, 2008;Föllmi and Gainon, 2008;Föllmi, 2008Föllmi, , 2012Huck et al, 2010Huck et al, , 2012Rameil et al, 2010;Masse and Fenerci-Masse, 2011;Graziano, 2013). As discussed by Yamamoto et al (2013), early Aptian shallow-water carbonates show different facies changes according to their paleogeographic position: the northern Tethyan platforms were episodically drowned, while in the central to southern NeoTethys margins carbonate deposition continued, although affected by a profound faunal shift from rudist-coral-stromatoporoid communities to Lithocodium-Bacinella dominance.…”

Organic geochemistry, stable isotopes, and facies analysis of the Early Aptian OAE—New records from Spain (Western Tethys)

“…In addition, notable faunal and facies changes in carbonate platforms have been related to the OAEla (i.e. Huck et al, 2012), including the remarkable wide spread development of Lithocodium-Bacinella episodes in the Tethys domain Bover-Arnal, 2010;Rameil et al, 2010).…”