Early post-mortem formation of carbonate concretions around tusk-shells over week-month timescales

Abstract

Carbonate concretions occur in sedimentary rocks of widely varying geological ages throughout the world. Many of these concretions are isolated spheres, centered on fossils. The formation of such concretions has been variously explained by diffusion of inorganic carbon and organic matter in buried marine sediments. However, details of the syn-depositional chemical processes by which the isolated spherical shape developed and the associated carbon sources are little known. Here we present evidence that spherical carbonate concretions (diameters φ : 14 ~ 37 mm) around tusk-shells (Fissidentalium spp.) were formed within weeks or months following death of the organism by the seepage of fatty acid from decaying soft body tissues. Characteristic concentrations of carbonate around the mouth of a tusk-shell reveal very rapid formation during the decay of organic matter from the tusk-shell. Available observations and geochemical evidence have enabled us to construct a ‘Diffusion-growth rate cross-plot’ that can be used to estimate the growth rate of all kinds of isolated spherical carbonate concretions identified in marine formations. Results shown here suggest that isolated spherical concretions that are not associated with fossils might also be formed from carbon sourced in the decaying soft body tissues of non-skeletal organisms with otherwise low preservation potential.

Generalized conditions of spherical carbonate concretion formation around decaying organic matter in early diagenesis

“…Spherical, isolated carbonate concretions occur throughout the world in marine argillaceous sedimentary rocks of widely varying geological ages. These concretions are characteristically highly enriched in CaCO 3 compared to the surrounding sedimentary rock matrices and are typically separated from these rock matrices by sharp boundaries 1 – 3 . These sharp boundaries mean that the isolated carbonate concretions are readily identifiable and both geologists and non-scientists alike have been motivated to consider how such concretions could have formed.…”

“…It is also known that many of these concretions have various kinds of well-preserved fossils at their centres 3 . Isotopic analyses have been used to identify the source of carbon that formed the concretions 4 – 6 and to understand the diagenetic processes, including microbial activity, that occurred during sediment burial and concretion formation 1 , 7 , 8 .…”

Looking for the key to preservation of fossil marine vertebrates in the Pisco Formation of Peru: new insights from a small dolphin skeleton

“…In particular, the widespread presence of dolomite concretions enclosing many fossil vertebrates of the Pisco Formation has been recently recognized. The formation of mineral concretions during early diagenesis as a consequence of biomediated processes related to organic matter decay has been described elsewhere (McCoy et al, 2015;Yoshida et al, 2015). In the Pisco Formation, early precipitation of dolomite around vertebrate carcasses, although not always present, has been pointed out for its major contribution in allowing the preservation of complete and articulated skeletons, delicate anatomical structures, and even soft organic tissues (Gariboldi et al, 2015;Gioncada et al, 2016Gioncada et al, , 2018.…”

Preservation of overmature, ancient, sedimentary organic matter in carbonate concretions during outcrop weathering

“…Organic matter enrichment within concretion interiors may also arise if concretions preferentially precipitate around sediments with relatively high organic carbon content. Organic nuclei have been found in concretions of other localities (Allison & Pye, ; Gaines et al ., ; Grazhdankin et al ., ; Yoshida et al ., ; Plet et al ., ), and this association may result from localized alkalinity production upon organic nucleus degradation (Berner, ). In Holz Shale concretions, discrete organic nuclei are lacking, however, enrichments in disseminated organic matter could promote similar localized concretion precipitation.…”

“…Cementation may act to preserve primary sedimentary components by filling host rock pore space, thereby reducing local permeability, providing rigidity, and limiting the potential for chemical and physical degradation. In some cases, carbonate cementation has been shown to preserve unstable igneous minerals (Bramlette, 1946;Dickinson et al, 1969), magnetic minerals (McNeill & Kirschvink, 1993;Maloof et al, 2007), sedimentary structures (Lattman, 1973;Plet et al, 2016), microfossils (Blome & Albert, 1985), shells (Allison & Pye, 1994), biogenic particles (El Albani et al, 2001), soft organic tissues (Gaines et al, 2005;Grazhdankin et al, 2008;Yoshida et al, 2015), and specific biomolecules/biomolecular compositions (Pearson & Nelson, 2005;Melendez et al, 2013;Plet et al, 2016). In other cases, primary sedimentary components are relatively degraded within concretions compared to the host rock (Walker, 1984;Molenaar & Martinius, 1990;Vasconcelos & McKenzie, 1997;Taylor et al, 2000), reflecting a dichotomous relationship between cementation and preservation.…”