Carbonate Preservation in Shallow Marine Environments: Unexpected Role of Tropical Siliciclastics

Abstract

Coordinated taphonomic, geochronologic, and geochemical studies of bivalve death assemblages and their sedimentary environments of San Blas, Caribbean Panama, permit us to identify the major factors controlling skeletal degradation in mixed carbonate-siliciclastic tropical shelf sediments. Ten sites were studied along environmental gradients including water nutrients, grain size, and sediment chemistry (carbonate, organic carbon, and reactive iron contents). Taphonomic data were derived from naturally occurring bivalve death assemblages and experimentally deployed specimens of Mytilus edulis and Mercenaria mercenaria to determine environmental controls on types and intensities of postmortem damage to skeletal hardparts and to quantify short-term rates of damage accrual. Death assemblage shells were dated using 14 C and amino acid racemization techniques to examine shell persistence, scales of time averaging, and long-term rates of damage accrual, including correlations between shell damage and shell age. Pore water and sediment geochemical analyses were used to determine the pathways and extent of early diagenetic change in the different sediment-pore water environments. We found that carbonate shell preservation is enhanced in dominantly siliciclastic sediments compared to dominantly carbonate sediments. The most important factors limiting the postmortem persistence of shell material are (1) exposure above the sediment-water interface, which is enhanced in coarser-grained carbonate sediments and permits attack by bioeroders and encrusters; (2) the availability of abundant reactive iron mineral phases in the sediments, which promotes supersaturated pore waters and limits acid production; and (3) shell microstructure (rather than mineralogy), particularly organic content that is the focus of intense microbial attack. Thus, there is significant potential for enhanced carbonate shell preservation in areas receiving ferric-rich tropical weathering products, which are common in much of the tropics today and are associated with subduction systems in the geologic past. This suggests that paleodiversity estimates from carbonate tropical settings are minima and that siliciclastic settings are probably underestimated regions for carbon burial, given the large proportion of tropical shelf area characterized by such conditions and the relatively high proportional capture there of local carbonate production.

Taphonomic bias and time-averaging in tropical molluscan death assemblages: differential shell half-lives in Great Barrier Reef sediment

“…Because shell microborers are most active at the sediment-water interface, rapid burial promotes shell survival (Tudhope and Scoffin 1984;Perry 1998;Bradshaw and Scoffin 2001;Best et al 2007). Following the death of an infaunal individual (here, Ethalia, Natica, or Tellina) its shell may never be exposed to the sediment-water interface, as it is likely to be buried rapidly below 20 cm (Figs.…”

Carbon mineralization and carbonate preservation in modern cold-water coral reef sediments on the Norwegian shelf

“…Boudreau and Canfield, 1993;Ku et al, 1999;Walter et al, 1993;Walter and Burton, 1990) have shown that the relationship between sulfate reduction, hydrogen sulfide oxidation and iron-sulfide mineral formation can drive carbonate dissolution and precipitation in shallow marine sediments. Best et al (2007) noted that shell preservation of bivalve shells is enhanced in siliciclastic sediments compared to carbonate dominated sediments. A recent comparison of bauxite-contaminated and uncontaminated sites at Discovery Bay, Florida, (Perry and Taylor, 2006) revealed carbonate grain dissolution at the uncontaminated sites compared to the contaminated sites, where carbonates are preserved.…”

Assinaturas tafonômicas em moluscos recentes e seu significado paleoambiental

“…Neste, devido à alta turbidez da água, associada às elevadas taxas de sedimentação, os restos esqueléticos são soterrados mais rápida e intensamente, e também há uma menor colonização por organismos epibiontes causadores de bioerosão. Além disso, o enriquecimento em ferro do sedimento siliciclástico favorece um pH maior da água percolante, prevenindo a dissolução, ao contrário das fácies carbonáticas (Best et al 2007). …”

“…10), que funcionam diminuindo a alcalinidade (Aller 1982, Cai et al 2006, Best et al 2007. No meio marinho, as taxas de dissolução variam enormemente, podendo ultrapassar a taxa de produção de carbonato bioclástico (Aller 1982Davies et al 1989a).…”

“…Recentemente, o núme-ro de trabalhos tafonômicos atualísticos aumentou consideravelmente, por exemplo, com iniciativas como o SSETI (Shelf and Slope Experimental Taphonomy Initiative) (veja Parsons-Hubbard et al 2011 e suas referências). A tafonomia de moluscos em plataformas carbonáticas tropicais está bem descrita e quantificada (Best & Kidwell 2000a, 2000b, Best et al 2007, Best 2008, mas ainda pouco é conhecido em ambientes subtropicais, siliciclásticos (e.g.…”