Unveiling the Consequences of Environmental Variation and Species Abundances on Beach Taphofacies in Bahamas: The Role of Cementation and Exhumation

Abstract

Although environmental variability generates differences in the preservation of shell assemblages, intrinsic variations in shell characteristics can confound the effects of environment on preservation. However, several studies proposed that the composition of shell supply only affects the intensity of alteration but not its preservation trend along the environmental gradient and that environmental variability represents a major driver of taphofacies preservation. Here, we examine whether taphonomic differences among four infaunal and aragonitic bivalve species differing in shell thickness affect the definition of beach taphofacies in tropical carbonate environments on San Salvador Island (Bahamas). We show that (1) taphofacies can be discriminated with respect to (a) wave and storm activity as a function of exposure to Trade Winds, and (b) sandy beaches versus beaches with a mixture of sands and beach rock (representing a source of exhumed and cemented shells), and (2) species-specific bivalve assemblages show similar gradients in preservation, documenting that differences in preservation between species have minor effects on taphonomic discrimination of beach environments. Environments with a mixture of sands and beach rock are characterized by higher frequency of external cementation and abrasion than sandy beaches. Shells from low-energy beaches are more fragmented and discolored than shells from high-energy beaches. Previous studies showed that shells from San Salvador sandy beaches are more time-averaged than shells from rocky beaches. Differences in preservation between these two environments indicate two pathways: (1) assemblages on sandy beaches are degraded at higher rate but are enriched by old exhumed and lithified shells, and (2) assemblages on rocky beaches are cemented at higher rate. Old and lithified shells on sandy beaches are probably derived from submerged or exposed beach rock patches, leading to the mixture of young, well-preserved shells with old, poorly preserved shells. Shells on sandy beaches thus experience a complex history of burial and exhumation before their final deposition. Therefore, shell assemblages in lower energy carbonate environments from San Salvador Island are highly time-averaged, with the presence of old, poorly preserved shells. The depositional environment is thus the dominant factor controlling the structure of San Salvador beach taphofacies.

Taphonomic Signature of Glycymeris longior Shells (Bivalvia) and its Potential as Paleoenvironmental Proxy for the Quaternary of Northern Patagonia (Argentina)

“…The samples of G. longior shells from Punta Delgado showed two types of preservation patterns that allow for the reconstruction of the paleoenvironment in the Holocene of the SMG. The environmental factors affecting the different assemblages have left a signature on the shell preservation at a different intensity (according to Brett & Baird, 1986; Kidwell & Bosence, 1991; Bayer et al, 2019).…”

“…For the choice of taphonomic attributes and taphonomic grades corresponding to this species, various sources were consulted (Brett & Baird, 1986; Kidwell et al , 1986; Kidwell & Bosence, 1991; Gordillo et al , 1993; Kowalewski et al , 1995; Zuschin et al , 2003; Gordillo, 2009; Ritter et al , 2013, 2016; Bayer et al , 2016, 2019, among others). The taphonomic attributes of the specimens were described using taphonomic grades when quantifiable or binary character states.…”

“…The advantage of considering a target species for taphonomic analysis, i.e. the same species from different assemblages throughout the same region, is the elimination of interspecific variations associated with intrinsic factors such as the shell's microstructure, life habits, and behavior, among others (Bayer et al, 2016(Bayer et al, , 2019 . Thus, the purpose of this study is to describe the taphonomical signature registered on G. longior shells from San Antonio Bay (SMG, Río Negro) as a target species to contribute to future taphonomical comparisons in the SW Atlantic Ocean, as well as in other Quaternary deposits in which it can be found.…”

Millenial-Scale Time Averaging Inferred by Discolored Shells in Beach Death Assemblages

“…Most skeletal concentrations (death assemblages) present on siliciclastic beaches are formed by a complex admixture of shells with different abundances, postmortem ages, preservational grades, and provenances (e.g., Nawrot 2012; Bayer et al 2019). This complexity in age and preservation has been studied from different points of view (e.g., paleoenvironmental analysis, time averaging, fidelity of life and death assemblages, decay) during the last few decades, including the field of Conservation Paleobiology (Fürsich and Flessa 1987; Kidwell 1997, 2013; Zuschin and Oliver 2003; Kowalewski and Labarbera 2004; Kosnik et al 2009; Kidwell and Tomašových 2013; Dietl and Flessa 2017).…”