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

Abstract

Abstract.-Radiocarbon-calibrated amino acid racemization ages of 428 individually dated shells representing four molluscan taxa are used to quantify time-averaging and shell half-lives with increasing burial depth in the shallow-water carbonate lagoon of Rib Reef, central Great Barrier Reef, Australia. The top 20 cm of sediment contains a distinct, essentially modern assemblage. Shells recovered at depths from 25 to 125 cm are age-homogeneous and significantly older than the surface layer. Taxon age distributions within sedimentary layers indicate that the top 125 cm of lagoonal sediment is thoroughly mixed on a sub-century scale. The age distributions and shell half-lives of four taxa (Ethalia, Natica, Tellina, and Turbo) are found to be largely distinct. Shell half-lives do not coincide with any single morphological characteristic thought to infer greater durability, but they are strongly related to a combined durability score based on shell density, thickness, and shape. These results illustrate the importance of bioturbation in tropical sedimentary environments, indicate that age estimates in this depositional setting are sensitive to taxon choice, and quantify a taxon-dependent bias in shell longevity and death assemblage formation.

Sediment accumulation, stratigraphic order, and the extent of time-averaging in lagoonal sediments: a comparison of 210Pb and 14C/amino acid racemization chronologies

“…This is in stark contrast to the shell assemblages spanning similar burial depths from Rib and Bramble reefs that yielded age homogenous samples spanning the last *3000 yr (Kosnik et al 2007(Kosnik et al , 2009. Consistent with previous work, both 210 Pb and 14 C/ AAR chronologies indicate that the top 0.15-0.2 m of sediment is actively mixed.…”

“…Coral Reefs (2015) 34:215-229 223 skewed (e.g., Kidwell 2013), and observations made in previous studies that the top 0.2 m of surficial sediment typically sampled by grab samplers do not reflect the properties of deeper sediments (Kosnik et al 2007(Kosnik et al , 2009. Estimates of sedimentation or time-averaging derived from the surficial layer are indicative of internal depositional dynamics and not the aggrading sedimentary column.…”

“…The ponded water depth at the collection site (23.49677°S, 152.06587°E) was 4.5 m. Here, we focus on a single 1.6 m deep 80 mm diameter percussive hand core and a single 1.2 m deep 0.25 m 2 excavation both collected by divers. The method of excavation differs slightly from previous work at Rib and Bramble Reefs (Kosnik et al 2007(Kosnik et al , 2009 in that the retaining wall system was assembled without mechanized assistance and the layers were collected using an 80 mm diameter water dredge rather than an 80 mm diameter airlift. The water dredge was fitted with a small water jet to facilitate the gentle breakup of the compacted mud.…”

“…Now, however, paleoecological studies spanning the period of colonial expansion and/or anthropogenic impact require a detailed understanding of the formation of the fossil assemblages being utilized. Although time-averaging in tropical sedimentary deposits is still comparatively understudied (see Kidwell et al 2005;Krause et al 2010;Kosnik et al 2013), previous work has shown that reefal sediment accumulating over the past few hundred yrs is not necessarily stratigraphically ordered (Kosnik et al 2007(Kosnik et al , 2009. One needs look no further than volume 11 issue 2 of Coral Reefs to see the controversy caused by the use of paleontological data derived from reefal sediment to examine the Holocene history of Crownof-thorns outbreaks on the Great Barrier Reef (GBR).…”

“…Third, sediment grain durability determines the length of time that a sedimentary particle will remain intact and recognizable. Fragile sedimentary particles are more likely to be destroyed (e.g., fragmented, eroded, or dissolved) during mixing, leading to less time-averaging, whereas durable particles can be thoroughly mixed without breaking, leading to more time-averaging (Cummins et al 1986;Kosnik et al 2009). …”