Tracing Burial History and Sediment Recycling in a Shallow Estuarine Setting (Copano Bay, Texas) Using Postmortem Ages of the Bivalve Mulinia Lateralis

Abstract

Age spectra of Mulinia lateralis shells from the top 0-10 cm of the sediment column in Copano Bay, Texas, show three distinct populations: a young population with a highly skewed distribution ranging from 0 to 1 years, a middle-aged population that is more symmetrical ranging from 1 to 10 years with a peak , 4 years, and a small, very old population ranging from 100 to . 10000 years. The young population is interpreted to record the rapid loss of shells from the taphonomically active zone at or near the sediment surface. The middle-aged population is interpreted to record a sequestered population of shells that has had time to accumulate below the taphonomically active zone. Although surface age spectra differ among depositional environments and at different locations, both of these populations are present in all sampled facies at multiple locations within the bay, indicating that the overarching controls on surface age spectra affect the entire bay. The very old surface population is present only in bay-margin sites and is interpreted to represent shells exhumed from eroding Holocene deposits. An 86-cm-long core taken at a bay-margin site near the mouth of the Aransas River contains shells spanning , 1 to 14,000 years but not preserved in stratigraphic order. The lack of stratigraphic order and the presence of distinct breaks in the postmortem age distribution suggest that 67% of shells in the core were recycled from older deposits, resulting in a 2-3 order of magnitude increase in the amount of time averaging in the sediment column relative to surface samples.

Life span bias explains live–dead discordance in abundance of two common bivalves

“…If the death assemblages were dominated by populations with different maximum ages or AFDs due to time averaging of material representing different climates or environments, their comparison with the living assemblages would be inappropriate. Though we did not date the death assemblage, back-barrier environments can have time averaging that spans centuries or millennia (Olszewski and Kaufman 2015; Dominguez et al 2016; Tomašových et al 2017; Albano et al 2018). However, death assemblages are usually dominated by the newest cohorts (see Kidwell 2013), and even several centuries ago, it is unlikely that North Carolina sea-surface temperatures differed enough to change maximum ages appreciably (Shearman and Lentz 2010; Cléroux et al 2012).…”

Millennial‐Scale Age Offsets Within Fossil Assemblages: Result of Bioturbation Below the Taphonomic Active Zone and Out‐of‐Phase Production

“…When measuring the effect of mixing on the temporal resolution of fossil assemblages, this timescale should correspond to the timescale of shell final burial (i.e., the elapsed time to entering the zone of final burial). Increment‐specific age‐frequency distributions (AFDs) of shells (e.g., Kosnik et al, , ; Olszewski & Kaufman, ; Tomašových et al, ) can reveal the depths of complete and incomplete mixing at such timescales and can directly unmix stratigraphic signals, without any assumptions about the depth or frequency of shell mixing (Tomašových & Kidwell, ). AFDs from successive, downcore increments can thus resolve the effects of incomplete mixing both on the temporal resolution of oceanographic and biological data and on the existence of age‐homogeneous intervals within a sedimentary core (Tomašových et al, , ) and can be informative about the conditions that promote significant age offsets within and between co‐occurring species (Kosnik et al, ; Scarponi et al, ; Tomašových et al, ).…”

Reconstructing geographic range-size dynamics from fossil data

“…This will likely affect the minimum number of occurrences required to accurately reconstruct ranges but will also remove some of the problems associated with the unusual geometries of terrestrial preservable areas (i.e., for alpha hulls). (3) Although preservation potential differs in marine settings, decades of research (e.g., Kidwell and Bosence 1991; Kowalewski et al 2003; Kidwell et al 2005; Kosnik et al 2009; Darroch 2012; Olszewski and Kaufman 2015) have worked toward calibrating the taphonomic biases associated with different taxa in many of these settings, potentially allowing taphonomic potential to be traced onto regional-scale maps of the world’s coastlines and ocean floor. (4) The number of occurrences for marine species is typically higher than it is for terrestrial species, and perhaps promises even better news for workers studying macroecological and macroevolutionary patterns in range-size dynamics through deep time.…”