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.

You do not currently have access to this article.