TRACING BURIAL HISTORY AND SEDIMENT RECYCLING USING POSTMORTEM AGES OF THE BIVALVE MULINIA LATERALIS (COPANO BAY, TX)

In contrast to detrital sediment grains, biogenic hardparts are produced autochthonously within a depositional system. As such, after death they can serve as tracers of sediment burial and recycling pathways. The pilot study presented here demonstrates how the distribution of postmortem ages of the bivalve Mulinia lateralis in an 86-cm-long core taken near the mouth of the Aransas River reflects Holocene sedimentary dynamics in Copano Bay, Texas. Postmortem ages of shells determined by amino acid racemization range from <1 to >20,000 years with a median age ~600 years. There is no significant relationship between absolute age and depth (Pearson’s r = 0.103, df = 55, p-value = 0.446), although there is a significant but weak correlation between relative stratigraphic order and postmortem age (Kendall’s τ = 0.260, p-value = 0.0045). Plotting the core data as a survivorship curve (i.e., a cumulative frequency distribution) indicates the presence of distinct shell populations that represent different burial and recycling histories. Based on comparison with a numerical model of exhumation and reburial, the 18 youngest shells (<40 years old) were incorporated into the sediment column shortly after death and are in their first cycle of burial. This population shows a strong correlation between age and depth (Pearson’s r = 0.783, p-value = 9.3x10^-4), and based on their ages, net aggradation rate is ~2.25 cm/yr. Shells >40 years old appear to have been recycled (exhumed and reburied) at least once and are not in stratigraphic order. Numerical simulation indicates that recycled shells younger than ~2500 years (n=24) have gone through only one cycle of exhumation and reburial; this age coincides with the establishment of the current depositional regime of the Aransas River delta and indicates the degree to which it is cannibalizing its own deposits. Shells older than ~2500 years (n=15) appear to have gone through multiple cycles of exhumation and burial over the entire Holocene history of Copano Bay. These results shed light on the potential influence of sediment recycling on the span of time averaging in coastal marine deposits.