SCLEROCHRONOLOGICAL COMPARISON OF TURRITELLA ALTILIRA FROM WITHIN AND WITHOUT TURRITELLINE-DOMINATED ASSEMBLAGES IN THE GATUN FORMATION (MIOCENE), PANAMA

Turritellines are among the most abundant and widespread marine gastropod clades and have a fossil record extending into the early Cretaceous. High abundance assemblages of turritellines are also frequent occurrences in post-Cretaceous marine packages. Turritelline-dominated assemblages (TDAs) were defined by Allmon (2007) as macrofaunal assemblages in which “turritelline gastropods comprise either at least 20% of the total actual or estimated biomass or at least 20% of macroscopic individuals in the assemblage and are at least twice as abundant as any other macroscopic species in the assemblage.” Previous studies have suggested that TDAs are linked to high primary productivity, and related upwelling, a relationship which holds for living turritellines. Previous isotopic analysis has indicated upwelling associated with turritelline-rich beds (Allmon et al., 1995, Jones and Allmon 1995), but has not compared the isotopic data from high-abundance layers with data from conspecifics sourced from low-abundance layers.

In order to explore the environmental context of TDA formation, especially differences in nutrient input, specimens of Turritella altilira were collected from 6 TDA beds and several non-TDA layers within the Late Miocene Gatún Formation of central Panama. We compared δ13C and δ18O sclerochronologies sourced from both TDA beds and non-TDA beds to ascertain whether there are detectable differences in upwelling strength, freshwater input, or differences in growth rate which may be related to the formation of these layers. T. altilira which grew under strong upwelling conditions should exhibit a strong negative correlation between δ13C and δ18O, while freshening would be indicated by a strong positive correlation (Jones and Allmon, 1995, Tao et al., 2012).

Data from this analysis are consistent with the hypothesis that TDAs were deposited under mesotrophic to eutrophic condition. However neither average initial growing conditions nor lifetime growing conditions appear to differ significantly between beds in which Turritella are present at low abundance and those in which they are highly abundant. Therefore the presence of these layers may be indicative of slight shifts in depth of deposition within the formation rather than changes in nutrient conditions (Hendy, 2013).