A downcore increase in time averaging is the null expectation from the transit of death assemblages through a mixed layer

Abstract

Understanding how time averaging changes during burial is essential for using Holocene and Anthropocene cores to analyze ecosystem change, given the many ways in which time averaging affects biodiversity measures. Here, we use transition-rate matrices to explore how the extent of time averaging changes downcore as shells transit through a taphonomically complex mixed layer into permanently buried historical layers: this is a null model, without any temporal changes in rates of sedimentation or bioturbation, to contrast with downcore patterns that might be produced by human activity. Assuming stochastic burial and exhumation movements of shells between increments within the mixed layer and stochastic disintegration within increments, we find that almost all combinations of net sedimentation, mixing, and disintegration produce a downcore increase in time averaging (interquartile range [IQR] of shell ages), this trend is typically associated with a decrease in kurtosis and skewness and by a shift from right-skewed to symmetrical age distributions. A downcore increase in time averaging is thus the null expectation wherever bioturbation generates an internally structured mixed layer (i.e., a surface, well-mixed layer is underlain by an incompletely mixed layer): under these conditions, shells are mixed throughout the entire mixed layer at a slower rate than they are buried below it by sedimentation. This downcore trend created by mixing is further amplified by the downcore decline in disintegration rate. We find that transition-rate matrices accurately reproduce the downcore changes in IQR, skewness, and kurtosis observed in bivalve assemblages from the southern California shelf. The right-skewed shell age-frequency distributions typical of surface death assemblages—the focus of most actualistic research—might be fossilized under exceptional conditions of episodic anoxia or sudden burial. However, such right-skewed assemblages will typically not survive transit through the surface mixed layer into subsurface historical layers: they are geologically transient. The deep-time fossil record will be dominated instead by the more time-averaged assemblages with weakly skewed age distributions that form in the lower parts of the mixed layer.

Simulating our ability to accurately detect abrupt changes in assemblage-based paleoenvironmental proxies

“…Previous studies of time averaging in macrofaunal shelly assemblages also suggest that even if assemblages overlap in time by 50%, the older assemblage will only contribute 10% of the fossils in the resulting sampled assemblage due to the exponential taphonomic loss of the fossils (Olszewski, 1999), thus the preserved assemblage mostly retains individuals of similar age. We do not include this loss of older specimens in the simulations herein, thus, the DCA-1 values we obtain from the simulations likely depart further from the event value than they would in a natural sedimentary system-unless the age frequency distribution of subsurface assemblages are more symmetric, as suggested by recent modeling work by Tomašových et al (2023), and assemblage mixing would be greater than in Olszewski (1999), as in our model.…”

Stratigraphic paleobiology

“…This creates not only clusters of first and last occurrences in marine settings, but also abrupt changes in community composition and attributes of communities (such as richness, evenness), as well as abrupt changes in the degree of time averaging and taphonomic modification, as those are also correlated with marine ecological gradients. Such clusters can also be modified over centimeters to decimeters by sediment mixing through bioturbation, as can fossil occurrences in general (Tomašových et al 2023b). In addition, marine deposits contain intervals of progressive change in fossil occurrences that reflect longer-term changes in water depth that are entirely predictable from sequence-stratigraphic architecture.…”

Challenges of Conservation Paleobiology: From Baselines to Novel Communities to the Necessity for Granting Rights to Nature

“…Accordingly, estimating the degree of time-averaging and the shape of the post-mortem age frequency distribution of skeletal elements in a given layer requires age-dating many biogenic components; and this approach is time- and resource-intensive (Kosnik et al 2013, 2015). Age-frequency distributions of shells in surface carbonate and siliciclastic environments are distinctly right-skewed (e.g., Kosnik et al 2007; Kidwell 2013), while downcore increase in time averaging is typically associated with a shift towards more symmetrical age frequency distributions (Olszewski and Kaufman 2015; Tomasovych et al 2023b). To avoid premature conclusions on potential anthropogenic impacts, extensive age-dating is also necessary to understand the population dynamics of those species that are abundant and dead but absent in the living assemblage (Albano et al 2016).…”