Temporal scales, sampling designs and age distributions in marine conservation palaeobiology

Abstract

Conservation palaeobiology informs conservation and restoration of ecosystems by using the fossil record to discriminate between baseline and novel states and to assess ecosystem response to perturbations. Variability in the time scale of palaeobiological data can generate patterns that either exaggerate or mute the magnitude of biotic changes. We identify two approaches that remedy the challenges associated with the mixing of baseline and post-impact states and with the transformation of the stratigraphic depth to time. First, combining surface death assemblages with both (1) fossil assemblages preserved in the subsurface historical layers and (2) living assemblages can better resolve the nature of ecosystem shifts than within-core surveys or live-dead analyses alone. Second, post-mortem age distributions of skeletal particles and their preservation states are not only informative about stratigraphic resolution and time averaging of death assemblages but also about the timing of changes in abundance of skeletal producers. High abundance of the youngest age cohorts in surface death assemblages is a null expectation of disintegration and burial dynamic. When this dynamic is accounted for, age distributions of benthic invertebrates from Holocene sediments often reveal high volatility, prolonged turn-offs in production, or pervasive regime shifts that are obscured in the raw stratigraphic record. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6487005

Taxonomic, phylogenetic, and functional diversity of mollusk death assemblages in coral reef and seagrass sediments from two shallow gulfs in Western Cuban Archipelago

“…These age models were based solely on sediments, but they have the advantage that samples were taken at nearby sites (< 100 m) using a barrel corer that effectively samples the top 10 cm of sediments [ 37 ]. A limitation is that estimations of the stratigraphic resolution and time averaging in the Holocene record ultimately requires the coupling of age models and post-mortem age-frequency distributions of dated shells [ 3 ]. Nevertheless, simple age models derived from sediment cores still can be informative about the temporal scale of fossil assemblages even in the absence of post-mortem age-frequency data [ 3 ].…”

“…A limitation is that estimations of the stratigraphic resolution and time averaging in the Holocene record ultimately requires the coupling of age models and post-mortem age-frequency distributions of dated shells [ 3 ]. Nevertheless, simple age models derived from sediment cores still can be informative about the temporal scale of fossil assemblages even in the absence of post-mortem age-frequency data [ 3 ].…”

“…We identified other indirect lines of evidence supporting this rather short time averaging of about one century. First, the sedimentation rate in GG was high [ 37 ] promoting as consequence a low time averaging, regardless the mixing depth and rate of disintegration [ 3 ]. Second, the large taxonomic-, phylogenetic- and functional β-diversity suggested a limited homogenization of assemblage composition due to time averaging [ 2 , 68 ].…”

“…Death assemblages are defined as the taxonomically identifiable skeletal remains from recent and past generations that are present in the surficial mixed layer on landscapes and seafloors [ 1 , 2 ]. Studies about ecology of death assemblages provide historical baselines (e.g., [ 3 ]) but also insights about modern spatial patterns (e.g., [ 4 ]). Two important taphonomic processes influencing the diversity and ecological structure of death assemblages are the time averaging and the postmortem lateral transport of shells.…”

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

“…Surface death assemblages are valuable sources of historical ecological data (Kidwell 2013), but may well be too young to capture post-impact- or mixtures of pre- and post-impact communities, especially in areas with high sedimentation rates. It is therefore necessary to compare living assemblages not only to surface death assemblages but also to buried death assemblages from historical layers below the mixed layer, which may serve as a pre-impact ecological baseline (Tomasovych et al 2023a). Without geochronological context it is often difficult if not impossible to know whether skeletal remains in the death assemblage are from a pre- or post-impact community (e.g., Zuschin and Ebner 2015).…”