Dead shells speak: detecting unsuspected ecosystem change on continental shelves

Dead shells speak: detecting unsuspected ecosystem change on continental shelves

Fri 25th Feb 2022
2 pm
Booking required

Speaker: Prof Susan Kidwell, Chicago

Prof Kidwell is a sedimentary geologist whose research focuses on the postmortem fates of skeletal remains, especially in marine settings. Her work initially addressed controls on the concentration of shells and bones and their application to sequence and basin analysis, mostly using the usefully young Neogene record. For the past 20 years, however, her work has focused largely on quantifying the dynamics of shell preservation in modern settings, using live-dead analysis, meta-analysis, and extensive shell-age dating in tropical, temperate, and, most recently, Arctic settings. As a byproduct, that work has led to some very good news for paleobiologists concerning the faithfulness of species-level community information, notwithstanding remarkably long intervals of time-averaged accumulation and the perils of postmortem bias.

Abstract: One of the major challenges for environmental management and conservation biology is simply discovering ‘what was natural’ before human impacts. This problem is especially pressing in marine systems, where biological monitoring and other records are brief or lacking. This question can be tackled locally in a geologically common-sensical way. First, we compare local dead-shell assemblages – sieved from the surface mixed layer — to a census of the local living community, identifying which species occur only as dead individuals (suggesting their populations have waned) and those that occur only alive, despite having mineralized tissues that should leave a trace, suggesting they are new arrivals to the habitat or region. We can then “age un-mix” the dead-shell assemblage using radiocarbon or other methods, which permits us to establish when taxa declined relative to local timelines of natural and cultural stressors, among other insights (e.g., rates of shell carbonate loss and sequestration). This simple approach, strongly supported by meta-analysis and mechanistic modeling, provides a powerful tool for recognizing ecological change retrospectively. Importantly, it has the power to exonerate as well as to incriminate human activities. The method will be illustrated with case studies, especially from southern California, where approximately 300 years of shifting land use in the Los Angeles watershed has transformed seafloor communities in completely unsuspected ways.

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