Ideas and perspectives: Human impacts alter the marine fossil record

Ideas and perspectives: Human impacts alter the marine fossil record Rafał Nawrot1 , Martin Zuschin1 , Adam Tomašových2 , Michał Kowalewski3 , and Daniele Scarponi4 1 Department of Palaeontology, University of Vienna, Vienna, 1090, Austria Science Institute, Slovak Academy of Sciences, Bratislava, 84005, Slovakia 3 Florida Museum of Natural History, University of Florida, Gainesville, FL 32611, USA 4 Alma Mater Research Institute on Global Challenges and Climate Change, Università di Bologna, Bologna, 40126, Italy 2 Earth Correspondence: Rafał Nawrot () Received: 9 December 2023 – Discussion started: 26 January 2024 Revised: 26 March 2024 – Accepted: 28 March 2024 – Published: 3 May 2024 Abstract. The youngest fossil record is a crucial source of data documenting the recent history of marine ecosystems and their long-term alteration by humans. However, human activities that reshape communities and habitats also alter sedimentary and biological processes that control the formation of the sedimentary archives recording those impacts. These diverse physical, geochemical, and biological disturbances include changes in sediment fluxes due to the alteration of alluvial and coastal landscapes, seabed disturbance by bottom trawling and ship traffic, ocean acidification and deoxygenation, removal of native species, and introduction of invasive ecosystem engineers. These novel processes modify sedimentation rates, the depth and intensity of sediment mixing, the pore-water saturation state, and the preservation potential of skeletal remains – the parameters controlling the completeness and spatiotemporal resolution of the fossil record. We argue that humans have become a major force transforming the nature of the marine fossil record in ways that can both impede and improve our ability to reconstruct past ecological and climate dynamics. A better understanding of the feedback between human impacts on ecosystem processes and their preservation in the marine fossil record offers new research opportunities and novel tools for interpreting geohistorical archives of the ongoing anthropogenic transformation of the coastal ocean. 1 Introduction The fossil and sedimentary archives (geohistorical records) of the Anthropocene (sensu Gibbard et al., 2022; see Head et al., 2022, for an alternative view) are an indispensable source of data on ecosystem and climate states preceding the onset of ecological monitoring and instrumental records, allowing reconstruction of long-term human impacts on marine ecosystems (Dietl and Flessa, 2011; Kidwell, 2015; Yasuhara et al., 2020; Dillon et al., 2022). As with any historical record, the geohistorical data are most valuable when their information quality is well understood. However, humans are altering not only marine ecosystems but also the nature of the stratigraphic archives that record those changes (Wilkinson, 2005; Tyrrell, 2011; Oberle et al., 2016; Syvitski et al., 2022). In other words, humans affect not only what is preserved but also how it is preserved because key ecosystem processes affected by human impacts, such as bioturbation and remineralization of organic matter, also control the burial and preservation of skeletal remains. Although the resulting taphonomic and stratigraphic signatures may pose a challenge for accurately reconstructing past ecological and climate dynamics, they can also pinpoint historical shifts in ecosystem functioning (e.g., Gooday et al., 2009; Yasuhara et al., 2019; Tomašových et al., 2021) and thus improve our understanding of the consequences of global change. Here, we propose a conceptual framework for understanding how human alteration of marine ecosystems changes the completeness and spatiotemporal resolution of geohistorical records that form in marginal marine, continental shelf, and slope environments, where human impacts are concentrated Published by Copernicus Publications on behalf of the European Geosciences Union. Ideas and perspectives Biogeosciences, 21, 2177–2188, 2024 https://doi.org/10.5194/bg-21-2177-2024 © Author(s) 2024. This work is distributed under the Creative Commons Attribution 4.0 License. 2178 R. Nawrot et al.: Human impacts alter the marine fossil record (Halpern et al., 2008). We highlight the implications of this phenomenon for marine paleoecology, conservation paleobiology, and paleoclimatic studies and suggest research strategies that can maximize the information value of the geohistorical data extracted from sediment cores and surface death assemblages (i.e., skeletal remains accumulating in the surface mixed layer of present-day seabeds). By providing an overview of mechanisms by which humans modify the incipient fossil record, we hope to encourage a more explicit consideration of these effects during interpretation of marine sedimentary and fossil archives. 2 What determines the quality of geohistorical records? The quality of geohistorical records formed by fossils embedded in sedimentary successions is determined by their spatiotemporal resolution and completeness. Spatiotemporal resolution corresponds to the extent of spatial mixing and time averaging of fossil assemblages, i.e., the co-occurrence of remains of organisms that lived at different times and/or places in a single sedimentary layer (depositional resolution sensu Kowalewski and Bambach, 2008). Completeness of the record can be understood both as completeness of fossil assemblages relative to their source communities (controlled by variability in preservation potential, both within and across taxa) and as stratigraphic completeness (Sadler, 1981) determined by the duration of hiatuses (stratigraphic resolution sensu Kowalewski and Bambach, 2008). Here, we will primarily focus on fossil assemblage completeness, which determines the fidelity of a given eco-environmental variable preserved in the geological record relative to its original signal. For example, a fossil sample limited to thickshelled specimens varying in age by 3000 years has low completeness and coarse temporal resolution and thus low fidelity with respect to the original composition of the source living assemblage. In the marine realm, the completeness and resolution of geohistorical records are primarily controlled by four parameters: (1) net sediment accumulation rate, (2) depth and intensity of sediment mixing below the seafloor, (3) disintegration rates determined mainly by the pore-water saturation state and bioerosion, and (4) skeletal production and durability (Kidwell, 1986; Olszewski, 2004; Kowalewski and Bambach, 2008; Tomašových et al., 2019). Skeletal production depends on community composition and population dynamics, which control the durability of skeletal remains and the rate at which they enter a death assemblage. Subsequently, sedimentation, sediment mixing, and pore-water chemistry determine whether the remains disintegrate near the sediment–water interface and if and at what rate they undergo burial to historica (...truncated)