In the face of accelerating anthropogenic carbon dioxide emissions, the ocean surface acts as a critical but vulnerable sink, absorbing a substantial fraction of atmospheric CO2. This uptake intensifies ocean acidification, imposing profound ecological stress on planktonic communities—microscopic marine organisms fundamental to global biogeochemical cycles and marine food webs. Understanding how these communities respond to elevated CO2 and associated warming is paramount for predicting future marine ecosystem dynamics under continued climate change scenarios. Insights into such responses can be gleaned from paleontological investigations of past rapid warming events, notably the Paleocene-Eocene Thermal Maximum (PETM), approximately 56 million years ago. The PETM serves as an analog for modern climate disruption, characterized by a rapid surge in carbon emissions and profound oceanic changes, evidenced globally in deep-sea sediment archives.
Recent research led by a team from MARUM at the University of Bremen focuses on the sensitivity of high-latitude phytoplankton to environmental shifts during the PETM. High-latitude marine ecosystems are particularly important yet historically underrepresented in paleoceanographic research, despite their ecological sensitivity and biogeographic distinctiveness. The researchers utilized sediment cores retrieved from the Campbell Plateau in the Southern Ocean during International Ocean Discovery Program Expedition 378, facilitating a novel examination of calcareous nannoplankton assemblages preserved in deep-sea deposits. These microscopic algae biomineralize calcium carbonate shells, leaving detailed fossil records that chronicle shifts in community composition and abundance across climatic perturbations.
Calcareous nannoplankton species exhibit distinct ecological preferences, with some taxa adapted to warmer, oligotrophic surface waters, while others favor cooler, nutrient-rich conditions. By quantifying fossil nannoplankton assemblages preceding and during the PETM, the researchers reconstructed community adaptations to ocean warming and acidification. Contrary to expectations of dramatic PETM-driven turnover, the study reveals a more nuanced response, marked by prior destabilization of communities approximately 200,000 years before the PETM onset. This earlier warming episode appears to have primed phytoplankton assemblages for subsequent environmental stressors, suggesting that background climatic variability plays a critical yet often overlooked role in mediating ecosystem resilience.
Dr. Heather L. Jones, first author of the study, emphasizes the importance of incorporating pre-event intervals when assessing paleobiological responses to climatic crises. The findings highlight that even modest, incremental environmental changes can exert outsized ecological impacts, particularly in sensitive polar marine environments. The research calls for a broader temporal framework in paleoecological investigations to capture the cumulative effects of successive and overlapping stress events on marine communities, which may have direct relevance to forecasting ongoing planktonic responses under progressive anthropogenic climate change.
The study’s identification of this previously undocumented pre-PETM warming event invites further exploration within the extensive global repository of legacy deep-sea sediment cores. The Bremen Core Repository (BCR), housed within MARUM, offers an invaluable archive enabling comparative analyses to determine the spatiotemporal extent and ecological ramifications of this early phase climatic disturbance across multiple ocean basins. Such endeavors will refine paleoceanographic models, adding depth and resolution to our understanding of ecosystem dynamics at critical transitional intervals in Earth’s climate history.
These findings underscore the intricacy of biotic responses to rapid environmental change and emphasize the utility of calcareous nannoplankton as sensitive bioindicators for reconstructing past ocean conditions. The MARUM team’s work contributes significantly to the broader Cluster of Excellence “The Ocean Floor – Earth’s Uncharted Interface,” which seeks to unravel the complex interactions at the junction of geosphere and biosphere. Investigating how fundamental productivity drivers react to stressors enhances predictive capacity for future ocean health and carbon cycle feedbacks under continued warming and acidification.
The revelation of the pre-PETM event also prompts reconsideration of vulnerability thresholds in marine ecosystems. It appears that ecosystems may exhibit cumulative stress effects, where prior exposure to moderate environmental fluctuations modulates subsequent ecological trajectories. This has significant implications for current climate change impacts in regional high-latitude seas, where warming is occurring at an accelerated pace, and ecosystems may already be operating near critical tipping points.
Furthermore, the study illustrates the value of integrating fossil evidence with present-day ecological theory to develop holistic understandings of how marine life adapts or succumbs to rapid environmental shifts. The documentation of such ecological preliminary changes offers a magnified lens for interpreting contemporary observations, where rapid yet subtle shifts in plankton composition can have cascading effects through food webs and global biogeochemical cycles.
By providing a temporal context extending well before the PETM interval, the research challenges the notion of abrupt biotic change confined narrowly to peak warming periods. Instead, a protracted prelude of environmental destabilization may underlie the most severe ecosystem transformations, emphasizing the need for long-term, multidimensional perspectives in climate impact assessments.
As the ocean continues to absorb anthropogenic CO2, the structured analysis of fossil plankton communities holds promise for deciphering the evolutionary and ecological mechanisms that will govern the resilience or decline of marine primary producers. The MARUM team’s pioneering insights form a cornerstone for future high-resolution paleoecological studies, bridging past and present in the quest to understand climate-driven ecosystem shifts in a warming world.
Subject of Research:
High-latitude phytoplankton community responses to Paleocene-Eocene Thermal Maximum warming and precursor climatic disturbances.
Article Title:
Palaeoecological change preceded the Palaeocene-Eocene Thermal Maximum by 200 kyr in the high latitude south-west Pacific Ocean
News Publication Date:
12-Sep-2025
Web References:
http://dx.doi.org/10.1038/s43247-025-02749-5
Image Credits:
MARUM – Center for Marine Environmental Sciences, University of Bremen; M. Toyos Simón
Keywords:
Paleocene-Eocene Thermal Maximum, ocean acidification, calcareous nannoplankton, high-latitude phytoplankton, paleoceanography, climate warming, deep-sea sediment cores, Southern Ocean, carbon cycle, marine ecosystems, International Ocean Discovery Program, paleoecology
