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UNravelling BIvAlve Shell chemistry: Advanced Techniques for Accurate Reconstructions of Sub-annual Climate

Periodic Reporting for period 1 - UNBIAS (UNravelling BIvAlve Shell chemistry: Advanced Techniques for Accurate Reconstructions of Sub-annual Climate)

Reporting period: 2020-04-01 to 2022-03-31

The action “UNBIAS: UNravelling BIvAlve Shell Chemistry” aims to improve our understanding of the mineralization of mollusk shells and how shell composition can function as an archive of changes in the shallow marine environment. Shells and skeletons of marine organisms (such as corals or mollusks) are widely used as archives for reconstructing climate and environmental change in Earth’s past. These carbonates preserve in rocks, serving as “time capsules” containing information on climate millions of years ago. Mollusk shells are particularly suitable for this because, like tree rings, they grow incrementally, depositing layers of carbonate. The composition of these layers reflects the environment in which the animals grew and how that environment changed throughout the seasons or even at the scale of days. This type of information is exceedingly rare in geology, since most rocks and minerals grow very slowly and therefore only yield information on a timescale of thousands to millions of years.

We can therefore take “snapshots” of climate and environment from fossil shells of animals that lived during warm periods in the past to learn about the effect of global warming on our seasons or even on weather phenomena. These highly precise climate reconstructions help us to create better models for projecting climate of the future and informing policymakers about the effects of anthropogenic climate change under various future climate scenarios.

However, we do not know precisely how mollusks build shells and how their shell composition depends on the environment. Therefore, UNBIAS tests new methods to characterize the chemistry of mollusk shells and link changes in this chemistry to variability in their environment. Through a combination of growth experiments, in which modern bivalves (e.g. mussels, oysters and cockles) are grown under controlled conditions, and chemical measurements in fossil bivalve shells from past greenhouse periods, UNBIAS aims to improve our understanding of shell formation in mollusks and subsequently apply the lessons learned from modern mollusks to produce more accurate reconstructions of climate in the past.
Work within the UNBIAS project was organized in 4 work packages:

WP1 focused on my personal development as a researcher. The project kicked off with a 3-month secondment at the University of Mainz (carried out in September – December 2019) in which I studied growth structures in bivalve shells and learned to create accurate shell chronologies. WP1 also covered analytical training in the labs of the partner institutions, including setting up the growth experiments with modern bivalve shells, project organization meetings and some academic teaching. At the end of the project, WP1 helped me obtain new lab skills and experience leading a research project and teaching at the academic level.

WP2 bundled the methodological objectives. Primarily, WP2 involved testing of the new clumped isotope temperature proxy on bivalve shells. Later, this method was combined with other proxies in multi-proxy studies. WP2 included work on modern bivalves, testing these new proxies on shell material grown under known environmental conditions. The multi-proxy approach was also applied on fossil shells to test its application for climate reconstructions and testing the preservation of fossil shell material.

WP3 focused on climate reconstructions. The methods developed in WP2 were applied on fossil shells obtained from partnering museums to create high-resolution climate reconstructions of past greenhouse periods. The focus was on reconstructions of the Pliocene Warm Period, a period ~3 million years ago in which global climate was about 2-3 degrees warmer than today, with CO2 concentrations comparable to the present-day. This period forms an ideal analogue for future climate under stable present-day CO2 concentrations. UNBIAS provided precise climate reconstructions from this period, resolving differences between seasons. The methods were also tested on hotter periods in the past, with case studies in the Eocene hothouse (45-55 million years ago), the Late Cretaceous greenhouse period (~75 million years ago) and the extreme Triassic hothouse period (~230 million years ago) to test the effect of various degrees of global warming on short-term climate fluctuations.

WP4 focused on dissemination of UNBIAS results to both specialized and wider audiences. Results were communicated to peers in conference contributions, invited research seminars and academic papers. Some of these studies were disseminated to the popular media with help of public relations assistants of the partnering institutes. In addition, several outreach activities were organized, including public lectures, blogposts, and engagement on social media. Finally, the ideas worked out within the UNBIAS project culminated in the funding of follow-up proposals by the Flemish Research Council and the UU-NIOZ collaboration grant. These projects allow the UNBIAS workflow to yield more accurate climate reconstructions of past greenhouse periods.
UNBIAS pushed the boundaries of high-resolution climate reconstructions. Firstly, the project established the clumped isotope temperature proxy in mollusk shells using calibration studies on lab-grown bivalve shells. This resolves a long-standing debate about potential biological controls on clumped isotope composition in biogenic carbonates and demonstrates that the proxy can be applied on aragonitic shells without a mineral-specific calibration. Aided by routines to optimize sampling strategies and establish accurate chronologies in shells developed in UNBIAS, the clumped isotope proxy allows high-resolution climate reconstructions through geological history.

These state-of-the-art sub-annual climate reconstructions critically revise our understanding of seawater chemistry in the shallow marine realm by demonstrating that seawater chemistry in the shallow marine realm is highly seasonally variable. This result merits reevaluation of previous climate reconstructions which may be seasonally biased.

Strategies for optimal sampling of geochemical records in mollusks were worked out into open-source software packages. These models and guidelines enable the high-resolution climate records required to link climate change to short-term environmental variability, and have wide applications for climate reconstruction studies overall. Similarly, the in situ trace element measurements in UNBIAS help to screen fossil shell material to select optimally preserved specimens for future climate reconstructions.

Finally, UNBIAS actions were communicated to a wider audience through a range of outreach events. These efforts increased awareness of the impacts of climate change on short-term (days to decades) weather and climate variability with a wider audience. We emphasized and explained the importance of climate reconstructions for our understanding of the climate system, which is of fundamental importance for projecting future climate scenarios resulting from anthropogenic global change.
Overview of UNBIAS actions