In the ongoing anthropogenic climate crisis, successful adaptation to future climate requires a detailed understanding of the response of Earth’s climate system to warming. Past warm climates constitute a valuable natural laboratory for studying this response, but reconstructions of past climate variability on human timescales (days to decades) remains challenging. Biogenic carbonates such as mollusk shells are uniquely suitable for these high-resolution climate reconstructions for three reasons:

Firstly, mollusk shells grow incrementally, depositing annual, daily, or even tidal laminae of carbonate marking time at unique detail.

Secondly, mollusks are diverse, abundant, and highly evolutionarily successful: Their fossil record spans the entire Phanerozoic and they produce shells of various shapes, sizes, mineral structures and compositions, making them versatile climate archives.

Thirdly, carbonate shells have a high preservation potential, retaining their original chemical composition, and the climate information locked therein, also on long geological timescales.

I will present some of the latest developments in sclerochronology, the study of reading the skeletal diaries of these fascinating invertebrates, and highlight how the information they reveal changes our understanding of past climate. A few case studies will showcase the full potential of fossil shells as climate archives. Finally, I will discuss some open questions in the field and the ongoing and future projects in which we hope to answer them. The goal is to demonstrate how collaborations between biology, marine science and (geo)chemistry enable us to unlock the full potential of these unique archives and contribute to understanding shallow marine ecosystems and climate.

Tridacna are important archives for (sub)tropical marine palaeoenvironmental conditions. Their longevity (up to 100 years), large aragonitic shells (up to 1m) and rapid shell accretion (mm-cm/year) make them ideal to give insights into region specific climate and environmental variability. Highly spatially-resolved analytical techniques such as LA-ICPMS mean that geochemical data can be retrieved at a high temporal resolution (subdaily). Tridacna, with their daily banding and growth rates of tens of µm/day, are ideal candidates for applying this methodology evaluating seasonality and extreme weather events in (sub-)tropical reefs since their appearance in the early Miocene. Studies of seasonal records and information on extreme weather events from past climate settings can help inform model assessment exercises regarding how seasonality might change in future climate scenarios. In this study we present a multiproxy record from a 250 mm large late Miocene Tridacna from East Borneo spanning several decades, with subseasonally resolved stable δ¹⁸O and δ¹³C data and sub-daily resolved elemental ratio data (B, Na, Mg, Sr, Ba to Ca). By applying Daydacna, a recently developed python script that enables daily cycle based internal age modelling, we can create an age model of the shell, which forms the basis for the temporal reassignment of the geochemical data. Displaying geochemical data against time rather than shell distance improves multi-annual as well as inter-organism comparisons for palaeoseasonality reconstructions. We reconstruct seasonal growth rate variability and compare it to the corresponding elemental and isotopic ratios to evaluate the relationships between geochemical signals, shell growth and environmental parameters.

Climate change, in particular the rise in tropical sea surface temperatures, is the greatest threat to coral reef ecosystems today and causes climatic extremes affecting the livelihood of tropical societies. Assessing how future warming will change coral reef ecosystems and tropical climate variability is therefore of extreme urgency. Ultra-high resolution (monthly, weekly) coral geochemistry provides a tool (1) to understand the temporal response of corals and coral reefs to ongoing climate and environmental change, (2) to reconstruct past tropical climate and environmental variability, and (3) to use these data in conjunction with advanced statistical methods, earth system modelling and observed ecosystem responses for improved projections of future changes in tropical climate and coral reef ecosystems. The DFG Priority Programme “Tropical Climate Variability and Coral Reefs” (SPP 2299, https://www.spp2299.tropicalclimatecorals.de/) aims to enhance our current understanding of tropical marine climate variability and its impact on coral reef ecosystems in a warming world, by quantifying climatic and environmental changes during both the ongoing warming and past warm periods on timescales relevant for society. The programme aims to provide an ultra-high resolution past to future perspective on current rates of change to project how tropical marine climate variability and coral reef ecosystems will change in a warming world. Information on the organisational structure, research topics and preliminary results of this collaborative programme, which involves more than 40 scientists from ten universities, three Helmholtz Centres, one Max-Planck Institute and one Leibniz Centre from all over Germany, will be provided.

The Triassic-Jurassic boundary (TJB) and the early Toarcian are characterized by greenhouse warming, caused by the emplacement of large volcanic provinces. Despite similar trigger mechanisms, the two events differ in their character. Most significantly, the early Toarcian records the genesis of an Oceanic Anoxic Event (T-OAE), while the TJB event lacks robust evidence for widespread marine anoxia and organic matter (OM) accumulation.

Here, we present bulk, isotope, and molecular geochemical data from a continuous drill core, taken in the northeastern part of the North-German Basin that spans upmost Triassic and Lower Jurassic strata (Rhaetian-Toarcian). The sediment archive represents a near-shore environment, proximal to an estuary. This setting at the land-sea-interface was particularly susceptible to sealevel, climate, and environmental change and records the response of a shallow marine environment to major Late Triassic-Early Jurassic climate events.

In contrast to the predominantly OM-lean Triassic-Jurassic strata, the TJB interval and the T-OAE are characterized by increased OM contents, but their compositions differ significantly. At the TJB, increased abundances of soil and land plant OM accumulated under semi-arid conditions in mesosaline and well-oxygenated shallow marine setting. By contrast, during the T-OAE, OM-rich sediments accumulated under short-lived anoxic-euxinic conditions. Development of oxygen-deficient conditions was favored by a high sea level and persistent freshwater stratification caused by enhanced riverine discharge under humid climate conditions. Land plant wax lipid, algal molecular fossils, and wildfire combustion residues further revealed that the TJB and the T-OAE were accompanied by substantial changes in both the continental and marine ecosystems.

The Eastern Paratethys (EP) is a former epicontinental basin that unified the Black Sea, Caspian Sea and the Dacian Basin and played a crucial role in shaping of the west Eurasian paleoecosystems.

In the late Middle-Late Miocene, during Sarmatian sensu lato Stage, the EP underwent a gradual hydrological isolation from the global ocean. This process was accompanied by adaptation and radiation of endemic faunas in the early and middle Sarmatian s.l. (Volhynian and Bessarabian) and by the near complete extinction of marine life forms in the late Sarmatian s.l. (Khersonian). The drivers of this ecological crisis still remain ununderstood. We present our integrated stratigraphic study of 130-m-thick outcrop Karagiye, Caspian Basin. The preliminary data demonstrate:

1. Incomplete Volhynian record (12.3–12.05 Ma). Proximal lagoon with molluscs Obsoletiformes and Ervilia, ostracod zones Cytheridea hungarica and Euxinocythere turpe, foraminfera zones Veridentella reussi and Elphidium reginum, rich marine vertebrate fauna;
2. Bessarabian offshore to lagoon record (11.9–9.8 Ma) with molluscs Plicatiformes plicatofittoni, Sarmatimactra vitaliana, ostracod zones Euxinocuthere grave odessoensis and Loxoconcha subcrassula, foraminifera zones Dogielina sarmatica and Porosononion aragviensis, rich marine vertebrate fauna.
3. Khersonian shallow water barrier island to foreshore (9.4–7.6 Ma) with molluscs Chersonimactra caspia and Ch. bulgarica, ostracod zones – Euxinocythere immutata and E.dilecta, foraminifera and marine vertebrate fauna missing.

Our ongoing study for the first time provides well-dated mollusc and microfauna zonations of the Sarmatian s.l. substages in the Caspian Basin.