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Seasonal Temperatures and Acidification of sensitive Marine settings: Insight of an uNmatched macro-invertebrate Archive.

Periodic Reporting for period 1 - STAMINA (Seasonal Temperatures and Acidification of sensitive Marine settings: Insight of an uNmatched macro-invertebrate Archive.)

Reporting period: 2021-04-07 to 2023-04-06

Since the industrial revolution began, the concentration of carbon dioxide in the atmosphere has increased and it is now more than 50% higher than pre-industrial levels. The oceans absorb about 30% of the carbon dioxide, which reacts with seawater and carbonate ions, resulting in a decrease of seawater pH. As the pH decreases, oceans became more acidic, and the fitness of many marine organisms is negatively impacted in a number of different ways. Corals and shell builders with a calcium carbonate skeleton are harmed, as well as other organisms such as fishes, posing a threat to humanity, since billions of people depend on food from the ocean as their main source of protein and many activities rely on the fish and shellfish living in the ocean, posing a threat to the economy.
Seasonality is a key factor to understand possible climate changes. Indeed, analyses of surface ocean observations from 1982 to 2015 has demonstrated that the seasonal difference in concentration of carbon dioxide has increased globally because of changes in the seasonality of superficial temperature. Climate models show an enhancement in the amplitude of the seasonal cycle of the sea surface in the middle-high latitudes, reaching a difference between summer and winter up to 8°C by the year 2100, implying more substantial stress for organisms sensitive to pH due to change in the seasonal dissolution of the carbon dioxide.
Studying seasonal time series of superficial temperature and pH is thus critical to forecast climatic changes, but the record of seasonality, especially for the pH, does not go far into the past, covering about the last 40 years. Therefore, the challenge of this project was to understand if it is possible to obtain seasonal time series for pH and seawater temperature beyond the instrumental record using recent and fossil shells of brachiopods.
Brachiopods, known also as lamp-shells, are sessile marine organisms, which produce a shell of calcium carbonate. Estimates for pH and temperature can be obtained by measuring the Boron and Oxygen isotopes of the brachiopod shell, after understanding the biological factors affecting the isotopic values. The broad objectives of this project were to study the seasonal signal in shells of brachiopods which live in middle-high latitudes and to understand the meaning of seasonal time series to make correct esteem of past seasonal parameters (pH and Temperature) estimated by analysing fossil shells.
This research project generated a relevant understanding of the interpretation of the seasonal signal in brachiopods shells. The study of the shell morphology and microstructure highlighted that their shell is able to produce seasonal growth lines. Since studying growth permitted to assign an ontogenetic age to the shell material, it was possible to compare the seasonal variation of Oxygen (δ18O) and Boron isotopes (δ11B), which are proxies for temperature and pH respectively, with the environmental parameters independently instrumentally measured. The main investigated case, the species Calloria inconspicua, is an intertidal brachiopod living in Otago Harbour (New Zealand), where the temperature and pH are monitored daily.
This brachiopod species shows different vital and biological effects affecting the record of the geochemical proxies. The Oxygen isotopes of the juvenile shell (<3 years old) reflect the seasonal variation of the temperature, while the adult part of the shell (>3 years old) does not record the full seasonal variation of temperature. This behaviour is caused by a shift in the growing season when the brachiopods reach the adult stage, with the shell expanding only in the favourable season and decreasing its growth rate.
The Boron isotopes proved to be difficult to interpret in terms of seasonal variation in pH values. When the shell is growing fast and extending its length, the organism is able to buffer the pH of the fluid from which the shell is mineralized, reducing the acidity to favour the calcification. The values of Boron isotopes reflect the pH measured in the environment at Otago Harbour only when the shell grows slowly. Therefore knowledge of pH past seasonality seems more challenging to achieve at the current state of the art.
This acquired knowledge allowed the interpretation of past seasonality inferred from Oxygen isotopes measured on fossil brachiopods from the Permian period (~251-290 M.a.). Taking into account the effect related to growth, the seasonal signal seems to change greatly between glacial and interglacial intervals during the demise of the Late Palaeozoic Ice Age. A few studied fossil reveal that the temperature difference between summer and winter during interglacial times could be more than double compared to glacial intervals, indicating that global warming leads to an increase of extreme seasonal temperature at the middle-high latitudes.
The research output of this project was disseminated to experts at four international conferences and one paper has been published in the scientific peer-reviewed journal, with two papers currently in progress and to be published after the closure of the action. The communication activities were performed in different contests. The researcher opened a website page explaining to the public the meaning of the action and he used a Facebook web page to share information about climate change and action to undertake. Both web pages are still operational. The researcher has also released public lectures and published one opinion article on a business magazine published in his native country, Italy.
The achieved knowledge contributes to the interpretation of the brachiopods shell as a biological archive of past climate change. It is now clear that studies of seasonality using brachiopod shells need to account for their ontogenetic history. Although this aspect is well known and taken into account for other archives, such as for bivalved molluscs, previous studies of the paleotemperature record based on brachiopods have not taken account for this issue. It is clearly necessary to revise this standard for future studies.
Brachiopods are able to buffer their physiological fluid and control the pH during the phase of shell accretion. This implies the necessity of calibration for the Boron isotopes as a proxy of pH, which entails correction for the growth rate if the aim to acquire high-resolution record of ancient pH in future studies.
The study of past seasonality in the Permian provides additional clues about the compelling necessity of mitigating global warming and its effect since the change in the seasonal extreme temperature that humanity will experience in the future are potentially much more dramatic than what we are already experiencing. Indeed, during the demise of the Late Paleozoic Ice Age, when glaciers were melting and Carbon dioxide was increasing in the atmosphere, the mean seasonal difference between summer and winter temperatures might have increased up to 8 ºC at the middle-high latitudes. A future scenario such as this, coupled with the overall increase in mean temperatures, entails an unpredictable and dramatic change in the marine coastal ecosystem, but also in the climate, harming both biodiversity and human prosperity.
Dr. Claudio Garbelli speaks to the public about brachiopods and climate changes