Concern about the oxygenation of the oceans has increased, making it essential to study the past in order to understand changes in the oceans and anticipate future scenarios. The ocean's chemical composition has undergone significant changes over geological time, and analysing these variations is crucial to predicting the impacts of climate change. In the past, over millions of years, the Earth has faced more extreme conditions than today, and studying these periods allows us to understand how ecosystems have reacted.
Analysing these changes is fundamental, since - for example - the sharp drop in oxygen levels in the oceans is associated with mass extinction events and changes in biogeochemical cycles. The formation of hypoxic and anoxic zones, i.e. areas with low oxygen levels, can seriously jeopardise marine life.
‘To understand the context of these changes over time, it's important to consider the geological scale. In a nutshell, the history of the Earth is divided into Eons, Eras, Periods and Epochs. This study focuses on the Cenozoic Era, the era in which we currently live and which began 66 million years ago. The aim is to better understand the changes in the oceans and their implications for the future.’ - explains Giulia Molina.
This work is the result of the collaboration of several international experts in the PO2: Past Ocean Oxygenation working group of the PAGES (Past Global Changes) programme. Initially, the aim was to systematise a list of oxygen proxies, culminating in this publication. The result is a study that is highly technical and complex to convey to the general public. However, the second phase of this group's work, which is currently underway, aims to create an integrated, global record of the history of marine oxygen under different climatic conditions. This data will be key to improving future forecasting models and predicting impacts on ecosystems, allowing for the development of more effective solutions.
To reconstruct past ocean conditions, scientists use proxies, which are indirect indicators of these conditions. As it is not possible to take direct measurements of environmental conditions in periods prior to human presence, these proxies provide valuable information on palaeo-oxygenation - the study of marine oxygen concentrations throughout geological history. These indicators can be geochemical, such as carbon isotopes, or biological, such as fossils of microorganisms.
‘This review on palaeo-oxygenation proxies aims to provide an overview of the use of these indicators, presenting detailed descriptions, an analysis of the current state of knowledge, their advantages, limitations and applicability in reconstructing ocean oxygenation levels over geological time.’ - says Lélia Matos.
Based on these proxies, we can investigate how oxygen concentration has varied with natural changes and extreme events in the past, allowing scientists to make more accurate projections about the challenges facing the oceans today. This makes it easier to develop strategies for the conservation of marine ecosystems. By learning from the past, we can act in the present to ensure a sustainable future for marine life and the global climate balance.
Some of the main findings included in this study are:
- The use of carbon isotope gradients in epifaunal and infaunal foraminifera has advanced to provide more quantitative estimates of oxygen concentration in bottom water, a big step since the original proposal of this method in the 1980s.
- Some uncertainties of the proxies are still not well quantified. Deposition environments subject to processes such as sediment denitrification and sulphate reduction may not be suitable for certain proxies.
- The rapid evolution of proxies based on trace elements, such as the use of elemental ratios I/Ca, Mn/Ca and U/Ca in foraminifera, opens up new opportunities for palaeo-oxygen reconstructions, although they require more rigorous calibration.
- The development of ‘non-traditional’ stable metal isotope systems has opened up new possibilities for more quantitative and globally integrated redox reconstructions. Advances in lipid biomarkers and genomic techniques have also increased the ability to identify changes in the carbon, nitrogen and sulphur cycles in the oceans.
Although palaeo-oxygen proxies have evolved significantly, there are still challenges, especially in the precise quantification and standardisation of reconstruction methods. Continued research in these fields will help improve understanding of past changes in ocean oxygenation and their impact on marine ecosystems.
For more information on these oxygenation indicators in palaeoceanographic reconstructions, we recommend reading the article: Review of proxies for low-oxygen palaeoceanographic reconstructions
