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How does the Earth stop global warming? Using metal isotopes to understand climate recovery processes

Periodic Reporting for period 1 - EnvironMetal (How does the Earth stop global warming? Using metal isotopes to understand climate recovery processes)

Reporting period: 2018-03-01 to 2020-02-29

Earth’s history is punctuated by climate disturbances, often marked by abrupt changes in the carbon cycle, and by mass-extinctions. Episodes of rapid, large-scale carbon release are linked to global warming events that last for 100,000s years. Eventually the climate recovers from these perturbations via a number of negative feedback processes that act to slowly remove carbon from the atmosphere. The aim of this project was to use metal isotope proxies to reconstruct some of these important feedback processes and compare their operation between different ancient warming events. Of particular interest is the role of silicate weathering, heightened primary productivity and oceanic anoxia in burying carbon. By studying these processes we are examining the long-term fate of atmospheric carbon dioxide. Whilst these mechanisms are important for climate recovery, anoxia (little or no dissolved oxygen) also represents a direct driver of mass extinction events. The expansion of anoxic waters today are already having impacts on marine life and studying past events helps us understand how rapidly and to what extent ocean de-oxygenation can occur.

The primary focus of the project to date has been on reconstructing the global extent of anoxia for the Paleocene Eocene Thermal Maximum (PETM), which is a warming event that occurred around 54 Myrs ago. The PETM is characterized by a large and rapid carbon emission, probably from massive volcanic activity, which resulted in ~5 degrees C warming, ocean acidification and ocean de-oxygenation. With these characteristics, the PETM is often compared to modern climate change. Whilst much work has focused on the local development of anoxia for the PETM there are still questions with regard to the global scale extent of anoxia.

By measuring uranium isotopes in carbonate sediments from the PETM interval it is possible to reconstruct the global extent of seafloor anoxia. This approach has been used for a number of mass extinctions and warming events in Earth history, allowing a direct comparison of the magnitudes of change for different warming scenarios. In combination with a biogeochemical model, we are able to quantify the maximum amount of seafloor anoxia during the PETM. Ongoing work will integrate these results with estimates of weathering changes and primary productivity in order to better quantify the Earth system response during the PETM, and compare it to other types of warming events.
The project has:

1) Undertaken extensive method development studies to improve laboratory protocols for measuring metal isotopes in carbonate sediments. This work demonstrates the importance of sample treatment and digestion protocols in extracting carbonate bound metals from bulk sediment samples, which have implications for paleo-reconstructions. Test results have been published for uranium and molybdenum isotopes, and are in preperation for othe metal isotope systems. These data and interpretations will be of significant benefit to the isotope geochemistry field as an increasingly diverse suite of proxies are used.

2) produced metal isotope records for the PETM hyperthermal event as a proxy for the global extent of oceanic anoxia. These measurements provide an important target for a biogeochemical model, which together are used to estimate upper limits on the scale of environmental change during the PETM. The publication of this work is still ongoing.

3) advanced a biogeochemical model approach to integrate metal isotope systems as tracers of past environmental change, thereby providing a tool to better understand geochemical records.

The results have been presented to scientific audiences with one published paper and one submitted, and also as oral or poster presentations at international scientific conferences.
Method development work represents a significant advancement for the state of the art, allowing multiple metal isotope proxies to be measured simultaneously on a single sample. These methods will of be of use to a large scientific community interested in various proxies in carbonate sediments, and help improve the quality of geochemical dataset interpretation.
T
he estimates of global anoxia for the PETM are part of an increasingly detailed quantification of past global warming events. These ancient events act as targets for models in order to improve predictions for modern climate change. The more constraints we have for past events like the PETM, the better our understanding of the climate system response to current and future global warming.
Taking core samples for the PETM at the Bremen IODP core repository