Understanding climate
for the benefit of society

Sevasti Eleni Modestou at one of the locations where you can observe the geological heartbeat of Mediterranean sediments – the south coast of Sicily at Punta di Miata, otherwise known as Scala dei Turchi. Here each package of white, red, white, grey equals about 20 thousand years of time. Photo: Ryan Ickert

Assigning ancient time with lead

Sediments are archives of past climates – provided you know when they were deposited. Sevasti Eleni Modestou uses lead to clock events of the past. Here she writes about her recent study. 

Written by Sevasti Eleni Modestou from the Bjerknes Centre and the Department of Earth Science at the University of Bergen

Studying past climate is very important to understanding future climate change. However, we also need to know the age of events in our archives, otherwise when events happened and rates of change in the environment remain unknown. To assign time, researchers generate age models. The Geologic Time Scale (GTS) is a key tool for Earth Scientists to help build age models for climate archives.

For a large part of more recent geologic time (from about 2 to 14 million years ago), the GTS is pinned to sediments that were deposited in the Mediterranean. This is because those sediments were deposited with a very regular ‘heartbeat’ of short duration, which not only allows age models to be accurate but also precise. Of course, this short duration and high precision is on geological time scales, and still needs improvement in order to better apply paleoceanographic information to some of the important climate questions of today.
 
While researching the behavior of isotopes of the trace metal lead (Pb) in Mediterranean sediments, we found that there are other ways to ‘see’ this geological heartbeat. We also found that this new heartbeat was beating at a different time in the already known cycles.

Combined with what is already known, the Pb isotope proxy has potential to improve the precision of age models further, perhaps cutting uncertainties in half. It also can help in areas where the commonly used sapropel layers are either poorly expressed or missing altogether.

This improvement can help us bridge the gap between the tools we have and the tools we need in order to understand past change on timescales relevant to answering modern climate questions.

Pb timeseries
Both the sedimentary layers and the Pb (lead) isotope time series show cyclicity. Sedimentologists often use the (usually) visually apparent sapropelic layers to 'tune' their age models; these layers are deposited in the warmer, wetter part of an insolation cycle.  The Pb isotope values peak at a different time in the cycle, probably due to increases in dust input during the colder, drier part of the insolation cycle.  This means that the combination of information from sediment lithology and Pb isotope cyclicity could help pinpoint time more precisely within an insolation cycle. Illustration: Sevasti Eleni Modestou

Reference

Modestou, S. E., Gutjahr, M., van der Schee, M., Ellam, R. M., & Flecker, R. (2019). Precessional cyclicity of seawater Pb isotopes in the late
Miocene Mediterranean. Paleoceanography and Paleoclimatology, 34,2201-2222. https://doi.org/10.1029/2018PA003538