How Can We Use Oxygen Isotopes from Ostracods to Reconstruct Abrupt Climate Changes from the Holocene?

by Joanna Tindall

Why are we interested in past abrupt climate changes?

The earth’s climate has always fluctuated. However, with the current anthropogenic climate crisis, the importance of understanding climatic change has never been greater. You may be familiar with big climatic changes in Earth’s history such as the so-called ‘ice ages’ or extended periods of long cold conditions. Our current geological period is the Quaternary which covers the past 2.6 million years. During the Quaternary, climate has switched between extended periods of cool, glacial conditions and warm interglacial conditions. We currently live in one of these interglacial periods, the Holocene, which spans the last ∼11,700 years. We generally consider the Holocene climate to be relatively stable, but our records show that abrupt changes have occurred. Abrupt climatic changes have a sudden onset and are short-lived, typically lasting only a few hundred years. However, they are large enough to be detected in palaeoclimate records and may have been significant enough to affect humans by changing the landscape. One example of a Holocene abrupt climate event is the Little Ice Age (∼16th to 19th century). This was a regional cooling event to parts of Europe and is captured in works of art (Figure 1).

Figure 1: 'A Frost Fair on the Thames at Temple Stairs 1684' by Abraham Hondius (1631-1691). Image made available to Wikicommons from the Museum of London (accessed 28th September 2020). Image description: The Thames river is frozen, several people and horses and one horse-drawn carriage are moving across the river where lots of market stalls have been set up.  

Although we know that a number of abrupt events have occurred during the Holocene, scientists are still trying to understand their causes and how they are expressed across the world. This is important for our understanding of how the climate system works. Human activity is both dramatically warming the climate and changing its stability. These changes will impact how society operates and there will be a need to adapt to wide-ranging consequences, including for food and water security. Researching and understanding palaeoclimate and how abrupt shifts affected societies in the past is valuable for contemplating how the changing climate might impact us both now and in the future.

There are lots of ways to investigate past climate change. In this post I share the method I use in my research: oxygen isotope ratios in ostracod shells.

Figure 2: Left: Living ostracods in a puddle. S. Rae from Scotland, UK / CC BY (https://creativecommons.org/licenses/by/2.0) Right: Candonid. Example of a fossil ostracod, these are typically up to a couple of millimetres along their longest edge once they reach adulthood, dependent on the species; Photo by Joanna Tindall, 2020.

What are ostracods and what can they tell us about palaeoclimate?

Ostracods are micro-crustaceans with calcium carbonate (CaCO₃) shells (Figure 2) and have been on the planet for millions of years. Today, you can find them in all types of water, fresh or saline, across the world. Here, I refer to the processes affecting freshwater ostracods, the species that I work on. Some freshwater ostracods live in or on the bottom of lakes and can swim, whilst others are found associated with plants. Depending on the species, their life cycle can last a few days to weeks, moulting their shells eight times as they grow from juvenile to adult [1]. Each new shell is influenced by the water chemistry and temperature at the time the shell is formed, providing a valuable climate signal — a record which is used to answer a broad range of scientific questions. I use the oxygen isotope ratio from the ostracod shells to reconstruct the climate over time. 

What are oxygen isotopes and how do we use them to reconstruct climate?

Isotopes are different forms of the same element. Inside each isotope the number of protons is the same but the number of neutrons varies. This means they behave the same way chemically but they have different atomic masses. Stable isotopes do not undergo radioactive decay. The majority of oxygen (over 99%) has an atomic mass of 16 (¹⁶O), a minority has a mass of 18 (¹⁸O), and a even smaller amount has a mass of 17 (¹⁷O). ¹⁸O is heavier than ¹⁶O. In nature, the relative amount of the heavier ¹⁸O compared to the lighter ¹⁶O is dependent on climatic conditions. For example, in temperate lakes, ostracod shells typically have more of the ¹⁸O when it is warmer. To describe the relative change in the amount of ¹⁸O we use delta notation (δ). In lake sediments (“mud”) with lots of carbonate (which includes ostracod shells), the δ¹⁸O is controlled by temperature and the oxygen isotope composition of lake water, which is in turn controlled by precipitation (Figure 3). When ostracods make their new calcium carbonate shells, they take on the isotopic signature of the water that they are in. The carbonate component (CO₃^2-) contains the oxygen isotopes which we measure and, because the ¹⁸O:¹⁶O ratio is temperature dependent, we can use the δ¹⁸O record to reconstruct past climate. 

There are some important considerations to be made when we begin interpreting our δ¹⁸O record to understand climate. These include developing a good understanding of the local conditions such as the geographical location of your lake sediment record and atmospheric circulation patterns as these may influence the δ¹⁸O record. Furthermore, temperature has several influencing factors for example the season the ostracod forms its shells in (e.g. winter or summer). A summer signal is more likely to be impacted by evaporation; water depth is important because in deep lakes temperature may vary with depth, therefore the habitat preferences of the ostracod species chosen may influence the temperature reconstruction — a deep water species may predict temperatures cooler than those actually experienced at the surface. A good understanding of the controlling conditions and influencing factors allows a better reconstruction of past climate. 

Figure 3: Controlling factors of oxygen isotopes composition of ostracods in lakes. The δ¹⁸O_ostracod is controlled by temperature, biological processes and δ¹⁸O_{lake_water}, these controls allow us to make temperature and δ¹⁸O_{precipitation} reconstructions when we account for the various controlling factors. The δ¹⁸O of the lake water is affected by catchment and in-lake processes. Temperature and δ¹⁸O_{precipitation} are influenced by different factors. The more of these factors we have data for, the more reliable our reconstruction. In contrast, δ¹⁸O_{precipitation} is influenced only by mean annual temperature. Mean annual temperature may affect how much evaporation there is and the amount of rainfall. Diagram by Joanna Tindall, adapted from Leng and Marshall [2].

So… how can ostracods tell us about past abrupt climate change in the Holocene?

The nature of abrupt climatic events means they are sudden and short-lived, so you need a proxy that will capture these quick and often brief changes. Ostracods have short life spans; therefore, their isotopic record will respond quickly to changes in the water chemistry and temperature of the lake. This has great potential for high-resolution research into past climate change. In the last 11,700 years there have been multiple abrupt climatic events, which ostracods may well have captured in the chemistry of their shells. Preliminary findings from my PhD work from Orkney (Northern Scotland) suggests that detailed reconstructions are possible and that between ∼10,200 and ∼9,000 years ago not only is an abrupt climatic event recorded, but the internal structure of this event is expressed in the oxygen isotope record [3]. Detailed records like these tell us a lot about past climate change, particularly climate stability, and this knowledge can be utilised to better understand the possible expression and implications of present-day global warming.

Joanna Tindall,
PhD Researcher, University College London and Royal Holloway, University of London, UK

If you have questions or comments concerning Joanna's post, please leave a comment below, or send her an email. You can also connect with her on ResearchGate, LinkedIn or Twitter.

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References:

[1] Holmes, J.A. and Chivas, A.R., 2002. Geophysical Monongraph 131: The Ostracoda: Applications in Quaternary Research. American Geophysical Union.

[2] Leng, M.J. and Marshall, J.D. (2004) 'Palaeoclimate interpretation of stable isotope data from lake sediment archives', Quaternary Science Reviews, 23, pp.811–831.

[3] Tindall, J., Holmes, J., Candy, I., Leng, M., Timms, R., Francis, C., Petts, D., Blockley, S., Matthews, I. and Palmer, A. (2020) ‘A high-resolution ostracod-derived δ18O record of early Holocene abrupt climatic change from N. Scotland’, In: EGU General Assembly May 2020. p.18990.