GEOLOGY | Climate Proxies
by Noam Vogt Vincent
Noam Vogt Vincent is Oxford Geology Group's Communications Secretary and sits on the Board of Trustees.
He read Earth Sciences at the University of Oxford and is currently studying for a DPhil, an advanced degree by research, known as a PhD at other institutions. He is a member of the The Physical Oceanography group which uses analytical theory, numerical models and ocean observations to understand the circulation of the oceans and their role in the climate system. It includes scientists in both the Earth Sciences Department and in Atmospheric, Oceanic and Planetary Physics.
What is a climate proxy?
Classical geological observations such as lithology and fossil assemblages can give geoscientists insights into the kind of environment and climate that a rock was deposited in. However, as the boundaries between geoscience and climate science have blurred, it has become increasingly important to be able to quantify these climatic parameters and how they vary in time and space. This has given rise to the use of climate proxies, one of the most powerful and widely used tools in palaeoclimatology.
A climate proxy is a measurable signal that responds in some known and predictable way (calibration) to a climate variable, and is naturally recorded and stored for a long time in some medium. To understand what this means, we’ll use a very famous example of a climate proxy – tree rings (also known as dendroclimatology). Due to the seasonal cycle in tree growth, a ring is added to the outside of the tree trunk for every year that a tree is alive. When the climatic conditions are favourable for growth, the tree grows faster and the ring for that year is thicker. As a result, if we look at the cross section of a tree and measure how the tree ring thickness changes the closer you get to the outside (the point at which the tree stopped growing), you can see how climatic conditions around the tree changed through time. To relate this back to our definition, the climate variable is how ‘tree-friendly’ the climate was, the measurable signal was tree ring thickness, a signal which is naturally recorded (trees naturally generate tree rings) and stored for a long time (trees can become thousands of years old and can be fossilised). ‘Tree-friendly’ may sound like a wishy-washy climate variable (and it is) but this is mainly a function of temperature and precipitation and by comparing tree rings to observed climate records of the past few hundred years, scientists can generate known and predictable relationships between the signal (tree ring thickness) and the climate variables this depends on (primarily temperature and precipitation).
Tree rings. The youngest rings are at the centre and each ring further out represents another year of the tree’s life.
Tree rings are a very useful proxy for the past few hundreds to thousands of years but few trees live longer than this and although dendroclimatology can be carried out on fossilised trees, these fossils are uncommon and it is challenging to calibrate records based on extinct trees. Fortunately, geoscientists have established countless proxies based on longer-term geological archives, including: the analyses of gases trapped in ancient ice, lake varves, pollens, foraminifera and speleothems.
One of the most widely used and influential geological climate proxies is based on oxygen isotopes stored in deep-marine sediments. Oxygen exists as several isotopes, which are atoms with the same number of protons (and therefore chemical behaviour) but a different number of neutrons (and therefore a different mass). Two such isotopes are O-16 and O-18. Although O-16 and O-18 are chemically identical, their different masses mean that their physical behaviour is slightly different. In particular, because O-16 is lighter than O-18, it evaporates slightly more easily than O-18. To understand why this is important, let’s imagine an ocean that starts off with 50% O-16 and 50% O-18. If we start evaporating water from the sea surface, slightly more 16O will leave the ocean than 18O, so the water vapour forming from the ocean will have >50% O-16 and the water left behind in the ocean will have <50% 16O. If this water that we’ve just evaporated later falls as rain and returns to the ocean, the ocean will return to 50% O-16 and 50% O-18. However, what if this rain falls as snow over an ice sheet such as Antarctica? In this case, that water is then trapped (as ice) and the ocean permanently remains with <50% O-16 (or at least for a very long time).
In other words, the more water is trapped as ice in ice sheets, the less O-16 the ocean will have relative to O-18. We could therefore use the ratio of O-16 to O-18 in seawater (expressed using a notation known as δ-O-18, effectively a measure of how much O-18 there is compared to O-16) as a proxy for the mass of ice stored in Earth’s ice sheets, which would be very useful for understanding how Earth’s ice sheets have changed in size over time in response to other climate events. But is δ-O-18 a good climate proxy? Comparing it to our definition, we have a measurable signal (we can measure the δ-O-18 seawater) which responds in a known and predictable way (it can be calculated theoretically or measured empirically) to a climate variable (the mass of the Earth’s ice sheets)… but is it naturally recorded and stored for a long time? Sort of. There is a kind of marine algae called foraminifera that build shells of calcite. Calcite (CaCO^3) contains oxygen which is obtained from seawater and, as a result, the δ-O-18 of foraminifera shells reflects the δ-O-18 of the seawater when the foraminifera was alive. When it dies, it falls to the sea floor and the organism itself decays, but the calcite shell remains and forms sediment. Geoscientists can then obtain this sediment using drills (similar to those used by the oil industry in deep-sea drilling) and can measure how the δ-O-18 of the sediment changes with depth (and therefore time) to find out how the size of Earth’s ice sheets have changed through time.
Birch Pollen Grains
The famous LR04 Benthic Stack showing how the oxygen isotopes in deep-marine sediments (from foraminifera, specifically foraminifera living at the sea floor) has varied over the past 5 million years. In fact, this record does not only measure global ice volume but is also sensitive to temperature, but these two variables affect the record in a similar way. Higher values of δ-O-18 (lower on the y-axis) represent greater ice volume and/or lower temperature. This record shows the incredible dynamic variability of Earth’s climate over the past few million years.