The study of past climates (paleoclimatology) offers an opportunity to improve the predictability of future climate change. Examples from the past provide information on climatic variability and abrupt events on timescales that exceed the relatively short period of instrumental climate data. We are able to reconstruct past climates by analysing “natural archives”, including trees, ice-cores, lake sediments, corals, and stalagmites. These archives are any natural material that has been preserved through time, with distinct growth or depositional layers which can be analysed and traced back through time. Natural archives can be biotic or abiotic in nature – meaning they come from formerly living organisms (e.g. corals, trees) or are non-living geological components (e.g. ice cores, lake sediments, speleothems), respectively.
The natural archives can be analysed in terms of visible properties or chemical characteristics. For example, the layers from core of lake sediment can be analysed in terms of the sediment composition, particle textural variability, structure (e.g. water flow, seismic deposits), colour (e.g. rust due to anoxic conditions), and magnetic properties among many other variables. Additionally, layers of ancient snow in ice cores can be analysed for annual accumulation, seasonal distribution and air bubble composition. On the other hand, the chemical characteristics of different geological deposits or growth layers can provide a wealth of information, including the reconstruction of temperature, precipitation, atmospheric circulation and composition, natural disasters and seawater composition through time. These chemical characteristics are measured based on stable isotopes, including those of boron (δ11B), oxygen (δ18O) and carbon (δ13C).
These visible and chemical characteristics in natural archives vary in time due to changes in environmental and climate conditions in distinct and predictable ways, including the relationship between photosynthetic organisms and temperature. One can use the understanding of the relationships between archives and their surrounding environment to reconstruct variability in both elements in the past – the use of such relationships allow one to develop climate proxy records. For example, the width of growth in a tree-ring is a proxy for temperature in climates when temperature is the limiting factor for tree growth.
The stable isotopes of boron (δ11B) and oxygen (δ18O) are two important variables used to reconstruct climate in a variety of natural archives. A single element can have slightly different species or “isotopes” due to differences in the atomic weight of the nucleus. Those isotopes with a heavier atomic weight (e.g. 18O is heavier than 16O) are larger and thus cycle through the environment in a different and predictable way compared to their lighter counterparts. For example, when clouds rain out across a landscape from a shoreline into a continent, the heavier oxygen (18O) in water tends to rain out more readily than lighter oxygen (16O), creating a relationship between rainout history and isotopic species. Rainfall events along a coastline would likely have a stronger 18O signature compared to rainfall events occurring far inland, known as the Continentality Effect. This preferential movement of one isotope species over another in natural circumstances is known as isotopic fractionation. This fractionation forms the basis for stable isotope climate proxy relationships.
Oxygen isotopes (δ18O) are present in precipitation in the form of H2O, and can be used to trace the origin/source, amount and type of precipitation; all of which vary depending on geographic location and dynamics in the climate system. For example, tree-ring stable oxygen isotopes have been utilised to reconstruct atmospheric reorganization and storm activity during a major climate event at the end of the last glaciation (Pauly et al. 2018). Furthermore, these isotopes are commonly measured in ice cores to reconstruct temperature in Greenland, allowing one to track hemispheric changes in temperature correlated to glacial and interglacial phases at annual resolution (Steffensen et al. 2008) and spanning multiple millenia (Rasmussen et al. 2006). Speleothems have also been extensively studied using δ18O, including a global study finding the strong correlation between δ18O and meteoric precipitation in cool climates (Baker et al. 2019).
Boron isotopes (δ11B) are an important variable in the reconstruction of past ocean conditions due to the correlation between fractionation of δ11B, oceanic pH and CO2 (Henehan and Jurikova, 2019), which is not greatly impacted by diagenetic effects (Edgar et al. 2015). This relationship is particularly important in reconstructing the trends in ocean acidification in both recent time (McCulloch et al. 2012) due to anthropogenic climate change (Caldeira and Wickett 2003) as well as in deep time (Müller et al. 2020). For example, Müller et al. (2020) reconstructed ocean acidification from the Toarcian Oceanic Anoxic Event (~183 million years ago) demonstrating its contribution to a significant marine extinction event.
In addition to the analysis of stable isotopes, it is very important to develop a robust chronology so the layers of growth or deposition can be accurately dated relatively (e.g. within a single core) as well as absolutely on calendar time. As such, additional chemical analyses (e.g. 14C, U-Th) must be undertaken to complement the proxy-climate reconstructions. Read more about Carbon-14 and U-Th dating methods.
Stable isotopes can also provide clues into the migration patterns and paleodiets of past populations for archaeology studies which can be traced through time and correlated with evidence of climate change.
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