Welcome to my digital lab! I am Professor Proxy, and my research focuses on paleoclimatology, the study of how the climate has evolved in the past. To reconstruct paleoclimate, we rely on proxies, which are physical and chemical evidence from the geologic record that offers unique and crucial insights into past climate changes. We have a diverse range of proxies to choose from, each revealing a distinct and valuable story about the past. Meet my carefully selected team of Pokémon, chosen for their potential as paleoclimate proxies, and learn more about them below. Thank you for visiting!

Corsola: Coral Cores

When you think of coral the image that pops into your mind may look something like Corsola. While many species of coral do share the same features as Corsola, other species look completely different. When we look to create records of past climate we want to include as much time as possible. To do that we look for what are known as “massive” coral. These species of coral resemble underwater boulders and some colonies can grow higher than 10 meters! The living part of the coral colony is only a few centimeters deep, but beneath it is carbonate rock that was once the organism’s skeleton. This skeletal material captures the climate of the time it was growing in its chemical makeup.

After we find the type of coral we want to work with, we drill into it from the top to bottom. This process is completely harmless to the coral colony and we make sure to protect the coral from any future damage that may come from the coring process. We take the core back to dry land where we use a dental drill to sample the coral down its annual growth bands. Depending on the species of coral and its growth rate, each sample we take from the core represents roughly one month. These cores can extend as far back as 500 years, allowing us to reconstruct climate at near monthly resolution over several centuries!

These powder samples may be small but they pack a lot of useful climate information. In each sample scientists measure the ratio of the elements strontium (Sr) and calcium (Ca), as well as the ratio of stable isotopes of oxygen (δ¹⁸O). Both the Sr/Ca ratio and δ¹⁸O of the coral skeleton are dependent on temperature, meaning we can use fluctuations in these chemical properties to calculate changes in temperature! We can also use δ¹⁸O to understand changes in the salinity of the oceans, which is controlled by the regional balance between evaporation and precipitation.

There are other chemical tracers of past climate captured in the coral skeleton that we can use to reconstruct other climate processes. For example, we can use the ratio of barium (Ba) and Ca to understand changes in the movement of dust and sediment from land into the oceans. Increased Ba concentration in the ocean can reflect an increase in wind speeds or rainfall depending on where the coral was growing. Changes in other chemical constituents can yield information about volcanic activity, aquatic ecosystem change, and terrestrial sediment input. Finally, changes in the concentration of heavy metals like aluminum and lead can be used to track non-climatic ecosystem changes, primarily from human caused coastal pollution.

Shellder: Bivalve and Brachiopod Shells

Species that create carbonate shells (like Shellder here) are among the most iconic marine organisms. Shelled organisms can generally be split into two major categories: brachiopods and bivalves. Organisms of each group look similar but have distinct physical traits and evolutionary histories. Bivalves, like oysters, scallops and mussels, are more common and identifiable to the average person than brachiopods are. That being said, brachiopods are abundant in the fossil record and both groups can be used to reconstruct paleoclimate.

Both bivalves and brachiopods grow shells out of calcium carbonate. These organisms have varying lifespans but some can grow for over 200 years! They grow in such a way that the youngest part of the shell is towards the hinge while the oldest part of the shell is towards the edge. Scientists can collect both modern and fossil specimens from various ecosystems (rivers, shallow oceans, under ice shelves, in bedrock, etc). In the lab we use a drill to sample along the growth bands of the shells to measure shell chemistry. Changes in the chemistry of the shells allow us to understand how the climate changed while they were growing.

The chemistry of the shells of these organisms reflects the temperature and salinity of the waters that they grew in. Similar to corals, scientists use the ratio of trace elements like strontium (Sr) and magnesium (Mg), in relation to calcium (Ca), in the growth bands of these shells to reconstruct temperature. The ratio of stable isotopes of oxygen (δ¹⁸O) in the shells are also used to calculate water temperature. But, δ¹⁸O is also impacted by the salinity of the water the organism grew in. Salinity of these waters is primarily impacted by changes in evaporation and precipitation. Thus, shelled organisms can be used to reconstruct marine and riverine water temperatures as well as local precipitation variability.

Another popular tool scientists look at is the ratio of stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) in the shell. Both δ¹³C and δ¹⁵N can be used to understand how nutrient availability has changed in the past. Fluctuation in nutrient availability may be related to changes in temperature, precipitation, or even ice shelf extent. Scientists can also use δ¹⁵N to track the impacts and sources of pollution into rivers and the ocean. Other aspects of shell chemistry can be used to reconstruct how the climate around the organism changed through time.

Avaluug: Ice Cores

Ice cores are a well known paleoclimate proxy that inform us about changes in temperature, humidity, atmospheric chemistry, and more over hundreds of thousands of years. Ice cores can be drilled anywhere that permanent ice is found (eg. glaciers, ice sheets, and ice caps). These form as layers of snow accumulate on top of each other. Over time, the weight and pressure of overbearing snow causes deeper layers to transform into ice. These layers of ice remain there unless they melt; in many cases the ice is over 100,000 years old!

The most famous ice cores tend to come from Greenland and Antarctica, creating records up to 800,000 years! But, many cores are also extracted from places outside of the poles. For example, glaciers on the tops of mountains in the Tropical Peruvian Andes, European Alps, the Himalayas, New Zealand, and Mount Kilimanjaro have been used to understand regional changes in temperature and precipitation.

The chemistry of each layer of ice can tell scientists about changes in regional temperature and humidity. The most common proxies used for this are the ratios of stable isotopes of oxygen (δ¹⁸O) and hydrogen (δD). The way in which δ¹⁸O is related to air temperature is complex, but in short, the δ¹⁸O value of ice is expected to be higher when it is colder. Scientists use this relationship to calculate the temperature the air must have been to achieve the δ¹⁸O value recorded in the ice. Changes in δD can be related to relative humidity in a similar fashion. When there is a greater rate of evaporation from the oceans there is typically a higher δD value in atmospheric moisture. Thus, when it is less humid, and there is a greater degree of evaporation from the ocean, the δD value of precipitation and ice should be higher than when it is more humid.

Bubbles of gas are also often trapped within layers, providing a means of measuring the atmospheric concentration of the past. The fact the ice cores capture atmospheric chemistry this way was pivotal in understanding the modern climate system. We see in ice core records that the atmospheric concentration of CO2 has remained within a value of 280 and 180 ppm for the last 500,000 years. With increases in CO2 we also observe increases in temperature, likewise when CO2 is lower we observe cooler air temperatures. Currently, due to human combustion of fossil fuels, the atmospheric concentration of CO2 is nearly 420 ppm. This increase in CO2 is unprecedented in the last 500,000 years and has led to a corresponding rise in global temperatures, a relationship observed by scientists over the last 150 years.

There are a number of other climate processes that can be reconstructed using ice cores. Particulates are often trapped in the ice and can be used to understand past changes in atmospheric dust concentration, aridity, and more. Scientists also use changes in the concentration of trace elements like sodium (Na) to track fluctuations in regional storminess.

Crustle: Lake and Ocean Sediment Cores

Sediment that settles at the bottom of lakes and oceans record a number of climatic processes. Each core is unique and the information that scientists can extract from a core depends on where the core was extracted, the type of sediment that is cored, and the chemistry of the water above the sediment.

Ocean sediment cores allow scientists to reconstruct ocean chemistry, temperature, salinity, and biologic activity on long timescales. What makes ocean cores so powerful is the presence of microscopic photosynthetic organisms known as foraminifera, or forams for short. Forams create a carbonate test (similar to a shell) when they are alive. When these organisms die the tests sink to the bottom of the ocean and end up as a part of the sediment. The ratio of stable isotopes of oxygen (δ¹⁸O) in these tests is dependent on the temperature of the water they grew in, allowing scientists to reconstruct changes in ocean temperature through time. The ratio of stable isotopes of carbon (δ¹³C) in these tests to understand changes in the carbon cycle and biologic activity in the oceans. Other factors, like the color and size of sediment grains in the core can yield information regarding biologic activity and ocean chemistry.

Lake sediment cores are some of the most versatile paleoclimate proxies. Past temperature can be calculated from lake cores using a number of techniques. For example, scientists can calculate the temperature of the lake water using δ¹⁸O values from carbonate that forms in the lake and/or shells of organisms that lived in the lake. Air temperature can be calculated in some lake cores by comparing the assemblage of pollen present in the sediment and relating that to the temperature bounds in which those plants live. Scientists also reconstruct the level of the lake using physical and chemical aspects of the lake sediment. Reconstructing lake level provides insight into changes in precipitation and evaporation dynamics. Another common technique is to measure the amount of organic material in different layers of the sediment to understand how changes in the climate impacted the chemistry and ecology of the lake. Changes in erosion and human caused pollution are also commonly reconstructed in lakes using changes in the concentration of heavy metals through the core. Finally, terrestrial processes like wildfires, flooding, and ecosystem change can also be recorded in the chemistry and physical characteristics of the lake sediment.

Regirock: Speleothems

Caves form through the gradual dissolution of limestone or other soluble rocks by acidic groundwater. Speleothems, such as stalactites and stalagmites, form inside caves when mineral-rich water drips into a cave where calcium carbonate (CaCO3) from the water precipitates into a solid. Speleothems that grow on the floor of a cave are referred to as stalagmites, while speleothems that grow on the ceiling of a cave are referred to as stalactites. The chemistry of the carbonate in speleothems, mainly stalagmites, can be used to reconstruct changes in precipitation, biologic activity, and relative temperature among other things.

Speleothems grow at different rates and for different periods of time. The rate and length of speleothem growth is mostly dependent on the conditions of the cave and how much water is percolating underground. Collectively, speleothems can record climate variability on multi-decadal to hundred thousand year timescales over the last 500,000 years.

Speleothems are collected from caves and taken back to the lab where they are cut vertically down their growth axis. Scientists then drill into the speleothem at different points to measure the ratio of uranium (U) and thorium (Th) to determine how old each portion of the speleothem is. This works because the water that the carbonate precipitates from is rich in U but typically poor in Th. Thus, when the carbonate forms there should be X amount of U and zero Th. U is radioactive and decays into Th over time, meaning that the concentration of Th in a given sample is a direct product of the amount of time it has taken the U to decay into Th. Scientists can calculate the amount of time this process took using measurements of the concentration of both U and Th. The age of the speleothem is calculated in spots along its growth axis and then stitched together to create an age model.

After the age model is constructed, scientists then drill smaller samples along the growth axis to measure changes in the ratio of stable oxygen (δ¹⁸O) and carbon (δ¹³C) isotopes. Fluctuations in δ¹⁸O can reflect changes in precipitation amount or source, while δ¹³C primarily reflects changes in biologic activity and vegetation cover above the cave. A more detailed review of how these stable isotope proxies work can be found here. Comparison of both δ¹⁸O and δ¹³C can inform scientists about how changes in precipitation led to changes in the type and extent of vegetation in the past.

Other chemical tracers in speleothems provide insight into past climate fluctuation. The ratio of magnesium (Mg) and calcium (Ca), as well as the ratio of strontium (Sr) to Ca, can reflect changes in precipitation and input of soil material. Likewise, changes in speleothem magnetism have been shown to reflect input of soil material due to changes in precipitation. Speleothems have also been used to reconstruct past wildfires, floods, and human activity in caves.

Trevenant: Tree Rings

Tree rings are some of the most widely used paleoclimate proxies to date because they are widespread and easy to collect. When selecting a location, scientists have to think about what questions they want to try to answer with the record they are constructing. If they are looking to answer questions about changes in precipitation then they will likely choose trees in a location where the growing season is defined by rainfall. Alternatively, if they are trying to reconstruct temperature then they will likely pick an area where seasonal growth is limited by temperature.

Once a location is chosen then scientists will survey individual trees to make sure they haven’t been impacted by other processes (eg. wildfire, storms, wind damage, etc). In some cases, these trees may be selected to answer specific questions about these processes, but it is more common for these trees to be excluded from sampling to eliminate errors and bias. After the trees are selected, scientists then use a handheld drill (mechanical or motor driven depending on the size of the tree) and extract a thin core from the outside towards the center. Scientists use multiple trees in an area to reduce the amount of noise and error in their data.

Once the samples are obtained the important process of cross dating the samples is performed. Scientists do this by counting the rings in the cores they extrated from a location to make a shared chronology for all of the cores. In the lab scientists carefully note changes in the physical characteristics (width, color, density) of the tree rings, which relate to changes in growth that are controlled by temperature and precipitation. Changes in the ratio of stable oxygen (δ¹⁸O), hydrogen (δD), and carbon (δ¹³C) isotopes can also provide a chemical record of changes in temperature and precipitation.

Other aspects of three ring morphology and chemistry can be used to reconstruct changes in climate. Scientists have used the ratio of stable isotopes of nitrogen (δ¹⁵N) to understand changes in soil health and chemistry. Concentrations of trace elements like iron, magnesium, and nickel have been used to track human pollution at the onset of the industrial revolution. Radioactive isotopes of carbon (¹⁴C) have been used to reconstruct solar activity. Wildfire history can be reconstructed by determining the age of fire scars present in the sample. Likewise, landscape changes like landslides and slope creep can be understood by looking at how the width of growth rings changes around the circumference of the tree. The versatility of tree rings and their widespread nature makes them powerful paleoclimate archives.

Sources and Further Reading

Coral

[1] Brenner, L.D., Linsley, B.K. and Dunbar, R.B., 2017. Examining the utility of coral Ba/Ca as a proxy for river discharge and hydroclimate variability at Coiba Island, Gulf of Chirquí, Panamá. Marine pollution bulletin, 118(1-2), pp.48-56.

[2] Bryan, S.P., Hughen, K.A., Karnauskas, K.B. and Farrar, J.T., 2019. Two hundred fifty years of reconstructed South Asian summer monsoon intensity and decadal‐scale variability. Geophysical Research Letters, 46(7), pp.3927-3935.

[3] Corrège, T., 2006. Sea surface temperature and salinity reconstruction from coral geochemical tracers. Palaeogeography, Palaeoclimatology, Palaeoecology, 232(2-4), pp.408-428.

[4] Correge, T., 2006. Monitoring of terrestrial input by massive corals. Journal of Geochemical Exploration, 88(1-3), pp.380-383.

[5] Scott, P.J.B., 1990. Chronic pollution recorded in coral skeletons in Hong Kong. Journal of Experimental Marine Biology and Ecology, 139(1-2), pp.51-64.

[6] Thompson, Diane M. “Environmental records from coral skeletons: A decade of novel insights and innovation.” Wiley Interdisciplinary Reviews: Climate Change 13, no. 1 (2022): e745.

Shells

[1] Elliot, M., Welsh, K., Chilcott, C., McCulloch, M., Chappell, J. and Ayling, B., 2009. Profiles of trace elements and stable isotopes derived from giant long-lived Tridacna gigas bivalves: Potential applications in paleoclimate studies. Palaeogeography, palaeoclimatology, palaeoecology, 280(1-2), pp.132-142.

[2] Gillikin, D.P., Lorrain, A., Jolivet, A., Kelemen, Z., Chauvaud, L. and Bouillon, S., 2017. High-resolution nitrogen stable isotope sclerochronology of bivalve shell carbonate-bound organics. Geochimica et Cosmochimica Acta, 200, pp.55-66.

[3] Heilmayer, O., Brey, T., Chiantore, M., Cattaneo-Vietti, R. and Arntz, W.E., 2003. Age and productivity of the Antarctic scallop, Adamussium colbecki, in Terra Nova Bay (Ross Sea, Antarctica). Journal of experimental marine biology and ecology, 288(2), pp.239-256.

[4] Mii, H.S., Shi, G.R., Cheng, C.J. and Chen, Y.Y., 2012. Permian Gondwanaland paleoenvironment inferred from carbon and oxygen isotope records of brachiopod fossils from Sydney Basin, southeast Australia. Chemical Geology, 291, pp.87-103.

[5] Yan, H., Chen, J. and Xiao, J., 2014. A review on bivalve shell, a tool for reconstruction of paleo-climate and paleo-environment. Chinese Journal of Geochemistry, 33, pp.310-315.

Ice Cores

[1] Alley, R.B., 2000. Ice-core evidence of abrupt climate changes. Proceedings of the National Academy of Sciences, 97(4), pp.1331-1334.

[2] Alley, R.B., Andrews, J.T., Brigham-Grette, J.T.A.J., Clarke, G.K.C., Cuffey, K.M., Fitzpatrick, J.J., Funder, S., Marshall, S.J., Miller, G.H., Mitrovica, J.X. and Muhs, D.R., 2010. History of the Greenland Ice Sheet: paleoclimatic insights. Quaternary Science Reviews, 29(15-16), pp.1728-1756.

[3] Jouzel, J. and Masson‐Delmotte, V., 2010. Paleoclimates: what do we learn from deep ice cores?. Wiley Interdisciplinary Reviews: Climate Change, 1(5), pp.654-669.

[4] Seltzer, G.O., 1990. Recent glacial history and paleoclimate of the Peruvian-Bolivian Andes. Quaternary Science Reviews, 9(2-3), pp.137-152.

[5] Thompson, L.G., 2000. Ice core evidence for climate change in the Tropics: implications for our future. Quaternary Science Reviews, 19(1-5), pp.19-35.

[6] Vimeux, F., Ginot, P., Schwikowski, M., Vuille, M., Hoffmann, G., Thompson, L.G. and Schotterer, U., 2009. Climate variability during the last 1000 years inferred from Andean ice cores: A review of methodology and recent results. Palaeogeography, Palaeoclimatology, Palaeoecology, 281(3-4), pp.229-241.

Sediment Cores

[1]Binford, M.W., Deevey, E.S. and Crisman, T.L., 1983. Paleolimnology: an historical perspective on lacustrine ecosystems. Annual Review of Ecology and Systematics, 14(1), pp.255-286.

[2] Birks, H.H. and Birks, H.J.B., 2006. Multi-proxy studies in palaeolimnology. Vegetation history and Archaeobotany, 15, pp.235-251.

[3] Lisiecki, L.E. and Raymo, M.E., 2005. A Pliocene‐Pleistocene stack of 57 globally distributed benthic δ18O records. Paleoceanography, 20(1).

[4] Lyle, M.W., Prahl, F.G. and Sparrow, M.A., 1992. Upwelling and productivity changes inferred from a temperature record in the central equatorial Pacific. Nature, 355(6363), pp.812-815.

[5] Park, J., Kim, M., Lim, H.S. and Choi, J., 2013. Pollen and sediment evidence for late-Holocene human impact at the Seonam-dong archeological site, Gwangju, Korea. Review of Palaeobotany and Palynology, 193, pp.110-118.

[6] Reeves, C.C., 2014. Introduction to paleolimnology. Elsevier.

[7] Saulnier-Talbot, É., 2016. Paleolimnology as a tool to achieve environmental sustainability in the Anthropocene: An overview. Geosciences, 6(2), p.26.

[8] Shakun, J.D., Lea, D.W., Lisiecki, L.E. and Raymo, M.E., 2015. An 800-kyr record of global surface ocean δ18O and implications for ice volume-temperature coupling. Earth and Planetary Science Letters, 426, pp.58-68.

[9] Siani, G., Michel, E., De Pol-Holz, R., DeVries, T., Lamy, F., Carel, M., Isguder, G., Dewilde, F. and Lourantou, A., 2013. Carbon isotope records reveal precise timing of enhanced Southern Ocean upwelling during the last deglaciation. Nature communications, 4(1), p.2758.

[10] Vinayachandran, P.N.M., Masumoto, Y., Roberts, M.J., Huggett, J.A., Halo, I., Chatterjee, A., Amol, P., Gupta, G.V., Singh, A., Mukherjee, A. and Prakash, S., 2021. Reviews and syntheses: Physical and biogeochemical processes associated with upwelling in the Indian Ocean. Biogeosciences, 18(22), pp.5967-6029.

Speleothems

[1] Baker, A., Smith, C., Jex, C., Fairchild, I., Genty, D. and Fuller, L., 2008. Annually laminated speleothems: a review. International Journal of Speleology, 37(3), pp.193-206.

[2] Campbell, M., McDonough, L., Treble, P.C., Baker, A., Kosarac, N., Coleborn, K., Wynn, P.M. and Schmitt, A.K., 2023. A Review of Speleothems as Archives for Paleofire Proxies, With Australian Case Studies. Reviews of Geophysics, 61(2), p.e2022RG000790.

[3] Feinberg, J.M. and Hobart, K.K., 2021. Attraction in the dark: the magnetism of speleothems. Elements: An International Magazine of Mineralogy, Geochemistry, and Petrology, 17(2), pp.113-118.

[4] Feinberg, J.M. and Johnson, K.R., 2021. Cave and Speleothem Science: From Local to Planetary Scales. Elements: An International Magazine of Mineralogy, Geochemistry, and Petrology, 17(2), pp.81-86.

[5] Johnson, K.R., 2021. Tales from the underground: speleothem records of past hydroclimate. Elements: An International Magazine of Mineralogy, Geochemistry, and Petrology, 17(2), pp.93-100.

[6] Rodbell, D.T., 2012. Marching in near lock-step. science, 335(6068), pp.548-549.

[7] Wendt, K.A., Li, X. and Edwards, R.L., 2021. Uranium–thorium dating of speleothems. Elements: An International Magazine of Mineralogy, Geochemistry, and Petrology, 17(2), pp.87-92.

Tree Rings

[1] Anchukaitis, K.J., 2017. Tree rings reveal climate change past, present, and future. Proceedings of the American Philosophical Society, 161(3), pp.244-263.

[2] George, S.S., 2014. An overview of tree-ring width records across the Northern Hemisphere. Quaternary Science Reviews, 95, pp.132-150.

[3] Hughes, M.K., 2002. Dendrochronology in climatology–the state of the art. Dendrochronologia, 20(1-2), pp.95-116.

[4] Miyake, F., Nagaya, K., Masuda, K. and Nakamura, T., 2012. A signature of cosmic-ray increase in AD 774–775 from tree rings in Japan. Nature, 486(7402), pp.240-242.

[5] Sheppard, P.R., 2010. Dendroclimatology: extracting climate from trees. Wiley Interdisciplinary Reviews: Climate Change, 1(3), pp.343-352.

[6] Swetnam, T.W., 2002. Fire and climate history in the Western Americas from tree rings. Corso di Cultura in Ecologia, 31, p.31.

[7] Turkyilmaz, A., Sevik, H., Isinkaralar, K. and Cetin, M., 2019. Use of tree rings as a bioindicator to observe atmospheric heavy metal deposition. Environmental Science and Pollution Research, 26, pp.5122-5130.