Paleoclimate proxies are physical, chemical and biological materials preserved within the geologic record (in paleoclimate archives) that can be analyzed and correlated with climate or environmental parameters in the modern world. Scientists combine proxy-based paleoclimate reconstructions with instrumental records (such as thermometer and rain gauge readings) to expand our understanding of climate variability to times before humans began measuring these things. These reconstructions of past climate and environment span all timescales, from year-to-year variations to those that occurred over millions of years. These data help us understand how the Earth's climate system varied both before and after human alteration of the landscape.
The use of a proxy to reconstruct past climate requires an understanding of how that proxy is related to some aspect of climate. For example, some proxies, such as atmospheric gases trapped in glacial ice (e.g., carbon dioxide and methane), provide a relatively direct measurement of atmospheric chemistry at the time the ice formed and was sealed off from the atmosphere. Other proxies are less direct, such as stable isotope measurements (e.g., oxygen and carbon) from shells of marine organisms. These indirect proxies require calibration studies in the modern system to establish the relationship between climate processes and the proxy.
Physical proxies include characteristics such as sediment composition, texture, structure, color, density, and magnetic properties, among others. Scientists use changes in physical properties of archive materials to infer past climate conditions. Environmental change, driven by climate, human activity, or other factors, can alter the physical properties of sediments in predictable ways. Understanding these relationships provides a means to reconstruct the climate conditions at the time the sediments were deposited.
The types of minerals and fossils that are preserved in a climate archive can inform scientists about salinity, temperature, ice cover, oxygen levels, nutrient levels, whether sediments washed into the lake/ocean from the landscape or were formed in the water, how the landscape evolved through time, and volcanic eruption histories.
Scientists use the size and shape of sediment particles to determine if the sediment was transported, how far it was transported, and how energetic the environment of transportation was (for example: waves crashing on a beach leave behind coarse sand particles, whereas very small grains are deposited in very still conditions).
The shape and thickness of sediment layers provides a wealth of information about past processes, including whether the sediment was deposited on land or underwater, which direction(s) the wind or water was flowing, local earthquake activity, and whether conditions were suitable for biological activity.
Sediment color can be measured very precisely to aid in determining aspects of sediment composition, such as the amount of green chlorophyll, which is produced when plants photosynthesize, and/or to identify rust on the surface of minerals that were formed under very low oxygen conditions and were later exposed to oxygen. Color can also provide clues about the sources of different sediment layers. For example, red soil that was eroded and transported following Colonial era land clearance is easily identifiable in coastal plain sediments along the East Coast.
Sediment density is the mass of a sediment sample per unit volume. The density of sediment is controlled by the composition of the sediment, the amount of void space between sediment grains, and whether the void spaces are filled with water, air, or other fluids. Density measurements can therefore be used to approximate changes in sediment composition, grain size, and the abundance of void spaces in a sediment sample.
By measuring how easily sediments can become magnetized, scientists can identify where sediments originated (for example, from a volcanic eruption or a specific geologic unit on the landscape). Minerals that are sensitive to magnetism tend to be aligned with the magnetic field of the earth, which allows scientists to date older sediments that were deposited throughout several reversals of the earth's magnetic field.
Biological proxies include remains of living organisms, such as pollen, foraminifera (single-celled, microscopic organisms that bear an external chambered shell), mollusks, and ostracodes (small members of the Crustacean (shrimp) family that are encased by two shells). Because the distribution of these organisms is controlled by temperature, moisture availability, and other environmental factors, their presence in a sample allows scientists to make inferences about the climate when the sample was deposited. Some examples of biological proxies are shown below.
Terrestrial Biological Proxies
Pollen and spores
Pollen and spores are microscopic-sized structures that are part of the reproductive cycle of plants. Pollen grains are produced by seed plants (such as flowering plants and conifers), and spores are produced by more primitive vascular plants such as mosses and ferns. Fossil pollen and spores typically are dispersed from the source plant by wind, insects, and other means. The oldest known land-plant spores are of Upper Ordovician age (~440 million years old). Pollen from seed plants dates to the Late Devonian (~365 million years old), with the first definitive pollen from flowering plants found in Cretaceous rocks (at least 125 million years old). By analyzing pollen and spores preserved in sediments, scientists can reconstruct patterns of past vegetation and climate.
Plant macrofossils are plant remains large enough to be visible without a microscope, including leaves, flowers, cones, and other plant fragments. When possible, scientists identify the plant species represented by the macrofossil. The oldest known plant macrofossils are liverworts found in Middle Ordovician rocks (~475 million years old). The oldest vascular land plant was Cooksonia, preserved in from Middle Silurian (~425 million years old) rocks in Ireland. This is the oldest plant with a stem and vascular tissue, and it represents a transitional form from the older bryophytes to vascular plants such as ferns and seed plants. Based on knowledge of the environmental requirements of living plant taxa, scientists use their presence to infer past climate and environment.
Charcoal is the carbon residue that persists after plants and other organic materials are burnt. Fossil charcoal is preserved in sediments as fallout from fires burning in the surrounding vegetation. Scientists use fossil charcoal to reconstruct changes in the frequency and magnitude of fires in an ecosystem. As vegetation and climate change through time, the frequency, intensity, and area of fires also changes.
Aquatic Biotic Proxies
Foraminifers are single-celled, microscopic organisms that live in water and bear an external chambered shell. Because foraminifer species have distinctive shell morphologies and environmental requirements, scientists can use the composition of foraminifer assemblages to interpret changes in water temperature and quality. The earliest known foraminifers were benthic forms that live on the ocean floor; these are found in rocks as old as the early Cambrian (nearly 500 million years old). Planktic forms, which live in shallower water above the ocean floor, first occur in the mid-Jurassic. Their rapid diversification makes them valuable paleoclimate proxies from Cretaceous (~145.5 million years) to modern time.
Ostracodes are small members of the Crustacean (shrimp) family that live in aquatic environments and are encased by two shells. Their fossil record extends to the early Ordovician (nearly 500 million years), and their sensitivity to changes in water temperature, salinity, oxygen level, and other parameters makes them valuable tools to reconstruct past variations in climate and water quality.
Diatoms are photosynthetic golden brown algae that form skeletons made of silica. Because diatoms are sensitive to changes in temperature, nutrients, salinity, and other physical factors, they provide a means to reconstruct changes from both fresh-water and marine sediments. The oldest diatoms are found in rocks dating to the Early Jurassic, about 190 million years ago, and they first appeared in large numbers in Eocene sediments (45-40 million years ago).
Corals are marine invertebrates that typically live in colonies that contain many individuals. Corals are important reef builders that live primarily in tropical oceans and secrete calcium carbonate to form a hard skeleton. Because corals build sequential layers, they can be used to reconstruct changes in water chemistry and temperature on an annual to decadal scale. The oldest known corals are from Cambrian rocks, deposited ~540 million years ago. Some modern reefs, including the Great Barrier Reef in Australia, began their formation as long as 18 million years ago.
Dinoflagellates are a group of single-celled aquatic organisms that have whip-like flagella (threadlike structures) that propel them through water. Most dinoflagellates live in marine waters as plankton, but some are found in freshwater, also. Some dinoflagellates form dormant cysts (dinocysts) as part of their life cycle. Because of their resistant cell wall, dinocysts are preserved in sedimentary rocks as old as the middle Triassic (~235 million years ago). The presence of dinocysts in sediment archives can tell scientists information about salinity and nutrient status of the water these organisms were living in.
Mollusks are a diverse group of invertebrates that include clams, snails, squid, and many other commonly recognized animals. Most mollusks secrete a hard shell, which is usually well preserved in sediments. Mollusks can be found in terrestrial, freshwater, estuarine, and marine ecosystems, and they span over 540 million years of Earth's history. Aquatic mollusks provide information about a number of environmental parameters including salinity, temperature, nutrients, water depth, and substrate.
The chemical composition of shells of aquatic organisms is affected by the chemistry of the water in which they form. Water, in turn, is influenced by temperature and precipitation. Consequently, shell chemistry (stable isotope and elemental composition) is one example of a chemical proxy of temperature and precipitation. For organisms such as corals and mollusks that secrete sequential layers, these layers can provide an archive of change over the lifespan of the animal. Organic biomarkers are another type of chemical proxy; these are molecular fossils derived from living organisms (such as plants), and they can serve as proxies for other physical and chemical properties of their environment (e.g., temperature, pH, salinity).
Isotopes are atoms of the same element, such as carbon (C) or oxygen (O), that have different numbers of neutrons, giving them slightly different atomic weights. Ratios of stable isotopes from the same element can be measured from archive material to infer a wide range of information about past climate. For example, the ratio of 18O to 16O in rain or snow is controlled by temperature, humidity and atmospheric circulation. Any archive that faithfully preserves these isotopes can provide information about changes in these climatic parameters. Because isotopes can provide climate information from every environment on earth where there are archives of water or plant material, they represent a very useful proxy.
Concentrations of chemical elements, such as iron, titanium, and phosphorus, in sediments and other archives can be used to determine past changes in erosion, lake and ocean productivity, and land use. Erosion intensity is sensitive to changes in precipitation and stream flow as well as changes to the landscape such as deforestation. Changes in aquatic productivity may reflect precipitation-related fluctuations in nutrient input from the land. Changes in land use, such as agriculture or urbanization, can cause deposition of elements released from fertilizers, sewers, and other systems.
Biomarkers are organic molecules that are unique to a specific organism or group of organisms. Biomarkers can be preserved in sediments and rocks after the organism itself has disintegrated, and measurements of their abundance can be used as a proxy for the past distribution and abundance of the source organisms. Some biomarkers can be used to reconstruct past physical parameters such as temperature. For example, alkenones are biomarkers produced by marine algae called coccolithophorids. The molecular structure of alkenones is related to the water temperature in which the algae grew.
Biogenic silica, also known as opal, is one of the most important chemicals found in marine and freshwater sediment. It is primarily created by microscopic algae called diatoms, but it also is produced by other organisms, such as radiolarians and silicoflagellates. Measurements of opal in aquatic ecosystems are a proxy for biological productivity, or the amount of biomass produced in the ecosystem. Productivity changes also can reflect factors such as temperature, salinity, and circulation. Because biogenic silica is so stable in sediments, it has been used to study past marine ecosystem primary productivity in samples that are more than 48 million years old.