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Scientists reconstructed the 3,000-year history of fire by dating fire scars on ancient giant sequoia trees, Sequoiadendron giganteum, in the Giant Forest of Sequoia National Park.
The new research extends Swetnam’s previous tree-ring fire history for giant sequoias another 1,000 years into the past.
Knowing how giant sequoia trees responded to a 500-year warm spell in the past is important because scientists predict that climate change will probably subject the trees to such a warm, dry environment again, said Swetnam, a UA professor of dendrochronology and director of UA’s Laboratory of Tree-Ring Research. During the Medieval Warm Period extensive fires burned through parts of the Giant Forest at intervals of about 3 to 10 years, he said.
The team also compared charcoal deposits in boggy meadows within the groves to the tree-ring fire history. The health of the giant sequoia forests seems to require those frequent, low-intensity fires, Swetnam said. Researchers Peter Brown and Tom Swetnam dating the tree-rings and fire scars on a cross-section of a giant sequoia tree displayed near the General Sherman Tree in Sequoia National Park. Such charcoal-based fire histories can extend much further into the past than most tree-ring-based fire histories, he said.
Increasingly, researchers all over the world are using charcoal to reconstruct fire histories, Swetnam said. National Geographic VoicesResearchers, conservationists, and others around the world share stories and commentary on the National Geographic Society's conversation platform Voices. Society Field Inspection 2016: Gulf of CaliforniaDescribed by Jacques Cousteau as the "aquarium of the world," the Gulf of California in Mexico is threatened by coastal development, climate change, and other human activities. Support National GeographicWe've supported more than 11,000 grants to scientists and others in the field.Learn more about our work. Two years after being discovered deep in a South African cave, the 1,500 fossils excavated during the Rising Star Expedition have been identified as belonging to a previously unknown early human relative that National Geographic Explorer-in-Residence Lee Berger and team have named Homo naledi. With at least 15 individuals of all ages and both sexes represented, the find adds an unprecedented amount of information to our understanding of early human evolution in Africa.
In addition, the absence of any other animal remains or large debris in the fossil chamber strongly suggests that these non-human beings intentionally deposited their dead within this cave.
Analysis by tree-ring dating has demonstrated that two distinct periods of felling are represented.
Students develop a basic knowledge of Dendrochronology using the specially designed simulated tree cores in this engaging kit. Your students will learn the important effects of climate on living things as they unravel secrets about the age and history of trees.
Absolute dating is the process of determining an age on a specified time scale in archaeology and geology. In archeology, absolute dating is usually based on the physical, chemical, and life properties of the materials of artifacts, buildings, or other items that have been modified by humans and by historical associations with materials with known dates (coins and written history). In historical geology, the primary methods of absolute dating involve using the radioactive decay of elements trapped in rocks or minerals, including isotope systems from very young (radiocarbon dating with 14C) to systems such as uranium-lead dating that allow acquisition of absolute ages for some of the oldest rocks on earth. Radiometric dating is based on the known and constant rate of decay of radioactive isotopes into their radiogenic daughter isotopes.
One of the most widely used and well-known absolute dating techniques is carbon-14 (or radiocarbon) dating, which is used to date organic remains.
The relatively short half-life of carbon-14, 5,730 years, makes the reliable only up to about 75,000 years.
An additional problem with carbon-14 dates from archeological sites is known as the "old wood" problem.
Optically stimulated luminescence (OSL) dating constrains the time at which sediment was last exposed to light.
Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree rings, also known as growth rings.

In some areas of the world, it is possible to date wood back a few thousand years, or even many thousands. Amino acid dating is a dating technique [5][6][7][8][9] used to estimate the age of a specimen in paleobiology, archaeology, forensic science, taphonomy, sedimentary geology and other fields.
Individual giant sequoias can live more than 3,000 years, the university said in a news statement. Other research has found that the period from 800 to 1300 was warm and dry,” the university said. The chronology of charcoal deposits closely matches the tree-ring chronology of fire scars, the university statement said.
He added that as the climate warms, carefully reintroducing low-intensity fires at frequencies similar to those of the Medieval Warm Period may be crucial for the survival of those magnificent forests, such as those in Sequoia and Kings Canyon National Parks.
The charcoal history of fire in the giant sequoia groves extends back more than 8,000 years. Many scientists are analyzing the global record of charcoal to study relationships between climate, fire and the resulting addition of carbon dioxide to the atmosphere.
Geological Survey, Mountain Home Demonstration State Forest and Calaveras Big Trees State Park provided funding. Posters and commenters are required to observe National Geographic's community rules and other terms of service.
To learn firsthand what's going on in and around this marine treasure of global importance, the National Geographic Society sent its top scientists and officials to the region in January 2016. The first or eastern part comprises the main carriageway opening and the adjacent pedestrian portal; it is believed to have been constructed in the fourteenth century, with stone to the ground floor and a mainly timber-framed upper floor. All the structural timbers of the Gatehouse proper, and the majority of timbers from the mezzanine floor here, were probably felled in the latter half of the AD 1330s and the early AD 1340s. They will also be asked to estimate the climate of a particular year in the distant past by carefully studying a trees annual rings. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty of accuracy.[1][2] Absolute dating provides a numerical age or range in contrast with relative dating which places events in order without any measure of the age between events. Techniques include tree rings in timbers, radiocarbon dating of wood or bones, and trapped charge dating methods such as thermoluminescence dating of glazed ceramics.[3] Coins found in excavations may have their production date written on them, or there may be written records describing the coin and when it was used, allowing the site to be associated with a particular calendar year. Particular isotopes are suitable for different applications due to the type of atoms present in the mineral or other material and its approximate age. The technique often cannot pinpoint the date of an archeological site better than historic records, but is highly effective for precise dates when calibrated with other dating techniques such as tree-ring dating.
It is possible, particularly in dry, desert climates, for organic materials such as from dead trees to remain in their natural state for hundreds of years before people use them as firewood or building materials, after which they become part of the archaeological record. This technique is based on the principle that all objects absorb radiation from the environment. Fluctuating levels can skew results – for example, if an item went through several high radiation eras, thermoluminescence will return an older date for the item. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year. This technique relates changes in amino acid molecules to the time elapsed since they were formed. You may redistribute it, verbatim or modified, providing that you comply with the terms of the GFDL.
However, ancient giant sequoias can have trunks that are 30 feet in diameter–far too big to be sampled using even the longest coring tools, which are only three feet long, the University of Arizona said.
Hallett looked for charcoal in sediment cores taken from meadows within the sequoia groves. They toured the desert islands off the southeastern coast of the Baja Peninsula and they listened to presentations by more than a dozen experts, including several whose Baja California research is funded by the Society.

Adjoining the west end of the Gatehouse proper, having a slightly different roof line, but likewise running parallel to the road, is the second or western part of the Gatehouse structure, the ‘annex’, which includes a single-bay gabled arm to its south. The roof timbers of the annex roof, and its south arm, were probably all felled in AD 1582. For example, techniques based on isotopes with half lives in the thousands of years, such as Carbon-14, cannot be used to date materials that have ages on the order of billions of years, as the detectable amounts of the radioactive atoms and their decayed daughter isotopes will be too small to measure within the uncertainty of the instruments. Cosmic radiation entering the earth’s atmosphere produces carbon-14, and plants take in carbon-14 as they fix carbon dioxide. Thus dating that particular tree does not necessarily indicate when the fire burned or the structure was built. Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger.
Upon burial, the sediment accumulates a luminescence signal as natural ambient radiation gradually ionises the mineral grains.
Brown of the Rocky Mountain Tree-Ring Research and Colorado State University in Fort Collins; R.
Carbon-14 moves up the food chain as animals eat plants and as predators eat other animals.
For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating. The half-life of potassium-40 is 1.3 billion years, far longer than that of carbon-14, allowing much older samples to be dated.
Heating an item to 500 degrees Celsius or higher releases the trapped electrons, producing light. Careful sampling under dark conditions allows the sediment to be exposed to artificial light in the laboratory which releases the OSL signal. All amino acids except glycine (the simplest one) are optically active, having an asymmetric carbon atom. The development of [accelerator mass spectrometry] (AMS) dating, which allows a date to be obtained from a very small sample, has been very useful in this regard.
Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated. The amount of luminescence released is used to calculate the equivalent dose (De) that the sediment has acquired since deposition, which can be used in combination with the dose rate (Dr) to calculate the age.
This means that the amino acid can have two different configurations, "D" or "L" which are mirror images of each other.
It takes 5,730 years for half the carbon-14 to change to nitrogen; this is the half-life of carbon-14. Argon, a noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay. With a few important exceptions, living organisms keep all their amino acids in the "L" configuration. The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice. When an organism dies, control over the configuration of the amino acids ceases, and the ratio of D to L moves from a value near 0 towards an equilibrium value near 1, a process called racemization.
By measuring the carbon-14 in organic material, scientists can determine the date of death of the organic matter in an artifact or ecofact.

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