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The curve showed two types of variation from the straight line: a long term fluctuation with a period of about 9,000 years, and a shorter term variation, often referred to as "wiggles", with a period of decades.
Suess said he drew the line showing the wiggles by "cosmic schwung" – freehand, in other words.
Dendrochronology or the study of tree rings led to the first such sequence: tree rings from individual pieces of wood show characteristic sequences of rings that vary in thickness because of environmental factors such as the amount of rainfall in a given year.
These factors affect all trees in an area, so examining tree-ring sequences from old wood allows the identification of overlapping sequences.
The graph to the right shows the part of the INTCAL13 calibration curve from 1000 BP to 1400 BP, a range in which there are significant departures from a linear relationship between radiocarbon age and calendar age.
In places where the calibration curve is steep, and does not change direction, as in example t in blue on the graph to the right, the resulting calendar year range is quite narrow.
Programs to perform these calculations include Ox Cal and CALIB.
Simply reading off the range of radiocarbon years against the dotted lines, as is shown for sample t Variations in the calibration curve can lead to very different resulting calendar year ranges for samples with different radiocarbon ages.
A third possibility is that the curve is flat for some range of calendar dates; in this case, illustrated by t The method of deriving a calendar year range described above depends solely on the position of the intercepts on the graph.
These are taken to be the boundaries of the 68% confidence range, or one standard deviation.
The improvements to these curves are based on new data gathered from tree rings, varves, coral, and other studies.
Significant additions to the datasets used for INTCAL13 include non-varved marine foraminifera data, and U-Th dated speleothems.