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As you might imagine, scientists have been attempting to discover other organic objects that can be dated securely steadily since Libby's discovery.Other organic data sets examined have included varves (layers in sedimentary rock which were laid down annually and contain organic materials, deep ocean corals, speleothems (cave deposits), and volcanic tephras; but there are problems with each of these methods.Although we don't have any 50,000-year-old trees, we do have overlapping tree ring sets back to 12,594 years.So, in other words, we have a pretty solid way to calibrate raw radiocarbon dates for the most recent 12,594 years of our planet's past.Carbon dioxide is distributed on a worldwide basis into various atmospheric, biospheric, and hydrospheric reservoirs on a time scale much shorter than its half-life.Measurements have shown that in recent history, radiocarbon levels have remained relatively constant in most of the biosphere due to the metabolic processes in living organisms and the relatively rapid turnover of carbonates in surface ocean waters.

What you need is a ruler, a reliable map to the reservoir: in other words, an organic set of objects that you can securely pin a date on, measure its C14 content and thus establish the baseline reservoir in a given year.

Radiocarbon dating estimates can be obtained on wood, charcoal, marine and freshwater shells, bone and antler, and peat and organic-bearing sediments.

They can also be obtained from carbonate deposits such as tufa, calcite, marl, dissolved carbon dioxide, and carbonates in ocean, lake and groundwater sources.

Cave deposits and varves have the potential to include old soil carbon, and there are as-yet unresolved issues with fluctuating amounts of C14 in ocean corals.

Beginning in the 1990s, a coalition of researchers led by Paula J.

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