Termination

Termination may refer to:

Science

Termination (geomorphology)

Termination, as used by Quaternary geologists, oceanographers, and paleoclimatologists is the period of time during an glacial cycle when there is a relatively rapid transition from full glacial climates to full interglacial climates. For the Quaternary period, terminations are numbered using Roman numerals from the most recent termination as “I” and with increasing value, e.g. “II”, “III”, and so forth, into the past. Termination I, also,known as the Last Glacial Termination, is the end of Marine isotope stage 2; Termination II is the end of Marine Isotope Stage 6; Termination III is the end of Marine Isotope Stage 8; Termination III is the end of Marine Isotope Stage 10, and so forth.

During the Quaternary, global climate experienced a recurring pattern of ice-sheet growth and decay. The length of Late Quaternary cycles varied between 80,000 and 120,000 years, with an average recurrence interval of about 100,000 years. The typical Late Quaternary glacial cycle was asymmetric having a long cooling interval that was characterized by an oscillating buildup of ice sheets to maximum volume. The long cooling interval was then followed by a relatively short warming period. During this warming period, called a termination, huge Northern hemisphere ice sheets melted away; sea level rose about 120 meters (390 ft); and interglacial climate emerged across the planet in a few thousand years. In case of the termination of the last glacial cycle, the retreat of continental ice sheets in the Northern hemisphere began about 20,000 calendar years ago. By about 7,000 calemdar years ago, a small ice cap on Baffin Island was all that was left of the great Laurentide Ice Sheet that had once covered northern North America. In Antarctica, the last termination began about 18,000 years ago and interglacial climate was attained close to 11,000 years ago.

Radical (chemistry)

In chemistry, a radical (more precisely, a free radical) is an atom, molecule, or ion that has unpaired valence electrons. With some exceptions, these unpaired electrons make free radicals highly chemically reactive towards other substances, or even towards themselves: their molecules will often spontaneously dimerize or polymerize if they come in contact with each other. Most radicals are reasonably stable only at very low concentrations in inert media or in a vacuum.

A notable example of a free radical is the hydroxyl radical (HO•), a molecule that has one unpaired electron on the oxygen atom. Two other examples are triplet oxygen and triplet carbene (:CH
2) which have two unpaired electrons. In contrast, the hydroxyl anion (HO−
) is not a radical, since the unpaired electron is resolved by the addition of an electron; singlet oxygen and singlet carbene are not radicals as the two electrons are paired.

Free radicals may be created in a number of ways, including synthesis with very dilute or rarefied reagents, reactions at very low temperatures, or breakup of larger molecules. The latter can be affected by any process that puts enough energy into the parent molecule, such as ionizing radiation, heat, electrical discharges, electrolysis, and chemical reactions. Indeed, radicals are intermediate stages in many chemical reactions.