<descriptor thcode="2"><thesaurus thcode="2" name="USGS Thesaurus" scope="Topics and methods of scientific study carried out by USGS, with product types, scientific disciplines, geologic time, and types of institutional structure and activities.  Broad and shallow, used to help people find scientific information." creator="U.S. Geological Survey" rights="Public domain" edition="2.0" date="2020-01-09" codetype="number" prefix="topic" uri="https://www2.usgs.gov/science/USGSThesaurus" tblname="term" nonpref="nonpref" relterm="relterm" root_code="1" contact="pschweitzer"><category><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/></category><altlabel><altlabel>USGSThesaurus</altlabel><altlabel>USGSThesaurus</altlabel></altlabel></thesaurus><term thcode="2" code="435" name="geochemical processes" parent="101" scope="Processes affecting the amount, distribution, or structure of chemical elements in air, water, soil, rocks, and minerals."/><bt><term thcode="2" code="101" name="biological and physical processes" parent="1174" scope="All continuing activities, functions, and phenomena associated with organisms and non-living matter."/><term thcode="2" code="1174" name="topics" parent="1" scope="Themes, subjects, and concerns for which USGS information resources are relevant."/><term thcode="2" code="1" name="Categories" scope="Thematic terms for categorizing information resources.  This thesaurus is intended to support finding and understanding scientific information."/></bt><nt><term thcode="2" code="1346" name="biodegradation" parent="435" scope="Reduction in the concentration of harmful chemicals by the action of animals and plants."/><term thcode="2" code="1731" name="carbon sequestration" parent="435" scope="Long-term storage of carbon in soils and terrestrial organic material, either by natural or engineered processes. Typically of importance for the balance of CO2 in the global climate system. "/><term thcode="2" code="2058" name="ocean acidification" parent="435" scope="Decrease in the pH of ocean waters as a result of the increase in atmospheric carbon dioxide concentration."/><term thcode="2" code="1703" name="ore formation" parent="435" scope="Geochemical processes resulting in concentration of useful chemical elements and compounds, in earth materials.  Typically refers to metal ores."/><term thcode="2" code="1708" name="oxidation and reduction" parent="435" scope="Addition or reduction of oxygen in a compound accompanied by exchange of electrons among atoms."/><term thcode="2" code="1711" name="reaction transport" parent="435" scope="Chemical reactions occurring during ground-water flow."/><term thcode="2" code="1715" name="sorption" parent="435" scope="Attachment of chemical substances to materials, either externally (by adsorption) or internally (by absorption)."/><term thcode="2" code="2279" name="thermal maturation" parent="435" scope="Chemical alteration of organic material within sediments and sedimentary rocks as a result of heat. Measures of thermal maturation indicate the extent of transformation of detrital organic material into coal and petroleum."/></nt><rt><term thcode="2" code="93" name="biogeochemical cycling" parent="310" scope="The cycling of chemical constituents through a biological system."/><term thcode="2" code="437" name="geochemistry" parent="291" scope="Study of the distribution of chemical elements and natural compounds on the earth and in the atmosphere and the chemical processes that affect the earth."/><term thcode="2" code="1710" name="soil formation" parent="451" scope="Geological, geochemical, and ecological processes that work together to produce soils."/></rt></descriptor>