A rhythmite consists of layers of

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

which are laid down with an obvious periodicity and regularity. They may be created by annual processes such as seasonally varying deposits reflecting variations in the runoff cycle, by shorter term processes such as tides, or by longer term processes such as periodic floods. Rhythmites serve a significant role in unraveling prehistoric events, providing insights into

sea level change Globally, sea levels are rising due to human-caused climate change. Between 1901 and 2018, the globally averaged sea level rose by , or 1–2 mm per year on average.IPCC, 2019Summary for Policymakers InIPCC Special Report on the Ocean and Cryo ...

, glaciation change, and earth's orbital variations which serve to answer questions about

climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

Annually-laminated rhythmites

Annually-laminated deposits (

varve A varve is an annual layer of sediment or sedimentary rock. The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as ''Hvarfig lera'' (va ...

s) are rhythmites with annual periodicity: annual layers of

are laid down through seasonal variations that result from precipitation, or from temperature, which influences precipitation rates and debris loads in runoff. Of the many rhythmites found in the geological record, varves are among the most important and illuminating to studies of past climate change. Varves are amongst the finest resolution events easily recognised in

stratigraphy Stratigraphy is a branch of geology concerned with the study of rock layers ( strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithost ...

Periodically laminated rhythmites

Rhythmites may be deposited with periodicities other than annual. The geologic record captures both more frequent events (e.g., tides) and less frequent events (glacial floods).

Tidal rhythmites

Geologic tidal rhythmites display layered

Carboniferous Period The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonif ...

beds which record tidal cyclical events such as semi-diurnal, diurnal or

neap tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables c ...

spring tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tabl ...

cycle. The geologic record captures layered beds comparable to those found currently in sediments in the

Bay of Fundy The Bay of Fundy (french: Baie de Fundy) is a bay between the Canadian provinces of New Brunswick and Nova Scotia, with a small portion touching the U.S. state of Maine. It is an arm of the Gulf of Maine. Its extremely high tidal range is t ...

in Canada and the Bay of

Mont Saint-Michel Mont-Saint-Michel (; Norman: ''Mont Saint Miché''; ) is a tidal island and mainland commune in Normandy, France. The island lies approximately off the country's north-western coast, at the mouth of the Couesnon River near Avranches and ...

in France. The Storm Mountain area of

Big Cottonwood Canyon Big Cottonwood Canyon is a canyon in the Wasatch Range southeast of Salt Lake City in the U.S. state of Utah. The -long canyon provides hiking, biking, picnicking, rock-climbing, camping, and fishing in the summer. Its two ski resorts, Brig ...

, Utah, has rhythmites which record sea-level sedimentary deposit fluctuations consistent with the cycle of the tides.

Proglacial rhythmites

One common mechanism is the episodic flooding which results from glacial dam bursts. In one such example geologists estimate that the

Missoula Floods The Missoula floods (also known as the Spokane floods or the Bretz floods or Bretz's floods) were cataclysmic glacial lake outburst floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the las ...

cycle of flooding and reformation of the lake took an average of 55 years and that the floods occurred approximately 40 times over the 2,000-year period between 15,000 and 13,000 years ago. Distinct rhythmites with an approximately 55-year periodicity have been observed.

Glacial epicycle rhythmites

Sea-level changes which correspond to the glacial periods also show up as extremely long-term rhythmites. As an example, the ice surge in the Quaternary resulted in changes in sea level of 127 meters to 163 meters. The regression and transgression of the sea level from waxing and waning glaciers have been identified in the rhythmites of the Pennsylvanian and Permian periods.

References