Varves are distinct layers of sediment that form annually in bodies of water subject to strong seasonal changes. A varve is defined as an annual layer or couplet of sediment. This pair of contrasting sediment types acts as a precise geological calendar, reflecting the distinct environmental conditions of a single year. The term originates from the Swedish word “varv,” meaning revolution or circle, emphasizing their cyclical formation.
The Mechanics of Varve Formation
Varve formation relies on a strict, repeating annual cycle that separates sediment input into a couplet of two distinct layers. Deposition begins during warmer months, forming the summer layer, which is typically lighter and coarser. This layer consists primarily of silt and fine sand delivered by increased water flow, such as glacial meltwater runoff. These coarse particles settle quickly, often close to the source, a process termed proximal deposition.
As the season transitions to winter, water inflow decreases sharply, and the water surface often freezes, creating a quiescent environment. This lack of movement allows only the finest clay particles and organic material to slowly settle out of suspension, known as distal settling. This fine sediment forms the overlying darker, thinner layer, which is rich in organic carbon and clay. The sharp boundary between the winter layer and the coarse layer of the following summer confirms that one dark and one light layer constitute a single year’s deposit.
The thickness of the varve couplet provides clues about the year’s conditions. The summer layer, subject to variable meltwater discharge, often shows the greatest annual variation in thickness. Conversely, the winter layer, formed by the slow settling of fine material, tends to be more uniform year to year. This repeatable process makes the varve couplet a reliable chronological marker, allowing scientists to count the layers backward in time, similar to counting tree rings.
Environments Where Varves Occur
For varves to form and be permanently preserved, specific environmental conditions must be met within the water body. The primary requirement is a deep, quiescent basin where sediment layers accumulate without disturbance from strong currents or wave action. A second necessary condition is the absence of biological disturbance, or bioturbation, which is the mixing of sediments by burrowing organisms. This mixing would destroy the fine annual layering, making the sediment unsuitable for annual dating.
Varves are typically found where the bottom waters are anoxic, meaning they lack dissolved oxygen, which prevents most life forms from inhabiting the sediment. Classic varves occur in proglacial lakes, such as those in Scandinavia and the Connecticut Valley, where the seasonal melt-freeze cycle provides regular sediment input. Other locations include deep, anoxic marine basins, like the Santa Barbara Basin off the coast of California, and deep fjords. In some instances, such as Crawford Lake in Canada, varves are biogenic, with the annual cycle driven by biological productivity rather than strictly clastic sediment input.
Varves as a Tool for Geological Dating
The annually resolved nature of varves makes them a valuable tool in geochronology and paleoclimatology. Scientists create a varve chronology by counting and measuring the successive annual layers in a sediment core, establishing a continuous, year-by-year timeline. This technique is comparable to dendrochronology (tree ring dating) but can extend much further back in time, sometimes providing records tens of thousands of years long.
A varve chronology that is counted but not yet tied to the present-day calendar is known as a floating chronology. To establish an absolute date, this chronology must be anchored to a known calendar year using independent dating methods, such as radiocarbon dating or matching the sequence to a known historical event. A common anchoring technique involves identifying an instantaneous geological marker bed, like a layer of volcanic ash (tephra), which can be dated independently. For example, the Swedish Time Scale was anchored by identifying a distinct, basin-wide drainage event and tying it to a precise radiocarbon date.
The physical characteristics of the varve couplets offer detailed information about past climate conditions. The overall thickness of a varve is directly related to the amount of sediment delivered to the lake. In glacial environments, this thickness is a proxy for the intensity of the summer melt season. Thicker varves often indicate warmer, wetter summers with greater glacier melt, while consistently thin varves suggest prolonged periods of cooler temperatures.
The composition of the layers also reveals environmental shifts, such as the ratio of clastic sediment to organic matter, which indicates changes in biological productivity or erosion rates. Scientists use advanced techniques like micro-X-ray fluorescence (micro-XRF) scanning to analyze the chemical composition at a sub-millimeter scale. This allows them to track the seasonal input of specific elements, such as titanium (a proxy for clastic input) or sulfur (an indicator of anoxia). Combining the precise annual count with these compositional proxies allows researchers to reconstruct past climate and environmental changes with high resolution.