Box 31 Extracting the climate signal from varved sediments

Varved sediments have been used to generate palaeoenvironmental reconstructions based on variations in the varve thickness and composition, as well as on variations of fossil or other signals in the sediments. However, it may be difficult to identify the type of environmental signal recorded in the sediments, and to quantify the relationship between sediment variations and a given environmental parameter (temperature, precipitation, runoff). It can be hard to estimate the rate at which a sediment-related change responds to a given environmental parameter. Sometimes, it has been difficult to quantify the varve-climate relationship, which can result from: (1) the lack of suitably long instrumental time series of appropriate environmental data near sites with laminated sediments; (2) an inaccurate varve age model; (3) the complex multivariate or evolving nature of the sediment-environment link; (4) the dominance of sedimentological/environmental noise over environmental signal; and (5) a slow or uneven rate of response to environmental change.

The environmental response times of many potential palaeoenvironmental signal-carriers are rapid enough to eliminate concern. Most biotic and abiotic factors respond almost immediately to environmental changes, and carry this signal quickly to the sediment. Varved sediments are by definition not significantly biotur-bated, but it is possible for periodic down-slope sediment movement to occur.

in building a varve-based chronology are analogous to those dealt with in dendrochronology. The first step in generating a varved-sediment record is to obtain undisturbed samples of the sediment. For most applications, it is important to obtain the uppermost sediments, including the sediment-water interface. This allows the varve counts to be placed in an exact calendar-age framework.

A well-dated varved sediment record requires multiple cores. Single cores are subject to many core-specific interpretational problems which can be minimized using comparisons between individual cores. These problems commonly include core gaps, localized core disturbance, missing varves, or an anomalous number of laminae deposited per year. In practice, the identification of varves is subjective due to sedimentological noise, which can be reduced using comparisons between multiple cores. In most cases, it is unlikely that the varve-based ages are determined without error, and errors in the order of 5-10 per cent should be assumed, unless a lower error estimate can be justified. An essential first step in using laminated sediments for geochronology and palaeoclimatic reconstruction is to confirm that the laminations are truly annual (varved). This can be accomplished by varve-dating known strati-graphic events, by radiocarbon dating (calendar dates), and by bomb-nuclide profiles. 210Pb is one way to confirm that the varves are annual. The temporal resolution of radiocarbon dates is, however, too coarse to be ideal for confirming varve chronologies, but can help to date sediments older than 150 years.

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