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Controls on Formation and Strategies for Study |
, Room 725, National Science Foundation, 4201 Wilson Blvd, Arlington, VA 22230, USA
This paper summarizes observations and ideas developed over a decade studying a variety of laminated sediments. Laminations of alternating lithic (dark) and biogenic (light) particles occur near land masses, where the lithic laminae may be interpreted as a record of precipitation, and the biogenic laminae as a record of primary production resulting from wind stress. The seasons corresponding to each type can be predicted from regional climatology. Biogenic laminae frequently consist of sublaminae representing a sequence of production events, so that several seasons of information may be included. Laminations of two types of biogenic particles are unusual, but have been reported from deep-sea settings. The physical control on these laminations is very indirect, and their occurrence is more probably a result of complex ecologic interactions. Although many laminar couplets are true varves, problems can occur when the normal cycle of alternating conditions is disturbed. Examples include the occurrence of a heavy rainstorm producing a dark lamination within the light lamina of a productive season, juxtaposition of two dark laminae from separate rainy seasons if the intervening productive events did not occur, and continuous rainfall throughout a productive season such that the light biogenic material is masked by the dark lithic material. Such cases, while complicating the use of laminae for time control, may provide useful information on interannual variability if they can be accurately identified. As an alternative to generating very long time series, it is suggested that selected clusters of samples combined with the use of analyses of variance may provide such information. Another strategy is the use of factor analysis to define seasonal proxies from the top part of a section; the proxies may then be projected downcore to derive information on seasonal trends of the past.
