Required but disguised: environmental signals in limestone–marl alternations
The nature of rhythmic carbonate-rich successions such as limestone^marl alternations has been, and still is, 
subject to controversy. The possibility of an entirely diagenetic origin for the rhythmic calcareous alternations is 
discarded by most authors. One problem with an entirely diagenetic, self-organized development of limestone^marl 
alternations is the fact that limestone and marl beds in many examples are laterally continuous over hundreds of 
meters or even kilometers. In an entirely self-organized system, lateral coupling would be very limited; thus one would 
expect that, rather than laterally continuous beds, randomly distributed elongate nodules would form. We address the 
origin of limestone^marl alternations using a computer model that simulates differential diagenesis of rhythmic 
calcareous successions. The setup uses a cellular automaton model to test whether laterally extensive, rhythmic 
calcareous alternations could develop from homogeneous sediments in a process of self-organization. Our model is a 
strong simplification of early diagenesis in fine-grained, partly calcareous sediments. It includes the relevant key 
mechanisms to the question whether an external trigger is required in order to obtain laterally extensive limestone^ 
marl alternations. Our model shows that diagenetic self-organization alone is not sufficient to produce laterally 
extensive, correlatable beds. Although an external control on bedding formation could be considered to have solved 
the problem as commonly assumed, we here suggest an interesting third possibility: the rhythmic alternations were 
formed through the interaction of both an external trigger and diagenetic self-organization. In particular we observe 
that a very limited external trigger, either in time or amplitude, readily forms correlatable beds in our otherwise 
diagenetic model. Remarkably, the resulting rhythmites often do not mirror the external trigger in a one-to-one 
fashion and may differ in phase, frequency and number of couplets. Therefore, the interpretation of calcareous 
rhythmites as a one-to-one archive of climate fluctuations may be misleading. Parameters independent of diagenetic 
alteration should be considered for unequivocal interpretation.