I Nl tsUrnschafien Naturwissenschaften 78,505- 507 (1991) © Springer-Verlag 1991 Stromatolitic Origin for Desert Laminar Limecrusts A New Paleoenvironmental Indicator for Arid Regions E. P. Verrecchia, J. Ribier, M. Patillon and K. E. Rolko Laboratoire "G6omicrobiologie et S6dimentologie en Zone Aride", Centre de G6omorphologie du C.N.R.S., F-14000 Caen, and U.F.R. de Biologie Sciences de la Nature, Universit6 Paris VII, F-75005 Paris Terrestrial limestones (called calcretes, caliches, kankar, or nari) in hot and arid regions are presently interpreted as calcium carbonate accumulations within ancient soils which begin to form under wetter conditions than the present day [1]. Where these calcretes are exposed at the surface, it is thought that the soft overlying soils have been eroded. The upper part of these cal- cretes is called the laminar limecrust; it is constituted by thin layers up to several centimeters in thickness which differ in color, hardness, and composi- tion from the underlying formation (Fig. 1). A recent review of the many possible origins of these laminar lime- crusts points out that their genesis can be very easily confused [2]. Formerly considered as intrapedical formations related to the downward leaching of calcium carbonate within the soil profile or as lateral accumulations during calcrete formation [3], laminar limecrusts have recently been attribut- ed to biogenic processes within the soil [4]. The biogenic (stromatolitic) origin of desert laminar crusts has previously been suspected [2, 5- 8]. One of the most essential characteristics of these laminar limecrusts is the presence of organic matter and fibro-radial calcitic spherulites which have been docu- mented by researchers worldwide. Strong resemblances have been found between calcitic spherulites and: 1) cal- cified cells of microorganisms (bac- teria, fungi, etc.) [9], 2) Microcodium [10], and 3) late diagenetic infilling of Naturwissenschaften 78 (1991) micropores. In addition, numerous "calcifications" enclosing coccoid (e.g. Pleurocapsa sp., Gloeocapasa sp.) or filamentous (e.g. Rivularia sp., Nostoc sp.) cyanobacteria have been reported from hot desert [5], freshwater (la- custrine to fluviatile) [11], and marine Fig. 1. Thin section of a Plio-Pleistocene limecrust from New Mexico, USA. Arrows point to the contact between the gravelly limestone (at the bottom) and the overlying laminar crust: grooves indicate that this is an erosional surface on a slightly prehard- ened deposit. The amount of quartz grains (which appear white) is much higher and the grain size is very different than those in the upper part. The presence of the erosional surface and the difference in the quartz skel- eton demonstrate that the laminar crust and the underlying formation are of totally dif- ferent origins. The lower formation is a sedi- mentary deposit that has undergone pedogen- esis. The upper part is of biogenetic origin, constituted b3( successive undulating layers of calcified cyanobacterial mats which have trapped aeolian dust (white specks) © Springer-Verlag 1991 or hypersaline environments [12, 13]. But in all cases, their crystallographic nature is either micritic or microsparitic (calcite or monohydrocalcite) [6] and none of these crystalline features ex- hibits the same structure as spherulites found in desert laminar crusts. Calcite mineralizations also occur in environ- ments with roots [14], lichens [15], or fungi [16] but they are in the form of isolated needles or cell pseudomorphs. Spherulite-like calcitic crystals have been obtained in vitro by the reaction between CaC12 and Na2CO 3 in the pres- ence of chlorophyll in solution with CaC12 [17]. In addition, petrographic structural similarities have been recog- nized between laminar limecrusts and stromatolites [5]. This emphasizes the potential role of photosynthetic mi- croorganisms in spherulite genesis and suggests that the crust was formed on the surface [5]. However, similar fibro- radial spherulites like those in lime- crusts have never been reported from present-day subaerial environments [181. The following results show that spher- ulites with a true fibro-radial structure, identical to those observed in desert limecrusts, can develop in cultures of cyanobacteria strains: • Petrographic observations from Late Tertiary and Pleistocene calcretes (New Mexico, USA; Oran Basin, Al- geria; Alicante Region and Madrid Basin, Spain; Galilee and Shephela, Is- rael) show that laminar limecrusts are always composed of a succession of micritic to microsparitic light-colored bands covering darker brown laminae, organomicritic in composition (Fig. 1). The microsparitic layer is mainly com- posed of isolated or coalescent spher- ulites, with a fibro-radial structure (Fig. 2a, b). In numerous cases, they are composed of concentric layers with an episyntaxic crystal growth. In addi- tion, ultraviolet epifluorescence in- dicates conclusively the presence of organic matter. 505