ANNUALLY LAMINATED SEQUENCES IN THE INTERNAL STRUCTURE OF SOME BELGIAN STALAGMITES - IMPORTANCE FOR PALEOCLIMATOLOGY

Fifteen stalagmites from four caves and one sealed tunnel in southern Belgium are composed of alternations of annually deposited white-porou s and dark-compact laminae. This is demonstrated by comparing the numb er of laminae with the local history of the site for modern stalagmite s and with radioisotopic ages for Late Glacial and Holocene stalagmite s. Annual cyclicity in the internal structure of these speleothems is explained by the highly seasonal variations of the water excess, which influences underground water flow. Comparison between climatic data a nd modern stalagmites of a closed tunnel shows that growth laminae can record climatic variations: (1) there is a good correlation (R = 0.84 ) between lamina thickness in a stalagmite and water excess; (2) durin g years with a high water excess, dark-compact laminae are more develo ped, which makes the speleothem darker. Vertical successions of severa l laminae represent microsequences that may have recorded climatic var iations with a time resolution of 1/2 year. In a Late Glacial stalagmi te, successive laminae microsequences form very regular cycles of 11 y ears separated by a thick dark-compact lamina. It is supposed that, as for modern stalagmites, the thick dark-compact lamina corresponds to a period of high water excess. Hence, this 11-year cycle may reflect a climatic cycle.