Growth lines evident in cross sections through stalactites from Nani Cave provide a temporal record of their growth. Many of these dark, organic-rich laminae developed as biofilms that are recognized by the presence of (1) a diverse microbial biota that is dominated by actinomycetes, (2) calcified filaments, (3) films formed of extracellular polymeric substances (EPS), (4) grain-coating sheets of calcite crystals that grew in EPS, (5) biterminal calcite crystals, and (6) etching. This biosignatures suite encompasses a variety of constructive and destructive processes. Where fully developed, the features generated by the biofilms form a distinctive microstratigraphic succession, collectively < 50 μm thick, which can be traced laterally across the stalactite's surface.
The use of speleothems in paleoclimate studies is commonly framed against a chronology that relies, at least in part, on annual growth couplets. The dark, organic-rich lamina that forms one part of the growth couplet is typically ascribed to abiotic precipitation that incorporated exogenic organic matter that was flushed into the cave following the first major rainfall of the wet season. This assumption ignores the possibility that dark, organic-rich growth laminae can be the record of biofilms that developed on the surface of the stalactites and hence, may not be a record of annual events.
