Quaternary pollen analysis in the Iberian Peninsula: the value of negative results

Most unsuccessful palynological work is never published. As a consequence, pollen analysts waste time re-processing sterile sediments, and the available literature exhibits a uniformly positive record of success in pollen extraction. Here we report failures with Quaternary pollen analyses in the Iberian Peninsula; that is, case studies where it was not possible to extract palynomorphs for pollen counting. Both totally sterile and partially sterile sites are considered. Sites and perspectives for future studies are suggested. The majority of the failed studies are open-air archaeological and palaeontological sites, caves and rockshelters, but there are prominent cases of success. Peat bogs have provided positive results, but only with sequences formed under continuous sedimentation processes in marshy environments. Lakes are often successful sites, but a multi-core strategy, following the facies change along a transect from the shore to the depositional centre, is recommended for saline lacustrine deposits, salt marshes and lagoons, especially when there is evidence of temporary desiccation. Cave and rockshelter infills should be considered case-by-case, and these sites definitely require a palyno-taphonomical approach to post-depositional processes. Indurated deposits are sometimes surprising in their high pollen concentration, but one must be prepared for sterility. Coprolites have been insufficiently exploited, and offer a great potential, especially those of Pleistocene Crocuta. This article shows that venturing into sediments assumed a priori to be 'difficult', like fluvial terraces, slope deposits, speleothems, cave travertines, and palaeosols, may nevertheless be successful. A summary is proposed of the various factors causing sterility, before, during and after sedimentation.