de Boever, E, Birgel, D, Thiel, V, Muchez, P, Peckmann, JL, Dimitrov, L and Swennen, R (2009) The formation of giant tubular concretions triggered by anaerobic oxidation of methane as revealed by archaeal molecular fossils (Lower Eocene, Varna, Bulgaria). Palaeogeography, Palaeoclimatology, Palaeoecology, 280(1-2). 23-36. doi:10.1016/j.palaeo.2009.05.010