PRESERVATION OF ORGANIC-WALLED MICROFOSSILS IN RED SEDIMENTS: RECOVERY OF USEFUL PALYNOMORPH TAXA IN LACUSTRINE SEDIMENTS IN WESTERN AUSTRALIA

One of the limitations of palynological analyses is the susceptibility of palynomorphs and dispersed organic matter (DOM) to destruction by oxidation. Therefore, the traditional approach when red sediments are present in a sequence is to avoid sampling those sediments because they are expected to be barren. This assumption that oxidizing conditions cause poor preservation of organic remains often prevents studies of sequences that may preserve valuable information. A study of pollen, spores, algae and DOM in red sediments in Western Australia may challenge this widespread assumption. These sediments were retrieved from six ephemeral saline lakes and one interdunal flat in the southern part of the Yilgarn Craton to study the evolution of acidic and hypersaline conditions. The geochemical and sedimentary characteristics of these ephemeral lakes create a unique suite of minerals, such as halite, gypsum, alunite, jarosite and iron oxides, which have been deposited in a very dynamic environment. Many of the sediments recovered in the drill cores had red and orange color, suggesting the presence of iron oxides. Although previous studies suggested that post-Eocene lacustrine sediments in Western Australia were barren of organic remains, these sediments were sampled for palynology. A poorly preserved, low diversity but paleoclimatically and paleoecologically useful palynomorph assemblage and DOM were recovered in the palynological preparations of these sediments, the preservation of which could be due to rapid coating of organic particles by early diagenetic minerals. Common palynomorphs include Chenopodiaceae, Asteraceae, Myrtaceae, Poaceae, Haloragaceae and the halophilic alga Dunaliella. Contamination from upper sediments has been discarded since this palynological assemblage is much poorer than that of the overlying sediments at the top of each drill core. Moreover, the Rotosonic technique used for drilling minimized contamination because it did not utilize fluids. The recovery of useful palynomorph taxa in red sediments may have stratigraphical and paleontological implications in sequences considered as barren, as well as the exploration of extraterrestrial life, given that similar environmental conditions have been interpreted for the Burns Formation on Mars.