Observation and implication of the paleo-cave sediments in Ordovician strata of Well Lundong-1 in the Tarim Basin

Well Lundong-1 is located in the periclinal area on the eastern flank of the Tahe-Lunnan paleo-uplift in the Tarim Basin. A 25-m-high cave fill sequence was observed in the Upper Ordovician interval of the well at 6800-6825m. A third cut of cores was obtained from the top of the cave. The following conclusions were obtained by studying the cave sediments and depositional sequence, and by undertaking paleontological and elemental geochemistry analyses. 1. The cave sediments contain abundant brachiopod, gastropod, echinoderm, ostracod, and acritach fossils, which can be classified into two groups: cave autochthonous and cave allochthonous fossils (from collapse breccia dissolution or transportation by underflow). The fossils indicate that the cave was formed before the Carboniferous and partly-filled and buried during Carboniferous resubsidence. 2. Elemental geochemistry shows that the mud that filled the cave is sourced from calcareous paleo-soil and weathered crust that came from a salty environment with poor water circulation. 3. The formation and evolution of the cave occurred in three stages. The first stage occurred after the deposition of the Late Ordovician Lianglitage Formation, the second stage took place after the deposition of the Late Ordovician Sangtamu Formation, and the last stage happened after Silurian deposition. Major dissolution occurred in the latter two stages as a result of bedding-confined deep underflow karstification. Based on the reconstruction of the cave formation history, favorable paleokarst targets can be predicted and estimated to aid paleogeography and paleokarstology studies.

Distinct distribution revealing multiple bacterial sources for 1-Omonoalkyl glycerol ethers in terrestrial and lake environments

The 1-O-monoalkyl glycerol ethers （MAGEs） were initially viewed as the biomarkers for sulfate-reducing bacteria （SRB） me- diating anaerobic oxidation of methane in the marine environments. However, limited information is known about their distri- bution in terrestrial and other aquatic settings including soils, fresh water lakes, and cave sediments, which may obscure our understanding of their biological sources. Here we found that MAGEs were ubiquitous but differed obviously in distributional pattern among those environments. The surface soils are dominated generally by iC15：0-MAGE, followed by nCI6：o-MAGE whereas the lake sediments show the opposite, resulting in significantly higher iC15：0/nC16：0 ratios in soils than in lake sedi- ments. The cave deposits are characterized by considerably higher proportions of branched MAGEs than the former two envi- ronments. The logarithm of iC15：0/aC15：0 ratio shows a significant negative correlation with soil pH, likely reflecting an adapta- tion of microbial cell membrane to change in the ambient proton concentration. The MAGE profiles in cultured bacteria cannot fully explain the MAGE distribution in all the samples analyzed. Therefore, MAGEs in soil, lake sediments, and cave deposits likely have additional biological source（s） other than SRB and cultured MAGE-producing bacteria. The difference in MAGE pattern among environments is likely to be attributed to change in microbial communities.