EFFECTS OF OXIDATIVE DEGRADATION BY HYDROGEN PEROXIDE ON TETRAETHERS-BASED ORGANIC PROXIES

The organic proxies such as TEX86(the tetraether index of tetraethers consisting of 86 carbon atoms),MBT/CBT(the methylation index/the cyclisation index),BIT(the branched and isoprenoid tetraether index)and Ri/b(the ratio of isoprenoid GDGTs to branched GDGTs)based on the archaeal and bacterial glycerol dialkyl glycerol tetraethers(GDGTs)have been widely applied to reconstruct palaeoenvironment in marine,lacustrine sediments and loess-paleosol sequences.However,degradation like oxidation usually has a significant effect on GDGTs,which should be considered when these organic proxies,especially those branched GDGTs-based proxies,are used in palaeoenvironmental reconstruction.Therefore it's of great significance to evaluate the effects of oxidation on these proxies.Here,we simulate the geological oxidation with hydrogen peroxide to investigate the changes of the different proxies.A soil sample with abundant GDGTs was collected from Chongqing(29°34.0'N, 106°25.2'E) where the soil profile is developed on an anticline formed by Permian and Triassic limestone.Sixty grams of soil sample were then separated into six aliquots equally.The samples were named as VSL-0-1,VSL-0-2,VSL-2,VSL-4,VSL-6 and VSL-8,which means they are oxidized for 0 hr,2 hrs,4 hrs,6 hrs and 8 hrs respectively.We then use hydrogen peroxide to oxidize soil powders and stirred samples by the magnetic mixer to homogenize the samples for the fixed time series at the room temperature.Once times up,we gathered the soil residuals with filters as soon as possible.The soil residuals were freeze-dried and ground into powders again.Finally,we extracted and detected GDGTs in the soil residuals to examine the changes of different proxies.The results showed that cyclopentane-containing GDGTs were less resistant to oxidation than acyclic GDGTs,and that the isoprenoid GDGTs are more easily degraded than the branched GDGTs in the oxidation experiment.The TEX86 index based on isoprenoid GDGTs is more easily to be affected by oxidation and decreases gradually in the oxidation process compared to the MBT/CBT proxy.In addition,the BIT index became higher with increasing oxidation,suggesting a potential overestimation of terrestrial organic input in palaeoenvironment interpretation.Meanwhile,Ri/b index,a proxy of soil alkaline condition induced by aridity,reduced with the increasing oxidation,which might lead to an underestimated reconstruction of salinization and aridity.This study shows that oxic degradation of the organic matter may have a profound effect on the biomarker fingerprint and may result in a severe bias in the palaeoenvironmental reconstruction with long-term exposure in O2,such as loess-paleosoil.It should be noted that the oxidation of hydrogen peroxide is much stronger than oxygen in the natural environment.So the results in this study can be considered as an extreme case.