北京高压科学研究中心
Center for High Pressure Science &Technology Advanced Research

P5-WU Weiyan, Zhang Chuanlun

Archaeal Lipids from the South China Sea Subsurface SedimentsImplications for Source Identification and Proxy Fidelity in Highly Diagenetic Sediments

Weiyan Wu a, Yang Xu a, Suning Hou b, Huanye Wang c, Zenghao Zhao c , Haodong Liu a, Huangmin Ge d, Weiguo Liu c, Chuanlun Zhang e*

a State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China

b Utrecht University, 3584CD Utrecht, The Netherlands

c State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China

d Hadal Science and Technology Research Center, Shanghai Ocean University, Shanghai 201306, China

e Department of Ocean Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China

* Correspondence: zhangcl@sustc.edu.cn


Glycerol dibiphytanyl glycerol tetraether lipids (GDGTs) are core lipids in archaeal membranes, which can be decorated with polar head groups like monohexose (1G), dihexose (2G), and hexose and phosphohexose (HPH) to form intact polar lipids (IPLs). IPLs mostly occur in living archaea and are assumed to degrade to core GDGTs upon cell lysis. However, the IPL-GDGTs have been ubiquitously detected in subseafloor sediments and suggested to be fossil archaeal remains, which questions IPL-GDGTs as biomarkers for living archaea. Here we explored the IPL- and CL-GDGT distributions in deep sediments retrieved at site U1431 in the South China Sea (SCS) by IODP 349 Expedition. The sediments were dominated by clay with turbidite intervals like volcanic ash, ooze and silt, which were transported from shallow water by turbidite currents. We found that HPH-GDGTs rapidly declined with depth and became undetectable at 31 mbsf, suggesting their origin from living benthic archaea in sediments. Total IPL-derived GDGTs that were primarily composed of 1G- and 2G-GDGTs significantly correlated with CL-GDGTs. Furthermore, 1G- and 2G-GDGT abundances showed no obvious overall decreasing trends with depth. These results indicate that 1G- and 2G-GDGTs are mostly contributed by fossil pelagic archaeal remains. 1G- and 2G-GDGTs also demonstrated lithologic patterns in concentration and distribution above 300 mbsf, with 1G- and 2G-GDGTs being more abundant and containing less GDGT-0 in clay than non-clay sediments. Below 300 mbsf, 1G- and 2G-GDGTs showed decreased concentrations and elevated relative abundances of GDGT-0 with increasing depth, with the eradication of the difference between clay and non-clay sediments. Cyclopantane numbers (ring index, RI) for 1G-GDGTs were scattered over different lithological sequences, whereas 2G-RI varied little, implying that 2G-GDGTs could be better preserved than 1G-GDGTs during transport and depositional processes. This can be hypothesized that 2G-GDGTs likely have tighter associations with minerals because they have relatively more hydrogen bonds provided by their two sugar heads. Our study indicates that the preservation of archaeal lipids can be dictated by lithology, which is complicated by or suffers from diagenesis of sediments at greater depths. Our results help better constrain the applicability of GDGTs-related proxies in ancient sedimentary systems.