AUTHOR=Wang Aiguo , Li Chunyu , Li Long , Pu Renhai , Yang Zeguang , Zhu Nan , Guo Kai 

TITLE=C20-C21-C23 tricyclic terpanes abundance patterns: Origin and application to depositional environment identification

JOURNAL=Frontiers in Earth Science

VOLUME=11

YEAR=2023

URL=https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2023.1128692

DOI=10.3389/feart.2023.1128692

ISSN=2296-6463

ABSTRACT=<p>Reconstruction of paleo-depositional environments in a sedimentary basin is often obstructed by the absence of typical environmental indicators in sedimentary rocks. Here, we propose a biomarker method using C<sub>20</sub>-C<sub>21</sub>-C<sub>23</sub> tricyclic terpanes (TTs) as a tracer, which is simple in analysis but robust to provide reliable and detailed environmental information. Based on the analysis of 271 C<sub>20</sub>-C<sub>21</sub>-C<sub>23</sub>TT data from 32 basins in 18 countries, we observed a relationship between C<sub>20</sub>-C<sub>21</sub>-C<sub>23</sub>TT abundance patterns and depositional environments. This relationship was attributed to the control of depositional environments on the input proportions of plankton and terrigenous plants, which act as two end-member precursors for the TTs in a depositional system. The various mixing proportions between these two end-members result in different C<sub>20</sub>-C<sub>21</sub>-C<sub>23</sub>TT abundance patterns associated with different depositional environments, e.g., C<sub>20</sub>&gt;C<sub>21</sub>&gt;C<sub>23</sub>TT in river-lake transitional, C<sub>20</sub>&lt;C<sub>21</sub>&lt;C<sub>23</sub>TT in marine or saline lacustrine environments, C<sub>20</sub>&lt;C<sub>21</sub>&gt;C<sub>23</sub>TT in freshwater lacustrine and C<sub>20</sub>&gt;C<sub>21</sub>&lt;C<sub>23</sub>TT in marine-continental transitional environments. In addition, the C<sub>23</sub>/C<sub>21</sub>TT ratio increases with elevated salinity of depositional water, and the C<sub>21</sub>/C<sub>20</sub>TT ratio increases with increasing water depths. Based on these observations, a discrimination diagram using C<sub>23</sub>/C<sub>21</sub>TT vs. C<sub>21</sub>/C<sub>20</sub>TT was developed for environmental identification. The validity of this C<sub>20</sub>-C<sub>21</sub>-C<sub>23</sub>TT biomarker method is well demonstrated by the rock samples with typical environmental indicators. This method is applicable in a broad spectrum of rocks and in maturities up to 2.4%Ro. Its strength was shown by a case study of a complex depositional system in the East China Sea Basin, which has been strongly affected by eustasy.</p>