Multi-proxies Reconstruction For The Changes In The Composition Of The Sedimentary Organic Matter In The Min-Zhe Coastal Area Of East China Sea

Marginal seas connecting the continents and the ocean plays an important role inthe global carbon cycling. As an important carbon sink, the changes of sedimentaryorganic matter （SOM） in the East China Sea （ECS） would influence carbon cycleand climate change. In this study, three cores spanning about one hundred years wererecovered to reconstruct the variations of SOM composition in the ECS by use ofmulti-proxies. In addition, the samples of a247cm long core were also measured toreconstruct long-term evolution of SOM in the ECS.Multiple-proxies including the total organic carbon （TOC） contents and theirδ¹³C and C/N values, the contents and MARs of odd long-chain n-alkanes,phytoplankton biomarkers （sterols and alkenones） and GDGTs were analyzed in coresediments. Furthermore, binary and three end-member mixing models based on theδ¹³C, TMBR and BIT proxies are employed to evaluate relative contribution ofterrestrial organic matters. TOC contents and the MARs of all biomarkers showdecreasing trends from near-shore to the shelf in accordance with the sedimentarytransport and dispersal pattern in the ESC. During the past hundred years, both thecontents and MARs of∑(C₂₇+C₂₉+C₃₁),∑（B+D+A）, crenarchaeol and BranchedGDGTs increased. The ranges of δ¹³C values are-21.5^-20.9‰,-21.82^-21.38‰and-21.31^-20.72‰, and the ranges of BIT values are0.032⁰.059,0.033⁰.056and0.026⁰.051in cores DH5-1, DH6-2and DH6-3respectively, indicating dominantcontribution of marine organic matter for SOM. With the exception of δ¹³C valuesof the core DH6-3, the values of δ¹³C, TMBR and BIT proxies of the three coresreveal decreasing TOM contribution since the1980s. The results calculated by abinary end-member mixing model based on δ¹³C, TMBR and BIT proxies also revealdominant contribution of MOM, while the calculated fraction of TOM are different based on different indices. Fraction based on δ¹³C proxy is similar to the resultcalculated by TMBR proxy. However, lower TOM fraction based on BIT index wasachieved in comparison with δ¹³C and TMBR proxies, the reason for that still needsmore investigate. δ¹³C values of TOC indicate bulk OM composition, and TMBRindex represents the relative contribution of terrestrial high plant OM. However, BITindex traces soil OM transported into marine sediments by rivers. The threeend-member mixing model based on δ¹³C, TMBR and BIT proxies shows that themarine OM, terrestrial high plant debris and soil OM account for47⁸7%,9.7³8%and3.5⁶.5%in core DH5-1;58⁸6%,12³0%and3.6⁶.2%in core DH6-2;49⁸0%,16³4%and2.8⁵.6%in core DH6-3, respectively.TOC contents in core T08ranged from0.35%to0.86%, increasing from thecore-bottom to the core-top. δ¹³C values in core T08ranged from-22.42‰to-21.51‰, indicating enhanced TOM contribution from the core-bottom to the core-top.The values of BIT index in core T08varied from0.029to0.059, with an increasingtrend below the55cm and a decreasing trend in the upper55cm, which indicatesTOM contribution increases from the core-bottom and then decreases towardcore-top.