| Distinguishing the sources of organic matter in marginal sea sediments is an important aspect of carbon cycle research. There are two approaches to reconstruct the relative amounts of terrestrial and marine organic matters: one based on the total organic matter and another based on biomarkers. The BIT index, as a new proxy for indicating terrestrial organic matters, has been recently used to quantitatively estimate sources of organic matter in estuary and marginal sea sediments. Based on the ratio of Branched GDGTs (produced by soil bacteria) to Crenarchaeol (derived from aquatic crenarchaeota), the BIT index has the advantages that the biomarkers have specific origins and they are less affected by diagenesis. This study reports the C/N,δ(13)~C, BIT index and the contents of microbial biomarker of Branched GDGTs of 54 surface sediment samples from the South Yellow Sea to investigate the source and distribution of organic matter in sediments. This study also use binary and three end-member mixing models to estimate the terrestrial organic matter input and use the MBT/CBT index to estimate soil pH and regional Mean Annual Temperature (MAT).The TOC contents of the surface sediments vary between 0.08% and 1.11%, with the highest value in the middle of the south Yellow Sea, and the C/N ratios and theδ(13)~C values vary from 5.4 to 12.8 and -22.8‰to -19.95‰respectively, indicating the organic matters origin from both the sea and the land. The lowerδ(13)~C values distribute in Shandong province and Subei shoals. The results show higher BIT values for samples close to the Old-Huanghe River, and lower BIT values seaward, except the higher BIT values in the southeast of the Shandong Peninsula. Because of the influence of the high sedimentation rate to the south of the Shandong Peninsula,the contents of the Branched GDGTs have maximum values there. Branched GDGTs/TOC can eliminate the influence of the sedimentation rate, with the similar distribution of BIT index.Estimates of the percent terrestrial organic matter in these sediments can be made using a simple binary mixing model, based on the C/N ratio, theδ(13)~C content and the BIT index. We also use a three end-member mixing model based on the C/N ratio, theδ(13)~C content and the BIT index to estimate the distinction of soil and plant organic matter as separate contributors to the organic matter in marine sediment. As to the contribution of the terrestrial organic matter in the Old-Huanghe estuary and in the south of Shandong Peninsula, in contrast toδ(13)~C values, BIT index values indicate low terrestrial organic carbon. This may be contributed to the origins ofδ(13)~C which comprise both soil and vegetation,both river and dust. The BIT index is best interpreted as a proxy for soil or peat derived organic matter and a proxy for riverine input. In addition, terrestrial Crenarchaeol may affect the BIT index values. The results of the application of the three end-member mixing model show that marine organic matter accounts for 46% of the total organic matter present in the surface sediments, soil organic matter accounts for 13%, and land plant organic matter accounts for 32%, which suggests that land plant organic matter represents the majority of the terrestrial organic matter delivered to the sediments in the South Yellow Sea, and the terrestrial organic matter contributes nearly half of the total organic matter.The Cyclisation ratio of Branched Tetraethers (CBT) and the Methylation index of Branched Tetraethers (MBT) results based on the analysis of Branched GDGTs in the Yellow Sea sediments show that the average pH is 8.59, which indicates that the terrigenous soil of the Yellow Sea is alkaline. And the alkalinity of the soil in the north of the Old-Huanghe River is higher than soil in the south. The average of MAT is 17.2℃, which is higher than the average air temperature in the downstream of the Huanghe River and similar with the air temperature in the coast of the Yellow Sea. |
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