Factors Controlling The Transport And Deposition History Of Fossil Organic Carbon In The Yellow River

The export of fossil organic carbon(OC)in rivers usually provides a potential CO2 source into the atmosphere through its oxidation during erosion and transport processes.Recent studies have shown that the export of fossil OC in the rivers correlates positively to the sediment yield.The Yellow River was the most sediment-laden river in the world.Previous studies reported that fossil OC composes-31%particulate OC in suspended particle materials transported by the Yellow River.On the other hand,the Yellow River has experienced large reductions in sediment load over the last several decades,due to both climate change and human interventions.Reconstructing the history of fossil OC export under such a background is the key to decipher the factors controlling these processes,which can provide guidelines for regional watershed management and have implications for the global carbon cycle.In this study,we pin down the specific sources of fossil OC transported by the Yellow River and calculate the burial efficiency of fossil OC in Laizhou Bay,the Yellow River Mouth.Using a sedimentary core from Lake Wuliangsu in the upper reaches of the Yellow River,we further reconstruct a 150-year history of fossil OC export in the Yellow River and further decipher the factors controlling these processes.The highlights of this study are:1.Distributions and stable carbon isotopic compositions of individual n-alkanes from 45 samples demonstrate three sources of OC in the sediments from Laizhou Bay—soil OC,fossil OC and in-situ productivities.Our results show that short-chain n-alkanes,with even carbon-number predominance,are most likely microbial products from degradation of riverine fossil OC.2.Concentration ratios of n-C26+28+30 to n-C27+29+31 from the riverine particles to surface sediments in Laizhou Bay were constructed to calculate the burial efficiency of fossil OC.Results show that the burial efficiency fossil OC ranges from 36%to 76%,with an TOC-weighted mean value of 42%.The 58%fossil OC lost during the transport and burial processes in the Yellow River mouth may provide a significant C02 source.3.The geological configuration pentacyclic triterpenoids(GCTs)in 8 samples from from the middle reach to the lower reache of the Yellow River course show similar patterns,which is identical with those detected in sediments from Laizhou Bay,the Yellow River Mouth.This strongly suggests that the fossil OC in the Yellow River has a stable source,which is from the rock erosion in the upper reaches of the Yellow River.4.Based on the significance of fossil OC in global carbon cycle as well as its stable source from the rock erosion,we interrogate a 150-yr sedimentary record from Lake Wuliangsu for information about the changes in fossil OC transport since the Industrial Era.Biomarker assemblages of GCTs in sediments suggest that the fossil OC in the sediments from Lake Wuliangsu has a stable source in the last 150 years,which is mostly from Ordovician rocks exposed in the catchment upstream of Lake Wuliangsu.5.The flux of GCTs and%OCfossii based on radiocarbon age spectra measured by Ramped Pyrolysis Oxidation(RPO)clearly show an onset of fossil OC export since?AD 1954 and a rapidly increasing trend since-AD 1978,which is accompanied by a decrease in sediment loads of the Yellow River.6.Our results highlight that the exposure of rocks caused by the degradation of land surface ecology can be the main factor controlling the evolution of OCpetro export in a single catchment.This means in developing countries where great changes in land use and land cover take place,the export of fossil OC in rivers may increase in recent years,which can cause a significant increase of carbon source to the atmosphere.