Sources,Transport And Burial Of Sedimentary Organic Matter In The Kermadec Trench

The lateral transport of allochthonous terrestrial organic carbon?OC?and vertical deposition of autochthonous marine OC are two important components of the marine carbon cycle.The source apportionment of OC has been a hot topic in the oceanographic community.Most of the previous studies paid attention to estuaries,coastal and marginal seas,while the studies for the hadal zone with water depths exceeding 6,000 m are very limited.Hadal trenches,as the deepest area of the ocean?6,000?11,000 m?,are one of the least understood habitats on Earth.Recent studies,however,have revealed higher sedimentation rates,higher biomass and microbial activity in hadal trenches compared to abyssal plains,which indicate that the hadal trenches may play an important role in marine biogeochemical cycle.Meanwhile,whether the hadal trench is a sink of terrestrial organic carbon(OC_terr)in the ocean is a scientific issue worthy of attention.The Kermadec Trench is one of the top ten deepest trenches in the world.It is located in the southwestern Pacific Ocean and is close to the Kermadec Islands.So far,the Chinese researches focus on Mariana and Yap trenches,and have rarely reported OC in the Kermadec Trench.Therefore,the study for the OC in the Kermadec Trench will fill the gap in understanding the source and fate of OC in the southwest Pacific.This study is based on core sediments from two hadal stations?site 4,6?and one abyssal station?site 7?in the Kermadec Trench,and has conducted comprehensive analyses for bulk and molecular characteristics of sedimentary organic matter.The former includes total organic carbon?TOC?,total nitrogen?TN?content,carbon stable isotopes(?¹³C),radioactive carbon isotopes(?¹⁴C)and ²¹⁰Pb activity,while the latter includes multiple biomarkers such as alkanes,fatty acids,alcohols and sterols.Based on these data,three main conclusions were reached:1.Biomarkers:n-alkanes are dominated by short-chain compounds(<C₂₀), presenting a unimodal pattern.This suggests that marine organic carbon(OC_mar)is the main source of OC in the Kermadec Trench.The Carbon Preference Index ?CPI?values of n-alkanes in some samples are close to 1,indicating high maturity of sedimentary organic matter.The Odd Even Preference?OEP?index in some surface sediments is close to that of petroleum hydrocarbons,reflecting the existence of fossil fuel-derived n-alkanes in the trench.The fatty acids are maximized at C₁₆,again reflecting the predominance of OC_mar.The abyssal plain station?site 7?was more affected by bacterial activity than the hadal stations on the axis of the trench?site 4,6?.The unimodal pattern of fatty alcohols predominated by short chain homologues also reflects OC_mar as a major source to sedimentary OC.The higher abundance of C₂₂ n-alcohol at the hadal sites 4 and 6 suggests the contribution of marine plants likely from nearby shallow water, supporting a hypothesis of the trench resource accumulation depth.In the two stations on the trench axis?site 4,6?,sterols are derived from terrestrial plants and zooplankton,but the sterols at the abyssal site 7,the sterol compositions suggest pure marine inputs.Our result suggests that an efficient sequestration of OC_terr in the hadal trench prevents it disperse to the abyssal plain.Despite that, the OC in the Kermadec Trench is mainly of a marine autochthonous source.2.The analysis of sedimentary environment found that:?1?At site 6,the biomarker concentration,TOC and TN contents and C/N ratio all showed a decreasing trend with the increasing depth,reflecting an obvious diagenetic process.?2?Site 7 has the smallest deposition rate,and is characterized by the strongest predominance of OC_mar.The strong bacterial activity in sediments at site 7 not only changed the TN content,but also caused the large fluctuation of biomarker concentrations.?3?At site 4,the bulk parameters revealed the presence of turbidity current in the sediment core,which contributed to large fluctuations in the concentrations of the four biomarkers.Therefore,it should be caution for the interpretation of biomarker indicators under the influence of turbidity current,and the combination of multiple indicators is recommended for determining OC source.The special deposition environment at site 4 is helpful for the burial of OC.3.Transport and burial of OC:?1?Based on bulk parameters and four molecular biomarkers,the horizontal comparison of the three sampling sites revealed that the turbidite layer at site 4 site resulted in a dramatic change in the total biomarkers concentration,TOC,TN,C/N,²¹⁰Pb,?¹³C and?¹⁴C index.However,the content of terrestrial biomarkers?long-chain lipids?at site 4 did not show dramatic changes.Similarly,the long-chain lipid contents indicative of OC_terrinputs at site 6 were also stable.These results suggest that the OC_terr input are relatively stable at the long-time scales.The physical disturbance in sediment core did not cause large fluctuations in the content of long-chain lipids.These terrestrial biomarkers were delivered by ocean current,during which they were well homogenized.As a result,the terrestrial biomarkers in sediment cores presented relatively constant concentrations.?2?The biomarker concentration in sediments from the axial trench was 1-2 folds higher than that in abyssal plain sediments.This is attributed to the unique V-shaped topography of the hadal trench which can facilitated the sediment supply and enhanced the OC burial in trench axis.?3?The site 4 of the Kermadec Trench has strong turbidity current activity,which is obviously different from the nearby hadal and non-hadal sites.It demonstrates the high heterogeneity of OC deposition within the trench.This feature has a profound effect on ecological environments of the trench.In addition,the comparison of TOC contents among three sampling sites suggested that turbidity current is not necessarily for the enhancement of OC contents in sediments,but it can substantially increase burial flux of sediment and OC due to rapid sedimentation rates.