Heterogeneity Of Sediment Composition In The Mariana Trench:Insights Into Sediment Provenance And Hadal Sedimentary Environment

The hadal trench（with a water depth of more than 6000-6500 m）is usually located at the boundary of the convergent plate.Active geological activities make the sedimentation in the hadal trench unstable.At the same time,the funnel-shaped topography and high-frequency current activities of the trench are conductive to promoting the focusing of particulate matter to the axis of the trench,forming a"hotspot"for microbial activity probably with unique and rich microbial communities.The typical feature of unstable deposition also leads to the heterogeneity of sediment composition and source in different water depths within the trench.As sediments act as the basic support for benthic life activities,the heterogeneity of sediment composition will affect the distribution and activities of benthic communities.Therefore,uncovering the differences in the sediment composition and corresponding information regarding sedimentary environment in different water depths within the hadal trenches has great implications for understanding the distribution of benthos and the evolution of sedimentary environment in the extreme environment of the trench.In order to understand the differences of sediment composition,source and sedimentary environment at different water depths and locations in the trench,the sediments collected from five sites with a water depth of 5800-10954 meters in the Mariana Trench are selected as the research object.The five sites（from MBR02 to MBR06）cover different sedimentary units such as trench axis（MBR02）,basin（MBR03,MBR04）,seaward slope（MBR05,MBR06）.Based on the major,trace and rare earth elements,clay minerals,particle size,Sr-Nd isotope,organic carbon content and AMS-¹⁴C dating of sediment samples,this paper discusses the source,transportation and accumulation process of sedimentary materials in the Mariana Trench and compares the differences of sedimentary characteristics in space,and analyzes the evolution of sedimentary material sources,driving factors and indicated paleoenvironment differences since the last glaciation of the Mariana Trench.The results of geochemical elements and mineralogy show that the composition of sediments in different depths of the Mariana Trench is quite different The whole MBR05 and the interval of 8-20 cm of MBR06 are dominated by valve fragments of the giant diatom Ethmodiscus rex（E.rex）,forming laminated diatom mats（LDMs）.AMS-¹⁴C dates of bulk organic carbon show that the LDMs were formed between 15and 20 kyr B.P.,corresponding to the Last Glacial Maximum（LGM）.Both Sr-Nd isotope compositions and clay-mineral assemblages of the detrital fraction reflect a two-component mixing pattern consisting of Luzon arc volcaniclastics and Asian eolian dust,with LDMs show greater aeolian dust contribution,indicating that the bloom of E.rex may be related to Asian aeolian dust input.Meanwhile,the rare earth elements（REEs）distribution pattern along with weak or absent Ce anomalies in the LDM point to suboxic conditions during the LGM,while the non-LDM samples exhibit little to no enrichment of redox-sensitive elements as well as negative Ce anomalies,indicating deposition under oxic bottom-water conditions.We propose that the more reducing bottom-water conditions during the LGM were linked to the bloom of E.rex and increased primary productivity stimulated by increased nutrient contribution associated with enhanced Asian dust input.This study is of significance for understanding the heterogeneity of sediment composition in hadal trenches and its impact on the distribution,ecology and activity intensity of benthic microorganisms.