Biomarker Records From The Zhejiang-Fujian Coast, East China Sea: Implications For Paleoclimatic And Paleoenvironmental Changes In Holocene

The East China Seas?ECSs?are typical marginal seas located between the Eurasian continent and west Pacific Ocean,and provide a significant aquatic-terrestrial interface,where large rivers with high runoff transfer huge amounts of particulate and dissolved material from terrestrial sources to the ocean.Moreover,there is a large amount of marine autogenous material produced here.Apparently,they play an important role in the global matter cycle.As one part of the ECSs,the East China Sea?ECS?is mainly affected by the East Asian Monsoon?EAM?,the the Yangtze?Changjiang?River?YR?and the nearshore circulation system.The Holocene ECS has experienced the complicated relative sea level rise,and the formation process of the nearshore circulation system and the modern mud area.Recently,there has been a decreasing sediment flux,heavy floods,increasing red tides,frequent occurrence of hypoxic events in the YR mouth,and rising sea water temperature.The Zhejiang–Fujian coastal mud area?ZFCMA?was formed during the highstand system tract after the last glacial maximum,with high deposition rate,continuous sedimentary records and complex organic matter source,recording the large amount of information about the natural climate environment evolution and human activities.Thus,the ZFCMA sediment is an ideal object in the study of paleoclimate and paleoenvironment evolution.In this study,three sediment cores from the ZFCMA were used to examine burial and changes in sources of organic matter at these locations over time.Specifically,we aim to?1?reveal the region sedimentary evolution responded to the influence of paleoclimate and paleoenvironment change,and to?2?decipher the biomarkers records in response to recent natural and human-induced environmental changes.Two short cores DZ-28?28.63°N,122.36°E,water depth 52.8m?and DZ-41?28.07°N,121.95°E,water depth 42.2m?from the ZFCMA were analyzed using bulk(TOC,TN and?¹³C_TOC)and molecular biomarker?n-alkane,brassicasterol,dinosterol and glycerol dialkyl glycerol tetraether lipids:GDGTs?techniques to clarify the sources,spatiotemporal distribution and fate of SOM in the Yangtze River Estuary and adjacent shelf.Results from this study indicated that the effects of diagenesis and diffusion on different sedimentary biomarkers resulted in overestimation of the relative contribution of terrestrial organic matter(%OM_terr),compared with those based on?¹³C_TOC.The amounts of terrestrial plant organic matter(OM_plant)and%OM_terr in sediments decreased offshore.In contrast,the amounts of marine organic matter(OM_marine)increased offshore,but closer to the YR mouth,the amounts of soil organic matter(OM_soil)increased.Moreover,the amounts of TOC,OM_plant and OM_marine biomarkers increased,but OM_soil and%OM_terr decreased over time in recent decades.Our study suggests that spatial organic matter distribution patterns in marine shelf sediments were controlled primarily by hydrodynamic sorting and nutrient concentration,and temporally diverse patterns were controlled predominantly by anthropogenic influence?e.g.,dam construction and soil conservation,reclamation and agricultural plantations,anthropogenic nutrient input,dust storms,eutrophication,etc?and climate events?e.g.,interdecadal climatic jump and heavy rain events?in the geological period.The distribution pattern of free fatty acids over the last 160 years in the sediment cores—DZ-28 and DZ-41—is investigated in the present paper.Results show that the free fatty acids are dominated by the marine autochthonous fatty acids derived from planktonic and bacterial sources,and followed by a small amount of allochthonous higher plant-derived fatty acids from the land.In response to the changes in the intensities of the EAM,the Kuroshio Current?KC?intrusion and the Pacific Decadal Oscillation?PDO?,the amounts of marine free fatty acids are significantly higher in the20th century than those in the 19th and abnormally high values found in the period from the late 1970s to early 1990s.Meanwhile,heavy floods?such as those in 1998,1954and 1931 mentioned in literatures?from the whole of the YR catchment may led to the sharp increase in terrestrial higher plant-derived free fatty acids instead of the decrease in marine autochthonous free fatty acids.Furthermore,human activities have become the dominating factor that caused the durative increase in total free fatty acids in particular the marine autochthonous ones since the late 1960s.Our study based on the indicators of free fatty acid further reveals that the construction of the Three Gorges Dam has inhibited to certain extent the growth of diatoms,and the i-C_15:0%can be used effectively to invert the history of the hypoxia events in this mud area.The low-oxygen events have been in a trend of increase since 1950s followed by a more dramatic one after the 1980s.The cores DZ-28 and DZ-41 sediments were also studied in detail to investigate temporal changes in the source and distribution of tetraether lipids in the area,and the implications of such data for paleoclimate change.The results indicate that isoprenoid GDGTs?isoGDGTs?in the ZFCMA were derived mainly from Thaumarcheaota,with a minor contribution from in situ and allochthonous methanogenic archaea in upwelling areas,whereas branched GDGTs?brGDGTs?originated from mainly terrigenous bacteria and in situ organisms.However,the minor methanogen-derived and allochthonous isoGDGTs were considered to have had no effect on inferred sea surface temperatures?SSTs?in this study.The presence of brGDGTs produced in situ might corrupt the relationship between the methylation index of branched tetraethers or the cyclization index of branched tetraethers?MBT/CBT?and mean annual air temperature?MAAT?or pH,particularly in hypoxic areas.However,#ring_tetraetra data,used here as an index to evaluate the sources of the brGDGTs,indicated that the brGDGTs buried in the cores sediments are predominantly derived from soil erosion of the YR catchment.Furthermore,temporal changes in the GDGT-based proxy records indicate that climate changes and events?e.g.relating to the EAM,PDO/KC intensity,and flood events?,as well as anthropogenic influences?e.g.dam construction,soil conservation,fertilizer usage,and discharge of industrial waste water and domestic sewage?affect the signal in the core sediments.SSTs fluctuated over the past 160 years in the ZFCMA,with higher values in the 20^th century than in the 19^th century.In addition,the humidity,as inferred from the relationship between precipitation/soil water content and p H,and continental MAAT in the mid–lower YR basin switched between dry–hot/dry–cool and wet–hot/wet–cool over the last 160 years.The reconstructed paleoclimate change agrees well with literature records and satellite monitoring data.The results also indicate that PDO/KC intensity plays a key role in temperature changes and has a larger impact on terrestrial ecosystems than on ocean ecosystems in the ZFCMA and the adjacent continent.Eutrophication has caused drastic changes to marine ecosystem of the ECS during the past decades.However,there is still relatively sparse evidence of historical changes,as well as the explicit effects of climatic changes and anthropogenic activities on the primary productivity of marine coastal ecosystems.In this study,surface and core sediments from the ZFCMA,China were analyzed using the bulk and molecular biomarkers.Results showed that ecosystem changes were characterized by increased phytoplankton productivity and a fluctuant transition from blooms mostly dominated by diatoms to red tide events dominated by dinoflagellates.Variations from 1852 to 2013can be divided into a nature-dominated?1852-1960s?and a human-impacted?1960s-2013?periods.Particularly,natural forcing such as heavy floods?e.g.,1998,1954 and 1931?in the whole of the YR catchment,variations in the intensity of EAM,and strengthened or weakened KC intrusion/positive or negative PDO in the coastal mud area have substantially affected phytoplankton productivity and community structure during the nature-dominated period.In contrast,changes in nutrients were more apparent during the human-impacted period,which could have been due to increased fertilizer usage,discharges of industrial waste water and domestic sewage,and large-scale human projects?e.g.,Danjiangkou Reservoir and Three Gorges Dam?in the YR drainage area,leading to significant phytoplankton productivity and community structure variations in the coastal mud area system of ECS.Long core ECS-1302?122.37°E,29.02°N?recovered from a water depth of 38.2 m in the muddy area off the ZFCMA was analyzed for Holocene paleoclimatic and paleoenvironmental changes.On the basis of the lithology,the foraminiferal and ostracod assemblages,and the accelerator mass spectrometry ¹⁴C and optically stimulated luminescence ages,we divided the core into two depositional units representing the Holocene sedimentary sequence.The early and middle Holocene unit was deposited in a littoral to tidal-flat environment,a far end of the continental shelf tidal sand ridge/sand table,and a condensed section formed after the maximum sea surface appeared during ca.12,000–7,700 cal yr BP.Our bulk organic matter and biomarker proxies indicated that the local ecological environment was mainly controlled by relative sea level change.During this period,the KC had invaded the ECS inner shelf repeatedly,and the coastal current was weak.The results also recorded that the rapid rise of sea level induced by MWP-1B during ca.11.5–11.0 cal kyr BP and MWP-1C after ca.9.5 cal kyr BP;the palaeo-Yangtze had shifted into the south Yellow Sea ca.10.5 cal kyr BP;and the maximum flooding surface occurred at ca.8.0 cal kyr BP,then the ECS entered the stage of high sea level.During the stage of high sea level,the relative sea level change was relatively small,and the KC intrusion was strong.Following the progradation of the YR delta,a huge number of terrestrial sediments were transported and deposited along the Zhejiang-Fujian coast.Thus the modern muddy sedimentary framework in the continental shelf was generally formed since ca.7.7 cal kyr BP.Until ca.3.0 cal kyr BP high sea level gradually subsided to the modern sea level.Meanwhile,bulk organic matter and biomarker proxies recorded several weakened KC intrusion,which occurred at from ca.3.5 to 4.0,2.0,1.4,and 1.0 cal kyr BP in the late Holocene,and 1.4 ka BP,8.2 ka BP,and suspected 9.4 ka BP cold events.The inner shelf sedimentary evolution and marine primary productivity in the ZFCMA,however,were dramatically influenced by human activities during the last ca.1.5 cal kyr BP.