| Multiple sedimentary dynamics coexist and interact in deep-water environments,yielding complex sedimentation patterns subject to climate changes at different scales and preserving key information on paleoceanography and paleoclimatology in the evolution of deep-water sedimentary systems.It is thus an important basis to identify each sediment facies for a better understanding of depositional processes.However,distinguishing finegrained sedimentary facies in sediment cores is still under much debate,driving this frontier into a hotspot in deep-sea research.Ice raft debris(IRD)releasing is a way of sediment supply unique to glacial continental margins.Its distribution characteristics in sediments are indicative of the changes in sedimentary processes and dynamics,which provides a new way to differentiate fine-grained sedimentary facies.This study is focused on the SW Grand Banks off Newfoundland in the North Atlantic.Sedimentological techniques such as sedimentary structure description,grain size analysis,geochemical elements analysis,and chronostratigraphic analysis are used to identify sedimentary features in the deep-water depositional systems developed at 700-1600 m.By recognizing sedimentary facies and analyzing key factors during the development of deep-water sedimentary systems,this study reveals the sedimentary processes related to the evolution of sedimentary patterns and explores their significance on paleoceanography.1.Eight typical lithofacies in all cores within the study area were identified,based on the deposition background of glacial evolution and sediment provenance,colour changes,sediment grain size,sediment composition,sedimentary structures,and the presence and distribution features of ice-rafted debris.These lithofacies are homogeneous light grey mud,mottled olive-grey silty mud with IRD,fine-grained dark grey sandy mud with IRD,alternating grey silty to sandy mud devoid of IRD,bioturbated olive green silty mud,greyish-red mud with layered IRD,detrital carbonate-rich mud with IRD and conglomerate with dispersed mud clasts.They are further identified as contourites,turbidites and mixed deposits considering the depositional mechanism.2.Fine-grained sedimentary facies and their characteristics were described.X-ray images and geochemical element variations show:(1)Turbidites are mainly characterized by horizontal to sub-horizontal laminations,and the thickness of their sandy layers is commonly at 0.5 mm to 3 cm.They are typical bypassing deposits of turbidity currents at the upper slope.Turbidites devoid of IRD show no trace of bioturbation,and the boundaries become clearer with the increasing sand content.Where there are frequent turbidity currents,the turbidites and interbedded mud construct a well-layered structure with sharp changes in bulk density and elemental ratios.(2)The overall grain size of contourites varies from being muddy,silty to sandy.Muddy contourites are dominated by grey mud,which is formed under a weak hydrodynamic condition unable to generate any obvious sedimentary structures.They appear to be homogeneous except where are truncated by bioturbation,but changes in thickness are significant regionally.Silty contourites have relatively distinct lamination,Laminae has indistinct boundaries and are mostly undulating,lenticular or irregular in shape.The distribution of IRD is relatively uniform and can be found within the laminae.The sand content of fine-grained sandy contourites is up to 30-45%,and none of these sandy beds are graded.They are characterized by a notable absence of primary sedimentary structures,in places showing faintly rare lamination and bioturbation.(3)Hybrid deposits are produced when turbidity current and bottom current interact,with sedimentation rates at an intermediate level.Bioturbation is abundant throughout this silty mud,and sediments tend to be homogeneous.3.Key factors constructing the deep-water sedimentary system in the study area were analyzed.The activity of turbidity currents,the role of bottom currents,and the input of IRD were identified:(1)Turbidity currents mainly flowed along the canyon in which turbidites were deposited.With the influence of ice sheet retreating,sea-floor morphology and sea level changes,the sediment dynamics feeding turbidity currents include meltwater discharge,longshore drift,storminess and rapid marine transgressions.(2)Contourites accumulated where the deposition was not dominated or disrupted by turbidity currents,developing contourites depositional systems.The biological components have little influence on the sortable silt from grain size in contourites.However,before the Heinrich Event 1(～16 ka BP),The release of IRD disturbed the sortable silt record documenting the sorting capacity of the bottom current.End-member analysis is an effective tool for isolating the influence of IRD input and extracting a proxy for the strength of the bottom current.Overall,the bottom current was stronger during the last glacial maximum-Heinrich Event 1 period with frequent fluctuations at small scales,it was weaker and remained relatively stable during the middle-late Holocene.In addition,changes in the vigor of the bottom current exhibit cyclicity with periods of about 8,860 years,2,190 years,and 1,517 years,which are well coupled with regular climate oscillations at millennial scales.(3)Along with the northward retreating of the Laurentide ice sheet,the distance between the source area of icebergs and the study area was increased,resulting in a decrease in the abundance and size of IRD kept in the deposits.Iceberg rollover is an event independent of the melting processes,which led to sediments dumping and pouring into the deep sea,most of which were accumulated in-situ,forming thick deposits.4.The evolution of deep-water sedimentary patterns and related sedimentary processes since the last glacial maximum in the study area was summarized.During the last glacial maximum,the southern Laurentide ice sheet was in a relatively stable condition,limiting the meltwater supply,and turbidity currents were infrequent.In the deglaciation period at ～23-18 ka BP,the continuous melting of the ice sheet drove the meltwater to transport a large amount of sediment,which resulted in frequent turbidity currents,and thick turbidites developed in the canyon.At the same time,the bottom current interacted with turbidity currents in the canyon,yielding hybrid deposits and asymmetrical outer levees outside the canyon.From ～18-15 ka BP,coastal erosion and longshore drift were the main driving processes for sand supply and the triggering of turbidity currents,which is characterized by a higher sand content in turbidites and a decrease in sediment flux.At ～15 ka BP,a rapid rise in sea level submerged coastal sand banks,and turbidity currents were decreased.During the Younger Dryas period corresponding to ～12.9-11.7 ka BP,increased storm activity led to turbidity currents at a higher frequency.Also,meltwater discharge enhanced bottom currents,corresponding to the regionally developed bi-gradational sequences.In the early Holocene,at ～11.7-11.2ka BP,continuous marine transgression rapidly submerged large areas of previously exposed continental shelves,resuspending sufficient sands and triggering frequent turbidity currents.After ～11.2 ka BP,turbidity current activity was almost stagnant,bottom current activity became dominant.The intensity of the bottom current was relatively low,with muddy sediment as the main component of its deposits.The axis of the current is likely at 1200–1350 m water depth.The Holocene deposition is mainly a process of muddy drift building,with spatial deposition patterns being largely controlled by an along-slope current interacting with pre-existing morphology.5.It is revealed that the Labrador Current was characterized by a spatially gradual decrease in strength.Regionally developed sandy contourites during the Younger Dryas period can be distinguished from the overlying mud-dominated Holocene,marking the onset of the Holocene and indicative of Holocene thickness.On the SE Grand Banks slope,there were obvious erosion/non-sedimentation processes during the Holocene,but depositional processes dominated on the SW Grand Banks slope,and the Holocene thickness gradually increased along the path of the Labrador Current.Since the Labrador Current was in a relatively stable condition during the Holocene,the described features indicate that the current vigor gradually decreased along the Grand Banks slope during the Holocene.6.The significance of the turbidity currents in the early Holocene as a sedimentary response to the meltwater pulse 1b was proposed.The complete record of early Holocene turbidity current activities with high frequency was discovered for the first time in eastern Canada.During 12-11 ka BP,the largest area of the previously exposed continental shelf was inundated,indicating a rapid rise in sea level.The width and flatness of the Grand Banks shelves may also have amplified the transgression effect,providing massive sands that could be efficiently resuspended.It was onset at ～11.7 ka BP,corresponding to the end of the Younger Dryas and the initial of the Holocene.Thus,the early Holocene turbidites serve as evidence of meltwater pulse 1b in deep-sea sediment at the North Atlantic,indicating that meltwater pulse 1b may have occurred during the transition from cold to warm periods. |
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