| The diversity,assemblages and geochemical indicators of reef-associated foraminifera are closely related to the depositional environment.However,so far,in the South China Sea,where reefs are in serious degradation,few studies were focused on the foraminiferal indication of reefal paleoenvironment.This paper selects 47 surface sediments and a deep gravity piston core sediment-CK-2(928.75 m,the upper 873.55 m of carbonate rocks with a time span of nearly20 Ma)of Yongle Atoll from Xisha Islands in the South China Sea to conduct a systematic study on the spatio-temporal change of reef-dwelling foraminifera and their indicative significance to the environmental changes using the method of analyzing the foraminifera,biological components,Oxygen and carbon isotope,granularity and geochemical data.The study Four basic issues were explored:(1)The spatial distribution of recent sediment structure and foraminiferal biofacies in Xisha Islands,and their environmental control factors;(2)The temporal sequence of foraminiferal diversity and assemblages of CK-2;(3)The evolution process of sedimentary facies of CK-2 recorded mainly by foraminiferal data;(4)The change process of relative sea-level,climate,and water nutrient level of Xisha Islands recorded by foraminifera from CK-2 in the last 20 Ma.The main conclusions were obtained as follows:(1)A total of 192 foraminiferal species belonging to 3 orders,31 families,9 subfamilies and 80 genera(benthic 29 families,78 genera,188 species;planktonic 2 families,2 genera,4species)were identified from surface sediments of Yongle atoll,among which 21 genera were most common.The foraminiferal richness and diversity show gradually increased from shallow fore-reef facies toward lagoon facies.(2)The recent foraminiferal biofacies model of Yongle Atoll were established which were closely related to the regional ecological and environmental factors.1)The fore-reef slope facies: The well-sorted medium-coarse sand dominated,and became finer with as the body of water deepens.Dominant Foraminifera were thick-shelled and robust Rotallines(>0.25 mm),such as Amphistegina lobifera in the shallow depth.The thinner foraminiferal tests(such as A.lessonii and A.radiata)and more planktonic foraminifera increased with the increasing water depth.2)Reef flat and lagoon slope facies: The thick large-medium Rotaliines(the Calcarina hispida–Neorotalia calcar assemblage)are the significant components for the shallow,clear and oligotrophic conditions.Toward the lagoon slope,with the weakening of the hydrodynamic force,small benthic foraminifera increased,and also,the sediments change from well-sorted coarse-medium sand to medium-sorted medium-fine sand.3)Lagoon facies: The sediments change from poor sorted fine sand at the edge of the lagoon(depth 6-10 m)to well sorted silt in the middle of the lagoon(depth >10 m)for the case of the weaker hydrodynamics.The miliolids and small rotallines,such as,the Quinqueloculina crassa subcuneata–Elphidium advenum assemblage,are dominated.Foraminifera showed a patchy distribution in this facies.Canonical Correspondence Analysis and Bivariate Correlation Analysis indicate that the spatial distributions of foraminifers in Yongle Atoll are associated with water depth and sediment texture.Other environmental factors,including hydrodynamic energy,substratum type and light intensity also influence the foraminiferal biofacies.(3)A total of 141 foraminiferal species belonging to 46 families and 74 genera(benthic42 families,61 genera,101 species;planktonic 4 families,32 genera,40 species)were identified from CK-2 core,among which,11 species are chronological significant,and were used to indicate 3 planktonic foraminifera fossil belts(N21,N20 and N18)and 3 larger benthic foraminifera fossil belts(Tf1,Tf2 and Tf3).The sedimentation rate of CK-2 showed high from the early Miocene-early Middle Miocene and Early Pleistocene-present,however,it was low from the early Middle Miocene-Early Pleistocene.(4)Foraminiferal richness and diversity show relatively high at 18.5-17.58 Ma,16.37-10.19 Ma,4.28-3.50 Ma and 2.86-2.56 Ma.The most strata are dominated by hyline shells,except in the Miocene period(about 16.96-16.78 Ma;16.24-10.6 Ma),where the porcelaneous shells are concentrated.The planktonic foraminifera are abundant from Pliocene to early Pleistocene(4.21-2.18 Ma).Agglutinated tests are scarce in the drill hole.(5)According to the dominant foraminiferal species and diversity,9 foraminiferal assemblages(FAs)were identified,corresponding to 7 sediment facies.1)FA1:Nephrolepidina–Miogypsina assemblage,distributed at the strata of 19.6-18.67 Ma and 17.54-17.38 Ma,indicating reef flat facies,with water depth <3 m.2)FA2: Nephrolepidina–Amphistegina–SBF assemblage,distributed at the strata of 18.67-17.54 Ma and 17.38-16.78 Ma,indicating shallow open platform facies,with water depth <20 m;3)FA3: miliolids–small Rotaliines assemblage,distributed at the strata of 16.78-15.77 Ma and 14.79-10.19 Ma,indicating shallow lagoon facies,with water depth 6-10 m;4)FA4: Nephrolepidina–Miogypsina–miliolids assemblage,distributed at the strata of 15.77-14.79 Ma,indicating lagoon slope facies,with water depth 2-6 m;5)FA5: A.lobifera–A.lessonii assemblage,distributed at the strata of 10.19-10 Ma,10-9.77 Ma,9.77-8.49 Ma,8.49-6.5 Ma and 5.30-4.35 Ma,mainly indicating reef flat facies and fore reef slope facies,with water depth <15 m;6)FA6: Amphistegina–Cycloclypeus–Operculina assemblage,distributed at the strata of 6.5-5.30 Ma,indicating shallow fore-reef facies,with water depth 20-40 m;7)FA7: planktonic foraminifera–Cycloclypeus–Operculina assemblage,distributed at the strata of 4.35-3.15 Ma,indicating submerged platform facies,with water depth about 40-50 m;8)FA8: planktonic foraminifera–Amphistegina(A.radiata and A.lessonii)assemblage,distributed at the strata of3.15-2.09 Ma,indicating submerged platform facies,with water depth abut 30-40 m;9)FA9:A.lobifera–Calcarina assemblage,distributed at the strata of 2.09-0 Ma,indicating the transition between reef flat and the shallow reef front facies,with water depth <10 m.(6)Based on the water depth and sedimentary facies indicated by the foraminiferal diversity and assemblages,combined with foraminiferal indexes(morphology and planktonic/benthic ratio)and sediment particle size,we reconstructed the paleo-water depth sequence of the Xisha Islands since about 20 Ma.The paleo-water depth of the Xisha carbonate platform experienced two sea-level cycles.The first sedimentary cycle(19.6-10 Ma): The paleo-water depth decreased briefly from 19.6-18.8 Ma(water depth <3 m),and then experienced two fluctuations and rose to about 20 m at 16.78 Ma.After that,it decreased slowly to <1 m at 10 Ma.The paleo-water depth of this cycle had a small fluctuation(<20 m).The second sedimentary cycle(10-0 Ma): The paleo-water depth increased again from 10 Ma,rising to 40-50 m at 4.28-3.15 Ma.After a brief dropping at 3.15-2.99 Ma(<3 m),it rose rapidly to30-40 m.After 2.09 Ma,the Xisha paleo-water depth was in the stage of decreasing with many fluctuations.The fluctuation amplitude of this water cycle varied greatly(<1 m to ～50 m).The two cycles of shallow-deepening-shallowing presented by the Xisha paleo-water depth change are basically coincided with the global secondary sea level change cycle.(7)The foraminiferal assemblages,biological components and element content of CK2 recorded the effects of nutrients on the sediment environment.According to the enrichment of small benthic foraminifera,such as Anomalinella rostrate,Rosalina terquemi,Planorbulinella larvata,Heterolepa praecinctus and Cibicides,and also,the mixing of shallow-dwelling species(A.lobifera)with deep-dwelling foraminifera(such as,Cycloclypeus and Operculina),in combination with other biological indicators of eutrophic water(rhodoliths and bryozoans),the nutrient content in the study area mainly appeared high values in four periods,such as,18.8-17.54 Ma,17.38-16.78 Ma,6.4-5.45 Ma and 2.7-2 Ma.The high values in the first two periods may be caused by the intensification of the East Asian summer monsoon.The strong summer monsoon enhances eutrophic upwelling,increases surface productivity,and reduces transparency,resulting in the thriving of small foraminifera,deep-dwelling large benthic foraminifera and other non-reef-building organisms.The high values of the latter two periods are consistent in time with the phenomenon of "Late Miocene Biological Bloom" and the formation of bipolar ice sheets,which may be related to the uplift of the Tibetan Plateau,the intensification of the East Asian winter monsoon,and the climate cooling.These lead to the shallowing of thermocline as well as nutricline,and changes the nutrient distribution in the South China Sea.At the same time,the accelerated oceanic surface current enhanced the edge upwelling,forming a high-nutrient of the surface water.(8)Foraminifera of CK-2 recorded the impact of a series of paleoclimatic events on carbonate deposition.For example,around 540 m(about 13.5 Ma)and 521 m(about 10.19 Ma),the foraminiferal abundance and diversity decreased sharply,and foraminiferal assemblages changed,which recorded the oxygen isotopes Mi3,Mi5 cold events.At 482-476 m(about 9.7-9.5 Ma),the dominant species of foraminifera changed from the deep-dwelling A.lessonii to the shallow-dwelling A.lobifera,recording the oxygen isotope Mi6 cold event.At 521-437 m(about 10.19-8.6 Ma),the low foraminiferal richness and diversity,together with the poorly preserved fossils may be related to the "carbonate crash"(12-9 Ma)events that occurred in the world.The glaciers expanded,resulting the sea level fall,and shallow water level in Xisha carbonate platform.At 306 m(about 4.2 Ma),the foraminifera richness and diversity significantly increased and shells were well preserved,indicating that the depositional environment was suitable;The thrive of planktic foraminifera,the increasing of the foraminiferal planktonic/benthic ratio,as well as the transformation to deep-water community,and the greatly reduced sediment particle size synthetically indicated the deepening of water,which correspond to the Pliocene Climate Optimum(4.5-3 Ma).(9)Based on sedimentary facies,paleo-water depths and deposition rates recorded by foraminifera,the evolution process of the Xisha carbonate platform since Early Miocene was reconstructed.1)Initial stage(mid Early Miocene-late Early Miocene): At 19.6 Ma,the regional tectonic subsidence and global sea level rise caused submerge of the Xisha Uplift,resulting the tropical shallow water organisms such as reef-building corals,coralline algae began to aggradate on the platform margin with high deposition rate.Except for short deepening periods of 18.67-17.54 Ma and 17.38-16.78 Ma,when possible increased nutrient flux reduced carbonate-accretion rates.The intensification of the East Asian summer monsoon resulted in the upwelling that increased surface productivity.2)Colonization stage(late Early Miocene-early Late Miocene): At 16.78-14.79 Ma,due to tectonic subsidence and higher global sea level,as well as the influence of the MMCO(Middle Miocene Climatic Optimum),coral reefs developed extensively.Because the accommodation creation couldnot keep pace with reef growth,progradation and platform expansion occurred,and the lagoon facies deposition was formed in CK2.During the period of 14.79-10 Ma,the low deposition rate was resulted from the expansion of the Antarctic ice sheet,and the influence of short-lived climate cooling events(Mi3,Mi5 events).3)Slow development stage(Early Late Miocene-Early Pliocene): At 10-9.77 Ma,corresponding to the Middle–Late Miocene Climate transition,the global sea level fell rapidly,resulting the low accumulation rate of carbonate sediments(about 2.12 m/Ma).From Late Miocene-Early Miocene(about 8.5-5.3 Ma),with the rise of global sea level,the reef developed vertically.From 5.3-4.28 Ma,the shallow paleo-water depth indicated that the accumulation rate of carbonate rocks exceeded the rate of global sea level rise.4)Drowning stage(early Pliocene–early Pleistocene): In the early Pliocene(4.28-2.09 Ma),with the relative sea level rise in the South China Sea,Biocomponents shifted from reef-facies shallow-water organisms to deeper-water organisms,such as planktonic foraminifera,mosses,and deep-water coralline algae.The deposition rate of carbonate rocks is low.5)Rapid development stage(since the early Pleistocene): Since 2.09 Ma,with the decrease of sea level,the water body of the carbonate platform has gradually become shallowing.Reef-flat facies alternated with reefcore facies with cycles of sea level.High carbonate deposition rate and increasing coral content indicate recovery(catch-up)and rapid aggradation(keep up)of the coral reefs. |
