Holocene Coral Reef Development And Ecological Significance At Meiji Reef,Nansha Islands

Exploring the development process of Holocene coral reefs and their relationship with the climate environment is of great scientific significance for understanding the response of modern coral reefs to global warming.In this study,the Holocene section of the cores from the drilled well（MJK-1,top elevation 0.19m,well depth 200.88 m）of Meiji Reef in Nansha Islands was used as the object of study.The chronological framework of the Holocene stratigraphy of Meiji Reef and the sequence of sedimentary phase zone changes were constructed by U-Th dating of 13 samples and analysis of particle size,biological components,foraminiferal genera and mineral composition of 20 samples,and the development history of Holocene coral reefs in Meiji Reef was reconstructed.On this basis,the controlling factors and ecological significance of the evolution of the Holocene coral reef development in Meiji Reef were analyzed.The following main insights were obtained from the paper:（1）The growth of the MJK-1 Holocene reef at MJK Reef initiated at5934±46 a BP（relative to 1950 AD）,and the present-day water depth at the growth site is 9.65 m.The top of MJK-1 is dated at 1195±4 a BP,0.19 m above present sea level,and the thickness of the MJK-1 Holocene reef is 9.84 m.The MJK-1Holocene reef is not integrated above the reef formed during the Late Pleistocene.The MJK-1 Holocene reef is not integrated on top of coral reef（U-Th age>147ka）formed during the Late Pleistocene.（2）The depth-dating framework shows that the average accretion rate of MJK-1 Holocene reefs is 2.07 m/ka,with a phased change.5934-4497 a BP,the accretion rate of reefs fluctuates greatly,averaging 2.23 m/ka;4497-3682 a BP,it has the fastest accretion rate of reefs,averaging 5.60 m/ka;3682-1195 a BP,the accretion rate of reefs decreases,averaging 0.82 m/ka.（3）By comparing the initial development age and elevation of the Holocene coral reef adjacent to drilling,it is concluded that the time of the formation of the Holocene coral reef depends on the basement elevation.For example,the Holocene initial age（and elevation,present water depth）of well Nanyong-1（NY-1）in Yongshu Reef of Nansha Islands is 7350 a BP(¹⁴C age)（-17.3 m）,well Nanyong-2（NY-2）is 8220 a BP(¹⁴C age)（-13.7 m）,well Nanke-1（NK-1）in Meiji Reef is 8215 a BP（-14.8 m）,well Chenke-2（CK-2）of Xisha Islands is 7846 a BP（-13.8 m）,combined with 5934 a BP（-9.65 m）of MJK-1.In this paper,it is inferred that the initial development age of the Holocene coral reef in the South China Sea depends on the top elevation of the Pleistocene coral reef,and the top geomorphology of the Pleistocene coral reef should be an uneven topographic structure.（4）Based on core observation and analysis of sediment grain size and biological composition,the Holocene section of core MJK-1 can be divided into reef flat facies,lagoon slope facies and lagoon patch reef facies.0-1.19 m（well depth）,the content of coral,mollusks,coral algae,Halimeda and foraminifera is41.61%,25.13%,25.13%,21.95%,9.53%and 1.22%,respectively.The sediment is poorly sorted medium sand（1.54 grains）,which belongs to the reef flat facies.1.19-6.69 m,the contents of coral,mollusks,coral algae,Halimeda and foraminifera are 44.70%,20.12%,17.35%,11.64%and 3.74%,respectively.Foraminifera mainly indicates the reef flat-lagoon slope by Calcarina hispida and Neorotalia calca,and the sediment is medium-fine sand,belonging to lagoon slope facies.6.69-9.84 m,the contents of corals,Halimeda,coral algae,mollusks and foraminifera are 43.37%,22.28%,21.34%,9.52%and 2.61%,respectively.The sediment is a poorly sorted medium sand,which has a high content of Halimeda and belongs to lagoon patch reef facies.The mineral composition of the Holocene core is mainly aragonite.（5）According to the phased changes of Holocene coral reef development rate in MJK-1 and its corresponding sedimentary facies,combined with the relationship between coral reef development and sea level,it is inferred that the relative sea level of Nansha Islands is generally rising in 5934-3682 a BP,and the rising rate is fluctuating,but the sea level decreases after 1195 a BP.5934-4497a BP,MJK-1 is lagoon patch reef facies,indicating that the sea level rise rate is faster than the reef accretion of MJK-1 during this period.4497-3682 a BP,MJK-1 is lagoon slope facies,indicating that the sea level rise rate slows down,which is lower than the reef accretion of MJK-1,and Meiji Reef shows lateral development to a certain extent.3682-1195 a BP,MJK-1 is the reef flat facies,indicating that the sea level is basically stable or very slow rising at this time,the coral reef mainly develops laterally,and the vertical development almost stops.The elevation of the top of MJK-1（0.19 m above the present sea level）and the age（1195±4 a BP）indicate that the sea level has dropped by at least 0.19 m since～1195 a BP.（6）By comparing the development process of Holocene coral reefs in MJK-1 with the characteristics of climatic and environmental factors in the same period,it is concluded that the development of coral reefs in Meiji Reef is mainly controlled by sea-level and sea surface temperature（SST）,as well as by El Nino-Southern Oscillation（ENSO）,East Asian winter wind intensity,storms and so on.5934-5279 a BP,driven by sea-level,relatively low SST,weak ENSO activity and enhanced winter monsoon are conducive to the rapid development of coral reefs in Nansha Islands;5279-4497 a BP,high SST,strong ENSO activity and weak winter monsoon restrict the accretion of coral reefs and slow the accretion of coral reefs;4497-3682 a BP,slow sea-level rise,relatively low SST and weak ENSO activities promote the accelerated accretion of coral reefs.3682-1195 a BP,the stability of sea level limits the development space of coral reefs,the increasing SST and the increase of storm frequency restrict the accretion rate of coral reefs,and the accretion of reefs is slow.MJK-1 Holocene coral reef rapid accretion period corresponds to relatively lower SST,slow accretion period corresponds to higher SST.In the future,the rise of SST will bring pressure on Meiji Reef corals,and Meiji Reef corals accrete slowly.