| Coralline algae(Corallinophycidae,Rhodophyta)are essential biogenic components in coral reef ecosystems,major producers of carbonate in coral reefs,and key builders of mass structures of carbonate rock in coral reefs.Coralline algae widely distribute into the global marine environment since the early Cretaceous(Hauterivian,136 Ma).The composition and assemblage characteristics of coralline algal genera or species are closely associated with environmental settings,such as water depth.Coral reefs are valuable archives of information on the paleoenvironmental changes,but to date few studies have been used coralline algae for long-term paleoenvironmental reconstructions,and the development and evolution of coralline algae remain to be elucidated.To solve these problems,the composition,assemblage,and paleoenvironmental interpretations of coralline algae were investigated in this study using the whole drilling core Well CK2(depth 928.75 m),which consists of 55.2 m of volcanic basement overlain by 873.55 m of carbonates.Based on strontium isotopic stratigraphy and chronological framework,the time span of carbonates in Well CK2 is about 20 million years.On the basis of careful observation of rock core,771 samples of 873.55 m carbonate sections were collected from top to bottom and then made them into thin sections(total of 1,198).The composition and distribution characteristics of coralline algae were identified in thin sections under a polarizing microscope.The characteristic changes in time series of coralline algal genus in Well CK2 were analyzed,including the distribution,assemblage,morphology,abundance,and diversity.Combined with the relationship between the growth distribution of different coralline algae and the water depth,the sea-level changes in the Xisha Islands over the past 20 million years as recorded by coralline algal assemblages were reconstructed.The following main conclusions were drawn:(1)Coralline algae in Well CK2 can be divided into 3 orders,8 families,5subfamilies,12 genera.Among them,Corallinales mainly includes Spongites and Neogoniolithon(Spongitaceae,Neogoniolithoideae),Lithophyllum and Amphiroa(Lithophyllaceae,Lithophylloideae),Lithoporella(Mastophoraceae,Mastophoroideae),Hydrolithon(Hydrolithaceae,Hydrolithoideae),and Corallina and Jania(Corallinaceae,Corallinoideae);Hapalidiales mainly includes Aethesolithon and Lithothamnion(Hapalidiaceae)and Mesophyllum(Mesophyllumaceae);Sporolithales mainly includes Sporolithon(Sporolithaceae).(2)Over the past 20 million years,coralline algae in the Xisha Islands mainly consist of Hapalidiales and Corallinales,with relatively few Sporolithales.The algal type of Amphiroa,Corallina,and Jania is articulated,while Aethesolithon,Lithothamnion,Mesophyllum,Lithoporella,Spongites,Lithophyllum,Neogoniolithon,Sporolithon,and Hydrolithon are non-geniculate.The range zones of Corallina,Lithoporella,Lithophyllum,and Hydrolithon are the longest.They distributed in different degrees into the carbonate sections over the past 20 million years.Posteriorly,the age span of Jania,Mesophyllum,and Lithothamnion is 19.6-0.71 Ma,19.48-119 Ma,and 17.45-2.18 Ma,respectively.The distribution of Aethesolithon and Spongites was relatively concentrated at 14.13-8.77 Ma and 6.35-4.28 Ma,respectively.Neogoniolithon,Amphiroa,and Sporolithon sporadically distributed at 18.89-5.02 Ma,6.94-2.17 Ma and 16.5-6.74 Ma,respectively.The statistical results showed that the mean abundance of coralline algae in the whole carbonate sequences was 6.72 ± 0.45%,of which the highest abundance was the interval of 312-309m(4.36-4.28 Ma),with an average of 34.06 ± 9.09%,which was significantly different(p < 0.05)from other Subunits.In addition,a large amount of rhodoliths developed during this period,which can account for up to 80% of the core.The results of coralline algal abundance showed that it was highest at789.85-689.07 m(17.73-16.78 Ma),10 genera,followed by 9.77-7.35 Ma and8 genera.The results of core observation showed that coralline algae in the Xisha Islands over the past 20 million years had formed many ellipsoidal and spherical rhodoliths at 17.3-0.31 Ma with a diameter of ～1-10 cm.(3)Statistics on the biological components in thin sections showed that coralline algae in the Xisha Islands had produced large amounts of calcium carbonates for coral reef ecosystems over the past 20 million years,accounting for 38.51 ± 1.52% of the total amount of calcium carbonates.Over a20-million-year time series,the variations in calcium carbonate contents as produced by coralline algae in the Xisha Islands for coral reef ecosystems can be divided into three different intervals: 873.55-514.57 m(19.6-10.11 Ma),514.57-309 m(10.11-4.28 Ma),and 309-0 m(4.28-0 Ma),which accounted for 26.69%,76.87%,and 20.67% of the total calcium carbonates,respectively.At 19.6-10.11 Ma,the amount of calcium carbonate produced by coralline algae overall increased.At 10.11-4.28 Ma,the amount of calcium carbonates produced by coralline algae was relatively high.At 4.28-0 Ma,the overall calcium carbonate contents of coralline algae increased and decreased in the boundary of 1.05 Ma(75 m).(4)According to the dominant genus of coralline algae in different Subunits,they were grouped into 10 coralline algal assemblages,including Corallina-Jania-Hydrolithon-Lithoporella assemblage(CAA1),Mesophyllum assemblage(CAA2),Hydrolithon-Lithoporella-Lithophyllum assemblage(CAA3),Corallina-Jania-Hydrolithon-Lithophyllum assemblage(CAA4),Mesophyllum-Lithophyllum assemblage(CAA5),Hydrolithon-Aethesolithon assemblage(CAA6),Mesophyllum-Lithothamnion-Lithophyllum assemblage(CAA7),Spongites assemblage(CAA8),Lithothamnion assemblage(CAA9),Lithoporella-Lithophyllum assemblage(CAA10).Of these,at 19.6-18.67 Ma(873.55-838.05)and 16.31-14.79 Ma(637.92-560.07 m),the coralline algal assemblage was CAA1;at 18.67-17.98 Ma(838.05-801.15 m),14.79-13.72Ma(560.07-545 m),and 5.23-4.36 Ma(342.45-312 m),the coralline algal assemblage was CAA2;at 17.98-17.73 Ma(801.15-789.85 m)and 0.28-0 Ma(30-0 m),the coralline algal assemblage was CAA3;at 17.73-16.78 Ma(789.85-689.07 m)and 13.72-10 Ma(545-506.1 m),the coralline algal assemblage was CAA4;at 16.78-16.31 Ma(689.07-637.92 m),9.77-7.35 Ma(489.44-410 m),and 2.2-1.16 Ma(200-80 m),the coralline algal assemblage was CAA5;at 10-9.77 Ma(506.1-489.44 m),the coralline algal assemblage was CAA6;at 7.35-5.23 Ma(410-342.45 m),the coralline algal assemblage was CAA7;at 4.36-4.28 Ma(312-309 m),the coralline algal assemblage was CAA8;at 4.28-2.2 Ma(309-200 m),the coralline algal assemblage was CAA9;at 1.16-0.28 Ma(80-30 m),the coralline algal assemblage was CAA10.(5)According to the distribution characteristics of coralline algal genera and assemblages in Well CK2,the paleo-water depth variations have been reconstructed over the past 20 million years.At 19.6-18.67 Ma and 16.31-14.79 Ma,CAA1 developed in a water depth between 0 and 25 m.At 18.67-17.98 Ma,14.79-13.72 Ma,and 5.23-4.36 Ma,CAA2 developed in a water depth between15 and 25 m.At 17.98-17.73 Ma and 0.28-0 Ma,CAA3 developed in a water depth of ～10 m.At 17.73-16.78 Ma and 13.72-10 Ma,CAA4 developed in a water depth between 0 and 10 m.At 16.78-16.31 Ma,9.77-7.35 Ma,and2.2-1.16 Ma,CAA5 developed in a water depth between 15 and 20 m.At10-9.77 Ma,CAA6 developed in a water depth of 5 m or less.At 7.35-5.23 Ma,CAA7 developed in a water depth of > 20 m.At 4.36-4.28 Ma,CAA8 developed in a water depth between 0 and 5 m.At 4.28-2.2 Ma,CAA9 developed in a water depth of > 25 m.At 1.16-0.28 Ma,CAA10 developed in a water depth between 0 and 15 m.Overall,the paleo-water depth variations as recorded by coralline algal assemblages in the Xisha Islands have shown a water depth range from < 5 m to > 25 m over the past 20 million years.And three large sedimentary cycles were presented in the time series: 19.6-16.31 Ma,16.31-4.36 Ma,and 4.36-0 Ma,respectively.(6)The paleo-water depth variations as recorded by the coralline algal assemblages in Well CK2 are basically consistent with global sea-level changes.The first sedimentary cycle(19.6-16.31Ma): The global sea level showed an upward process on a long-term scale.During this period,the paleo-water depth in the Xisha Islands also showed a deepening upward process,and showed a cyclical pattern of rise with 18.67 Ma and 16.78 Ma,in which CAA3,which developed in a shallow water environment,confirmed that the global sea level provisionally decreased from 17.98 Ma to 16.78 Ma,and then the paleo-water depth rose from 0-10 m to 15-20 m.The second sedimentary cycle(16.31-4.36Ma): The global sea level showed a shallow downward process at 16.31-10 Ma and a deepening upward process at 10-4.36 Ma.During this period,the paleo-water depth variations as recorded by coralline algal assemblages in Well CK2 was similarly bounded at 10 Ma,with an overall trend of decreasing first and then rising,and rising from the lo West < 5 m to > 20 m.Among them,the paleo-water depth increased from 0-5 m to 15-25 m at 16.31-13.72 Ma,and then decreased to 0-10 m at 13.72-10 Ma.During this period,the global climate experienced the maximum expansion and formation event of Antarctic ice sheet,resulting in a drop in global sea level.At 10-4.36 Ma,the paleo-water depth increased at different rates.The third sedimentary cycle(4.36-(15)Ma): The global sea level showed a pattern of periodic fluctuations.The paleo-water depth variations as recorded by coralline algal assemblages in Well CK2 showed an deepening upward process at 4.36-2.2 Ma and a decreasing downward process at 2.2-0 Ma.After 4.28 Ma,the Xisha Islands entered a period of rapid transgression,when the paleo-water depth rose from < 5 m to > 25 m,and then decreased in a depth of < 15 m at 1.16 Ma.The global sea level also showed a distinct decrease in a remarkable transition of the Pleistocence climate(namely mid-Pleistocene transition,MPT).Overall,a comparison of the reconstructed paleo-water depth in this study with global sea-level changes showed similar long-term changes,thereby indicating that the coralline algal assemblages since the Miocene can accurately reconstruct past sea-level histories,which also validates that variations in genus diversity and assemblage evolution of coralline algae are more susceptible to environmental changes.(7)The distribution characteristics of coralline algae in the Xisha Islands clearly respond to major events over the past 20 million years,showing the potential of coralline algae to record major climatic and environmental events.In the early Miocene,the coralline algal abundance showed a distinct increase at17.55 Ma(770.55 m),17.45 Ma(744.74 m),and 17.44 Ma(741.74 m),respectively,corresponding to the Miocene Climate Optimum,and was a response to global warming during the period.The decrease in the distribution of Mesophyllum corresponded to a series of cooling events over the past 20 million years,of which were the most obvious at 13.72 Ma and 1.19 Ma,the former corresponded to the maximum expansion and the permanent formation of the Antarctic ice sheet,and the latter corresponded to the beginning of the mid-Pleistocene transition.At 14.13-8.77 Ma,the concentration distribution of Aethesolithon was corresponding to the sea-level fall on a long term,and its extinction indicated the rapid rise of sea level.At 6.71 Ma,the increasing abundance of Lithothamnion corresponded to rapid rise of sea level.(8)The developmental evolution of coral reefs in the Xisha Islands has been reconstructed over the past 20 million years based on the variations in paleo-water depth as recorded by coralline algal assemblages in Well CK2.At19.6-18.67 Ma,the genera corallina and jania prolifically developed,indicating a shallow-water environment.During this period,the sedimentation rate of coral reefs was basically consistent with sea-level rises,indicating that coral reefs were in a “keep-up” stage.At 18.67-17.98 Ma,the genus Mesophyllum mainly developed,during which the paleo-water depth deepened rapidly.During this period,the sedimentation rate of coral reefs lagged behind that of sea-level rises,indicating that coral reefs were in a “drowned give-up” stage.At 17.98-14.79 Ma,the common genera represented by Jania,corallina,Lithophyllum,and Hydrolithon,during which the paleo-water depth decreased downward trend,and also experienced the Miocene Climate Optimum.During this period,the sedimentation rate of coral reefs caught up with or even exceeded the sea-level rises,indicating that coral reefs were in a “catch-up” stage.At 14.79-10 Ma,Mesophyllum was replaced by Lithophyllum and Hydrolithon.During this period,the paleo-water depth decreased downward trend.And there were obvious unconformities.Meanwhile,the global climate also experienced the maximum expansion and the permanent formation of Antarctic ice sheet.The sedimentation rate of coral reefs was higher than that of sea-level rises,indicating that coral reefs were in a “emergent give-up” stage.At 10-7.35 Ma,Hydrolithon and Aethesolithon were replaced by Mesophyllum and Lithophyllum.During this period,the paleo-water depths deepened upward trend.During this period,the sedimentation rate of coral reefs caught up with the sea-level rises,indicating that coral reefs were in a “catch-up” stage.At7.35-5.23 Ma,there were large numbers of Lithothamnion,Mesophyllum,and Lithophyllum,showing a deepening upward process.During this period,the sedimentation rate of coral reefs lagged behind that of sea-level rises,indicating that coral reefs were in a “drowned give-up” stage.At 5.23-4.28 Ma,Mesophyllum was gradually replaced by Spongites.The paleo-water depth decreased gradually during this period,and the sedimentation rate of coral reefs caught up with or even exceeded the sea-level rises,indicating that coral reefs were in a “catch-up” stage.At 4.28-2.2 Ma,Lithothamnion was the dominant genus,and representative of a deepening upward process.And the sedimentation rate of coral reefs lagged behind that of sea-level rises,indicating that coral reefs were in a “drowned give-up” stage.At 2.2-1.16 Ma,the genera represent by Mesophyllum and Lithophyllum.During this period,the water depth was suitable for coral reef’s development,and the sedimentation rate of coral reefs caught up with the sea-level rises,indicating that coral reefs were in a “catch-up” stage.At1.16-0 Ma,the common genera represented by Hydrolithon and Lithophyllum.During this period,water depth was relatively shallow,and the sedimentation rate of coral reefs was basically consistent with that of sea level rise,indicating that coral reefs were in a “keep-up” stage. |
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