CCD Evolution And Implications Since The Miocene

The Central Indian Ocean Basin(CIOB)is the largest deep-sea basin in the Indian Ocean.It is very difficult for this area to receive terrestrial debris because it is far away from the mainland.So,the sediments are mainly marine biological and chemical depositions,and the deposition rate is very slow.The average water depth of the CIOB is 5100m,which is close to Carbonate Compensation Depth(CCD).The sediment components and characteristics were very sensitive to the changes of CCD in the geological history.So,the sediments record the information of CCD changes in history,and this is also an ideal area to study the change of CCD and the evolution of sedimentary environment.In this paper,by analyzing the particle size,elemental geochemistry and microfossils assemblage of the core GC18 sediments in the CIOB,the evolutionary history of sedimentary environment in this area is discussed.Host minerals and the reasons for the enrichment of REY is also discussedBased on analysis of the elemental geochemistry and microfossils assemblage in the sediments of core GC18 in the CIOB,the sediments in this core can be divided into five sections downwards.In section I(0-30cm),CaO and TIC display a decreasing trend with the depth while SiO2 and ?REY increases.Microfossils are mainly foraminifer detritus,and a small quantity of calcareous nannofossils are found as well,such as Calcidiscus leptoporus and Ceratolithus cristatus.In section ?(30-150cm),there are low CaO and high SiO2 and they both are relatively stable.TIC is very low with no microfossils co-existed.?REY first showed a gradual downward trend,then stabilized until the end of this section.Sediments are mainly clay minerals.Section ?(150-280cm)shows a significant shift in the CaO and TIC compares with section ?,which both experienced an up-stable-down process with the depth.However,the trend of SiO2 is opposite to CaO and TIC.?REY shows a gradual decline at first,and goes into a stable state in the end.Calcareous nannofossils are found,which mainly are Discoaster deflandrei.Section ?(280-300cm)is similar with section ?.It is also relatively stable and characterized by low CaO and high SiO2.TIC is extremely low.?REY is higher than Section ? and ?.In Section ?(300-415cm),sediment components fluctuate frequently.The increases of CaO and TIC are coeval with lower values of SiO2 and?REY.A high relative abundance of calcareous nannofossils are found in section ?,mainly composed of Umbilicosphaera jafari,Umbilicosphaera rotula,and Discoaster deflandrei.According to the calcareous nannofossils assemblages,their ages of different sections are also discussed:Quaternary in section ?,the late Miocene in section ?,and the middle Miocene in section ?.Calcareous nannofossils were preserved in section ?,? and ? with high CaO and TIC,reflecting that the sediments were all above Carbonate Compensation Depth(CCD)at those times.Calcareous nannofossils were not found in Section ? and ? with low CaO&TIC and high SiO2,indicating that the sediments were all below CCD.Overall,CCD gradually became shallower from section? to section ?(middle Miocene to late Miocene).It is consistent with the global CCD evolution trend in this time period.CCD in CIOB has experienced a process of deep-shallow-deep-shallow-deep trend since the Miocene.The average mass fraction of ?REY in core GC18 is 757 ppm,and the REY-rich sediment(?REY>400ppm)accounts for 92.8%.The sediments have obvious negative anomalies of Ce and positive anomalies of Eu and Y.The REY distribution curve is slightly inclined to the left,indicating that LREE is slightly deficient and HREE is enriched.The host mineral of REY is apatite.The enrichment of REY is mainly affected by the deposition rate and changes in sediment composition.The lower the deposition rate,the higher the REY content.The composition of the sediment is controlled by the CCD.When the seabed depth is below the CCD,a large amount of deep-sea clay is developed,and the content of REY is relatively high.The calcareous ooze area where core GC18 is located is above the CCD,the calcareous ooze layer in the sediments is very thin.Due to the slow deposition rate and slightly influence of hydrothermal and volcanic activities,the content of REY is still rich.It is an ideal area for the development of REY-rich sediments.The pelagic clay and siliceous ooze areas are in the north of the calcareous ooze area.The water depth is deeper and the sediments especially in pelagic clay areas may also be rich in REY and it is a potential development area for REY-rich sediments.The terrigenous sediment area in the northern part of CIOB has high content of terrestrial deposits.This area is close to the equator and the CCD is relatively deeper So,it does not have the conditions for the development of REY-rich sediments and it is possible to find REY-rich sediments in deep layers.