Thrusting and Exhumation Processes of a Bounding Mountain Belt: Constraints from Sediment Provenance Analysis of the Hefei Basin

Lithic (or gravel) composition analyses of the Jurassic Sanjianpu Formation and Fenghuangtai Formation in the Hefei basin show that the sediment provenance consists mainly of four kinds of rock units: the basement metamorphic complex, granitic rocks, medium- and low-grade metamorphic rocks, and sandy and muddy sedimentary rocks, which are distributed along the bounding thrust belt. The whole stratigraphic section can be divided into 2 lithic sequences and 7 subsequences. The regular distribution and changes of lithic fragments and gravels in lithic (or gravel) sequences reflect that the bounding thrust belt of basin has undergone 2 thrusting cycles and 7 thrusting events. Lithic (or gravel) composition analyses of the basin fully reveal that the northern Dabie basement metamorphic complex was exhumed on the earth's surface in the Middle and Late Jurassic, and extensive intermediate and acid intrusive rocks were developed in the southern North Huaiyang or northern Dabie Mountains during the basin's syndepositional stage.

Research advance in sediment provenance of the Qiongdongnan Basin

1 Introduction Sediment provenance study,as an important part of basin analysis,is a key for source area definition,paleogeographic reconstruction,sediment transport route identification,and tectonic evolution(Haughton et al.,1991;Morton et al.,1999;Fontanelli et al.,2009;Cawood et al.,2012).The Qiongdongnan Basin(QDNB),lies in the northwestern passive continental margin of the South

Geochemical-geomorphological Evidence for the Provenance of Aeolian Sands and Sedimentary Environments in the Hunshandake Sandy Land, Eastern Inner Mongolia, China

Identifying the provenance of aeolian sediments in the Hunshandake Sandy Land is of great importance for understanding the formation of the dune fields in the mid-latitudes and for deciphering information about desert＇s responses to global change. By determining the major and trace elements concentrations of aeolian sands in three grain size fractions from the central and western parts of the Hunshandake Sandy Land, we systematically study the provenance and the depositional history of aeolian sands in this desert environment. Our results show that aeolian sands from the Hunshandake Sandy Land are enriched in SiO2 and are depleted in many other elements compared to those of the Upper Continent Crust （UCC）. Variations of the immobile elements ratios like Zr/Hf, La/Yb, Th/Nb, La/Nb, LaN/YbN, GdN/YbN are relatively large in the coarse and medium fractions but minor in the fine fractions. Eu anomalies are quite different in the coarse fractions, but mostly positive in the medium fractions and all negative in the fine fractions. Decreasing tendency of Zr concentrations from the west to the east in the Hunshandake Sandy Land is evident in the coarse sands but rather weak in the fine grain size fractions. Our geochemical data indicate that the sources for the coarse and medium fractions of aeolian sands are diverse, influenced by local geology and geomorphology, while the fine sand fractions are more homogenous due to intensive mixture mainly by aeolian processes. Various ratios of immobile elements suggest that these sands should be sourced primarily from the surrounding mountains by fluvial/alluvial processes rather than from any remote territories. Aeolian sands with Ce negative anomalies are widely distributed in the Hunshandake Sandy Land, indicating that aquatic environments have occurred extensively prior to the occurrence of the dune field.

Rare earth element geochemistry in the inner shelf of the East China Sea and its implication to sediment provenances

Gravity core EC2005, located in the mud wedge off the Zhejiang-Fujian coast in the inner shelf of the East China Sea （ECS）, was analyzed for its lithology, grain size, rare earth elements （REE） and AMS14C dating. Results revealed high-resolution paleoenvironmental evolution and multiple switches of sediment provenances in the inner shelf of ECS. The lithology, grain size and REE concentrations of sediments varied significantly down the core. Mean grain size ranged from 8 to 121 μm, and the values of ΣREE were 152.8-227.9 μg/g. The chondrite- and shale-normalized REE patterns indicated their similarity to the terrigenous sediments in the study area. Between 17.3-12.3 ka BP the sediments in the inner shelf of ECS were mainly originated from local drainage basins. The source then gradually switched to the Yangtze River between 12.3 and 9.8 ka BP. Since about 10-9.8 ka BP, the sediments in the study area were dominantly derived from the Yangtze River. The continuous rising of sea level and formation of coastal current along the inner shelf played a key role in the switches of sediment provenances.

Sediment provenance discrimination in northern Okinawa Trough during the last 24 ka and paleoenvironmental implication:rare earth elements evidence

Rare earth elements （REE） compositions and discriminant function were successfully used to examine high resolution sediment source changes in the northern Okinawa Trough over the last 24.1 ka, especially for the influence from the Yellow River and the Tsushima Warm Current （TWC） that has not been well solved. Variations of these parameters were clearly divided into three distinct depositional units. During Interval 1 （24.1-16.0 ka BP）, the paleo-Yellow River and the paleo-Yangtze River mouths were situated near the studied area and could have played major roles in the sedimentation therein. In Interval 2 （16.0-7.3 ka BP）, these river mouths gradually retreated with global sea-level rise, leading to less fluvial inputs from them to the northern Okinawa Trough. Meanwhile, formation of the TWC could carry some sediment loads of Taiwan to the studied core, especially during its late phase （8.0-7.3 ka BP）. Modem oceanographic conditions, with a predominance of the TWC, were finally established since the beginning of Interval 3, causing more ten＇igenous contribution from Taiwan to the studied area. Subsequently, modem depositional environments mainly influenced by the Yellow River, the Yangtze River, and the TWC were finally formed.

Detrital Zircon Geochronology of the Radial Sand Ridge System of Jiangsu Coast, East China: Implication for Sediment Provenance

The radial sand ridge system （RSRS） located at Jiangsu coast of China attracts much attention on its origin and mechanic of formation for its special structure and potential land resource. Due to complicated hydrodynamic condition, the Jiangsu RSRS is a hot debated on its potential sources, Yangtze River or Yellow River？ We collected ten sand samples from surface sediments along the west coast of Bohai Sea and Yellow Sea from the modern Yellow River estuary to Yangtze River estuary in summer, 2013. The samples are analyzed by method of detrital zircon age for source identification of the RSRS sediments. The U-Pb age spectra of detrital zircon grains of the samples show a wide range from Cenozoic to Late Archean with several age peaks. Comparing the age spectra between the Yangtze River and the Yellow River, the detrital zircons have younger age （〈100 Ma） group in the Yangtze River. These age distribution of the Jiangsu coastal RSRS sediments are similar to that of the Yangtze River, but different from the Yellow River. The samples located adjacent to the old Yellow River Delta show more wide-range age distribution, implying a compounded origination from the both rivers. Based on these findings it is proposed that, contrary to common opinion, the main sediment source of the Jiangsu RSRS is the Yangtze River, rather than the Yellow River. By implication, there should be evidence of hydrodynamic mechanics of oceanic currents and tidal motion. This aspect awaits confirmation in future research.

A preliminary reassessment of δEu_N-ΣREEs plot for distinguishing sediment provenances

Several REE data sets were collected in Core DD2 and EC2005 in the inner shelf of East China Sea,Core Y127 in the northern Okinawa Trough,Core NT1,C9247 and C92169 in the south Yellow Sea to check the applicability of the δEuN-ΣREEs model for quantita-tively distinguishing sediments from the Yangtze River and the Yellow River.By comparison,several sediments in cores DD2,EC2005,Y127 are generally in accordance with this model.The cores NT1,C9247 and C92169,which have been influenced by the Yangtze River and the Yellow River together,however,were inconsistent with the δEuN-ΣREEs model.The δEuN-ΣREEs plot and its corresponding regression equation,which could be used to distinguish sediments ＂intuitively and quantitatively＂ from the Yangtze River,and the Yellow River should be reconsidered or reassessed.

Clay minerals in Arctic Kongsfjorden surface sediments and their implications on provenance and paleoenvironmental change

Kongsfjorden is a typical fjord on the edge of the ice cap of the Arctic Svalbard-Barents Sea. Its inner bay is connected with a modern glacier front along the direction of the fjord axis with a significant gradient change in the parameters of hydrology, sedimentation, and biology. In summer, ice and snow melt-water and floating ice collapse continuously and thus transport the weathering products on the surrounding land into the sea. Thus Kongsfjorden is regards as a natural laboratory for the study of unique sedimentation in polar fjords under modern glacial-sea water conditions. In this study, fifty-two surface sediments were collected in Kongsfjorden for clay mineral analysis to study the sediment source and sediment-transport process. Our results indicate that clay minerals in the surface sediments from Kongsfjorden are mainly composed of illite, chlorite, and kaolinite, and no smectite is found. Rocks from different periods exposed extensively in the surrounding areas of Kongsfjorden provide an important material basis for clay minerals in the Kongsfjorden. Kaolinite may be mainly derived from the fluvial deposits, weathered from reddish sandstones and conglomerates during the Carboniferous Period.Illite is mainly derived from Proterozoic low-grade and medium-grade metamorphic phyllite, mica schist, and gneiss. While chlorite is mainly from Proterozoic low-grade metamorphic phyllite and mica schist. In the direction from the fluvio-glacial estuary to the sea of the glacier front of Kongsfjorden, illite increase gradually,and the content of kaolinite declines gradually. However, the change pattern of chlorite is insignificant, which may be related to the provenance. Kongsfjorden detritus is mainly transported by the fluvio-glacial streams and icebergs into the sea and deposited in the inner bay. Coarse sediments are rapidly deposited in the glacier front,estuary, and near-shore areas. Clay fraction begins to deposit significantly by 200–400 m after flowing into the sea,which due to the crystal behavior of clay minerals, hydrodynamic condition and flocculation. Kaolinite and chlorite on the south of the bay near the Blomstrandhalv？ya Island is mainly affected by ice-rafted detritus and thus can reveal the trajectory of transportation by the floating ice while entering the sea.

Provenance of Cored Sediments from Active Margin off Southwestern Taiwan Deduced from Geochemical Constraints

The cored sediments sampled by R/V Marion Dufrense in the active margin off southwestern Taiwan on the east side of the Manila Trench are mostly muds consisting mainly of quartz, feldspar, illite, chlorite ＋ kaolinite with a trace amount of montmorillonite. The cored sediments have higher Fe, Ti, Cr, Cs, Li, Ni, and V but lower Ca, Na, Mn, Hf, Nb, Sr, Ta, U and Zr contents when compared with the upper continental crust（UCC, Taylor and McLennan, 1985）. The Eu/Eu＊ ratios of the sediments averaging 0.69 indicate that the recycling effect on the Eu anomaly may be minor and the contribution of mafic source rock was unimportant, which can also be depicted in the La-Th-Sc plot（Cullers, 1994） showing a ＂mixed origin＂ close to the felsic end member. The chemistry of the cored sediment can be explained by a mixing model designed by Ho and Chen（1996） with four end members i.e., greywacke, shale, quartzite and limestone, which are widely distributed in the Cenozoic formations of southern Taiwan with greywacke as the dominant contributor followed by shale.

Provenance of Detrital Magnetites from Quaternary Sediments in the Yichang Area and its Significance to the Birth of the Three Gorges, Yangtze River

Objective In recent years, the birth time and evolution of the Three Gorges, Yangtze River has become a focused topic. Different from previous studies, this study used provenance analysis of Quaternary sediments to discuss this question. Among those minerals in Quaternary sediments, magnetite was rarely studied. This paper presents element geochemistry and backscatter images of detrital magnetites from the Quaternary sediments in the Yichang area of Hubei Province. By discussing the provenance changes of detratic magnetites, we suggested the birth time of the Three Gorges of the Yangtze River.

Tracing the Provenance of the Huguangyan Maar Lake Sediments in Coastal Regions of South China

The Huguangyan Maar Lake（HML）（21°9＇N,110°17＇E）,situated on the Leizhou Peninsula in the southernmost of China's Mainland,is the deepest recent crater lake among the identified volcanic structures in the Leiqiong Volcanic Field.The bi-lobate lake.

Geochemical and Grain-Sized Implications for Provenance Variations of the Central Yellow Sea Muddy Area Since the Middle Holocene

Based on high-resolution analysis to a 280-cm long sediment core obtained from the muddy area in the central Yellow Sea, we examined the provenance of muddy sediments and discussed the changing marine sedimentary environment since the middle Holocene. The results indicated that fine-grained sediments in the muddy area were mainly derived from the Huanghe(Yellow River) and Changjiang(Yangtze River) with considerable stepwise variations during the past 6.6 kyr. The Yellow Sea Warm Current was initiated at 6 kyr when the sea level was high together with the enhanced East Asian Winter Monsoon. These in combination established the framework of shelf circulation in the Yellow Sea that began to trap the river-derived fine-grained sediments. From 4.9 kyr to 2.8 kyr, both the Kushiro Current and East Asian Monsoon were significantly weakened, reducing the delivery of Changjiang sediments to the muddy area. As a result, the sediments were mainly originated from the Huanghe. From 2.8 kyr to 1.5 kyr the continuously weakened East Asian Winter Monsoon and enhanced Yellow Sea Warm Current entrapped more fine-grain sediments. Whereas the enhanced East Asian Winter Monsoon and the human caused increase in sediment load of the Huanghe since 1.5 kyr, and direct delivery of Huanghe sediments to the Yellow Sea during 1128–1855 AD might dominated the sedimentation in the study area. The stepwise variations of the sediment provenance and composition of the Central Yellow Sea muddy sediments are of importance to understanding the formation of muddy deposit in the central Yellow Sea and the associated variations of marine environment since the middle Holocene.

Geochemistry of Harappan potteries from Kalibangan and sediments in the Ghaggar River： Clues for a dying river

The ephemeral Ghaggar-Hakra River of northwestern India has always been considered to be the remnant of an ancient perennial glacier-fed river（Vedic Saraswati）. The exact reason and timing of major hydrological change of this river remains speculative. The river＇s purported association with the zenith of the Harappan civilisation remains a conjecture because the timings of its fluvial past are still being debated. In this study we have made an attempt to resolve this issue using geochemical provenance of sediments from some dated horizons in the Ghaggar flood plain and that of the material used in the potteries from the Mature Harappan period（4600-3900 yr BP） at Kalibangan. Sampled sedimentary horizons were dated by radiocarbon and optically stimulated luminescence（OSL） methods. Results of our study from the Ghaggar alluvium indicate that the river did have glacial sources during the early Holocene. However, the data from the potteries suggest that during the Mature Harappan period, the sediments in the Ghaggar as used by the potters did not have a higher Himalayan provenance and hence, were not derived from glaciated Himalayas.These findings imply that during the time of the Mature Harappans the Ghaggar had already become a foothill-fed river.

Response of Terrigenous Sediment Input to Sea Level Change and East Asian Monsoon Evolution Since 30kyr in the Southwestern Taiwan Basin

The compositions of grain size,clay minerals,and geochemical elements in core sediments(TS4)from the southwestern Taiwan Basin(South China Sea)were investigated to assess the response of terrigenous sediment input to sea level change and the East Asian monsoon evolution since 30 kyr.The chronology was discussed based on foraminiferal AMS14C dates.Our results indi-cated that rivers in Taiwan have been the major sediment contributors since 30 kyr,followed by the Pearl River;and Luzon Island contribute little sediments to the southwestern Taiwan Basin.In this study,we reconstructed the variations in terrigenous sediment input by using the proxies such as Al2O_(3)(%),F1 score,and TiO_(2)/CaO.The F1 score can be used to indicate the flux of terrigenous sediments.The contribution of Taiwan Residents rivers and the Pearl River were evaluated by using the value of(illite+chlorite)/kaolinite.The variations of Al2O_(3)(%),F1 score,and TiO_(2)/CaO values along the core were clearly correlated to the evolution of the East Asian summer monsoon since 30 kyr.Based on these records,we recognized three evolutionary stages of terrigenous sediment input in the southwestern Taiwan Basin.During stage Ⅰ(29-24kyr),the input of terrigenous sediments continued to increase.During stage Ⅱ(24-11.5 kyr),the sediment input decreased at first and then increased,with the lowest value during the last glacial period(21-17kyr).Terrigenous sediment input during stage Ⅲ(11.5 kyr-)showed the decreasing first and then increasing trends,generally higher than those in the first and second stages.The variations of terrigenous sediment input in the study area corresponded well with the evolu-tion of the East Asian summer monsoon since 30 kyr.Increased terrigenous sediment input during 4-1.8 kyr was suggested to be caused by the enhanced ENSO activity.Sea level change does not alter the overall trend of terrigenous sediment input,but does change the relative contributions of rivers in Taiwan and the Pearl River.

Composition, Source and Environmental Indication of Clay Minerals in Sediments from Mud Deposits in he Southern Weihai Offshore, Northwestern Shelf of the South Yellow Sea, China

Studying the sedimentary characteristics of the muddy area along the Yellow Sea shelf is of great significance for deep-ening the understanding of the climate and environment evolution since the last glacial period.Recently,a small muddy area developed in the southern Weihai offshore has attracted a lot of attention.Based on high-resolution grain size,clay mineral,AMS14C,and OSL data of sediments from the core WHZK01 in the muddy area,we acquired a new understanding of sedimentary characteristics,sources and paleoclimatic environment during the last 13 kyr.The results show that the core WHZK01 sediments were mainly from the Yellow River and local rivers along the coast.However,the sources varied for different deposition units.The riverbed(before 13 kyr BP)and fluvial plain deposits(13-10 kyr BP)were mainly from local river inputs.The very thin littoral deposits from 10 kyr BP to 8 kyr BP also mainly originate from coastal river inputs,while the littoral-neritic deposits since 8 kyr BP were dominated by the Yellow River materials and partly supplied by the coastal rivers and the island erosion.In addition,five events related with the strong East Asian winter monsoon(EAWM)during the intervals of 13.0-11.0 kyr BP,10.0-8.2 kyr BP,7.0-5.0 kyr BP,3.5-2.5 kyr BP,and 1.2-0.5 kyr BP were iden-tified.Three events related with the enhancement of the EAWM since the middle Holocene have strengthened the transport capacity of the coastal currents of the Shandong Peninsula.Meanwhile,more Yellow River-derived distal sediments were deposited to the southern Weihai offshore and formed wedge-shaped muddy bodies.In short,the ratio of smectite/illite could be used as an effective EAWM indicator since 13 kyr BP on the northwestern shelf of the South Yellow Sea.

Origin and geochemistry of surface sediments in the mud deposit area offshore the Shandong Peninsula,China

The waters offshore the Shandong Peninsula separate the North Yellow Sea from the South Yellow Sea,and receive a large amount of terrestrial material from Chinese and Korean rivers,making it an ideal area for studying land-sea interactions.However,little attention has been given to measuring sediment transportation in most previous studies.Based on an analysis in composition of major and trace elements and particle size characteristics from 62 surface sediment samples from the northeastern region off Shandong Peninsula,the type,element composition,and controlling factors of the surface sediments were investigated.In addition,the transportation of sediments from source to sink was described from measured thermohaline data.The results show two types of surface sediments:sandy silt and silt.The sediments were mainly terrestrial;and marine carbonate had little effect on sediment composition.Shown on a binary diagram of Rb/Sc to Co/Sc,the Huanghe(Yellow)and Changjiang(Yangtze)Rivers are the main sources of surface sediments in the study area.The component mixing model showed that the relative contribution of sediment from the Huanghe River was up to 92%,followed by the Changjiang River(8%).The Yellow Sea Warm Current and the North Shandong Peninsula Coastal Current met at∼37.7°N in the study area,and were the main forces carrying sediment from the two main river sources.However,there was a deficit of transported material into the study area in summer.The secondary distribution of sediment from the two river sources was controlled by tidal currents and waves.

One size does not fit all: Refining zircon provenance interpretations via integrated grain shape, geochronology, and Hf isotope analysis

Sediment provenance studies commonly utilize isotopic signatures to resolve detrital mineral sources and routing.However,non-unique ages and geochemical characteristics across geographically distinct crystalline source regions can lead to significant ambiguities in mineral provenance interpretations.Such ambiguity is apparent in southern Australia’s Cenozoic Eucla Basin,which hosts world-class heavy mineral sand resources.Here,new Hf isotope data are provided from four heavy mineral prospects(N=8,n=844[N=samples,n=grains]).Zircon grain shape data are also presented for a suite of detrital Eucla Basin samples(N=22,n=35,604)and the basin’s underlying basement,the Coompana Province(N=13,n=824).The data are integrated with published detrital and non-detrital primary zircon data to investigate the efficacy of grain shape analysis to better resolve the basin’s mineral provenance.Zircon Hf isotope compositions indicate a primary Mesoproterozoic juvenile source for zircon melts(~1250-1000 Ma,-2.5<εHf>~+5)with additional contributions from a range of juvenile to evolved late Archean to Phanerozoic-aged zircon bearing magmas(-28.0<εHf>+11).U-Pb geochronology and Hf isotopes are incapable of differentiating Mesoproterozoic-aged source rocks bounding the region for the majority of heavy mineral deposits analyzed as potential sources express overlapping crystallization ages and similarities in Hf-isotope characteristics.However,distinct zircon grain shapes(i.e.,perimeter,major axis and circularity)facilitate improved differentiation across these Mesoproterozoic sources.Filtering of U-Pb age,Hf isotope and shape data implicate the underlying Madura and Coompana provinces as dominant sediment sources for Eucla Basin detritus aged~1400-1000 Ma.The lack of direct sediment pathways between the underlying basement provinces and placer sediments analyzed demonstrates the significance of zircon reworking from intermediate sedimentary basins in the formation of the economically significant Eucla Basin beach placers.Zircon grain shape represents a cheaply acquired and readily incorporated grain characteristic that can enhance provenance investigations.

Composition, origin and weathering process of surface sediment in Kumtagh Desert, Northwest China

Kumtagh Desert is one of the eight biggest deserts in China, but poorly investigated before our interdisciplinary study because of the difficulty of access. In this paper, 33 representative surface sediment samples were collected from the Kumtagh Desert and analyzed in the laboratory to obtain heavy mineral components and geochemical element contents. Results show that various kinds of heavy minerals are present in these samples, with high levels of epidote and hornblende. Si and AI take up a large part of chemical composition. Compared with the average composition of geochemical elements of the upper continental crust （UCC）, except Si and Ca, all elements are depleted to a certain degree; Fe, Mg, Ca, P, Ti and Mn have high correlation coefficients in their contents. The mineral and geochemical composition of the Kumtagh Desert sediments have a similarity with that of rocks of Altyn Tagh Mountains, and the surface sediments of the alluvial/diluvial fans around the Altyn Tagh Mountains and that of the Taklamakan Desert, indicating that one major source of the Kumtagh Desert sediments is located in the Altyn Tagh Mountains. Alluvial deposits and lake sediments in Aqik valley and lower reaches of Shule River are prone to be eroded and transported by the strong northeasterly wind into the Kumtagh Desert, forming another source of the desert deposits. An A-CN-K ternary diagram shows that a weak degree chemical weathering by the loss of Na and K occurred in these sediments, whereas A-CNK-FM ternary diagram suggests that Fe and Mg have undergone a significant chemical differentiation. Physical weathering processes cause easy erosion and enrichment in fine particles for mafic minerals, thus coarse desert sand particles can be relatively depleted in Fe and Mg. The mineral and geochemical composition of sediments in arid regions experiencing less chemical weathering are mostly affected by physical weathering.

Evolution of Asian drying since 30 Ma revealed by clay minerals record in the West Pacific and its tectonic-climatic forcing

As the second largest dust source on the globe,the tectonic and climatic evolution of continental Asia has an important impact on regional and global climate change.The West Pacific is the main sediment sink for eolian dust transported eastward from the Asian interior.Clay minerals,as the major fine-grained weathering products of continental rocks,can be readily transported by wind or currents over long distances and thus have been widely used in the reconstruction of paleoclimate and weathering history.However,the overall evolutionary tendency and response mechanism of clay mineral records over large spatial and long timescales across Asia remain unclear.Here,two continuous and high-resolution clay mineral records since 30 Ma were reconstructed from sediments at Deep Sea Drilling Program(DSDP)Sites 292 and 296 in the Philippine Sea.Clay minerals and Sr-Nd isotope compositions were used to constrain provenance and reconstruct the drying history of the dust source region since the Oligocene.The results show that the clay-sized detrital sediments in the Philippine Sea are a mixture of Asian eolian dust and volcanic materials from the West Pacific arcs.Based on the clay mineral compositions and eolian flux,we reveal that the Asian interior has been continuously drying since the late Oligocene and that stepwise enhanced aridification occurred at approximately 20,14,7,and 3 Ma.Compared with other regions of the world,the relative contents of illite and chlorite increased more dramatically in Asia during the late Cenozoic,and the inconsistency became more obvious at approximately 20 Ma.Since 3 Ma,illite and chlorite have increased consistently across the globe.Combined with the Asian tectonic and climatic history,we suggest that the increase in illite and chlorite from Asia between 20 and 3 Ma was mainly in response to the uplift of the Himalayan-Tibetan Plateau,whereas after 3 Ma,it was primarily controlled by global cooling driven by the expansion of the Arctic ice sheet.