Low palaeoelevation of the northern Lhasa terrane during late Eocene: Fossil foraminifera and stable isotope evidence from the Gerze Basin

The Lhasa terrane is a key region for understanding the paleoelevation of the southern Tibetan Plateau after India-Asia collision. The Gerze Basin, located in the northern part of the Lhasa terrane, is a shortening-related basin. We discovered Lagena laevis (Bandy) fossils in upper Eocene strata of the Gerze Basin. This type of foraminifera is associated with lagoon and estuarine environments, indicating that the northern part of the Lhasa terrane was near sea level during the late Eocene. We speculate that these foraminifera were transported inland by storm surges to low elevation freshwater lakes during times of marine transgressions. This inference is consistent with the relatively positive δ18O values in carbonate from the same deposits that indicate low palaeoelevations close to sea level. Considering the palaeoelevation results from the nearby Oligocene basins at a similar latitude and the volcanic history of the Lhasa terrane, we infer that large-magnitude surface uplift of the northern Lhasa terrane occurred between late Eocene and late Oligocene time.

Scientific RepoRts | 6:27508 | DOI: 10.1038/srep27508 a detailed summary of 121 geological maps (1:250000) of the Tibetan Plateau area, and they estimated that the average palaeoelevation in central Tibet was ~500 m at ca. 34 Ma.
Based on conflicting evidence for high paleoelevations from sparse stable isotope studies and low paleoevelations from fossil, stratigraphic, and thermochronology/thermal modelling studies, it remains unclear whether the Lhasa terrane was at a high elevation at and following the collision of the Indian and Eurasian plates. To determine the paleoelevation history of the Lhasa terrane, we collected both fossil and stable isotope data from a late Eocene section in the Gerze Basin. We provide age information for these deposits, based on magnetostratigraphy with additional age constraints from fossils and zircon U-Pb dates.

Results
Lithostratigraphy. Eocene strata in the northern Lhasa terrane are defined as the Dingqinghu Formation.
This study is focused on the upper part of the Dingqinghu Formation, located at 31°54.655′ N, 85°7.693′ E (4618 m), with a thickness of 330 m (Fig. 2a). The base of the section is marked by a thick layer of conglomerate, and the top of the section is the core of a syncline. The lower part (layers 1-26) of the section consists of mediumto thickly bedded brown conglomerate, pebbly sandstone, and cross-stratified sandstone. Occasionally interbedded with clastic deposits are thick beds of grey bioclastic limestone that contain freshwater ostracods, gastropods and charophyte fossils. These facies suggest that the depositional environment for the lower part of the section was lacustrine to marginal lacustrine. The upper part (layers 27-86) mainly consists of thin-medium beds of grey limestone intercalated with grey shale and two pebbly sandstone layers (<20 cm thick). These facies suggest that the depositional environment of the upper part of the section was a stable shallow lacustrine environment. The bedding thins upward in the section, suggesting a transition to deeper water environments. Chronostratigraphy. We use three different methods to constrain the ages of the investigated section: detrital zircon U-Pb ages, index fossils and magnetostratigraphy. We collected three detrital zircon samples from layers 1, 12, and 25, which in total yielded 198 analysis points for the selected zircon grains. The three youngest detrital zircon grains yielded a mean age of ca. 42 Ma (Fig. 2b), and the youngest zircon population provides the maximum depositional age for our section.
Previous research has shown that the ostracoda genus Austrocypris is an important index fossil that was short-lived in the late Eocene 11,12 . This genus is widely observed in the lower member of the late Eocene Xiaganchaigou Formation in the Qaidam Basin [11][12][13] and in the fourth member of the Shahejie Formation in the Dongpu depression 14 and Bohai Sea Basin in eastern China 15 . Their extinction may correspond with the Eocene-Oligocene climate transition that occurred at ca. 34 Ma 12,16 . This ostracoda is observed in our study section between layer 8 and layer 76 and can be used for regional stratigraphic correlation to bracket the timing of events. Moreover, Gyrogona qianjiangica, which is another species of chara, is an index fossil of the late Eocene Xiaganchaigou Formation 13 that occurs from layer 9 to layer 83.
Based on the above late Eocene fossil age constraints, the age of this section is bracketed between 42 Ma and 34 Ma. A total of 197 oriented palaeomagnetic samples were collected in our section to acquire high-resolution  Foraminifera fossils. We found a foraminifera fossil shell in layer 2 and five foraminifera fossil shells in layer 23 (Fig. 2c). The lithology of layer 2 is purple-red medium-thick bedded laminated siltstone, whereas the lithology of layer 23 is purple-red thinly bedded massive mudstone.
These fossils are well preserved with obvious identifying characteristics, indicating that they are the same species. The fossils are globular, ovoid, and widest just below the middle section and presented circular cross sections. Additionally, they have smooth rounded bases, short, round, tapering necks approximately one-ninth the length of the body, and smooth, finely perforated walls. The plesiotype length is 0.23 mm, and the diameter is 0.20 mm. The elemental composition was detected via an energy dispersion spectrometry analysis, and the results indicate that they are mainly calcareous (Fig. 2c). Based on the neck shape, clear-walled burrow structure and standard calcareous composition, these fossils are identified as Lagena laevis Bandy. This species belongs to the Lagena genus, Nodosariacea family, and Rotaliina order and has been found in the Eocene Oujiang Formation in the East China Sea shelf and late Eocene strata in Alabama, USA 18 . Lagena laevis Bandy is a benthic 19 warm water foraminifera species that occupies a narrow range of salinities and can adapt to a strong hydrodynamic environment. This organism occurs in estuaries or coastal bay sedimentary environments and has been reported in offshore gulfs around the world 20-23 . Oxygen isotope paleoelevation reconstructions. 126 limestone samples were collected for oxygen isotope analysis to reconstruct palaeoelevation. All of the carbonate results are reported in δ 18 O c (VPDB-Vienna PeeDee belemnite). Evaporation during deposition and post-depositional diagenesis are two major factors that can influence oxygen isotope values, resulting in biased paleoelevation reconstructions, and thus we carry out a detailed evaluation of these factors. Analysis of diagenetic phases provides insights into whether the δ 18 O c values of the primary micritic carbonates have been modified 24 . Thin section analyses show that the lacustrine carbonates from layers 1-30 of our section contain significant (>50%) coarse-grained spar that is characterized by extensive dissolution cavities filled with sparry calcite (Fig. 3a,b). However, the micrites from layers 31-86 of our section generally preserve primary pore spaces (Fig. 3c) that are usually filled with fine-grained blocky calcite textures associated with early diaganesis, when pore fluids reflect surface water compositions and temperatures. Some samples show vein calcite (Fig. 3d). To determine the possible influence of veins and coarse-grained blocky calcite that filled dissolution cavities, primary micritic carbonate and late stage diagenetic phases were sampled  Supplementary Table S3).
In addition to diagenesis, evaporation is another factor that can influence the δ 18 O values of lake water and carbonate. In a hydrologically open system, oxygen isotopes are less likely to be affected by evaporation. δ 13 C and δ 18 O values that present variation coefficients of r > 0.7 are considered to indicate a closed lake system or long-term residence times 25 . Lacustrine micrite from layers 31-86 show poor correlations between δ 18 O c and δ 13 C c values (r = 0.0173) (Fig. 3e), which implies a hydrologically open condition. Sparite from layers 1-30 shows a distinct isotopic composition from micrite, indicating that micrite is a faithful recorder of the primary isotopic composition of carbonates. The very negative δ 18 O c end-member of about − 20‰ for sparite indicates that these diagenetic carbonates either precipitated from meteoric water with a low δ 18 O value, consistent with modern high altitude precipitation, or from waters of similar composition to what precipitated the micrites, but at burial temperatures higher than surface temperature 24 .

Discussion
Paleogeography inferences from foraminifera. Although foraminifera fossils mainly occur in marine strata, they are also distributed in Late Quaternary strata located hundreds of kilometres away from current shorelines associated with marine transgressions 26,27 . Inland basins that have little contact with the ocean may also contain a small amount of foraminifera 28,29 associated either spatially or temporally with marine sources. The airborne colonization of these inland water bodies via migratory birds has been suggested by previous research, although this type of colonization is occasional and does not result in selectivity of species. The foraminifera fossils from our study section all belong to the same species and have a relatively high abundance. This species has a nearly hollow spherical shape, and its selectivity is likely related to the large specific surface area of its structure, which facilitates its transport during storm surges. Research has also shown that foraminifera from older marine sources can adapt to and thrive in suitable inland saltwater environments 30 . However, in the investigated section, the underlying and overlying lacustrine limestone strata contain rich chara and lacustrine ostracoda and represent clear and shallow lacustrine environments. Additionally, this species of foraminifera has not been previously reported in older marine strata across the Lhasa terrane and thus is unlikely to have survived from older marine sources.
The Lagena laevis Bandy foraminifera could have been transported hundreds of kilometres inland from the ocean by marine transgression or by large storms if a passageway was available 31 . Two possible passageways include the fluvial volcanic depression through Gangdese volcanic arc, which is connected to the Himalayan Sea in southern Tibet, and the channels that occur through the coastal-alluvial plain connected to Sumxi Bay of the Pamir Sea in western Tibet (Fig. 1) 32 . Both the Himalayan Sea and Pamir Sea belong to the Neo-Tethys Sea.
(1) Passageway from the Himalayan Sea. The Palaeogene Himalayan seaway in Tibet was an east-west trending remnant sea that covered southern Tibet and the Gangdese areas. Burrard 33 and Douville et al. 34 discovered Palaeogene Himalayan marine strata in Gamba and Tingri (Fig. 1), and researchers have also discovered many foraminifera, ostracoda and chara fossils in these areas and conducted further studies of the marine strata 35,36 . Previous research has shown that the Zongpubei Formation in Gamba and Tingri contain the youngest documented marine strata 37,38 . The Zongpubei Formation is dated to the early Priabonian stage of the Eocene (ca. 35 Ma) and in both areas yields larger foraminifera, including Nummulites willcoxi and Nummulites manila, and other smaller foraminifera. The strata at the top of the Zongpubei Formation in these areas are cut by faults and are sparsely outcropping. However, the deposition in southern Tibet was stable during the Eocene 37,38 , thus, the latest time of regression may have been after the early Priabonian stage of the late Eocene. Furthermore, float rocks that contain rich foraminifera have been found in both Niumagou and Zhongba, which are located to the north of Lhasa City. The foraminifera are identified as Keramosphaera tergestina (Stache), which only lived during the Palaeocene-Eocene 39 . Although the fossil positions are not clear, seawater certainly reached the area near Lhasa City during the early Tertiary. Deltaic deposits have been documented in several Eocene stratigraphic sections along the southern margin of the Gangdese volcanic arc 32 , indicating a fluvial drainage basin sourced in the Gandese volcanic arc. This palaeogeography could have enabled the foraminifera to be transported through channels to be deposited in the Gerze Basin during transgressions and/or extreme storms if the Gerze Basin and the fluvial system was located at a low elevation. However, previous research suggested that portions of the magmatic arc were at high elevation at this time 9 , which seems inconsistent with a low elevation fluvial volcanic depression through the Gangdese volcanic arc.  (Fig. 1). We infer that rivers connected the Sumxi Bay to inland lakes, similar to the Yangtze River in eastern China and the Mississippi River in the USA. These channels may have enabled eastward intrusions of seawater during large transgression events, which could have transported the foraminifera Lagena laevis Bandy from an estuary environment to the Gerze Basin on the northern margin of the Lhasa terrane. This process requires that this region was located at low paleoelevation to allow for occasional intrusion if marine water containing foraminifera.

Evaluation of palaeoelevation.
Based on the discovery of foraminifera fossils, the Gerze Basin appears to have been close to sea level during the late Eocene (ca. 39 Ma). Oxygen isotopes from lacustrine micrites enable evaluation of the paleoelevation of this region. Comparisons between the δ 18 O values of modern surface water and palaeo-surface waters can be used to evaluate temporal changes in the surface elevation and climate 44 . Study of modern surface water and air mass trajectories across the Tibetan Plateau shows that most of the moisture that reaches the southern plateau (Lhasa terrane and the area to the south) is derived from a southern source region 45 . The δ 18 O values of modern surface water in the southern plateau, including the Lhasa terrane, conform well with a simple Rayleigh distillation model of vapor mass depletion in 18 O, indicating that stable isotopes of meteoric water provide reliable reconstructions of palaeoelevation 44 . We assume that 25 ± 10 °C reflects a reasonable temperature uncertainty for lacustrine carbonate precipitation that could account for the overall warmer Eocene conditions and low latitude 46 , even at moderate to high paleoelevations. We then used the mean δ 18 O c (VPDB) of lacustrine micrites of − 5.1 ± 1.1‰ (1σ ) and the temperature-dependent fraction equation between water and calcite 47 to calculate the δ 18 O psw (psw -paleo-surface water) relative to SMOW -Standard Mean Ocean Water of − 2.8 ± 1.7‰. The 1σ uncertainty in this δ 18 O psw estimate is determine by a bootstrap Monte Carlo approach that propagates errors associated with uncertainty in the temperature of carbonate precipitation, error in the temperature-fractionation equation 47 , and scatter in the δ 18 O values of lacustrine micrite. Because of global cooling and the build-up of the continental ice sheets, the oxygen isotope value of the sea water has changed by approximately + 1‰ between the Eocene and the present day 48 . For comparison to modern meteoric water, the δ 18 O psw value is therefore corrected to −1.8 ± 1.7‰. We compared this value of δ 18 O psw to modern meteoric water isotope results (annual average weighted by precipitation amount) of the GNIP (Global Network of Isotopes in Precipitation) sites in Asia 49 . Considering the mean δ 18 O psw of −1.8 ± 1.7‰, with values as low as − 3.5‰ at 1σ , these values are observed in modern rainfall that occurred at 50 ± 600 m asl (Fig. 4). The high δ 18 O psw values in Eocene deposits of the northern Lhasa terrane are therefore consistent with fossil foraminifera results, which both indicate low paleoelevations, close to sea level. This low palaeoelevation inference agrees with sedimentary characteristics and the overall pattern of deposition and erosion of the Tibetan Plateau during the Eocene 3 (Fig. 5a).
The Nima Basin and Lunpola Basin are also located to the south of the Bangong-Nujiang orogenic belt at approximately the same latitude as the Gerze Basin. According to previous palaeoelevation study results, in the late Oligocene (~26 Ma), the Nima Basin had reached approximately 4,000 m 5 and the Lunpola basin had reached 3,190 ± 100 m 50,51 . Combining these results with the results of our study suggests that the northern Lhasa terrane experienced surface uplift of at least 3,000 m over approximately 10 million years from the late Eocene to the late Oligocene.
Evidence from low temperature thermochronometers and the history of magmatism in the Lhasa terrane are consistent with a low paleoelevation of the northern Lhasa terrane in the late Eocene. Thermal modelling based on the (U-Th)/He ages of apatite and zircon, as well as apatite fission track data from the northern Lhasa terrane indicate that a peneplain formed at least 3-4 km beneath its current elevation of ~5300 m 10 . The formation of this low elevation peneplain was completed by ca. 50 Ma, and the resistant bedrock surface has undergone very slow erosion since then. Zircon age records [52][53][54] indicate an approximately 10 million-year period of magmatic quiescence in the Lhasa terrane between ca. 40-30 Ma with a significant magmatic event occurring after ca. 30 Ma (Fig. 5b(1)). Widespread magmatism initiated with adakitic rocks by ~26 Ma, followed by potassic-ultrapotassic rocks by ~24 Ma 55 . The chemistry of these magmas has been attributed to break-off the Indian continental lithosphere that had subducted beneath the Lhasa terrane and/or convective removal of the Tibetan lithospheric mantle 55,56 . Ninety-six apatite fission track ages collected from the Lhasa terrane indicate three cooling age peaks at 22-15, 10-7, and 5-0 Ma, whereas the Palaeogene ages are more scattered 57 (Fig. 5b(2)). Magmatic zircon ages and apatite fission track ages combined suggest minimal exhumation of the Lhasa terrane before the Oligocene. Additionally, widespread unconformities and related basal-conglomerates are dated at ca. 34 Ma, and alluvial fan and fluvial conglomerates resulting from rapid erosion are documented in the peripheral areas of the plateau, such as the Shimagou, Huoshaogou and Tala Formations along the north of the plateau 3 .  Fossil foraminifera and oxygen isotope studies of Eocene Gerze basin lacustrine deposits indicate that the northern Lhasa terrane resided at paleoelevations close to sea level between ~39 to 36 Ma. In combination with results from the Nima basin and Lunpola basin that indicate high paleolevations of ~4000 m by 26 Ma, we infer >3 km surface uplift of the northern Lhasa terrane between 36 Ma and 26 Ma. The temporal association between this large magnitude surface uplift and widespread adakitic and potassic-ultrapostassic magmatism across the Lhasa terrane after ~30 Ma suggests that this surface uplift event was associated with break-off Indian continental lithosphere and/or convective removal of the lithospheric mantle beneath the Lhasa terrane.

Methods
Detrital zircon age. A total of 198 analysis points of selected zircon grains were analysed using a Neptune MC-ICP-MS (multicollector inductively coupled plasma mass spectrometer) (Thermo Fisher, Lafayette, Colorado, USA) with a UP193-FX ArF-excimer laser-ablation system (ESI, Sunnyvale, California, USA) at the Isotope Laboratory of Tianjin Center, China Geological Survey (CGS). Data were collected with a 35 μ m spot and the ablation depth was 20-40 μ m. Zircon 91500 was the external standard used for age calculation, NIST SRM 610 silica glass was the external standard used for calculation of concentrations, and Si was the internal standard. For ages younger than 1,000 Myr ago, the discordance is defined as ( 207 Pb/ 235 U-206 Pb/ 238 U)/ 207 Pb/ 235 U*100 and 206 Pb/ 238 U ages were adopted for them. All the ages of the three zircon grains in Fig. 2b present a low discordance degree (<10%).

Magnetostratigraphy.
A total of 197 oriented palaeomagnetic samples were collected in the field to acquire high-resolution magnetostratigraphy data. Because most of the palaeomagnetic samples originated from limestone, which contains small amounts of magnetic minerals, we used different methods to conduct the magnetism tests. Samples with NRM (natural remanent magnetization) ≥10 −5 T were subjected to stepwise (averaging 17 steps) thermal demagnetization using a Thermal Demagnetizer

Microfossils.
We collected 183 bulk samples for a microfossil analysis from the study section. A 15% solution of H 2 O 2 was used for the sample preparation of microfossils from mudstone and siltstone. The heating-acid-digestion method with pure acetic acid (>99.5%) was used for the sample preparation of microfossils from limestone. More than 1300 ostracoda fossils and 6 foraminifera fossils were handpicked and sorted. We selected well-preserved specimens for photography using a scanning electronic microscope. An energy dispersion spectrometry analysis of the foraminifera fossils was performed at the State Key Laboratory of Geological Processes and Mineral Resources of the China University of Geoscience.
Oxygen isotope. We collected 126 limestones samples from the bottom to the top of the cross section, and they were generally evenly distributed. The δ 18 O data are reported with respect to the V-PDB standard. Analyses of the stable isotope were performed at the SIREAL (Stable Isotope Ratios in the environment, Analytical Laboratory) of the University of Rochester using a "Delta Plus XP" mass spectrometer (ThermoFinnigan, San Jose, California, USA). δ 18 O psw can be calculated from the δ 18 O c , which was fractionated from paleo-waters. The fractionation factor (α ) is defined as: α (calcite-water) = (1000 + δ 18  We assume 25 ± 10 °C as a reasonable lacustrine carbonate precipitation temperature to account for overall warmer conditions, the low latitude position of southern Tibet in Eocene 59 .