Drastic environmental changes are expected in the next 10s to 100s of years when atmospheric CO2 will attain levels that were last reached 65.5 to 23 million years ago.
To understand ongoing global warming and predict future environmental conditions, it is crucial to know how and when our world changed from the past Greenhouse state into the Icehouse state of today.
The prevailing hypothesis states that global cooling was caused by uplift of the Tibetan Plateau and land sea redistributions, following the onset of the Indo-Asia continental collision.
However, proving this far-sighted theory is upsetting the geosciences community because of the inability to establish unequivocal relationships between tectonism, global climate and major environmental and biodiversity changes in Asia such as continental aridification, and the intensification of Asian monsoons.
The interplay between the Indo-Asia collision, uplift of the Tibetean Plateau and climate belongs to te most significant and fascinating issues of tectonics and paleoclimate (Fig. 1). According to prevailing hypotheses supported by various tectonic and climate models the impact of the continental collision on climate is twofold:
(1) Globally, the orogenesis increases rock weathering and organic carbon burial which enhances consumption of atmospheric CO2 leading to Cenozoic global cooling. This mechanism, rather than the opening of a sea passage around Antarctica, is now believed to have pre-conditioned the Eocene-Oligocene transition (EOT), an abrupt cooling event associated with the onset of Antarctic ice-sheet formation 34.0 million years (Myrs) ago.
(2) Regionally, uplift of the Tibetan Plateau and the retreat of the Paratethys (an epicontinental sea formerly extending over Eurasia in the Paleogene) triggers dramatic aridification and cooling of continental Asia and the onset of the Asian monsoons.
Despite the profound implications of these hypotheses, the critical information required to testing them is still essentially lacking for the key Paleogene period, when these events are taking place.
In order to answer these questions, we have further diversified mutli-disciplinary approach, and applied it to three well-targeted research areas to provide an integrated tectonic and climatic dataset over the entire Indo-Asia collision system (Fig. 1).
We have developed a multi-disciplinary approach tailored for the research questions and geologic settings to be investigated in this new project. The aim is to constrain climatic and tectonic evolution independently during basin formation. Tectonic and paleo-environmental proxies are recorded within the basin strata that are calibrated to the astronomically tuned timescale using combined magnetostratigraphy and cyclostratigraphy. Tectonic events are recorded independently by thermochronologic analysis of the exhumation of neighbouring mountain ranges (Fig.2).
A complete record of magnetic polarity zones (=chrons) throughout the sections is obtained by high-resolution sampling down to the shortest chron duration. This improves our ability to (1) determine a perfect correlation between our sections and the geomagnetic polarity time scale and (2) recognize astronomically driven cyclicity (eccentricity, obliquity and precession) expressed in the sediments, which improves correlation to the timescale and provides additional time resolution. This is complemented by biostratigraphy from macro- and microfossils. Along with the age of sediments, magnetostratigraphy provides the timing and magnitude of tectonism through analysis of tectonic rotations, paleolatitude variations and changes in sediment accumulation rates.
Paleoenvironmental proxies are jointly gathered for paleoclimatic reconstruction and astronomical calibration of observed cyclicity. Sedimentologic facies analysis, grain-size, palynology, stable isotopes, biomarkers and any other proxies at hand are performed depending on depositional environments and fossil content, paleontologic analysis performed with collaborators.
Low temperature thermochronology (Apatite Fission Track and U-Th/He) are performed both on the basin-bounding mountain ranges and in the sediments drained from those ranges. These tools provide timing and rate of exhumation as well as lag time (=exhumation age – depositional age) and provenance related to basin formation during mountain uplift.
1. Northeastern Tibet: Exceptional paleoenvironmental records
The aim of this sub-project is to recognize how Paleogene Asian environment (e.g. biotic changes, monsoons, aridification and cooling) responded to regional tectonic forcing (sea retreat or Tibetan uplift associated to local and regional tectonism) and to Paleogene global climate and astronomical forcing parameters before and during the Eocene Oligocene Transition (EOT). The EOT is the most dramatic climate event since the onset of the collision of India and Asia but its cause remains uncertain.
Continental records of atmospheric changes during this major transition are still critically rare. Continental climate records are needed globally – especially in the Paleogene – to test climate hypotheses and models. In Asia however, poor time resolution of the few existing Paleogene records (with typical Myr uncertainty), precludes understanding of links between continental and oceanic climate during major episodes of global change. In contrast, the accurately dated climate proxy records that we build and extend in Northeastern Tibet (Fig. 3), provide the opportunity to solve outstanding issues:
- Is it associated to atmospheric cooling and/or to biotic events as suggested by floral changes and mammal turnovers of the ‘Mongolian remodelling’?
- Can the aridification be attributed to cooling of global (surface) ocean temperatures and reduction of moisture supply to continental interiors?
- Or, is aridification related to a large Paratethys retreat induced by glacio-eustatic sea level lowering driven by Antarctic glaciation (see next project)?
- Ultimately, we want to test the exciting possibility that global cooling was caused by the Tibetan uplift which in turn caused the aridification.
Fig. 3. Northeastern Tibetan Plateau: Sampling of cyclic records of Paleogene global climate trends and Tibetan uplift by Roderic Bosboom, Hemmo Abels and Huang Wentao.
2. Northwestern Tibet: Sea Retreat, Aridification and Global Climate Change.
The aim of this sub-project is to determine the timing, the cause and the effects of the westward retreat of the epicontinental sea formerly covering Eurasia in the Paleogene (Fig. 1).
Climate models suggest that the impact of this sea retreat on Asian environment (aridification and monsoons) was possibly more important than uplift of the Tibetan Plateau.
To understand what has governed Asian environments it is therefore necessary to accurately date and quantify this sea retreat.
This will enable to distinguish whether observed regional paleo-environmental changes can be associated to either sea retreat or Tibetan uplift. In addition, we want to understand the cause of the sea retreat. A logical hypothesis is that it is related to eustatic sea-level drop such as the ~60 m lowering at the EOT as we previously proposed as a possible explanation for Asian aridification at this time. However, this remains to be proven. Regression could be related to other Paleogene eustatic fluctuations or to tectonics. The sea may have been disconnected from oceans to the west by northward indentation along the Western Kunlun Shan during the Indo Asia collision.
To test these hypotheses we have scouted for the perfect record of the Paratethys retreat and found it in the remote region of the Western Tibetan Plateau (Fig. 4).
3. Southern Tibet: Paleolatitude, age and convergence of the Indo-Asia collision
Despite the profound and widespread impact of the Indo-Asia collision estimates for the age of the onset of the collision remain uncertain.T o complicate matters, it has been established that sediments usually yield paleomagnetic inclinations that are too shallow giving paleolatitude error up to ~3000 km. Fortunately, techniques now allow testing and correction of shallow inclinations.
Collision ages range from 70 to 35 Ma which equates to an uncertainty of several thousand kilometres in the magnitude of subsequent intra-continental shortening (Fig. 5). The relative positions of the Indian and Eurasian continents throughout the Cenozoic are well constrained by paleomagnetic poles and marine magnetic anomalies, but reconstructing the original geographic boundaries of these continents before being affected by collision and shortening is highly controversial. In principle, paleomagnetism can estimate paleolatitudes of terranes found on either side suture zone (Lhasa terrane vs. Tethyan Himalaya representing the northern extent of India). However, paleolatitude estimates for the southern Lhasa terrane range from 6–20°N (~1500 km uncertainty) based on sediments and volcanics loosely constrained in age. Paleolatitudes for the northern margin of India before suturing are based on very few datasets from Tethyan Himalayan sedimentary rocks.
We acquire large new datasets and apply and develop these correction techniques and carefully select reliable data to show that the evolution of India–Asia suturing can be vastly improved by analyzing large paleomagnetic datasets from well-dated sections and by using innovative processing techniques. By constraining latitudes to within 5° (~550 km) uncertainty, we can significantly improve the resolution and accuracy of estimating the age of initial India–Asia contact and the history and kinematics of intracontinental shortening.Results are associated to kinematic reconstructions as well as tomographic and seismic images to estimate the crustal and lithospheric material disappeared beneath Asia during subduction.
Ultimately, our data is used to discriminate hotly debated geodynamic models of collision and uplift such as eastward crustal flow, tectonic escape of lithospheric fragments in central Asia and in particular provide essential constraints for numerical models of the lithospheric collision integrated in this project.