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 changes in Asia such as continental aridification and the intensification of the monsoons in Asia.
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. lWe previously provided evidence for regional aridification on the Tibetan Plateau, precisely dated at the Eocene Oligocene Transition (EOT). This remarkable correlation demonstrated that global climate, and not only Tibetan uplift and Paratethys retreat, must be recognized as a major contributor to Asian palaeoenvironment. Our study illustrated that distinguishing climate effects from tectonism can be accomplished using the multidisciplinary approach developed in Utrecht during my Veni (2005-2008). Building on this work, I have now identified outstanding key questions that remain to be answered in the Paleogene.
Fig. 1. The Paleogene world. In 3 target areas (red boxes) westudy links between (a) global cooling, (b) northward indentation of India into Asia, (c) uplift of the Tibetan Plateau, (d) westward retreat of the Paratethys sea over Eurasia, (e) onset of Asian monsoons, and (f) Asian aridification.
What triggered aridification, cooling and monsoon intensification?
How and when are these changes associated to Tibetan uplift, Paratethys retreat and/or global climate changes? What are the timing, cause and environmental impacts of the Paratethys retreat?
What is the precise timing of the Indo-Asia collision and how was continental shortening accommodated by continental deformation?
During my Veni, I 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).
Fig. 2: Our multi-disciplinary approach constrains tectonic and climatic events recorded within a basin during orogenesis.
Magnetostratigraphy and cyclo-stratigraphy is performed to obtain a chronostratigraphic framework with a resolution down to the precession cycle (~20 kyr). 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. 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. Grain-size analysis, palynologic analyses are performed and depending on environment and fossil content, paleontologic analysis of macro- and microfossils, such as ostracods and/or foraminifera, is performed in collaboration with various collabortive institutes. Complementing sedimentologic facies analysis, major element and stable isotope geochemistry, Scanning Electron Microscopy (SEM), colour reflectance, susceptibility and rock magnetic properties are processed.
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) related to basin formation during mountain uplift.
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.
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).
TheThe EOT is the most dramatic climate event since the onset of the collision of India and Asia but its cause remains uncertain. However, continental records of athmospheric changes during this major transition are still essentially lacking. 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.
TheIn contrast, the accurately dated climate proxy records that we propose to build and extend in Northeastern Tibet (Fig. 3), provide the opportunity to solve outstanding issues.
The- Is it associated to atmospheric cooling and/or to biotic events as suggested by floral changes and mammal turnovers of the 'Mongolian remodelling'?
The- Can the aridification be attributed to cooling of global (surface) ocean temperatures and reduction of moisture supply to continental interiors?
The- Or, is aridification related to a large Paratethys retreat induced by glacio-eustatic sea level lowering driven by Antarctic glaciation (see next project 2)?
TheUltimately, we want to test the exciting possibility that global cooling was caused by the Tibetan uplift which in turn caused the aridification.
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).
￼Fig. 4. Left: Location of targeted sedimentary sections on geologic map of Northwestern Tibet. Right: Field view of lower part of one of the targeted sections (road for scale). Inset shows fossil from marine horizons.
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.
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).
MAGIC : Monsoons in Asia caused Greenhouse to Icehouse Cooling
DSP-TIBET- ANR : Probing Deep and Surface Processes in Central Tibet
Francois Chirouze (Grenoble University) PhD thesis finished January 2011 "Contrôles tectonique et climatique du drainage himalayen et son évolution depuis 15MA".