Late Cretaceous–Paleogene Indian monsoon climate vis-à-vis movement of the Indian plate, and the birth of the South Asian Monsoon
Graphical abstract
Introduction
The voyage of the Indian plate, after separation from Gondwana and before eventually colliding with Eurasia, took place over ~160 million years and covered a distance of ~9000 km (Chatterjee et al., 2013). Initially travelling at approximately 20 cm/year from the Late Cretaceous (67 Ma) to the early Eocene, India eventually slowed to ~5 cm/year after collision with the Eurasian plate (Chatterjee et al., 2013; Pusok and Stegman, 2020). A recent study suggests that the Réunion mantle plume played an important role in driving India's movement (Pusok and Stegman, 2020). During its journey from the Southern to the Northern Hemisphere, the Indian plate experienced a range of climatic conditions, due to secular (global) climate change influenced by oscillating insolation caused by fluctuating orbital configurations, continually varying ocean and land configurations, changes in atmospheric composition and India's latitudinal position (Chatterjee et al., 2013). Such a dynamic climate inevitably must have had a profound impact on the evolution, diversification and distribution of many life forms on the India raft (Wheeler et al., 2017; Kapur and Khosla, 2018).
This unique journey of the Indian plate as an isolated Island continent, makes it a natural laboratory to understand the relationship between climate and biota. In south Asia, proxy records from terrestrial and marine archives indicate the presence of a monsoonal climate during the Late Cretaceous and Paleogene (Ghosh et al., 1995; Srivastava et al., 2012; Licht et al., 2014; Shukla et al., 2014; Spicer et al., 2016, Spicer et al., 2017; Liu et al., 2017; Ghosh et al., 2018; Farnsworth et al., 2019), but the exact nature of such a monsoon remains poorly resolved. Previous work has shown that fossil leaves exhibit distinctive morphological trait combinations, which allow palaeoclimatic conditions to be reconstructed in detail and different monsoon types to be recognized (Yang et al., 2015; Spicer et al., 2016, Spicer et al., 2017). Tracking the climate of India as it journeyed across the equator, offers an opportunity to characterise low latitude climate during a period of exceptional global warmth, not unlike the condition to which we may now be headed. In particular, there is a need to understand better, terrestrial equatorial temperatures in the hyperthermal Paleogene, and associated monsoon dynamics.
A monsoon can be defined as the annual reversal of surface winds that generally produce marked seasonal contrasts in rainfall (Ramage, 1971; Wang et al., 2017) and today's low latitude monsoons are typified by rainy summers and dry winters. Monsoon circulation is a planetary phenomenon which can regionally be divided into eight domains: a South Asia Monsoon (SAM), East Asia Monsoon (EAM), Western North Pacific Monsoon (WNPM), Australian Monsoon (including Indonesia) (I-AM), North America Monsoon (NAmM), South America Monsoon (SAmM), North Africa Monsoon (NAfM) and a South Africa Monsoon (SAfM) (Wang et al., 2017). These monsoon domains are confined to the tropics except the EAM and SAmM, which penetrate to higher latitudes due to region-specific land-sea contrasts and distinctive topography. The planetary-scale phenomenon of global monsoon system is mainly due to the manifestation of the movement of the Intertropical Convergence Zone (ITCZ) and different domains of the monsoon are shaped by prevailing regional topography and land-ocean configurations. The ITCZ migrates meridionally towards the warmer hemisphere driven by the oscillations of seasonal insolation. The associated rainfall intensity depends on shifts in the atmospheric energy balance (Schneider et al., 2014).
The presence of a monsoon climate over India in the past does not necessarily mean it was the same as the modern SAM which dominates the Indian sub-continent today. For example, it seems that in the Paleogene leaf physiognomy across southern China was adapted to an Indonesian–Australian (I-AM) ITCZ type of monsoon system (Spicer et al., 2016, Spicer et al., 2017) and did not display features characteristic of today's EAM. To properly understand monsoon evolution affecting India we need relevant data throughout the Paleogene.
In this work, we reconstruct past climate, quantitatively, as experienced by different parts of India using fossil leaf morphological traits (physiognomy) from the latest Maastrichtian to the end of Paleogene. This reconstruction is intended to address the following questions:
- 1.
How did the climate of India change at different palaeolatitudes during its voyage from the Southern to the Northern Hemisphere?
- 2.
What were the changes in leaf physiognomy at different palaeolatitudes?
- 3.
Was leaf physiognomy adapted to a monsoon or non-monsoon climate during the voyage of the Indian plate?
- 4.
If a monsoon was present over India in the Paleogene what kind of monsoon was it?
Section snippets
Materials and methods
In this study, we investigate leaf physiognomy exhibited by five fossil leaf assemblages recovered from latest Maastrichtian to late Paleogene deposits at different locations across India (Fig. 1). We use plant megafossils because they cannot be transported far from their original growth site before deposition without relevant features being destroyed (Ferguson, 1985), so they represent the prevailing local climatic conditions at the time of their growth. They are also immune to diagenetic
Thermal regime
All the climate parameter reconstructions for all the studied sites are given in Table 3. The reconstructed temperature data show that the climate was warm (tropical–sub-tropical) at all the studied fossil sites. The CMMT varies from 16.3–21.3 °C with a standard deviation at each site of ±3.5 °C, while the maximum temperature during the coolest part of the year (Max_Temp_C) varies from 22 to 27 °C also with an uncertainty of ±3.5 °C. The estimated CMMTs for the Deccan Intertrappean beds and
Thermal regime
For all fossil assemblages the warm thermal regime indicated by CLAMP is consistent with the low palaeolatitudes and the palaeofloral compositions. However, in the context of the Paleogene being an interval of extreme global warmth, including numerous hyperthermal events (Lauretano et al., 2015; Anagnostou et al., 2016; Barnet et al., 2019), some of the temperatures for the Paleocene and Eocene appear surprisingly cool, being cooler than that of the late Oligocene. While the reconstructed
Conclusions
Fossil leaf traits from the Paleogene of India track changing climate as India traversed the tropics. A combination of CLAMP and CCA analysis of fossil leaf traits from assemblages spanning the Paleogene shows that India was exposed to an Indonesia-Australia type of monsoon dominated by seasonal migrations of the ITCZ. However, by combining temperature, rainfall, and humidity measures we infer that the Paleogene ITCZ seems to have migrated over a wider latitudinal range, or that the ITCZ was
Declaration of Competing Interest
None.
Acknowledgements
GS, HB and RCM are thankful to Dr. Vandana Prasad, Director of the Birbal Sahni Institute of Palaeosciences, Lucknow for providing necessary facilities and encouragement during the research work. MK, TH, MH and KR gratefully acknowledge the Department of Botany, Sidho-Kanho-Birsha University for providing infrastructural facilities to accomplish this work. SB acknowledges the Centre of Advanced Study (Phase-VII), Department of Botany, University of Calcutta for providing necessary facilities.
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