Shifts in the Intertropical Convergence Zone, Himalayan exhumation, and late Cenozoic climate

Abstract

The Early Miocene (ca. 23-16 Ma) was the time of peak Himalayan exhumation and postulated mid-crustal channel fl ow in the High Himalayas, a tectonic confi guration not recognized before or after in the range. Previous models have correlated this rapid exhumation with the onset of the Asian monsoon, and/or suggested that it contributed ultimately to mid-Miocene global cooling via silicate weathering and/or organic carbon burial, and consequent atmospheric CO 2 drawdown. Based on published Pacifi c Ocean sedimentation records and India-Eurasia plate reconstructions, we compare the position of the Intertropical Convergence Zone (ITCZ) with the paleolatitude of the Himalayas, and show that these converged in the Early Miocene. Because the ITCZ is a zone of high precipitation, we suggest that this convergence was an important driver of Early Miocene High Himalayan exhumation and erosion. Continued northward motion of the Indian plate took the Himalayas north of the ITCZ, which migrated south through the remainder of the Miocene. High Himalayan exhumation and ductile shearing correspondingly decreased after the Early Miocene. This mechanism for increased Himalayan erosion is independent of the timing of the onset or enhancement of the Asian monsoon, which is debated. 25 20 15 This paper resulted from a chance meeting by the laser printer. We acknowledge the comments of James Baldini and Peter Sauer on an early draft of the manuscript, and the helpful review comments received.

Wet tropical climate in SE Tibet during the Late Eocene

“…Modeling studies also suggested that ITCZ migration extended poleward in the Eocene 42 . In a similar Early Miocene’s synopsis, some authors proposed that the ITCZ paleolatitude was possibly more northerly posited (over SE Asia) at the Oligocene–Miocene boundary 45 . According to these authors, the Mi-1 glaciation at the Oligocene–Miocene boundary would have cooled the Southern Hemisphere relative to the « ice-free » Northern Hemisphere, leading to a shift of more northern (equatorial) peak sea-surface temperatures and a northward drift of the ITCZ.…”

“…According to these authors, the Mi-1 glaciation at the Oligocene–Miocene boundary would have cooled the Southern Hemisphere relative to the « ice-free » Northern Hemisphere, leading to a shift of more northern (equatorial) peak sea-surface temperatures and a northward drift of the ITCZ. The scenario proposed for the Oligocene-Miocene boundary 45 may be tentatively explored for the LED in SE Tibet. Cooling of the austral subpolar regions and/or ice sheet formation in Antarctica – which may have occurred as early as around 36.5 Ma in the Weddell Sea 46 – would cool the Southern Hemisphere, mimicking the seasonal pattern during the austral winter and forcing the ITCZ to shift northwards over monsoonal Asia and SE Tibet.…”

Eocene monsoons

“…Little emphasis has been put on these features in the pre-Quaternary (excepting for example: Greenwood, 1996;Herold et al, 2011;Fricke et al, 2009), so we attempt to address these unexplored avenues using Eocene conditions. One related question that has been asked in the warm climates of the pre-Quaternary, but not answered, is the degree to which the ITCZ's location and persistence might have been different (Pettke et al, 2002;Ziegler et al, 2003;Gallagher et al, 2004;Lyle et al, 2008;Armstrong and Allen, 2011). As described further below, the global monsoon system and the IAM in particular are impossible to separate from the strength, meridional extent, intensity, and seasonal shifts of the ITCZ.…”

Deep ocean carbonate ion increase during mid Miocene CO2 decline

“…S2e). Although dating is not yet able to precisely constrain the timings of all these events, if enhanced silicate weathering and erosional deposition from the Himalaya was responsible for global cooling during the mid Miocene, as has been previously suggested153342, then an increase in ocean ALK and hence [CO 3 2− ] would be expected to be coeval. Other possible terrestrial sources of ALK were the Tethys Ocean, which underwent uplift and intermittent closure in the mid Miocene48, and the East Africa Plateau which also experienced peak uplift in the mid Miocene49 (Supplementary Fig.…”