Cyclone trends constrain monsoon variability during late Oligocene sea level highstands (Kachchh Basin, NW India)

Abstract

Climate change has an unknown impact on tropical cyclones and the Asian monsoon. Herein we present a sequence of fossil shell beds from the shallow-marine Maniyara Fort Formation (Kachcch Basin) as a recorder of tropical cyclone activity along the NW Indian coast during the late Oligocene warming period (~ 27–24 Ma). Proxy data providing information about the atmospheric circulation dynamics over the Indian subcontinent at this time are important since it corresponds to a major climate reorganization in Asia that ends up with the establishment of the modern Asian monsoon system at the Oligocene–Miocene boundary. The vast shell concentrations are comprised of a mixture of parautochthonous and allochthonous assemblages indicating storm-generated sediment transport from deeper to shallow water during third-order sea level highstands. Three distinct skeletal assemblages were distinguished, each recording a relative storm wave base. (1) A shallow storm wave base is shown by nearshore molluscs, reef corals and <i>Clypeaster</i> echinoids; (2) an intermediate storm wave base depth is indicated by lepidocyclinid foraminifers, <i>Eupatagus</i> echinoids and corallinacean algae; and (3) a deep storm wave base is represented by an <i>Amussiopecten</i> bivalve-<i>Schizaster</i> echinoid assemblage. These wave base depth estimates were used for the reconstruction of long-term tropical storm intensity during the late Oligocene. The development and intensification of cyclones over the recent Arabian Sea is primarily limited by the atmospheric monsoon circulation and strength of the associated vertical wind shear. Therefore, since the topographic boundary conditions for the Indian monsoon already existed in the late Oligocene, the reconstructed long-term cyclone trends were interpreted to reflect monsoon variability during the initiation of the Asian monsoon system. Our results imply an active monsoon over the Eastern Tethys at ~ 26 Ma followed by a period of monsoon weakening during the peak of the late Oligocene global warming (~ 24 Ma)

Nummulitids, lepidocyclinids and Sr-isotope data from the Oligocene of Kutch (western India) with chronostratigraphic and paleobiogeographic evaluations

“…In our opinion this species is identical with the one that was mentioned in the previous literature (Biswas, 1992;Reuter et al, 2013) as Operculina complanata, which, however, was neither described nor illustrated. Most probably the internal morphology of these forms has never been studied until now.…”

“…All the rare radiate Nummulites present in our samples belong to N. kecskemetii, although they were traditionally determined as Operculina complanata (Biswas, 1992;Reuter et al, 2013 Nummulites clipeus n. sp. -Nuttall, 1925, pp.…”

“…Moreover, the Oligocene LBF of the Kutch Basin were mostly determined typologically (with the exception of Nephrolepidina, studied by van Vessem, 1978and Saraswati, 1994, while the most widely used LBF biostratigraphic schemes are based on morphometrically defined chrono-species (see Pignatti & Papazzoni, 2017). Reuter, Piller, Harzhauser, and Kroh (2013) recently assigned the Coral Limestone to the early Chattian SBZ 22B Zone and the Bermoti Member to the late Chattian SBZ 23 Zone. However, they provided only a list of LBF with neither description nor illustration.…”

“…genera, Heterostegina and Spiroclypeus, differing from each other in the absence or presence of lateral chamberlets, respectively. Genus Operculina, widely reported from the Oligocene of Kutch (Biswas, 1992;Reuter et al, 2013), has not been found.…”

Stepwise Onset of the Icehouse World and Its Impact on Oligo- Miocene Central Asian Mammals

“…This cool phase was followed by the global Late Oligocene Warming (LOW), which was interrupted only by the minor isotope event Oi2c. Though widely recognized in various marine and continental settings 13,[18][19][20] , the cause of this warming is still under debate and in Antarctica no warming occurred at all during the late Oligocene 21 . The LOW was rapidly ended by the Mi1 glacial 22 , marking…”