Abstract
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 Clypeaster echinoids; (2) an intermediate storm wave base depth is indicated by lepidocyclinid foraminifers, Eupatagus echinoids and corallinacean algae; and (3) a deep storm wave base is represented by an Amussiopecten bivalve-Schizaster 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).
Highlights
Ocean ScienceCyclones are one of the most devastating weather phenomena that affect tropical coasts (Henderson-Sellers et al, 1998)
The Maniyara Fort Formation in the Kachcch Basin (NW India) comprises a succession of vast shell beds composed of larger benthic foraminifers, molluscs and echinoids that were deposited in an isolated inner ramp environment during the late Oligocene
Three major biotic assemblages point to variable storm intensities: (1) shallow storm reworking is indicated by nearshore gastropods, Clypeaster echinoids and reef corals; (2) an intermediated storm wave base is reflected in larger benthic foraminiferal deposits with abundant Eupatagus echinoids and corallinacean algae; (3) a deep storm wave base caused high amounts of Amussiopecten bivalves and Schizaster echinoids in the tempestites
Summary
Ocean ScienceCyclones are one of the most devastating weather phenomena that affect tropical coasts (Henderson-Sellers et al, 1998). Future predictions consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms (Solomon et al., 2007), the limited inTstrhuemeCntarlyroecsoprdhaendredeficient availability and quality of global historical records impede extensive analyses of the natural cyclone variability in most of the tropical cyclone basins (Henderson-Sellers et al, 1998). It remains uncertain whether current changes in tropical cyclone activity have exceeded the variability expected from natural causes (Knutson et al, 2010). In the Arabian Sea, most storms tend to be small and dissipate quickly because the Indian summer monsoon and associated vertical wind shear effectively prevent large storms from developing at the time sea surface temperatures (SST) are at their maximum
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