Low-latitude vegetation and climate dynamics at the Paleocene-Eocene transition - A study based on multiple proxies from the Jathang section in northeastern India

We present a multi-proxy study of an upper Paleocene-lower Eocene succession from the paleo-equatorial region. The study is carried out on a coal-bearing, shallow-marine succession exposed at Jathang, east Khasi hills, Meghalaya, northeastern India. The succession was deposited in a low-energy, coastal marsh-bay complex. Dinoflagellate cyst biostratigraphy yields a late Paleocene to early Eocene age for the section. The deposits of the lower part of the succession represent a transgressive systems tract (TST) defined by seven parasequences, each starting with bay sediments deposited during transgression, followed by a shallowing-upward bay fill-marsh deposit. In the vertical succession, each parasequence acquires an increasingly marine character, culminating in a maximum flooding surface at the Paleocene-Eocene boundary. It is followed by four shallowing upward parasequences deposited in a highstand systems tract (HST). Enhanced chemical weathering and high terrestrial supply are testified by raised SiO2 and Al2O3 contents and high percentages of terrestrial palynomorphs. The pollen flora recovered from the Jathang section was used for quantitative paleoclimate and vegetation reconstructions. The Coexistence Approach was applied based on Nearest Living Relatives (NLRs) of sixty fossil species recorded at different stratigraphic levels. Seven climate variables were determined for the fossil assemblages, and, as a measure of the seasonality of climate, the number of dry months was estimated. Our study shows that during the Paleocene there existed warm, seasonally dry tropical climate conditions with mean annual temperature at ca. 24-26 °C and mean annual precipitation at ca. 700-1800 mm, and with a dry season of 5-6 months. Particularly warm and wet, perhumid climate conditions with 26-27 °C and 2200-3200 mm mean annual precipitation with a dry period of 2-3 months were reconstructed for the latest Paleocene-earliest Eocene interval. The study shows a distinct vegetational turnover from palm-dominated, seasonally dry tropical forest during the Paleocene to highly diversified dicotyledonous megathermal rainforest during the latest Paleocene-early Eocene. The present study demonstrates that the reduced duration of the dry period during the latest Paleocene-earliest Eocene, due to a more active hydrological cycle, played a major role in determining the climate and shaping the vegetation cover in the paleo-equatorial region. There is evidence from our data that seasonality of rainfall is the determining factor for the tropical forest vegetation pattern of the equatorial region rather than mean annual rainfall condition. As the main trigger for the observed step-wise changes of the hydrology along the studied succession, the fast northward movement of the Indian Plate is inferred.