Was the Late Paleocene-Early Eocene Hot Because Earth Was Flat? An Ocean Lithium Isotope View of Mountain Building, Continental Weathering, Carbon Dioxide, and Earth's Cenozoic Clima

Hothouse climates in Earth’s geologic past, such as the Eocene epoch, \ud are thought to have been caused by the release of large amounts of carbon dioxide \ud and/or methane, which had been stored as carbon in biogenic gases and organic \ud matter in sediments, to the ocean-atmosphere system. However, to avoid runaway \ud temperatures, there must be long-term negative feedbacks that consume CO 2 on \ud time scales longer than the ~ 100,000 years generally ascribed to ocean uptake of \ud CO 2 and burial of marine organic carbon. Here, we argue that continental chemical \ud weathering of silicate rocks, the ultimate long-term (multi-million year) sink for \ud CO 2 , must have been almost dormant during the late Paleocene and early Eocene, \ud allowing buildup of atmospheric CO 2 to levels exceeding 1,000 ppm. This reduction \ud in the strength of the CO 2 sink was the result of minimal global tectonic uplift of \ud silicate rocks that did not produce mountains susceptible to physical and chemical \ud weathering, an inversion of the Uplift-Weathering Hypothesis. There is lack of \ud terrestrial evidence for absence of uplift; however, the δ 7 Li chemistry of the Paleogene \ud ocean indicates that continental relief during this period of the Early Cenozoic was \ud one of peneplained (flat) continents characterized by high chemical weathering \ud intensity and slow physical and chemical weathering rates, yielding low river fluxes \ud of suspended solids, dissolved cations, and clays delivered to the sea. Only upon \ud re-initiation of mountain building in the Oligocene-Miocene (Himalayas, Andes, \ud Rockies) and drifting of these continental blocks to low-latitude locations near the \ud Inter-Tropical Convergence Zone and monsoonal climate belts did continental \ud weathering take on modern characteristics of rivers with high suspended loads and \ud incongruent weathering, with much of the cations released during weathering being \ud sequestered into secondary clay minerals. The δ 7 Li record of the Cenozoic ocean \ud provides another piece of circumstantial evidence in support of the Late Cenozoic \ud Uplift-Weathering Hypothesis