Lithium isotope composition of the Carboniferous seawater: Implications for initiating and maintaining the late Paleozoic ice age

Abstract

• δ 7 Li values the Carboniferous seawater are reported for the first time. • Carboniferous continental silicate weathering changed from congruent to incongruent. • Hercynian orogeny and radiation of tropical forests are key drivers for the LPIA . The reasons for decreasing atmospheric CO 2 concentration in the Carboniferous and initiation of the Late Paleozoic Ice Age (LPIA) are still in debate. To assess the contributions of continental silicate weathering, we obtain lithium isotope composition (δ 7 Li) of the Carboniferous seawater based on the East Fenghuangshan carbonate section in South China. Original seawater records are recognized after systematic evaluation of diagenetic impacts, showing that seawater δ 7 Li declined from ∼ 20 ‰ before the late Visean to ∼ 12 ‰ around the Visean-Serpukhovian boundary followed by a gradual rise to ∼ 21 ‰ until the Moscovian. Combined Li-Sr isotopic modeling indicates that decline of seawater δ 7 Li requires decrease of riverine δ 7 Li and twofold increase of riverine Li flux, suggesting a transition to more congruent continental silicate weathering regime that likely results from uplift of the Hercynian orogen. High relief due to the Hercynian orogeny exposes fresh rocks for weathering by physical denudation of the thick soil and saprolite cover, which is beneficial for rapid CO 2 consumption and probably initiates the LPIA. By contrast, radiation of tropical forests is responsible for the subsequent increase of seawater δ 7 Li. It turns the weathering regime to a more incongruent one by promoting clay mineral formation. The colonization of plants with deep roots also accelerates the continental weathering, which together with intense photosynthesis, should contribute significantly to keeping low atmospheric CO 2 concentration and maintaining the prolonged LPIA. Therefore, we suggest that uplift of the Hercynian orogen and radiation of tropical forests are the key drivers for initiating and maintaining the LPIA.