Manganese Anomalous Behavior Through the PETM Sea Level Rise in the Roman Bridge Section (Tunisia)

The Paleocene–Eocene Thermal Maximum event is worldwide associated with significant paleoenvironmental changes and global sea level rise. The eustasy across the late Paleocene-early Eocene, particularly in the PETM interval, was studied in the Roman Bridge section (southern Tethyan margin, Tunisia). The eustatic PETM sequence, its parasequences, and its key surfaces (sequence boundaries (SB), maximum flooding surface (MFS)) are disentangled by the evolution of Sr/Ca, (Zr/Rb)/Ca ratios and Mn content. The various system tracts of the third-order depositional PETM sequence, spanning the lowermost Zone P5-middle part of Zone E2, are well resolved in the PETM interval. During the transgressive system tract, a conspicuous rise in Sr/Ca, (Zr/Rb)/Ca ratios, and the P/B ratio is noticed. The MFS is indicated by a prominent peak in (Zr/Rb)/Ca and Sr/Ca ratios and punctuated glauconite percentage increase. The Sr/Ca, (Zr + Rb)/Ca ratios, and P/B ratio perceptibly decrease continuously during the highstand system tract. The Mn element is highly sensitive to sea level fluctuations. Traditionally, Mn should mimic the same trend of the afore-cited geochemical elements and ratios. However, in our case, the Mn flux exhibits anomalous behavior. The Mn content falls with rising sea level, reaching its minimum around the maximum flooding surface before increasing again in the overlying highstand system. The exceptional transient change in Mn content stems from the prevalence of suboxic conditions and oxygen–water depletion during the PETM in the studied section. This ascertainment suggests that the Mn element is unreliable for sea level variations depiction during oxygen-depleted hyperthermal periods.