The onset of the Early Eocene Climatic Optimum at Branch Stream, Clarence River valley, New Zealand

Abstract

We present new lithologic, biostratigraphic and carbon isotope records for a calcareous‐rich ∼84m thick, early Eocene, upper continental slope section now exposed along Branch Stream, Marlborough. Decimetre‐scale limestone‐marl couplets comprise the section. Several marl‐rich intervals correspond to carbon isotope excursions (CIEs) representing increased 13 C ‐depleted carbon fluxes to the ocean. These records are similar to those at nearby Mead Stream, except marl‐rich intervals at Branch Stream are thicker with a wider δ 13 C range. Comparison to other sites indicates the section spans ∼53.4–51.6 Ma, the onset of the Early Eocene Climatic Optimum (EECO). The most prominent CIE is correlated with the K/X event (52.9 Ma). Prominent marl‐rich intervals resulted from increased fluxes of terrigenous material and associated carbonate dilution. We find multiple warming events marked lowermost EECO, each probably signaling enhanced seasonal precipitation. Branch Stream bulk isotopic records suggest ‘differential diagenesis’ impacted the sequence during sediment burial.