The Paleocene–Eocene transition at Mead Stream, New Zealand: a southern Pacific record of early Cenozoic global change

Abstract

Mead Stream in Marlborough, New Zealand, exposes a spectacular 650-m-thick stratigraphic section of well-bedded micritic limestone, chert, and marl that was deposited on a South Pacific upper continental slope from Late Cretaceous to middle Eocene. The ∼200 m upper Paleocene–lower Eocene succession was examined for its lithology, bulk carbonate carbon isotopes, and assemblages of radiolarians, calcareous nannoplankton and foraminifera, so that it could be placed into a global context. The interval displays several prominent carbon isotope anomalies and is correlated with South Pacific Radiolarian Zones RP5 to RP9 and Calcareous Nannofossil Zones NP6–8 to NP12. Additionally, Planktic Foraminiferal Zones P4 to P6b are identified in uppermost Paleocene and lower Eocene strata. Mead Stream has a near-continuous Paleocene–lower Eocene sediment record, and three globally significant climate events—the late Paleocene carbon isotope maximum (PCIM), the initial Eocene thermal maximum (IETM), and the early Eocene climatic optimum (EECO)—have obvious expressions. The PCIM is a ∼50-m-thick interval of biosiliceous micritic limestone in which δ 13C is ∼3‰. The lower part contains two organic-rich biosiliceous mudstone units that may represent expansion of an oxygen minimum zone (OMZ) during a global increase in marine biological productivity. The IETM is a ∼4-m-thick interval in which δ 13C drops below 1.5‰ in a pattern seen at other locations. The basal 2.4 m is a distinctive recessed marl-rich unit that is defined herein as the Dee Marl. The IETM is marked by a rapid decline in the nannoplankton genus Fasciculithus, short-lived occurrences of Discoaster cf. araneus and Morozovella aequa aequa, and significant radiolarian faunal turnover. Owing to impoverished benthic faunas, the benthic foraminiferal extinction event (BFEE) is poorly defined. The onset of the EECO is marked by a ∼1‰ negative δ 13C excursion, a transition from limestone-rich to marl-rich facies, and a marked decrease in radiolarian abundance. Compacted sedimentation rates vary between 1.4–2.7 cm/kyr for upper Paleocene–lower Eocene strata at Mead Stream. Although individual beds with average thickness of ∼10 cm were deposited too fast to directly represent cycles in orbital parameters, time series analysis of bed thicknesses suggests that groups of beds may record Milankovitch-scale periodicity, perhaps with a significant obliquity component. Thus, the relative frequency and thickness of marl and limestone beds in this section is shown to be strongly influenced by climatic changes at a wide range of temporal scales, from suborbital and orbital cycles to aberrant short-term events and long-term trends. Predominance of marl in IETM and EECO intervals indicates that episodes of extreme global warming resulted in reduced oceanic productivity and increased terrestrial discharge in the high-latitude southwestern Pacific.