Unsettled puzzle of the Marlboro clays

Abstract

Global warming during the Paleocene-Eocene thermal maximum (PETM) was associated with the release of large amounts of 12C-enriched carbon, as reflected in the carbon isotope excursion (CIE) observed globally in sedimentary records. The New Jersey shelf PETM sequences are expanded relative to deep-sea settings, offering high temporal resolution to unravel the pathway of carbon release, although the exact amount of time represented is unknown because of unconformities (1, 2). Wright and Schaller (1) propose an almost instantaneous atmospheric carbon release, based on the assumption that rhythmic couplets in the New Jersey sediments represent annual layers, and that the CIE onset occurred over 13 couplets. Understanding these sequences remains challenging because of complex and extreme environmental changes during the PETM, including eustatic sea-level changes (3) and the development of a river-influenced shelf with high accumulation rates of fine-grained sediment (2). We argue that: (i) there is no compelling evidence for annual rhythmic layers; (ii) the accumulation rates of 2 cm/y are highly improbable in view of the microfossil content; (iii) the cross-shelf δ13C gradient in bulk carbonate does not reflect overall dissolved inorganic carbon; and (iv) the paleodepth estimates are significantly less than published estimates (2, 3).