Unsettled puzzle of the Marlboro clays.

Global warming during the Paleocene-Eocene thermal maximum (PETM) was associated with the release of large amounts of 12 C-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 uncon-formities (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 δ 13 C gradient in bulk car-bonate does not reflect overall dissolved inorganic carbon; and (iv) the paleodepth estimates are significantly less than published estimates (2, 3). First, Wright and Schaller (1) do not present compelling evidence for seasonality in the rhythmic layers, such as internal microstruc-tures. Modern offshore laminated muddy sediments may be influenced by various mechanisms, such as variable freshwater discharges, irregular passages of hurricanes, and erosional to nondepositional events (4), all potentially variable on a time scale similar to that of the Atlantic multidecadal oscillation (5). Therefore, the rhythmic New Jersey sedimentary patterns are not necessarily in conflict with published age models and rates of the PETM onset (2) (Fig. 1A). Second, in the offshore areas of modern deltas, very high sedimentation rates occur, but in highly turbid waters in which symbi-ont-bearing planktic foraminifera and calcar-eous nannoplankton are extremely rare or absent. However, such microfossils are common and widespread in the New Jersey CIE-sediments (2), so that foraminiferal accumulation rates would be several orders-of-magnitude higher than in similar modern environments. The microfossil evidence thus indicates a clear photic zone in a rather deep shelf setting during the PETM. Third, the CIE shelf gradient during the PETM is not present in species-specific fora-miniferal carbonate, but only in the bulk carbonate record (Fig. 1B). This record is easily affected by fluctuations in the proportions of nannofossil species, …