Constraints on hyperthermals

The abrupt warming event 56 million years ago, known as the Palaeocene–Eocene Thermal Maximum (PETM), was associated with the large scale release of 13C-depleted carbon into the ocean–atmosphere system. In sedimentary records, the event is reflected by a negative carbon isotope excursion1. Cui et al. used a carbon-cycle model to estimate the rate of carbon release during the PETM. The model assumed that the onset of the carbon isotope excursion occurred over approximately 20,000 years, an estimate based on a cyclostratigraphic model. Here we highlight several issues that weaken the conclusions of Cui et al

Warm saline intermediate waters in the Cretaceous tropical Atlantic Ocean

During the mid-Cretaceous period, the global subsurface oceans were relatively warm, but the origins of the high temperatures are debated. One hypothesis suggests that high sea levels and the continental configuration allowed high-salinity waters in low-latitude epicontinental shelf seas to sink and form deep-water masses1, 2, 3. In another scenario, surface waters in high-latitude regions, the modern area of deep-water formation, were warmed through greenhouse forcing4, which then propagated through deep-water circulation. Here, we use oxygen isotopes and Mg/Ca ratios from benthic foraminifera to reconstruct intermediate-water conditions in the tropical proto-Atlantic Ocean from 97 to 92 Myr ago. According to our reconstruction, intermediate-water temperatures ranged between 20 and 25 °C, the warmest ever documented for depths of 500–1,000 m. Our record also reveals intervals of high-salinity conditions, which we suggest reflect an influx of saline water derived from epicontinental seas around the tropical proto-North Atlantic Ocean. Although derived from only one site, our data indicate the existence of warm, saline intermediate waters in this silled basin. This combination of warm saline intermediate waters and restricted palaeogeography probably acted as preconditioning factors for the prolonged period of anoxia and black-shale formation in the equatorial proto-North Atlantic Ocean during the Cretaceous period.<br/

Orbital forcing of Cretaceous river discharge in tropical Africa and ocean response

The tropics have been suggested as the drivers of global ocean and atmosphere circulation and biogeochemical cycling during the extreme warmth of the Cretaceous period1, 2; but the links between orbital forcing, freshwater runoff and the biogeochemistry of continental margins in extreme greenhouse conditions are not fully understood. Here we present Cretaceous records of geochemical tracers for freshwater runoff obtained from a sediment core off the Ivory Coast that indicate that alternating periods of arid and humid African climate were driven by orbital precession. Our simulations of the precession-driven patterns of river discharge with a global climate model suggest that ocean anoxia and black shale sedimentation were directly caused by high river discharge, and occurred specifically when the northern equinox coincided with perihelion (the minimum distance between the Sun and the Earth). We conclude that, in a warm climate, the oceans off tropical continental margins respond rapidly and sensitively to even modest changes in river discharge

Shell growth, microstructure and composition over the development cycle of the European abalone Haliotis tuberculata

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