Christian Linnert

Evidence for global cooling in the Late Cretaceous.

The Late Cretaceous ‘greenhouse’ world witnessed a transition from one of the warmest climates of the past 140 million years to cooler conditions, yet still without significant continental ice. Low-latitude sea surface temperature (SST) records are a vital piece of evidence required to unravel the cause of Late Cretaceous cooling, but high-quality data remain illusive. Here, using an organic geochemical palaeothermometer (TEX86), we present a record of SSTs for the Campanian–Maastrichtian interval (~83–66 Ma) from hemipelagic sediments deposited on the western North Atlantic shelf. Our record reveals that the North Atlantic at 35 °N was relatively warm in the earliest Campanian, with maximum SSTs of ~35 °C, but experienced significant cooling (~7 °C) after this to <~28 °C during the Maastrichtian. The overall stratigraphic trend is remarkably similar to records of high-latitude SSTs and bottom-water temperatures, suggesting that the cooling pattern was global rather than regional and, therefore, driven predominantly by declining atmospheric pCO2 levels.

Calcareous nannofossils from Eastbourne (southeastern England) and the paleoceanography of the Cenomanian–Turonian Boundary interval

Abstract The Cenomanian–Turonian (C–T) boundary interval is marked by one of the most prominent perturbations of the Mesozoic carbon cycle, Oceanic Anoxic Event 2 (OAE2). Increased fertilization of surface waters caused by greater fluvial input of nutrients may have caused the widespread deposition of organic-rich black shales during the OAE2 (productivity model). Alternatively, sluggish oceanic circulation may have enhanced stratification of the water column favoring the preservation of organic matter due to anoxic bottom-water conditions (preservation model). In order to gather evidence for the driving mechanism behind the deposition of the OAE2 black shales, calcareous nannofossils from the midlatitudinal Holywell section (Eastbourne, southeastern England) were studied. Ten bioevents, including last occurrences of six species and first occurrences of four, were recognized throughout the 11-m-thick interval. Preservation of calcareous nannofossils was moderate to good in all studied samples. The C–T interval here contains an abundant (mean 2.4 × 109 specimens/g sediment) and highly diverse (mean 58 spp./sample) calcareous nannoflora, with Watznaueria, Zeugrhabdotus, Biscutum, and Prediscosphaera the most common taxa. The most remarkable change in assemblage composition through the OAE2 is the decrease of Biscutum spp. Low abundances of Biscutum, combined with elevated numbers of Watznaueria spp. and/or Polycyclolithaceae, indicate reduced surface-water fertility during the OAE2 in midlatitudinal European shelf areas. A reduction of primary productivity seems to be quite common in midlatitudinal sections, whereas calcareous nannofossils and geochemistry indicate an increase in primary productivity in low-latitudinal sections. It is therefore likely that the origin of the OAE2 in mid latitudes was caused by sluggish ocean circulation, which intensified stratification. Reduced rates of mixing prevented the oxygenation of bottom waters in these regions, causing black shale deposition.

BOREAL EARLY TURONIAN CALCAREOUS NANNOFOSSILS FROM NEARSHORE SETTINGS—IMPLICATIONS FOR PALEOECOLOGY

Abstract Nutrients and sea-surface temperature were likely the most important paleoecological parameters that controlled the composition of Late Cretaceous calcareous nannofossil assemblages. The extinct nannofossil taxa Biscutum spp., Tranolithus orionatus and Zeugrhabdotus spp. are commonly thought to have preferred nutrient-rich surface waters. In order to test this hypothesis, calcareous nannofossils from two lower Turonian nearshore sections (Bochum, Herne; northwest Germany) were studied. These sections, which reflect a mesotrophic and perhaps cool paleoenvironment, contain well-preserved and highly diverse calcareous nannofossil assemblages yielding 92 species. Based on the first occurrences of Eprolithus eptapetalus, Quadrum gartneri, and Eiffellithus eximius, both sections have been assigned an early Turonian age. The most common taxa are Watznaueria barnesiae, Zeugrhabdotus spp., Biscutum constans, Prediscosphaera cretacea, and Tranolithus orionatus. In combination with previously published data from pelagic settings of the same area, these new findings allow the reconstruction of a nearshore-offshore transect. In proximal sections the abundances of B. constans and T. orionatus are higher than in coeval assemblages from distal localities. Zeugrhabdotus spp. shows no specific preference, whereas W. barnesiae is more common in open ocean settings. These findings suggest that B. constans and T. orionatus preferred more fertile and perhaps cooler nearshore habitats whereas W. barnesiae was adapted to oligotrophic and probably warmer paleoenvironments.