Elizabeth V. Nunn

Valanginian isotope variation in glendonites and belemnites from Arctic Svalbard: Transient glacial temperatures during the Cretaceous greenhouse

Oxygen and carbon isotope data from Cretaceous (Valanginian) glendonites and belemnites from Arctic Svalbard are presented. Oxygen isotope data from well-preserved glendonites, in conjunction with the ikaite to glendonite pseudomorph transition temperature, are used to provide an estimate of the oxygen isotope composition of ambient seawater. Calculation of such a factor is essential for robust paleotemperature estimates. Using this methodology, our paleotemperatures calculated from the oxygen isotope compositions of coexisting belemnites yield cool temperatures (4–7 °C) consistent with transient glacial polar conditions during the Cretaceous greenhouse. Cool polar temperatures during the Cretaceous help reconcile geologic data with the simulations of general circulation models. Nevertheless, beyond this postulated and transient cool event within the Valanginian, the remainder of the isotope data are interpretable in terms of warm polar conditions during the Cretaceous greenhouse.

Isotopic signals from Callovian–Kimmeridgian (Middle–Upper Jurassic) belemnites and bulk organic carbon, Staffin Bay, Isle of Skye, Scotland

Abstract: The stable isotope data presented here significantly extend and expand upon previous isotopic investigations of the Middle to Late Jurassic interval. The belemnite samples collected from the Staffin Bay and Staffin Shale formations from the Isle of Skye, Scotland, yielded oxygen isotope values consistent with Callovian–Kimmeridgian palaeotemperatures of 6.7–20.6 °C. The carbon isotope data comprise one of the first moderately high-resolution investigations of the relationship between terrestrial δ13Corg (predominantly fossil wood debris) and marine δ13Ccarb (belemnites) as derived from a geologically coeval record. The Staffin Bay data reveal a broad Early to Mid-Oxfordian positive carbon isotope excursion. The excursion maximum occurs in the cordatum Zone (British Boreal ammonite zonation), although high values persist into the tenuiserratum Zone. The correspondence between the marine and terrestrial records indicates a strong coupling of the ocean–atmosphere system and suggests that the total exchangeable carbon reservoir would have been affected at this time. The Mid-Oxfordian negative carbon isotope excursions identified in published Tethyan records and commonly attributed to methane release are not recorded in the Staffin Bay data, which may suggest that the Tethyan excursions do not represent fluctuations in the global carbon reservoir and that the fidelity of the methane hypothesis should be re-evaluated. Supplementary material: Isotopic and geochemical data are available at http://www.geolsoc.org.uk/SUP18339.

Investigation of Li/Ca variations in aragonitic shells of the ocean quahog Arctica islandica, northeast Iceland

Interannual and intra-annual variations in lithium-to-calcium ratio were investigated with high temporal resolution in the aragonitic outer shell layer of juvenile Arctica islandica (Mollusca; Bivalvia) collected alive in 2006 off northeast Iceland. Li/Ca shell ranged between 7.00 and 11.12 µmol mol -1 and presented well-marked seasonal cycles with minimum values recorded at the annual growth lines; a general pattern was a progressive increase in Li/Ca shell from March to May, followed by a plateau in June and a decrease down to minimum values in July-August. Li/Ca shell was correlated with d 18 O shell -derived temperature, but the strength of this relationship was weak ( r 2 p shell in A. islandica may most likely be explained (1) by calcification rate and/or (2) by significant river inputs of Li-rich silicate particles flowing to the sea as soon as snow melts. In the first case, Li/Ca shell may be a useful proxy for addressing seasonal variations of growth rate in bivalves that lack discernable microgrowth patterns. Abrupt decreases of Li/Ca shell may, in turn, help identify growth retardations due to harsh environmental conditions. Alternatively, if Li/Ca shell variations are linked to particulate Li inputs by rivers, this could be a new proxy for the intensity of mechanical weathering of Icelandic basalts, with interesting perspectives for the reconstruction of frequency and intensity of past jokulhlaups (subglacial outburst floods). Further works, including experimental studies, are needed to test these hypotheses.