U–Pb dating of cements in Mesozoic ammonites

Dating sedimentary carbonates using the U-Pb method can help improve the Phanerozoic timescale. Using a novel combination of laser-ablation, multi-collector, inductively-coupled-plasma, mass-spectrometry (LA–MC–ICP–MS) and thermal ionization multi-collector mass spectrometry (TIMS), U-Pb numerical ages were obtained on early-diagenetic calcite cements in Jurassic ammonites in which concentrations of U range from 0.47 to 5.3 ppm. The calcite cements of two ammonites, IS1 and IS2, from the uppermost Bifrons Zone of the Toarcian (179–180 Ma) of the UK, gave TIMS-normalized LA U–Pb dates of 164.9 ± 5.3 Ma and 166.7 ± 4.8 Ma respectively. Normalizing LA–ICP–MC–MS data to an in-house calcite standard gave a more precise date of 165.5 ± 3.3 Ma for IS1 cement. An unzoned ammonite, SS2, of Bajocian age (168–170 Ma) yield a TIMS-normalized LA U–Pb age of 158.8 ± 4.3 Ma for its early-diagenetic cement. Both the combined LA–MC–ICP–MS and TIMS approach, and the use of a calcite laser ablation standard can result in accurate ages of cements with uncertainties of 2–3% (2σ). The later, however, is more efficient and precise. These U-Pb dates of cements are 10 to 20 Myr younger than the numerical ages of the biostratigraphic intervals from which the ammonites derive. The U-Pb dates are taken to represent the time at which the aragonite shell of the ammonite inverted to calcite and released its U to precipitate in a late-diagenetic alteration of early-diagenetic fringing cements. Concentrations of U and Pb in a range of other pristine biogenic carbonates were found too low (U < 0.01 ppm) for meaningful dating using laser ablation method

The Proteoglycan Metabolism of Articular Cartilage in Joint-Scale Culture

Understanding and controlling chondrocyte and cartilage metabolism in osteochondral tissues may facilitate ex vivo maintenance and application, both for allografts and tissue-engineered grafts. The hypothesis of this study was that maintenance of chondrocyte viability and matrix content and release of sulfated glycosaminoglycan (sGAG) in the articular cartilage of joint-scale osteochondral fragments are temperature and metabolism dependent. The aims were to assess, for adult goat joints, the effects of incubation temperature (37°C vs. 4°C) on cartilage chondrocyte viability and tissue matrix content and mechanical function, and the effects of temperature and cellular biosynthesis on sGAG release. Chondrocyte viability was maintained with 37°C incubation for 28 days, but decreased by ∼30% with 4°C incubation. Concomitantly, with 37°C incubation, cartilage sGAG was depleted by ∼52% with the lost sGAG predominantly unable to aggregate with hyaluronan, whereas collagen content, tissue thickness, and tissue stiffness were maintained. The depletion of sGAG was diminished by slowing metabolism, with 4°C decreasing release by ∼79% compared with 37°C incubation, and cycloheximide inhibition of cell metabolism at 37°C decreasing release by ∼47%. These results indicate that the articular cartilage of joint-scale grafts have enhanced chondrocyte viability with incubation at 37°C, but may need anabolic stimuli or catabolic inhibitors to maintain sGAG content