IMPLICATIONS OF NEW 40Ar/39Ar AND U-PB AGES FOR CENOMANIAN-TURONIAN OAE2

New ⁴⁰Ar/³⁹Ar dating of sanidine from Cenomanian-Turonian ash beds in the Western Interior Basin has implications for the Cretaceous time scale and possible drivers of OAE2. Samples include those of Obradovich (1993) plus material recently collected from the E. septemseriatum, N. juddii, W. devonense, and V. birchbyi ammonite zones which give ages of 94.23±0.11, 94.11±0.14, 93.83±0.15, and 93.82±0.20 Ma, respectively, relative to the astrochronologically-calibrated age of 28.201 Ma for Fish Canyon sanidine (Kuiper et al., 2008). Four of seven zircons from ash in the juddii zone measured using CA-TIMS and the ET535 tracer yield a ²⁰⁶Pb/²³⁸U age of 94.11±0.11 Ma (analytical uncert.). Propagating full uncertainties (standard or tracer composition+decay constant errors), the ⁴⁰Ar/³⁹Ar age of this ash is 94.11±0.17 Ma, identical to the ²⁰⁶Pb/²³⁸U age of 94.11±0.30 Ma, and supporting the Kuiper et al. (2008) calibration.

In GTS04 the Cenomanian-Turonian boundary is 93.55±0.80 Ma based on ⁴⁰Ar/³⁹Ar ages of Obradovich (1993) that assume Fish Canyon sanidine is 28.02 Ma. Interpolation between revised ages of the juddii and devonense zones shifts the boundary to 93.97±0.20 Ma. In turn, the time between the astrochronological K-Pg and Cenomanian-Turonian boundaries is 27.99±0.20 Myr, with an uncertainty smaller than a single long eccentricity cycle, a finding that will help test forthcoming astrochronologic time scales for the Mesozoic.

Our revised ages provide a direct test of Sageman et al.'s (2006) astrochronologic age model which suggests a duration of the C-isotope excursion associated with OAE2 of between 563 and 601 ka. Three ash beds spaning OAE2 are 400±182 ka apart. Thus ⁴⁰Ar/³⁹Ar dating supports the hypotheses that sedimentation was orbitally-forced and that OAE2 lasted 500-600 kyr. The new age of 93.97 Ma for the Cenomanian-Turonian boundary is, however, problematic for the hypothesis of Mitchell et al. (2008) that ocean anoxic events of the mid-Cretaceous, including OAE2, are coincident with, and may even be triggered by, simultaneous nodes in eccentricity, obliquity and precession, predicted by Laskar et al. (2004). Our ages imply that OAE2 began 590 kyr before the proposed nodal insolation minimum. This finding lends credence to the hypothesis that submarine volcanism, rather than orbital forcing, is the trigger for OAE2.