THE BARREMIAN-APTIAN BOUNDARY CONTROVERSY: EVALUATION OF THE NUMERICAL AGE

The Late Jurassic through Barremian geologic time scale is built using the M-sequence magnetic polarity time scale (Hawaiian pattern) starting with the reversed-polarity Chron M0r. Although the Barremian–Aptian boundary is defined as the base Chron M0r, its numerical age is highly debated. GTS2012 has the base Aptian as 126.3 Ma, derived from the Aptian–Albian cyclostratigraphy analysis of the Piobbico borehole in Italy and sections in southeastern France corroborated by a linear fit to spreading rates derived from cycle-durations on magnetostratigraphic intervals (Ogg and Hinnov, 2012). However, the MHTC12 time scale has the base age of Chron M0r at 121.5 Ma, based on a modified version of the M-sequence magnetic polarity time scale obtained using a Monte Carlo sampling method (Malinverno et al., 2012). Erba et al., 2015 stated that there is a better temporal consistency with the Ontong Java Plateau flood basalts (124–120 Ma) and paleoenvironmental perturbations during the latest Barremian-early Aptian (OA1a event, negative carbon isotopic excursion, nannoconid crisis) with the MHTC12 time scale than the GTS2012. Patruno et al., 2015 also used the age of 121.5 Ma for the boundary in their biostratigraphic analysis of the Gorgo a Cerbara stratotype section in Italy. Independent evidence from U-Pb dating on a zircon-bearing bentonite unit and palynomorph biostratigraphy in Barents Sea Svalbard cores estimated the absolute age of the Barremian–Aptian boundary as 121–122 Ma (Midtkandal et al., 2016). Despite the studies in 2012–2016 claiming a younger age for the boundary, GTS2016 favored the age of 126.3 Ma based on studies supporting the older age (e.g., Shimokawa, 2010) and the implications of using the younger age, such as causing a 5-myr shortening of the Aptian. Changing the age of the Barremian–Aptian boundary will also create the need for re-evaluation of the cyclostratigraphic studies estimating 100.5 Ma age for the base of Cenomanian and 113.1 Ma for the Aptian–Albian boundary. Further chronostratigraphic, magnetostratigraphic and radiometric analyses are needed to resolve the issue, but current incompatibilities indicate that the Early Cretaceous geologic time scale might be ready for revision.