EARLY CRETACEOUS CARBON-ISOTOPE EXCURSIONS, OAES, AND ~9 M.Y. AND ~400 K.Y. SEA LEVEL CHANGES, ADRIATIC PLATFORM, CROATIA

The 700 m thick Hauterivian to Albian Adriatic Platform section mainly from Mljet Island, Croatia underwent little post-Mesozoic burial or little later diagenesis, consequently its smoothed δ¹³C and δ¹⁸O record from precursor calcite lime mudstone matrix provides one of the most continuous stable-isotope curves from an Early Cretaceous platform. This record captures the carbon isotope excursions and oceanic anoxic events (OAE1a, b, c) evident in published hemipelagic sections, with the added advantage of preserving a record of associated relative sea level changes. Only OAE1a was associated with influx of poorly oxygenated waters into local platform downwarps. Excluding the slowly accumulating Aptian, the smoothed δ¹³C and δ¹⁸O platform curves exhibit cycles with mean thickness of 15 m (~400 k.y. duration) driven by eccentricity modulation in which many of the negative excursions (climate warming) tie to periods of platform submergence. Conversely, most positive peaks of 400 k.y. cycles on the smoothed δ¹³C and δ¹⁸O record match short periods of minimum accommodation, suggestive of cooling and relative sea level fall. Three of the four positive δ¹³C peaks of long term (~9 m.y) cycles tie to peak accommodation events on the platform.

The study suggests that calcite-rich platform-top sediments may be better recorders of global carbon cycling than their metastable aragonite-rich Neogene counterparts, and that similar Mesozoic platforms that underwent little post-Mesozoic burial could provide detailed δ¹³C records for the Jurassic-Cretaceous and other times of calcite seas.