Mid-Cretaceous oceanic anoxic events in the Pacific Ocean revealed by carbon-isotope stratigraphy of the Calera Limestone, California, USA

The paleoceanographic history of the Pacific Ocean during the mid-Cretaceous is poorly constrained due to the loss of much of the contemporaneous Pacific seafloor to subduction and difficulties in recovering chert-rich sediments by ocean drilling. Pelagic sediments that were originally deposited in the Pacific Ocean but that have been subsequently accreted during subduction potentially provide an alternative paleoceanographic archive. This study presents micropaleontological and carbon-isotope data from the Calera Limestone, part of the classic Franciscan Complex exposed in Permanente Quarry, central California, USA. In the three stratigraphic sections studied, pelagic limestones with low organic-carbon contents are the dominant lithology. However, two stratigraphic intervals are recognized that contain organic carbon, and these date to the early Aptian and late Albian-early Cenomanian. These time intervals correspond to two mid-Cretaceous oceanic anoxic events (OAEs): early Aptian OAE1a (equivalent to the Selli level) and late Albian OAE1d (equivalent to the Breistroffer event). It is well established that both of these events were associated with significant carbon-isotope excursions, which are also shown to exist in the Calera Limestone. The record of OAE1a from the Calera Limestone complements ocean drilling records by providing further evidence for variability in the sedimentary and stratigraphic record of this event. The carbon-isotope data from the late Albian-early Cenomanian provide the first detailed chemobiostratigraphic record of this period for the Pacific Ocean, confirming that environmental change occurred at this time in the Pacific, possibly related to OAE1d.