Carbon-Isotope Stratigraphy of the Cretaceous Calera Limestone, California, USA: Insights into Pacific Ocean Paleoceanography and Oceanic Anoxic Events

Unfortunately, the pelagic paleoceanographic history of the Pacific Ocean during the mid-Cretaceous is poorly constrained due to the loss of much of the contemporaneous Pacific sea floor to subduction and difficulties in recovering chert-rich sediments by ocean drilling. The Calera Limestone, part of the classic Fransican Complex of California, was originally deposited in the Pacific Ocean but was subsequently accreted during subduction of the Farallon Plate. The Calera Limestone provides a unique insight into the pelagic environments of the north-eastern Pacific during the mid-Cretaceous that is complementary to more proximal records. We present here micropaleontological and carbon-isotope data from the Calera Limestone, exposed in the 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, which date to the Early Aptian and Late Albian–Early Cenomanian. These time intervals correspond to two mid-Cretaceous ‘oceanic anoxic events': 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 are associated with significant carbon-isotope excursions, which are also shown to exist in the Calera Limestone. The record of OAE1a from the Calera Limestone compliments 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 chemo-biostratigraphic record of this period for the Pacific Ocean, confirming that environmental change occurred at this time in the Pacific, possibly related to OAE1d.