Cenomanian-Turonian and δ13C, and δ18O, sea level and salinity variations at Pueblo, Colorado

Stable isotopes of the surface dwelling planktic foraminifera. Hedbergella planispira, its abundance variations, and mineralogical analysis of the Cenomanian-Turonian at Pueblo, CO, reveal cyclic variations in surface salinity due to changes in precipitation, freshwater influx, marine incursions and long-term sea-level fluctuations. Hedbergelia planispira is a proxy for salinity variations, as indicated by 2-4parts per thousand more negative delta(18)O values in intervals of peak abundances as compared to intervals with reduced populations. Negative delta(18)O values reflect periods of brackish surface waters caused by freshwater influx during wet humid periods, accompanied by increased elastic transport. More positive (delta(18)O values reflect more normal marine salinities as a result of and periods and/or marine incursions and correlate with intervals of increased biogenic carbonate deposition. The magnitude of salinity variations during the low sea-level of the Hartland Shale is twice that during the sea-level transgression of the Bridge Creek Limestone. The rapid positive delta(13)C shift that marks the onset of Oceanic Anoxic Event 2 (OAE 2) at Pueblo occurred over a period of about 100 ky (93.90-94.00 Ma), and coincided with the major sea level transgression that culminated in the deposition of the basal Bridge Creek Limestone. A positive delta(13)C shift also occurred in the Rotalipora cushmani zone prior to OAE 2 and coincided with a sea level rise and enhanced preservation of terrestrial organic matter. The likely cause for OAE 2 is depletion of C-12 in the water column as a result of high primary productivity, whereas an earlier R. cushmani zone event was primarily caused by increased input of terrigenous organic matter. Both delta(13)C events are associated with enhanced organic matter preservation and anoxic or dysoxic bottom waters. (C) 2004 Elsevier B.V. All rights reserved.