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Cenomanian–Turonian and δ<sup>13</sup>C, and δ<sup>18</sup>O, sea level and salinity variations at Pueblo, Colorado
Auteur(s)
Date de parution
2004
In
Palaeogeography, Palaeoclimatology, Palaeoecology, Elsevier, 2004/211/1-2/19-43
Résumé
Stable isotopes of the surface dwelling planktic foraminifera <i>Hedbergella planispira</i>, 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. <i>Hedbergella planispira</i> is a proxy for salinity variations, as indicated by 2–4‰ more negative δ<sup>18</sup>O values in intervals of peak abundances as compared to intervals with reduced populations. Negative δ<sup>18</sup>O values reflect periods of brackish surface waters caused by freshwater influx during wet humid periods, accompanied by increased clastic transport. More positive δ<sup>18</sup>O values reflect more normal marine salinities as a result of arid 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 δ<sup>13</sup>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 δ<sup>13</sup>C shift also occurred in the <i>Rotalipora cushmani</i> 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 12C in the water column as a result of high primary productivity, whereas an earlier <i>R. cushmani</i> zone event was primarily caused by increased input of terrigenous organic matter. Both δ<sup>13</sup>C events are associated with enhanced organic matter preservation and anoxic or dysoxic bottom waters.
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Type de publication
journal article
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