| Title: | Climatic forcing of nannoplankton evolution during Oceanic Anoxic Event 1-d |
| Author: | David K Watkins |
| Date Submitted: | 05/02/2002 |
| Address: | Department of Geosciences,
214 Bessey Hall
Lincoln
NE
USA
68588-0340
|
| Phone: | 1-402-472-2177 |
| Email: | dwatkins@unl.edu |
| Co-Authors: | |
| Affiliation: | University of Nebraska |
| | |
| Abstract URL: | http://cis.whoi.edu/science/GG/ccod/viewAbstracts.cfm?RefNumber=19725616 |
| Keywords: | nannofossils, OAE-1d, Albian |
| Abstract: | Changes in the nature of oceanic surface water masses during oceanic anoxic events (OAE) were significant drivers of phytoplankton evolution. As an example, well-preserved nannofossil assemblages in upper Albian and lower Cenomanian hemipelagic sections from Ocean Drilling Program (ODP) Leg 171b record the early history of the establishment and adaptive radiation of the genus Eiffellithus. Seven distinct taxa (herein grouped as "early eiffellithids," "small eiffellithids" and "large eiffellithids") evolved during the relatively short interval from 101.5 to 100.0 Ma. Newly evolved species tended to remain at low abundance levels until a significant disruption in the pelagic realm resulted in the precipitous decline of the dominant species. This decline provided open niche space in the pelagic realm into which the new species could rapidly rise to dominance. These major disruptions correspond to significant changes or shifts in the sedimentological and carbon isotopic records associated with the late Albian Oceanic Anoxic Event (OAE-1d), suggesting that major changes in the strength of deep mixing and the structure of the surface water mass drove the early adaptive radiation of this genus. The two oldest taxa ("early eiffellithids") dominated the first 700 K.y. of eiffellithid history, but were suddenly replaced by the "small eiffellithids" and "large eiffellithids" coincident with the first occurrence of black shale (signaling the onset of OAE-1d) and its attendant negative carbon isotope shift. "Small eiffellithids" dominated the assemblages associated with OAE-1d, while "large eiffellithids" remain a stable, secondary component. The final black shale interval (approximately coincident with a positive carbon isotope excursion) corresponded with the rapid decline and subsequent extinction of one of the "small eiffellithids." This horizon marked the end of the peak in the conditions that resulted in significant organic carbon storage, although the sediment continued to be deposited under dysoxic conditions until the end of the Albian, as is illustrated in the elevated total organic carbon (TOC) levels in the uppermost Albian from ODP Hole 1050C. The end of deposition of organic-rich facies at the Albian/Cenomanian boundary brought the ultimate decline and extinction of the "small eiffellithids" and the rise to dominance of the "large eiffellithids," especially the large morphotype of Eiffellithus turriseiffelii. The close correspondence of species originations, changes in dominance, and species extinctions to changes in sediment TOC and significant carbon isotope shifts indicates that the variability in the surface water mass during OAE-1d was the principle environmental forcing mechanism behind the adaptive radiation of this genus during the late Albian. |
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