Paper No. 8
Presentation Time: 9:45 AM
DEPOSITION-DRIVEN CHANGES IN HABITAT AREA: THE SHOREFACE AS A DIVERSITY FACTORY
HOLLAND, Steven M., Department of Geology, Univ of Georgia, Geology Building, Athens, GA 30602 and CHRISTIE, Max, Geology, Pennsylvania State University, 434 Deike Building, University Park, PA 16802, stratum@uga.edu
Previous work has emphasized how the area of shallow-marine habitat is controlled by the rate and direction of sea-level change, the starting position of sea level, and regional physiography. Because our previous work assumed static topography, we use the basin simulation model Sedflux (Hutton and Syvitski, 2008) to examine changes in area of the shoreface (0-15 m), transition zone (15-45 m), and offshore (45-105 m) on a siliciclastic margin during sinusoidal changes in sea level of varying periods and amplitudes. During a single cycle of sea-level change on a ramp lacking a shelf break, the offshore and transition zone show moderate increases in area during the falling-stage (FSST) systems tract, balanced by similar decreases in area during the lowstand (LST), transgressive (TST), and highstand systems tracts. The presence of a shelf break can strongly modify this behavior, producing a more intuitive decrease in habitat area of these environments during sea level fall and an increase during sea level rise. The shoreface, however, undergoes a rapid increase in area at the onset of the HST and an equally rapid decrease in area at the onset of the FSST. This distinctive behavior of the shoreface was observed in all runs and does not appear to be a model artifact. When multiple scales of eustatic cyclicity are superimposed, these changes in the shoreface are observed at both scales of cyclicity. As the amplitude of eustatic fluctuations increase, the window of elevated shoreface area shortens as the onset of the HST is delayed.
When coupled with Hubbell's neutral model of biogeography and biodiversity, these changes in shoreface area predict two features widely recognized from the fossil record. First, the rapid decrease in shoreface area at the onset of the FSST should cause the preferential extinction through ecological drift of stenotopic and geographically restricted, giving rise to a eurytopic, geographically widespread, and geologically long-lived shoreface fauna. Second, the rapid increase in shoreface area during the HST should promote elevated rates of speciation and the episodic and repeated production of evolutionary novelty. If correct, these models may be a partial explanation of the common cause hypothesis, that evolutionary and stratigraphic patterns are driven by the same cause.
