Carbonate Productivity and Population Dynamics: Redefining Carbonate Sediment Supply in Terms of Ecologic Life-Adaption Strategies

Katie Joe McDonough

Carbonate sediment supply fluctuation is a critical component of carbonate sedimentation models and carbonate sequence stratigraphy. This study proposes that carbonate productivity variation is internally regulated by population dynamics, and is predictable from biotic attributes.

Full clinoform topset-to-toeset analysis of Cretaceous ramp carbonates (Vercors Platform, SE France) reveals dramatic fluctuations in both the amount and type of carbonate production through time. The accommodation/sediment supply (A/S) regime governs carbonate microfacies differentiation. Units deposited under increasing A/S are characterized by low species diversity (mostly miliolid foraminifera), high peloid content, and small organism size. Units deposited under decreasing A/S are characterized by high species diversity, low peloid content, and large organism size (e.g., orbitolinids).

This variation in biotic attributes suggests massive temporal shifts in the ecologic adaptive strategies of the biota, fluctuating between extremes of adaption and K-adaption. Evolutionary selection for r-adapted ("opportunist") species occurs under conditions of environmental instability ("transgression"?), favors high reproductive rates and short lifespans, and produces small, low diversity populations (e.g., bacteria). Evolutionary selection for K-adapted species occurs under conditions of environmental stability, favors specialization into multiple niches, low reproductive rates and long lifespans, and gives rise to large, high diversity populations (e.g., dinosaurs).

Differentiation parallels changing clinoform geometry, suggesting that population dynamics responding to environmental conditions tied to evolving physiography both respond and contribute to A/S variation. Biota attributes and A/S trends provide a proxy for carbonate productivity, with seaward-stepping units indicating highest productivity, and landward-stepping the lowest.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California