Using Community Structure to Delineate Environmental Change; Rank-Abundance Curves as a Useful Method in Biostratigraphy

Amelinda Webb
San Diego State University Department of Geological Sciences San Diego, California
[email protected]

Turnover of taxa in fossil assemblages provides the framework for biostratigraphy; however taxonomic decline is not instantaneous. This preliminary study uses an ecological tool (rank-abundance curves) to quantify the structure of micro-fossil communities during the Paleocene-Eocene Thermal Maximum. Rapid global warming during the event is recorded by carbon isotopes, allowing for close correlation of environmental and faunal changes. Rank-abundance curves reflect partitioning of resources in a community, and respond to changes or disturbances of the environment. After a disturbance, resource partitioning is uneven; as the community matures, partitioning becomes more even. When a community is exposed to continuing disturbances, then the community never matures, and the community structure reflects high stress levels. Extinction is caused by taxa being unable to cope with high stress levels. Rank-abundance curves detect increasing levels of environmentally-induced community stress before loss of taxa and allow for environmental reconstruction based on the faunal response. This is useful in studying and correlating extinction events and subsequent recoveries. When rank-abundance curves are generated for benthic foraminiferal and ostracode communities across the Paleocene-Eocene Thermal Maximum, both groups show significant change in community-structure related to the event. Benthic foraminifera experience an extinction, and rank-abundance curves reflect increasing stress prior to the extinction event. Ostracodes do not have an extinction, and the faunal turnover associated with the event actually results in more stable community-structures. This technique allows a better understanding of conditions before and after an extinction, which supports development of higher resolution biostratigraphy for such events.

AAPG Search and Discovery Article #90070 © 2007 AAPG Foundation Grants in Aid