ANTHROPOCENE STRESS IN REEF-DWELLING FORAMINIFERA: TAPHONOMIC EVIDENCE

Members of the genus Amphistegina have been important contributors to carbonate sediments in mid-and low-latitude shelf and reef environments through most of the Cenozoic. As a consequence of abundance and nearly circumtropical distribution of modern species, along with their relative ease in collection and maintenance in culture, the shallowest-dwelling Amphistegina spp. are among the most widely studied benthic foraminifera, especially of taxa that host algal endosymbionts. Investigations of their biology, population dynamics, and functional morphology began in the early 1970s. The discovery in 1991-92 of widespread bleaching in Amphistegina spp., analogous to bleaching in zooxanthellate corals, was followed by intensive field studies of populations on Florida reefs, as well as opportunistic sampling in all tropical oceans through the 1990s and since. Populations were observed under both acute and chronic stress. The primary symptom, loss of golden-brown color indicating breakdown and digestion of symbionts, cannot be preserved in the fossil record. However, secondary signs of chronic stress that affect shell structure during the life of an individual can be preserved. The degenerative nature of bleaching stress interferes with calcification and production of organic matrix, weakening shells so individuals are more susceptible to infestation and predation, and increasing the occurrence of morphological anomalies in asexually-produced broods. Examination of Amphistegina specimens collected live prior to the 1990s revealed that percentages of all shell anomalies, including chips, breaks, borings and deformities, seldom exceeded 5%. In the 1990s such features commonly were found on more than 25% of the live populations examined. Although shell breakage and overgrowth obviously can occur post mortem, elevated incidences of certain taphonomic features in modern or fossil shells, particularly the combination of breakage and repair, borings, and early-growth deformities, can provide evidence for a chronic stress event. To elucidate responses to environmental change in recent decades, museum collections and other sample repositories can provide invaluable material for comparative studies.