MULTIPLE LEVELS OF MOLLUSCAN FAUNAL RESPONSE TO THE DEVELOPMENT OF HIGH-ORGANIC, DYSOXIC SHALLOW-WATER BENTHOS DURING EARLY MIOCENE WARMING (ASTORIA FORMATION, OREGON)

Low oxygen conditions are being observed more frequently on modern coasts and the role of global warming is an increasing concern. Here I use a fossil record from the Early Miocene Newport Member to test for low-oxygen conditions in coastal waters during a well-known past warming event and to evaluate co-occurring changes in benthic mollusks at the community, population, and individual levels.

Faunal and geochemical analyses of benthic foraminifera provide a paleoclimatic framework for the record spanning ~20.2-18 mya. The δ¹⁸O values from Pseudononion costiferum, Buccella mansfieldi, and Bolivina astoriensis suggest a warming trend of ~3.2^OC. Increases in the proportional abundances of buliminids, Nonionella spp., and Fursenkoina spp. after ~19 mya suggest an increase in organic carbon flux and/or decrease in bottom-water oxygen after warming began. The abundances of these taxa are positively correlated with an increase in the δ¹³C difference between shallow infaunal to epifaunal and deeper infaunal taxa, supporting an increase in organic carbon flux. An increase in laminated sediments suggests a concurrent reduction in oxygen.

At the community level, (1) molluscan diversity decreases, (2) the small-bodied deposit-feeder, Saccella sp., increases in dominance, and (3) mean bivalve body size decreases after ~ 19 mya. These changes in the community composition are most strongly correlated with the decrease in δ¹⁸O and increase in buliminids and Nonionella spp., suggesting that both temperature and bottom water oxygenation were contributing factors.

At the population-level, the mean body length of suspension-feeding Anadara devincta, decreases from a median of 43 mm in assemblages >19 mya to 23 mm in younger assemblages, while Saccella sp. increases from 11.7 to 13.3 mm, parallel to changes in the relative abundance of these taxa. Sclerochronological analyses indicate that declining body size in A. devincta was driven by decreases in individual growth rate. Individual physiological rates thus underlie changes at the population level, which in turn underlie changes in community composition.

The environmental stress of low-oxygen conditions that developed during Early Miocene warming thus contributed to changes in the molluscan fauna at multiple ecological levels.