PALEOZOIC PELAGIC PROFILES: DEVONIAN DACRYOCONARID MICROFOSSILS FOR MULTIPROXY CHEMOSTRATIGRAPHY

Planktic calcifiers play key roles in biogeochemical cycles, yet their abundance and utility in the Middle Paleozoic has been mostly overlooked by chemostratigraphers. The Devonian Period spans more than a dozen global bioevents associated with abrupt climatic and eustatic perturbations, several of which are implicated in the onset of oceanic black shale episodes. With benthic shelf faunas suppressed by ocean anoxia, calcite microfossils from the globally-distributed extinct zooplankton Orders Dacryoconarida and Homoctenida dominate the faunas of dark-gray to black limestone and shale facies. In New York's Appalachian Basin, dacryoconarids from Eifelian to Givetian shales are often pristinely preserved, with visible microlaminae as fine as 200 nm in photomicrographs. Shell biocalcite is thought to reflect upper-water-column composition. As with Cenozoic planktic foraminifera, dacryoconarid shells can facilitate higher-resolution, geographically widespread chemostratigraphic records of changes in the Earth system.

We applied to very well-preserved dacryoconarid shells established methods for analyzing stable isotope ratios (δ¹³C, δ¹⁸O values) of small calcite microfossils (Kiel carbonate device with MAT253 mass spectrometer), and now add the I/Ca redox proxy previously applied to bulk rock carbonate (ICPMS). We analyzed shell composition from common genera, compared by taxon, rock unit, locality, and preservation level. We present reconstructions of seawater temperature and oxygenation changes.

Dacryoconarid δ¹³C and δ¹⁸O values, consistent across taxa and independent of shell size, vary systematically through time with water temperature changes and bio-events, and differ from bulk rock carbonates. I/Ca was lower in black shale facies, indicating low oxygen conditions. Multi-proxy comparison is inconsistent with strong diagenetic overprinting. Dacryoconarids are highly sensitive recorders of paleoenvironmental change, providing new windows on Paleozoic mixed layer geochemistry and chemostratigraphic detail in pelagic facies from the upper Silurian to upper Devonian. Complementing other geochemical sources from mid-level and benthos, these microfossils warrant further exploration as Paleozoic marine geochemical profiling tools.