STABLE ISOTOPE ANALYSIS OF DACRYOCONARID CARBONATE MICROFOSSILS: A NEW TOOL FOR DEVONIAN OXYGEN AND CARBON ISOTOPE STRATIGRAPHY

Devonian stable isotope stratigraphy has been limited to analysis of rare macroinvertebrate and conodont fossils or bulk rock calcites of uncertain provenance. We present methods and results from exploration of the stable isotope composition of Dacryoconarids and related Tentaculites, extinct marine zooplankton known from abundant, globally distributed calcite microfossils in Devonian strata. We applied standard stable isotope techniques for small carbonate sample analysis to Eifelian aged single Dacryoconarid and Tentaculites shells from six genera collected from Onondaga Formation and Marcellus Subgroup strata in the Northern Appalachian Basin of central to western New York as well as Emsian units in the Central Appalachian Basin of Pennsylvania. Calcite shell δ¹³C and δ¹⁸O values were compared by taxonomic group, rock unit, and locality. Shell δ¹³C values (-4.7 to 2.3‰) and δ¹⁸O values (-10.3 to -4.8‰) were consistent across taxa independent of shell size or part, and varied systematically through time. Lower fossil δ¹⁸O values were associated with warmer water temperature and more variable δ¹³C values were associated with major bioevents. Microfossil δ¹³C and δ¹⁸O values differed from bulk rock carbonate values. We find that isotopic analysis of dacryoconarid shells, like Cenozoic planktonic foraminifera, could facilitate higher-resolution, geographically widespread stable isotope records of paleoenvironmental change, including marine hypoxia events, climate changes, and biocrises. Dacryoconarid microfossil δ¹³C and δ¹⁸O values are highly sensitive to paleoenvironmental changes, thus providing a promising new avenue for studying paleoenvironmental changes in upper Silurian to upper Devonian strata. We encourage exploring these planktonic microfossils for their Paleozoic chemostratigraphic potential in stable and clumped isotopes and trace elements, as well as the impact of burial histories on relative preservation and diagenetic alteration of shell material.