Stable isotope analysis of Dacryoconarid carbonate microfossils: A new tool for high-resolution Devonian oxygen and carbon isotope stratigraphy, paleotemperatures, and cyclostratigraphic timescales?

The Devonian Period spans more than a dozen global bioevents, associated with geologically abrupt climatologic and eustatic perturbations, several of which are implicated in the onset of oceanic black shale episodes. With benthic shelf faunas suppressed during these times of ocean anoxia, calcite microfossils from the globally-distributed extinct Order Dacryoconarida dominate the faunas of dark-gray to black limestone and shale facies. In New York State, dacryoconarids from several Eifelian shales are often pristinely preserved, with visible microlaminae as fine as 200 nm thick. Devonian stable isotope stratigraphy was limited to less-common macroinvertebrates or bulk rock analyses of uncertain provenance. As with Cenozoic planktonic foraminifera, isotopic analysis of dacryoconarid shells could facilitate higher-resolution, geographically widespread stable isotope records of paleoclimate change.

We applied to very well-preserved dacryoconarid shells an established method for analyzing stable isotope ratios (δ13C, δ18O values) of small calcite microfossils (Kiel carbonate device with MAT253 mass spectrometer). We analyzed individual shell δ13C and δ18O values from five common genera, compared by taxonomic group, rock unit, locality, and preservation level. We present reconstructed seawater temperatures, additional taxa in class Tentaculita, and photomicrographs documenting the remarkably pristine microstructure of well-preserved shells.

Dacryoconarid δ13C and δ18O values were consistent across taxa and independent of shell size or part, but varied systematically through time and differed from bulk rock carbonate values. Fossil δ18O values were lower in warmer water, and δ13C values more variable at major bioevents. Dacryoconarid δ13C and δ18O values are highly sensitive to paleoenvironmental change, thus providing a promising avenue for stable isotope chemostratigraphy to better resolve regional/global paleoceanographic changes from the upper Silurian to upper Devonian. Dacryoconarids may help refine the timescale with independent tests of orbital-scale interpretations of cyclostratigraphic packets. Further exploration of dacryoconarids as Paleozoic paleoclimate indicators is warranted (Mg/Sr/Ca, 87/86Sr, microsampling, clumped isotopes). 4870 4900 3030 4954 9617