Frappier Amy Benoit, Lindemann Richard H, Frappier Brian R
Department of Geosciences, Skidmore College, Saratoga Springs, NY, USA.
Stable isotope and Palaeoclimate Analysis (SPA) Laboratory, Skidmore College, Saratoga Springs, NY, USA.
Rapid Commun Mass Spectrom. 2015 Apr 30;29(8):764-74. doi: 10.1002/rcm.7159.
Dacryoconarids are extinct marine zooplankton known from abundant, globally distributed calcite microfossils in the Devonian, but their shell stable isotope composition has not been previously explored. Devonian stable isotope stratigraphy is currently limited to less common invertebrates 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 paleoenvironmental change, including marine hypoxia events, climate changes, and biocrises. We explored the use of Dacryoconarid isotope stratigraphy as a viable method in interpreting paleoenvironments.
We applied an established method for determining stable isotope ratios (δ(13) C, δ(18) O values) of small carbonate microfossils to very well-preserved dacryoconarid shells. We analyzed individual calcite shells representing five common genera using a Kiel carbonate device coupled to a MAT 253 isotope ratio mass spectrometer. Calcite shell δ(13) C and δ(18) O values were compared by taxonomic group, rock unit, and locality.
Single dacryoconarid calcite shells are suitable for stable isotope analysis using a Kiel-IRMS setup. The dacryoconarid shell δ(13) C values (-4.7 to 2.3‰) and δ(18) O values (-10.3 to -4.8‰) were consistent across taxa, independent of shell size or part, but varied systematically through time. Lower fossil δ(18) O values were associated with warmer water temperature and more variable δ(13) C values were associated with major bioevents. Dacryoconarid δ(13) C and δ(18) O values differed from bulk rock carbonate values.
Dacryoconarid individual microfossil δ(13) C and δ(18) O values are highly sensitive to paleoenvironmental changes, thus providing a promising avenue for stable isotope chemostratigraphy to better resolve regional to global paleoceanographic changes throughout the upper Silurian to the upper Devonian. Our results warrant further exploration of dacryoconarid stable isotope proxy sensitivity, the isotopic contrast among dacryoconarids, other taxa, and bulk rock, as well as other potential dacryoconarid proxies (Mg/Ca, Sr/Ca, (87) Sr/(86) Sr, microlaser and ion microprobe isotope techniques, and clumped isotopes) for stratigraphic research.
泪管螺是已灭绝的海洋浮游动物，在泥盆纪有大量全球分布的方解石微化石为证，但此前尚未对其壳体稳定同位素组成进行研究。泥盆纪稳定同位素地层学目前仅限于不太常见的无脊椎动物或来源不确定的块状岩石分析。与新生代浮游有孔虫一样，泪管螺壳体的同位素分析有助于获得更高分辨率、地理分布广泛的古环境变化稳定同位素记录，包括海洋缺氧事件、气候变化和生物危机。我们探讨了将泪管螺同位素地层学作为解释古环境的一种可行方法。
我们将一种用于确定小型碳酸盐微化石稳定同位素比率（δ(13)C、δ(18)O值）的既定方法应用于保存非常完好的泪管螺壳体。我们使用与MAT 253同位素比率质谱仪联用的基尔碳酸盐装置分析了代表五个常见属的单个方解石壳体。根据分类群、岩石单元和产地比较方解石壳体的δ(13)C和δ(18)O值。
单个泪管螺方解石壳体适用于使用基尔-同位素比率质谱仪装置进行稳定同位素分析。泪管螺壳体的δ(13)C值（-4.7至2.3‰）和δ(18)O值（-10.3至-4.8‰）在各分类群中是一致的，与壳体大小或部分无关，但随时间有系统变化。较低的化石δ(18)O值与较高水温相关，δ(13)C值变化较大与主要生物事件相关。泪管螺的δ(13)C和δ(18)O值与块状岩石碳酸盐值不同。
泪管螺单个微化石的δ(13)C和δ(18)O值对古环境变化高度敏感，因此为稳定同位素化学地层学提供了一条有前景的途径，以便更好地解析整个志留纪晚期至上泥盆纪的区域到全球古海洋学变化。我们的结果值得进一步探索泪管螺稳定同位素代用指标的敏感性、泪管螺与其他分类群及块状岩石之间的同位素对比，以及用于地层研究的其他潜在泪管螺代用指标（Mg/Ca、Sr/Ca、(87)Sr/(86)Sr、微激光和离子微探针同位素技术以及聚集同位素）。
