Graptolite graveyard: Re-Os dating of macroplankton at the Lower-Middle Ordovician boundary (Floian-Dapingian stage boundary), Tøyen Shale, southern Sweden
Abstract
There exist only 16 radiometric ages for the entire 42 m.y. Ordovician Period. Stage boundaries are biostratigraphically defined by the first appearance of agreed on graptolite and conodont species. Cosmopolitan graptolites are common in the Ordovician and their relatively brief stratigraphic durations make them ideal for global correlations. The Floian-Dapingian stage boundary (Lower-Middle Ordovician boundary) is very poorly constrained, with an absence of radiometric dates for several million years below the boundary and poor statistics on ages in the lower Dapingian [1]. Here we use the Ordovician Tøyen Shale, widespread across southern Sweden and Norway with a highly-refined graptolite biostratigraphy, to add a new age constraint [2]. With drill core from Lerhamn, Sweden (samples from 35.75-36.70 m depth), we employ a novel approach to directly date the fauna. We physically extracted a well-preserved 5-cm fossil of macroplankton (graptolite) from organic-rich shales (up to 4% TOC) for Re-Os dating. The graptolite and its hosting shale together define a well-constrained Model 1 isochron of 469.4 ± 1.7 Ma (2s, MSWD = 1.7, n = 9) and an initial 187Os/188Os (Osi) of 0.802 ± 0.002 for seawater. The Osi documents sustained radiogenic Os input to seawater from the Neoproterozoic-Cambrian through the Early Ordovician, in concert with the Sr isotope seawater curve. The analyzed graptolite belongs to the species Pseudophyllograptus augustifolius, a member of the upper Floian fauna [2]. Our nominal age for the dated graptolite and the shale is lower Dapingian according to the 2017 GTS [1]. Therefore, the Re-Os age suggests the Floian-Dapingian stage boundary may be younger than currently accepted. As defined in the GTS, the Dapingian stage is only 2.7 m.y. (470.0 ± 1.4 to 467.3 ± 1.1 Ma); combined uncertainties could give the Dapingian a mere 0.2 m.y. duration (or a maximum of 5 m.y). Although uncertainties overlap, our first dating of the Lower-Middle Ordovician boundary suggests the Floian-Dapingian stage boundary may be somewhat younger, potentially shortening the duration of the Dapingian by 20% or more. Funding - NSF EAR0745946 and The Gyllenstierna Krapperuṕs Foundation [1] Cooper and Sadler, 2012, in The Geologic Time Scale (GTS)[2] Maletz and Ahlberg, 2011, Lethaia, DOI: 10.1111/j.1502-3931.2010.00246.x
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMPP41B1294G
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCES;
- 1030 Geochemical cycles;
- GEOCHEMISTRY;
- 5225 Early environment of Earth;
- PLANETARY SCIENCES: ASTROBIOLOGY