Transcript Strontium isotope stratigraphy of the Early Silurian (Llandovery)

Strontium isotope stratigraphy of the Early Silurian (Llandovery): Implications for tectonics and
weathering
Abstract # 127751
Jeremy C. Gouldey, Dr. Matthew R. Saltzman, Dr. Kenneth A. Foland, Jeffrey S. Linder
The Ohio State University, School of Earth Sciences, Columbus, OH 43210
Geologic Background
-Weathering from Silurian continent-continent collisions and from large scale felsic
volcanics could explain the trend to more radiogenic Sr ratios.
-Data from other Llandovery deposits (i.e. Pancake range, Nevada) could help to assess
the probability of global volcanics affecting the Sr record.
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0.7080
0.7081
0.7082
0.7083
0.7084
0.7085
-2.00
-1.00
0.00
1.00
2.00
3.00
304.8 m
Argillaceous limestone
Telychian
crispus
Llandovery
turriculatus
maximus
5
6
Micritic limestone w ith some
thicker argillaceous intercalations
7
Aeronian
Argillaceous limestone
397.5 m
459.4 m
8
Figure 2. Map of Baltic region where Ikla Estonian core was
extracted (after Kaljo et al., 2000).
Micritic limestone w ith some
thicker argillaceous intercalations
365.7 m
480.5 m
494.6 m
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Introduction
528.0 m
Mudstone/marlstone w ith
carbonate nodules and lenses
Micritic limestone
Mudstone/marlstone w ith
carbonate nodules and lenses
Figure 4. Stratigraphy and lithology of Estonian Ikla core (after Kaljo et al., 2000). Graptolite
zones: 1, C. insectus-O. spiralis; 2, Mcl. crenulata-Mcl. griestoniensis; 3, Str. crispus-Spir.
guerichi; 4, St. sedgwickii; 5, D. convolutus; 6, M. argenteus; 7, D. pectinatus-D. triangulatus; 8,
Cor. cyphus; 9, Cys. vesiculosus; 10, Par. acuminatus. 87/86Sr and inorganic 13C curves,
respectively, through Ikla core. Blue lines indicate unconformities.
sedgwicki
Aeronian
convolutus
References
Azmy et al., 1999, Silurian strontium isotope stratigraphy, GSA Bulletin, v. 111, no. 4, p. 475-483.
Rhuddanian
87/86
Sr/Sr
Shields et al. (2003)
Bergstrom et al., 1992, Silurian K-bentonites in the Iapetus Region: A preliminary event-stratigraphic and tectonomagmatic assesment: Geologiska
Foreningens I Stockholm Forhandlingar, v. 114, pt. 3, p. 327-334.
Bergstrom et al., 1998, The Lower Silurian Osmundsberg K-bentonite. Part I: stratigraphic position, distribution, and palaeogeographic significance:
Geological
Magazine v. 135 (1), p. 1-13.
Azmy et al. (1999)
Ruppel et al. (1996)
0.7086
0.7085
Figure 3. Stratigraphic succession of Silurian Estonian KBentonites with graptolite zones (after Bergstrom et al., 1992)
Berner, R. A., 2006, Inclusion of the Weathering of Volcanic Rocks in the GEOCARBSULF Model: American Journal of Science, v. 306, p. 295-302.
Dahlqvist et al., 2005, The lowermost Silurian of Jamtland, central Sweden: conodont biostratigraphy, correlation and biofacies: Transactions of the
Royal Society of Edinburgh: Earth Sciences, v. 96, p. 1-19.
0.7084
0.7083
0.7082
0.7081
0.7080
0.7079
0.7078
Caradoc
Mohawkian
Ashgill
Cincinnatian
Upper Ordovician
Rhuddanian Aeronian
Telychian
Llandovery
Lower Silurian
Figure 1. 87/86Sr record of the late Ordovician and early Silurian based on studies by Ruppel
et al., 1996, Shields, 2003, and Azmy et al., 1999 (after Azmy et al., 1999 and Shields,
2003).
4.00
Mudstone/marlstone
4
Tely.
Ikla C ore
0.7079
322.3 m
Ikla Core
Ikla Core
Changes in the 87/86Sr isotopic record of seawater represent one of the most useful
paleoceanographic tools in the study of ancient climates. These Sr isotope trends are
accurately recorded in marine carbonate rocks and phosphatic shells back at least a half
billion years, particularly in research done in recent years (Veizer et al., 1999, Azmy et al.,
1999; Ruppel et al., 1996; Shields et al., 2003). The two sources of marine Sr are from
the weathering of continental silicates and hydrothermal exchange of seawater at midocean ridges. These two sources differ greatly in their 87/86Sr ratio and may thus affect the
ocean composition. In the case of continental silicate rocks, either an increase in the rate
of weathering or a change in the 87/86Sr ratio of the rocks being weathered can change the
riverine flux of Sr into the oceans (Azmy et al., 1999; Veizer et al., 1999; Ruppel et al,
1996). Tectonic uplift resultant from continental collisions may allow for older, highly
radiogenic silicate rocks to be exposed, which can cause a rising trend in Sr. In addition, if
overall silicate weathering rates were to increase, global temperatures would be expected
to drop due to uptake of atmospheric CO2. This enhanced weathering has been proposed
for the Himalayan uplift, which may have caused cooling that led ultimately to the Late
Cenozoic Ice Age (Raymo et al., 1988).
-87/86Sr ratios rapidly increase starting in the mid Aeronian through to the end of the
Llandovery.
Rhuddanian
A high resolution Sr isotope data set, generated from a fossiliferous drill core through the
Llandovery in Estonia, shows that the 87/86Sr ratio reaches a minimum in the early
Llandovery, and then trends to more radiogenic ratios in the mid to late Llandovery. The
range of values is in general agreement with previous sample sets of calcitic brachipods
and conodonts recovered from localities in North America and Europe that record a rising
trend in the 87/86Sr ratio throughout the Silurian from approximately 0.70787 to 0.70835.
Our data, however, show a general decreasing trend in the 87/86Sr ratio from the end of the
Ashgillian/beginning of the Rhuddanian until the early Aeronian, with values ranging from
0.70834 to 0.70804. During the Aeronian, 87/86Sr ratios slowly trend towards more
radiogenic ratios. Starting in the late Aeronian, the isotope record shows a rapid shift to
radiogenic ratios, ranging from 0.70807 to 0.70844 through the remainder of the Aeronian
and the Telychian. Increases in the 87/86Sr ratio during the late Llandovery may be due to
increased riverine flux of radiogenic Sr into the oceans due to weathering of non-volcanic
continental silicate rocks that were uplifted during early Silurian continent-continent
collisions. Alternatively, or in addition to non-volcanic weathering, a radiogenic Sr flux from
exposed felsic volcanics in the Balto-Scandanavian region is also consistent with the
presence of K-bentonites in late Aeronian and early Telychian strata.
Conclusions
L l a n d o v e r y
Abstract
An abundant series of K-bentonite beds has been well documented in the British Isles and
Baltoscandanavian region, and recently in the Appalachian region of North America.
These ash beds may correlate between Llandovery sections in North America and
Estonia (Bergstrom et al., 1992; Bergstrom et al, 1998; Lehnert et al., 1999), and may
also help to explain changes in Sr ratios. A high resolution, more complete Llandovery Sr
record may aid in this correlative process, which would place the Sr changes in a more
detailed geologic context to address tectonic and climate interactions. In the Llandovery,
there have been several stratigraphic and biostratigraphic studies conducted, including
conodont biozonation in Anicosti Island (Zhang et al., 2002), marine sediment and
conodont biozonation in Scandanavia (Dahlqvist et al., 2005), and graptolite, conodont,
and chitinozoan biozonation in the Baltic region (Loydell et al., 2003; Kaljo et al., 2000).
These studies can be used to assist in regional correlation. Previous Llandovery Sr
records are based essentially on biostratigrapically correlated brachiopods, and record
only a few data points. The Sr data in this study is based upon whole rock carbonate
samples collected from a well dated and stratigraphically known Estonian drill core (Ikla
core), and provides a much more complete Sr record for the early Llandovery due to a
higher sampling resolution.
Discussion
There are several possible explanations as to the rapid increase to more radiogenic Sr
ratios during the late Llandovery. One explanation is that older, more radiogenic
continental rocks became exposed and weathered during early Silurian continentcontinent collisions, causing a flux of more radiogenic Sr into the oceans. Another
possibility is that weathering of large scale felsic volcanics caused an increase in the
radiogenic Sr flux to the oceans. This correlates well with abundant well-dated Kbentonite beds in the Estonian regions associated with the sedgwicki and convolutus
graptolite zones, which is the same time when Sr in the Llandovery begins to rapidly
become more radiogenic. As further evidence of this, a large influx of volcanic CO2, low in
13C, could cause a more negative shift in the 13C record, which is shown in the
accompanying 13C record as occurring just after the rise in radiogenic Sr levels (Kaljo et
al., 2000). A similar negative 13C excursion during this time can also be seen in the
Canadian Arctic (Melchin et al., 2006), which could be evidence that this is a global
excursion. Since these two events were probably acting in conjunction, it is possible that
a combination of weathering due to tectonic activity and weathering of large scale
volcanics, which both lead to a more radiogenic Sr flux to the oceans, is responsible for
the rapid trend to more radiogenic Sr ratios in the carbonate rocks. Further research of
the Sr and C record in the Llandovery, specifically work done in the Pancake range of
Nevada, will help to assess if this shift to more radiogenic Sr levels, occurring with a
negative 13C excursion, is a global occurrence. Also, better understanding of the
composition of mid-Llandovery volcanics, primarily in Balto-Scandanavia, will help to link
the volcanic weathering to radiogenic Sr ratios.
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