Ordovician Glaciations Were Complex, Implicated in Mass Extinction

Nd isotope records of late Ordovician sea-level change—implications for glaciation frequency and global stratigraphic correlation

Authors:

1. C. Holmden (a)

2. C.E. Mitchell (b)

3. D.F. LaPorte (a)

4. W.P. Patterson (a)

5. M.J. Melchin (c)

6. S.C. Finney (d)

Affiliations:

a. Saskatchewan Isotope Laboratory, Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, S7N 5E2 966-5712

b. Department of Geology, University at Buffalo, 876 Natural Science Complex, Buffalo, NY 14260

c. Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS, , B2G 2W5Canada

d. Department of Geological Sciences, California State University at Long Beach2, 1250 Bellflower Boulevard, Long Beach, CA 90840

Abstract:

Detailed records of ice-sheet advances and retreats are reconstructed for the Hirnantian and Katian ages of the Late Ordovician using Nd isotopes (εNd) as a sea-level proxy in three study sections from the western margin of Laurentia: two shallow water platform sections located south and north of the paleo-equator, and one deep water section located in a continental slope-rise setting. When sea-level was high and paleo-shorelines had migrated eastward, the εNd value of seawater in the vicinity of each of the study sections shifted toward the εNd value of the eastern Panthalassa Ocean (~ –4.0). By contrast, when sea-level was low and paleo-shorelines had migrated westward, the εNd value of seawater shifted toward the εNd value of the continental weathering flux from Laurentia (–8.5 ± 0.2, 2σmean). These stratigraphic patterns of changing εNd values are interpreted to reflect the eustatic sea-level fluctuations that previous studies have documented in response to Gondwanan ice-sheet advances and retreats, thus linking the εNd sea-level proxy to Late Ordovician global-scale climate changes. The εNd profiles for the two platform sections yielded similar proxy sea-level curves with five cycles of oscillation recorded during the latest Katian and Hirnantian. Three additional cycles of late Katian sea-level change are recognized in the εNd profile of the deep water continental slope-rise section.

The combination of εNd, δ13C and graptolite biostratigraphic data facilitates a precise interregional correlation of the Hirnantian Age and the paleoclimate changes that took place during this interval. The new correlations support previous findings that the Hirnantian ice age comprised two major glacial periods separated by a minor interglacial during the early part of the M. persculptus Biozone. The younger glacial (confined to mid M. persculptus Biozone time) led to more extensive sea surface cooling than did the earlier one, and resulted in extensive eustatic sea-level drawdown and C-cycle changes. It records the strata most often recognized as HICE (the Hirnantian Carbon Isotope Excursion) in sedimentary successions worldwide, such as Anticosti Island, Scotland, Estonia, Siberia, and South China. The results of this study support and strengthen the view that glaciation predated the Hirnantian Age in the Late Ordovician, and that the record of small positive δ13C excursions in Katian successions from Baltica and eastern North America are themselves proxy indicators of glaciation frequency and eustatic sea level changes.