Tracking the Hirnantian glaciation using Os isotopes

Here we present initial Os-187/Os-188 (Os-i) values integrated with delta C-13(org) for the first Paleozoic section - the Ordovician/Silurian boundary GSSP at Dob's Linn, Scotland. Our Os-187/Os-188 data tracks major changes in climate that occurred during the Late Ordovician (Hirnantian glaciation), which coincides with the second largest known mass extinction. During the complanatus and early anceps Biozones Os-i values increase from 0.28-1.08. This provides evidence for a period of increased silicate weathering of radiogenic continental crust, likely from the Caledonian Orogen. This increase in weathering was likely the driving mechanism for the drawdown in atmospheric CO2 and global cooling that resulted in the onset of the Hirnantian Glaciation. A decrease to less radiogenic Os-i occurs at the base Hirnantian extraordinarius Biozone and coincides with the trend to more positive delta C-13(org), values that mark the onset of the Hirnantian Glaciation. The trend in Os-i during this interval is ascribed to Hirnantian ice cover and reduced chemical weathering rates cutting the supply of radiogenic material to the Iapetus Ocean. The reduction in silicate weathering enabled atmospheric CO2 to return back to greenhouse levels, causing rapid deglaciation during the mid persculptus Biozone. This period is marked by an abrupt increase in Os-i values from 0.6 to 1.08 over 19 cm of stratigraphy and coincides with the deglacial limb of the delta C-13(org) profile. We interpret the Os-i data to reflect the leaching of exposed radiogenic Os-187/Os-188 bearing glacial deposits and increased weathering of radiogenic Os-187/Os-188 silicate terrane during the deglaciation. Previous workers have identified the Hirnantian glaciation primarily through delta C-13 stratigraphy. However, our Os isotope data indicate that an initial mechanism (i.e. increased silicate weathering) was the driving mechanism behind the Hirnantian Glaciation and subsequent mass extinction. Thus, by coupling Os; and delta C-13(org) proxies we provide the most direct evidence for the initiation and cessation of the Hirnantian glaciation. Furthermore, this study demonstrates the first use of Os-817/Os-188 chemostratigraphy for the Paleozoic as a proxy for reconstructing the Earth's climate system, particularly palaeoceanography. (C) 2010 Elsevier B.V. All rights reserved.