NEW WENLOCK-LUDLOW (SILURIAN) 87SR/86SR DATA AND A REVISED SILURIAN SR CURVE: IMPLICATIONS FOR THE SILURIAN TIMESCALE

Recalibrations of the Silurian numerical timescale and improvements to global chronostratigraphic correlation over the past two decades have altered the shape of the Silurian ⁸⁷Sr/⁸⁶Sr curve significantly. Previously published Silurian Sr composites have shown a generally monotonic increase in the ⁸⁷Sr/⁸⁶Sr curve throughout the Silurian, however, the current numerical timescale for the Silurian produces a major positive inflection point at the onset of the Ludlow Epoch. Here, we show new biostratigraphically well-constrained brachiopod ⁸⁷Sr/⁸⁶Sr values from Gotland, Sweden, that confirm the presence of an early Ludlow positive inflection from a single chronostratigraphically well-constrained region.

It is tempting to suggest that the inflection toward more radiogenic ⁸⁷Sr/⁸⁶Sr values near the onset of the Ludlow was the result of increased weathering of old sialic strata during the accretion of Laurentia, Avalonia, and Baltica into the Old Red Continent, however, the present Silurian numerical timescale requires that interpretations of the Silurian ⁸⁷Sr/⁸⁶Sr curve remain provisional. At present only a handful of radiometric age determinations exist for the Silurian that pass modern analytical scrutiny and are sufficiently chronostratigraphically controlled by biostratigraphy, chemostratigraphy, or any other chronostratigraphic tool.

In order to produce a Silurian timescale without an inflection at the onset of the Ludlow, a >12 Myr-long Ludlow Epoch would be required. As currently recognized by the International Commission on Stratigraphy, the Ludlow Epoch is 4.2 Myr-long and the Silurian Period has a total duration of only 27.7 Myr. Therefore, the present data indicate that either the duration of the Ludlow is substantially too short and the Ludlow Epoch accounts for nearly half of the Silurian Period, or that global tectonic and climatic changes combined to produce a significant positive inflection in the Silurian ⁸⁷Sr/⁸⁶Sr curve that resulted in a positive rate of change in Silurian ⁸⁷Sr/⁸⁶Sr values more that twice as rapid as the Cenozoic ⁸⁷Sr/⁸⁶Sr rise.