The “Guttenberg” positive δ13C excursion (GICE) interval of Latest Sandbian–early Katian (Late Ordovician) spans ca. 1.2 Myr in Asian reference sections

Abstract

The boundary between the Sandbian and Katian stages of the Late Ordovician is directly followed by the onset of the Guttenberg positive isotope carbon excursion (GICE), which is potentially a means to aid in global correlation of this stage boundary. The sedimentary record of the GICE in mid-continent North American sections often spans only a few meters and has been estimated to span <400 kyr. In contrast, a major sustained positive excursion that has been considered equivalent to the GICE spans over 100 m within some other sections in North America and in the Qilang Formation of the Tarim Basin in northwest China. In addition to enhancing the carbon-isotope records of this expanded record of the GICE, we applied cyclostratigraphy analysis to detailed natural-gamma logging from the Dawangou section, Tarim Basin, which is the auxiliary GSSP for the Sandbian Stage, and from the nearby Sishichang section. The average proportion of carbonate to marl in the outer shelf facies of these sections exhibits cycles of ca. 30 m. Superimposed on these large-scale cycles are ca. 1.5-m oscillations. We interpret these cycles as a sedimentary record of long-eccentricity (405 kyr) and precession cycles. Three secondary low-amplitude oscillations in δ13C are superimposed on the broad GICE plateau and are in phase with the long-eccentricity cycles. The astronomical-tuned sedimentary stratigraphy of these sections implies that the main GICE interval has a duration of ca. 1.2 Myr, with an uncertainty of ca. 200 kyr depending upon how the onset and termination of the GICE is defined. Similar secondary peaks in δ13C on the main GICE excursion reported from other regions have been termed G1, G2 and G3. In the Dawangou section, the estimated placement of the Sandbian-Katian stage boundary is midway between G1 and G2. The GICE record in the composite δ13C stratigraphy from the Cincinnati region of Ohio-Kentucky exhibits an initial “Logana” positive excursion followed by a “Macedonia” positive excursion, and we suggest that these are correlative to the G2 and G3 secondary peaks on the broad GICE as identified in Asia. In both the Tarim region and in the Cincinnati composite, these two secondary peaks in δ13C occur during intervals of greater clay content relative to carbonate. If verified, this astronomically tuned reference scale of the δ13C modulations will enable high-resolution global correlation of biozones and other events near the Sandbian-Katian stage boundary and during the Early Katian.