THE EARLY CRETACEOUS CHEMOSTRATIGRAPHIC AND PALEOCLIMATE RECORD OF NORTHWEST CHINA AND WESTERN NORTH AMERICA

Early Cretaceous strata in NW China and western North America are widespread, fossil-rich, and an archive for continental paleoclimate. Use of C-isotope chemostratigraphy has improved correlations between basins and continents. We will review our work from the Xinminpu and Hekou groups of Gansu Province as well as the North American Cedar Mountain Formation (CMF).

C-isotope analysis of sedimentary organic C from the Xinminpu Group (Xiagou Formation in the Changma Basin), reveals a δ¹³C_org curve characterized by a negative C-isotope excursion (CIE) of ~7 ‰ followed by a positive CIE of ~4 ‰. To the east, the Yujingzi Basin spans the Xiagou and overlying Zhonggou Formation and also has a negative CIE (~6.5 ‰) followed by a positive CIE (~3.5‰) in the C-isotope profile. These records are similar to the Early Aptian Selli Event. At Yujingzi, the transition from the Xiagou to Zhonggou Formation is punctuated by at least two negative CIEs similar to those associated with Late Aptian to Early Albian anoxic events. The Hekou Group of the Lanzhou-Minhe Basin, in southern Gansu, reveal a ~9‰ negative CIE near the top of the group, and is preliminarily correlated to Selli Event CIEs.

The North American record is also developing, with a number of C-isotope records established from the CMF presumed to be Barremian to Cenomanian using both organic δ¹³C and soil carbonate δ¹³C. The lower CMF, expected to be similar in age to the Xiagou Formation, reveal highly variable C-isotope records however, and suggest C-isotope chemostratigraphy may not always be useful in correlation of continental strata. Improved dating techniques are vital to correlation between these two continents.

While dating and stratigraphic correlation remain a barrier to synchronizing Asian and American paleoclimate records, a wealth of paleoclimate proxies can be applied to these strata. Clumped isotope paleothermometry, for example, has opened a door to developing temperature records in continental strata. Clumped isotope analyses from the Xiagou Formation average 31°C. Preliminary analyses from soil carbonates of the Hekou Group average 27°C. Preliminary analyses from the lower CMF reveal an average of 34°C. All data suggests significant warmth for mid-latitudes in the Early Cretaceous.