Stable isotopic evidence for climate and basin evolution of the Late Cretaceous Songliao basin, China

Abstract

Because of the need to understand the links and feedbacks of the carbon cycle during times of global greenhouse conditions numerous studies have focused on the Cretaceous climate. Much of what we know about this warm period in Earth's history comes from the study of ocean sediments recovered from both ocean drill cores and marine sediments exposed at the Earth's surface. In contrast, there are few studies of Cretaceous terrestrial sediments. The Songliao basin located in northeast China offers a unique opportunity to understand Cretaceous paleoclimate of terrestrial settings because it contains a nearly complete record of lacustrine sediments deposited throughout the Cretaceous and there is an active drilling program to recover core from this paleolake. We present carbon, oxygen and strontium isotopic data from ostracods collected from two drill cores (SK-1 (N) and SK-1 (S)) that cover a time interval that extends from the Turonian through the Maastrichtian. These data record robust isotopic trends with numerous carbon and oxygen isotope shifts that are both rapid and long-term. We tentatively interpret this record to reflect changes in both global climate and regional basin evolution. In the Turonian and Coniacian Qingshankou Formation we observe several carbon isotope shifts that appear to be correlative to marine isotopic records based upon timing and magnitude of the isotopic changes. We suggest that the carbon isotope record in the Songliao basin reflect the decrease in carbon isotope ratios following the strong positive excursion at the Cenomanian/Turonian boundary, a positive isotope excursion in the late Turonian, and the negative isotope shift that occurs at the Turonian/Coniacian boundary. Upward in the section, however, the marine and Songliao isotopic records diverge as sediment sources shift from the southwest, east and north to more northerly. Strontium isotopes record the change in source region as they increase markedly between the Coniacian/Santonian Yaojia and Santonian/Campanian Nenjiang Formations. As this is the first isotopic record of the Songliao basin we are cautious about many of our interpretations of these isotopic data. Yet it is clear that with tighter age constraints and focused sampling centered on many of the isotope excursions presented here will allow for a deeper understanding of the terrestrial response to changing climate and the hydrologic response to basin evolution.