Stable isotopic evidence for climate and basin evolution of the Late Cretaceous Songliao basin, China
Highlights
► The first detailed isotopic record of terrestrial Cretaceous climate change in Asia. ► Carbon isotope shifts appear to be correlative to marine isotopic records. ► Strontium isotopes increase reflecting a change in regional drainage patterns.
Introduction
Decades of ODP and IPOD coring have yielded a robust isotopic proxy record of inferred temperature change in the Cretaceous ocean (Douglas and Savin, 1975, Barrera and Savin, 1999, Fassell and Bralower, 1999; among many others). This record is especially significant because it include periods of extreme global greenhouse climate conditions (Huber et al., 2002), as well as several temporally restricted times of widespread ocean anoxia (e.g. Arthur et al., 1985, Arthur et al., 1987, Arthur et al., 1988, Elrick et al., 2009). Cretaceous nonmarine stratigraphic sections are extensively exposed around the world and have yielded, for instance, abundant and excellent reptilian remains, yet there is currently no terrestrial isotopic proxy record even remotely similar in completeness to the Cretaceous marine record.
An ambitious effort is now underway to develop an oxygen and carbon isotopic record for a nearly complete Cretaceous nonmarine section in the Songliao basin of northeastern China (Fig. 1). This basin initiated as a rift basin in Late Jurassic time and persisted as a thermal-cooling sag depression throughout the Cretaceous (Feng et al., 2010) (Fig. 1). As such, it was a persistently subsiding topographic low spot in which lakes, sometimes quite deep, developed for most of the Cretaceous. Certain of the lacustrine deposits are especially rich in oil-prone organic matter, and as a result, the Songliao basin houses the Daqing oilfield, the largest oil field in China (Zhao et al., 2010). Given this basin history, and the intense drilling and seismic investigation of the basin by the Chinese petroleum industry, it is difficult to imagine a better location for a scientific coring program for a Cretaceous terrestrial climate record.
As a result, a coring program, the “Cretaceous Continental Scientific Drilling Program of China” (the SongKe, or simply SK drilling program), was initiated by the Ministry of Science and Technology of China and Daqing oilfield, which has to-date yielded nearly 2.5 km (2485.9 m) of continuous (> 90% recovery) (96.46%) core of Upper Cretaceous nonmarine strata (Feng et al., 2010). The deposits largely reflect lacustrine and palustrine environments, and contain ostracods, which form the focus for our isotopic study.
In this paper, we provide the first reported isotopic results from this extraordinary stratigraphic section, and interpret them in terms of global climatic and regional hydrologic changes. We determined carbon and oxygen isotopes, elemental analyses, and strontium isotopes of ostracod shells from two coreholes. Our results show an incredibly textured and rich isotopic record, some of which can be related to global climate and other portions to basin evolution. Our interpretation of this record is provisional and awaits other geochemical proxies and better age control. Although the interpretations require further testing, in this paper we show that the Songliao basin holds considerable promise as an archive of Cretaceous terrestrial climate. Specifically, we demonstrate that the carbon isotope records in Turonian and Coniacian sediments Songliao basin seemingly track the corresponding contemporaneous isotopic marine records, whereas the uppermost portion of this section appears to reflect the evolution of the basin itself.
Section snippets
Geologic setting
At 26 × 104 km2 preserved area, the Songliao basin is among the largest basins in China. Feng et al. (2010) recently published a thorough summary of the evolution of stratigraphic filling patterns and paleogeographic evolution of the basin, and we draw heavily on this excellent source for this discussion of the context for the cores we studied. The basin is especially well known, since it has been penetrated by 3000 petroleum exploration and exploitation boreholes and saturated by more than 10,000
Selection of corehole sites
Extensive previous investigations of Songliao basin demonstrate that no single corehole could recover the entire Cretaceous stratigraphic section with samples appropriate for isotopic analysis. In many places, great depth of burial predicts substantial diagenetic alteration (Fig. 1). As a result, the Cretaceous Continental Scientific Drilling Program of China selected three drill sites for the coring project (Fig. 1), SK-1 (N), SK-1 (S), and SK-2, each targeting a different part of the
Results
The ostracods examined in this study cover the time range from the Turonian upward into the Maastrichtian. For clarity, we discuss the results obtained here from oldest to youngest samples examined and assign numbers to the general trends (in text and on figures) that we observe. These numbers are then used in the interpretation section.
Interpretation and discussion
The isotopic and elemental composition of lake water reflects the balance between inputs from rivers and precipitation, modification of lake water through evaporation, and outputs from river discharge (e.g., Carroll and Bohacs, 1999, Carroll and Bohacs, 2001). The inflow and outflow of lakes can be modified by global and regional climate change, as well as evolution of the basin drainage system. As a result, the isotopic and elemental record will be a measure of the interplay between climate
Conclusions and implications
We argue that the ostracod isotopic record for the Songliao basin coreholes reflects both global climate and basin evolution during the Late Cretaceous. If correct, then the most important conclusion that can be drawn from this study is the possibility that these proposed cores will provide an unprecedented terrestrial record of Cretaceous climate. In the forthcoming years there is an ambitious plan as part of the Continental Scientific Drilling Project to recover drill core from the Songliao
Acknowledgments
This study was financed through a grant from the National Science Foundation (EAR 1045003) to S. Graham. We greatly appreciate the review of an anonymous reviewer.
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