Clay mineralogical evidence for mid-latitude terrestrial climate change from the latest Cretaceous through the earliest Paleogene in the Songliao Basin, NE China
Abstract. From the latest Cretaceous (late Campanian to Maastrichtian, ~ 76−66 Ma) through the earliest Paleogene, a fluctuating greenhouse climate prevailed and climatic changes were linked to catastrophic geological events and massive biotic extinction. Paleoclimate reconstructions during this time period primarily rely on marine sediments, with limited high-resolution terrestrial records. Here we present a high-resolution clay mineralogical record from the Sifangtai Formation and the Mingshui Formation of the Songliao Basin, northeast China, which are continuously deposited fluvial to lacustrine strata, and have been tightly age constrained as late Campanian to early Danian. Smectite and illite are the dominant clay species, whereas kaolinite and chlorite are minor components. Clay minerals are derived from the weathering of parent rocks and/or paleosols, and their relative weight percentages are primarily controlled by regional paleoclimate and sedimentary environment. We use three clay mineralogical proxies, including the percentage ratio of smectite and illite, illite chemistry index and the percentage ratio of phyllosilicate clay minerals and quartz in clay fractions, for paleoclimatic reconstruction. We correlate these proxy timeseries with basin-scale and global paleoclimate timeseries. Our results show that from the latest Cretaceous through the earliest Paleogene, values of all three clay mineralogical proxies in the Songliao Basin are generally higher during warming intervals than those during cooling intervals. We interpret this dataset to suggest that warming caused strengthened moisture delivery from the Pacific, increasing precipitation and intensified chemical weathering, whereas cooling was accompanied by increasing dryness and physical weathering. Before the Cretaceous-Paleogene (K-Pg) boundary (approximately 66.4 Ma to 66.0 Ma), the warming likely related to Deccan volcanism and the transient cooling afterwards are characterized by paleosol carbonate stable isotopic excursions and changes in the illite chemistry index recorded in the Songliao Basin sediments, reflecting fluctuations in precipitation and weathering intensity. However, changes in clay mineral assemblages are not clear before and at the K-Pg boundary. This is probably due to the relatively long-response time of terrestrial weathering regimes (up to 500 kyrs) to the short duration of the K-Pg boundary impact and the degassing by the preceding Deccan Traps volcanism (~ 200 kyrs). In the earliest Paleogene, after the K-Pg boundary, all clay mineralogical and stable isotopic proxies indicate a warmer and more humid climate with stronger chemical weathering. Our work demonstrates that terrestrial climate and weathering intensity in the mid-latitude Songliao Basin fluctuated during the latest Cretaceous through the earliest Paleogene and sensitively responded to global climate changes.
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