[M-IS18] Paleoclimatology and paleoceanography

[MIS18-27] Cretaceous Paleoenvironmental changes across the Albian-Cenomanian boundary in high latitude Southern oceans

*Takashi Hasegawa¹, Yusuke - Takagi² (1.Faculty of Geoscience and Engineering, Institute of Science and Engineering, Kanazawa University, 2.Graduate School of Natural Science and Technology, Kanazawa University)

Keywords:Cretaceous, alkenones, Albian, Cenomanian, biomarkers

Cretaceous is a period of very warm, greenhouse climate due to high atmospheric CO₂ concentration derived partly from high oceanic crustal production, and is characterized by high sea level, small latitudinal temperature gradients, and no or small ice sheets. Contrary to such long-term feature of the Cretaceous, a short-term decrease in sea surface temperature (SST) in late Albian through middle Cenomanian were reported in the North Atlantic, Tethys, and the Indian Oceans, suggesting a provisional cooling trend of the regions before the rapid warming toward Cenomanian/Turonian age boundary. While this warming interval has been extensively studied, little attention has been paid to the cooling interval, and a shortage of SST and vegetation data makes it difficult to discuss the extent of the cooling event. It is not yet clear how the terrestrial environment responded to the declined SST or climatic cooling. Oceanic anoxic event 1d (OAE-1d) has also been studied in the northern low latitudes across the Albian/Cenomanian stage boundary interval, but its causal mechanism has not yet been clarified due to poor paleoenvironmental information from high latitudes and insufficient understanding of the global state of the paleoclimate.
In the present study, DSDP Leg 36 Site 327, DSDP Leg 40 Site 364, ODP Leg 122 Site 763, IODP Exp.369 Sites U1513, U1514 were analyzed in order to estimate paleoenvironments through the Cretaceous Albian/Cenomanian boundary interval in southen high latitudes, focusing particularly on organic geochemistgry. Significance of this study is derived from two reasons: 1) it focuses on southern high latitudes. The paleoenvironmental information has been overlooked across the Albian/Cenomanian boundary interval; 2) it focuses on biomarkers, which has not been studied for this period.
Alkyl ketones with forty carbons and two or three unsaturation bonds (C_40:2 Et and C_40:3 Et alkenones, respectively) were detected from some samples from Sites U1513, U1514, and 763 updating the oldest of record of them in the southern oceans. Considering previous reports from the Northern Hemosphere, haptophyte group that biosynthesized C₄₀ alkenones may have prevailed worldwide by late Albian. The index U^K’₄₀ (U^K’₄₀= C_40:2 Et/(C_40:2 Et + C_40:3 Et)) was tentatively introduced to evaluate relative change of SST. Based on the index, overall decrease of SST was suggested through the studied interval of U1513. It was accompanied by gradual domination of the alkenones over other marine plankton biomarkers suggesting replacement of primary producer. Indices of terrestrial aridification (relative decline of perylene and increase of chain length of long-chain n-alkanes) also showed concordant increase with the marine transition.
In summary, the present study demonstrated that a biomarker approach is useful to discuss global and local paleoenvironmental changes in the mid Cretaceous Albian/Cenomanian boundary interval. This study and following researches will lead to a better understanding of the global environmental changes through the Albian/Cenomanian boundary transition and of the mechanism of OAE-1d development.