4:00 PM - 4:15 PM
[BPT04-03] Seasonal temperature gradients during the middle Cretaceous northwestern Pacific region recorded in bivalve fossil carbonates
Keywords:Turonian, Seawater temperature seasonality, Bivalve, Growth line analysis, Stable isotope analysis, Deep-water formation
The middle Cretaceous is characterized by a warm climate and low meridional temperature gradients associated with high pCO2 level. Understanding the middle Cretaceous climates is important not only for predicting future anthropogenically progressed future earth but also for understanding climate dynamics that operated in the middle Cretaceous. Deep-water circulation is/was a significant factor of global climate setting, and northwestern Pacific Ocean is considered as one of the deep-water producing sites during the Cretaceous by numerical simulation. Such a simulation assumed that deep-water formation in the Cretaceous was promoted by subducting less than 20℃ cold surface water to deep ocean. However, studies about shallow-water temperature during the Cretaceous, especially those with its seasonal range, have yet to be done in the northwestern Pacific region. In this study, we conducted growth increment analysis, and analysed stable carbon and oxygen isotope compositions (δ13C and δ18O, respectively) of well-preserved bivalve fossils, in order to understand bivalve life traits and to reconstruct shallow-water temperatures at a seasonal resolution.
Well-preserved bivalves were collected from the middle Turonian shallow-marine deposits of the Mikasa Formation, Middle Yezo Group (Mikasa City, Hokkaido, Japan, 43°N paleolatitude). Two specimens of Cucullaea (Idonearca) delicatostriata and one specimen of Aphrodina pseudoplana were used for the analyses. Samples were cut along their growth direction, and they were used for the observation of thin section and the polished surface of cross section. Growth lines were firstly compared between the cross section and the thin section, in order to correlate the timing of growth line formation between the two. Then, growth increments were measured following its growth direction. Shell δ13C and δ18O analysis were performed by Thermo Fisher Delta V Advantage isotope ratio mass spectrometer coupled to a Thermo Quest Kiel-Ⅲ automated carbonate device at the Institute of Geology and Paleontology, Tohoku University, Sendai, Japan. Paleotemperatures recorded in fossil bivalves were estimated by using the equation of temperature and aragonite δ18O from Grossman and Ku (1986).
SEM and Raman spectroscopy clarified that the crossed-lamellar and complex crossed-lamellar structures were clearly present in the shell outer layer and inner layer, respectively showing that the studied shells were affected no or little diagenetic alteration. Growth lines in thin section were conducted as fortnightly growth lines according to the comparison with modern bivalves. From 60 to 90 fortnightly increments can be recognized from the thin section of C. (I.) delicatostriata shell. The maximum width was 0.4 mm in the juvenile of the shell and varied by about 0.2 mm, while the width near the ventral margin was almost constant at less than 0.1 mm. δ18O of the shell showed a cyclic pattern ranging from -3.0 to -4.5 ‰, and reconstructed temperatures from the δ18O values ranged from 28 to 34°C, reflecting seasonal temperature changes up to five years.
A comparison of the fortnightly increment width and the δ18O values revealed that the seasonal growth rates of C. (I.) delicatostriata were maximized in spring and minimalized in winter, but their shell carbonate precipitation was continued almost through the year. Compared with the previous studies dealing with deep-water temperatures of the northwestern Pacific region, it is concluded that the mid-latitudinal northwestern Pacific region during the Turonian were not a deep-water production site because the water temperature was clearly higher than that of the bottom water and far higher than the temperature that would enable deep water production. Obtained small seasonality and tropical temperature during the mid-Cretaceous than modern Northwestern Pacific Ocean in the same latitude is consistent with the result of rudist-based late Turonian shallow-water temperatures in the Tethyan region and this represents wide distribution of tropical oceans in the mid-latitude distributed not only in Tethys Ocean but also in the Pacific. This study provides the first data about seasonal temperature fluctuations during the middle Cretaceous from non-Tethyan region and could become a clue to reconstruct the middle Cretaceous global climate with a seasonal resolution.
Well-preserved bivalves were collected from the middle Turonian shallow-marine deposits of the Mikasa Formation, Middle Yezo Group (Mikasa City, Hokkaido, Japan, 43°N paleolatitude). Two specimens of Cucullaea (Idonearca) delicatostriata and one specimen of Aphrodina pseudoplana were used for the analyses. Samples were cut along their growth direction, and they were used for the observation of thin section and the polished surface of cross section. Growth lines were firstly compared between the cross section and the thin section, in order to correlate the timing of growth line formation between the two. Then, growth increments were measured following its growth direction. Shell δ13C and δ18O analysis were performed by Thermo Fisher Delta V Advantage isotope ratio mass spectrometer coupled to a Thermo Quest Kiel-Ⅲ automated carbonate device at the Institute of Geology and Paleontology, Tohoku University, Sendai, Japan. Paleotemperatures recorded in fossil bivalves were estimated by using the equation of temperature and aragonite δ18O from Grossman and Ku (1986).
SEM and Raman spectroscopy clarified that the crossed-lamellar and complex crossed-lamellar structures were clearly present in the shell outer layer and inner layer, respectively showing that the studied shells were affected no or little diagenetic alteration. Growth lines in thin section were conducted as fortnightly growth lines according to the comparison with modern bivalves. From 60 to 90 fortnightly increments can be recognized from the thin section of C. (I.) delicatostriata shell. The maximum width was 0.4 mm in the juvenile of the shell and varied by about 0.2 mm, while the width near the ventral margin was almost constant at less than 0.1 mm. δ18O of the shell showed a cyclic pattern ranging from -3.0 to -4.5 ‰, and reconstructed temperatures from the δ18O values ranged from 28 to 34°C, reflecting seasonal temperature changes up to five years.
A comparison of the fortnightly increment width and the δ18O values revealed that the seasonal growth rates of C. (I.) delicatostriata were maximized in spring and minimalized in winter, but their shell carbonate precipitation was continued almost through the year. Compared with the previous studies dealing with deep-water temperatures of the northwestern Pacific region, it is concluded that the mid-latitudinal northwestern Pacific region during the Turonian were not a deep-water production site because the water temperature was clearly higher than that of the bottom water and far higher than the temperature that would enable deep water production. Obtained small seasonality and tropical temperature during the mid-Cretaceous than modern Northwestern Pacific Ocean in the same latitude is consistent with the result of rudist-based late Turonian shallow-water temperatures in the Tethyan region and this represents wide distribution of tropical oceans in the mid-latitude distributed not only in Tethys Ocean but also in the Pacific. This study provides the first data about seasonal temperature fluctuations during the middle Cretaceous from non-Tethyan region and could become a clue to reconstruct the middle Cretaceous global climate with a seasonal resolution.