Carbon and Oxygen Isotopic Oceanography of Indian and South Atlantic Ocean : Paleoclimate and Paleo-ocean Circulation

インド洋と南大西洋における酸素と炭素同位体による海洋学的研究 : 古気候と古海洋大循環

瀬戸 浩二

Journal of science of the Hiroshima University. Series C, Earth and planetary sciences

The ocean circulation pattern is mainly associated with the continent arrangement and the global climate. The purposes of this study are to reconstruct the water mass structure and the deep ocean circulation within the Cenozoic at the Indian and South Atlantic Ocean, and to reveal the global climate change and the mechanism about the remarkable event of oxygen and carbon isotopes.

For those purposes, the oxygen and carbon isotope records are revealed in the Cenozoic sediments at 6 sites (Sites 752, 754, 756, 757, 758, and 762) in ODP Legs 121 and 122 of the northeastern Indian Ocean. Those records are related to the important global event such as the shift of middle Miocene and Eocene / Oligocene boundary, the increase of Eocene (Miller et al., 1987) for the oxygen isotope record, and the drastical change of Paleocene / Eocene boundary for the carbon isotope record. To compile those records and a number of published isotopic data at the Indian and South Atlantic Ocean, the adjustment values has been calculated from foraminiferal interspecific different of isotope ratio, and the benthic and planktonic foraminiferal isotopic data converted into δ value of DIC of marine water. The general trends of isotopic records are similar in all ocean. However, the oxygen and carbon isotopic values tend to increase southward.

The average isotopic values of 1 Ma interval are calculated in each ODP and DSDP site, and the vertical and horizontal distributions of the oxygen and carbon isotopic values are shown from the estimated paleodepth and paleocoordinate in those sites. In the Paleocene water column section, the vertical change of isotope ratio is not present, but in the Miocene the remarkable oxygen isotopic discontinuity and carbon isotopic low values are recognized around 1500m in the Northeastern Indian Ocean. From those results, it is assumed that the water column structure became to complicate from Paleocene to resent. In this study, the original location of water-mass assumed from the distribution of oxygen and carbon isotope ratio, and the direction of the most important deep water flows is reconstructed from these gradients.

Further, this study estimates the ranking of relative circulation velocity from the coefficient of linear function between the beeline distance and δ13C value. The reconstructions of deep water circulation pattern are illustrated from that (Figs. 77-82). According to reconstruction, AABW series have been formed at the Weddell Sea around the Antarctica through the Cenozoic. In the Paleocene, the water-mass was formed around the Tethyan Sea at the low latitude, and this water may be warm and high salinity judging from the characteristic of oxygen isotope ratio. This water-mass corresponds to WSDW that is discussed by Brass et al. (1982), and WSDW may be confronted with Proto-AADW around the mid latitude in the Paleocene. This water mass rapid reduced with the closing of Tethyan Sea around the Paleocene / Eocene boundary, and influenced-until 50 Ma in the Indian Ocean and until 40 Ma in the South Atlantic Ocean. Proto-CPDW begin to flow into the Pacific Ocean around 33 Ma due to the opening between the Antarctica and the Australia. AAIW are recognized since 30 Ma in the Indian Ocean. Proto-NADW rapid developed around 3 Ma, and influenced to Proto-AABW and Proto-CPDW, and consequently present ocean circulation pattern is formed.

Three shift events are recognized in the oxygen isotopic record. Those events have many common characteristics. In the general trend, the sea level decrease together with the shift event, and the amplitude of sea level change becomes larger. These trends may suggest that the ice sheet of Antarctica is growing together with the shift event. The existence of mechanism that affect to stop the cooling is suggest from closely net magnitude in all shift events. The mechanism may be that the continental margin exposes by regression with the growth of ice cap, and that the CO2 gas directly releases by the oxidative decomposition of organic matter where accumulated on the continental margin.

The most remarkable event among the Cenozoic carbon isotopic records is the drastical decrease of 613C value around the Paleocene / Eocene boundary. This study considers the scenario and mechanism about this event. This event may be caused by the simultaneous stagnation of two water masses: Proto-AADW activating by the cooling until 61 Ma rapid stagnate by the warming around the latest Paleocene, and the active WSDW through Paleocene reduced with the closing of Tethyan Sea. Then the bottom water rapidly changes from oxic to anoxic environment, as a result, many of benthos may be extinguished within the environmental change.

地球科学・地学・地質学 [ 450 ]

英語

Copyright(c) by Author

Jour. Sci. Hiroshima Univ., Ser. C, Vol.10, No.3, 393-485 (1995) ～を参照している

[NCID] AA11095875

[ISSN] 0075-4374

広島大学