139736eo.pdf (20MB) - Japan Oceanographic Data Center

MESOZOIC OCEANOGRAPHY AND CLIMATESModels of oceanic circulation can be constructed by employing both the epirical data andanalogies from the present-day hydrographic and meteorological principles. The reconstructions canbe reconciled iteratively with the known stratigraphic data and modified, if necessary. Climatic scenariaof the distant geological past can be constructed indirectly from the sedimentary and fossil record andfrom the paleogeographic and oceanographic reconstructions. When available, the oxygen-isotopicrecord, pieced together from marine microplankton and benthos, can yield direct paleoteniperaturedata.The oxygen-isotopic (l80/16O) ratios of calcareous plankton and benthos can yieldinformation about the surface, deep and bottom water temperatures at the time the organisms wereliving. From these data the temperature structure of the water column can be constructed. Carbonisotopes, on the other hand, supply information about the marine fertility and productivity, as theirratios ( 13O/l*O) are dependent on the supply and storage of organic carbon.Oxygen-isotopic data from the Jurassic are lacking, but both the sedimentary and fossilevidence point to generally warm global climates, with relatively dry conditions in the early and middleJurassic. Well-developed reefs along the Tethyan margins indicate warm climates in this embayment(FRAKES, 1979).The sparse oxygen-isotopic data from the Cretaceous suggest that the global climates remainedwarm but became somewhat more equable than the Jurassic. Humidity may have increased due to thegenerally high seastands. A climatic optimum occurred from the Albian to Santonian times (1 10-80Ma). The vertical thermal structure of the water column was relatively homogeneous, with a lowthermal gradient (DOUGLAS and SAVIN, 1978). Atmospheric modeling of the mid Cretaceouspredicts polar surface temperatures from about 15 to 19"C, with tropical temperature only a fewdegrees above the present (BARRON and WASHINGTON, 1982). This simulation also shows a beltof variable high and low pressure zones in the mid latitudes along the edge of the continents borderingthe Tethys, related to land-sea thermal contrasts. The northern margins of Tethys are believed to havereceived extensive rainfall during this time. Such conditions would have produced strongmonsoonal-type circulation and excessive evaporation over the mid and low latitude epicontinentalseas. Thus these sites over tropical-temperate latitudes may have been the source of Warm SalineBottom Waters (WSBW) that characterized the Cretaceous deep water regime (BRASS et al., 1981).To appreciate the circulation within the ancestral Tethys and in the developing Indian Ocean, itis necessary to see it in a global context. The mid-Jurassic and mid-Cretaceous circulation modelsbefore and after the opening of the Central American passage and the initiation of the Tethys Currenthave been presented by HAQ (1984) and need not be repeated here (Figs. 3 and 4). These patternsshow the eastern Tethys to be dominated by an anti-clockwise gyre with an extension into the westernTethys that may have reached the proto-North Atlantic during high seastands. After the CentralAmerican opening, the Tethys Current became the dominant feature of the world ocean and the mainsource of hydrographic, climatic and biotic dispersals (Fig. 4). Circulation in the newly opened IndianOcean south of India was still restricted to some surface communication with the Tethys.The Rapid Flight of IndiaIndia broke loose from Madagascar about the same time as Australia separated fromAntarctica, around 90 m.y. ago. Initial rate of the northward flight of India was slow, but rapidspreading began around 80 Ma. Half spreading rates between magnetic anomalies 33 and 28 averagedabout 6 cm/yr. At anomaly 28 time (64 Ma) the rates increased to between 7 and 8 cm/yr, and aroundanomaly 24 time (ca 53 Ma) they accelerated further to about 18-19 cm/yr. The rate slowed downdramatically after anomaly 23.Figure 5 shows a comparison between the half spreading rates in the three major Ocean basins.The Indian Ocean rates stand out from the other basins in showing the greatest variation in thespreading rates, being exceptionally high almost throughout the latest Cretaceous and earliest Tertiary.Somewhere between anomaly 23 and 21 (52-48 Ma) the rates decelerated and a dramatic reductionoccurred after anomaly 21, when India's northward motion nearly came to a standstill. This is the time60