白亜紀における地球システム変動(<特集>白亜紀海洋無酸素事変の解明)

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Global warming during the mid-Cretaceous is investigated using a carbon geochemical cycle model. The atmospheric CO_2 level may have increased owing to enhanced volcanic activity during the mid-Cretaceous, while the organic carbon burial rate increased during the same period and may have suppressed further warming. Mantle plume activity, forming large igneous provinces (LIPs), would have released a large amount of CO_2 to the atmosphere, but the effects of this on the climate may have been small compared to those of increased seafloor spreading rates. However, the effects of plume activity could have been significant if most of the LIP-forming magma eruptions were limited to very short periods. Ocean anoxic events may occur when ocean circulation becomes either active or inactive. According to a reconstruction of the abrupt warming event (the PETM event) at 55.5 Ma using a one-dimensional ocean biogeochemical cycle model, ocean circulation may have strengthened, resulting in an increase of primary productivity in the surface ocean. Thus the oxygen minimum zone may have extended vertically, which could have resulted in an extinction of benthic foraminifera at this event. According to the analysis of a two-dimensional ocean biogeochemical cycle model, the ocean circulation pattern would change from polar sinking to equatorial sinking as the climate becomes warm. Under intermediate temperature conditions, however, the ocean circulation pattern is periodic sinking (short-period polar sinking replaced by long-period (<50 kyr) shallow equatorial sinking). In this case, deep-water becomes anoxic because the ocean is stagnant during the periods of shallow equatorial sinking. This could have been the case for the anoxic events during the mid-Cretaceous. If so, however, anoxic conditions should not be maintained for more than 50 kyr, but have repeated periodically. Ocean anoxic events tend to occur under the warm climate conditions.
日本古生物学会の論文
2003-09-20

白亜紀における地球システム変動(<特集>白亜紀海洋無酸素事変の解明)

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