Fossils

Holocene marine clay deposits from the Shimonada core in Iyonada Sea are examined by an integrated analysis of molluscan fossil assemblages, sand content, sedimentation rate and radiometric ages. The Shimonada core lithostratigraphically consists of five depositional units, A to E in upward sequence : A) gravel, B) massive sandy clay with rootlet and brackish molluscs, C) bioturbated sandy clay with thin very fine sand layers, containing brackish and tidal-flat molluscs, D) bioturbated clay with inner-bay stagnant-water molluscs, and E) bioturbated sandy clay with two very fine sand layers and inner-bay mud-bottom and nearshore sandy mud-bottom molluscs. Five molluscan fossil assemblages are recognized through species composition as follows : 1) brackish, 2) tidal flat, 3) inner-bay stagnant-water, 4) inner-bay mud, and 5) nearshore muddy sand assemblages. Tide-influenced salt marsh and estuary environments represented by B with assemblage 1) appeared before 12, 000-11, 000 cal. yBP. Tidal flat environment [C with 1) and 2)] was prevailing during 11, 000-10, 000 yBP. After rapid sea-level rise at about 10, 000 yBP, possibly resulting from subsidence by an event of the Shimonada-oki-minami and Shimonada-oki-kita faults, inner-bay stagnant environment [D with 3)] had appeared in the graben off Shimonada, Iyonada Sea during 10, 000-8, 000 yBP. Inner-bay muddy environment [E with 4) and 5)] have continued since the graben had filled by mud with high sedimentation rate until 8, 000 cal. yBP. While mud sedimentation rate decreased, sand content relatively increased at about 8, 000 cal. yBP, possibly reflecting muddy sediment bypassing. This seems to have resulted from tidal currents and transgression associated with the formation of Seto Inland Sea.

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Cretaceous greenhouse conditions were characterized by the expansion of Tethyan carbonate platforms on the continental shelf and by widespread deposition of organic-rich sediments representing "oceanic anoxic events (OAEs)". Many Cretaceous carbonate platforms were temporarily drowned several times, indicating that environmental factors frequently stressed carbonate factories on these platforms. Nine global Cretaceous episodes of carbonate platform drowning are recognized. Recent bio- and chemostratigraphic studies revealed that three episodes were probably coincident with OAEs and with events of global carbon accumulation : the mid-Valanginian carbon isotope event, the mid-Aptian event (OAE1a), and the Cenomanian-Turonian boundary event (OAE2). These three breaks in carbonate platform development were associated with mass extinctions of carbonate platform biota, such as rudists, benthic foraminifers and calcareous algae. The major changeovers of Cretaceous hermatypic fossil assemblages came during the periods following these biotic crises. These lines of evidence suggest some causal relationships among some OAEs, carbonate platform drowning episodes and extinctions of carbonate platform biota.

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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.

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This paper documents extinction-recovery patterns of ammonoids and inoceramids across the Cenomanian/Turonian boundary (CTB) including the Oceanic Anoxic Event 2 (OAE2) in the Hakkin-zawa River section, Oyubari area, Hokkaido, Japan, and the Pueblo section, Western Interior, USA. The timing of extinction and recovery in these molluscan faunas occurred synchronously in both areas, based on micro-and macrofossil biostratigraphy and carbon-isotope chemostratigraphy. In the Hakkin-zawa, an ammonoid diversity decreased 0.5 to 0.9 m.y. prior to the CTB (extinction interval), reached a minimum just after the CTB (survival interval), and recovered 0.2 to 0.5 m.y. after the CTB (recovery interval). Inoceramids became increasingly dominant during the extinction and survival intervals, and the genus Inoceramus was replaced by the genus Mytiloides in the latter part of the survival interval. In the Western Interior, the extinction interval spanned 0.42 m.y. before the CTB, and the recovery of faunas took place after 0.15 m.y. from the CTB. In the Western Interior, nekto-benthic ammonoids of acanthoceratids disappeared earlier than planktonic heteromorph ammonoids such as Sciponoceras and Allocrioceras in the extinction interval. By contrast, the nektobenthic desmoceratids also appeared in the later part of the extinction interval in Hokkaido. This inconsistency presumably resulted from different expansion processes for oxygen-depleted water in an open ocean setting (Hokkaido) and a restricted seaway (Western Interior).

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