麻豆传媒

Quantitative studies of benthic foraminifera and stable isotopes from 4 DSDP/ODP holes (552A, 608, 547A, 646B in the North Atlantic Ocean), and one inland borehole (Sal茅 Briquetterie, Morocco) have been conducted to determine the deep-water circulation in the North Atlantic and its possible relationship to the Mediterranean Salinity Crisis during the Messinian. The Messinian sequences in these holes are well controlled by integrated magneto-, isotope- and bio-stratigraphy. Our data show a distinct relationship between the Atlantic deep-water circulation and the Mediterranean Salinity Crisis.

Benthic foraminifera from Sal茅 core clearly show three major stages of water exchange, which are constrained by local tectonic movements during the Messinian: Stage 1 occurred between 7.12-6.2 Ma is characterized by paleo-Mediterranean outflow water (PMOW) leaking, suggesting an anti-estuarine circulation system. This model suggests that the paleo-Mediterranean Basin probably was equivalent to an euxinic environment where the bottom water had become stagnant and poorly oxygenated below sill depth. Stage 2 (6.2-5.9 Ma) is marked by a reversed current (i.e. Atlantic inflow into the Mediterranean). This reversed current reflects a strong negative water balance in the Mediterranean Basin caused by strong evaporation, and marks the onset of the Mediterranean salinity crisis. Stage 3 (5.9-5.32 Ma) is characterized by a significant uplift of Morocco and salinity crisis in the Mediterranean Basin. In Stage 3, seven glacioeustatic sea-level lowerings occurred, once every 100,000 yrs, coinciding with the eccentricity orbital parameter. These sea-level fluctuations may have served as an "automatic choke" in controlling the Atlantic water infilling, and have caused the cyclic evaporite deposits in the Mediterranean Basin. At about 5.6 Ma, a major transgression is observed in Sal茅 core, which is considered to have caused the unconformity that separates the Lower and Upper Evaporites in the Mediterranean Basin. Such sea-level changes are also observed in abyssal North Atlantic indicated by similar numbers of deep-water turbidite cycles at Site 646 and climate cycles recorded at Site 552. It is clear that the cause of the Mediterranean Salinity Crisis has been associated with both Moroccan uplift and sea-level lowerings that coincide with eccentricity forcing.

In the deep ocean, three major paleoceanographic phases similar to those in Morocco are also determined in the intervals of 7.12-6.2 (Stage 1), 6.2-5.9 (Stage 2), and 5.9-5.32 Ma (Stage 3), respectively. During Stage 1, the North Atlantic was characterized by relatively strong, altered North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) fluxes, indicating relatively good ventilation in the open ocean. This is also supported by the presence of an unconformity at Site 547 that cut off the early Messinian section (7.12-6.2 Ma). AABW may have pinched out at least as far as 60oN in the North Atlantic Ocean at this time. During Stage 2, AABW was significantly reduced during the onset of Mediterranean Salinity Crisis. In Stage 3, deep-water circulation varied significantly. In particular, both NADW and AABW were greatly reduced at 5.75 and 5.55 Ma, respectively. These two major reductions are coeval with two major glaciations recorded in stable oxygen isotopes, indicating the cause coupled with climate cooling and Mediterranean Salinity Crisis. The salinity crisis could have caused open ocean to be more sluggish. However,brief returns of NADW and AABW have occurred during the "interglacial" at about 5.6 Ma. This return has been linked to the major unconformity (resulting from inundation) that separates the Lower and Upper Evaporites in the Mediterranean Basin

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Pages: 388
Supervisor: David Scott