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Marine sediments exceptionally rich in organic carbon, known as black shales, occur globally but intermittently in well correlated Cretaceous successions. The presence of black shales indicates that sporadic, ocean-wide interruption of normal respiration of marine organic matter during oxygen-deficient conditions has occurred. Submarine volcanism on a massive scale, related to the construction of ocean plateaus, could be responsible for the abrupt onset and conclusion of these Ocean Anoxic Events (OAEs), via the oxidation of magmatic effluent, the stimulation of increased primary productivity, and the resultant respiration of sinking organic matter. These discrete periods of global ocean anoxia are accompanied by trace metal enrichments that are coincident with magmatic activity and hydrothermal exchange during plateau construction.
The link between submarine volcanism associated with the emplacement of the Ontong Java - Manihiki plateau (~122 Ma) and Cretaceous Ocean Anoxic Event 1a is explored in this study. Two marine sedimentary sections, recovered in cores from Deep Sea Drilling Program (DSDP) Site 167 (Magellan Rise) and Site 463 (Mid-Pacific Mountains), were analyzed for a suite of major, minor, and trace elements. Trace element abundance patterns for these locations were compared to similar data from the CISMON core (Belluno Basin, Northern Italy) to determine if a relationship existed between the timing of trace metal anomalies and global biogeochemical events.
To account for the variable effects of terrestrial input, trace element data were normalized to Zr and major element data were normalized to Al. Distinct, stratigraphically correlatable peaks could be seen deviating from background levels in the trace element data from the three sites. These anomalous trace metal intervals are related to melt-gas partitioning associated with magmatic degassing and hydrothermal activity resulting from chemical exchange of seawater with cooling plateau rocks. The stratigraphic coincidence of trace metals with other biogeochemical events is consistent with the idea that submarine plateau volcanism, on a massive scale, pushed the global deep ocean into anoxic conditions at discrete intervals of the Cretaceous period. |
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