Cenomanian-Turonian boundary event
|Subdivision of the Cretaceous system|
according to the ICS, as of 2017.
The Cenomanian-Turonian boundary event, or the Cenomanian-Turonian extinction event, the Cenomanian-Turonian anoxic event (OAE 2), and referred to in Europe as the Bonarelli Event, was one of two anoxic extinction events in the Cretaceous period. (The other being the earlier Selli Event, or OAE 1a, in the Aptian.) The OAE 2 occurred approximately 91.5 ± 8.6 Ma, though other estimates are given as 93-94 Ma. The Cenomanian-Turonian boundary has recently been refined to 93.9 ± 0.15 Ma There was a large carbon disturbance during this time period. However, apart from the carbon cycle disturbance, there were also large disturbances in the oxygen and sulfur cycles of the ocean.
The event brought about the extinction of the Pliosauridae, and many Ichthyosauria. Coracoids of Maastrichtian age were once interpreted by some authors as belonging to ichthyosaurs, but these have since been interpreted as plesiosaur elements instead. Although the cause is still uncertain, the result starved the Earth's oceans of oxygen for nearly half a million years, causing the extinction of approximately 27 percent of marine invertebrates, including certain planktonic and benthonic foraminifera, mollusks, bivalves, dinoflagellates and calcareous nannofossils. The global environmental disturbance that resulted in these conditions increased atmospheric and oceanic temperatures. Boundary sediments show an enrichment of trace elements, and contain elevated δ13C values.
The Cenomanian-Turonian boundary event was first noted by D'Orbigny who named the two stages. The global type section for this boundary is located in the Bridge Creek Limestone Member of the Greenhorn formation near Pueblo Colorado, which are bedded with the Milankovitch orbital signature. Here, a positive carbon-isotope event is clearly shown, although none of the characteristic, organic-rich black shale is present in this type-section. It has been estimated that the isotope shift lasted approximately 850 kyrs longer than the black shale event, which may be the cause of this anomaly in the Colorado type-section. A significantly expanded OAE2 interval from southern Tibet documents a complete, more detailed, and finer-scale structures of the positive carbon isotope excursion that contains multiple shorter-term carbon isotope stages amounting to a total duration of 820±25 kyrs.
In Europe, the boundary is known as the Bonarelli event because of 1-2 meter layer of thick black shale that marks the boundary. This layer was first studied by Guido Bonarelli in 1891 and is characterized by interbedded black shale, chert and radiolarian sands and has been known to span 400,000 years. Planktonic foraminifera do not exist in this Bonarelli level, and the presence of radiolarians in this section indicates relatively high productivity and an availability of nutrients.
One possible cause for the occurrence of this event is sub-oceanic volcanism, possibly the Caribbean large igneous province, with increased activity approximately 500,000 years earlier. During that period, the rate of crustal production reached its highest level for 100 million years. This was largely caused by the widespread melting of hot mantle plumes under the oceans at the base of the lithosphere. This resulted in the thickening of the oceanic crust in the Pacific and Indian Oceans. This volcanism would have sent large quantities of carbon dioxide into the atmosphere, leading to global warming. Within the oceans, the emission of SO2, H2S, CO2, and halogens would have increased the acidity of the water, causing the dissolution of carbonate, and a further release of carbon dioxide. When the volcanic activity declined, this run-away greenhouse effect would have likely been put into reverse. The increased CO2 content of the oceans could have increased organic productivity in the ocean surface waters. The consumption of this newly abundant organic life by aerobic bacteria would produce anoxia and mass extinction. The resulting elevated levels of carbon burial would account for the black shale deposition in the ocean basins.
The δ13C isotope
The δ13C isotope found at the Cenomanian-Turonian boundary is one of the main carbon isotope events of the geologic record. It represents one of the largest disturbances in the global carbon cycle from the past 110 million years. This δ13C isotope indicates that the ocean was depleted of oxygen at the time and that there was widespread deposition of organic carbon-rich sediments. Within the positive carbon isotope excursion, short eccentricity scale carbon isotope variability is documented in a significantly expanded OAE2 interval from southern Tibet.
Large igneous provinces and their possible contribution
Several independent LIP events occurred within the time near the OAE2. Within the time period from 90-95 million years ago, two independent LIP events occurred - the Madagascar and the Caribbean-Colombian. Trace metals such as chromium, scandium, copper and cobalt have been found at the C-T boundary and this suggests that an LIP could have been involved in the occurrence of the event. However, the presence of these trace metals was found corresponding to the time right in the middle of the event as opposed to the beginning, suggesting that the LIP may have occurred during the time, but may not have been the initiator for the event. However, other studies linked the lead isotopes of the OAE2 to the Caribbean-Colombian and the Madagascar LIPs. A modeling study performed in 2011 also confirmed that it is possible that a LIP may have been initiated the event, as the model revealed that the peak amount of carbon dioxide degassing from volcanic LIP degassing could have resulted in more than 90% global deep ocean anoxia.
Changes in oceanic biodiversity and its implications
The alterations in diversity of various marine invertebrate species such as calcareous nannofossils indicate a time when the oceans were warm and oligotrophic, in an environment with short spikes of productivity followed by long periods of low fertility. A study performed in the Cenomanian-Turonian boundary of Wunstorf, Germany, reveal the uncharacteristic dominance of a calcareous nannofossil species, Watznaueria, present during the event. Unlike the Biscutum species, which prefer mesotrophic conditions and were generally the dominant species before and after the C/T Boundary event; Watznaueria species prefer warm, oligotrophic conditions.
At the time, there were also peak abundances of green algal groups Botryococcus and prasinophytes, coincident with pelagic sedimentation. The abundances of these algal groups are strongly related to the increase of both the oxygen deficiency in the water column and the total organic carbon content. The evidence from these algal groups suggest that there were episodes of halocline stratification of the water column during the time. A species of freshwater dinocyst - the Bosedinia was also found in the rocks dated to the time and these suggest that the oceans had reduced salinity during the time period.
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