‘The Cretaceous Greenhouse World’ refers to a part of geological history that lasted from about 110 to 90 million years ago. During this period, submarine volcanic carbon dioxide emissions were released high enough to cause atmospheric CO2 concentrations in excess of 1000ppm. (Caldeira, K.,Rampino, M.R., 1991) This then caused rapid sea-floor spreading and the break up of continents. This lasted for ten million years and the global warming led to major radiation in the diversity of flowering plants, insects, birds and mammals. At the beginning of the Cretaceous, the super-continent, Pangaea, had broken up into two continents; the northern one being Laurasia and the southern one being Gondwana, which intern broke up even further into Africa, South America, India, Antarctica, and Australia. The Gondwanan break-up led to an increase in seaways and shorelines, which may have affected the diversification of terrestrial plants and animals by creating several isolated continental regions. (Lovett, Richard, 2008)
Wonderfully preserved forminifera from the low-latitude Demerara Rise, mid-latitude Blake Nose and the high-latitude Falkland Plateau in the Atlantic Ocean, have been especially useful for reconstructing temperature gradients of the mid to Late Cretaceous ocean. The isotopic readings of surface-dwelling planktic forams, sea surface temperatures reached a maximum of 42˚C at the Demerara Rise (Bice et al., 2006), 33°C at the Blake Nose (Huber et al., 2002), and 31°C at the Falkland Plateau (Huber et al., 2002; Bice et al., 2003) during the Turonian, which was approximately 93–89 million years ago. It seems that Cretaceous warming was more prominent at the higher latitudes, where the different between the mid-Cretaceous temperature and now is almost 30˚C.
Using a more recent climate model, Bice et al. (2006) conclude that 3500 ppm or greater atmospheric CO2 concentration is required to reproduce the estimated maximum sea surface temperatures of the Mesozoic tropical ocean. Mesozoic palaeotemperatures are based on geochemical proxies and are still insufficient in sediments older than the Albian (112MA). Furthermore, in areas outside of the Atlantic Ocean, more investigation is needed to reconstruct a more accurate climate (Takashima, 2006).
There is also a huge amount of limestone all over the world dating back to the Cretaceous. This shows that there were universal high sea levels and warmer conditions. But, distinct black, laminated organic-rich layers, black shale, are occasionally intercalated within these sequences. Black shales are often an indication of anoxic conditions, so it has been suggested that this is evidence of a widespread anoxic environment, known as ‘Oceanic Anoxic Events’. Two schools of thought have been proposed to explain the black shale in the ‘Oceanic Anoxic Events’. The ‘stagnant ocean’ model attributes OAE (Oceanic Anoxic Events) to depletion of bottom water oxygen as a result of dense vertical ocean stratification. The ‘expanded oxygen-minimum layer’ model proposes that increased surface ocean productivity caused expansion of the oxygen-minimum layer in the water column. The diagram below shows these two models. The two models predict different vertical thermal gradient profiles of the water column that can be inferred from the oxygen isotopes of planktic and benthic forams. The OAE in the early Albian is characterised by the drastic increase in surface water temperatures and strengthening of the vertical stratification of the water column, which is similar to the stagnant ocean model.
However, the OAE 93.5 MA shows sudden warming of deep water and the collapse of vertical stratification (Huber et al., 1999). This probably induced enhanced upwelling and productivity similar to the expanded oxygen-minimum layer model. Warming of deep water could have contributed to depletion in oxygen solubility in the deep ocean and may have triggered the disassociation of large volumes of methane hydrate buried in sediments of the continental margins. Oxidation of the released methane could have further consumed dissolved oxygen in the water column, while simultaneously releasing CO2 to the atmosphere (Gale, 2000; Jahren, 2002). As one can imagine, OAE have a significant affect on the diversity and evolution of marine organisms. Numerous records demonstrate a high turnover rate of microfossils at or near OAE intervals (Jarvis et al., 1988; Erbacher et al., 1996; Premoli Silva and Sliter, 1999; Leckie et al.,2002; Erba, 2004). For example, during the Cenomanian-Turonian boundary, OAE expanded below the photic zone, (Damsté and Köster, 1998; Pancost et al., 2004) to greater than 3500m depth in the Atlantic Ocean (Thurow et al., 1992). The result was that about 20% of marine organisms became extinct in less than one million years.
It is worth a mention that black shales, especially in OAE 1 and 2, yield no calcareous nanofossils, suggesting that these anoxic conditions had expanded to within the euphotic zone (Hart and Leary, 1991; Coccioni and Luciani, 2005). Anoxic conditions also occasionally occurred in very shallow water during the OAE 2.
Furthermore, these Oceanic Anoxic Events served as a natural thermostat for the Cretaceous Greenhouse Earth; because there was greater organic carbon burial in the OAE pelagic sections, this may have drawn down CO2 from the ocean–atmosphere by burying organic carbon in black shales, thereby causing long-term global warmth (McElwain et al., 2005). Organic-rich sediments are formed under anoxic conditions, and many petroleum source rocks were formed during greenhouse warming peaks between the Middle Jurassic to mid-Cretaceous. Isotopic analyses of wood fragments found throughout the section allow correlation to the standard secular marine δ13C curve because of characteristic peaks at the Aptian-Albian and Cenomanian-Turonian boundaries. Consistency of the north-central Texas δ13C curve with the marine standard facilitates correlation among non-marine and marine environments on a global scale (Jacobs et al., 2005). δ13C records provide an interpretation in terms of global partitioning of carbon between reduced and oxidised reservoirs; this provides a fairly reliable marker, although various things can affect it such as diagenesis and local carbon cycle, tend to obscure such chemostratigraphic signals (Bice et al., 2002). There seems to be an abrupt -3 ‰ excursion in δ 13C that immediately preceded a more gradual 3-4‰ positive that is consistently expressed in multiple sections. Pratt et al. discussed how δ 13C studies of lacustrine organic matter could provide better estimates of pCO2 during times when temporal trends in δ 13C or carbonate and organic matter are de-coupled. Mass balance considerations indicate that biomass burning is an under-explored explanation for pronounced negative excursions in Cretaceous d13C records.
Many important factors must be taken into account in order to explain such high sea levels. First of all, there was a change in volume of ocean ridges; this is dependent upon sea-floor spreading rates. There was also a contraction of thermal sea water, depending on sea water temperature and a formation of volcanic plateaus. Finally, there were no ice caps during this period, which changed the temperature drastically. Sea levels peaked in mid- to Late
Cretaceous, approximately 100-75 million years ago, when the total land area flooded was more than 40% greater than today, resulting in the expansion of continental shelf environments (Hays and Pitmann III, 1973) (See figure 5). Through integration of data on occurrences of ice-rafted and/or glacial deposits around polar regions, positive oxygen isotope values of forams, and intervals of rapid sea-level fall, it is quite possible that glaciers waxed and waned during the greenhouse climate of the Mesozoic (Miller et al., 2005a). Uncertainty still exists over ice cap volume and age but, several geologically short-term glacial events during the Cretaceous have been proposed, such as middle Cenomanian, middle Turonian,middle Campanian and earliest and late Maastrichtian (Takashima et al., 2006). In contrast to previously proposed models, these proposals imply that greenhouse periods can exhibit short-term climate variability.
Ocean circulation was also affected during this greenhouse world. In the Cretaceous, the westerly winds developed only seasonally. In the absence of these winds, the subtropical ocean gyres would have weakened leading to an ocean circulation dominated by eddies. Circulation in the coastal ocean differs fundamentally from that of the open ocean in that the large ocean basins are under the influence of trade winds and westerlies, whereas coastal regions are much more influenced by passing weather systems (Csanady, 1982).
There seems to be an abundance of organic plankton in the Cretaceous, suggesting that shelf-break frontal systems that divide shelf and epicontinental seawaters from the open ocean today did not exist then (Hay, 2008). The breakdown of shelf-break fronts, it is argued, occurred because of the increase water exchange between the ocean and epeiric seas, all of which would have had either a positive or negative fresh-water balance relative to the ocean proper (Hay, 2008). However, in many areas the onset of chalk deposition occurs well after the flooding of the epeiric sea began.
Warm deep water is usually associated with high-latitude deep-water sources. Nevertheless, because the sea water density depends on temperature and salinity, it may be questioned whether the deep-ocean water temperature reflects the warm polar surface-ocean regions (Valdes, 2000). The less dense surface water means that convection shuts and reduces meridional overturning. The reduction of overturning leads to a reduction of poleward heat transfer and vice versa, cooling and even sometimes an increase of salinity. This can lead to a stronger overturning and stronger poleward heat transport (Valdes, 2000). It may be assumed that past ocean circulation was sensitive to high-latitude salinity distribution (Stocker et al., 1992); in some instances, the meridional overturning might have behaved as a bipolar seesaw with a periodicity of hundreds to a thousand years or more (Broecker, 1998). In the Cretaceous, warm, ice-free climates, poleward water vapor transport or river runoff are the causes of low salinity, whereas increased evaporation, which is unlikely in high latitudes, might be a cause of increased high-latitude surface salinity (Pond and Pickard, 1983).
As previously mentioned, there were no polar ice caps during the Cretaceous, or if there were, they were temporary and episodic. It is suggested that this may be due to atmospheric feedbacks, in conjunction with the increased greenhouse gases (Haupt and Seidov, 2001). Because the Earth rotates around the Sun on a tilt, it means that in mid-winter the sun would not appear over the horizon for a month or more and that for much of summer the sun didn’t fully set. Quite a lot of fossil discoveries have confirmed that fact that the Polar Regions were indeed, a lot warmer in the Cretaceous. Recently, fossil remains of turtles and a tropical to sub-tropical champsosaur, have been found on Axel Heiberg Island at 79°N in the Queen Elizabeth Islands of north-eastern Canada (Tarundo, 1998). This crocodile-like reptile is said to have had the same living habits as a modern crocodile- being cold blooded and having to live in relatively warm climates. Furthermore, it cannot migrate or hibernate during winter, as is thought possible for some polar dinosaurs and turtles. Therefore, the champsosaur is considered to be an ideal climatic indicator. Therefore, the climate in the Polar Regions must have been sub-tropical to tropical; more specifically, the warm season temperatures very likely ranged from 25°C to 35°C with the coldest month mean temperature of about 5.5°C (Tarundo et al., 1998). This means that the mean annual temperature would have been 14°C, which is drastically warmer to today’s -20°C (for the present annual mean temperature for the area).
The Cretaceous climate was indeed very different to our own; CO2 concentrations were in excess of 1000ppm; the surface temperature was possibly up to 30°C hotter than the present; there were no polar ice caps and sea circulation did not bring cool water to high latitudes. The ocean thermohaline conveyor, although very different to present day, may have operated similarly to the present-day mode. The only difference was that the amplitude might have been lower than today, due to smaller equator-to-pole density gradients, and the driving deep-water sources may have switched roles. It should be noted that during the Turonian several pronounced, yet short-lived, cooler intervals punctuate the extreme warmth and shows that rapid tropical sea surface temperature changed during the Cretaceous Greenhouse World. This implies that even the mid-Cretaceous ‘super greenhouse’ climate may have been less stable than previously thought (Foster et al., 2009).
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