Abstract
This study aims at examining different methods used to study the composition and variation of marine organisms and plants. Various methods create different results that show the effect of the geological environment structure in the ecosystem at various oceanic conditions. Temperature variation is used as a major determinant of the population distribution of the organisms and plants found in various ecosystems. For instance, this study will use crocodilian fossils and mangroves to assess the composition of ancient oceanic environments and the trend of the climate and geological change in these conditions (Häder 1999).
This study uses some isotopic approaches to understanding ocean environments, and isotopes are used in paleoclimatology to determine and estimate the temperature of precipitation at a different period. The temperature of ice can be measured assuming that conditions at the polar have not changed through the use of isotopes. This study will calculate these parameters as a fractionation between varying phases of oceanic masses. Crocodilian and other organism fossils will be used to assess early evolution in the ocean. However, it is important to note that the reconstruction of climate is controversial and not always accurate due to the volatility of the data used (James and Austin 2008).
Introduction
It is possible to group paleoclimate proxies into two distinctive groups. There is a difference between the two proxies’ types the variation palaeotemperatures can be tracked from isotopic approaches and palaeontological methods that are based on habitat distribution. There are some isotopic approaches which include; planktonic, foraminifera and oxygen isotope. The paleontological approaches are derived from the palaeodistribution of fossil crocodilians and some distinctive shallow-water organisms that include, mangroves, zooxanthellate corals, and large foraminifera. The above approaches are used to interpret the uneven dispersal of palaeoclimate during their Cretaceous and Cenozoic phases (Kennett and Ingram 1995). They both display different composition of the sea surface temperatures derived from palaeontological and isotopic sources. The isotopic evidence in both cases is insufficient to account for the tertiary tropical, subtropical habitat and shallow-water organisms distribution and diversity. This report is inclusive of the two methods that try to explain the distribution and also their statistical evidence that tries to explain the disparities between the two approaches that show different climate and temperature for Late Cretaceous and Cenozoic periods.
Isotopic methods
With the discovery of oxygen isotope proportion (d18O) of calcite is reliant on the water medium in which they precipitated temperatures. A number of studies have used this approach as a measure of the temperatures for the fossil oceans by assessing the fossil shells, planktonic foraminifera, and other organisms. It is essential to reflect on isotope breakdown by the organism, the composition of oxygen isotopic of the ocean water, earth surface volume covered by ice, the rate of evaporation and precipitation, the merging trends of the local water mass, and changing pH and alkalinity composition of the ocean waters. Besides, it is important for the researchers to assume that original shell d18O remains constant. Nevertheless, an evaluation of the diagenetic factor established that the overgrowth and infilling of foraminifera shells by calcite could be responsible for reducing temperatures at a maximum rate of 58 degrees Celsius (Shackleton 1974).
Planktonic foraminifer oxygen isotope
(Paul N. Pearson) Accounted for the stable isotope proportions of foraminifera from thirteen samples of varying ages. The samples of planktonic foraminifer used were well-preserved assemblages from the Paleocene and Eocene of Tanzania. The results were later augmented with TEX86 analyzes that had been retrieved from the same or adjacent samples of the Tanzanian stable isotopes and TEX86 measurements. Mr. Pearson came to a conclusion that the maximum sea-surface temperature ranged from more than 30 °C. This level of temperature is rarely reached in the modern open ocean. Mr. Pearson suggested that the late Eocene and early Oligocene recorded values show slightly cooler temperatures although this data could also be interpreted as a result of the effect of the increasing amount of ice on the planet during these times. The Paleocene and earliest Eocene reading indicate an optical warming trend through this period. This can be proved by investigations on Benthic foraminifer shells found in the coastal sites that reveal diagenetic characteristics comparable to planktonic forms (Fig. 2.1-2). However, with restraint, their oxygen isotope reading can be deliberated as a diminished bottom water temperature record reason being both biomineralization and recrystallization could only have happened if the two were in contact with the cold surface or pore waters. These oxygen isotope readings, therefore, indicate Eocene was responsible for polar cooling.
(Pearson et al.) Analyzed data retrieved from well-preserved planktonic foraminifer shells from water-resistant hemipelagic clays by VG Prism mass spectrometer for those parts of the Late Cretaceous and Eocene epochs. They measured the O18 value in a number of species of stable isotope within the shale. These analyzed samples fail to show any significant overgrowth or infilling of diagenetic calcite whereas preservation of foraminiferal shells appears right on a micrometer scale. Mr. Pearson had insinuated that diagenesis would decrease the recorded temperatures, interspecies differentials, and reduce the apparent meridional temperature gradients that are derived from recrystallized substance. The reason behind this assessment is that deep-sea temperatures and early diagenetic conditions are should be relatively homogeneous all the time regardless of the latitude. On the other hand, because of the even un-encrusted planktonic foraminifera can precipitate calcite in slightly deeper environmental conditions, they used the most negative O18 value in the assemblage as a representative of the maximum seasonal SST. Besides, the benthic foraminifera in the samples show bottom-water temperature conditions on the continental shelf, and cannot be comparable to marine environment coeval temperature. These readings pointed out that the tropical sea surface temperatures were similar to today temperature conditions and most likely a number of degrees warmer between 28 ± 32 C.
The paleontological methods
The paleontological methods are drawn exclusively from the geological record. The Palaeoclimate can be explained from geological data invariably based on the theory with the Recent, the variance of which depends on corroboration from multiple lines of evidence such as the paleo and modern distribution of living organism that is influenced by temperature. This requires the compilation and investigation of large, global datasets of well-constrained geological climate proxies. In this report, the paleoclimate has been interpreted based on the distribution of fossil crocodilians and the biological evidence from a varied group of shallow-water organisms.
Fossil crocodilians
(Paul J. Markwick) Suggested that the primary determinant of crocodilians global distribution was temperature. Crocodilians live within a certain range of body temperatures; the 'critical maximum' and 'critical minimum' temperatures and represent temperature gradient above and below which the crocodilians can no longer be able to escape independently from imminent threats for survival. The extant crocodilians critical maximum is has been established as 38- 39°C and the critical minimum about 4-5°C. Paul likeness these temperature boundaries with the palaeodistribution of fossil crocodilians to come up with palaeoclimate distribution during Late Cretaceous and Cenozoic. This concluded that during the Late Cretaceous crown group crocodilians were distributed around the globe and their spatial distribution also expands (Figs. 34-38), such that by the Santonian period (Fig. 36) the crocodilians were already evenly distributed throughout the Northern Hemisphere, except in the Far East.
(Upchurch and Wolfe, 1993), is credited for interpreting the tropical climates (MAT >20°C) that extends to about 40 ° 50°N palaeolatitude and mesothermal conditions (MAT 13- 20°C) stretching to about 60 °. They provide an avenue to interpret the palaeoclimate of a much larger region. The Americas from Patagonia in the south (~ 50 °) to Alberta and Saskatchewan in the north (~60°N) experienced during the Paleocene CMM's >5.5°C and MAT's > 14.2. This points out that there might have been a transition from the greenhouse world of the Cretaceous to the 'ice-house' world in our current environment.
The biological evidence from a varied group of shallow-water organisms:-
(C. GEOFFREY ADAMS 1) Suggest the sea surface temperatures at low latitudes during the Tertiary used the biotic evidence from a different group of shallow-water organisms including mangroves, zooxanthellate corals, and larger foraminifera. The shallow marine organism was ideal for the test because temperature limits their distribution.
Mangroves
Mangroves cover about 60-70% of the world's tropical climates and extend to 32°N and 38°S of the equator. Developed mangrove species can be found along shores where the average temperature of the coldest month is more than 20°C, and the seasonal range is around 5°C. In the Palaeogene, mangroves originated from the Palaeocene of Brazil (Dolianiti, 1955), the Early Eocene of England and Belgium (Collinson, 1983). In the Eocene mangroves have been documented in the Deccan, India, and Borneo. During the Late Eocene mangroves were reported in southwest Australia (Churchill, 1973) and the Gulf Coast States. Using the distribution of the mangroves, Churchill (1973) estimated minimum temperatures of not less than 20-25°C for the in-shore coastal waters of southwestern Australia during the Late Eocene.
Zooxanthellate corals
Larger foraminifera
The larger foraminifera is today confined to an area bounded on the north and south by the 18-20°C means temperature’s for the warmest month of the year (see Fig.2). The larger foraminifera is at their farthest north in September, and farthest south in March (Anon., 1976). Most of this species are found within the 25°C mean summer isotherm. The maps (Figs.2-7) Show that the geographical (latitudinal) distribution of larger foraminifera was wide before, and their diversity was greater during Middle Eocene and Middle Miocene times than they are today. During the Early and Middle Oligocene, they occupied a narrower latitudinal zone and were less diverse compared to today statistics.
Basing on the biotic evidence, (C. GEOFFREY ADAMS 1) concluded that:-
1- A large extent of the world's ocean has had tropical (>25°C) sea-surface temperatures since late Cretaceous times.
2- Most surrounding areas of the tropical zone continue to experience subtropical (20 25°C) sea-surface temperatures levels since the late Cretaceous and this area has been varying in size during periods of global climatic warming and cooling.
3- In the Eocene, and the Early Miocene, the area surrounded by the subtropical and tropical sea-surface isotherms was much larger than in present-day oceans.
1. Results and Discussion:-
Age Late Cretaceous and Eocene epochs Paleocene and Eocene late Eocene and early Oligocene
Research
(Pearson et al.) the tropical sea surface temperatures 28 ± 32 C optical warming
(Paul N.Pearson) Maximum Sea-Surface Temperature >30 Cooler temperatures.
Isotropic evidence insinuates that during late Cretaceous and Early Eocene the tropical sea surface temperature ranged between 28 to 32 C, this variation shows that there was a global warming effect experienced during the two periods. In the Paleocene, the earth had an optical warming with the maximum sea-surface temperature above 30C. Finally, at late Eocene and early Oligocene the planet earth had cool temperatures.
Age Late Cretaceous and Cenozoic Late Eocene-Early Miocene
Fossils
Crocodilians MAT >20 C
Mangroves 20-25 C
Zooxanthellate corals 25°C mean summer isotherm.
The above table reveals the temperature of the tropics sea surface from late Cretaceous and Cenozoic to early Miocene, these temperatures are based on the fossil distribution. It is evidential in the table that the temperatures remain relatively constant. However, both data shows variations in result due to some factors and problems that can affect both sides due to some occurrences. These causes of the differences in the results include;
The arising problems of isotopic data.
Some variety of foraminifera (Savin, 1977; Erez, 1978) and zooxanthellate scleractinian corals (Erez, 1978; Swart and Coleman, 1980) reveal that they do not necessarily deposit calcium carbonate which is in isotopic equilibrium with the sea water. This makes foraminifera fractionate l8o/16o varieties unreliable indicators of water temperature (Savin, 1977; Erez, 1978). The result is affected not only by the organism group Killingley (1983) has insinuated that the effects of diagenetic alteration in deep-sea sediments may be enough to produce the resulting pattern of gradually declining temperatures since the beginning of the Tertiary to the end of the Neogene. He holds that absolute values for ocean bottom temperatures are very consistent for stratigraphically equivalent times which would be unlikely the case if these values were as a result of recrystallization. Besides, Matthews (1984) assumptions about the global ice volume at various times during the Tertiary may have a created an effect on the interpretation of isotopic data.
Arising problems from paleodistribution methods
It was identified that the older the fossil assemblages, the more they vary from those found in the present day. The existence of extinct taxa requires assumptions to be made based on the existing ecological requirements of the nearest living relatives of the organisms. This may is hard to justify because old extinct taxa can also cause a problem as well as major geological events and the geographical distribution of the continents which can affect the geographic distributions of marine animals associated with major palaeocurrent systems (Kennett and Ingram 1995).
Bibliography
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Schneider, S., Fürsich, F.T., Schulz-Mirbach, T. and Werner, W., 2010. Ecophenotypic plasticity versus evolutionary trends-morphological variability in Upper Jurassic bivalve shells from Portugal. Acta Palaeontologica Polonica, 55(4), pp.701-732.
Shackleton, N.J., 1974. Attainment of isotopic equilibrium between ocean water and the benthonic foraminifera genus Uvigerina: isotopic changes in the ocean during the last glacial.
