Interaction Between Ocean-Atmospheric Cycles and their effects

Interaction Between Ocean-Atmospheric Cycles and their effects:

Indian Ocean Dipole and El Nino Southern Oscillation

Both the ENSO and the Indian Ocean Dipole are ocean atmospheric systems that consist of the relative sea-surface temperature and the global wind systems, with the ENSO a system which affects predominantly the pacific ocean and the Indian Ocean Dipole affecting the Indian ocean. These Ocean temperature and wind systems are an important control system on the nutrient cycle within the oceans but also for agriculture as they affect rainfall patterns and intensity and as such we will be looking at the effects of both these cycles within the Indian Monsoons.

ENSO: When People think of monsoons they often think of the Indian Monsoons which are also closely related to torrential rainfall within Indian. However, this is completely true as monsoons are common on other parts of the globe and monsoons are also associated with periods of drought as the monsoons as a system are related to the seasonal change in wind direction and rainfall patterns driven by shifts in temperature differences between the ocean and land causing pressure differentials as depicted in the diagram below. It is this temperature dependency of the monsoons on oceans that the Indian Ocean Dipole and ENSO become important systems to understand.   

Indian Seasonal Monsoons.

ENSO is a climatic pattern involving changes in the temperature of the waters in the central and eastern tropical Pacific Ocean. It involves the meeting of the Northern and Southern Hadley cells at the ITCZ, in which surface Easterly winds are weak causing the Western boundary of the pacific ocean surface to rise by 0.5-0.75m compared to the Eastern boundary during the normal Walker Cycle. However, under El Nino conditions the Easterly winds weaken further causing the Western boundary seal level to drop causing the warm water to be evenly spread throughout the pacific ocean increasing the depth of the thermocline and causing rain to fall over the central pacific ocean as depicted below.

El Nino Conditions over the pacific ocean.

However, the opposite occurs during La Nina conditions with the normal Walker cycle conditions becoming more exaggerated increasing rainfall over the Western boundary of the pacific ocean, (Link).

Indian Ocean Dipole: The Indian Ocean Dipole (IOD) is similar to the ENSO because it speaks about the difference in sea surface temperatures of the Eastern and Western Boundaries of the Indian Ocean. The IOD has three phases, just like the ENSO, with the three phases being neutral, positive and negative. Neutral Conditions are when sea surface temperatures over the whole Indian ocean are relatively normal but during the positive phase of the IOD the Westerly winds weaken allowing warmer waters to concentrate on the tropical Eastern boundary of Africa increasing land rainfall within this region. The IOD is also an important driver of the Australian climate having a strong impact on the agriculture of the region and more recently the bush fires which have occurred as this years positive IOD hit 2 degrees Celsius exaggerating the dry conditions experienced in Australia.

Now that we have a basic understanding of how the Indian Monsoons work, the ENSO and the IOD, how do the two ocean atmospheric systems interact in influence the Indian Monsoons. A study conducted by Cherchi, showed that rainfall decreased during the Indian summer when the El Nino and positive IOD occurred concurrently with a study by Ashok, confirming this trend. However, both studies also showed how the different phases of both system when occurring together but not in synergy either have amplifying or reducing effects on the monsoon rains, but it was also noted by Ashok, that the IOD had weakened the ENSO-monsoon relationship.  

In Conclusion each of the different ocean-atmospheric system when looked in isolation have differing effects but when occurring together have both positive and negative effects on each other depending on the feedback mechanisms of the climates they influence.

Reference List:
  Ashok, K., Guan, Z., Saji, N.H. and Yamagata, T., 2004. Individual and combined influences of ENSO and the Indian Ocean dipole on the Indian summer monsoon. Journal of Climate, 17(16), pp.3141-3155.

 Cherchi, A. and Navarra, A., 2013. Influence of ENSO and of the Indian Ocean Dipole on the Indian summer monsoon variability. Climate dynamics, 41(1), pp.81-103.

  Hashizume, M., Chaves, L.F. and Minakawa, N., 2012. Indian Ocean Dipole drives malaria resurgence in East African highlands. Scientific reports, 2(1), pp.1-6.

Oxygen isotope uses:

Oxygen Isotopes

In order to understand the different uses of Oxygn isotopes one first needs to know what an isotope is and be able to differentiate between stable and radioactive isotopes. Oxygen as an isotope is widely used within the scientific community to accomplish a number of things some of which will be explored in this blog.

What is an isotope:

An isotope is a variation of an element which either has more or less neutrons than the most abundant form of that particular element in question. This variation in the number of neutrons within the elements nucleus increases or decreases the atomic mass of the element and allow for the element to have different chemical reaction rates even though all the different elemental isotopes will have the same chemical reaction principles. There are however, two different types of isotopes namely stable and unstable isotopes. Radioactive and stable isotopes are not very different in the sense that they all contain differing numbers of neutrons however radioactive isotopes have excess nuclear energy which allows them to radioactively decay and give off that excess energy with it known that radioactive decay is responsible for the earths internal heat budget, while stable isotopes do not have excess nuclear energy and do not radioactively decay. Just as every element has an isotope each element has both stable and radioactive isotopes which is important to known since they each have different applications.

Oxygen Isotope Earth Science uses:

Oxygen has three known stable isotopes: 16-oxygen, 17-oxygen and 18-oxygen, and a number of Radioactive isotopes. The main use of oxygen isotopes specifically the ratio between 16-oxygen and 18O is in paleoclimate reconstruction studies. An example of such paleoclimate reconstruction is using the stable isotope composition of marine calcite which looks at the 18-oxygen signature of shell marine life to determine the seawater temperature which has been done by Tindal .Tindal in a study found that the use of 18-oxygen composition within shell marine life is useful as the marine life incorporates the 18-oxygen signature of the seawater with the fractionation of which is temperature dependent allowing us to reconstruct the temperature at which these marine creatures lived and therefore the temperature of the sea body. 18-oxygen is used to determine the water temperature because 16-oxygen is more readily evaporated than 18-oxygen, this relation is also important as it allows for the origin of precipitation to be found. This reaction rate differential between 16-oxygen and 18-oxygen is important in paleoclimate reconstruction studies as the lighter oxygen isotope will have a higher concentration within glaciers and terrestrial waterbodies compared to oceans which would have greater concentrations of 18-oxygen.

Oxygen isotopes are extensively used within the field of petrology because of the important role which hydrothermal fluids play in magmatic processes and metamorphic processes. Most studies that use Oxygen isotopes look at the interaction between hydrous and anhydrous minerals within whole rocks. Schiffman used 18-oxygen isotope analysis of hydrothermal zonation within the northern Troodos complex to indicate that the rock complex was formed in the upper Cretaceous within a seawater hydrothermal system. Schiffman found out that the complex is rich in subgreenschist facies mineral assemblages that contain high ratios of 18-oxygen isotopes which is indicative of rocks formed within areas where seawater diffuse recharge is an important component of rock formation.

Other uses of Oxygen Isotope:

Most Earth Science studies use the stable isotopes of oxygen within their studies to get to whatever answer they desire to however Earth Scientist are not the only users of isotopes, medical researchers like Ter-pogossian have previously used 15-oxgen to study the cerebral blood flow, blood volume, and oxygen metabolism because of the short half-life of 15-oxygen, which is 2 minutes. The use of radioactive 15-oxygen within the medical world has also been used to determine the functionality of the human respiratory system with a study by Dyson used 15-oxygen to determine the regional lung function within humans. These are two of many examples of the application of radioactive oxygen isotopes outside of the Earth sciences.

Overall the use of Oxygen isotopes goes beyond just paleoclimate reconstruction with this blog giving a number of different applications both within and outside the Earth Science stream for the different applications of both stable and radioactive oxygen isotopes. However, there are a lot more applications out there which this blog has not looked into but hopes that an interest within the topic has been ignited within my readers.

Reference List:

 Schiffman, P. and Smith, B.M., 1988. Petrology and oxygen isotope geochemistry of a fossil seawater hydrothermal system within the Solea graben, northern Troodos ophiolite, Cyprus. Journal of Geophysical Research: Solid Earth, 93(B5), pp.4612-4624.

 Tindall, J., Flecker, R., Valdes, P., Schmidt, D.N., Markwick, P. and Harris, J., 2010. Modelling the oxygen isotope distribution of ancient seawater using a coupled ocean–atmosphere GCM: implications for reconstructing early Eocene climate. Earth and Planetary Science Letters, 292(3-4), pp.265-273.

 Ter-Pogossian, M.M. and Herscovitch, P., 1985, October. Radioactive oxygen-15 in the study of cerebral blood flow, blood volume, and oxygen metabolism. In Seminars in nuclear medicine (Vol. 15, No. 4, pp. 377-394). WB Saunders.

 Dyson, N.A., Hugh-Jones, P., Newbery, G.R., Sinclair, J.D. and West, J.B., 1960. Studies of regional lung function using radioactive oxygen. British medical journal, 1(5168), p.231.

Atmospheric Composition during the Last Glacial maximum:

Atmospheric Composition during the Last Glacial maximum:

Image 1: Last Glacial Maximum Ice cover. 

The last Glacial Maximum has been studied in great detail but there does exist areas where greater research must be done and one of these areas is the composition of the atmosphere during this period compared to the current atmosphere pre-industrialization.

Most of the effort when it comes to studying the atmosphere of the last glacial maximum has been focused on atmospheric carbon dioxide and its sequestration, with little attention being paid to methane and dust and the effects they have on the global climate during this glacial period.

Carbon Dioxide vs Methane:
One of the main reasons as to why CO2 is so extensively studied is the fact that during the Devonian the drop in CO2 levels caused a major Glaciation at the end of the Devonian. However, as a Greenhouse gas CO2 is not the strongest naturally occurring greenhouse gas as methane per ton is four times more potent that CO2 however it does have a shorter life span than CO2 with a lifespan of CH4 in the order of a decade as Kaplan indicated. Kaplan found that Methane as a natural gas is not produced as rapidly as CO2 with the main producers of Methane being wetlands and boreal wetlands in the northern hemisphere.

Image 2: Natural Methane Production

Methane Importance:
Dust is an important component in the atmosphere as it plays a number of roles which range from absorbing and scattering incoming solar and outgoing infrared radiation and indirectly by acting as ice nuclei as well as providing micronutrients for organisms in the ocean affecting the biochemical cycles within the ocean which are responsible for the reduction and increase in atmospheric CO2. The priority nutrient within dust is Iron as it the main nutrient needed for phytoplankton to grow and remove CO2 from the atmosphere and store it in deep ocean sinks.

The atmosphere of the Last Glacial Maximum is known to be very different in terms of composition with studies conducted by Lambert showing that during the last glacial maximum dust deposition was 2-3 times higher in the tropics and South Pacific, 5 times higher in the South Atlantic and 20-30 times higher in polar regions than present. Lamberts study also crucially found out that the Last Glacial Maximum contained 3-4 times more dust than the Holocene. The increase in dust partial concentrations during the last glacial maximum is partially due to the increase in erosion caused by advancing glaciers which mechanically abraded underlying rock turning it into dust.

The concentration of Methane during the Last Glacial Maximum is thought to have been around 385ppb, with an increase to 450ppb from the last glacial maximum to pre-industrialization, with this figure coming from a study conducted by Kaplan. Kaplan’s study also gave the reasons as to why there was a great decrease in the concentration of methane, the biggest contributor to the low methane concentrations was the reduction in the production of methane as wetlands reduced in total land coverage so did that of plants and the number of animals.

As common knowledge the concentration of atmospheric Carbon dioxide was lower during the last Glacial maximum than pre-industrialization and present, with atmospheric CO2 concentration during the last glacial maximum being 40% lower than the Holocene which sat at 280ppm and 368ppm post-industrialization. The most intriguing aspect about the concentration of atmospheric CO2 and methane is that they have a synchronous increase and decrease pattern as Monnin found in his study, with it also being noticed today that as ice caps melt trapped methane gas is being released which adds more greenhouses gases into the atmosphere increasing temperature as it would have at the termination of the Last Glacial Maximum.

The different greenhouse gases and other atmospheric components vary depending on the climatic reign at the time, with an interconnected relation existing between dust, methane and carbon dioxide. Dust increase in atmosphere increase phytoplankton production and therefore atmospheric carbon sequestration which decreases global temperatures reducing the production of methane and shortening its lifespan as more OH is available to react with the remaining methane. All these factors contribute to the continuation of a glaciation and the termination of one and subsequent warming of the Earth.

Reference List:

 Monnin, E., Indermühle, A., Dällenbach, A., Flückiger, J., Stauffer, B., Stocker, T.F., Raynaud, D. and Barnola, J.M., 2001. Atmospheric CO2 concentrations over the last glacial termination. Science, 291(5501), pp.112-114.

 Lambert, F., Tagliabue, A., Shaffer, G., Lamy, F., Winckler, G., Farias, L., Gallardo, L. and De Pol‐Holz, R., 2015. Dust fluxes and iron fertilization in Holocene and Last Glacial Maximum climates. Geophysical Research 

 Kaplan, J.O., Folberth, G. and Hauglustaine, D.A., 2006. Role of methane and biogenic volatile organic compound sources in late glacial and Holocene fluctuations of atmospheric methane concentrations. Global Biogeochemical Cycles, 20(2).

Use of Geology to state the timing of the Great Oxidation Event:

Use of Geology to state the timing of the Great Oxidation Event:

Oxidation of the oceans.

The timing of the Great Oxidation event has been of great debate within the scientific community, because of the different number of techniques which can be used to postulate the timing of the Great Oxidation event. However, a large number of studies have focused on the use of geological data.

The exact reason as to why scholars continue to focus on geological data to create time constraints for the great oxidation event will be explored in this blog.

The most obvious reason as to why geological data is often used is the fact that geological features are physical objects which can be studied, they do however also allow for a range of data sets to be uncovered through their geochemical analysis and correlation with other geological features which have been found to have the same age. The common method of getting the date of a rock formation is through Zircon dating, however one first has to establish if the zircon is of syngenetic or epigenetic origin. The timing of deposition is only one of the major concerns when it comes to geological features and their inclusions, reworking and metamorphism can create major timing issues.

Geochemical Analysis:

Geochemical data can also be gathered which give relative dates depending on the isotopes which one chooses to study. Crowe studied the distribution of Cr isotopes of Nsuze paleosol from the Pongola Supergroup of South Africa. Crowe used Cr isotopes because they are sensitive indicators for oxidation weathering. S-isotopes have also been used to determine the timing of the Great Oxidation event, in a study Gumsley analyzed S-isotopes from the Duitschland Formation of the Transvaal Supergroup. Gumsley's study focused on the S-isotope fractionation of mass-independent fractions as they have been found to provide the tightest constraint on the timing of the Great oxidation event. These two studies studied two different geological features and used two different isotope analysis methods which yielded two different times for the great oxidation event with Crowe's study indicating that the event occurred 300-400 million years earlier than, Gumsley's study which stated that the event occurred 2.4-2.3Ga, indicating the uncertainty that still exist when using geochemical data.

Another method which has been recently adopted by scientist is the use of bio-markers within rock formations. Bio-markers are known to be sensitive to changes in climate since they were the living organisms during that specific climatic regime and as such would be the best detectors of Oxygen production during the Great Oxidation event and other atmospheric and environmental changes.

Gumsley, went beyond just studying S-isotope fraction, he also studied the assembly of large continental mass, extensive magmatism and continental migration to near-equatorial latitudes. This was done because the extensive magmatism created large igneous provinces which would have released a large amount of greenhouse gases especially carbon dioxide, while the study of the large continental mass and its migration would allow for better correlation between different rock formation. The study of the large igneous provinces was also found to be responsible for triggering a nutrient flux which would have increased the photosynthetic activity and therefore the production of Oxygen possibly signaling the beginning of the great Oxidation, 2.5-2.4 Ga.

Even though a number of challenges do exist when it comes to using geological data as evidence for the timing of the great oxidation, when the different data sets are used together as one a more accurate and coherent date is found as to when the great oxidation event occurred 2.4 Ga.  

Reference List:

 Crowe, S.A., Døssing, L.N., Beukes, N.J., Bau, M., Kruger, S.J., Frei, R. and Canfield, D.E., 2013. Atmospheric oxygenation three billion years ago. Nature, 501(7468), pp.535-538.

 Gumsley, A.P., Chamberlain, K.R., Bleeker, W., Söderlund, U., de Kock, M.O., Larsson, E.R. and Bekker, A., 2017. Timing and tempo of the Great Oxidation Event. Proceedings of the National Academy of Sciences, 114(8), pp.1811-1816.

 Sessions, A.L., Doughty, D.M., Welander, P.V., Summons, R.E. and Newman, D.K., 2009. The continuing puzzle of the great oxidation event. Current Biology, 19(14), pp.R567-R574.