May 28, 2010 at 10:30 pm | Posted in Earth, History, Research, Science & Technology, World-system | Leave a comment









CLAW hypothesis

The CLAW hypothesis proposes a feedback loop that operates between ocean ecosystems and the Earth‘s climate.[1] The hypothesis specifically proposes that particular phytoplankton that produce dimethyl sulfide are responsive to variations in climate forcing, and that these responses lead to a negative feedback loop that acts to stabilise the temperature of the Earth’s atmosphere. The CLAW hypothesis was originally proposed by Robert Charlson, James Lovelock, Meinrat Andreae and Stephen Warren, and takes its acronym from the first letter of their surnames.[2]

The CLAW hypothesis

The hypothesis describes a feedback loop that begins with an increase in the available energy from the sun acting to increase the growth rates of phytoplankton by either a physiological effect (due to elevated temperature) or enhanced photosynthesis (due to increased irradiance). Certain phytoplankton, such as coccolithophorids, synthesise dimethylsulfoniopropionate (DMSP), and their enhanced growth increases the production of this osmolyte. In turn, this leads to an increase in the concentration of its breakdown product, dimethyl sulfide (DMS), in first seawater, and then the atmosphere. DMS is oxidised in the atmosphere to form sulfur dioxide, and this leads to the production of sulfate aerosols. These aerosols act as cloud condensation nuclei and increase cloud droplet number, which in turn elevate the liquid water content of clouds and cloud area. This acts to increase cloud albedo, leading to greater reflection of incident sunlight, and a decrease in the forcing that initiated this chain of events. Note that the feedback loop can operate in reverse, such that a decline in solar energy leads to reduced cloud cover and thus to an increase in the amount of solar energy reaching the Earth’s surface.

A significant feature of the chain of interactions described above is that it creates a negative feedback loop, whereby a change to the climate system (increased/decreased solar input) is ultimately counteracted and damped by the loop. As such, the CLAW hypothesis posits an example of planetary-scale homeostasis or complex adaptive system, consistent with the Gaia hypothesis framed by one of the original authors of the CLAW hypothesis, James Lovelock.[3]

Some subsequent studies of the CLAW hypothesis have uncovered some evidence to support its mechanism,[2][4] although this is not unequivocal.[5] Other researchers have suggested that a CLAW-like mechanism may operate in the Earth’s sulfur cycle without the requirement of an active biological component.[6]

The Anti-CLAW hypothesis

In his 2007 book, The Revenge of Gaia, Lovelock proposed that instead of providing negative feedback in the climate system, the components of the CLAW hypothesis may act to create a positive feedback loop.[7]

Under future global warming, increasing temperature may stratify the world ocean, decreasing the supply of nutrients from the deep ocean to its productive euphotic zone. Consequently, phytoplankton activity will decline with a concommitant fall in the production of DMS. In a reverse of the CLAW hypothesis, this decline in DMS production will lead to a decrease in cloud condensation nuclei and a fall in cloud albedo. The consequence of this will be further climate warming which may lead to even less DMS production (and further climate warming …). The figure to the right shows a summarising schematic diagram.

Evidence for the anti-CLAW hypothesis is constrained by similar uncertainties as those of the sulfur cycle feedback loop of the CLAW hypothesis. However, researchers simulating future oceanic primary production have found evidence of declining production with increasing ocean stratification.[8]


1. a b Charlson, R. J., Lovelock, J. E., Andreae, M. O. and Warren, S. G. (1987). Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature 326, 655-661.

2. a b Andreae, M. O., Elbert, W. and Demora, S. J. (1995). Biogenic sulfur emissions and aerosols over the tropical South Atlantic, 3. Atmospheric dimethylsulfide, aerosols and cloud condensation nuclei. J. Geophys. Res. 100, 11335-11356.

3. Lovelock, J.E. (2000) [1979]. Gaia: A New Look at Life on Earth (3rd ed. ed.). Oxford University Press. ISBN 0-19-286218-9.

4. Cropp, R.A., Gabric, A.J., McTainsh, G.H., Braddock, R.D. and Tindale, N. (2005). Coupling between ocean biota and atmospheric aerosols: Dust, dimethylsulphide, or artifact? Global Biogeochemical Cycles 19, GB4002.

5. Vallina, S. M., Simo, R., Gasso, S., De Boyer-Montegut, C., del Rio, E., Jurado, E. and Dachs, J. (2007). Analysis of a potential “solar radiation dose-dimethylsulfide-cloud condensation nuclei” link from globally mapped seasonal correlations. Global Biogeochemical Cycles 21, GB2004.

6. Shaw, G.E., Benner, R.L., Cantrell, W. and Clarke, A.D. (1998). The regulation of climate: A sulfate particle feedback loop involving deep convection – An editorial essay. Climate Change 39, 23-33.

7. a b Lovelock, James (2007). The Revenge of Gaia. Penguin Books Ltd. ISBN 0141025972.

8. Cox, P. M., Betts, R. A., Jones, C. D., Spall, S. A. and Totterdell, I. J. (2000). Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model. Nature, 408, 184-187.

External links

· Gaia and CLAW, Max Planck Institute for Chemistry, Mainz

· DMS and climate, Pacific Marine Environmental Laboratory, Seattle


Tephrochronology is a geochronological technique that uses discrete layers of tephra—volcanic ash from a single eruption—to create a chronological framework in which paleoenvironmental or archaeological records can be placed. Such an established event provides a “tephra horizon”. The premise of the technique is that each volcanic event produces ash with a unique chemical “fingerprint” that allows the deposit to be identified across the area affected by fallout. Thus, once the volcanic event has been independently dated, the tephra horizon will act as time marker.

The main advantages of the technique are that the volcanic ash layers can be relatively easily identified in many sediments and that the tephra layers are deposited relatively instantaneously over a wide spatial area. This means they provide accurate temporal marker layers which can be used to verify or corroborate other dating techniques, linking sequences widely separated by location into a unified chronology that correlates climatic sequences and events.

Tephrochronology requires accurate geochemical fingerprinting (usually via an electron microprobe).[1] An important recent advance is the use of LA-ICP-MS (i.e. laser ablation ICP-MS) to measure trace-element abundances in individual tephra shards.[2] One problem in tephrochronology is that tephra chemistry can become altered over time, at least for basaltic tephras.[3]

Early tephra horizons were identified with the Saksunarvatn tephra (Icelandic origin, ca 10.2 cal. ka BP), forming a horizon in the late Pre-Boreal of Northern Europe, the Vedde ash (also Icelandic in origin, ca 12.0 cal. ka BP) and the Laacher See tephra (in the Eifel volcanic field, ca 12.9 cal. ka BP). Major volcanoes which have been used in tephrochronological studies include Vesuvius, Hekla and Santorini. Minor volcanic events may also leave their fingerprint in the geological record: Hayes Volcano is responsible for a series of six major tephra layers in the Cook Inlet region of Alaska. Tephra horizons provide a synchronous check against which to correlate the palaeoclimatic reconstructions that are obtained from terrestrial records, like fossil pollen studies (palynology), from varves in lake sediments or from marine deposits and ice-core records, and to extend the limits of carbon-14 dating.

A pioneer in the use of tephra layers as marker horizons to establish chronology was Sigurdur Thorarinsson, who began by studying the layers he found in his native Iceland.[4] Since the late 1990s, techniques developed by Chris S. M. Turney (QUB, Belfast; now University of Exeter) and others for extracting tephra horizons invisible to the naked eye (“cryptotephra”)[5] have revolutionised the application of tephrochronology. This technique relies upon the difference between the specific gravity of the microtephra shards and the host sediment matrix. It has led to the first discovery of the Vedde ash on the mainland of Britain, in Sweden, in the Netherlands, in the Swiss Lake Soppensee and in two sites on the Karelian Isthmus of Baltic Russia. It has also revealed previously undetected ash layers, such as the Borrobol Tephra first discovered in northern Scotland, dated to ca. 14.4 cal. ka BP,[6] the microtephra horizons of equivalent geochemistry from southern Sweden, dated at 13,900 Cariaco varve yrs BP[7] and from northwest Scotland, dated at 13.6 cal. ka BP.[8]


1. Smith & Westgate (1969)

2. Pearce et al. (2002)

3. Pollard et al. (2003)

4. Alloway et al. (2007)

5. Turney et al. (1997)

6. Turney et al. (1997)

7. Davies (2004)

8. Ranner et al. (2005)


· Alloway B.V., Larsen G., Lowe D.J., Shane P.A.R., Westgate J.A. (2007). “Tephrochronology”, Encyclopedia of Quaternary Science (editor—Elias S.A.) 2869–2869 (Elsevier).

· Davies S.M., Wastegård S., Wohlfarth B. (2003). “Extending the limits of the Borrobol Tephra to Scandinavia and detection of new early Holocene tephras”, Quaternary Research, 59: 345–352.

· Davies, S. M., Wohlfarth, B., Wastegård, S., Andersson, M., Blockley, S., & Possnert, G.,(2004). “Were there two Borrobol Tephras during the early Late-glacial period: implications for tephrochronology?”, Quaternary Science Reviews, 23, 581–589.

· Dugmore A., Buckland P.C. (1991). “Tephrochronology and Late Holocene soil erosion in South Iceland”, Environmental Change in Iceland: Past and Present (eds. J.K. Maizels and C. Caseldine) 147–159 (Dordrecht: Kluwer Academic Publishers).

· Keenan D.J. (2003). “Volcanic ash retrieved from the GRIP ice core is not from Thera“, Geochemistry, Geophysics, Geosystems, 4, doi: 10.1029/2003GC000608.

· Pearce N.J.G., Eastwood W.J., Westgate J.A., Perkins W.T. (2002). “Trace-element composition of single glass shards in distal Minoan tephra from SW Turkey”, Journal of the Geological Society, London, 159: 545–556. doi:10.1029/2003GC000672

· Pollard A.M., Blockley S.P.E., Ward K.R. (2003). “Chemical alteration of tephra in the depositional environment”, Journal of Quaternary Science, 18: 385–394. doi:10.1002/jqs.760

· Ranner, P. H., Allen, J. R. M. & Huntley, B. (2005). A new early Holocene cryptotephra from northwest Scotland. Journal of Quaternary Science, 20, 201–208.

· Smith D.G.W., Westgate J.A. (1969). “Electron probe technique for characterizing pyroclastic deposits”, Earth and Planetary Science Letters, 5: 313–319.

· Þórarinsson S. (1970). “Tephrochronology in medieval Iceland“, Scientific Methods in Medieval Archaeology (ed. R. Berger) 295–328 (Berkeley: University of California Press).

· Turney C.S.M., Harkness D.D., Lowe J.J. (1997). “The use of microtephra horizons to correlate late-glacial lake sediment successions in Scotland“, Journal of Quaternary Science, 12: 525–531.

External links

· USGS tephrochronology technique

· Tephra and Tephrochronology, The University of Edinburgh

· Antarctic Research Group


· TephraBase

· International Arctic Workshop, 2004. Stefan Wastegård et al., “Towards a tephrochronology framework for the last glacial/interglacial transition in Scandinavia and the Faroe Islands”: (Abstract)

Oxygen isotope ratio cycle and Climate Change

Oxygen isotope ratio cycles are cyclical variations in the ratio of the abundance of oxygen with an atomic mass of 18 to the abundance of oxygen with an atomic mass of 16 present in some substance, such as polar ice or calcite in ocean core samples. The ratio is linked to water temperature of ancient oceans, which in turn reflects ancient climates. Cycles in the ratio mirror climate changes in geologic history.

Isotopes of oxygen

Oxygen (chemical symbol O) has three naturally occurring isotopes: 16O, 17O, and 18O, where the 16, 17 and 18 refer to the atomic mass. The most abundant is 16O, with a small percentage of 18O and an even smaller percentage of 17O. Oxygen isotope analysis considers only the ratio of 18O to 16O present in a sample.


1. Miller, Dana L.; Mora, Claudia I.; Grissino-Mayer, Henri D.; Mock, Cary J.; Uhle, Maria E.; Sharp, Zachary (July 31, 2006/September 19, 2006). “Tree-ring isotope records of tropical cyclone activity”. Proceedings of the National Academy of Sciences, 2006 – National Acad Sciences. 103 no. 39. National Acad Sciences. pp. 14294–14297. doi:10.1073/pnas.0606549103. Retrieved 2009-11-11.

· Encyclopædia Britannica under Climate and Weather, Pleistocene Climatic Change

· Harmon Craig. 1961. “Isotopic variations in meteoric waters”. Science. 133. pp 1702

· S. Epstein, T. Mayeda. 1953. “Variation of O18 content of waters from natural sources”, Geochemica et Cosmochemica Acta. 4. pp 213

The calculated ratio of the masses of each present in the sample is then compared to a standard, which can yield information about the temperature at which the sample was formed – see Proxy (climate) for details.

Connection between isotopes and temperature/weather

18O is two neutrons heavier than 16O and causes the water molecule in which it occurs to be heavier by that amount. The addition of more energy is required to vaporize H218O than H216O, and H218O liberates more energy when it condenses. In addition, H216O tends to diffuse more rapidly.

Because H216O requires less energy to vaporize, and is more likely to diffuse to the liquid surface, the first water vapor formed during evaporation of liquid water is enriched in H216O, and the residual liquid is enriched in H218O. When water vapor condenses into liquid, H218O preferentially enters the liquid, while H216O is concentrated in the remaining vapor.

As an air mass moves from a warm region to a cold region, water vapor condenses and is removed as precipitation. The precipitation removes H218O, leaving progressively more H216O-rich water vapor. This distillation process causes precipitation to have lower 18O/16O as the temperature decreases. Additional factors can affect the efficiency of the distillation, such as the direct precipitation of ice crystals, rather than liquid water, at low temperatures.

Due to the intense precipitation that occurs in hurricanes, the H218O is exhausted relative to the H216O, resulting in relatively low 18O/16O ratios. The subsequent uptake of hurricane rainfall in trees, creates a record of the passing of hurricanes that can be used to create a historical record in the absence of human records.[1]

Connection between temperature and climate

The 18O/16O ratio provides a record of ancient water temperature. Water 10 to 15 degrees Celsius (18 to 27 degrees Fahrenheit) cooler than present represents glaciation. Precipitation and therefore glacial ice contain water with a low 18O content. Since large amounts of 16O water are being stored as glacial ice, the 18O content of oceanic water is high. Water up to 5 degrees Celsius (9 °F) warmer than today represents an interglacial, when the 18O content is lower. A plot of ancient water temperature over time indicates that climate has varied cyclically, with large cycles and harmonics, or smaller cycles, superimposed on the large ones. This technique has been especially valuable for identifying glacial maxima and minima in the Pleistocene.

Earth‘s climate.


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