Wednesday, July 23, 2014

Bahama Carbonate Precipitation Triggered By Saharan Dust

Interesting paper in Geology : The fertilization of the Bahamas by Saharan dust: A trigger for carbonate precipitation? - P.K. Swart, A.M. Oehlert, G.J. Mackenzie, G.P. Eberli and J.J.G. Reijmer

Abstract

The enigma of the Bahamas is that this highly productive carbonate system has existed for at least 100 m.y., building a vast edifice of carbonates, thousands of meters thick, in an essentially nutrient-poor environment. Based on measurements of the insoluble material, the Fe and Mn in the carbonate fraction, and the δ15N of the sedimentary organic matter, we suggest a paradigm shift in order to explain the formation of the Bahamas and possibly other similar platforms. We propose that the Great Bahama Bank is currently, and may in the past have been, fertilized by atmospheric dust, promoting the fixation of atmospheric N2 by cyanobacteria. These cyanobacteria provided a source of nitrogen to the rest of the community in this nutrient-poor environment. The fixation of N has imparted a characteristic δ15N signal and has been responsible, through the drawdown of CO2, for initiating the precipitation of carbonate in the shallow waters. This phenomenon might be responsible for the formation of vast amounts of sediments in the oceans, not only within recent times, but throughout geological history, particularly in the early history of the Earth prior to the existence of calcium carbonate–secreting organisms.

Many carbonate sedimentologists suspect that microbial activity has played an important role in carbonate sediment precipitation and this paper provides more evidence for it from a familiar setting. The geology of the Bahamas has been extensively studied but sometimes basic questions remain "Why such  prolific sediment production by organisms in a nutrient poor environment?".  The answer in this case may be an exogenous source of nitrogen. Whether this is a more general explanation remains to be answered. There may be other controls, for example in the Proterozoic sea water chemistry was different with elevated levels of calcium carbonate saturation. This means that there were lots of Ca and CO3 molecules available to aggregate as CaCO3 crystals. This enabled -with and without the aid of microbial activity-  vast deposits of carbonates to accumulate.

Tuesday, July 1, 2014

Geology Ignored In The Planning And Building Of Himalayan Dams

In Current Science K.S Valdiya explains (Open Access):

It will be obvious from the distribution of dam locations (Figure 1; Tables 3–6) based on information culled from reports of Central Electricity Authority, Uttara-khand Hydroelectricity Nigam, Uttarakhand Renewable Energy Development Authority, etc. that the existing hydroelectric projects and those that are under construction or planned are sited close to the terrane-defining thrusts known to be active. The sites were chosen presumably in the narrowest stretches of the river valleys, little realizing that the otherwise wide valleys with gentle valley sides become narrow with steep to nearly vertical walls due to uplift of the ground and attendant accelerated riverbed erosion as explained earlier. The ground rises as a conse-quence of upward movement on active faults/thrusts (Figure6). Moreover, the belts of active faults are made up of deformed rocks –many-times folded, sheared, shattered an even crushed rocks. These rocks understandably easily break-up, fall -off, creep and slide or slump down when excavated or shaken by earthquakes and explosions,and sink under loads. These incidences are bound to pose a threat to the various structures built in the project areas.

The development of hydroelectric projects not only entails excavations for the head race dams and associated coffer dams, diversion tunnels, main tunnels for carrying water to turbines, and multitudes of adits, but also for thenetwork of roads, for residential colonies for work force,and for power generators. Obviously, a dam site–nomatter if it is just a small one–is excessively subject to tampering with the natural balance in a zone of very weakened rocks.

Reactivation of the active thrusts is bound to impact the stability of the engineering structures. One of the impacts could be the displacement or disruption of the structures due to sudden release of stress that the thrust movements entail. The effects on the tunnels associated with dams would be far more severe – there would be dis-ruption or offsetting of tunnel, roof collapse, sudden on-rush of interstitial groundwater with crushed material,and severe damage to tunnel lining. The very making of a tunnel is like opening an underground drainage and thus altering the groundwater regimes of the mountains, resulting in drastic lowering of groundwater table and at tendant drying up of springs and dwindling of surface flow in streams.

Figure 1 is self-explanatory. Needless to state that a large number of existing and planned hydroelectric projects are bound to encounter serious problems, particularly if and when movements take place on the thrusts in the proximity of the project locations.


Uttarakhand has plans for 180 big and small hydroelectric projects with 95 dams in the middle and upper reaches of the Alaknanda and Bhagirathi rivers in the vicinity south of the Main Central Thrust. K.S Valdiya suggests that the sites should be chosen preferably north of the Main Central Thrust in regions with much lower population density which will lead to less environmental, social and economic problems. The current government has indicated that it will lean towards rapid environmental clearances for infrastructure projects, so just how much attention will be given to warnings like this one?

Tuesday, May 27, 2014

A Survivor From The Namib Desert

This beautiful passage from Robert Krulwich's essay on the plant Welwitschia mirabilis, a survivor, the last representative of its genus, isolated and endemic to the Namib desert in South Africa:

Welwitschia, when you finally get to see one, sits apart. It's very alone. All its relatives, its cousins, nieces, nephews have died away. It is the last remaining plant in its genus, the last in its family, the last in its order. "No other organism on earth can lay such a claim to being 'one of its kind,' " writes biologist Richard Fortey. It comes from a community of plants that thrived more than 200 million years ago. All of them slowly vanished, except for Welwitschia. It has survived by doing very little, very, very slowly — sipping little wafts of dew in the early mornings, otherwise minding its own business, as the big, busy world goes by.

With much recent attention given to the question of de-extinction i.e. bringing back extinct species, a more urgent focus needs to be on species and populations which are alive today and hanging by a thread. Genetics will play a role here too along with old fashioned conservation of the ecology in which these creatures persist despite all odds.

Thursday, May 15, 2014

Sherlock Holmes And Long Term Evolutionary Patterns In Dinosaur Body Size

This week's (in Pune, India) Sherlock Holmes Elementary featured a murder mystery involving a smuggled dinosaur fossil. The fossil is believed by some palaeontologists to be entombed in rocks of the earliest Paleocene, making that dinosaur a survivor of the end Cretaceous mass extinction. Other palaeontologists strongly disagree with this survivor fauna scenario, which the drama turns into a motive for murder. There was a fair amount of geology and palaeontology in the episode and a reference to a term "dead clade walking".  This term is real life was coined by David Jablonski a palaeontologist from University of Chicago.  The term means that a clade or a lineage has survived a mass extinction but its fate has been sealed. Over a period of time say a few million years after the mass extinction that group does eventually go extinct.

Some dinosaur species may have survived the mass extinction but by early Palaeocene they were certainly all gone... except one lineage.. the Availea or birds. They prospered in the Cenozoic, radiating into a hundreds of species. Their success may have had deep roots and one important factor may have been the small size of their ancestors.

A paper in PLOS Biology explores the long term evolutionary patterns of dinosaurs and finds a positive relationship between high rates of evolution and small size-

Rates of Dinosaur Body Mass Evolution Indicate 170 Million Years of Sustained Ecological Innovation on the Avian Stem Lineage:

Author Summary

Animals display huge morphological and ecological diversity. One possible explanation of how this diversity evolved is the "niche filling" model of adaptive radiation—under which evolutionary rates are highest early in the evolution of a group, as lineages diversify to fill disparate ecological niches. We studied patterns of body size evolution in dinosaurs and birds to test this model, and to explore the links between modern day diversity and major extinct radiations. We found rapid evolutionary rates in early dinosaur evolution, beginning more than 200 million years ago, as dinosaur body sizes diversified rapidly to fill new ecological niches, including herbivory. High rates were maintained only on the evolutionary line leading to birds, which continued to produce new ecological diversity not seen in other dinosaurs. Small body size might have been key to maintaining evolutionary potential (evolvability) in birds, which broke the lower body size limit of about 1 kg seen in other dinosaurs. Our results suggest that the maintenance of evolvability in only some lineages explains the unbalanced distribution of morphological and ecological diversity seen among groups of animals, both extinct and extant. Important living groups such as birds might therefore result from sustained, rapid evolutionary rates over timescales of hundreds of millions of years.


In general the rapid-evolvers would be the smallest-bodied species -- the ones that reach reproductive age quickly and while they are still small. Rates of evolution depend on generational time. Which predicts that large-bodied/long-generation-time species would have more difficulty adapting to rapidly-changing extinction conditions.

The authors also mention that body size evolution in many non-avian dinosaur lineages seem to follow Cope's Rule, an increase in body size of descendant species over time. The earliest species in that lineage would be small and over time there would be a trend towards evolution of larger bodied species.

What could cause such a trend? Is larger body size advantageous and hence being favored by natural selection? Again the relationship to mass extinctions is intriguing. The authors mention that during the late Triassic mass extinction many branches of dinosaurs became extinct. If larger bodied dinosaur species were disproportionately killed off the survivor species of dinosaurs in early Jurassic would have been small bodied. This could explain Cope's Rule in a rather novel way (as explained by my adviser Anthony Arnold - who has worked on size evolution in foraminfera - in an email to me) - "since it means that rather than evolution favoring size increase, mass extinction selectively removes larger-bodied species, leaving behind the smaller guys as survivors. Since they don't have much room to get smaller the survivors could even speciate at random and the only direction in which the variance has room to expand is toward larger size.

Maybe!.. arm waving.."

So ecological crises may lead to species sorting based on size with smaller size being favored and then evolution of a trend towards larger size that reflects simply an inability to become any smaller!..
 

Monday, May 12, 2014

Book- John Tyler Bonner- Randomness In Evolution

This book is surely worth reading. Randomness in Evolution by John Tyler Bonner. I have loved Bonner's  previous books The Evolution of Complexity and Life Cycles. This promises to be interesting too.

In Current Science Raghavendra Gadagkar gives a positive review. The main premise of the book is that morphologies of unicellular eukaryotes like radiolarians and diatoms to give two examples are neutral phenotypes. That means that the variation of size, shape, ornamentation on skeletal material did not become common because it contributed to reproductive fitness. Rather for example different shapes were selectively neutral i.e. that is they neither gave the organism any advantage over another shape nor a disadvantage. One of the shapes became common just by chance, through random genetic drift.

Size according to Bonner is an important constraint on whether natural selection or drift becomes important in shaping morphology. Randomness is more important in small organisms with relatively simple genetic controls on morphology, while natural selection is more important in larger organisms with elaborate interlocking developmental steps.

I'll reserve comments until I have read the book but just would like to point out that even small unicellular eukaryotes have very sophisticated cellular machinery for processes like photosynthesis and regulating cellular functions. These would have evolved via natural selection.