January 2014 – Earth-logs

Plate tectonics and the Cambrian Explosion

Published on January 27, 2014 by zooks777Leave a comment

A rough-and-ready way of assessing the rate at which silicic magmatic activity has varied through time is to separate out grains of zircon that have accumulated in sedimentary rocks of different ages. Zircon is readily datable using the U-Pb method, if you have access to mass spectrometry. While some of the zircons will date from much older continental crust that was exposed while the sediments originated, sometimes there are grains that formed only a few million years before the sediments accumulated. Those are likely to have crystallized from silica-rich volcanic rocks above subduction zones where ocean-floor has been driven beneath continental crust; i.e. at continental volcanic arcs. Such young zircons therefore help assess the tectonic conditions close to sedimentary basins. The potential of detrital zircon geochronology was first suggested to me by Dr M.V.N. Murthy of the Geological Survey of India in 1978, long before anyone could aspire to mass zircon dating. M.V.N. had by then amassed kilograms of zircon grains from every imaginable source in India, and may have been the first geologist to realise their potential. It has become a lot quicker and cheaper in the last two decades, thanks to methods of dating single zircon grains both precisely and accurately and M.V.N.’s prescient suggestion has been borne out globally.

Results for the late Precambrian to early Palaeozoic have recently been compiled (McKenzie, N.R. et al. 2014. Plate tectonic influences on Neoproterozoic-early Paleozoic climate and animal evolution. Geology, online publication doi:10.1130/G34962.1). One of the striking correlations is between the abundance of ‘young’ zircons relative to Cambrian sedimentary deposition and the pace of diversification of animal faunas during the Cambrian. During the Cambrian Period there may have been far more continental-margin arc volcanism than in the preceding late Neoproterozoic or later in the early Palaeozoic. That would match with evidence for the Cambrian atmosphere having reached the greatest CO₂ concentration of Phanerozoic times and the fact that the Gondwana supercontinent (comprising the present southern continents plus India) was assembled at that time by collision of several Precambrian continental masses. Global temperatures must have been rising.

The rapid emergence of all the major animal groups by the middle Cambrian – the Cambrian Explosion – took place during and despite climatic warming. Environmental stress, perhaps increased calcium and bicarbonate ions in sea water as a result of acid conditions, may have forced animals to develop means of getting both ions out of their cells to form carbonate skeletons: the Cambrian Explosion really marks the first appearance of shelly faunas and a good chance of fossilisation. Yet at the peak of volcanically-induced warming faunal diversity, especially of reef-building animals, fell-off dramatically to create what some palaeobiologsts have termed the Cambrian ‘dead interval’. Marine life really took-off in a big way during the Ordovician while temperatures were falling globally; so much so that the close of the Ordovician was marked by the first major glaciation focused on Gondwana. The zircon record indicates that continental-arc volcanism also declined during the Ordovician, and maybe the Cambrian silicic volcanics were chemically weathered during that Period to remove carbon-dioxide from the atmosphere, along with renewed reef building to bury carbonate fossils.

Volcanic mayhem drove major burst of evolution

Did ice-age climate changes across Europe happen at the same time?

Published on January 23, 2014 by zooks7771 Comment

Although the frigid conditions at the last glacial maximum, around 19 to 20 thousand years ago, gradually relinquished their grip through slow global warming, this amelioration came to sudden stop around 12 800 years before the present. Northern hemisphere ice-core and other climate records show that there was a return to glacial conditions over a period of a few decades at most, to launch what is known as the Younger Dryas stadial that lasted over a thousand years until about 11 500 years ago, with the onset of the warm, climatically more stable Holocene that launched the transformation of the human way of life. The start of the Younger Dryas had dramatic effects throughout the northern hemisphere, the cold conditions emerging suddenly from an immense oceanographic change; a weakening or the halt of the North Atlantic thermohaline circulation in which cold, very salty surface waters at the fringe of the Arctic Ocean sink to drag warmer water to high latitudes. In short, the Gulf Stream slowed or stopped its warming influence at high northern latitudes. Current thoughts centre on a freshening of surface sea water following the collapse of the North American ice sheet to gush meltwater and icebergs into the North Atlantic to buoy-up surface waters.

Most of the data about this climatic shock can only be dated accurately to within a few centuries: it is clear that the initial cooling was very rapid, on the scale of a few years, as was the warming that closed the Younger Dryas and marked the start of the Holocene, but the ‘when’ is known only to within a few hundred years. To resolve the start and stop ages needs records that include several indicators: clear signs of the beginning and end of the episode, an accurate means of dating them and confirmation from other sites, which presupposes a cast-iron means of correlating the records over large distances. The most reliable markers for correlation are volcanic ashes that can be dated radiometrically and which drift on the wind to be deposited over very large areas. If sedimentary sequences that accumulated continuously preserve such ashes, contain clear signs of climatic change and clearly record the passage of time in great detail, there is a chance of resolving climatic events very accurately; but they are no common. A British-German team have located and analysed two such promising sites (Land, C.S. et al. 2013. Volcanic ash reveals time transgressive abrupt climate change during the Younger Dryas. Geology, v. 41, p. 1251-1254). One of them is from the bed of a lake that formed by a single volcanic eruption (Meerfelder Maar) in the Eifel region of western Germany. Quiet sediment accumulation has occurred there continuously to form very narrow, alternating dark and light layers, the variegation being due to sedimentation under ice in winter and open water in summer respectively. Twelve thousand of these annual varves provide a means of dating potentially with a precision of ± 1 year, but calibration to absolute time is necessary. The maar sediments contain three ash layers, two of which are from small local eruptions; the older having an age of 12 900 years before 2000 AD, the other being 11 000 years old, showing that the entire Younger Dryas is spanned by the Meerfelder Maar sediments. The third was dated by varve counting, showing the eruption had taken place 12 140 years ago. That age coincides closely with that of major eruption in Iceland.

One prominent climatic feature of the Younger Dryas of Europe is a shift around halfway through: it started with the fiercest cold and then ameliorated. This change shows up in the Meerfelder Maar record as a reduction in mean varve thickness and an increase in the titanium content of the clays, the latter taking place in about a year (12 250 years ago) some 100 years before the Icelandic ash was deposited. The same kind of change occurs in records from lakes as far north as the Arctic Circle. One of the core records from Kråkenes in Northern Norway also contains the tell-tale Icelandic ash (as do ice cores from Greenland), but in its case it occurs 20 years before the abrupt climate shift. This clearly shows that major climate changes at the end of the last ice age occur at different times from place to place. The authors ascribe the 120 year difference between the two records to the times when prevailing, warm westerly winds began to affect central and northern Europe, linked to a gradual northward migration of the polar front. The data from both lakes also suggest that the Younger Dryas ended about 20 years earlier in Norway than in Germany, although Lane et al. do not comment..

Hitherto, correlation between climate records has been based on an assumption that major climate changes were at the same time, so that climate proxies such those discussed here have been ‘wiggle-matched’. Quite probably a lot of subtleties have thereby been missed.

Water cycle amplifies abrupt climate change

Land almost colonized during the Cambrian Explosion

Published on January 20, 2014 by zooks777Leave a comment

One of the major shale-gas source strata in the eastern USA, the Middle Cambrian Conasauga Shale, formed in a shallow inland sea. Consequently the sedimentology of the lowest Palaeozoic Era of the region and the strange structures affecting it during deformation that formed the Appalachian Mountains have become a focus of intense tectonic and stratigraphic interest – economic potential generally helps fund academic research at a time when money for pure science is short. This has extended into the deepest part of the Cambrian lying unconformably just above the crystalline Precambrian basement. The Lower Cambrian of the Appalachians marks the earliest stage of rifting that flooded former dry land and comprises the multicoloured mudstones, siltstones and sandstones of the Rome Formation. Though only sparsely fossiliferous, the Rome formation contains archetypical trilobites of the genus Olenellus, typical of the Lower Cambrian and used to correlate sedimentary rocks of this age far and wide. They occur far across the North Atlantic in coeval rocks of the Northwest Highlands of Scotland, but not in those a mere couple of hundred kilometres to the south in Wales. This faunal disparity forms a major line of evidence that the olenelid fauna occupied one side of a once major ocean – Iapetus – another different bunch of early trilobites being characteristic of its opposite flank. The almost hemispherical extent of similar faunas was long regarded as an indication that they inhabited open ocean water. In fact, their wide distribution is as much due to juvenile arthropods being planktonic, while adults may have occupied all sorts of marine environments. It now turns out that Olenellus lived in very shallow water (Mángano, M.G. et al. 2014. Trilobites in early Cambrian tidal flats and the landward expansion of the Cambrian explosion. Geology, online pre-publication doi:10.1130/G34980.1).

Gabriela Mángano of the University of Saskatchewan and colleagues from Argentina and the US found that the Rome Formation is full of sedimentary structures typical of modern intertidal zones. Tidal-flat strata are full of suncracks but are also criss-crossed by tracks made by substantial arthropods, only fossil olenellid trilobites being big enough to have made them while feeding , maybe on microbial mats formed on the mudflats or on worms that burrowed the muds. Clearly these animals were literally only a few steps away from colonising the land very shortly after abundant, sturdy animal life appeared in the Cambrian Explosion. Currently the dominant hypothesis for permanent entry of animals onto land is that the colonizers first adapted to fresh- or brackish water habitats. Yet, apparently, there was little to stop a direct invasion from the sea.

Human evolution: bush or basketwork?

Published on January 8, 2014 by zooks777Leave a comment

Analysis of DNA from ancient humans has revealed its power decisively in the last few years, and especially at the beginning of 2014 with publication of the sixth full genome of an individual who was not an anatomically modern human (Prüfer, K. and 44 others 2014. The complete genome sequence of a Neanderthal from the Altai Mountains. Nature, v. 505, p. 43-49). The newly sequenced material came from a toe bone found in the Denisova Cave in the Altai Mountains of southern Siberia; the same location made famous in 2010 by genetic evidence for unknown late hominins, the Denisovans . The bone occurred in the same layer of cave sediment, dated at 50.3 ka, which yielded the Denisovan finger bone, but from a lower sublayer. So there is no firm evidence that both groups cohabited the cave.

The genome reveals that the individual was female and related to the three Neanderthals from Croatia and another infant Neanderthal from the Caucasus, also analysed previously by Svante Pääbo’s team at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany (Note that the toe-bone team also includes co-workers from US, Chinese, Austrian, French and Russian institutions). The closest statistical link is to the Caucasian infant Neanderthal’s DNA. Interestingly, it proved possible to demonstrate that the Siberian Neanderthal woman was from a population that was clearly inbred, her parents having been related at the level of half siblings. Her mtDNA shows that she shared a common ancestor with all 6 Neanderthals from whom mtDNA has been analysed.

Comparing genomes from the single Denisovan, the 5 Neanderthals and living humans from sub-Saharan Africans gives an estimated 550 to 765 ka time of divergence of a population leading to anatomically modern humans from the progenitors of Neanderthals and the Denisovan. The Neanderthal-Denisovan split was roughly 380 ka ago. It was already known that non-African living humans contain genetic evidence for past interbreeding with Neanderthals and that some people in Asia, Australia, Melanesia and the Philippines had acquired genes from Denisovans. More refined comparisons now show Oceanians to have 3 to 6% Denisovan make-up with Asians in general sharing 0.2%. Neanderthal to modern non-African gene flow is now estimated at between 1.5 and 2.1%, with Asians and Native Americans being at the high end. Neanderthals and Denisovans also interbred, but only at the level of about 0.5% inheritance. However, that genetic sharing involved DNA regions known to confer aspects of immunity and sperm function, that also made their way into living non-African humans.

Since the common ancestor of Neanderthals and Denisovans left Africa long before modern humans appeared on the scene it would be expected that living Africans’ genomes would show the same level of similarity with both the now extinct groups, if all three originally shared a common ancestor. A surprising outcome from comparison of Neanderthal and Denisovan genomes with those of living sub-Saharan Africans is that there is a significant bias towards Neanderthal rather than Denisovan comparability. There are three possibilities for this bias. After the Neanderthal-Denisovan split the former group may have continued to interbreed with the group leading to modern Africans (and indeed to modern non-Africans): that would require Neanderthal genetics to have originated in Africa before they migrated to Eurasia. Secondly, the gene flow could have been from the ancestors of modern humans to Neanderthal progenitors, making descendant Neanderthals more like modern humans. Prüfer et al. suggest that the evidence is less supportive of both and weighs towards a third possibility; that the Denisovans interbred with an unknown contemporary hominin, whose genetic make-up was yet more different from that of all three known groups of the late Pleistocene and therefore their common ancestor . This may have been Homo antecessor or possibly H. erectus who survived until as late as 20 ka in SE Asia.

Family tree of the four groups of early humans living in Eurasia 50,000 years ago and the gene flow between the groups due to interbreeding. Image credit: Kay Prüfer et al.

As other commentators on the paper (Birney, E. & Pritchard J.K. 20113. Four makes a party. Nature, v. 505, p. 32-34) have observed, ‘…Eurasia during the late Pleistocene was an interesting place to be a hominin, with individuals of at least four quite diverged groups living, meeting and occasionally having sex.’ All this arises quite convincingly from the genetics of only 7 ancient individuals, to show that it may no longer be appropriate to consider human evolution as a tree or a bush linking permanently separated species. Either it is the history of a single, polymorphic species – remains of 1.7 Ma old Homo georgicus show clear evidence of such polymorphism – or a better metaphor for human development is an interwoven basket or twine. Rumour has it that attempts are being made to sequence an H. antecessor dated at 900 ka from Gran Dolina Cave in the Atapuerca Mountains in Northern Spain: as they say, ‘Watch this space’!

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