Category Archives: Climate change and palaeoclimatology

Alternative explanation for interglacial climate instabilities; and a warning

Read about this at Earth-logs

Maintenance of Earth-pages has stopped. If you wish to continue following reports on significant research developments in Earth science  you can register as a follower of my new blog at the Earth-logs site

Soluble iron and global climate

Read about this at Earth-logs

Maintenance of Earth-pages has stopped. If you wish to continue following reports on significant research developments in Earth science  you can register as a follower of my new blog at the Earth-logs site

Finding Archaean atmospheric composition using micrometeorites

Read about this at Earth-logs

Maintenance of Earth-pages has stopped. If you wish to continue following reports on significant research developments in Earth science  you can register as a follower of my new blog at the Earth-logs site

Closure for the K-Pg extinction event?

Read about this at Earth-logs

Maintenance of Earth-pages has stopped. If you wish to continue following my brief reports on significant research developments in Earth science  you can register as a follower of the new blog at the Earth-logs site

More on the Younger Dryas causal mechanism

Untitled-2
Colour-coded subglacial topography from radar sounding over the Hiawatha Glacier of NW Greenland (Credit: Kjaer et al. 2018; Fig. 1D)

Read about new data from lake-bed sediments, which suggest that a major impact around 12.8 thousand years ago may have triggered a return to glacial conditions at the start of the Younger Dryas.

 

What followed the K-Pg extinction event?

Taeniolabis_NT_small
Reconstruction of the 35 kg early Palaeocene mammal Taeniolabis (credit: Wikipedia)

Read about processes connected with the Chicxulub impact that may have influenced the K-Pg mass extinction and the evolution of mammalian survivors during the first million years of the Palaeocene, as revealed by a unique sedimentary sequence near Denver, Colorado, USA.

Chaos and the Palaeocene-Eocene thermal maximum

Read how chaotic behaviour in the Solar System may have affected Milankovich cycles in the late Palaeocene

Ediacaran glaciated surface in China

Read about a unique confirmation of Snowball Earth conditions during the Ediacaran Period at Earth-logs.

striae
29 Ma old striated pavement beneath the Carboniferous Dwyka Tillite in South Africa (credit: M.J Hambrey)

Geochemical background to the Ediacaran explosion

The first clear and abundant signs of multicelled organisms appear in the geological record during the 635 to 541 Ma Ediacaran Period of the Neoproterozoic, named from the Ediacara Hills of South Australia where they were first discovered in the late 19thcentury. But it wasn’t until 1956, when schoolchildren fossicking in Charnwood Forest north of Leicester in Britain found similar body impressions in rocks that were clearly Precambrian age that it was realised the organism predated the Cambrian Explosion of life. Subsequently they have turned-up on all continents that preserve rocks of that age (see: Larging the Ediacaran, March 2011). The oldest of them, in the form of small discs, date back to about 610 Ma, while suspected embryos of multicelled eukaryotes are as old as the very start of the Edicaran (see; Precambrian bonanza for palaeoembryologists, August 2006).

ediacaran-1

Artist’s impression of the Ediacaran Fauna (credit: Science)

The Ediacaran fauna appeared soon after the Marinoan Snowball Earth glaciogenic sediments that lies at the top of the preceding Cryogenian Period (650-635 Ma), which began with far longer Sturtian glaciation (715-680 Ma). A lesser climatic event – the 580 Ma old Gaskiers glaciation – just preceded the full blooming of the Ediacaran fauna. Geologists have to go back 400 million years to find an earlier glacial epoch at the outset of the Palaeoproterozoic. Each of those Snowball Earth events was broadly associated with increased availability of molecular oxygen in seawater and the atmosphere. Of course, eukaryote life depends on oxygen. So, is there a connection between prolonged, severe climatic events and leaps in the history of life? It does look that way, but begs the question of how Snowball Earth events were themselves triggered.

There are now large amounts of geochemical data from Neoproterozoic sedimentary rocks that bear on processes in the atmosphere, seawater, continental crust and the biosphere of the time. Some are indicative of the reducing/oxidising (redox) potentials of ocean water in which various sediments were deposited. Carbon isotopes chart organic burial and the abundance of CO2 in the oceans and atmosphere. Strontium isotopes give details of the rates of continental erosion. The age statistics of zircon grains in sediments are useful; the proportion of zircons close in age to the time of sediment deposition relative to older grains is a proxy for the rate of continental-arc volcanism and thus for subduction rates. Joshua Williams of Britain’s University of Exeter and colleagues from the universities of Edinburgh and Leeds have used complex modelling to assess the pace at which oxygen was added to the surface environment through the Ediacaran Period (Williams, J.J. et al. 2019. A tectonically driven Ediacaran oxygenation eventNature Communications, v.  10 (1); DOI: 10.1038/s41467-019-10286-x).

They estimate a 50% increase in atmospheric oxygen during the Ediacaran to about 0.25 % of the present concentration, which would be sufficient to support large, mobile animals. They attribute this primarily to a boost in the supply of CO2 to the atmosphere as a result of increased volcanic activity. This would have warmed the surface environment so that exposed rock on the continents underwent accelerated chemical weathering. By freeing from continental crust increased amounts of nutrients, such as phosphorus and potassium, the boost to photosynthesis would have increased the oceanic biomass, thereby emitting oxygen. Multicelled animals would have been beneficiaries of such a transformation. The trend continued into the Cambrian, thereby unleashing the explosion of animals and their evolution that continued through the Phanerozoic. Ultimately, the trigger was increased Late-Neoproterozoic tectonic activity that drove the massive Pan-African orogeny and the accretion of the Gondwana supercontinent.

See also: https://www.sciencedaily.com/releases/2019/06/190619130315.htm

Note added, 26 June 2019: Roger Mason has referred me to the carbon-isotope record during the Ediacaran. It shows some of the stratigraphic record’s largest negative δ13C excursions in carbonate rocks (Tahata, M. and 10 others 2013. Carbon and oxygen isotope chemostratigraphies of the Yangtze platform, South China: Decoding temperature and environmental changes through the EdiacaranGondwana Research, v.23, p. 333-353; DOI: 10.1016/j.gr.2012.04.005). Such isotopic excursions went on throughout the Ediacaran, along with sudden fossil appearances and disappearances – so-called ‘Strangelove’ oceans – plus fluctuations in sediment types and climate. The Ediacaran was a wild time in most respects.

3-gorge-c

Geochemical changes recorded in the complete Ediacaran sedimentary sequence of the Three Gorges of the Yangtze River, China (credit: Tahata et al. 2013; Fig. 4)

Soluble iron, black smokers and climate

 

Phytoplankton bloom in the Channel off SW England (Landsat image)

At present the central areas of the oceans are wet deserts; too depleted in nutrients to support the photosynthesising base of a significant food chain. The key factor that is missing is dissolved divalent iron that acts as a minor, but vital, nutrient for phytoplankton. Much of the soluble iron that does help stimulate plankton ‘blooms’ emanates from the land surface in wind blown dust (Palaeoclimatology September 2011) or dissolved in river water. A large potential source is from hydrothermal vents on the ocean floor, which emit seawater that has circulated through the basalts of the oceanic crust. Such fluids hydrate the iron-rich mafic minerals olivine and pyroxene, which makes iron available for transport. The fluids originate from water held in the muddy, organic-rich sediments that coat the ocean floor, and have lost any oxygen present in ocean-bottom water. Their chemistry is highly reducing and thereby retains soluble iron liberated by crustal alteration to emanate from hydrothermal vents. Because cold ocean-bottom waters are oxygenated by virtue of having sunk from the surface as part of thermohaline circulation, it does seem that ferrous iron should quickly be oxidised and precipitated as trivalent ferric compounds soon after hydrothermal fluids emerge. However, if some was able to rise to the surface it could fertilise shallow ocean water and participate in phytoplankton blooms, the sinking of dead organic matter then effectively burying carbon beneath the ocean floor; a ‘biological pump’ in the carbon cycle with a direct influence on climate. Until recently this hypothesis had little observational support.

The Southern Ocean surrounding Antarctica is iron-starved for the most part, but it does host huge phytoplankton blooms that are thought to play an important role in sequestration of CO2 from the atmosphere. Oceanographic research now benefits from semi-autonomous buoys set adrift in the deep ocean. The most sophisticated (Argo floats ) are able to dive to 2 km below the surface, measuring variations of physical and chemical conditions with depth for long periods. There are 4,000 of them, owned by several countries. Two of them drifted with surface currents across the line of the Southwest Indian Ridge through waters thought to be depleted in phytoplankton, despite having high nitrate, phosphate and silica contents – major ‘fertilisers’ in water. They showed up ‘spikes’ in chlorophyll concentrations in the upper levels of the Southern Ocean (Ardyna, M. and 11 others 2019. Hydrothermal vents trigger massive phytoplankton blooms in the Southern Ocean. Nature Communications, 5 June 2019, online; DOI: 10.1038/s41467-019-09973-6). Their location relative to a large cluster of hydrothermal vents on the Southwest Indian Ridge was ‘downstream’ of them in the circum-Antarctic Current, but remote from any known terrestrial source of iron (continental shelves, dust deposition melting sea ice). Earlier oceanographic surveys that detected anomalous helium isotope, typical of emanations from the mantle, show that hydrothermal-vent water moves through the two areas. Although the Argo floats are equipped for neither helium nor iron measurements, it is likely that the blooms benefitted from hydrothermal iron. Modelling of the likely current dispersion of material in the hydrothermal plumes also outlines a large area of ocean where iron fertilisation may encourage regular blooms where they would otherwise be highly unlikely. Unfortunately, the study does not include any direct evidence for elevated soluble iron.

One thing that the study does foster is renewed interest in deliberate iron-fertilisation of the oceans to speed up the ‘biological pump’ as a means of managing global warming (Boyd, P. & Vivian, C. 2019. Should we fertilize oceans or seed clouds? No one knows. Nature, v. 570, p. 155-157; doi: 10.1038/d41586-019-01790-7). Small scale pilots of such ‘geoengineering’ have been tried, but raised outcries from environmental groups. Other than detecting, or hinting at, soluble iron from a deep natural source, scientific research has provided scanty evidence of what iron-seeding at the surface might do. There could be unexpected consequences, such as methane emission from decay of the blooms – a worse greenhouse gas than carbon dioxide.

See also: An iron age for climate engineering? (Palaeoclimatology, July 2007); Dust in the wind: North Pacific Ocean fertilized by iron in Asian dust ( National Science Foundation 2019)