Thursday, 29 May 2014

Marine treasure – keep your nerves!

Jellyfish are different, at least some of them. The comb jellyfish “Pacific sea gooseberry” (Pleurobrachia bachei) hunts for food. It is a predator, paddling through the sea, and grasping its prey with sticky tentacles.

Pacific sea gooseberry
(from Wikipedia Commons)
Recent analysis of the Pacific sea gooseberry revealed a fascinating detail, making them much of an alien [1]. Their nervous system miss the habitual set of components that are found in most other animals. The chemical signals (neurotransmitter) of the Pacific sea gooseberry, which make the nerves work are different. The Pacific sea gooseberry does not use the set of chemical signals that we know from most animals.

Without question, the Pacific sea gooseberry has a fully developed nervous system. It consist of a network with a ring of nerves around the mouth. The nervous systems senses light, detects prey and coordinates moves of muscles. However, the nervous system performing these operations is using different chemical signals. That is less than a detail. It indicates that nervous systems evolved on Earth twice, at least. The deciphered gene code of the Pacific sea gooseberry shows this.

Other differences between Pacific sea gooseberry and other animals are found for immune and development genes, and respectively for the related physiological processes.

Whether comb jellies descend from ancient organisms that lived 580 million years ago (in the Ediacaran [*] ), as some speculate, is a provocative hypothesis. If that is the case, then the Pacific sea gooseberry may trace that ancient world to present times and indicate how complex this ancient world was already. Ecosystems with prey-predator relationship existing more than half a billion years ago!

A comb jellyfish
(from Wikipedia Commons)
A less precocious hypothesis would be that the evolution of Pacific sea gooseberry included the replacement of the chemical signals that we know to be common in nearly all animals.

What should be noted, beyond any speculation, that the sea is full of treasures. Many we only do not know; one more was discovered recently. 

Understanding that nervous systems, immune systems or development process can be constructed from different building blocks is deep insight with possibly far-reaching consequences, be it for regenerative medicine or synthetic biology [3].

Comb jellyfish [**] are classified as a sister group to jellyfish and sea anemones. This kind of animals are ancient and lived in the ocean since very long time, even if the modern species evolved more recently. These animals have in common that the distinction between head and rears does not apply to them as it applies for slugs, fish and humans. This feature makes them simpler and they are put at the bottom of the tree of life, comb jellies among them. Thus it is useful to assume that similar functions evolved several times along parallel paths.

[*] The Ediacaran (635 - 542 million years) is the geological period preceding the Cambrian Period. The Ediacaran biota have little resemblance to modern lifeforms and include the oldest organisms with tissues; hard-shelled animals had yet to evolve.

[1] Ewen Callaway (2014) Jelly genome mystery, Nature (509) p. 411

[2] Leonid L. Moroz et al. (2014), The ctenophore genome and the evolutionary origins of neural systems, NATURE, doi:10.1038/nature13400


Saturday, 10 May 2014

Dirty sea-bottom, litter everywhere – and: bon appetit !

To recall, sea-birds confuse small bits of plastic with food. They swallow the bits of plastic and it kills them. [a] To recall, abandoned fishing gear entangles fish, turtles and marine mammals, and its kills them. Floating plastic bags do the same. And so on, but that is not the end of the story [1].

The NOAA Marine Debris Program funds projects
to remove derelict fishing nets and other debris
from marine waters, where they can entangle marine life.
Each year some 6.4 million tonnes of litter are entering the oceans, or about one kilogram for each human being. Most litter stems from cities. The world's cities currently generate around 1.3 billion tonnes of municipal solid waste a year, or 1.2 kilogram per city-dweller per day.” [b Thus, the litter discharged into the sea corresponds to about 4-5% of the municipal solid waste produced in the cities of the world. In that sense its a small part of waste produced by the nine billion humans, but as any dumping it causes problems.

More generally, marine litter is defined as ‘‘any persistent, manufactured or processed solid material discarded, disposed of or abandoned in the marine and coastal environment." Persistent littering the sea likely has started with disposing “clinker” from steamships and currently has found its peak with “plastic”.

Clinker, the residue of burnt coal, was commonly dumped from steamships well into the 20th century. In the Mediterranean Sea, its occurrence on the deep sea-floor has been shown to coincide with such shipping routes.

Currently, the most abundant marine litter is plastic. Only part of the plastic items float at the sea surface or close to it. Two third of the plastic sinks to the bottom of the sea when converted by fouling organisms. Therefore plastic accumulation on the seabed is more abundant than in the open sea. Thus, below the floating garbage is an even bigger garbage dump at the sea bottom.

Litter is present in all marine habitats, from beaches to the most remote points in the oceans, such as the Midway Islands in the Pacific [a. The particular distribution and accumulation of litter in the ocean is influenced by water movements, bottom morphology and economic activities.

On the global scale, the ocean currents sweep the litter to the centre of the ocean-gyres where it accumulates, called the big garbage patch. On the local scale, litter is washed upon the beach. On the hidden scale, litters is channelled to the deep sea. Marine litters accumulates in particular high densities in submarine canyons.
(from coast down to > 4000m depth)

Submarine canyons act as passages for litter transport from continental shelves into deeper waters. Submarine canyons are areas where organic debris accumulates. The debris is food for bottom-dwelling fauna and suspension-feeding invertebrates. They are abundant in submarine canyons. The accumulation of plastics in submarine canyons likely has a detrimental effects on theses deep-sea organisms.

A survey [2] of European seas published in April 2014, confirmed again that marine litter is found everywhere, from the beach down to the deep sea. The survey uses standardized methods to categorize the litter and to quantify its abundance. Most common litter items included, non-surprisingly, plastic bags, glass bottles and abandoned fishing lines and nets:
  • Plastic represented 41% of the litter items in European seas, whilst abandoned fishing gear accounted for 34% of the total. Clinker, glass and metal are least common. Density and composition of litter in European seas is comparable to what has found in other parts of the world ocean.
  • Litter density in submarine canyons reached an average of 12.2 – 6,4 items per ten-thousand square meters, or the double of the litter density found elsewhere.
A litter density of 12.2 – 6,4 items per ten-thousand square meters means to find about 10 items of litter on a surface wide as a football field! 

A list of locations with highest litter densities in European waters includes for example the Lisbon Canyon in continental shelf of Portugal or the Blanes Canyon in the continental shelf of the Mediterranean sea of Spain. Theses canyons were formed when the sea-level was much lower than today.

Litter is a serious risk for the marine environment. Entanglement in abandoned fishing gear is a serious threat for birds, turtles and marine mammals, it is also causing high mortality of fish through ‘‘ghost fishing''. Beyond other threats, plastic is a source of toxic chemicals that is lethal to marine fauna.

The degradation of plastics generates micro-plastics that are ingested by organisms, leading to contaminants across trophic levels up to the fish [3] that we may eat. So plastic debris may return to their source, finally.

[1] José G.B. Derraik, 2002, The pollution of the marine environment by plastic debris, Marine Pollution Bulletin 44(9), pp. 842-852.
[2] Christopher K. Pham et. Al, 2014. Marine Litter Distribution and Density in European Seas, from the Shelves to Deep Basins. PLOS ONE, 9(4), pp. 1-13.
[3] Chelsea M. Rochman et al. 2013, Ingested plastic transfers hazardous chemicals to fish and induces hepatic stress, Nature, doi:10.1038/srep03263;

[b] Economist, 7th June 2012,

Thursday, 3 April 2014

Tami found its Niche, and swept the Sea

The majestic baleen whales are filter-feeders eating vast amounts of small organisms. They typically seek out a concentration of zooplankton and filter the prey from the water using their baleen. Great, actively swimming filter-feeders evolved among sharks, rays, fishes and well-known whales. 

Up to now, animals occupying the ecological niche of “actively swimming filter-feeders” had not been identified among the fossils of the early Palaeozoic era, about 500 Million years ago. The known large swimming animals of that time, the Anomalocaridids [*] were carnivore predators. 

However, that understanding was incomplete. 

Recently, the fossilized Tamisiocaris borealis (“Tami”) was found in North Greenland [1]. Its frontal body part clearly is specialized for suspension feeding. “Tami” bears long, thin and evenly spaced spines, which are are fitted out with dense rows of long and fine spines. Evidently, “Tami” was feeding on small plankton. It got its food by sweep-net capture of small food-particle (down to 0.5 mm), thus as small as a copepod.

Why get excited about that? 

Fossilized "Tami" - [2, **]
So far, large, swimming suspension feeders were found during the later Cambrian, a bit less than 500 Million years ago. Their existence indicates a deep-water ecosystem supported by high primary productivity and nutrient flux. 

The presence of swimming suspension feeders in the early Cambrian, more than 525 Million years ago indicate that a complex deep-water ecosystem supported by high primary productivity and nutrient flux existed already at that time. Thus, these Cambrian deep-water ecosystems seem to have been already quite modern – may be less in terms of species, but certainly in terms of viable ecosystem niches.

[1] A suspension-feeding anomalocarid from the Early Cambrian; Jakob Vinther, Martin Stein, Nicholas R. Longrich & David A. T. Harper; Nature 507, 496–499 (27 March 2014)

[*] from Wikipedia: Anomalocaridids are a group of very early marine animals known primarily from fossils found in Cambrian deposits in China, United States, Canada, Poland and Australia. Anomalocarids are the largest Cambrian animals known — some Chinese forms may have reached 2 m in length — and most of them were probably active carnivores.
[**] Artist's view at "" on

Sunday, 9 March 2014

No Paper Heavens, Marine Protected Areas Examined

Conservation of marine biological diversity calls to foster protected sea-life from exploitation. Putting part of the seas under protection is a means to achieve that. In the last years the size and the the number of Marine Protected Areas increase rapidly. Currently about 2% of the world's seas are under full protection. The target is to protect 10% of territorial waters by 2020. Currently the degree of protection varies, e.g. hook-and-line fishing may be allowed, and some of the Marine Protected Areas qualify just as "paper parks".

Marine Protected Areas should generate socio-economic benefits to make their case. Some marine Protected Areas are known not to reach their full potential because of illegal harvesting, mis-regulation that allow detrimental harvesting, or mis-sizing so that animals leave the Marine Protected Areas when living their habitual life.

The World Database on Protected Areas (WDPA) [1]. 
Recently Graham Edgar and colleagues [*] examined the conservation benefits of 87 Marine Protected Areas worldwide. Their insight: benefits of Marine Protected Areas increase dramatically with the accumulation of five key properties: no take, well enforced, age, size, and isolation.

By its very nature, isolation is difficult in marine environments. Water and species move. Nevertheless the natural gradients of marine environments provide for guidance about natural confinements.

More than half of the Marine Protected Areas examined by Graham Edgar had only one or two key properties. These protected areas were ecologically indistinguishable from unprotected areas. They conclude: meeting only two of the five properties does not have much effect, but bundling four or five has effect.

Comparing effective Marine Protected Areas, which have four or five key features, with non-protected seas is indicating that total fish biomass is about three times higher. Also, effective Marine Protected Areas have twice as many large fish species, five times more large fish biomass, and fourteen times more shark biomass than fished areas.

Global conservation targets based on area alone will not effectively protect marine biodiversity. Design of Marine Protected Areas and their durable management needs five for conservation: no take, enforce, age, size, and as much isolation as possible. That is a difficult task but not mission impossible.

[1]  IUCN and UNEP-WCMC (Oct 2013). The World Database on Protected Areas (WDPA). Available

[*] Global conservation outcomes depend on marine protected areas with five key features; Graham J. Edgar, Rick D. Stuart-Smith, Trevor J. Willis, Stuart Kininmonth, Susan C. Baker, Stuart Banks, Neville S. Barrett, Mikel A. Becerro, Anthony T. F. Bernard, Just Berkhout, Colin D. Buxton, Stuart J. Campbell, Antonia T. Cooper, Marlene Davey, Sophie C. Edgar, Günter Försterra, David E. Galván, Alejo J. Irigoyen, David J. Kushner, Rodrigo Moura, P. Ed Parnell, Nick T. Shears, German Soler, Elisabeth M. A. Strain & Russell J. Thomson; Nature 506, 216–220 (13 February 2014)

Sunday, 17 November 2013

Climate Choke Point East of Greenland

The Arctic Ocean has an important role in Earth’s climate. The Arctic is part of the globe that is most sensitive to climate change. The choke point for the Arctic Ocean is the Fram Strait.  Currently the Arctic is shifting to a new normal; sea-ice is thinning, permafrost is thawing, and tundra is greening [1]. 

Arctic sea ice minimum 2012 compared to 30 average minimum [a]
The extend of its summertime sea-ice cover determines how much sun-light is reflected back into space. The Arctic receives important freshwater inflow from North American and Siberian rivers. Sea ice is export through the Fram Strait into the North Atlantic and is freshening the most salty ocean of the globe. The Frame Strait between Greenland and Svalbard is the only deep passage linking the Arctic Ocean to the global ocean. The main export vein of deep Arctic waters goes through this strait. Deep Arctic waters are forming in the interior of the Arctic ocean basin.

Freezing and cooling produce deep water in the Arctic ocean. Only if it is exported at depth out of the Arctic Ocean, then it is contributing to the global thermohaline circulation. The thermohaline circulation of the oceans is the slow vertical overturning of its water that brings heat and oxygen into the depth of the world ocean. One of the drivers of this circulation is the deep, cold and salty water that is flowing through the Fram Strait out in depth from the Arctic Ocean. To understand climate change processes it has to be assessed how the outflow of deep water from the Arctic Ocean varied during last several ten-thousand years.
Flow through Fram Strait - top/in & bottom/out [b]
Did the Arctic Ocean export waters through cold and warm climates, or did this export shut-off in warmer climates? Ratios of the radionuclides thorium-230 (230Th) and protactinium-231 (231Pa) in the sediments at the sea-bottom can be used to assess this question [2].

These radioactive tracers are produced in sea water by radioactive decay of natural uranium, which is transported by the rivers into the sea. Thorium and protactinium are not soluble in seawater and attach in a different time-depending manner to particles made of different minerals. These particles drop to the sea bottom and so remove the radioactive traces from the water column. This process is called “scavenging”. The “scavenging” of thorium and protactinium happens with a different speed. Much of the thorium will drop to the bottom even if much deep water is flowing out of the Arctic ocean. However an important part of the protactinium would be swept out through the Fram Strait if the outflow of Arctic waters is happening. Thus, the thorium and protactinium concentrations in sediments of the Arctic Ocean vary with the strength of the outflow of deep water through the Fram Strait. Whether that outflow varied during glacial, de-glacial and interglacial conditions can be studied in sediment cores taken from the bottom of the Arctic Ocean.

Thorium [c]
It has been found that the measured thorium burial is in balance with its production in Arctic seawater but that, for all time intervals, the burial of protactinium is in deficit. Thus, protactinium has been exported out of the Arctic Ocean all time throughout the past 35,000 years [2]. The outflow has to have been so strong that the replacement time for deep waters in the Arctic Ocean is many centuries since the most recent glaciation. Thus, Arctic waters are a persistent contribution to the global ocean circulation throughout the end of the last glacial periode and the following warmer Holocene.

[1] M. O. Jeffries, J.E: Overland and D.K. Perovich, 2013. The Arctic shifts to a new normal, Physics Today, Vol 66(10)
[2] Sharon S. Hoffmann, Jerry F. McManus, William B. Curry & L. Susan Brown-Leger, 2013, Persistent export of 231Pa from the deep central Arctic Ocean over the past 35,000 years
Nature 497, 603–606,

[a] from: 
[b] from:
[c] from:

Sunday, 26 May 2013

Keeping the heat on, or powering Europe!

 Moving on to Toronto, Florence, Vladivostok or Seattle?

Toronto, Florence, Vladivostok and Seattle are particular cities with particular surroundings. All four cities are situated about half way between the Equator and the North-pole, but what for a different climate they offer! Definitely, it's not the same choice where to live. 

Toronto Skyline
This four cities are situated at about the same northern latitude. Toronto in eastern Canada is facing the cold western North Atlantic Ocean.  
Florence in Europe is facing the warm eastern North Atlantic Ocean. Vladivostok in the far east of Russia is situated at the cold western shores of the Pacific Ocean, and Seattle at its  warm eastern shores.

Florence and Seattle have warm summers and temperate winters. Toronto and Vladivostok have temperate summers and really cold winters. Florence has the most clement climate, followed by Seattle, Vladivostok coming well last. 

Florence annual mean day-time temperature is 20°C, and its annual mean night-time temperature is 9°C. For Seattle the annual mean temperatures are 15°C and 7°C, respectively.  Toronto has an annual mean day-time temperature of 13°C, and its annual mean night-time temperature is 5°C. In Vladivostok the annual mean day-time and night-time temperatures are 9°C and 2°C, respectively.  

The average day in Vladivostok is as warm as the average night in Florence!

Watch your neighbouring ocean

These differences of the local climate of  Toronto, Florence, Vladivostok or  Seattle are strongly determined by the temperature of the surface waters of their neighbouring ocean. The  surface waters at northern east-coast of the North-Atlantic Ocean are much warmer than surface waters as its northern west-coast. The same pattern is found in the North-Pacific; the northern east-coast of  North Pacific is much warmer than its northern west-coast. The cross-ocean temperature difference is more pronounced in the North Atlantic Ocean than in the North Pacific Ocean. 

Florence -  Arno River
Lucky Europe, is heated by the currents of the North Atlantic Ocean!  Not so lucky Japan, its northern islands are cooled by the Pacific Ocean but happily Japan's southern islands are swept by the warm Kurishio. The currents of the Kurishio does for the North Pacific Ocean and North America what the Gulf Stream does for the North Atlantic and Europe; warm surface waters are pushed north-eastward across the ocean. 

Kurishio and Gulf Stream are part of a much wider pattern of  variable global currents and related fluctuating transport of heat, salt [1] and other substances.

A kind of global 'conveyor belt'  links the oceans at the top and at the bottom, with surface currents transporting warm water northward to the Arctic while cold water in the depths flows back to the tropics and around the world. But that belt operates with "stop and go", with the strength of currents varying widely from year to year, decade to decade.  

This circulation, in the North Atlantic called the 'Atlantic Meridional Overturning Circulation' ferries vast amounts of heat from the tropics to northern latitudes [2]. One of its main components is the Gulf Stream. But far more is happening below the surface in the depth of the ocean that determines features and variable strength of  currents and related heat transport [3]. The 'Atlantic Meridional Overturning Circulation' forms the part of the global  'conveyor belt' that operates in the North Atlantic Ocean.

Warm conveyor belt waters from Cape Hatteras to Murmansk

Global Conveyor Belt
The Gulf Stream transports heat along the eastern shores of North America up to Cape Hatteras and than into the wider North Atlantic Ocean setting of as the North Atlantic Current. The warm surface water moves across the North Atlantic Ocean to Europe and up into the Greenland Sea between Norway and Greenland. There part of the warm water is cooled and sinks into the depth, and part moves further north into the Arctic Ocean keeping the very northern European ports (at 69° N) ice-free. So regular cargo-ships can sail to Kirkenes and Murmansk at latitudes where elsewhere at similar latitude only icebreakers may navigate the ocean. The warm sea heats the western winds that keep Europe's climate mild. Nowhere else in the northern hemisphere is the climate so clement at the same northern latitudes.

The North Atlantic Ocean is the powerhouse heating Europe.

Conveyor Belt in the North Atlantic Ocean
global/207lec2images.htm )
Year-to-year and longer term changes in the strength of the  'Atlantic Meridional Overturning Circulation' happen and likely are affecting seasonal wetter conditions across Europe, Africa and the Americas.

For example, observations from 2009 indicate that strength of the overturning circulation dropped by 30% for a year. This reduced the amount of heat transported to the North Atlantic by almost 200 trillion watts. That drop of heat transport into the North Atlantic ocean has been linked to the harsh winters in Europe 2009-10 [4]. 

The estimate drop of heat transport is only a minor part of the total heat transport around the globe by ocean currents, nevertheless  200 trillion watts  is a tremendous amount of heat. This amount of heat corresponds to about half of  the additional amount of heat that currently  is captured by the atmosphere because of the increased carbon dioxide concentration of the atmosphere;  increase beyond the pre-industrial values of 290 ppm currently we hit the 400 ppm [ppm = parts per million]. A fluctuating heat tarnsport of that size is important. Therefore detailed and lasting observation along transects of the entire Atlantic Ocean are planned for the next years [4].

Between Greenland and Caribbean Seas

Cape Hatteras 
A key element of the conveyor belt is found in the the Greenland Sea and the Labrador Sea. When reaching this region in the north-eastern North Atlantic Ocean the warm surface waters cool and sink; sink really deep down to the bottom of the sea [5]. In that cooling process heat is passed on to the atmosphere and carbon dioxide is carried to the depths, sequestering it from the atmosphere.  Winds move warmed air eastward into Europe.

Water in the depth is moving back towards the equator along the continental slopes of Greenland and North America. Thus a cold southward current runs in the depth of the ocean along the North American east cost. The deep current is  accompanied by cold surface waters, the Labrador current, that sweeps the shores of Canada and north-eastern states of the USA before - south of Cape Hatteras - the warm northward flowing Gulf Stream dominates the surface currents. Thus, the east-coast of North America is cooled in the north and heated in the south. However, in the depth all along  the continental slope a mighty vein of cold water runs southward, the back-loop of the 'conveyor belt' in the North Atlantic.

The cold water that was formed in European sup-polar seas moves south into the South Atlantic Ocean where waters from different sources meet, including water from the Indian Ocean and the Pacific Ocean. The South Atlantic Ocean has its own less vigorous overturning circulation. It is moving heat along the shores of South America from the tropics poleward linking into the mighty circumpolar current sweeping around the Antarctic continent. Likewise - as a key part of the global 'conveyor belt' -  heat is swept  by surface currents northward out of the tropical South Atlantic Ocean. Warm water flows into the western tropical North Atlantic Ocean and the Caribbean Sea from where after further heating the even warmer water is shifted forward to Europe.

"What would happen if the North Atlantic Current
should stop or change direction?"... climate in Labrador and Ireland. 
The net global oceanic heat transport is into the North Atlantic where the heat is released from the ocean into the atmosphere. This heat then is swept by the winds into Europe, Northern Africa and Asia.

The relative warm surface waters of the North Atlantic Ocean, which are already enriched in salt content for example by the outflow of salty water from the Mediterranean, pass humidity to the warm atmosphere. The surface waters get saltier than surface waters of any other ocean. This high salt content is a favourable precondition for the deep-water formation in  Greenland Sea and Labrador Sea, what in turn is engine moving around the 'conveyor belt'. That engine drives the North Atlantic powerhouse for keeping Europe's climate warm and clement.

[1] See related posts: 
[2] The similar feature does not occur in the Pacific Ocean with the same strength because the Bering Sea linking it with the Arctic Ocean is shallow and the salt content of the surface waters of the Pacific Ocean is lower. However the winter freezing of Okhotsk Sea causes formation of a modest volume of deep water.

[3] McCarthy, G. et al. (2012), Observed interannual variability of the Atlantic meridional overturning circulation at 26.5°N, Geophysical Research Letters: "The Atlantic meridional overturning circulation (MOC) plays a critical role in the climate system and is responsible for much of the heat transported by the ocean. A mooring array, ...  provides continuous measurements of the strength and variability of this circulation. With seven full years of measurements, we now examine the interannual variability of the MOC. While earlier results highlighted substantial seasonal and shorter timescale variability,... From 1 April 2009 to 31 March 2010, the annually averaged MOC strength was just 12.8 Sv[erdrup = 1.000.000 m³ / second], representing a 30% decline. This downturn persisted from early 2009 to mid-2010....  This rebalancing of the transport from the deep overturning to the upper gyre has implications for the heat transported by the Atlantic."

[4] Q. Schiermeier, Ocean under surveillance, Nature, Vol. 497 p.167-168. The article by Q. Schiermeier has motivated me to prepare this contribution for my blog "Mundus Maris... first news". 

[5] Rudels, B. Quadfasel D. (1991), Convection and deep water formation in the Arctic Ocean-Greenland Sea System, Journal of Marine Systems: "The processes of convection and deep water formation in the Nordic Seas.... The dense shelf waters sink on the continental slopes into the deep basins entraining ambient waters from the strongly stratified Arctic Ocean proper. In the European Polar Seas—the Nordic Seas—deep water is only formed in the Greenland Sea through haline convection, ...resulting in a weakly stratified water column...".

Saturday, 23 March 2013

Marine Heavens on Snowball Earth ?

Spriggina is an Ediacaran organism, possibly it is an animal.
 Spriggina grew to about three centimetres, it was segmented,
with one row of tough plates on top and two interlocking
rows on the bottom, and it may have been predatory.
Time has passed, admittedly, since the Proterozoic eon - meaning the "eon of early life" - had reached its last phase 635 Million years ago. The continents were barren land [1] under an oxygen rich atmosphere, dusty and reddish since more than a billion years. But for the very time first multicellular, mostly sessile marine organisms had emerged, populating  the Ediacaran seas [2]. They were descendants of the lucky survivors of global glaciation during the preceding geological era. Since the Ediacarian the global ocean stayed open and is prospering with life.  However sea-life flourished since billion of years. Stromatolites populated freshwater of inland seas and coastal marine environments long time ago and still today. Different varieties of chlorophyta (green algae), bacteria and other unicellular marine algae, Acritarchs, prospered in the Proterozoic eon and were preserved as micro-fossil in marine sediments [3]. Many of these species were living on sulphur chemistry in an oxygen-poor sea through the billion years of the Proterozoic eon, including too a period  that geologist like to call the "boring billion" [4], although certainly it was full of unknown adventures of evolution of life.  

Cyanobacterial (Earth’s earliest oxygenic organisms)
stromatolites from Sharks Bay, Australia
The  Neoproterozoic era - meaning "recent era of early life" - was a dramatic time [5]. Twice Earth was ice-covered and was looking more like a ball of slush or snow. Twice Earth was sweating in  run-away greenhouse grilling the barren land. Life in the sea went through big swings.

Carbon isotope records in carbonate rocks  and Neoproterozoic glacial deposits found in Namibia suggest that biological productivity in the surface ocean collapsed for millions of years. Thus life faced a dramatic bottleneck after nearly three billion years of evolution of multiple forms using either chemical process (chemotrophic) or light (phototrophic) as source of energy. Geological findings show that marine ice extended from the Poles to the Equator at least twice during the Neoproterozoic era.

It is a teasing question whether open water was found along the Equator during that periods. How would phototrophic life survive when the entire planet, land and seas, are covered by ice?

The  kilometre-thick layer of ice of the  Neoproterozoic ice-age made Earth looking like Jupiter's icy moon Europa. "Snowball Earth" looked far different from the blue ball that fascinated so much the first astronauts. However the global ocean kept active under the global ice cover. Survival of phototrophic life in a Snowball Earth climate possibly depended, as outcome of a  recent study [*] can be interpreted, on the ocean circulation and mixing processes that kept local patches of water open and thus created "marine heavens" in which  phototrophic life could survive.

Ocean circulation and mixing processes  set the melting and freezing rates of the  "marine glaciers" today and in the  Neoproterozoic era; the same physics apply. The melting and freezing rates determine ice-extend and the local ice thickness. Modern computer models of ocean circulation and ice-dynamics can simulate the physical behaviour of the global ocean of "Snowball Earth" and addressing too the question where to look for patches of open water for survival of phototrophic species.

The ocean of "Snowball Earth" was not freezing to the bottom. Salinity of seawater, high pressure and geothermal heat-flux from the bottom prevented that. The ocean is insulated at the surface from atmospheric forcing by the thick ice cover.  This ice-cover also is a very good thermal isolator preventing heat loss from the ocean. These features make up the very particular ocean dynamics of  "Snowball Earth". Water flows are driven by heat-flux from the sea-bottom and freezing at the surface. That is limiting vertical stratification. Today wind, radiation of the sun and evaporation drive the ocean currents from the surface and create a stable stratification of the water column. Geothermal heat-flux from the sea-bottom has not stopped, but its impact on the ocean dynamics is much less than the forces acting at the surface.  

A recent study [*], modelling dynamics of the Neoproterozoic ocean, highlights the processes that could maintain spots of open water (or thin ice) that phototrophic forms of life need for survival at the ice-margin. Today marine life prospers at the margin of sea-ice because of the particular dynamics of ice water interactions. Likewise life dwells at the bottom of sea-ice as long as some light penetrates through the ice. It is a teasing hypotheses that similar "marine heavens" were found in the ice-covered Neoproterozoic ocean.

Modern Earth: Geothermal heat-flux
The Neoproterozoic ocean of "Snowball Earth" is isolated at the surface by a kilometre-thick ice-layer. This layer cuts off effectively wind, solar radiation, evaporation and radiation losses.  Water freezing at the bottom of the "marine glaciers" and the heating of water at the sea bottom are the dominating processes, which drive water flows. Vertical convective mixing occurs. Water gets warmed at the sea-bottom by geothermal heat and rises towards the surface. Salt is leaking out of the water freezing to ice at the bottom of the marine glaciers. Adding salt to the cooled water is making it heavier so that it sinks towards the bottom. Therefore the ice-covered Neoproterozoic ocean is less stratified and properties of water, such as salinity vary with latitude. Lateral differences are maintained by the Coriolis-force [6] and related currents. Thus, in the Neoproterozoic ocean, close to the equator warmer and less-salty water rises to the surface, moving poleward and is sinking down again when cooled and enriched in salt-content. Similar overturning circulation cells are found in modern oceans but away from the equatorial zone.

The ocean temperature, salinity and density of  Neoproterozoic ocean was fairly uniform in the vertical direction but showed lateral differences. These lateral differences of density sustained, because of the rotation of earth, jet-currents along the Equator. These currents were unstable and were shedding off eddies. These eddies transported warm water away from the Equator to the ice-margin. There the water was melting ice, was cooled, partly frozen to the ice, partly enriched in salt  so that it sunk downward. A compensating upward flow of warm water occurred at the Equator to close the circulation cell. Ridges at the sea-bottom or continental margins brought the source of heat closer to the surface and were interacting with the equatorial jet-currents. This interaction caused local jets, eddies, coastal up-welling and down-welling as well as convective mixing . The weak stratification made up-welling and down-welling far easier to happen than today in our well stratified ocean. Thus "Snowball Earth" ocean was not a stagnant pool of cold water, it was highly dynamic; at least to the eye of the oceanographer.  

from [*]: Temperature at 1,200 m depth (colour scale), areas of enhanced geothermal heating (black contour lines) and land masses (white areas). b, Salinity at 1,200 m (colour scale). c, Ice thickness (colour scale), and ice velocity vectors 
These insights in dynamics of the Neoproterozoic ocean were  gained recently by computer simulations using models that couples ice flow and ocean circulation; and as the authors summarize [*]: "Compared with the modern ocean, the Snowball Earth ocean had far larger vertical mixing rates, and comparable horizontal mixing by ocean eddies. The strong circulation and coastal up-welling resulted in melting rates near continents as much as ten times larger than previously estimated."

And marine life? It survived "Snowball Earth". Both, the chemotrophic life that is using sulphur as source of energy and the phototrophic life that is using light as source of energy. Chemotrophic life would have survived in the depth of ocean under total ice-cover, but  phototrophic life would have needed patches open surface water or thin ice; at least temporarily. The physics of  ocean dynamics make it likely that these patches, "marine heavens" existed regularly in the ice-covered Neoproterozoic equatorial seas.

The geochemical carbon cycle on a Snowball Earth
An essential prerequisite for existence of these "marine heavens" is that geothermal heat-flux through the sea-bottom was bigger as heat loss through the layer of marine glaciers at the surface. This ice-layer was moved and cracked by tides (as for example satellite pictures from Jupiter's moon Europa show for its ice-cover) and therefore the very thick ice-layer was not a perfect isolator. Heat will have been lost through the ice-cover.  Further studies of ice-dynamics may constrain what minimum geothermal heat-flux is needed to keep patches of  the Neoproterozoic ocean ice-free where ocean dynamics causes heat to be accumulated. Geophysical research then may assess whether this geothermal heat-flux is likely to have happened.

The end of  Snowball Earth likely was caused by volcanism blowing carbon-dioxide into the very dry and cold atmosphere of that time [7].  Rain must have been seldom, and without rain little carbon-acid weathering of rocks occurs and no carbonates are flushed into the sea. Thus carbon-dioxide accumulates in the atmosphere building up a greenhouse effect that finally caused Earth to warm again.

If that scenario to end "Snowball Earth" happened, then Earth was saved from staying frozen in snowball stage by  its active geophysical processes. To note the difference, Jupiter's moon Europa is frozen in snowball stage; likely the moon is too small for having an active geophysical evolution as planet Earth. However, researchers are quite certain that under the icy surface of moon Europa an ocean is alive. If it bears life we don't now. But if, then it will be of a chemotrophic form. Luckily survival of phototophic life on Earth in the global glaciations of the  Neoproterozoic era has happened and likely because ocean dynamics on a geophysical active planet with continents moving, plate-tectonics and a robust geothermal heat-flux created some "marine heavens".

[*]  Dynamics of a Snowball Earth ocean; Yosef Ashkenazy, Hezi Gildor, Martin Losch, Francis A. Macdonald, Daniel P. Schrag & Eli Tziperman; Nature 495, 90–93, 7th March 2013 March 2013

[1] for a debate about life on land see:

[2] from Wikipedia (simplified):  (a) The Ediacaran Period  named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period. Its status as an official geological period was ratified in 2004 by the International Union of Geological Sciences (IUGS).  Although the Period takes its name from the Ediacara Hills where geologist Reg Sprigg first discovered fossils of the eponymous biota in 1946, the type section is located in the bed of the Enorama Creek within Brachina Gorge in the Flinders Ranges of South Australia, at 31°19′53.8″S 138°38′0.1″E. (b)  The Ediacara biota consisted of enigmatic tubular and frond-shaped, mostly sessile organisms. Trace fossils of these organisms have been found worldwide, and represent the earliest known complex multicellular organisms. The Ediacara biota radiated in an event called the Avalon Explosion, 575 million years ago, after the Earth had thawed from the Cryogenian period's extensive glaciation [and] disappeared contemporaneously with the rapid appearance of  Cambrian biota [which] completely replaced the organisms that populated the Ediacaran fossil record.

[3] from Wikipedia (simplified) Acritarchs have been recovered from sediments deposited as long as 3.2 billion years ago, but at about 1 billion years ago they started to increase in abundance, diversity, size, complexity of shape and especially size and number of spines. Their populations crashed during the Snowball Earth episodes, when all or very nearly all of the Earth's surface was covered by ice or snow, but they proliferated in the Cambrian explosion and reached their highest diversity in the Paleozoic. The increased spininess 1 billion years ago possibly resulted from the need for defence against predators, especially predators large enough to swallow them or tear them apart. Other groups of small organisms from the Neoproterozoic era also show signs of anti-predator defences. Further evidence that acritarchs were subject to herbivory around this time comes from a consideration of taxon longevity. The abundance of planktonic organisms that evolved between 1,700 and 1,400 million years ago was limited by nutrient availability – a situation which limits the origination of new species because the existing organisms are so specialised to their niches, and no other niches are available for occupation. Approximately 1,000 million years ago, species longevity fell sharply, suggesting that predation pressure, probably by protist herbivores, became an important factor. Predation would have kept populations in check, meaning that some nutrients were left unused, and new niches were available for new species to occupy.


[5]  from Wikipedia (simplified): The Neoproterozoic Era is the unit of geologic time from 1,000 to 541 million years ago. The terminal Era of the formal Proterozoic Eon (or the informal "Precambrian"), it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods. The most severe glaciation known in the geologic record occurred during the Cryogenian, when ice sheets reached the equator and formed a possible "Snowball Earth". The earliest fossils of multicellular life are found in the Ediacaran, including the earliest animals.

[6]   from Wikipedia (modified):  Coriolis force: A force exerted on a parcel of water (or any moving body) due to the rotation of the earth. This force causes a deflection of the body to the right in the northern hemisphere and to the left in the southern hemisphere.

[7] from (modified): The snowball Earth scenario does not require glaciation of the continents . The ice cover on the oceans prevented water from evaporating, and therefore the climate must have been very dry. Lack of precipitation likely caused at least parts of continents to be bare rock, as ice was sublimated or flowed into the sea, and was not replaced due to the lack of precipitation.  The commonly proposed scenario for the end of snowball Earth is through the accumulation of carbon dioxide. Volcanism produces carbon dioxide, which accumulates until it reaches a point where it triggers warming through its greenhouse effect. The ice sheets are melted rapidly and temperatures rise, perhaps reaching as high at 50 °C temporarily, before the carbon dioxide is removed from the atmosphere. There is strong evidence of such extreme rises in atmospheric carbon dioxide, in the form of cap carbonates.