Guest essay by Philip Mulholland
In my previous essay We Must Get Rid of the Carboniferous Warm Period I discussed the role of the polar seas around Antarctica in generating the cold dense oxygenated marine water that dominates the abyssal ocean depths of our modern world. I now want to discuss the role of shallow tropical seas in generating warm dense oxygen-poor marine water and how this fundamental and often overlooked process explains the presence of abyssal ocean warm water and high atmospheric carbon dioxide concentration during previous geological times, in particular the Cretaceous period.
It is a mid-June day in 1991, West Caicos, a small uninhabited tropical island in the Turks and Caicos archipelago, bakes in the hot summer sun. I am on a field trip to the British West Indies organised by Dr Hal Wanless of the University of Miami, to study the modern geology and natural depositional environments of a marine carbonate platform. A visit that, even now, I consider to have been the best field study trip of my entire geoscience career. Located in the trade wind belt, the Turks and Caicos Islands lie at the south-eastern end of the Bahamian chain of Atlantic Ocean carbonate-platform islands. With the Tropic of Cancer passing to the north of the group, at midday the June sun is directly overhead and your shadow falls exactly beneath you. By evening, the summer thunderstorms arrive tracking west across the ocean, passing by on their way to the Caribbean.
For most of the year, the climate of West Caicos is dominated by dry trade winds. These are derived from the downwelling of the Hadley Cell, centred over the Atlantic Ocean to the north-east.
The low rainfall and high evaporation rate make the climate too dry for sugar cane production, an economic enterprise tried by past entrepreneurs at this remote island location. Salt production, the original economic activity of the Turks and Caicos, was also attempted at West Caicos, but that enterprise failed too. At West Caicos the salt pans were located on the site of a major wash-over fan in the northwest of the island. The bedrock here consists of permeable limestone rubble and not impermeable mudflats, the place of choice for salt production on the other islands in the group. This site, with its poor hydrogeology, probably accounts for the failure of the West Caicos salt pan enterprise.
Now West Caicos is a nature reserve and the native bromeliad flora are left to grow undisturbed. We are here to undertake a west to east traverse across the island to see how the individual elements of its geology have been created by the natural marine processes of active carbonate deposition occurring over the past few thousand years, since the sea level rise at the end of the last ice age flooded the Caicos platform.
We begin our journey in the sea, swimming with mask and flippers off the island’s west coast; here we observe the corals thriving in the shallow warm waters of the reef flat, everyone’s ideal coral island setting. Swimming is easy in the warm water with its slight swell, as we make our way out to the drop-off, and spot the barracuda fish below, patrolling the reef edge, marking its location. Then everything suddenly changes, the seabed disappears from sight as the water depth precipitously increases, the water colour becomes a deep blue and its temperature abruptly falls. With the sudden temperature drop I experience cramps in both legs and am grateful for the life jacket I’m wearing and the presence of my safety buddy, as swimming becomes difficult in the now cold water. So where has the warm water gone? Leaving this question unanswered, we swim to the support boat and head back to the island’s shore.
Our next stop is just off the beach, here the corals are no longer thriving, they are being buried by carbonate beach sand and the burrows of innumerable marine creatures pockmark the seabed. This change to carbonate sand is not evidence of environmental degradation, this sand zone is also a thriving pristine environment, it is simply no longer the coral’s home and a new force of nature, sediment derived from the inorganic carbonate beach factory, dominates the scene. Carbonate geologists estimate that approximately 50% of all the carbonate rock on Earth is generated by inorganic means and our next stop is the factory floor, the sand generating swash zone of the carbonate beach environment.
We arrive on the western beach of West Caicos, standing in the shallows where the seawater reaches its warmest temperature. We observe the continuous back and forth motion of the water as each wave arrives, rolling the grains of carbonate sand and creating a smooth beach profile with a distinctive sedimentary pattern or facies. Hal draws our attention to the beach rock in a small cliff adjacent to our landing point. Here we can see, preserved in the vertical rock face and deposited at a time of previously higher sea level, the sedimentary facies of the same near shore environments we have just observed offshore.
In the base of the cliff we find the fossil corals, above them surrounding and smothering them we see the lithified carbonate sand grains and the distinctive cone shaped burrows of long dead marine animals. Above this zone are the smooth layers of sand from the old swash zone forming a structured Z shaped pattern in the cliff face marking the exact tidal limit of the ancient beach. This is a classic geological example of the “Principle of Superposition and Original Horizontality”, where the younger sediments of the proximal shallow-water beach environment extend over the older distal deeper-water coral reef, as the sea bed shallows and the island grows seaward. The effects of this principle are regularly observed in marine carbonate deposits, with each upward episodic sea level change defining the next level in a repeated pattern of sedimentary growth.
We climb off the beach, up onto the rock outcrop and on its upper level we find gigantic boulders of beach rock with the same three facies as before, but tumbled out of their original setting. Hal observes that these boulders have been ripped out from the cliff and deposited up here by a storm surge from a former hurricane. My personal opinion is that this could be a tsunami deposit, given that we are due north of Hispaniola and at the western end of the active Puerto Rico submarine trench, this explanation of a powerful wave, generated by a submarine earthquake, also seems plausible. It is my view that in geoscience it is always good to consider more than one possible explanation for any set of field observations.
We are now standing at the top of the cliff on the highest and oldest part of the island. Turning to face east, the land falls away in a gentle slope and in the distance, on the horizon, a line of sand dunes rises behind a blue saline lake, Lake Catherine. Following a straight track, laid out by the former sugar plantation enterprise, we are soon back down at sea level, walking out on a causeway across the brine lake. Half way across there is a break in the track, the site of a former culvert, where the lake water flows though the causeway gap from north to south. Hal explains that on every visit to West Caicos he has always observed the same continuous direction of flow, so a tidal explanation for the movement of the water can be discounted.
Lake Catherine occupies the site of an old bay on the island’s former east coast, now separated from the lagoon by lines of barrier dunes, but its waters are still connected to the sea by an underground limestone aquifer. The wind driven marine current flowing west towards West Caicos island across the shallow Caicos lagoon creates a hydraulic gradient on the islands east coast that forces seawater underground, through the island’s limestone core to emerge in and flow through this central blue lake, before the water again makes its way back underground to regain the open sea on the island’s west coast.
Beyond Lake Catherine the track rises to a cut through heavily vegetated small hills, the maturity of the bromeliad flora demonstrates the significant age of these now inactive sand dunes. At the crest line, a new vista appears, in the distance a second line of modern sand dunes lies beyond a sabkha mudflat. We descend and cross the sabkha, its fragile algal crust breaking under the pressure of our footsteps, to reveal soft gypsum mud below. The presence of natural gypsum (hydrated calcium sulfate) in this ocean island setting is a surprise and is a testimony to the effectiveness of the high evaporation rate of the Caicos climate in concentrating the seawater brine.
Leaving the sabkha we climb the line of modern dunes, the loose carbonate sand and the sparse vegetation of grasses demonstrate the young age of this second barrier to be crossed before we reach the modern east coast of West Caicos. Beyond the crest, a rapid descent brings us down to a wide wind swept beach. A continuous drying wind, blowing in our face, moves the loose sand off the shore, adding to the dunes behind us and raising the island’s surface above sea level by means of aeolian sedimentation.
Here on the wide eastern beach, sitting below a small Casuarina tree and facing the shallow lagoon, we see the true extent of the carbonate sediment factory, a prolific producer of inorganic carbonate sand. Oolitic (egg shaped) grains roll in the beach swash zone growing layer on layer to produce an onion ringed sand grain wrapped around an original seed crystal of aragonite. Out beyond the beach the shallow warm sea, with water depths of less than 10 metres, extends eastward for 100 km, it is dotted with small patch reefs of coral rising clear of the sandy bottom. Parrot fish, with their strong beaks, bio-erode the coral and excrete crystals of indigestible aragonite, mineral seeds that form an endless supply of crystals around which new oolitic sand grains grow, in a symbiotic union of organic and inorganic sedimentation.
It is now 22 years since that summer day, yet the memories of my short visit to West Caicos remain vivid. Looking back, it is time to place all the elements of that day into an environmental synthesis and answer the question of what happened to the warm surface water when I swam beyond the reef edge into the cold water of the Atlantic Ocean.
There are two major types of marine carbonate environment: carbonate platforms and carbonate ramps. Carbonate platforms are found throughout the tropical oceans of the modern world and consist of isolated flat topped carbonate banks that are very sensitive to global seawater drawdown. During the ice ages, when the sea level lowers as ice builds up on land, carbonate platforms are easily exposed and then become incapable of further sedimentary growth; warm water production ceases, inorganic calcium carbonate formation stops and the associated process of carbon dioxide gas liberation fails.
Marine inorganic carbonate sedimentation is a geological process that occurs in shallow warm-water, tropical seas. The crystalline chemical solid calcium carbonate is unusual in that it becomes more insoluble as water warms. Carbon dioxide gas dissolved in cold water creates the weakly acidic carbonic acid which can dissolve solid calcium carbonate crystals creating water soluble calcium bicarbonate, by this mechanism the carbon dioxide becomes chemically associated with the calcium, and not just simply dissolved in the water. Calcium bicarbonate however, unlike calcium carbonate, does not exist in a solid chemical form, it occurs only in solution. In the warm surface waters and beach zones of shallow tropical seas calcium bicarbonate solution becomes thermally unstable, calcite precipitates naturally from the seawater as the water soluble calcium bicarbonate reverts to insoluble calcium carbonate crystals, liberating carbon dioxide molecules.
The geological record shows that half of all marine limestones were formed from seawater by the mechanism of direct chemical precipitation in a purely temperature and evaporation driven process. These non-biological limestone rocks include oolitic carbonate sandstones; even now egg-shaped grains of these carbonate sands form abundantly in the shallowest and warmest waters of the modern Bahamian platform lagoons.
The Caicos Islands are an example of a modern active carbonate platform that, during our current interglacial high sea level, forms an area of shallow lagoon surrounded by the deep waters of the Atlantic Ocean. The dimensions of the platform are large, in the south it extends from West Caicos to Seal Cays, a distance of about 100 km, while in the north it extends from Providenciales to East Caicos a distance of about 80 km. The platform covers an area of approximately 5,400 sq. km, of which only 430 sq. km is land and about 5,000 sq. km is covered by shallow sea. This shallow lagoon is a gigantic solar energy collector, each day the tropical sun warms the seawater and all day and night the dry north-east trade wind enhances the surface evaporation, increasing the seawater salinity and driving the water westward across the lagoon towards West Caicos and the open ocean beyond.
As the temperature and salinity of the seawater increases in the lagoon a process of evaporitic precipitation of salts from marine waters becomes possible. The deposition of these salts occurs in a distinct sequence. Calcium carbonate, the least soluble salt, precipitates first. The water soluble calcium bicarbonate is converted to calcium carbonate precipitate with the release of gaseous carbon dioxide. This process takes place in the warmth of the beach swash zone and accounts for the prolific carbonate sand sedimentation found here and throughout the Bahamas.
The next salt that precipitates from the seawater concentrate is gypsum (hydrated calcium sulfate). This process takes place on the West Caicos sabkha, behind the dunes, where the ponded seawater, driven onto the island by the wind, concentrates by further evaporation. The third salt to precipitate is halite (sodium chloride) this is the most soluble mineral of the three and therefore the least likely to precipitate. The waters of the brine lake demonstrate that there is the potential for this process to occur on West Caicos, and would do so if a suitable natural salt pan existed here, as happens on other islands within the group.
As a consequence of the process of evaporation the sun warmed seawaters leaving the Caicos lagoon, on its western margin, are more saline and therefore denser than the colder open ocean waters that have flowed around the archipelago. At the reef edge this density difference allows the warmer lagoon water to sink down below the colder less saline ocean water and accounts for the sudden thermal contrast I experienced while swimming in the sea off West Caicos. It is interesting to note that the world freediving record was set at Providenciales, where the warm dense water exits from the Caicos lagoon and descends into the Atlantic Ocean depths.
Carbonate ramps are found on continental shelves in shallow tropical seas and form extensive coastal fringes. Unlike flat topped carbonate platforms, carbonate ramps are tilted and therefore robust to global sea level drawdown. They can maintain warm water production, calcite precipitation and carbon dioxide emission to the atmosphere throughout the sea level fall of a glacial cycle. Carbonate ramps are rare in the modern world. The best example is the Emirates coast on the southern margin of the relatively small (in geological terms) Persian Gulf. Because it is not the continental shelf margin of an open ocean, this shallow gulf, with its maximum water depth of 80m and restricted size, is vulnerable to global sea level fall, during ice ages the ramp ceased to function as the seabed turned into exposed land.
Although the modern world lacks major continental shelf tropical seas capable of hosting carbonate ramps, they occurred extensively in the geological past. For example, during the Cretaceous period a region of shallow tropical seas associated with the margins of the Tethys Ocean existed in the Horse latitudes of the northern hemisphere. In these shallow seas major carbonate ramps developed and abundant carbonate sedimentation occurred. The shallow waters of the carbonate ramp, warmed by the tropical sun, generated dense saline marine brines that filled the abyssal depths of the Cretaceous world ocean with warm anoxia prone bottom water, while at the surface inorganic carbonate sedimentation released carbon dioxide gas into the Cretaceous atmosphere.
The climatic difference between our modern cold ocean world and the ancient warm ocean world of the Cretaceous is simply due to the presence in the Horse latitudes of shallow tropical seas containing the carbonate ramps that form the planet’s “oceanic central heating system”. The physical location, areal size, and water depth of the world’s shallow tropical seas throughout geological time dictates the quantity of solar energy that these seas can collect from the tropics. Our modern world, with its carbonate platforms and restricted ramps (such as the Persian Gulf) that are sensitive to global sea level fall, has a much less efficient and less robust planetary “oceanic central heating system”.
In the argument of which comes first: atmospheric carbon dioxide levels or warm ocean water, the geological evidence is unequivocal: The “oceanic central heating effect” dog wags the “atmospheric greenhouse gas” tail.
I am a professional geoscientist with a BA in Environmental Sciences from The University of Lancaster in 1974 and an MSc in Conservation from University College London in 1981, where I studied the natural regeneration of woodland in Epping Forest using a Markovian Matrix technique to determine the temporal balance between Birch, Oak and Beech trees in a successional replacement cycle.
I started my career in the Institute of Geological Sciences (now the British Geological Survey) where I worked for 10 years learning about geology from experts, before moving on to continue my career in industry. Geology is a field science and the best geologist is the person who has seen the most rocks. I am a generalist by aptitude and therefore rely on the field work of experts when attempting to understand the interlocking complexities of geoscience.