Home to some very corrosive sulphur crags.
The Sintra volcanic complex, comprised mainly of granites and syenites, forms impressive, jagged sea cliffs that extend for several kilometres both north and south of the Cabo da Roca. Syenite is a low-silica form of granite, and looks very similar.
Unlike typical granitic intrusions with their massive, sparsely-jointed structures, these rocks are highly fractured with a marked NE/SW jointing. It is seems likely that they are highly permeable to ground water.
This area first came to my attention some years ago via an email chain circulating through UIAA Safe Com. A local climber, Rui Rosado had reported bolt failures at Praia da Ursa, on the syenite just north of the cape. This event was to become part of the “so it’s not just tropical karst” realisation that was slowly dawning.
As cases of severe corrosion began to appear outside of Railay/Tonsai, there was a reflexive action from various quarters of the climbing community to assert that all stainless steel, especially 304 (A2), was simply not resistant to sea water. To my skeptical mind, this was clearly a very poor generalization given the hundreds, if not thousands, of kilometres of architectural stainless steel installed in coastal cities across the world, not to mention my own experience of 304 bolts lasting decades in the subtropical, maritime environment of my home town.
There should be no magic in this matter. If there is a difference in the environment that causes extreme corrosion to occur at some locations and not others, then it will be measurable. We had already shown that wall-wash samples from Railay/Tonsai had anomalously high sulphate levels that could be explained only by assuming the agency of an outside sulphur source. Would the same be the case for the cliffs of Cabo da Roca?
The short answer is, yes. Like Tonsai/Railay, we are dealing with crags exhibiting high surface sulphate levels.
We were very lucky to have the services of local climber and equiper, Luis Fernandes Silva, to help us with this project. Luis has taken a number of wall-wash samples and spot samples that point clearly to high levels of sulphate circulating in the ground water.
The wall-wash samples themselves, aren’t so informative. Yes, there is one sample from Espinhaco that has anomalously high sulphate levels, but the others show pretty typical electrolyte levels for a sea cliff. In passing, it is worth noting the presence of the calcium bicarbonate system that keeps the pH of the surface environment not too far from neutral. Notice also that magnesium is displaced by calcium whenever sea-salt deposits on these surfaces. It is very rare to encounter a wall-wash sample where the electrolyte balance is representative of sea water. Consequently I believe it is naive to use sea water corrosion resistance as a benchmark for the corrosion resistance of installed hardware.
Leaving for the moment the question as to why we are picking up high sulphate levels in one sample and not others, let’s look at what else we know.
At multiple locations a white efflorescence can be seen where ground-water is seeping from the rock. Spot testing for sulphate, using our new test kit, reveals that such deposits are always a sparingly-soluble sulphate.
I took the opportunity to do a complete analysis of the efflorescence shown below. Once the sea water components are washed off, the fine granular material left behind is revealed to be gypsum, or CaSO4.2H2O, in a reasonably high state of purity.
So, where does this information take us? We know that prolific amounts of calcium sulphate seem to be circulating in the ground water. However, we need to remember that calcium sulphate is sparingly-soluble at neutral pH, so the actual concentrations of sulphate in such waters won’t be huge, but it will, most likely, be super-saturated given the tendency of calcium sulphate solutions to do so. Thus when it escapes to the surface on a cliff face, we can expect calcium sulphate to be deposited. If such seepage flows over an installed bolt and wicks into the internal spaces, it will feed any sulphate reducing bacteria (SRB) present. I doubt there will be anything uniform about this process, and thus we can expect some bolts will be fine, while others, less than a metre away, will be destroyed.
In a future post I will present the data we have to link the presence of sulphate to an SRB-mediated corrosion mechanism that accounts for the very corrosive nature of this cliff.
One of the big mysteries we have here is the identity of the sulphur source. Except for when contributions from volcanism arise, the natural sulphur cycle tends to be highly conserved and local in nature, For every sulphur crag we have thus far identified, we can point to a presumptive sulphur source emanating from a volcanic arc associated with plate tectonics.
I have a theory that elemental sulphur discharged from undersea vents accumulates in the organic film on the ocean surface, and from there is dispersed in the coarse marine aerosol to the cliff-top and cliff-sides. Sulphur oxidising bacteria (SOB) readily oxidise it to sulphuric acid, which then runs down the cliff face, being neutralised to sulphate as it goes.
For Cabo da Roca, we have two problems. Firstly, if we consider the fact that the Mediterranean is a reasonably closed basin, then the only points of volcanism are way out in the Atlantic ocean at the Azores. The second, and perhaps greatest difficulty, is to explain the highly pure calcium sulphate efflorescence. If we do indeed have an SOB-mediated process on the cliff-top, then the action of sulphuric acid on both granite and syenite would be to produce potassium/sodium aluminium sulphate, not calcium sulphate. We would know if this was happening, and it is not.
One possible explanation is that the ground water has a hydro-thermal source. We have previous experience of this phenomenon at two high sulphate crags on Kalymnos where hydro-thermal waters are seeping down the cliff depositing substantial amounts of sulphate. However, at these sites, we also see other markers such as aluminium and silica, which we do not see at Espinhaco. Given this evidence, it would be surprising if the ground water we find at Cabo da Roca had a hydrothermal source. The geology of the area is complex, and I don’t believe we are yet seeing the full picture.