What Rises from Beneath?….A Gas Seep in the Inner Hebrides?
Article by Peter Macalister Hall
Sailing off the West of Skye this summer I noticed a single thin trail of bubbles rising form submerged Tertiary basalts in the intertidal zone. (Picture). Curious. It was late evening and I was rowing the dog ashore for a short land-break before bed.
We had sought shelter from high winds, anchoring in the SW corner of Loch Bharcasaig, a small bay in the NW of Loch Bracadale. This is easily accessible by boat, but is a fairly remote corner of West Skye if on foot.
Collection of a gas sample was impractical at the time. The bubbles were small, their ascent slow. The trail varied with the slight swell and the rubber dinghy declined to hold still, similarly the dog. Lacking equipment and with plans for another week afloat, I gave up and moved on.
However, day dreams of Hydrocarbons or perhaps even Helium persisted. A chat with other geologists at Aberdeen University encouraged me to probe further. There are Jurassic rocks on Skye and Raasay, but no record of a gas seep in the Inner Hebrides. Finding a thermogenic hydrocarbon seep would be an interesting development. I really needed samples and gas analyses to avoid doubt –preferably at minimal cost.
I contacted Core Lab, a large Oil Industry Service Company with a long history in Aberdeen. They were intrigued and fully appreciated the potential significance. They agreed that even a long shot was worth a quick check. Very generously, they agreed to run a couple of analyses at a slack moment. Cost? “Hum. How about: please don’t forget us if this proves interesting.”
I returned to Skye by car and with some trouble relocated the seep. Things look different from land. Sample collection was in waist deep water on a rising tide. I placed a large inverted plastic funnel weighted with a lead collar over the source of the bubbles. Elastic bands from the collar held a water-filled glass jar over the funnel spout; the rising bubbles gradually displaced the water in the jar. I collected three half full jars, each taking about 15 minutes to accumulate. Probably a rather comic sight, but the location was so remote there were only Sea Eagles as observers. The jars were sealed underwater and remained inverted during transport. Collection of a headspace sediment sample was not possible. The bubbles were rising through a Tertiary basalt flow directly into sea water. No near-surface soft sediment -only rock.
Sadly, in the end, the bubbles proved to be air. Analysis of one of the samples is attached.
Jar 2 Headspace Gas
(Trace presence of Methane and Ethane. Atmospheric Methane is ~0.0001 Mole%).
Curiously, the CO2 Mole% seems rather high. Normal air is reckoned to have around 0.03 – 0.04 Mole%. Is some bug respiring somewhere down there, or is it due to intertidal (rich active biozone) sea water degassing in the sample jar ?
The locality comprises layered Tertiary-age lava flows gently dipping seaward. Sidestepping the CO2 issue, one possibility is that a seaward-dipping cavity beneath the upper flow drains at low tide and that air, compressed by the rising tide, is escaping very slowly through an up-dip crack.