We had a good turnout on a mild and bright Wednesday morning at Williams Beach for the second highest tide of the year. While confined to a narrow strip of shore between the gentle waves and the slippery, imbricate logs, we managed to see quite a lot. The following are some notes and observations related to the outing.
(1) Shoreline Erosion and Sea Level Change
Around these very high tides, particularly in wet and stormy weather, is when you can see the ocean eating away at the shore. With rising seas, shoreline erosion with increase. In our region, changes in sea level have both global and regional underpinning. On the global level, the melting of glaciers and ice sheets is taking a volume of water previously sequestered on land and returning it to the oceans. Increased ocean temperatures also also lead to some thermal expansion of the volume of the oceans. The IPCC 2017 assessment and modeling of sea level rise attributes about 20 cm of sea level rise over the last 100 years to this flux from ice to ocean. Furthermore, it predicts anywhere from another 20 to 100 cm rise by 2100.
Locally, sea level change is complicated by two other factors– land level changes associated with tectonic activity (earthquakes and crustal
flexure driven by the accumulation of strain near the subduction zone) and the slow rebound of the Earth’s crust in our area after it was buried under 1-2km of ice at the peak of the ice age. Locally, a recent paper by Fedje et al. provided evidence for sea levels being almost 200m higher. The map below is from the linked paper and shows the location of paleoshorelines in the Quadra Island area circa 14,000 years ago. We saw a much more recently exhumed shoreline just above the present beach.
(2) The Midden
We saw the dense concentration of clam shells in the bank from which Betty pulled a stone pestle some years ago. There was some discussion about how to distinguish human deposits of shell material from the “lags” of shells that typically accumulate by wave, tidal and current sorting. Some criteria: the biological composition of the shell material, vertical and lateral extent of the deposit and the prevalence of interstitial cobbles and sand. The troughs in which shells naturally accumulate tend to follow depositional bedforms that are either laminated or swooping and typically on the scale of ~1m or less in lateral extent and typically around cm-scale vertically.
(3) Stuff thrown up by the ocean.
We found a potpourri of seaweeds. Below from left to right are some of the common ones- Saccharina sp. (Sugar Kelp), Nerocystis sp. (Bull kelp), Sargassum sp. (Japanese Gum Weed), Ulva sp. (Sea Lettuce), Constantia sp. (Cup & Saucer), Chondracanthus sp. (Turkish Towel) and Gracilaria sp. (Red Spaghetti).
Gary discussed how
their colours and depths are controlled to some extend by the types of chlorophyll present in the species. Some of the more drmatic coloration at this time of year has more to do with the breakdown and loss of chlorophyll and pigments.
We saw some interesting animals, most notably a pacific sand lance (presumably hanging out in the abundant sands just below the high-tide line at Williams Beach) and a large sponge kicked up by the surf from the subtidal flats.
The identification of sponges is challenging as keys focus o
n spicule characters. The spicules are structural elements, typically rods of mineral material that are encased in organic compounds. I prepped out some spicules using standard household bleach and found these lovely ~100 micron spicules (“Tylostyles”) of SiO2. Turns out this thing is called “Dead Man’s Fingers”, Homaxinella amphispicula. It is a demosponge, which is to say a mixture of organic structural elements and mineralized ones. It is a subtidal species, so likely kicked up during a very strong storm around the wave base.
There are upcoming late night low tides. The plan is to head to miracle beach at night with headlamps to see what is stirring. I’ll send around a date later this week.
Thanks everyone for coming out and contributing.
See you soon,
Randal
