|Johnny Scow's Kwakwaka'wakw Kwakiutl House, 1918|
The day-to-day activities of each of these communities would much like our own. Babies were born, meals were served and life followed a natural cycle. As coastal societies lived their lives they also left their mark. Sometimes through totems and carvings but almost always through discarded shells and scraps of bone from their food.
These refuse heaps contain a wealth of information about how that community lived, what they ate and what environmental conditions looked like over time. They also provide insight into the local gastronomic record on diet, species diversity, availability and variation.
This physical history provides a wonderful resource for archaeologists in search of botanical material, artifacts, broken cooking implements and my personal favourite, mollusc shells. Especially those formed from enormous mounds of bivalves and clams. We call these middens. Left for a period of time, these unwanted dinner scraps transform through a process of preservation.
Shell middens are found in coastal or lakeshore zones all over the world. Consisting mostly of mollusc shells, they are interpreted as being the waste products of meals eaten by nomadic groups or hunting parties. Some are small examples relating to meals had by a handful of individuals, others are many metres in length and width and represent centuries of shell deposition. In Brazil, they are known as sambaquis, left between the 6th millennium BCE and the beginning of European colonization.
Younger shell middens are found in Latvia (associated with Comb Ware ceramics), Sweden (associated with Pitted Ware ceramics), the Netherlands (associated with Corded Ware ceramics) and Schleswig-Holstein (Late Neolithic and Iron Age). All these are examples where communities practised a mixed farming and hunting/gathering economy.
On Canada's west coast, there are shell middens that run for more than 1 kilometre (0.62 mi) along the coast and are several meters deep. The midden in Namu, British Columbia is over 9 metres (30 ft) deep and spans over 10,000 years of continuous occupation.
Shell middens created in coastal regions of Australia by Indigenous Australians exist in Australia today. Middens provide evidence of prior occupation and are generally protected from mining and other developments. One must exercise caution in deciding whether one is examining a midden or a beach mound. There are good examples on the Freycinet Peninsula in Tasmania where wave action currently is combining charcoal from forest fire debris with a mix of shells into masses that storms deposit above high-water mark.
Shell mounds near Weipa in far north Queensland are claimed to be middens but are actually shell cheniers, beach ridges re-worked by nest mound-building birds. The midden below is from Santa Cruz, Argentina. We can thank Mikel Zubimendi for the photo.
The Danish use the term køkkenmøddinger, coined by Japetus Steenstrup, a Danish zoologist and biologist, to describe shell heaps and continues to be used by some researchers.
So what about these ancient shells is so intriguing? Well, many things, not the least being their ability to preserve the past. Shells have a high calcium carbonate content.
Calcium carbonate is one of my favourite chemical compounds. It is commonly found in rocks — as the minerals calcite and aragonite, most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite — and is the main component of pearls, snails, eggs and the shells of marine organisms. About 4% of the Earth's crust is made from calcium carbonate. It forms beautiful marbles and the 70 million-year-old White Cliffs of Dover — calcium carbonate as chalk made from the skeletons of ancient algae.
Time and pressure leach the calcium carbonate, CaCO3, from the surrounding marine shells and help embalm bone and antler artifacts that would otherwise decay. Much of what we know around the modification of natural objects into tools comes from this preservation. The calcium carbonate (CaCO3) in the discarded shells tends to make the middens alkaline, slowing the normal rate of decay caused by soil acidity and leaving a relatively high proportion of organic material — food remnants, organic tools, clothing, human remains — to sift through and study.
Calcium carbonate shares the typical properties of other carbonates. In prepping fossil specimens embedded in limestone, it is useful to know that limestone, itself a carbonate sedimentary rock, reacts with stronger acids. If you paint the specimen with hydrochloric acid, you'll hear a little fizzling sound as the limestone melts and carbon dioxide is released: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l). I tend to use a 3-5 molar solution, then rinse with plain water.
Calcium carbonate reacts with water saturated with carbon dioxide to form the soluble calcium bicarbonate. Bone already contains calcium carbonate, as well as calcium phosphate, Ca2, but it is also made of protein, cells and living tissue. Decaying bone acts as a sort of natural sponge that wicks in the calcium carbonate displaced from the shells. As protein decays inside the bone, it is replaced by the incoming calcium carbonate, making the bone harder and more durable.
The shells, beautiful in their own right, make the surrounding soil more alkaline, helping to preserve the bone and turn dinner scraps into exquisite scientific specimens for future generations.