Taphonomy and organic matter in bone

The bone bed found on a beach at Bahia Thetis

All that history in my last post leads to the reason for this travel in the first place: the collection of historic samples. Up until this current trip, I've looked at samples of fur seals and sea lions form archaeological sites. The majority of these are from the earliest known site with animal remains, Tunel, named after the Estancia Tunel not far from Ushuaia. I also have some samples that were collected in other sites along the Beagle Channel, Peninsula Mitre, and Isla de los Estados that extend my data set both geographically and temporally to the end of the marine hunting and gathering lifestyle employed by the indigenous people of Tierra del Fuego. With this new collection, I'll be able to see if the massive changes that happened culturally with the arrival of industrial marine exploitation also had an impact ecologically. Significant changes have been observed in ecosystems around the world, including in California with the near elimination of sea otters. The extent of ecological impacts of industrial exploitation in comparison to subsistence exploitation of marine animals in Tierra del Fuego remains a mystery...until I run these samples in a few weeks!

Sea lion and whale bone in an archaeological site

But the understanding of the process of what happens to a body after death (the field of taphonomy) is a pretty important for interpreting stable isotopic results from my bone samples. Much of this field was pioneered by Behrensmeyer in multidecadal studies in Kenya with observations of the breakdown of bodies, separation into different parts, and ultimately into individual bones that break apart themselves. There are many different things that influence this process, including biological: scavengers, which can rip apart the body, limbs and chew on bones; microbes, which can feast on soft tissues, fat and oils of the body and the bones, and produce byproducts such as acids that chew away at bone mineral. Physical processes can include transport, for example, a bone traveling down a stream, bumping and grinding against rocks; damage from sunlight, which bleaches the surface and wears down the mineral, leading to cracking and flaking in the bone surface; and chemical weathering, the dissolution of bone mineral from acids.

One of the reasons I've been able to analyze samples from here is because the climate and conditions here in Tierra del Fuego generally buffer or slow down these taphonomic processes. Bones from the even the earliest sites have nearly the amount of organic matter in them that you would find in modern samples. This is because they've been buried almost immediately after being processed by humans. As a sub-antarctic climate, its generally very cold here, like throwing your steak into a refrigerator, it'll last longer than sitting out in the open on the Kenyan savannah, In addition, they've been buried in piles of shells. These shells are largely calcium carbonate, which counteracts acids that could be introduced by rain, trees, and soil bacteria.

Bones from partially articulated sea lions 

In contrast, some of the bones I have were buried in sand, allowing more of these chemical processes to reduce the organic matter in my bones, noticeably reducing the amount of organic matter. The samples we recovered from the seal greasery at Bahia Thetis are in an even worse state. After the carcasses were thrown into the bay, biological agents surely ate away at the soft tissue, leaving behind bones. Tidal action moved the bones/carcasses along the floor of the bay, before they were washed up onto the beaches during extreme storm events. Despite these destructive forces, a large number of identifiable bones can be found up on the beaches. However, bones on the surface tend not to last very long at all, and almost all the bones we found were in an advanced state of deterioration despite being only 70 years old. It may end up being difficult to extract the organic matter from these, but we have a large volume to extract from, so even if a small fraction is there, we'll get it.

We did find a fully articulated skeleton buried in the beach, with just a piece of its skull sticking out on the surface. The buried bones were in a much better state that the buried elements. I imagine this carcass was buried before it could be disarticulated by external forces. It stands to reason that there are probably many more articulated skeletons and bones either buried in the beaches or still in the bay where they were originally deposited. Likewise, there are surely many seals and sea lions buried in shell middens which have yet to be excavated, of which there are thousands known in Tierra del Fuego.