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.

Return from Bahia Thetis

Anchor from a shipwreck in Peninsula Mitre

After twelve days and much work, I've had a few days to rest and reflect on our mission to collect and sample sea lion, fur seals, kelp, water, birds, fish and shellfish from Bahia Thetis, one of the most remote parts of the island of Tierra del Fuego. And it was a absolute success: In total, we collected 101 left sea lion and fur seal jaws (the majority being adult female sea lions), around ~50 femurs, and 10 skin and fur samples from the 1940s factory site. In addition, we made three 50x50cm excavations of shell middens, containing numerous more bones, stone and glass artifacts and tools including the first harpoon point ever found in the peninsula.

I learned a lot about this part of the world, and Peninsula Mitre in particular, which I think is important for understanding the context of the work I'm doing here. Peninsula Mitre is different in a number of ways from the land surrounding the Beagle Channel. The Beagle Channel is characterized by steep mountains and sharp glacial features, drumlins, canyons and overall rocky with forest. The peninsula, and much of the Atlantic coast of Tierra del Fuego, has softer rolling hills, less forest, dominated by peat bogs, and shallow coastline with  a slowly descending continental slope that extends for hundreds of kilometers. This shallow coastline proved treacherous for navigating, resulting in many shipwrecks, many of which were scavenged to build ranch buildings.

A more recent shipwreck from the 1980s

Geographic differences also played out ethnographically between indigenous groups. The Yamana occupied the Beagle Channel and southern archipelago, the Selk'nam in the interior and northern parts of Tierra del Fuego, the Alakuf on the western archipelago in Chile and the Haush in Peninsula Mitre. It is important to note that boundaries were not absolute for these groups, language and culture mixed and created gradients. The Haush and Selk'nam commonly incorporated more terrestrial animals into their diets, especially Guanaco, but they also hunted sea lions and seals like the more marine oriented Yamana.

Bahia Policarpo, site of the Policarpo family sheep ranching operation

When European settlers arrived, such as the Anglican missionary Thomas Bridges, they first set up in the Beagle Channel. However, it didn't take long for others to settle in other parts of Tierra del Fuego. Estancia Policarpo was established in the late 1800s by a Chilean family southeastern Atlantic coast of Tierra del Fuego, extending from the easternmost part of the Island at Cabo San Diego to its northern border with Estancia Maria Luisa some 50 km away. Like the ranch at Harberton, Estancia Policarpo exploited sheep for their wool. Estancia Policarpo operated until the mid 20th century, succumbing to falling wool prices from technological innovations in synthetic fabrics and wool that was cheaper to produce elsewhere. A caretaker maintained the what was left of the ranch until the 1980s, but most of the ranch buildings fell into disrepair throughout the late 20th century. Today the estancia is operated as a cattle ranch.

Blubber boiling vats

One way to subsidize the cost of the operation of the failing ranch was the exploitation of pinnipeds. In the 1940s, over one hundred Chileans, experienced in seal in sea lion hunting more common on the western coast of South America, set up a seal oil production facility in Bahia Thetis. The company would raid the multiple sea lion colonies that existed along the shores of Estancia Policarpo, killing over 30,000 seals and sea lions over a decade. The pinnipeds would be killed at the site of their colonies, loaded onto boats and taken to Bahia Thetis, were they were skinned, stripped of blubber and thrown into the bay. The blubber was then boiled in large vats, producing oil, much in the same way as whale blubber. In 1952, sealing was banned and the operation abandoned, leaving behind ruins of both the seal greasery and the ranch.

Bahia Thetis, site of the 1940s seal greasery facilities

That's I've got for today, stay tuned for more!

Estancia Harberton

Estancia Harberton, home of missionary Thomas Bridges and family in 1886. Imported brick by brick from England to Tierra del Fuego. Located east of Ushuaia by 50 km or so.

Yesterday I joined the archaeologist crew to collect some of my own samples and help carry bags of rocks. They were very heavy and my arms are quite tired still. But it was quite a productive day trip! We managed to carry 145 rocks over several kilometers, which will eventually help my fellow graduate student Carmen determine the source of lithic tools that were used by the Yamana. For my own sampling, I took some samples of kelp and some water samples, one of each from a bay exposed to a river mouth and one from the bay facing the Estancia Harberton (pictured above) which does not have a direct freshwater input. I doubt there will be significant differences between the two areas but good to measure extra just in case. The waters of the Beagle Channel are known to be exceptionally fresh compared to ocean waters from other parts of the world, due to the high amount of precipitation and freshwater inputs (from snowmelt and glaciers), but I will find this out and verify this claim personally!

Loading a large bag of rocks to be carried back to the truck.

The purpose of sampling water here is two fold. First I'm interested in differences in the waters of the Beagle Channel and the southern Atlantic/Argentine Sea, specifically to see what differences in water chemistry there might be, especially when it comes to nutrients and trace elements. Different nutrient loads can have an impact on the productivity of food webs, which would in turn have effects on species high in the food chain, like top predators such as fur seals, sea lions, and cetaceans (whales and dolphins). Trace elemental analysis can help me not only differentiate bodies of water, but serve as an independent method that would potentially verify where my fur seals and sea lions spend their time. Trace elements are incorporated into your tissues just like stable isotopes are and can (hopefully) be used a tracer.

A figure showing the interaction of different bodies of water in South America (from  Acha, E. M., et al. Marine fronts at the continental shelves of austral South America: Physical and ecological processes. J. Mar. Syst. 44, 83–105 (2004).) 

The measurement of kelp will serve a similar purpose. Giant kelp, Macrocystis pyrifera, grows around the world, from the coasts of Alaska, New Zealand, Japan, all the way down to Tierra del Fuego, It's a very successful species, one of the fasting growing on the planet (a meter a day in the right conditions) and forms the basis of many highly productive food webs around the world. Constraining the variation between southern Atlantic kelp and Beagle Channel kelp will give me an understanding of what to expect when I compare the stable isotopic values between giant kelp, kelp based fish and ultimately fur seals and sea lions. Fur seals and sea lions might be moving back and forth from open ocean to kelp ecosystems or changing their degree of residency in kelp ecosystems over time so it's important to understand what the kelp ecosystem looks like versus the open ocean.

Soon I leave for the Atlantic Coast, Peninsula Mitre and Bahia Thetis, site of the abandoned sealing operation that ran from 1946 to 1952. Bahia Thetis was formerly known as the Bay of Good Success, I hope it lives up to its name!

Wild horses seen while sampling near Estancia Harberton