The pitfalls of transfer functions (and how to avoid them!)

By Erin Hayes-Pontius

Paleoecology, or the study of organisms and their environments in past times, has a specific branch known as paleolimnology, which deals with records left by lakes. Beyond being pleasant places for swimming, fishing, and other recreation, lakes are particularly valuable for paleo records. Not only do lakes archive the goings-on within themselves, but they also archive goings-on within their drainage basins. In the case of Lake Champlain, it has a drainage basin nearly 17 times that of its surface area (21,326 km2 : 1,269 km2), making it extra sensitive to changes in climate (Fig. 1).

Due to lakes’ positions at low points on the landscape, they integrate everything that happens within their drainage basin and are therefore important “sentinels of change” (Williamson et al. 2009).

Figure 1. Lake Champlain’s drainage basin. From Wikimedia Commons

Figure 1. Lake Champlain’s drainage basin. From Wikimedia Commons

Over time, a variety of things fall into lakes, such as diatoms (single-celled algae), pollen, chironomid midges, or needles, to name a few, and these things get buried by sediment. In some lakes- some, certainly not most- sediment deposits occur quickly enough to leave annual layers; if this interests you, definitely check out Rob’s post from a little while back. Unfortunately, this does not happen most of the time, and we’re left with a much coarser resolution to deal with. Rather than knowing about the diatom species every year, we may only know ‘who’ was there every few years, every decade, or even every few decades. Despite these challenges, lakes are still- and will continue to be- widely used in paleoecology. Continue reading

Splinters in Dendrochronological Analyses

By Audrey Cross

A tree corer used to drill into a trunk, and two tree cores. Rings are counted from these cores and used to date events or reconstruct climate change.  Wikimedia Commons.

A tree corer used to drill into a trunk, and two tree cores. Rings are counted from these cores and used to date events or reconstruct climate change. Wikimedia Commons.

In dendrochronology, most analyses rely on a number of underlying assumptions. For wood that was used by humans, a tree might have been used immediately after it died, and it could be found at a site nearby where the tree had lived. However, humans could obtain wood in different ways; it could be found, as with driftwood; recycled, as with beams in houses; or transported, as with boats.

The study of driftwood proposes an interesting twist to dendrochronological analysis because the amount of time that wood can spend in transport can vary greatly. Continue reading

Fecal proxies

By Dulcinea Groff

Dung fungus spores of Sporormiella australis. From Funghi Paradise.

Dung fungus spores of Sporormiella australis. From Funghi Paradise.

Feces of prehistoric organisms remaining in the sediment records harbor information that can lead to a picturesque reconstruction of an ecosystem from long ago.  It is quite remarkable how many examples of fecal proxies exist and provide more information than just an indication of the presence or absence of an animal.  In the early 1800’s, an eccentric paleontologist named William Buckland was the first to describe coprolites or fossilized feces.  When feces become fossilized the organic components are replaced with minerals and any clue as to what the organism ate is replaced.  Therefore, coprolites may not be very useful in understanding the ecology of past environments and organisms.  Instead, other things associated with feces become proxies in paleoecological studies. Continue reading

Attaching dates to lake sediment cores: precise dating using varves

By: Rob Brown

There are many proxies paleoecologists use to determine past environments and communities (insects, pollen, diatoms, packrat middens, tree rings, etc.), many of which have been discussed on this blog previously. These proxies can be used to answer questions ranging from seasonal to millennial time scales. With the exception of tree rings, which were previously discussed on this post by Erin our reconstructions are often limited by errors in dating methods. However in some lakes, sediments are deposited in visible annual layers called varves. Varved sediments offer a unique situation where the temporal resolution necessary to determine annual to decadal changes relevant to a human lifetime can be achieved.

Figure 1. Varve sediment from Newbury, Vermont, USA. Note the alternating light and dark bands and different thicknesses. From Tufts University North American Glacial Varve Project

Figure 1. Varve sediment from Newbury, Vermont, USA. Note the alternating light and dark bands and different thicknesses. From Tufts University North American Glacial Varve Project

What are varves and where are they found?

Simply put, a varve is an annual layer of sediment that forms in distinct layers (Figure 1). A single year’s deposit includes a light (summer) layer and a dark (winter) layer.

Varves don’t form in all lakes, in fact they are found in very few. The main factor controlling varve formation is climate variability; there must be large seasonal differences in both temperature and precipitation. This sets up the succession of biotic life and the physical and chemical structure of the lake necessary to form the contrasting layers. Additionally, there needs to be no disturbance of the sediment once it is deposited. Processes such as underwater currents, sediment slumping (think underwater mudslides), degassing (air bubbles within the sediment), or bioturbation (organisms physically mixing the sediment) all mix the sediment layers and the annual deposits are lost (O’Sullivan, 1983). Continue reading

Small but Mighty: Insects as Proxies

By Wayne Heideman

Insects have been on the Earth for a long time and their presence can affect their surrounding environment. It is important to look at insects in the past as they can provide us with insight on how they can act in the present and in the future. In a paleoecological sense, insects can be studied in a number of ways. One way is to look at plant-insect interactions through plant fossils (herbivory) and peatlands (habitat). Another technique is through amber and observing the insect in a snapshot of time. Lastly, sediment cores in lakes can capture insect presence, notably Chironomidae (non-biting midges) larvae and Coleoptera (beetles). All three are viable means of observing past insect use but they all have their strengths and weaknesses which should be assessed before using a specific method.

A picture of three different types of insect damage on plants. A) Shows a frass trail as well as an oviposition site marked by the arrow. B) Shows a high degree of herbivory, only leaving fine veins and C) shows areas of leaf case shelter sites. From Wilf 2008.

A picture of three different types of insect damage on plants. A) Shows a frass trail as well as an oviposition site marked by the arrow. B) Shows a high degree of herbivory, only leaving fine veins and C) shows areas of leaf case shelter sites. From Wilf 2008.

Continue reading

Through the looking glass: how diatoms can reconstruct wind

By Kelsey

Few object are more beautiful than the minute siliceous cases of the diatomaceae: were these created that they might be examined and admired under the high powers of the microscope? ~ Charles Darwin

As Darwin remarks, diatoms are beautiful.  They have unique, intricate cell walls that help in their identification, because in addition to their beauty, they can tell a lot about past environmental conditions.  My research uses diatoms as a proxy (a preserved item that acts as a ‘natural archive,’ capable of telling us something about climate in the past) to explore past environmental conditions in lakes.  Diatoms are a type of single celled organisms called algae.  These organisms are found in many wet environments including soils, but I focus on diatoms in lakes.

Figure 1: Images of various species of diatoms. Image from Wikimedia Commons.

Figure 1: Images of various species of diatoms. Image from Wikimedia Commons.

Diatoms are unique from other types of algae because they have siliceous or glass-like cell walls, and therefore are well preserved in lake sediments.  This makes them good proxies of past climates.  Additionally, diatom species, like other algae have a variety of environmental preferences.  These preferences can range from mixed or stable water conditions, to high nutrient or light levels and provide the basis for climate inferences. Continue reading

What proxies can tell us about drought

By Kelsey

We are all familiar with the images.  The barren landscape.  Brown, dead vegetation spanning as far as the eye can see.  Lakes and rivers shrinking in size, their relict banks cracked and dry.

These dryspells or droughts can last from days to years and they occur when a region receives below average precipitation.  Droughts are generally said to arise from insufficient precipitation amounts over an extended period of time.  This shortage in precipitation occurs typically in one season or more, and results in a water shortage for an environmental region.  The impact of the drought arises from the interaction between the natural events (less precipitation than expected) and the demand on the water supply (both from the environment and humans), which can exacerbate the impacts of the drought. Because of these factors that can influence the magnitude of the drought, they need to be taken into consideration when working to define a drought (National Drought and Mitigation Center). Frequently the definitions can vary from location to location as certain areas are less adapted to low water conditions than others.  But it can also dependent on timing.  For example, in the desert a month without rain doesn’t have the same effect as it would in the rainforest.  However, if the month without rain in the desert came during the period designated as the rainy season, the impacts would be drastically different.

Depending on how long these dryspells last there can be a huge impact on the ecosystem and economies associated with them.  One prime example is the Dust Bowl.  Continue reading

Putting waste to work: Bat guano & packrat middens as sources for ecological proxies

By Griffin Dill

Paleoecology is a discipline deeply rooted in the use of proxies. In order to develop an understanding of past ecosystems and climatic events, paleoecologists utilize a number of biological proxies, including pollen data, plant macrofossils, diatoms, charcoal particles, and isotope geochemistry, to name a few. These proxies provide researchers with quantitative data that can be used to examine a myriad of environmental variables and reconstruct ancient ecological communities. Proxies can be obtained from a variety of sources, but are commonly acquired from lake sediments and peat bog profiles and to a lesser extent marine sediments. As research into the paleoecological record intensifies, additional proxies and previously untapped proxy sources are sought. An often underappreciated source of ecological data is now providing additional information to paleoecologists: animal waste. Continue reading

Wildfire through time: shaped by climate and people

By Dulcinea Groff

Picture the landscape of a tropical savanna, composed of grasses, shrubs and a sparse number of trees.  The savanna biome is dominated by a wet season supplying thousands of herbivores with forage and a dry season accompanied by intense lightning and fire.  These wildfires maintain the savanna as grassland by killing the saplings (suppressing tree growth) and the grasses quickly regenerate.  Imagine early hominids in the fire-constructed savanna, they begin to use fire, control fire and even make it!  Fire is a permanent link now between biome and human.

Fire was used by humans for presumably many reasons:  communication, prepare food, drive and corral prey, warmth during cold periods, etc.  Humans have long suppressed and ignited fires for various reasons.  In fact, as a source of ignition, humans have been shaping landscapes since the earliest known hominids were thought to use fire one million years ago in South Africa (Berna et al. 2012). Continue reading