Lake sediment cores – exploring the past

Have you ever stood in a lake, wetland or estuary and wiggled your toes into the soft mud? No doubt you were thinking about the texture of the mud or maybe even the smell, but did you know that you were observing the past?

Sediments are the fragments of inorganic materials like sand, soil or residue from volcanic eruptions and organic materials like plant remains that are carried and deposited by wind, water or ice. Over the years, layer after layer of sediment builds up, trapping and preserving pollen, leaves, wood and inorganic materials. This process can happen over hundreds to thousands of years. Like tree rings or ice cores, these hidden records provide clues about past environments.

Sediments can be likened to the pages of a history book, continuously layering on top of one another recording environmental history, 24 hours a day, 7 days a week, year after year, for thousands of years. Everything that happens in or around the lake leaves a trace in the sediment.

Dr Marcus Vandergoes, co-leader, Lakes380 – Our lakes’ health: past, present, future

Lake sediments – a treasure trove of information

There are many ways that sediments end up in a lake. All of the materials that enter the lake are called inputs. This interactive looks at the four key types of inputs. The means of transport don’t really change over time but the materials they carry do. For example, the catchment in the interactive features land uses that are relatively modern. Go back 100–200 years and chances are the pollen in the atmosphere would have come from native plants rather than pasture grasses and plantation pine trees. Organic materials washed into the lake by rivers also reflect what was growing at the time.

Sediment cores create a timeline

The inputs from the atmosphere, land and the lake itself end up in the lake and eventually sink onto the lakebed. As the lakebed sediments build year after year, the pollen, plant remains, minerals and volcanic ash form a type of timeline. The newest deposits are at the top of the lakebed with the oldest at the bottom.

Scientists are able to ‘read’ this timeline by collecting sediment cores. A sediment core is a tube of mud carefully extracted from the lake so that the layers are intact. To collect a sediment core, scientists use ropes to lower a piece of equipment known as a sediment corer into the water. This contains a long plastic tube that is hammered into the lakebed. The top of the tube has a vacuum seal to keep the mud inside the tube as it is lifted from the lake floor. Imagine pushing a straw into a multilayer thickshake and then holding your finger over the top of the straw as it is lifted from the container. This analogy is a simplified but similar process.

Examining sediment cores

Extracting core samples from deep lakes has a number of challenges, and these continue once the sediment cores are on the boat. The cores need to be carefully stored in cool, dark places until they can be taken to a lab for analysis. This can be tricky when sampling lakes in remote locations around Aotearoa New Zealand!

Once in the lab, the tube holding the sediment core is split down the middle using a metal guillotine that slices through the sediment. Small amounts of sediment, called subsamples, are collected from the length of one half of the core. Scientists examine the subsamples for many things, including pollen, microscopic algae and environmental DNA. Pieces of organic matter are removed from the sediment for radiocarbon analysis. The other half of the sediment sample is placed in special high-resolution scanners. The scanners identify changes in the lake’s biological communities, geology and chemistry over time.

Paleolimnology – reconstructing the history of freshwater systems

‘Paleo’ means old or ancient, and limnology is the study of freshwater bodies. Paleolimnologists use physical and biological clues to explore the history of a lake system and how it has changed over the centuries. This information helps scientists understand the natural conditions of the lake and its catchment as they once were – prior to human impacts – and how it has evolved since.

The scientists working with Lakes380 – Our lakes’ health: past, present, future provide information they’ve gained from sediment and water sampling to help New Zealanders identify and protect habitats, manage freshwater resources and forecast future changes.