To do so, the researchers relied on existing data. “We had neither the time nor the funds to collect new data in the field, but there is a wealth of data already available that we were able to use,” said Flamand.
The existing data were incomplete, as they mostly came from studies that weren’t focused on carbon accumulation, but they often contained at least two of the three variables—sediment density, percentage of carbon content and dating—needed to calculate carbon accumulation. The team therefore developed a predictive model that links sediment density to organic matter content and is specifically adapted to the wide variety of boreal and temperate lake sediments.
“The existing models were not suited to boreal lakes, which generally contain very high levels of organic matter,” Flamand said.
The disparate data sources presented numerous challenges. For one thing, the dating methods varied—radiocarbon for the oldest samples and lead-210 for the more recent ones.
“Two dating methods are rarely used on the same core,” Lapierre explained. “It makes it difficult to compare accumulation rates across centuries and millennia.”
Variations growing wider
While there have been significant climatic disruptions at various points during the Holocene, the most pronounced changes in accumulation rates occurred in recent centuries. “It seems that the arrival of the first settlers already disrupted accumulation rates, but it was starting in 1850, with the beginnings of industrialization, that the variations intensified,” Lapierre said.
In some lakes, sporadic decreases in carbon accumulation rates were found: “We expected an increase in accumulation rates, and not necessarily an increase in variability,” Lapierre said. “This means the lakes are becoming more sensitive to disturbances,” Flamand added.
Nature’s balance disrupted
Ecosystems naturally sequester carbon, but the rate at which they do so depends on various factors: climate, nutrient inputs and the amount of carbon supplied by the watershed.
“The key takeaway from our study is that since industrialization, this function of aquatic ecosystems has been pushed far outside its natural range, for a variety of reasons linked to human activity,” Lapierre explained.
The researchers now want to identify the factors driving this trend so they can make projections and scenarios for the future, based on variables such as projected climate change or changes in land use. After analyzing the sediment data, they plan to focus on measurements of the water column over time.
This study therefore sheds new light on the impacts of human activity on the natural carbon cycle. “It’s important to quantify our carbon footprint, but also to compare it with the range of historically natural variations, which represent the true baseline,” Lapierre concluded.
