📄 earth_science

Sediment organic carbon sources, accumulation and stock estimates in a temperate lake over the past 10,000 years in Ireland

This study reconstructs the past 10,000 years of organic carbon accumulation in Ireland's Lough Feeagh using sediment cores and hydroacoustic data, revealing that terrestrial sources primarily drive sediment carbon stocks, which are estimated at a median of 205 kT OC, thereby highlighting the significant role of temperate lakes in global carbon storage.

Original authors: Ryan Smazal, Catherine Dalton, Karen Molloy, Aaron Potito, Suzanne Linnane, Elvira de Eyto, Eleanor Jennings

Published 2026-07-08

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Ryan Smazal, Catherine Dalton, Karen Molloy, Aaron Potito, Suzanne Linnane, Elvira de Eyto, Eleanor Jennings

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a lake not just as a body of water, but as a giant, slow-motion library. Over thousands of years, this library has been quietly collecting books made of dead leaves, decaying plants, and soil from the land around it. These "books" settle at the bottom, piling up layer by layer to form sediment. This paper is about reading the oldest, deepest books in the library of Lough Feeagh, a lake in western Ireland, to figure out how much "carbon" (a key ingredient in the global climate puzzle) is stored there and where it came from.

Here is a simple breakdown of what the researchers found:

1. The Library's History (The Last 10,000 Years)

The team drilled a long straw-like tube (a core) 8 meters deep into the bottom of the lake. This tube contained a timeline stretching back 10,000 years.

The Basement: The very bottom of the tube was made of hard, grey clay left behind by ancient glaciers. It was like the foundation of a building—very little "organic" material (plant life) was there.
The Main Library: Above that clay, the sediment turned dark and rich, like a thick chocolate cake. This is where the organic carbon started piling up.

2. Where Did the "Books" Come From?

The researchers wanted to know: Is this carbon coming from fish and algae living inside the lake, or is it dirt and leaves washing in from the land?

The Detective Work: They used chemical "fingerprints" (isotopes) and looked at the ratio of carbon to nitrogen in the mud.
The Verdict: The evidence pointed strongly to the land. The carbon is mostly from the surrounding peatlands and forests. Think of it like a river of soil and decaying vegetation constantly trickling into the lake, rather than the lake creating its own food. This has been the main source for the last 10,000 years.

3. The Speed of Piling Up

The researchers measured how fast this "library" was being built.

The Slow Start: For a long time, the pile grew slowly.
The Acceleration: About 3,700 years ago, the pile started growing faster. In the last 2,000 years, it sped up even more.
Why? It wasn't just because there was more plant life; it was because more stuff was washing into the lake. The study suggests this was due to a mix of climate changes and, more recently, humans clearing land and farming, which caused more soil to erode and wash into the water.

4. Counting the Total Treasure (The Stock Estimate)

This is the trickiest part. You can't just measure one spot and guess the whole lake.

The Problem: Sediment doesn't pile up evenly. It's like a snowdrift; the wind blows the snow deeper into the hollows. In a lake, currents push the sediment toward the deepest parts. If you only measure the deepest spot, you might think the whole lake is full of deep mud, when the shallow edges are actually bare rock.
The Solution: The team used a high-tech "sonar" (like a submarine's echo sounder) to map the thickness of the mud across the entire lake floor. They combined this map with the chemical data from their core.
The Result: They calculated that the lake holds a massive amount of carbon.
- The Estimate: The lake stores about 205,000 tonnes of organic carbon.
- The Range: They are pretty sure the number is between 115,000 and 246,000 tonnes.

5. Why Does This Matter?

The paper concludes that lakes like Lough Feeagh are not just pretty scenery; they are massive carbon vaults.

Because Ireland has so many lakes sitting right next to peatlands (which are carbon-rich), these lakes act as a long-term storage unit for carbon that would otherwise be in the air.
This study is the first time anyone has tried to count the total "carbon treasure" in an Irish lake over such a long time (10,000 years).

In a nutshell: The researchers found that Lough Feeagh has been acting as a giant carbon sponge for 10,000 years, soaking up mostly land-based material. Thanks to a mix of sonar mapping and mud sampling, they now know exactly how much carbon is sitting at the bottom of that lake, proving that these ecosystems are a crucial, though often overlooked, part of the Earth's carbon storage system.

Technical Summary: Sediment Organic Carbon Sources, Accumulation, and Stock Estimates in a Temperate Lake Over the Past 10,000 Years in Ireland

Problem Statement
While lake sediments are recognized as significant long-term sinks for organic carbon (OC), robust, data-driven estimates of these stores at the millennial scale remain limited, particularly for temperate lakes. Existing literature often focuses on boreal systems or short-term accumulation (decadal scales), leaving gaps in understanding the drivers of OC sequestration, source attribution, and total stock quantification in temperate, peat-dominated catchments. Furthermore, estimating total lake OC stocks is complicated by sediment focusing (the preferential accumulation of sediment in deeper basin areas) and the difficulty of accurately measuring total sediment volume without extensive hydroacoustic surveys. This study addresses these gaps by investigating the quantity, sources, and accumulation rates of sediment OC in Lough Feeagh, a temperate lake in western Ireland, over the last 10,000 years.

Methodology
The study employed a multi-proxy approach combining sediment coring, radiocarbon dating, geochemical analysis, and hydroacoustic surveying:

Sediment Coring and Chronology: Two cores were extracted from Lough Feeagh. A long piston core (LF17, 813 cm) provided a 10,000-year record, while a short surface gravity core (LF21, 43 cm) captured recent sedimentation. Chronology for LF17 was established using 12 Accelerated Mass Spectrometry (AMS) radiocarbon dates on macrofossils, modeled using the Bayesian rbacon R package. LF21 was dated using $^{210}$ Pb, $^{137}$ Cs, and $^{241}$ Am to establish a recent chronology and calculate a sediment focusing correction factor.
Geochemical Analysis: Sediment samples were analyzed for Dry Bulk Density (DBD), Loss-on-Ignition (LOI), and organic carbon (OC) content. Stable isotopes ( $\delta^{13}$ C and $\delta^{15}$ N) and the C:N ratio were measured to distinguish between autochthonous (in-lake) and allochthonous (terrestrial) OC sources.
Accumulation Rates: Mass Accumulation Rates (MAR) and Organic Carbon Accumulation Rates (OCAR) were calculated. OCAR values were corrected for sediment focusing using the ratio of sediment $^{210}$ Pb flux to regional atmospheric flux, derived from LF21 data.
Hydroacoustic Survey and Volume Estimation: A sub-bottom profiler (SES-2000) was used to map sediment thickness across the lake. Due to data gaps in shallow or gas-rich areas, a statistical relationship between water depth and sediment thickness was established using the available acoustic data points. This relationship was applied to bathymetric data to estimate the total sediment volume for the entire basin.
Stock Calculation: Total OC stock was estimated by multiplying the calculated sediment volume by median OC density values derived from LF17, LF21, and four historical cores. Uncertainty was quantified using 5th and 95th percentile ranges.

Key Results

Sediment Stratigraphy and Chronology: The core transitioned from glacial clay (bottom 17%) to organic-rich sediments at approximately 10,000 cal. YBP. The organic section showed homogenous, non-laminated sediments, suggesting prolonged anoxia was unlikely.
Accumulation Rates: A significant increase in both MAR and OCAR occurred around 3,700 cal. YBP, followed by a steady increase over the past 2,000 years. The Holocene average corrected OCAR was 7.19 g C m $^{-2}$ yr $^{-1}$ (SD $\pm$ 2.92), with a maximum of 14.69 g C m $^{-2}$ yr $^{-1}$ . Statistical analysis indicated that OCAR was most strongly driven by MAR ( $r^2$ = 0.90) rather than sediment OC content.
OC Sources: Geochemical indicators (high C:N ratios averaging 23.8, $\delta^{13}$ C values averaging -27.9‰, and $\delta^{15}$ N values) consistently pointed to a dominant terrestrial source of OC, likely derived from the surrounding peatland soils. This source signature remained relatively constant over the 10,000-year period.
Total OC Stock: The hydroacoustic survey estimated a total sediment volume of 9.9 $\times$ 10 $^6$ m $^3$ . Applying median OC density values, the study estimated a total median OC stock of 205 kT OC (5th–95th percentile range: 115–246 kT OC). The areal-weighted OC stock was estimated at 52 kg OC m $^{-2}$ .

Significance and Claims
The authors present the first millennial-scale estimate of carbon sediment stores, accumulation rates, and sources for a lake in Ireland. The study highlights that temperate lakes in peat-dominated catchments, such as Lough Feeagh, represent a substantial and currently under-quantified component of the national carbon stock.

The paper claims that its findings provide critical data for refining local and national carbon budgets. By demonstrating that OC accumulation is driven largely by sediment influx (MAR) rather than changes in OC concentration, and that the source is predominantly terrestrial, the study offers a mechanistic understanding of carbon burial in these systems. The authors assert that lake sediments should be explicitly considered in future carbon storage assessments and management strategies, particularly given the overlap between Irish lakes and extensive peatland distributions. The study also validates a methodology combining hydroacoustic volume estimation with core-derived density data as a viable approach for quantifying lake carbon stocks where direct volume measurement is challenging.

Limitations Acknowledged
The authors note that the sediment focusing correction factor was applied uniformly across the Holocene, assuming temporal invariance, despite evidence of changing deposition patterns. Additionally, the estimation of sediment volume in areas without acoustic data relied on a depth-thickness relationship, and OC density was assumed to be consistent across the basin based on available core data. The presence of gas pockets in the northern basin, which prevented acoustic data collection, introduces uncertainty, though the authors argue the established relationship is the best available proxy for this specific lake.

1. The Library's History (The Last 10,000 Years)

2. Where Did the "Books" Come From?

3. The Speed of Piling Up

4. Counting the Total Treasure (The Stock Estimate)

5. Why Does This Matter?

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