People and projects

How can we maximize ¹⁴C chronological information from lake sediments?

The ACE lab is developing approaches to optimizing the dual graphite and gas-source capabilities of the MICADAS to cost-effectively maximize the geochronological information from lake sediment macrofossils.

Figure from Zander et al. (2020)

Comparison between graphite and gas-source ¹⁴C ages of vegetation macrofossils from lake sediment (Lake Żabińskie, Poland) also dated by annual-layer (varve) counts. Ultrasmall samples were used for gas-source ages. Almost all graphite and gas ages overlap within analytical errors. Where the two differ, graphite-based ages tend to be slightly older, perhaps reflecting a higher proportion of woody material in these larger samples; wood tends to be stored on the landscape before washing into the lake. The data show that using the gas source to increase the sample density across a Holocene sedimentary sequence resulted in a superior age model, despite the lower precision of the gas-source ages. Data was generated on MICADAS at LARA, University of Bern.

Darrell Kaufman is interested in the geochronological applications of radiocarbon. He applies ¹⁴C to dating geologic archives of paleoclimate information and to quantifying the time-averaging of carbonate subfossils.

Is disturbance by wildfires causing a release of old C that indicates threshold changes in the ecosystem state?

One of the most rapid pathways through which climate warming could alter the C balance of high northern latitude ecosystems is through intensification of wildfire disturbance. The majority of C sequestered in arctic tundra and boreal forests resides in thick soil organic layers (SOL) that can be hundreds to thousands of years old, a C legacy of past fire cycles. Combustion of the SOL dominates C emissions during fire, and more intense fires result in deeper burning. Because rates of soil C accumulation vary across the landscape, deeper burning may not always combust old C. But deeper burning that does combust old C could rapidly shift ecosystems across a C cycling threshold, from net accumulation of C from the atmosphere over multiple fire cycles to net loss. Thus, the vulnerability of this C pool to combustion during more intense wildfires could ultimately determine the C balance of boreal ecosystems and their net feedback to climate warming.

Identifying when and where deep burning releases old C may also be key to predicting threshold changes in ecosystem structure and function. Combustion of old C indicates that the current fire event is outside of historic bounds of disturbance intensity. This makes it likely that these ecosystems will be vulnerable to additional threshold changes after fire that will have marked impacts on C cycling, such as permafrost degradation and loss. The potential for fire to destabilize permafrost depends upon its impact on the SOL. In ecosystems of the discontinuous permafrost zone, where insulation from the SOL protects permafrost from warm summer air temperature, deep burning can result in rapid destabilization and complete loss of permafrost. In colder regions, deep burning results in increased summer heat flux to permafrost, leading to deepening of the active layer that may take decades to centuries to re-freeze. Thaw of permafrost exposes previously stabilized old soil organic C to decomposition and release to the atmosphere as the greenhouse gases CO₂ and CH₄ and can also lead to subsidence and changes in drainage that impact both soil and plant C cycling processes.

The relationship between atmospheric ¹⁴C, the ¹⁴C of soil organic matter, and depth in the SOL of boreal and tundra ecosystems makes it possible to identify how much old C is vulnerable to loss during fire and relate depth of burning to old C loss. In this way, we can differentiate fires that only consume recently accumulated surface organic matter from fires that burn deeply into organic soils accumulated over hundreds to thousands of years. By relating age, depth, and carbon pool estimates, the mass of old C loss can be estimated to determine long-term net ecosystem carbon balance.

Darrell Kaufman is interested in the geochronological applications of radiocarbon. He applies ¹⁴C to dating geologic archives of paleoclimate information and to quantifying the time-averaging of carbonate subfossils.

How much old C is being currently released as CO and CH₄?

Permafrost thaw and increased microbial decomposition release stored organic C from the terrestrial biosphere into the atmosphere as greenhouse gases. At the same time, plant growth can remove atmospheric C, which becomes stored as new plant biomass or deposited as new soil organic matter. Climate change can stimulate both processes, and whether arctic ecosystems are currently a net C source (losses > gains) or sink (gains > losses) is still an area of intense research. This is a result of the inescapable fact that ecosystem C balance is the relatively small difference between two large, opposing fluxes: C uptake via plant photosynthesis and growth, and respiratory loss via metabolism by all living organisms. Across the landscape, this biological C cycle is then modified by physical disturbance processes such as fire and abrupt permafrost thaw that accelerate C losses while modifying rates of C gain. Disturbances that cause rapid C loss on a fraction of the landscape in any given year will accelerate change relative to that directly caused by changing climate alone. Sustained transfers of C to the atmosphere that could cause a significant positive feedback to climate change must come from old C, which has not been part of the active C cycle for centuries to millennia and forms the bulk of the permafrost C pool. But, this net movement of old C from permafrost to the atmosphere over years and decades is difficult to detect amidst large input and output fluxes due to rapid biological metabolism and disturbance.

Recent studies have begun to apply ¹⁴C to field measurements of C fluxes to detect release of old C. Because permafrost C has been accumulating in these ecosystems over thousands of years, there is a large difference between the ¹⁴C of organic matter deep in the soil profile and permafrost compared to contemporary C near the soil surface. This range in ¹⁴C provides a sensitive fingerprint for detecting the loss of old soil C as permafrost thaws. For example, permafrost degradation in Alaskan tundra initially enhanced plant growth, but measurements of respiration ¹⁴CO₂ emitted from the ecosystem revealed that microbial decomposition of old C had already begun. As permafrost degradation progressed, the loss of old C outpaced rates of new plant growth causing the ecosystem to shift from a C sink to a source. Permafrost thaw continued to expose more organic C to microbial decomposition, whereas plant uptake was limited by overall uptake capacity and did not respond to the same degree. In aquatic systems, ¹⁴CH₄ was similarly used to detect the mobilization and release of C from thaw lakes and in the coastal ocean that had been previously sequestered for tens of thousands of years.

Darrell Kaufman is interested in the geochronological applications of radiocarbon. He applies ¹⁴C to dating geologic archives of paleoclimate information and to quantifying the time-averaging of carbonate subfossils.

How do new plant C inputs influence ecosystem C balance?

With climate warming, arctic and boreal vegetation cover is changing and experiencing new environmental conditions. These changes have the potential to profoundly impact where plants allocate newly assimilated C from photosynthesis: whether it goes above or below ground, ground, grows new plant tissue, or maintains existing tissue, and whether it is stored within the plant for future use or is transferred to soil as exudates through roots. These allocation patterns determine how much C is retained in biomass C storage (sink) or non-structural carbohydrate storage (intermediate sink) and how much is quickly returned to the atmospherevia respired CO₂ (source). Additionally, the transfer of new C to the soil can occur either through root exudation or through plant tissue turnover and mortality. New C entering soil can both increase new soil organic matter pools and enhance microbial decomposition (source) simultaneously, including increasing losses of old soil C, called “priming”. Thus, tracing the fate of new C entering arctic ecosystems is essential for understanding plant and soil feedbacks for offsetting or enhancing soil C loss and future ecosystem C balance.

Quantifying the abundance of new C using radiocarbon has been done successfully with the atmospheric ¹⁴C bomb signal, but this pulse is diminishing over time (background/clean air ~11‰ in 2017) and will approach background pre-bomb levels mainly due to dilution from ¹⁴C-free fossil fuel inputs in the near future. In recent years, the rate of decline in ¹⁴C (combined with the precision of detection by AMS) does not enable distinguishing between annual differences in atmospheric C entering terrestrial ecosystems. Thus, newer and faster cycling (minutes to years) C sources can only be addressed by creating our own isotopic signal using tracer additions of ¹⁴C (or ¹³C) and following this through the study system. Traditional ¹⁴C labels, measured by decay counting techniques, contain large amounts (MBq) of radioactivity that is considered harmful and therefore have been limited to juvenile, short-stature vegetation in greenhouses or growth chambers. However, enhanced detection of ¹⁴C measured by AMS allows for low-level ¹⁴C label applications in natural ecosystems (i.e., amounts of ¹⁴C well below harmful or regulated levels). The low-level ¹⁴C approach is powerful for field applications because the strength of the label signal can be easily 6 orders of magnitude greater than possible with ¹³C, allowing the tracing of C in small plant and microbial C pools that cannot easily be quantified with ¹³C or current bomb ¹⁴C approaches.

A low-level ¹⁴C label measured by AMS was first used to study ecosystem C cycling in a boreal black spruce forest by Dr. Mariah Carbone. A pulse-chase approach was employed, where the “pulse” is an addition of the label substrate, in this case ¹⁴CO₂ which is assimilated by trees. This is then followed by the “chase” period, by which the concentration of the label is monitored over time into different ecosystem C pools. The ¹⁴C values in respiration reach a peak of ~1000‰, elevated over background values while still far below levels that would cause concern for cross-contamination. This approach gives a snapshot of where new C is allocated and how quickly it moves through the system at the time of the labeling and thus how it would be applied repeatedly to capture temporal variability such as seasonal patterns.

Darrell Kaufman is interested in the geochronological applications of radiocarbon. He applies ¹⁴C to dating geologic archives of paleoclimate information and to quantifying the time-averaging of carbonate subfossils.