Radiocarbon in modern carbon cycle research

<p>Atmospheric nuclear weapon testing in the 1950s and 1960s has been worrying, however, in many aspects it was extremely beneficial for environmental sciences. The artificial production of more than 6 x 10<sup>28</sup> atoms or about 0.6 tons of radiocarbon (<sup>14</sup>C), leading to a doubling of the <sup>14</sup>C/C ratio in tropospheric CO<sub>2</sub> of the Northern Hemisphere, has generated a prominent spike in 1963. This “bomb-spike” has been used as transient tracer in all compartments of the carbon cycle, but also to study atmospheric dynamics, such as inter-hemispheric and stratosphere-troposphere air mass exchange. Moreover, our attempt to accurately determine total bomb produced <sup>14</sup>C led to improved estimates of the atmosphere-ocean gas exchange rate and to a new constraint of the residence time of carbon in the terrestrial biosphere. Today, the transient bomb-radiocarbon signal has levelled off, and the anthropogenic input of radiocarbon-free fossil fuel CO<sub>2</sub> into the atmosphere has become the dominant driver of the <sup>14</sup>C/C ratio in global atmospheric CO<sub>2</sub>. The observed decreasing <sup>14</sup>C/C trend in atmospheric CO<sub>2</sub> may thus help scrutinising the total global release of fossil fuel CO<sub>2</sub> into the atmosphere. On the local and regional scale, atmospheric <sup>14</sup>C/C measurements are already routinely conducted to separate fossil fuel from biogenic CO<sub>2</sub> signals and to estimate trends of regional fossil fuel CO<sub>2</sub> emissions. Some prominent examples where the bomb <sup>14</sup>CO<sub>2</sub> disturbance has been successfully used to study dynamic processes in the carbon cycle are discussed as well as our current activities applying this unique isotope tracer for continental scale carbon cycle budgeting.</p>

Radiocarbon in modern Carbon Cycle research

<p>Atmospheric nuclear weapon testing in the 1950s and 1960s has been worrying, however, in many aspects it was extremely beneficial for environmental sciences. The artificial production of more than 6 x 10<sup>28</sup> atoms or about 0.6 tons of <sup>14</sup>C, leading to a doubling of the <sup>14</sup>C/C ratio in tropospheric CO<sub>2</sub> of the Northern Hemisphere has generated a prominent <sup>14</sup>C spike in 1963. This “bomb-spike” has been used as transient tracer in all compartments of the carbon cycle, but also to study atmospheric dynamics, such as inter-hemispheric and stratosphere-troposphere air mass exchange. Moreover, our attempt to accurately determine total bomb produced <sup>14</sup>C led to improved estimates of the atmosphere-ocean gas exchange rate and to a new constraint of the residence time of carbon in the terrestrial biosphere. Today, the transient bomb-radiocarbon signal has levelled off, and the anthropogenic input of radiocarbon-free fossil fuel CO<sub>2</sub> into the atmosphere has become the dominant driver of the decreasing <sup>14</sup>C/C ratio in atmospheric CO<sub>2</sub>; the observed trend may thus help scrutinising the total global release of fossil fuel CO<sub>2</sub> into the atmosphere. Prominent examples where the bomb <sup>14</sup>C disturbance has been successfully used to study dynamic processes in the carbon cycle are discussed as well as our current activities applying this unique isotope tracer for continental scale carbon cycle budgeting.</p>

Medusa-Aqua system: simultaneous measurement and evaluation of novel potential halogenated transient tracers HCFCs, HFCs and PFCs in the ocean

This study evaluates the potential usefulness of the halogenated compounds HCFC-22, HCFC-141b, HCFC-142b, HFC-134a, HFC-125, HFC-23, PFC-14 and PFC-116 as the time-dependent oceanographic transient tracers in order to better constrain ocean ventilation processes. We collected seawater samples and improved on an established analytical technique, the Medusa-Aqua system, to simultaneous measure them, and estimate their stability in seawater following previous work on the atmospheric history and solubility. HCFC-22, HCFC-141b, HCFC-142b, HFC-134a and HFC-125 have been measured in profiles in the Mediterranean Sea for the first time. We estimated the historic surface saturation anomalies of transient tracers in the Mediterranean Sea by evaluating the historic record. Their stability in seawater was estimated by analysis of their ocean partial lifetimes, seawater surface saturations and concentrations compared to CFC-12 measurements by a well-established technique. Of the investigated compounds, HCFC-141b was found to be the most promising transient tracer in the ocean; it fulfills several essential requirements by virtue of well-documented atmospheric history, established seawater solubility, inertness in seawater and feasible measurements and indication of conservative behavior in seawater by having mean ages in agreement to be expected from both CFC-12 and SF6 observations. However, more information on degradation is needed to further identify its stability in seawater, and HCFC-141b has restrictions on production and consumption imposed by the Montreal Protocol leading to its decreasing atmospheric mole fractions since 2017. The most potential oceanic transient tracers were PFC-14 and PFC-116 due to their stability in seawater, the long and well-documented atmospheric concentrations histories and constructed seawater solubility functions, although the low solubility in seawater creates challenging measurement conditions (i.e. low concentration). Measurements of PFCs can be potentially improved by modifying the Medusa-Aqua analytical system. With the exception of providing the information on the novel potential alternative oceanic transient tracers, this study also provides a method on how to evaluate the feasibility for a compound to be a transient tracer in the ocean.

Sources and accumulation of sediment and particulate organic carbon in a subarctic fjard estuary: 210Pb, 137Cs, and δ13C records from Lake Melville, Labrador

The sources and distribution of sediment and particulate organic carbon (OC) to Lake Melville, Labrador, were characterized to better understand impacts from climate and hydrological changes to the system. Mass accumulation rates (MARs) across the Lake Melville System (LMS) were established from 15 sediment cores collected in 2013 and 2014 by fitting excess 210Pb (210Pbex) profiles to a two-layer advection–diffusion model. MARs, validated using 137Cs, varied between 0.04 and 0.41 g cm−2 a−1, and overall decreased with increasing distance from the Churchill River, which drains into Goose Bay, a western extension of Lake Melville. The Churchill River is the greatest source of sediment to the system, but surprisingly, MARs were greatest in western Lake Melville rather than Goose Bay, reflecting the contribution of fine material carried eastward in the Churchill River plume and inputs from nearby tributaries. A comparison of 137Cs and 210Pbex inventories to expected atmospheric fallout (1.5 and 23.6 disintegrations per minute (dpm) cm−2, respectively) in sediment across the LMS suggests particles are largely sourced from the watershed. In eastern Lake Melville, elevated 210Pbex inventories and marine OC point to particle scavenging of dissolved 210Pb from inflowing marine water. A transient tracer mixing model was used to determine the depth in each core where >90% of sediment was deposited before and after hydroelectric development at Churchill Falls (1970) and applied to down-core profiles of OC and organic carbon isotopes (δ13Corg). We observed a significant increase of terrestrial OC to Lake Melville post 1970, which we interpret as change from climate or hydrology of the Churchill River.