210Po/210Pb Disequilibria and Its Estimate of Particulate Organic Carbon Export Around Prydz Bay, Antarctica

Due to the remoteness and difficulty of sampling, the 210Po and 210Pb data are scarce in the Southern Ocean. Here, the activity concentrations of 210Po and 210Pb around Prydz Bay in austral summer were determined to understand their spatial variation and evaluate the dynamics of particle organic matter (POM). The activity concentrations of dissolved 210Po (D210Po) and 210Pb (D210Pb) range from 0.47 to 3.20 Bq⋅m–3 and from 1.15 to 2.97 Bq⋅m–3, respectively, with the lower values in the shelf. The particulate 210Po (P210Po) and 210Pb (P210Pb) are lower in the open ocean and increase to the coastal waters, among which the circumpolar deep water (CDW) is the lowest. The activity concentration of total 210Pb (T210Pb) ranges from 1.26 Bq⋅m–3 to 3.16 Bq⋅m–3, with a higher value in CDW, which is ascribed to radiogenic production from 226Ra and subsequent lateral transport. Occasionally a high value of T210Po occurs in deep water (>3.00 Bq⋅m–3), which may be caused by the remineralization of POM. The disequilibria between T210Po and T210Pb appears throughout the water column at most stations. The average T210Po/T210Pb)A.R. in the euphotic zone is 0.66, reflecting the effect of strong particle scavenging. There is a good positive correlation between the solid-liquid ratio of 210Po and POC, while 210Pb does not, indicating that particulate organic matter regulates the biogeochemical cycle of 210Po around Prydz Bay. Based on the 210Po/210Pb disequilibria, the export flux of POC in the water column is estimated to be 0.8–31.9 mmol m–2 d–1, with the higher values in the shelf.

Review of the analysis of 234Th in small volume (2–4 L) seawater samples: improvements and recommendations

The short-lived radionuclide 234Th is widely used to study particle scavenging and transport from the upper ocean to deeper waters. This manuscript optimizes, reviews and validates the collection, processing and analyses of total 234Th in seawater and suggests areas of further improvements. The standard 234Th protocol method consists of scavenging 234Th from seawater via a MnO2 precipitate, beta counting, and using chemical recoveries determined by adding 230Th. The revised protocol decreases sample volumes to 2 L, shortens wait times between steps, and simplifies the chemical recovery process, expanding the ability to more rapidly and safely apply the 234Th method.

Modeling the partitioning of organic chemical species in cloud phases with CLEPS (1.1)

The new detailed aqueous-phase mechanism Cloud Explicit Physico-chemical Scheme (CLEPS 1.0), which describes the oxidation of isoprene-derived water-soluble organic compounds, is coupled with a warm microphysical module simulating the activation of aerosol particles into cloud droplets. CLEPS 1.0 was then extended to CLEPS 1.1 to include the chemistry of the newly added dicarboxylic acids dissolved from the particulate phase. The resulting coupled model allows the prediction of the aqueous-phase concentrations of chemical compounds originating from particle scavenging, mass transfer from the gas-phase and in-cloud aqueous chemical reactivity. The aim of the present study was more particularly to investigate the effect of particle scavenging on cloud chemistry. Several simulations were performed to assess the influence of various parameters on model predictions and to interpret long-term measurements conducted at the top of Puy de Dôme (PUY, France) in marine air masses. Specific attention was paid to carboxylic acids, whose predicted concentrations are on average in the lower range of the observations, with the exception of formic acid, which is rather overestimated in the model. The different sensitivity runs highlight the fact that formic and acetic acids mainly originate from the gas phase and have highly variable aqueous-phase reactivity depending on the cloud acidity, whereas C3–C4 carboxylic acids mainly originate from the particulate phase and are supersaturated in the cloud.

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.