Global database of ratios of particulate organic carbon to thorium-234 in the ocean: Improving estimates of the biological carbon pump

The ocean\u27s biological carbon pump (BCP) plays a major role in the global carbon cycle. A fraction of the photosynthetically fixed organic carbon produced in surface waters is exported below the sunlit layer as settling particles (e.g., marine snow). Since the seminal works on the BCP, global estimates of the global strength of the BCP have improved but large uncertainties remain (from 5 to 20 Gt C yr−1 exported below the euphotic zone or mixed-layer depth). The 234Th technique is widely used to measure the downward export of particulate organic carbon (POC). This technique has the advantage of allowing a downward flux to be determined by integrating the deficit of 234Th in the upper water column and coupling it to the POC∕234Th ratio in sinking particles. However, the factors controlling the regional, temporal, and depth variations of POC∕234Th ratios are poorly understood. We present a database of 9318 measurements of the POC∕234Th ratio in the ocean, from the surface down to \u3e5500 m, sampled on three size fractions (∼\u3e0.7 µm, ∼1–50 µm, ∼\u3e50 µm), collected with in situ pumps and bottles, and also from bulk particles collected with sediment traps. The dataset is archived in the data repository PANGAEA® under https://doi.org/10.1594/PANGAEA.911424 (Puigcorbé, 2019). The samples presented in this dataset were collected between 1989 and 2018, and the data have been obtained from published papers and open datasets available online. Unpublished data have also been included. Multiple measurements can be found in most of the open ocean provinces. However, there is an uneven distribution of the data, with some areas highly sampled (e.g., China Sea, Bermuda Atlantic Time Series station) compared to some others that are not well represented, such as the southeastern Atlantic, the south Pacific, and the south Indian oceans. Some coastal areas, although in a much smaller number, are also included in this global compilation. Globally, based on different depth horizons and climate zones, the median POC∕234Th ratios have a wide range, from 0.6 to 18 µmol dpm−1

Expanding Greenland seagrass meadows contribute new sediment carbon sinks

The loss of natural carbon sinks, such as seagrass meadows, contributes to grenhouse gas emissions and, thus, global warming. Whereas seagrass meadows are declining in temperate and tropical regions, they are expected to expand into the Arctic with future warming. Using paleoreconstruction of carbon burial and sources of organic carbon to shallow coastal sediments of three Greenland seagrass (Zostera marina) meadows of contrasting density and age, we test the hypothesis that Arctic seagrass meadows are expanding along with the associated sediment carbon sinks. We show that sediments accreted before 1900 were highly 13C depleted, indicative of low inputs of seagrass carbon, whereas from 1940’s to present carbon burial rates increased greatly and sediment carbon stocks were largely enriched with seagrass material. Currently, the increase of seagrass carbon inputs to sediments of lush and dense meadows (Kapisillit and Ameralik) was 2.6 fold larger than that of sparse meadows with low biomass (Kobbefjord). Our results demonstrate an increasing important role of Arctic seagrass meadows in supporting sediment carbon sinks, likely to be enhanced with future Arctic warming

Sampling device-dependence of prokaryotic community structure on marine particles: Higher diversity recovered by in situ pumps than by oceanographic bottles

Microbes associated with sinking marine particles play key roles in carbon sequestration in the ocean. The sampling of particle-attached microorganisms is often done with sediment traps or by filtration of water collected with oceanographic bottles, both involving a certain time lapse between collection and processing of samples that may result in changes in particle-attached microbial communities. Conversely, in situ water filtration through submersible pumps allows a faster storage of sampled particles, but it has rarely been used to study the associated microbial communities and has never been compared to other particle-sampling methods in terms of the recovery of particle microbial diversity. Here we compared the prokaryotic communities attached to small (1–53 mm) and large ( \u3e 53 mm) particles collected from the mesopelagic zone (100– 300 m) of two Antarctic polynyas using in situ pumps (ISP) and oceanographic bottles (BTL). Each sampling method retrieved largely different particle-attached communities, suggesting that they capture different kinds of particles. These device-driven differences were greater for large particles than for small particles. Overall, the ISP recovered 1.5- to 3-fold more particle-attached bacterial taxa than the BTL, and different taxonomic groups were preferentially recovered by each method. In particular, typical particle-attached groups such as Planctomycetes and Deltaproteobacteria recovered with ISP were nearly absent from BTL samples. Our results suggest that the method used to sample marine particles has a strong influence in our view of their associated microbial communities

Seagrass soils sequester up to half the metal emissions of one of the world\u27s largest smelters

One of the world\u27s largest smelters has been operating in South Australia since 1889, affecting environment and human health. Here we quantified the magnitude of Pb, Zn and Cd emissions from the smelter sequestered in the soil of an adjacent 110 km2 Posidonia australis seagrass meadows. Seagrass core records show that the smelter contaminated the entire area with decreasing sequestration with increasing distance from contamination points. The soil accumulated ~1300 t of Pb, ~3450 t of Zn, and ~ 90 t of Cd since 1889, and sequestered the equivalent of ~20 % of Pb, and ~50 % of Zn and Cd cumulative smelter emissions since 1999, showing that seagrass can be significant, long-term sinks of metal pollution in highly contaminated environments. Conservation efforts should prioritize these seagrass meadows to avoid the potential release of pollutants from their soils following habitat loss, which could turn seagrasses from a sink to a source of pollution

The ultracompact regions G40.54+2.59 and G34.13+0.47: A new detection of compact radio sources

We report the detection of three compact ($< 0.001$ pc) radio sources (CRSs) at K$_{a}$-band (0.9 cm) in the \uchiirs G040.54+2.59 (two CRSs) and G034.13+0.47 (one CRS). These CRSs have weak flux densities and are located at the center of their respective \uchii regions. We found no clear association between massive ionizing stars and CRSs but some radiative influence on the latter, as suggested by their large emission measures (> $10^7 \mathrm{cm}^{-6}\mathrm{pc}$), typical of photo evaporating neutral objects close to or associated with massive stars. Our modelling of G40.54+2.59 shows that their CRSs supply enough ionized material to shape its morphology while significantly extending its observable lifetime. On the other hand, despite the possible relation of the CRS with the large-scale outflow signatures observed in G034.13+0.47, the influence of this CRS on the evolution of the \uchii region is unlikely. Our results show that the presence of CRSs can alleviate the so-called lifetime problem of UCHII regions. Still, to address their dynamical evolution adequately, the scenario must include additional mechanisms like ambient confinement, or the role of the kinematics of their associated stellar objects.Comment: Published in Publications of the Astronomical Society of Japan, Volume 75, Issue 1, February 2023, Pages 90-102, https://doi.org/10.1093/pasj/psac09