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 Unpublished data have also been included.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

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

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

Bottom-trawling along submarine canyons impacts deep sedimentary regimes

Many studies highlight that fish trawling activities cause seafloor erosion, but the assessment of the remobilization of surface sediments and its relocation is still not well documented. These impacts were examined along the flanks and axes of three headless submarine canyons incised on the Barcelona continental margin, where trawling fleets have been operating for decades. Trawled grounds along canyon flanks presented eroded and highly reworked surface sediments resulting from the passage of heavy trawling gear. Sedimentation rates on the upper canyon axes tripled and quadrupled its natural (i.e. pre-industrialization) values after a substantial increase in total horsepower of the operating trawling fleets between 1960 s and 1970 s. These impacts affected the upper canyon reaches next to fishing grounds, where sediment resuspended by trawling can be transported towards the canyon axes. This study highlights that bottom trawling has the capacity to alter natural sedimentary environments by promoting sediment-starved canyon flanks, and by enhancing sedimentation rates along the contiguous axes, independently of canyons’ morphology. Considering the global mechanisation and offshore expansion of bottom trawling fisheries since the mid-20th century, these sedimentary alterations may occur in many trawled canyons worldwide, with further ecological impacts on the trophic status of these non-resilient benthic 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

Distributions of total and size-fractionated particulate 210Po and 210Pb activities along the North Atlantic GEOTRACES GA01 transect: GEOVIDE cruise

Vertical distributions of total and particulate polonium-210 (210Po) and lead-210 (210Pb) activities in the water column were measured at 11 stations in the North Atlantic during the GEOTRACES GA01 transect: GEOVIDE cruise in May–June 2014. Total 210Po activity was on average 24% lower than 210Pb activity in the upper 100m, and it was closer to unity in the mesopelagic (100–1000m). The partitioning coefficients (Kd) along the transect suggest the preferential association of 210Po relative to 210Pb onto particles. The prominent role of small particles in sorption was confirmed by the observation that over 80% of the particulate radionuclide activity was on small particles. To account for the observed surface water 210Po∕210Pb disequilibria, particulate radionuclide activities and export of both small (1–53µm) and large ( \u3e 53µm) particles must be considered. A comparison between the GEOVIDE total particulate 210Po∕210Pb activity ratios (ARs) and the ratios in previous studies revealed a distinct geographic distribution, with lower particulate ARs in the high-latitude North Atlantic (including this study) and Arctic in relation to all other samples. For the samples where apparent oxygen utilization (AOU) was calculated at the same depth and time as the 210Po∕210Pb AR (40 stations including this study), there was a two-phase correlation between the total particulate AR and AOU, likely reflecting the nature of the particles and demonstrating the forces of remineralization and radionuclide decay from particles as they age

Review of the scientific and institutional capacity of small island developing states in support of a bottom-up approach to achieve sustainable development goal 14 targets

Capacity building efforts in Small Island Developing States (SIDS) are indispensable for the achievement of both individual and collective ocean-related 2030 agenda priorities for sustainable development. Knowledge of the individual capacity building and research infrastructure requirements in SIDS is necessary for national and international efforts to be effective in supporting SIDS to address nationally-identified sustainable development priorities. Here, we present an assessment of human resources and institutional capacities in SIDS United Nations (UN) Member States to help formulate and implement durable, relevant, and effective capacity development responses to the most urgent marine issues of concern for SIDS. The assessment highlights that there is only limited, if any, up-to-date information publicly available on human resources and research capacities in SIDS. A reasonable course of action in the future should, therefore, be the collection and compilation of data on educational, institutional, and human resources, as well as research capacities and infrastructures in SIDS into a publicly available database. This database, supported by continued, long-term international, national, and regional collaborations, will lay the foundation to provide accurate and up-to-date information on research capacities and requirements in SIDS, thereby informing strategic science and policy targets towards achieving the UN sustainable development goals (SDGs) within the next decade

Seagrass soil archives reveal centennial-scale metal smelter contamination while acting as natural filters

The upper Spencer Gulf in South Australia hosts the world\u27s largest single stream Pb-Zn smelter, which has caused environmental and health issues related to elevated metal concentrations in the surrounding environment. The area also has extensive seagrass meadows, occupying \u3e4000 km2. We reconstructed the fluxes of heavy metals over the last ~3000 years through a multi-parameter study of the soil archives formed by the seagrass Posidonia australis. Pb, Zn and Cd concentrations increased up to 9-fold following the onset of smelter operations in the 1880s, and the stable Pb isotopic signatures confirmed the smelter has been the main source of lead pollution in the seagrass soils until present. Preliminary estimates suggest that over the past 15 years seagrass meadows within 70 km2 of the smelter accumulated ~7–15% of the smelter emissions in their soils. Here we demonstrate that seagrass meadows act as pollution filters and sinks while their soils provide a record of environmental conditions, allowing baseline conditions to be identified and revealing the time-course of environmental change

Sequestration of macroalgal carbon: the elephant in the blue carbon room

Macroalgae form the most extensive and productive benthic marine vegetated habitats globally but their inclusion in Blue Carbon (BC) strategies remains controversial. We review the arguments offered to reject or include macroalgae in the BC framework, and identify the challenges that have precluded macroalgae from being incorporated so far. Evidence that macroalgae support significant carbon burial is compelling. The carbon they supply to sediment stocks in angiosperm BC habitats is already included in current assessments, so that macroalgae are de facto recognized as important donors of BC. The key challenges are (i) documenting macroalgal carbon sequestered beyond BC habitat, (ii) tracing it back to source habitats, and (iii) showing that management actions at the habitat lead to increased sequestration at the sink site. These challenges apply equally to carbon exported from BC coastal habitats. Because of the large carbon sink they support, incorporation of macroalgae into BC accounting and actions is an imperative. This requires a paradigm shift in accounting procedures as well as developing methods to enable the capacity to trace carbon from donor to sink habitats in the ocean

Blue Carbon Opportunities: seagrass carbon storage and accumulation rates at Trang, Thailand

Report prepared as a contribution to the IKI Project “Conservation of biodiversity, seagrass ecosystems and their services – safeguarding food security and resilience in vulnerable coastal communities in a changing climate” funded through the International Klimate Initiative (IKI). The IKI Project is a partnership between the CMS, Edith Cowan University, Project Seagrass, Seagrass Watch, Murdoch University, MRS, Blue Ventures, SAN, C3, ZSL, MareCet and Yapeka. The collaboration enhances the understanding of seagrass ecosystem services and the capacity to develop and deliver science-based policy solutions in seagrass conservation. It brings together scientists, policy experts, business development experts and conservation NGOs across the globe to provide expert and independent advice on seagrass ecosystems services and how these might be relevant to policy and financial solutions to marine conservation issues. This report deals specifically with the assessment of seagrass blue carbon ecosystem services