Quantifying CO2 Removal at Enhanced Weathering Sites: a Multiproxy Approach.

Enhanced weathering is a carbon dioxide (CO2) mitigation strategy that promises large scale atmospheric CO2 removal. The main challenge associated with enhanced weathering is monitoring, reporting, and verifying (MRV) the amount of carbon removed as a result of enhanced weathering reactions. Here, we study a CO2 mineralization site in Consett, Co. Durham, UK, where steel slags have been weathered in a landscaped deposit for over 40 years. We provide new radiocarbon, δ13C, 87Sr/86Sr, and major element data in waters, calcite precipitates, and soils to quantify the rate of carbon removal. We demonstrate that measuring the radiocarbon activity of CaCO3 deposited in waters draining the slag deposit provides a robust constraint on the carbon source being sequestered (80% from the atmosphere, 2σ = 8%) and use downstream alkalinity measurements to determine the proportion of carbon exported to the ocean. The main phases dissolving in the slag are hydroxide minerals (e.g., portlandite) with minor contributions (<3%) from silicate minerals. We propose a novel method for quantifying carbon removal rates at enhanced weathering sites, which is a function of the radiocarbon-apportioned sources of carbon being sequestered, and the proportion of carbon being exported from the catchment to the oceans.

Global silicate weathering flux overestimated because of sediment-water cation exchange.

Rivers carry the dissolved and solid products of silicate mineral weathering, a process that removes [Formula: see text] from the atmosphere and provides a key negative climate feedback over geological timescales. Here we show that, in some river systems, a reactive exchange pool on river suspended particulate matter, bonded weakly to mineral surfaces, increases the mobile cation flux by 50%. The chemistry of both river waters and the exchange pool demonstrates exchange equilibrium, confirmed by Sr isotopes. Global silicate weathering fluxes are calculated based on riverine dissolved sodium (Na+) from silicate minerals. The large exchange pool supplies Na+ of nonsilicate origin to the dissolved load, especially in catchments with widespread marine sediments, or where rocks have equilibrated with saline basement fluids. We quantify this by comparing the riverine sediment exchange pool and river water chemistry. In some basins, cation exchange could account for the majority of sodium in the river water, significantly reducing estimates of silicate weathering. At a global scale, we demonstrate that silicate weathering fluxes are overestimated by 12 to 28%. This overestimation is greatest in regions of high erosion and high sediment loads where the negative climate feedback has a maximum sensitivity to chemical weathering reactions. In the context of other recent findings that reduce the net [Formula: see text] consumption through chemical weathering, the magnitude of the continental silicate weathering fluxes and its implications for solid Earth [Formula: see text] degassing fluxes need to be further investigated.

Freshwater monitoring by nanopore sequencing.

While traditional microbiological freshwater tests focus on the detection of specific bacterial indicator species, including pathogens, direct tracing of all aquatic DNA through metagenomics poses a profound alternative. Yet, in situ metagenomic water surveys face substantial challenges in cost and logistics. Here, we present a simple, fast, cost-effective and remotely accessible freshwater diagnostics workflow centred around the portable nanopore sequencing technology. Using defined compositions and spatiotemporal microbiota from surface water of an example river in Cambridge (UK), we provide optimised experimental and bioinformatics guidelines, including a benchmark with twelve taxonomic classification tools for nanopore sequences. We find that nanopore metagenomics can depict the hydrological core microbiome and fine temporal gradients in line with complementary physicochemical measurements. In a public health context, these data feature relevant sewage signals and pathogen maps at species level resolution. We anticipate that this framework will gather momentum for new environmental monitoring initiatives using portable devices.

Many water-dwelling bacteria can cause severe diseases such as cholera, typhoid or leptospirosis. One way to prevent outbreaks is to test water sources to find out which species of microbes they contain, and at which levels. Traditionally, this involves taking a water sample, followed by growing a few species of 'indicator bacteria' that help to estimate whether the water is safe. An alternative technique, called metagenomics, has been available since the mid-2000s. It consists in reviewing (or 'sequencing') the genetic information of most of the bacteria present in the water, which allows scientists to spot harmful species. Both methods, however, require well-equipped laboratories with highly trained staff, making them challenging to use in remote areas. The MinION is a pocket-sized device that ­ when paired with a laptop or mobile phone ­ can sequence genetic information 'on the go'. It has already been harnessed during Ebola, Zika or SARS-CoV-2 epidemics to track the genetic information of viruses in patients and environmental samples. However, it is still difficult to use the MinION and other sequencers to monitor bacteria in water sources, partly because the genetic information of the microbes is highly fragmented during DNA extraction. To address this challenge, Urban, Holzer et al. set out to optimise hardware and software protocols so the MinION could be used to detect bacterial species present in rivers. The tests focussed on the River Cam in Cambridge, UK, a waterway which faces regular public health problems: local rowers and swimmers often contract waterborne infections, sometimes leading to river closures. For six months, Urban, Holzer et al. used the MinION to map out the bacteria present across nine river sites, assessing the diversity of species and the presence of disease-causing microbes in the water. In particular, the results showed that optimising the protocols made it possible to tell the difference between closely related species ­ an important feature since harmful and inoffensive bacteria can sometimes be genetically close. The data also revealed that the levels of harmful bacteria were highest downstream of urban river sections, near a water treatment plant and river barge moorings. Together, these findings demonstrate that optimising MinION protocols can turn this device into a useful tool to easily monitor water quality. Around the world, climate change, rising urbanisation and the intensification of agriculture all threaten water quality. In fact, access to clean water is one of the United Nations sustainable development goals for 2030. Using the guidelines developed by Urban, Holzer et al., communities could harness the MinION to monitor water quality in remote areas, offering a cost-effective, portable DNA analysis tool to protect populations against deadly diseases.

An Abrupt Aging of Dissolved Organic Carbon in Large Arctic Rivers.

Permafrost thaw in Arctic watersheds threatens to mobilize hitherto sequestered carbon. We examine the radiocarbon activity (F14C) of dissolved organic carbon (DOC) in the northern Mackenzie River basin. From 2003-2017, DOC-F14C signatures (1.00 ± 0.04; n = 39) tracked atmospheric 14CO2, indicating export of "modern" carbon. This trend was interrupted in June 2018 by the widespread release of aged DOC (0.85 ± 0.16, n = 28) measured across three separate catchment areas. Increased nitrate concentrations in June 2018 lead us to attribute this pulse of 14C-depleted DOC to mobilization of previously frozen soil organic matter. We propose export through lateral perennial thaw zones that occurred at the base of the active layer weakened by preceding warm summer and winter seasons. Although we are not yet able to ascertain the broader significance of this "anomalous" mobilization event, it highlights the potential for rapid and large-scale release of aged carbon from permafrost.

Triple oxygen isotope insight into terrestrial pyrite oxidation.

The mass-independent minor oxygen isotope compositions (Δ'17O) of atmospheric O2 and [Formula: see text] are primarily regulated by their relative partial pressures, [Formula: see text]/[Formula: see text] Pyrite oxidation during chemical weathering on land consumes [Formula: see text] and generates sulfate that is carried to the ocean by rivers. The Δ'17O values of marine sulfate deposits have thus been proposed to quantitatively track ancient atmospheric conditions. This proxy assumes direct [Formula: see text] incorporation into terrestrial pyrite oxidation-derived sulfate, but a mechanistic understanding of pyrite oxidation-including oxygen sources-in weathering environments remains elusive. To address this issue, we present sulfate source estimates and Δ'17O measurements from modern rivers transecting the Annapurna Himalaya, Nepal. Sulfate in high-elevation headwaters is quantitatively sourced by pyrite oxidation, but resulting Δ'17O values imply no direct tropospheric [Formula: see text] incorporation. Rather, our results necessitate incorporation of oxygen atoms from alternative, 17O-enriched sources such as reactive oxygen species. Sulfate Δ'17O decreases significantly when moving into warm, low-elevation tributaries draining the same bedrock lithology. We interpret this to reflect overprinting of the pyrite oxidation-derived Δ'17O anomaly by microbial sulfate reduction and reoxidation, consistent with previously described major sulfur and oxygen isotope relationships. The geologic application of sulfate Δ'17O as a proxy for past [Formula: see text]/[Formula: see text] should consider both 1) alternative oxygen sources during pyrite oxidation and 2) secondary overprinting by microbial recycling.