Xiaomei Xu

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Verified

Xiaomei verified their affiliation via an institutional email.

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• Ph.D.
• Researcher at University of California, Irvine

Rapid warming is likely increasing primary production and wildfire occurrence in the Arctic. Projected changes in carbonaceous aerosols during the summer will impact atmospheric chemistry and climate, but our understanding of these processes is limited by sparse observations. Here, we characterize carbonaceous aerosol in Alaska, USA: Toolik Field S...

Climate change in the northern circumpolar regions is rapidly thawing organic-rich permafrost soils, leading to the substantial release of dissolved CO2 and CH4 into river systems. This mobilization impacts local ecosystems and regional climate feedback loops, playing a crucial role in the Arctic carbon cycle. Here, we analyze the stable carbon (δ¹...

Societal Impact Statement Over half of Earth's human population lives in urban areas where pollution from fossil fuel combustion threatens urban air quality and public health. The isotopic composition of plant tissues can be used as a tool to estimate local variation in the amount of fossil fuel–derived carbon dioxide. We present a case study that...

Climate change alters the sources and age of carbon in Arctic food webs by fostering the release of older carbon from degrading permafrost. Radiocarbon (¹⁴C) traces carbon sources and age, but data before rapid warming are rare and limit assessments over time. We capitalized on ¹⁴C data collected ~ 40 years ago that used fish as natural samplers by...

Water‐logged peatlands store tremendous amounts of soil carbon (C) globally, accumulating C over millennia. As peatlands become disturbed by human activity, these long‐term C stores are getting destabilized and ultimately released as greenhouse gases that may exacerbate climate change. Oxidation of the dissolved organic carbon (DOC) mobilized from...

Ooids (calcium carbonate coated grains) are common in carbonate environments throughout geologic time, but the mechanism by which they form remains unclear. In particular, the rate of ooid growth remains elusive in all but a few modern marine environments. In order to investigate the rate of ooid growth in a non-marine setting, we used 14C to date...

Northern high-latitude lakes are critical sites for carbon processing and serve as potential conduits for the emission of permafrost-derived carbon and greenhouse gases. However, the fate and emission pathways of permafrost carbon in these systems remain highly uncertain. Here, we used the natural abundance of radiocarbon to identify and trace the...

Dissolved inorganic carbon (DIC) in ocean water is a major sink of fossil fuel derived CO 2 . Carbon isotopes in DIC serve as tracers for oceanic water masses, biogeochemical processes, and air-sea gas exchange. We present a timeseries of surface DIC δ ¹³ C and Δ ¹⁴ C values from 2011 to 2022 from Newport Beach, California. This is a continuation o...

Foraminifera are widely used in paleoclimatic and paleoceanographic studies, providing information about past ocean conditions. However, in order to use these tracers, it is essential to obtain an accurate chronology. Radiocarbon has proven to be a powerful tool in developing robust chronologies. Sample sizes of a few milligrams of carbonate materi...

In recent decades, there has been a significant increase in annual area burned in California's Sierra Nevada mountains. This rise in fire activity has prompted the need to understand how historical forest management practices affect fuel composition and emissions. Here we examined the total carbon (TC) concentration and radiocarbon abundance (∆14C)...

Snow is critically important to the energy budget, biogeochemistry, ecology, and people of the Arctic. While climate change continues to shorten the duration of the snow cover period, snow mass (the depth of the snow pack) has been increasing in many parts of the Arctic. Previous work has shown that deeper snow can rapidly thaw permafrost and expos...

Radiocarbon (¹⁴C) is a powerful tracer of fossil emissions because fossil fuels are entirely depleted in ¹⁴C, but observations of ¹⁴CO2 and especially ¹⁴CH4 in urban regions are sparse. We present the first observations of ¹⁴C in both methane (CH4) and carbon dioxide (CO2) in an urban area (London) using a recently developed sampling system. We fin...

The Ocean University of China radiocarbon accelerator mass spectrometer center (OUC-CAMS) is equipped with a mini carbon dating system (MICADAS) from the Ionplus AG. The MICADAS allows measurements of both graphite and CO2 samples because of the design of a hybrid ion source. Also, both 13C and 14C of organic samples can be measured at the same tim...

The persistence of organic carbon (C) in soil is most often considered at timescales ranging from tens to thousands of years, but the study of organic C in paleosols (i.e., ancient, buried soils) suggests that paleosols may have the capacity to preserve organic compounds for tens of millions of years. However, a quantitative assessment of C sources...

Radiocarbon (14C) is a powerful tracer of fossil emissions because fossil fuels are entirely depleted in 14C, but observations of 14CO2 and especially 14CH4 in urban regions are sparse. We present the first observations of 14C in both methane (CH4) and carbon dioxide (CO2) in an urban area (London) using a recently developed sampling system. We fin...

Radiocarbon (∆14C) measurements of nonstructural carbon enable inference on the age and turnover time of stored photosynthate (e.g., sugars, starch), of which the largest pool in trees resides in the main bole. Because of potential issues with extraction-based methods, we introduce an incubation method to capture the ∆14C of nonstructural carbon vi...

Introduction The productivity of the Amazon Rainforest is related to climate and soil fertility. However, the degrees to which these interactions influence multiannual to decadal variations in tree diameter growth are still poorly explored. Methods To fill this gap, we used radiocarbon measurements to evaluate the variation in tree growth rates ov...

Fossil fuel carbon dioxide (CO2) emissions (ffCO2) constitute the majority of greenhouse gas emissions and are the main determinant of global climate change. The COVID‐19 pandemic caused wide‐scale disruption to human activity and provided an opportunity to evaluate our capability to detect ffCO2 emission reductions. Quantifying changes in ffCO2 le...

The persistence of soil organic carbon (C) in soil, defined as the mean residence time of organic C compounds in soils, is a critical measure for understanding the capacity of terrestrial ecosystems to regulate biogeochemical cycles. The persistence of organic carbon in soil is most often considered at timescales ranging from tens to thousands of y...

The terrestrial subsurface contains nearly all of Earth’s freshwater reserves and harbours the majority of our planet’s total prokaryotic biomass. Although genetic surveys suggest these organisms rely on in situ carbon fixation, rather than the photosynthetically derived organic carbon transported from surface environments, direct measurements of c...

Carbon- and nitrogen-containing aerosols are ubiquitous in urban atmospheres and play important roles in air quality and climate change. We determined the 14C fraction modern (fM) and δ13C of total carbon (TC) and δ15N of NH4+ in the PM2.5 collected in Seoul megacity during April 2018 to December 2019. The seasonal mean δ13C values were similar to...

Non‐growing season CO2 emissions from Arctic tundra remain a major uncertainty in forecasting climate change consequences of permafrost thaw. We present the first time series of soil and microbial CO2 emissions from a graminoid tundra based on year‐round in situ measurements of the radiocarbon content of soil CO2 (Δ¹⁴CO2) and of bulk soil C (Δ¹⁴C),...

Organic matter degradation and sequestration in marine sediments are important processes involved in carbon cycling in the ocean. Here, we present the results of carbon isotope (14C and 13C) and concentration measurements of sedimentary organic carbon (SOC), pore‐water dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) in sediment...

The terrestrial subsurface contains nearly all of Earth’s freshwater reserves ¹ and harbors upwards of 60% of our planet’s total prokaryotic biomass 2,3 . While genetic surveys suggest these organisms rely on in situ carbon fixation, rather than the translocation of photosynthetically derived organic carbon 4–6 , corroborating measurements of carbo...

Carbonaceous aerosols are major components in PM2.5 of both polluted and clean atmosphere. Accurate source apportionment of carbonaceous aerosols may support effective PM2.5 control. Dual-carbon isotope method (¹⁴C and ¹³C) was adopted to identify the contribution of three main air pollution sources biogenic and biomass (fbb), liquid fossil (fliq.f...

We developed a passive sampler for time-integrated collection and radiocarbon ( ¹⁴ C) analysis of soil respiration, a major flux in the global C cycle. It consists of a permanent access well that controls the CO 2 uptake rate and an exchangeable molecular sieve CO 2 trap. We tested how access well dimensions and environmental conditions affect coll...

Radiocarbon (14C) measurements offer a unique investigative tool to study methane emissions by identifying fossil-fuel methane in air. Fossil-fuel methane is devoid of 14C and, when emitted to the atmosphere, causes a strong decrease in the ratio of radiocarbon to total carbon in methane (Δ14CH4). By observing changes in Δ14CH4, the fossil fraction...

The history and mechanisms of dust storms in northern China remain unclear owing to the paucity of reliable long-term, high-resolution geological records. In this study, we reconstructed the dust storm history of the last ~500 years in northern China, based on sedimentary coarse fraction (>63 µm) of a well-dated core from Lake Daihai, Inner Mongoli...

Geochemical and stable isotope measurements in the anoxic marine zone (AMZ) off northern Chile during periods of contrasting oceanographic conditions indicate that microbial processes mediating sulfur and nitrogen cycling exert a significant control on the carbonate chemistry (pH, A T , DIC and p CO 2 ) of this region. Here we show that in 2015, a...

Lakes and ponds can be hotspots for CO2 and CH4 emissions, but Arctic studies remain scarce. Here we present diffusive and ebullition fluxes collected over several years from 30 ponds and 4 lakes formed on an organic‐rich polygonal tundra landscape. Water body morphology strongly affects the mixing regime—and thus the seasonal patterns in gas emiss...

Rationale The isotopic measurement of environmental sample CO2 via isotope ratio mass spectrometry (IRMS) can present many analytical challenges. In many offline applications, exceedingly few samples can be prepared per day. In such applications, long‐term storage (months) of sample CO2 is desirable, in order to accumulate enough samples to warrant...

Elemental carbon (EC) is a major light‐absorbing component of atmospheric aerosol particles. Here, we report the seasonal variation in EC concentrations and sources in airborne particulate matter (PM) and snow at Alert, Canada, from March 2014 to June 2015. We isolated the EC fraction with the EnCan‐Total‐900 (ECT9) protocol and quantified its stab...

While the complementarity of CO data in monitoring CO2 from fossil-fuel combustion (ffCO2) is widely known, a rigorous demonstration of its use in reducing uncertainties on top-down regional ffCO2 emissions is still warranted. Here, we report a case study investigating the regional covariation of observed and modeled abundances of CO, CO2, and ffCO...

Measuring isotopic ratios in aerosol particles is a powerful tool for identifying major sources, particularly in separating fossil from non-fossil sources and investigating aerosol formation processes. We measured the radiocarbon, stable carbon, and stable nitrogen isotopic composition of PM2.5 in Beijing (BJ) and Changdao (CD) in the North China P...

The cover image is based on the Original Article The sources and seasonal fluxes of particulate organic carbon in the Yellow River by Yuanxin Qu, Zhangdong Jin et al., https://doi.org/10.1002/esp.4861.

Nonstructural carbohydrates (NSCs) play a critical role in plant physiology and metabolism, yet we know little about their distribution within individual organs such as the stem. This leaves many open questions about whether reserves deep in the stem are metabolically active and available to support functional processes. To gain insight into the av...

Significance The vast majority of the world’s nations have pledged to reduce emissions of CO 2 and other greenhouse gases and to track and report emissions using accounting methods based on economic statistics and emissions factors. Here, we present an independent method of emissions monitoring based directly on atmospheric observations and the str...

The Yellow River transports a large amount of sediment and particulate organic carbon (POC), which is thought to mainly derive from erosion of the Chinese Loess Plateau (CLP). However, the compositions, sources and erosional fluxes of POC in the Yellow River remain poorly constrained. Here we combined measurements of mineralogy, total organic carbo...

Permafrost thaw in the Arctic is mobilizing old carbon (C) from soils to aquatic ecosystems and the atmosphere. Little is known, however, about the assimilation of old C by aquatic food webs in Arctic watersheds. Here, we used C isotopes (δ¹³C, Δ¹⁴C) to quantify C assimilation by biota across 12 streams in arctic Alaska. Streams spanned watersheds...

Plain Language Summary Dissolved organic carbon (DOC) produced in sediment porewater is a crucial intermediary during the early diagenesis of organic matter in marine sediments, and DOC production is also an important pathway of carbon cycling in the ocean. In this study, we present radiocarbon and stable carbon isotope results measured for porewat...

Graphitization of 0.5–1.5 mg C, and of smaller samples to a lesser extent, is routinely done at our Facility by reduction over zinc. The method yields low background, good accuracy but offers a limited throughput, requires dedicated equipment and considerable operator time. Sealed-tube graphitization is faster, easier and cost-efficient producing a...

Plain Language Summary Hadal trenches (depth > 6,000 m) are the remotest and the least explored places on our planet, with characteristics of low temperatures, high pressure, limited food, and frequent geological activity. Knowing the source of organic matter for hadal life is key in understanding the ocean carbon cycle and the biological adaptatio...

The lakes that form via ice‐rich permafrost thaw emit CH4 and CO2 to the atmosphere from previously frozen ancient permafrost sources. Despite this potential to positively feedback to climate change, lake carbon emission sources are not well understood on whole‐lake scales, complicating upscaling. In this study, we used observations of radiocarbon...

Glacial runoff exports large amounts of carbon (C) to the oceans, but major uncertainty remains regarding sources, seasonality, and magnitude. We apportioned C exported by five rivers from glacial and periglacial sources in northwest Greenland by monitoring discharge, water sources (δ¹⁸O), concentration and composition of dissolved organic carbon (...

Despite the global significance of the subsurface biosphere, the degree to which it depends on surface organic carbon (OC) is still poorly understood. Here, we compare stable and radiogenic carbon isotope compositions of microbial phospholipid fatty acids (PLFAs) with those of in situ potential microbial C sources to assess the major C sources for...

Determining the biogeochemical pathways utilized by microbes living in groundwater is essential for understanding the subsurface C cycle and the fate of organic compounds, including pollutants. The radiocarbon signature (Δ ¹⁴ C) of fatty acid methyl esters derived from microbial phospholipids (PLFA) provides useful information for differentiating m...

Supporting Information S1

Erosion of organic carbon from the terrestrial biosphere and sedimentary rocks plays an important role in the global carbon cycle across a range of timescales. Over geological timescales (>10 ⁴ years), erosion and burial of particulate organic carbon (POC) from the terrestrial biosphere (POC biosphere ) is an important CO 2 sink, while oxidation of...

Combustion of fossil fuel is the dominant source of greenhouse gas emissions to the atmosphere from California. Here, we describe radiocarbon (^(14)CO_2) measurements and atmospheric inverse modeling to estimate fossil fuel CO_2 (ffCO_2) emissions for 2009–2012 from a site in central California, and for June 2013–May 2014 from two sites in southern...

Advances in accelerator mass spectrometry (AMS) and environmental chemistry techniques have increased demand for the natural abundance ¹⁴C analysis of ultra-small (<25 µgC) and compound-specific samples. The sealed-tube zinc (Zn) method is used for the reduction of sample CO2 to graphite on an iron catalyst. This method provides reliable, low AMS ¹...

Radiocarbon ( ¹⁴ C) generated by the thermonuclear tests in the late 1950s to early 1960s has been used as a tracer to study atmospheric and oceanic circulations, carbon exchange between different reservoirs, and fossil fuel emissions. Here we report the first measurements of ¹⁴ C in atmospheric CO 2 of maritime air collected over the South China S...

In the last decade, production of shale gas has tremendously increased, and the need for local pre-exploitation baseline data on dissolved natural gas in aquifers has been stressed. This study investigated the origin of hydrocarbons naturally present in shallow aquifers of the Saint-Édouard area (Québec, eastern Canada), where the underlying Utica...

Significance We report radiocarbon ( ¹⁴ C) measurements of carbonaceous aerosol originating from fires on the islands of Sumatra and Borneo. These data provide information about what types of ecosystems burned and are critical for linking the human health effects of fires to the anthropogenic build-up of atmospheric CO 2 . Our measurements confirm...

Coastal lake sediments are valuable paleoclimate archives provided that they can be accurately dated. Here, we report radiocarbon ages of bulk sediment organic matter (OM), plants, shells, particulate OM, and dissolved OM from coastal lakes in Florida. Bulk sediment OM yielded ages that are consistently older than contemporaneous plants and shells,...

Isotopes are essential tools to apportion major sources of aerosols. We measured the radiocarbon, stable carbon, and stable nitrogen isotopic composition of PM2.5 at Taehwa Research Forest (TRF) near Seoul Metropolitan Area (SMA) during August–October 2014. PM2.5, TC, and TN concentrations were 19.4 ± 10.1 μg m⁻³, 2.6 ± 0.8 μg C m⁻³, and 1.4 ± 1.4...

Since ¹⁴C-tracer is widely used in determining oceanic primary production, most research vessels have a history of using ¹⁴C labeled sodium bicarbonate solutions for culture experiments. A contamination monitor test (swipe test) prior to each cruise can help to detect and prevent possible ¹⁴C contamination before sampling for ¹⁴C studies at natural...

Permafrost peatlands store globally significant amounts of soil organic carbon (SOC) that may be vulnerable to climate change. Permafrost thaw exposes deeper, older SOC to microbial activity, but SOC vulnerability to mineralization and release as carbon dioxide is likely influenced by the soil environmental conditions that follow thaw. Permafrost t...

Degradation of groundwater quality is a primary public concern in rural hydraulic fracturing areas. Previous studies have shown that natural gas methane (CH4) is present in groundwater near shale gas wells in the Marcellus Shale of Pennsylvania, but did not have pre-drilling baseline measurements. Here, we present the results of a free public water...

We use sediment cores collected by the Chinese National Arctic Research Expeditions from the Alpha Ridge to advance Quaternary stratigraphy and paleoceanographic reconstructions for the Arctic Ocean. Our cores show a good litho/biostratigraphic correlation to sedimentary records developed earlier for the central Arctic Ocean, suggesting a recovered...

Climate-sensitive Arctic lakes have been identified as conduits for ancient permafrost-carbon (C) emissions and as such accelerate warming. However, the environmental factors that control emission pathways and their sources are unclear; this complicates upscaling, forecasting and climate-impact-assessment efforts. Here we show that current whole-la...

In response to warming climate, methane can be released to Arctic Ocean sediment and waters from thawing subsea permafrost and decomposing methane hydrates. However, it is unknown whether methane derived from this sediment storehouse of frozen ancient carbon reaches the atmosphere. We quantified the fraction of methane derived from ancient sources...

Oral presentation

Dissolved organic carbon (DOC) is of primary importance to marine ecosystems and the global carbon cycle. Stable carbon (δ13C) and radiocarbon (Δ14C) isotopic measurements are powerful tools for evaluating DOC sources and cycling. However, the isotopic signature of DOC in the Gulf of Mexico (GOM) remains almost completely unknown. Here we present t...

Two stalagmites, XY07-8 and -11 collected from Xinya Cave in northeast Chongqing of China, were dated by ²³⁰Th/U and AMS ¹⁴C methods. The Holocene parts of the stalagmites contain high Th contents, resulting large uncertainties of the ²³⁰Th/U ages. Two precise ²³⁰Th/U dates in the pre-Holocene part of XY07-8 agree well with the AMS ¹⁴C ages. The Ho...

Black carbon (BC) and organic carbon (OC) aerosols are important components of fine particulate matter (PM2.5) in polluted urban environments. Quantifying the contribution of fossil fuel and biomass combustion to BC and OC concentrations is critical for developing and validating effective air quality control measures and climate change mitigation p...

The vast majority of radiocarbon measurement results (¹⁴C/¹²C isotopic ratios or sample activities) are corrected for isotopic fractionation processes (measured as ¹³C/¹²C isotopic ratios) that occur in nature, in sample preparation and measurement. In 1954 Harmon Craig suggested a value of 2.0 for the fractionation ratio b that is used to correct...

In lake sediments where terrestrial macrofossils are rare or absent, AMS radiocarbon dating of pollen concentrates may represent an important alternative solution for developing a robust and high resolution chronology suitable for Bayesian modelling of age-depth relationships. Here we report an application of the heavy liquid density separation app...

While most global productivity is driven by modern photosynthesis, river ecosystems are supplied by locally fixed and imported carbon that spans a range of ages. Alluvial aquifers of gravel-bedded river floodplains present a conundrum: despite no possibility for photosynthesis in groundwater and extreme paucity of labile organic carbon, they suppor...

Supplementary Figures 1-6 and Supplementary Tables 1-4.

Significance C-14 dating methods can be used to determine the time of death of wildlife products. We evaluate poaching patterns of elephants in Africa by using ¹⁴ C to determine lag time between elephant death and recovery of ivory by law enforcement officials. Most ivory in recent seizures has lag times of less than 3 y. Lag times for ivory origin...

The Intergovernmental Panel on Climate Change (IPCC) reported that all carbon dioxide (CO2) emissions generated by water resource recovery facilities (WRRFs) during treatment are modern, based on available literature. Therefore, such emissions were omitted from IPCC's greenhouse gas (GHG) accounting procedures. However, a fraction of wastewater's c...

We conducted an isotopic analysis of groundwater in Orange County, California, USA, around the Talbert Seawater Injection Barrier to determine if recycled water, used to artificially recharge local aquifers, carries a unique isotopic signature that can be used as a tracer. From September 2014 to April 2015, we collected groundwater from six private...

Radiocarbon in CO2 (14CO2) measurements can aid in discriminating between fast (<1 year) and slower (>5-10 years) cycling of C between the atmosphere and the terrestrial biosphere due to the 14C disequilibrium between atmospheric and terrestrial C. However, 14CO2 in the atmosphere is typically much more strongly impacted by fossil fuel emissions of...

Over the last few decades, radiocarbon laboratories have used different procedures for measuring a broad range of carbonaceous materials. To produce reliable results, the processes employed for sample processing, graphite target production, and spectrometer measurement must be rigorous, well tested, and reproducible. Most of the procedures have bee...

Oxygen minimum zones (OMZs), located below highly productive marine regions, are sites of microbially-mediated denitrification and biogeochemical cycling that have global significance. The intensity of OMZs fluctuate naturally, however, the degree of these fluctuations and a comprehensive understanding of the factors that drive these fluctuations o...

This chapter presents an overview of the steps required to prepare a sample for radiocarbon (14C) measurement by accelerator mass spectrometry (AMS). These include: (1) collection of an appropriate sample that can answer the question being asked; (2) pretreatment of samples to isolate a the most representative fraction of the bulk carbon (C) or to...