ENVIRONMENTAL SCIENCES
Skipjack tuna.
Human influence on mercury levels in Pacific tuna
As mercury emitted into the atmosphere through natural and anthropogenic processes enters the ocean, a fraction is naturally converted into neurotoxic methylmercury (MeHg) and bioaccumulates in marine food webs. High levels of MeHg have been reported in Pacific tuna, but the factors influencing spatial dispersal of MeHg in the ocean are unclear. Anaïs Médieu et al. measured mercury concentrations in skipjack tuna across six regions in the Pacific Ocean. The authors found that the levels were highest in the northwestern Pacific Ocean, which harbored 1.5–2 times higher levels than in the central, northern, and eastern regions and 4–5 times higher levels than in the southern, western, and western-central regions. Modeling suggested that biogeochemical factors, such as oxygen-poor conditions and shallow bacterial loops, might facilitate increased MeHg bioavailability at the base of marine food webs. However, modeling also estimated higher levels of atmospheric mercury, specifically along Asian coasts, likely derived from anthropogenic emissions as well as mercury inputs from rivers into the coastal shelf, correlating with high mercury levels in tuna from those regions. According to the authors, the findings provide insight into the spatial patterns of the natural and human influence on marine mercury bioaccumulation. — S.D.
EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES
Nucleation of Earth’s solid inner core
Earth’s solid inner core—a predominantly iron-based alloy in a stable, hexagonal close-packed phase (HCP)—is believed to have begun forming when molten iron cooled below its melting temperature. However, previous studies have determined that HCP nucleation requires an implausible degree of cooling, given ambient conditions and the core’s age. Using large-scale molecular dynamics, Yang Sun et al. demonstrate that molten iron at core conditions can crystallize via a two-step process of nucleating the metastable body-centered cubic (BCC) phase of iron followed by the HCP. The authors report that the intermediate BCC phase can occur with substantially less cooling than the HCP, corresponding to roughly 470 K versus 1,000 K, respectively. Although this temperature exceeds the core cooling maxima of roughly 200 K, the authors suggest this difference may be overcome by the existence of light elements that further reduce the necessary cooling for HCP nucleation and stabilize BCC phase nucleation relative to the HCP. The findings may help uncover the evolution of Earth’s inner core, according to the authors. — T.J.
SUSTAINABILITY SCIENCE
Species with small ranges, such as the black snub-nosed monkey (Rhinopithecus bieti), depend on forests in the mountains.
Reducing landslide risk and protecting biodiversity
Landslides are primarily caused by precipitation, and changes in land use, such as deforestation or road building, can increase the risk of landslides in mountainous terrain. Binbin Li et al. report that a cost-effective, natural solution to reduce landslide risk in mountainous areas could also protect critical habitat. The authors explored how landslide mitigation measures might affect biodiversity, mapping global rarity-weighted richness for mammalian, avian, and amphibian species. Overlapping the species richness map with landslide risk data revealed a strong spatial correlation between landslide risk and biodiversity. The authors identified 247 highly vulnerable mountains with both high species diversity and high landslide risk, representing 25.8% of mountainous areas worldwide. An additional 31 mountains were found to be vulnerable when risk from increasing climate change and deforestation was taken into account. All of the examined mountains had less than 17% of the area protected, and the authors suggest that forest protection and restoration should be a priority for 52 of the mountains. According to the authors, mitigating landslide risk by protecting natural areas can serve development and conservation goals while reducing costs associated with disaster recovery. — T.H.D.
SOCIAL SCIENCES
Impact of inequalities on science advancement
The scientific enterprise is plagued by disparities in both race and gender. Diego Kozlowski et al. investigated how the intersection of race and gender affect research topic choice and, consequently, the expansion of scientific knowledge. The authors analyzed the gender, race, research topic, and impact of the first authors of more than 5 million research articles that were published between 2008 and 2019 and indexed in the Web of Science database. The authors assigned the 1,609,107 US-based first authors a probable gender based on census and country-level data for first names, with the limitation that the method inferred gender in a binary way. Probable race, a social construct that differs from country to country, was inferred using data from the 2010 US Census for family names and racial groups. Impact was measured using citations. Among the study’s findings, the authors report that marginalized groups, such as Black and Latinx women, were overrepresented in topics with low citation counts and less cited than other groups across all health topics. Together, the results suggest that research topic choice is related to researcher race and gender, and a lack of diversity limits the expansion of scientific knowledge. According to the authors, the scientific enterprise should increase minority group participation and funding for historically underfunded research areas to enable equitable science advancement. — T.H.D.
ENVIRONMENTAL SCIENCES
Air quality and dementia risk
Exposure to air pollution late in life increases the risk of brain aging and dementia, but whether improving air quality lowers dementia risk is unclear. Xinhui Wang, Diana Younan, et al. conducted annual assessments of the cognitive function of a geographically diverse sample of 2,239 community-dwelling women in the United States between 2008 and 2018. The women ranged in age from 74–92 years and were dementia-free at baseline, between 2008 and 2012. The authors also analyzed yearly average concentrations of outdoor air pollution between 1998 and 2012. Individuals residing in locations with larger reductions in fine particulate matter and nitrogen dioxide had a lower risk of dementia than those living in places with less air quality improvement. The potential benefit was equivalent to the reduced dementia risk observed in women who were 2.4 years younger at baseline. According to the authors, the study bolsters findings on the putative health benefits of improved air quality in the United States and strengthens the evidence that late-life exposure to air pollution contributes to brain aging. — J.W.
PSYCHOLOGICAL AND COGNITIVE SCIENCES
Cognitive depletion and implicit language learning
Statistical learning allows humans to extract patterns in the environment, enabling the segmentation of continuous speech streams into words. Eleonore Smalle et al. explored whether cognitive depletion can improve the ability of adults to acquire linguistic knowledge through implicit statistical learning, which is a form of infant-like learning without awareness. A total of 96 adult volunteers participated in two experiments. First, the cognitive resources of some of the participants were drained either through theta-burst stimulation (TBS), which disrupted the activity of the left dorsolateral prefrontal cortex, or through a cognitively demanding task. The other set of control participants either received TBS over a control brain region or did not perform a cognitively effortful task. Next, the participants were exposed to continuous streams of syllables, including repeating pseudowords, while watching a silent film. The participants then performed a word-recognition task and reported their level of confidence in remembering the pseudowords. Both cognition-depleting interventions improved word-recognition accuracy—but only when memory confidence was low and knowledge was implicit. Together, the findings suggest that draining cognitive-control mechanisms in adults enhances implicit auditory word-segmentation abilities. The results also support the idea that higher cognitive functions can interfere with adult language learning. According to the authors, cognitive depletion could unlock infant-like implicit learning mechanisms to enhance foreign language learning in adults. — J.W.
JOURNAL CLUB
Highlighting recent, timely papers selected by Academy member labs
SUSTAINABILITY SCIENCE
Eyeing the impacts of the Anthropocene, a new framework sets out to combine existing models in order to better capture social and biophysical processes and their interactions. Image credit: Victor Lauer/Shutterstock.
A guidebook to incorporate changing human behaviors into planetary models
Posted on December 17, 2021
Amy McDermott
Humanity is now a geological force. Industries belch greenhouse gases into the sky; dams reroute waterways. People are changing major planetary cycles in the atmosphere, cryosphere, and ocean. These complex, multi-faceted effects have feedbacks that, in turn, affect society. And this presents a problem for researchers looking to understand and predict how Earth is changing in the midst of the Anthropocene: How do we factor our significant and ever-changing impacts and interactions into models of the planet’s biogeochemical cycling and other systems? While models simulating physical, chemical, and ecological cycles, as well as models simulating social dynamics, already exist in the literature, the two types typically aren’t combined.