Annual Review of Earth and Planetary Sciences - Early Publication

The oceans contain large reservoirs of inorganic and organic carbon and play a critical role in both global carbon cycling and climate. Most of the biogeochemical transformations in the oceans are driven by marine microbes. Thus, molecular processes occurring at the scale of single cells govern global geochemical dynamics, posing a challenge of scales. Understanding the processes controlling ocean carbon cycling from the cellular to the global scale requires the integration of multiple disciplines including microbiology, ecology, biogeochemistry, and computational fields such as numerical models and bioinformatics. A shared language and foundational knowledge will facilitate these interactions. This review provides the state of knowledge on the role marine microbes play in large-scale ocean carbon cycling through the lens of observational oceanography and biogeochemical models. We conclude by outlining ways in which the field can bridge the gap between -omics datasets and ocean models to understand ocean carbon cycling across scales.

• ▪  -Omic approaches are providing increasingly quantitative insight into the biogeochemical functions of marine microbial ecosystems.
• ▪  Numerical models provide a tool for studying global carbon cycling by scaling from the microscale to the global scale.
• ▪  The integration of -omics and numerical modeling generates new understanding of how microbial metabolisms and community dynamics set nutrient fluxes in the ocean.

Most seismicity in Latin America is controlled by the subduction process. Different zones have hosted earthquakes of magnitudes larger than Mw 8.5 that repeat every several centuries. Events around Mw 8.0 are more frequent; since the beginning of the twentieth century, some collocated earthquakes have occurred with differences of decades, which allows for comparison of old and modern seismological records. The rupture zones that have hosted mega-earthquakes continue to produce smaller earthquakes after three centuries. Therefore, the process of unlocking in the Latin America subduction zone occurs by giant (≥Mw 9.0), mega- (9.0 > Mw ≥ 8.5), and large (8.5 > Mw ≥ 7.5) earthquakes, and interaction between these events is not yet fully understood. We have less understanding of the earthquakes that occurred in the oceanic plates, which have not been correctly recorded due to poor seismological instrumentation and lack of knowledge about subduction during the first half of the twentieth century in Latin America. Slow earthquakes have been observed in some zones of Latin America, several of them with recurrence periods of a few years, as well as tectonic (nonvolcanic) tremors and low-frequency and very low-frequency earthquakes. How do these slow slip manifestations relate to ordinary earthquakes? This question is still difficult to answer for Latin America given the lack of dense geodetic and seismic networks that allow identification of all the slow earthquakes that likely occur more frequently than currently reported.

• ▪  Latin America subduction zones share similar seismic characteristics. They can host large-magnitude earthquakes and exhibit a variety of slow earthquakes.
• ▪  Giant earthquakes, with a magnitude greater than 9, have occurred so far in Chile, and mega-earthquakes have occurred in several Latin American countries.
• ▪  Additional slow earthquakes will be detected in Latin America as seismic and geodetic networks become denser.

The emergence of continental crust above sea level influences Earth's surface environments and climate patterns, and it creates diverse habitats that promote biodiversity. Earth exhibits bimodal hypsometry with elevated continents and a submerged seafloor. However, it remains elusive when and how this unique feature was first established. The geological record suggests the presence of subaerial landmasses between ca. 3.8 and 2.4 billion years ago (Ga), but their spatial extent and longevity remain unclear. Further, the tectonic processes governing the proportion of continental land to ocean basins and topography during this period are poorly understood. Here, we synthesize a variety of geological and geochemical proxies to suggest that crustal emergence did occur in the early-to-mid Archean, primarily exposing precratonized volcanic crust for brief time periods. Stable continental crust on a regional scale (as cratons) began emerging around ca. 3.2–3.0 Ga, facilitated by the development of thick, stable cratonic lithospheres. Over hundreds of millions of years, voluminous magmatism within a plateau-type setting led to the formation of thick, felsic crust and depleted mantle keels, allowing cratons to rise above sea level via isostatic adjustment. The areal extent of emergent land increased from ca. 3.0 to 2.5 Ga owing to the formation of more cratons, likely coinciding with the onset of plate tectonics, and culminated around ca. 2.5–2.2 Ga when land surface area and freeboard conditions resembled those observed today. These newly emerged landmasses possibly played a critical role in oxygenating the atmosphere and oceans, cooling the climate, and promoting biodiversity during the late Archean to early Paleoproterozoic.

• ▪  Continental emergence marks a pivotal moment in Earth's history, impacting the planet's atmosphere, oceans, climate, and life evolution.
• ▪  We review the rock record to infer the timing, nature, and tectonic drivers of continental emergence on early Earth.
• ▪  Emergence on early Archean Earth was mostly transient, exposing primarily volcanic crust.
• ▪  First stable continental land formed at ca. 3.2–3.0 Ga due to the development of thick cratons and their isostatic adjustment.
• ▪  Emergent land area increased from ca. 3.0 to 2.5 Ga as more cratons formed and plate tectonics began.

Field experiences are highly valued in geoscience education. However, logistical, financial, and accessibility challenges associated with fieldwork and rapid advancements in technology have all prompted geoscience educators to explore virtual field experiences (VFEs) as alternatives. Rigorous assessment of the effectiveness of VFEs has not kept pace with their implementation, but recent studies offer meaningful and actionable findings that can inform ongoing and future use of VFEs in geoscience education. We present a review of selected studies that address three significant aspects of this still-evolving modality. First, we examine current characterization and classification of VFEs. Second, we examine studies that evaluate the effectiveness of teaching with VFEs. Third, we extend this review to studies that compare VFEs with in-person field experiences (IPFEs). The studies we review demonstrate that VFEs are a valuable approach to teaching introductory geoscience content, even compared to IPFEs.

• ▪  Challenges associated with field geoscience education and improvements in technology have led geoscience educators to develop and implement virtual field experiences (VFEs) as teaching tools.
• ▪  VFEs are tested, practical, and effective alternatives to in-person field experiences in introductory geoscience education.

Methane (CH4) is a simple molecule that, due to its radiative forcing, wields an outsized impact on planetary heat balance. Methane is formed by diverse abiotic pathways across a range of pressures and temperatures. Biological methanogenesis for anaerobic respiration uses a terminal nickel-containing enzyme and is limited to the archaeal domain of life. Methane can also be produced in aerobic microbes during bacterial methylphosphonate and methylamine degradation and via nonenzymatic reactions during oxidative stress. Abiotic CH4 is produced via thermogenic reactions and during serpentinization reactions in the presence of metal catalysts. Reconstructions of methane cycling over geologic time are largely inferential. Throughout Earth's history, methane has probably been the second most important climate-forcing greenhouse gas after carbon dioxide. Biological methanogenesis has likely dominated CH4 flux to Earth's atmosphere for the past ∼3.5 billion years, during which time CH4 is thought to have generally declined as atmospheric oxygen has risen. Here we review the evolution of the CH4 cycle over Earth's history, showcasing the multifunctional roles CH4 has played in Earth's climate, prebiotic chemistry, and microbial metabolisms. We also discuss the future of Earth's atmospheric CH4, the cycling of CH4 on other planetary bodies in the Solar System (with special emphasis on Titan), and the potential of CH4 as a biosignature on Earth-like extrasolar planets.

• ▪  Before life arose on Earth, abundant atmospheric CH4 in Earth's early atmosphere was likely key for establishment of habitable conditions and production of organic molecules for prebiotic chemistry.
• ▪  Biological methanogenesis for anaerobic respiration is only known to exist in some groups of anaerobic archaea, but CH4 can also be produced via enzymatic and nonenzymatic biological pathways that are not directly coupled to energy conservation. The relative importance of each of these pathways to the global CH4 cycle is a topic of active research, but archaeal methanogenesis dominates all other biological pathways for CH4 generation.
• ▪  As atmospheric O2 rose over Earth history, models suggest that atmospheric CH4 declined; in the distant deoxygenated future, atmospheric CH4 is predicted to rise again.
• ▪  Future missions to Titan will aid in understanding the complex organic chemistry on the only other planetary body in our Solar System with an active methane cycle.

The brevity of the instrumental record limits our knowledge of tropical cyclone activity on multidecadal to longer timescales and hampers our ability to diagnose climatic controls. Sedimentary archives containing event beds provide essential data on tropical cyclone activity over centuries and millennia. This review highlights the advantages and limitations of this approach and how these reconstructions have illuminated patterns of tropical cyclone activity and potential climate drivers over the last millennium. Key elements to developing high-quality reconstructions include confident attribution of event beds to tropical cyclones, assessing the potential role of other mechanisms, and evaluating the potential influence of geomorphic changes, sea-level variations, and sediment supply on a settings’ susceptibility to event bed deposition. Millennium-long histories of severe tropical cyclone occurrence are now available from many locations in the western North Atlantic and western North Pacific,revealing clear regional shifts in activity likely related to intervals of large-scale ocean-atmosphere reorganization.

• ▪  Prior to significant human influence in Earth's climate, natural climate variability dramatically altered patterns of tropical cyclone activity.
• ▪  For some regions (e.g., The Bahamas and the Marshall Islands), earlier intervals of tropical cyclone activity exceeded what humans have experienced during the recent period of instrumental measurements (∼1850 CE–present).
• ▪  Risk assessments based on the short instrumental record likely underestimate the threat posed by tropical cyclones in many regions.
• ▪  Changes in atmospheric and oceanic circulation associated with the Little Ice Age (∼1400–1800 CE) resulted in significant regional changes in tropical cyclone activity.
• ▪  Given the past sensitivity of tropical cyclone activity to climate change, we should anticipate regional shifts in tropical cyclone activity in response to ongoing anthropogenic warming of the planet.

Coccolithophores are a major group of oceanic calcifying phytoplankton, and their calcite skeletal remains, termed calcareous nannofossils, are a major component of deep-sea sediments accumulating since the Jurassic. Coccolithophores play a role in both the biological pump and the carbonate pump, exporting organic and inorganic carbon, respectively, out of the surface ocean. This means that they are key responders to and recorders of ocean carbon cycle and climate changes over geological and shorter timescales, and studying these responses can help elucidate the uncertain fate of calcifying phytoplankton under projected climate change scenarios. Here, we review established and emerging approaches for reconstructing (a) mixed-layer ocean temperature, (b) marine productivity, and (c) aspects of the ocean carbon cycle, using calcareous nannofossils from deep-sea sediments. For each parameter, we discuss the different proxies that have been proposed, based on abundance or species composition, inorganic geochemistry, and/or coccolith morphology, and explore their applications and limitations in Cenozoic paleoceanography.

• ▪  Calcareous nannofossils can be used to reconstruct upper ocean conditions and changes over centennial to million-year timescales.
• ▪  Key coccolith-based proxies for temperature, productivity, and the carbon cycle are reviewed.
• ▪  Approaches based on assemblages, geochemistry, and morphology provide novel insights into the evolution and adaptation of coccolithophores and past climate.

Minna de Honkoku began as an online citizen science project to transcribe earthquake-related historical materials from the Earthquake Research Institute Library of the University of Tokyo. In Japan, almost all the documents are written in kuzushiji (old-style Japanese cursive script), a writing style used before ∼1900. Because the style of writing is different modern Japanese, transcription is necessary to use the historical documents as data for earthquake research. The workspace of Minna de Honkoku consists of a viewer of a document image and a vertical (Japanese-style) editor for transcription. Users can input transcribed text while viewing its image. The ranking of characters transcribed is displayed to keep users motivated. As of October 2024, more than 9,700 people were registered for the project, with the total number of characters transcribed at about 41 million. The text generated by Minna de Honkoku can be used for various academic research fields including seismology and can be used to enhance citizens’ disaster awareness. The paired kuzushiji characters and text data generated by Minna de Honkoku are beginning to be used as training data for artificial intelligence.

• ▪  Minna de Honkoku is an online citizen science project aimed at deciphering historical documents.
• ▪  The total number of participants is 9,700, and characters transcribed by Minna de Honkoku reaches 41 million.
• ▪  Minna de Honkoku began as a project to transcribe earthquake-related historical materials.
• ▪  The text generated by Minna de Honkoku is used in seismology and various research fields and for building artificial intelligence–based kuzushiji recognition.

Ecologists rely on a wealth of data, including field observations and light stable isotopes, to infer dietary preferences and other ecological and physiological properties in living mammals. But inferring such important traits (e.g., trophic position, metabolism, pathologies) in extinct animals, including humans, can be challenging because biological processes rarely mirror morphology as preserved in the fossil record. For instance, dietary behavior does not necessarily reflect tooth morphology. As an additional challenge, some isotopic mammal tissues commonly used in modern ecology, such as collagen in bone or dentin or keratin from hair, hoof, or horn, do not generally preserve in fossil remains older than ∼200 kyr. In contrast, major constituents of bioapatite often retain their initial isotopic composition through fossilization processes. Recent analytical developments in mass spectrometry now allow, using small samples, for assessment of isotopic variability of major and trace elements such as calcium or zinc. Here, we review the application potentials of metal (nontraditional isotopes) for (paleo)ecological, (paleo)physiological, and (paleo)mobility inferences as applied to mammalian research.

• ▪  Mammals are key elements of modern ecosystems and possess a rich evolutionary history, yet inferences about their past ecologies and physiologies are challenging to retrieve using traditional geochemical toolkits.
• ▪  Metal stable isotopes provide a novel and complementary approach to unveil paleoecological and paleophysiological characteristics of extinct mammal species.
• ▪  Within a 20-year time frame, the core of metal isotopic data in mammalian research remains small compared to traditional isotopic systems (C, O, N), which is inviting for designing cost-effective instrumentation and increasing dissemination across scientific disciplines.