ALBERT

Description: The large-scale dynamics of ocean oxygenation have changed dramatically throughout Earth's history, in step with major changes in the abundance of O2 in the atmosphere and changes to marine nutrient availability. A comprehensive mechanistic understanding of this history requires insights from oceanography, marine geology, geochemistry, geomicrobiology, evolutionary ecology, and Earth system modeling. Here, we attempt to synthesize the major features of evolving ocean oxygenation on Earth through more than 3 billion years of planetary history. We review the fundamental first-order controls on ocean oxygen distribution and summarize the current understanding of the history of ocean oxygenation on Earth from empirical and theoretical perspectives—integrating geochemical reconstructions of oceanic and atmospheric chemistry, genomic constraints on evolving microbial metabolism, and mechanistic biogeochemical models. These changes are used to illustrate primary regimes of large-scale ocean oxygenation and to highlight feedbacks that can act to stabilize and destabilize the ocean–atmosphere system in anoxic, low-oxygen, and high-oxygen states. Expected final online publication date for the Annual Review of Marine Science, Volume 14 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: The SUP05 clade of gammaproteobacteria (Thioglobaceae) comprises both primary producers and primary consumers of organic carbon in the oceans. Host-associated autotrophs are a principal source of carbon and other nutrients for deep-sea eukaryotes at hydrothermal vents, and their free-living relatives are a primary source of organic matter in seawater at vents and in marine oxygen minimum zones. Similar to other abundant marine heterotrophs, such as SAR11 and Roseobacter, heterotrophic Thioglobaceae use the dilute pool of osmolytes produced by phytoplankton for growth, including methylated amines and sulfonates. Heterotrophic members are common throughout the ocean, and autotrophic members are abundant at hydrothermal vents and in anoxic waters; combined, they can account for more than 50% of the total bacterial community. Studies of both cultured and uncultured representatives from this diverse family are providing novel insights into the shifting biogeochemical roles of autotrophic and heterotrophic bacteria that cross oxic–anoxic boundary layers in the ocean. Expected final online publication date for the Annual Review of Marine Science, Volume 14 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: A key Earth system science question is the role of atmospheric deposition in supplying vital nutrients to the phytoplankton that form the base of marine food webs. Industrial and vehicular pollution, wildfires, volcanoes, biogenic debris, and desert dust all carry nutrients within their plumes throughout the globe. In remote ocean ecosystems, aerosol deposition represents an essential new source of nutrients for primary production. The large spatiotemporal variability in aerosols from myriad sources combined with the differential responses of marine biota to changing fluxes makes it crucially important to understand where, when, and how much nutrients from the atmosphere enter marine ecosystems. This review brings together existing literature, experimental evidence of impacts, and new atmospheric nutrient observations that can be compared with atmospheric and ocean biogeochemistry modeling. We evaluate the contribution and spatiotemporal variability of nutrient-bearing aerosols from desert dust, wildfire, volcanic, and anthropogenic sources, including the organic component, deposition fluxes, and oceanic impacts. Expected final online publication date for the Annual Review of Marine Science, Volume 14 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Marine ecosystems are increasingly impacted by global environmental changes, including warming temperatures, deoxygenation, and ocean acidification. Marine scientists recognize intuitively that these environmental changes are translated into community changes via organismal physiology. However, physiology remains a black box in many ecological studies, and coexisting species in a community are often assumed to respond similarly to environmental stressors. Here, we emphasize how greater attention to physiology can improve our ability to predict the emergent effects of ocean change. In particular, understanding shifts in the intensity and outcome of species interactions such as competition and predation requires a sharpened focus on physiological variation among community members and the energetic demands and trophic mismatches generated by environmental changes. Our review also highlights how key species interactions that are sensitive to environmental change can operate as ecological leverage points through which small changes in abiotic conditions are amplified into large changes in marine ecosystems. Expected final online publication date for the Annual Review of Marine Science, Volume 14 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: A small subset of marine microbial enzymes and surface transporters have a disproportionately important influence on the cycling of carbon and nutrients in the global ocean. As a result, they largely determine marine biological productivity and have been the focus of considerable research attention from microbial oceanographers. Like all biological catalysts, the activity of these keystone biomolecules is subject to control by temperature and pH, leaving the crucial ecosystem functions they support potentially vulnerable to anthropogenic environmental change. We summarize and discuss both consensus and conflicting evidence on the effects of sea surface warming and ocean acidification for five of these critical enzymes [carbonic anhydrase, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), nitrogenase, nitrate reductase, and ammonia monooxygenase] and one important transporter (proteorhodopsin). Finally, we forecast how the responses of these few but essential biocatalysts to ongoing global change processes may ultimately help to shape the microbial communities and biogeochemical cycles of the future greenhouse ocean. Expected final online publication date for the Annual Review of Marine Science, Volume 14 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Many large marine predators make excursions from surface waters to the deep ocean below 200 m. Moreover, the ability to access meso- and bathypelagic habitats has evolved independently across marine mammals, reptiles, birds, teleost fishes, and elasmobranchs. Theoretical and empirical evidence suggests a number of plausible functional hypotheses for deep-diving behavior. Developing ways to test among these hypotheses will, however, require new ways to quantify animal behavior and biophysical oceanographic processes at coherent spatiotemporal scales. Current knowledge gaps include quantifying ecological links between surface waters and mesopelagic habitats and the value of ecosystem services provided by biomass in the ocean twilight zone. Growing pressure for ocean twilight zone fisheries creates an urgent need to understand the importance of the deep pelagic ocean to large marine predators. Expected final online publication date for the Annual Review of Marine Science, Volume 14 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Jupiter's Galilean satellite Io is one of the most remarkable objects in our Solar System. The tidal heating Io undergoes through its orbital resonance with Europa and Ganymede has resulted in a body rich in active silicate volcanism. Over the past decades, Io has been observed from ground-based and Earth-orbiting telescopes and by several spacecraft. In this review we summarize the progress made toward our understanding of the physical and chemical processes related to Io and its environment since the Galileo era. Io science has been revolutionized by the use of adaptive optics techniques on large, 8- to 10-m telescopes. The resultant ever-increasing database, mapping the size, style, and spatial distribution of Io's diverse volcanoes, has improved our understanding of Io's interior structure, its likely composition, and the tidal heating process. Additionally, new observations of Io's atmosphere obtained with these large optical/infrared telescopes and the Atacama Large Millimeter/submillimeter Array reveal the presence of volcanic plumes, the (at times) near-collapse of Io's atmosphere during eclipse, and the interactions of plumes with the sublimation atmosphere. ▪ Extensive new data sets of Io at ultraviolet, mid- to near-infrared, and radio wavelengths have been gathered since the Galileo era. ▪ New data and models inform us about tidal heating, surface properties, and magma composition across Io—although key questions remain. ▪ Atmospheric observations indicate a dominant sublimation-supported component and reinforce the presence of stealth volcanism. ▪ Observations of volcanic plumes show high gas velocities (up to ∼1 km/s) and their effect on Io's atmosphere. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Paleogeography is the study of the changing surface of Earth through time. Driven by plate tectonics, the configuration of the continents and ocean basins has been in constant flux. Plate tectonics pushes the land surface upward or pulls it apart, causing its collapse. All the while, the unrelenting forces of climate and weather slowly reduce mountains to sand and mud and redistribute these sediments to the sea. This article reviews the changing paleogeography of the past 600 million years. It describes the broad patterns of Phanerozoic paleogeography as well as many of the specific paleogeographic events that have shaped the modern continents and ocean basins. The focus is on the changing latitudinal distribution of the continents, fluctuations in sea level, the opening and closing of oceanic seaways, mountain building, and how these paleogeographic changes have affected global climate, ocean circulation, and the evolution of life. This review presents an atlas of 114 paleogeographic maps that illustrate how Earth's surface has evolved during the past 600 million years. During that time interval, Earth has witnessed the formation and breakup of two supercontinents: Pannotia and Pangea. The continents have been transformed from low-lying flooded platforms to high-standing land areas crisscrossed by the scars of past continental collisions. Oceans have opened and closed, and then opened again in a seemingly never-ending cycle. ▪ The changing configuration of the continents and ocean basins during the past 750 million years is illustrated in 114 paleogeographic maps. ▪ These maps describe how the surface of Earth has been continually modified by mountain building and erosion. ▪ The changing paleogeography has affected global climate, ocean circulation, and the evolution of life. ▪ The data and methods used to produce the maps are described in detail. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: The Cassini-Huygens mission that explored the Saturn system during the period 2004–2017 revolutionized our understanding of Titan, the only known moon with a dense atmosphere and the only body, besides Earth, with stable surface liquids. Its predominantly nitrogen atmosphere also contains a few percent of methane that is photolyzed on short geological timescales to form ethane and more complex organic molecules. The presence of a significant amount of methane and 40Ar, the decay product of 40K, argues for exchange processes from the interior to the surface. Here we review the information that constrains Titan's interior structure. Gravity and orbital data suggest that Titan is an ocean world, which implies differentiation into a hydrosphere and a rocky core. The mass and gravity data complemented by equations of state constrain the ocean density and composition as well as the hydrosphere thickness. We present end-member models, review the dynamics of each layer, and discuss the global evolution consistent with the Cassini-Huygens data. ▪ Titan is the only moon with a dense atmosphere where organic molecules are synthesized and have sedimented at the surface. ▪ The Cassini-Huygens mission demonstrated that Titan is an ocean world with an internal water shell and liquid hydrocarbon seas at the poles. ▪ Interactions between water, rock, and organics may have occurred during most of Titan's evolution, which has strong astrobiological implications. ▪ Data collected by the Dragonfly mission and comparison with the JUpiter ICy moons Explorer (JUICE) data for Ganymede will further reveal Titan's astrobiology potential. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Throughout Earth's history, CO2 is thought to have exerted a fundamental control on environmental change. Here we review and revise CO2 reconstructions from boron isotopes in carbonates and carbon isotopes in organic matter over the major climate transition of the past 66 million years. We find close coupling between CO2 and climate throughout the Cenozoic, with peak CO2 levels of ∼1,500 ppm in the Eocene greenhouse, decreasing to ∼550 ppm in the Miocene, and falling further into ice age world of the Plio–Pleistocene. Around two-thirds of Cenozoic CO2 drawdown is explained by an increase in the ratio of alkalinity to dissolved inorganic carbon, likely linked to a change in the balance of weathering to outgassing, with the remaining one-third due to changing ocean temperature and major ion composition. Earth system climate sensitivity is explored and may vary between different time intervals. The Cenozoic CO2 record highlights the truly geological scale of anthropogenic CO2 change: Current CO2 levels were last seen around 3 million years ago, and major cuts in emissions are required to prevent a return to the CO2 levels of the Miocene or Eocene in the coming century. ▪ CO2 reconstructions over the past 66 Myr from boron isotopes and alkenones are reviewed and re-evaluated. ▪ CO2 estimates from the different proxies show close agreement, yielding a consistent picture of the evolution of the ocean-atmosphere CO2 system over the Cenozoic. ▪ CO2 and climate are coupled throughout the past 66 Myr, providing broad constraints on Earth system climate sensitivity. ▪ Twenty-first-century carbon emissions have the potential to return CO2 to levels not seen since the much warmer climates of Earth's distant past. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Melanin and other pigments are now well known to be important in exceptional preservation of soft tissues in vertebrates and other animals. Because pigments confer coloration and even structural colors, they have opened a new field of paleocolor reconstruction. Since its inception about a decade ago, reconstruction of color patterns has been performed on several vertebrates, including feathered and scale-clad dinosaurs. Iridescence and other types of structural color can also be identified through melanosome shape and arrangement. How pigments and melanosomes fossilize and are altered has become an important research subject. Ancient color patterns that may range from crypsis to brilliant displays have revealed insights into the evolution and escalation of visual systems, the nature of ancient animal interactions, and how several unique characteristics of birds already arose among dinosaurs. ▪  Melanin and other pigments preserve in exceptional fossils; this opens paths for reconstructing coloration of extinct organisms, such as dinosaurs. ▪  The most abundant pigment is melanin, which can be identified chemically and through preserved melanosome microbodies. ▪  Melanosome shape reveals clues to original hue ranging from reddish brown and black to gray and structural coloration. ▪  Other pigments may preserve, such as porphyrin pigments in theropod dinosaur eggshells. ▪  Fossil color patterns contribute new insights into the evolution of visual systems, predator-prey interactions, and key innovations.

Description: The atmosphere is the synthesizer, transformer, and communicator of exchanges at its boundaries with the land and oceans. These exchanges depend on and, in turn, alter the states of the atmosphere, land, and oceans themselves. To a large extent, the interactions between the carbon cycle and climate have mapped, and will map, the trajectory of the Earth system. My quest to understand climate dynamics and the global carbon cycle has been propelled by new puzzles that emerge from each of the investigations and has led me to study subdisciplines of Earth science beyond my formal training. This article sketches my trek and the lessons I have learned. ▪  About half the CO2 emitted from combustion of fossil fuels and from cement production has remained airborne. Where are the contemporary carbon sinks? To what degree will these sinks evolve with, and in turn accelerate, climate change itself? ▪  The pursuit of these questions has been propelled by the integration of in situ and satellite observations of the atmosphere, land, and oceans, as well as by advances in theory and coupled climate–carbon cycle modeling. ▪  The urgency of climate change demands new approaches to cross-check national emission statistics.

Description: Carbonate sediments and rocks are valuable archives of Earth's past whose geochemical compositions inform our understanding of Earth's surface evolution. Yet carbonates are also reactive minerals and often undergo compositional alteration between the time of deposition and sampling and analysis. These changes may be mineralogical, structural, and/or chemical, and they are broadly referred to as diagenesis. Building on work over the past 40 years, we present an overview of key carbonate diagenesis terminology and a process-based framework for evaluating the geochemical impacts of carbonate diagenesis; we also highlight recent experimental and field observations that suggest metal isotopes as valuable diagenetic indicators. Our primary objectives are to demonstrate the value of coupling quantitative and analytical approaches, specifically with regard to metal isotopes and Mg/Ca, and to focus attention on key avenues for future work, including the role of authigenesis in impacting global geochemical cycles and the isotopic composition of the rock record. ▪  Quantitative frameworks utilizing well-understood diagenetic indicators and basic geochemical parameters allow us to assess the extent of diagenetic alteration in carbonate sediments. ▪  The reactivity, duration of reaction, and degree of isotopic or elemental/chemical disequilibrium determine the extent to which carbonates may be altered. ▪  Metal isotopic ratios (δ44Ca, δ26Mg, 87Sr/86Sr) can be used to constrain the extent and rate of carbonate recrystallization. ▪  Diagenetic signals may be globally synchronous, while diagenetic fluxes may impact global geochemical cycles.

Description: The strong ground motions, large crustal deformation, and tsunami generated by the 2011 Tohoku-oki earthquake ( Mw 9.1) reveal that a large coseismic slip likely propagated to shallow depth in the Japan Trench. Although data acquired by onshore networks cannot resolve the slip behavior of the updip fault rupture, marine geophysical and geological studies provide direct evidence of coseismic slip to the trench. Differential bathymetry data show ∼50 m of coseismic seafloor displacement extending to the central Japan Trench (38–39.2°N). Seismic data show that coseismic slip ruptured the seafloor within the trench. Pelagic clays may have promoted slip propagation to shallow depths, whereas disturbed/metamorphosed clays may have restricted slip to the main rupture zone. Those observations imply that a smooth, broadly distributed, weak, clay-rich sediment in a shallow part of a subduction zone is a characteristic factor that can foster a large coseismic slip to the trench and, consequently, the generation of a large tsunami. ▪  During the 2011 Tohoku-oki earthquake ( Mw 9.1), more than ∼50 m of slip occurred on a fault that ruptured the seafloor in the central Japan Trench. ▪  The fault rupture reaching the seafloor caused a large tsunami. ▪  Marine geophysical explorations revealed that a clay-rich sediment in the subduction zone was one factor fostering the large fault slip. ▪  Understanding of slip behavior in the shallow portion of a subduction zone will help us prepare for future large tsunamis along the Japan-Kuril Trench.

Description: The Human System is within the Earth System. They should be modeled bidirectionally coupled, as they are in reality. The Human System is rapidly expanding, mostly due to consumption of fossil fuels (approximately one million times faster than Nature accumulated them) and fossil water. This threatens not only other planetary subsystems but also the Human System itself. Carrying Capacity is an important tool to measure sustainability, but there is a widespread view that Carrying Capacity is not applicable to humans. Carrying Capacity has generally been prescribed a priori, mostly using the logistic equation. However, the real dynamics of human population and consumption are not represented by this equation or its variants. We argue that Carrying Capacity should not be prescribed but should insteadbe dynamically derived a posteriori from the bidirectional coupling of Earth System submodels with the Human System model. We demonstrate this approach with a minimal model of Human–Nature interaction (HANDY). ▪  The Human System is a subsystem of the Earth System, with inputs (resources) from Earth System sources and outputs (waste, emissions) to Earth System sinks. ▪  The Human System is growing rapidly due to nonrenewable stocks of fossil fuels and water and threatens the sustainability of the Human System and to overwhelm the Earth System. ▪  Carrying Capacity has been prescribed a priori and using the logistic equation, which does not represent the dynamics of the Human System. ▪  Our new approach to human Carrying Capacity is derived from dynamically coupled Earth System–Human System models and can be used to estimate the sustainability of the Human System.

Description: The connection between the geological record and dynamic topography driven by mantle convective flow has been established over widely varying temporal and spatial scales. As observations of the process have increased and numerical modeling of thermochemical convection has improved, a burgeoning direction of research targeting outstanding issues in ice age paleoclimate has emerged. This review focuses on studies of the Plio-Pleistocene ice age, including investigations of the stability of ice sheets during ice age warm periods and the inception of Northern Hemisphere glaciation. However, studies that have revealed nuanced connections of dynamic topography to biodiversity, ecology, ocean chemistry, and circulation since the start of the current ice-house world are also considered. In some cases, a recognition of the importance of dynamic topography resolves enigmatic events and in others it confounds already complex, unanswered questions. All such studies highlight the role of solid Earth geophysics in paleoclimate research and undermine a common assumption, beyond the field of glacial isostatic adjustment, that the solid Earth remains a rigid, passive substrate during the evolution of the ice age climate system. ▪  Dynamic topography is the large-scale, vertical deflection of Earth's crust driven by mantle convective flow. ▪  This review highlights recent research exploring the implications of the process on key issues in ice age paleoclimate. ▪  This research includes studies of ice sheet stability and inception as well as inferences of peak sea levels during periods of relative ice age warmth. ▪  This review also includes studies on longer timescales, continental-scale ecology and biodiversity, the long-term carbon cycle, and water flux across oceanic gateways.

Description: Ocean temperature variability is a fundamental component of the Earth's climate system, and extremes in this variability affect the health of marine ecosystems around the world. The study of marine heatwaves has emerged as a rapidly growing field of research, given notable extreme warm-water events that have occurred against a background trend of global ocean warming. This review summarizes the latest physical and statistical understanding of marine heatwaves based on how they are identified, defined, characterized, and monitored through remotely sensed and in situ data sets. We describe the physical mechanisms that cause marine heatwaves, along with their global distribution, variability, and trends. Finally, we discuss current issues in this developing research area, including considerations related to the choice of climatological baseline periods in defining extremes and how to communicate findings in the context of societal needs. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 4, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Hydrocarbon seeps, deep sea extreme environments where deeply sourced fluids discharge at the seabed, occur along continental margins across the globe. Energy-rich reduced substrates, namely hydrocarbons, support accelerated biogeochemical dynamics, creating unique geobiological habitats. Subseafloor geology dictates the surficial expression of seeps, generating hydrocarbon (gas and/or oil) seeps, brine seeps, and mud volcanoes. Biogeochemical processes across the redox spectrum are amplified at hydrocarbon seeps due to the abundance and diversity of reductant; anaerobic metabolism dominates within the sediment column since oxygen is consumed rapidly near the sediment surface. Microbial activity is constrained by electron acceptor availability, with rapid recycling required to support observed rates of hydrocarbon consumption. Geobiologic structures, from gas hydrate to solid asphalt to authigenic minerals, form as a result of hydrocarbon and associated fluid discharge. Animal-microbial associations and symbioses thrive at hydrocarbon seeps, generating diverse and dense deep sea oases that provide nutrition to mobile predators. ▪  Hydrocarbon seeps are abundant deep sea oases that support immense biodiversity and where specialization and adaptation create extraordinary lifestyles. ▪  Subseafloor geology shapes and defines the geochemical nature of fluid seepage and regulates the flux regime, which dictate the surface expression. ▪  High rates of anaerobic oxidation of methane require coupling to multiple processes and promote diversity in the anaerobic methanotroph microbial community. ▪  The recent discovery of novel phyla possessing hydrocarbon oxidation potential signals that aspects of seep biogeochemistry and geobiology remain to be discovered.

Description: Urban and periurban ocean developments impact 1.5% of the global exclusive economic zones, and the demand for ocean space and resources is increasing. As we strive for a more sustainable future, it is imperative that we better design, manage, and conserve urban ocean spaces for both humans and nature. We identify three key objectives for more sustainable urban oceans: reduction of urban pressures, protection and restoration of ocean ecosystems, and support of critical ecosystem services. We describe an array of emerging evidence-based approaches, including greening gray infrastructure, restoring habitats, and developing biotechnologies. We then explore new economic instruments and incentives for supporting these new approaches and evaluate their feasibility in delivering these objectives. Several of these tools have the potential to help bring nature back to the urban ocean while also addressing some of the critical needs of urban societies, such as climate adaptation, seafood production, clean water, and recreation, providing both human and environmental benefits in some of our most impacted ocean spaces. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: We have known for more than 45 years that microplastics in the ocean are carriers of microbially dominated assemblages. However, only recently has the role of microbial interactions with microplastics in marine ecosystems been investigated in detail. Research in this field has focused on three main areas: ( a) the establishment of plastic-specific biofilms (the so-called plastisphere); ( b) enrichment of pathogenic bacteria, particularly members of the genus Vibrio, coupled to a vector function of microplastics; and ( c) the microbial degradation of microplastics in the marine environment. Nevertheless, the relationships between marine microorganisms and microplastics remain unclear. In this review, we deduce from the current literature, new comparative analyses, and considerations of microbial adaptation concerning plastic degradation that interactions between microorganisms and microplastic particles should have rather limited effects on the ocean ecosystems. The majority of microorganisms growing on microplastics seem to belong to opportunistic colonists that do not distinguish between natural and artificial surfaces. Thus, microplastics do not pose a higher risk than natural particles to higher life forms by potentially harboring pathogenic bacteria. On the other hand, microplastics in the ocean represent recalcitrant substances for microorganisms that are insufficient to support prokaryotic metabolism and will probably not be microbially degraded in any period of time relevant to human society. Because we cannot remove microplastics from the ocean, proactive action regarding research on plastic alternatives and strategies to prevent plastic entering the environment should be taken promptly.

Description: Reactive oxygen species (ROS) are produced ubiquitously across the tree of life. Far from being synonymous with toxicity and harm, biological ROS production is increasingly recognized for its essential functions in signaling, growth, biological interactions, and physiochemical defense systems in a diversity of organisms, spanning microbes to mammals. Part of this shift in thinking can be attributed to the wide phylogenetic distribution of specialized mechanisms for ROS production, such as NADPH oxidases, which decouple intracellular and extracellular ROS pools by directly catalyzing the reduction of oxygen in the surrounding aqueous environment. Furthermore, biological ROS production contributes substantially to natural fluxes of ROS in the ocean, thereby influencing the fate of carbon, metals, oxygen, and climate-relevant gases. Here, we review the taxonomic diversity, mechanisms, and roles of extracellular ROS production in marine bacteria, phytoplankton, seaweeds, and corals, highlighting the ecological and biogeochemical influences of this fundamental and remarkably widespread process. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 4, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Oceanic uptake of anthropogenic carbon dioxide (CO2) from the atmosphere has changed ocean biogeochemistry and threatened the health of organisms through a process known as ocean acidification (OA). Such large-scale changes affect ecosystem functions and can have effects on societal uses, fisheries resources, and economies. In many large estuaries, anthropogenic CO2-induced acidification is enhanced by strong stratification, long water residence times, eutrophication, and a weak acid–base buffer capacity. In this article, we review how a variety of processes influence aquatic acid–base properties in estuarine waters, including river–ocean mixing, upwelling, air–water gas exchange, biological production and subsequent respiration, anaerobic respiration, calcium carbonate (CaCO3) dissolution, and benthic inputs. We emphasize the spatial and temporal dynamics of partial pressure of CO2 ( pCO2), pH, and calcium carbonate mineral saturation states. Examples from three large estuaries—Chesapeake Bay, the Salish Sea, and Prince William Sound—are used to illustrate how natural and anthropogenic processes and climate change may manifest differently across estuaries, as well as the biological implications of OA on coastal calcifiers. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 4, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: The Deepwater Horizon oil spill was the largest, longest-lasting, and deepest oil accident to date in US waters. As oil and natural gas jetted from release points at 1,500-m depth in the northern Gulf of Mexico, entrainment of the surrounding ocean water into a buoyant plume, rich in soluble hydrocarbons and dispersed microdroplets of oil, created a deep (1,000-m) intrusion layer. Larger droplets of liquid oil rose to the surface, forming a slick of mostly insoluble, hydrocarbon-type compounds. A variety of physical, chemical, and biological mechanisms helped to transform, remove, and redisperse the oil and gas that was released. Biodegradation removed up to 60% of the oil in the intrusion layer but was less efficient in the surface slick, due to nutrient limitation. Photochemical processes altered up to 50% (by mass) of the floating oil. The surface oil expression changed daily due to wind and currents, whereas the intrusion layer flowed southwestward. A portion of the weathered surface oil stranded along shorelines. Oil from both surface and intrusion layers were deposited onto the seafloor via sinking marine oil snow. The biodegradation rates of stranded or sedimented oil were low, with resuspension and redistribution transiently increasing biodegradation. The subsequent research efforts increased our understanding of the fate of spilled oil immensely, with novel insights focusing on the importance of photooxidation, the microbial communities driving biodegradation, and the formation of marine oil snow that transports oil to the seafloor. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 4, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: The dissolution of CaCO3 minerals in the ocean is a fundamental part of the marine alkalinity and carbon cycles. While there have been decades of work aimed at deriving the relationship between dissolution rate and mineral saturation state (a so-called rate law), no real consensus has been reached. There are disagreements between laboratory- and field-based studies and differences in rates for inorganic and biogenic materials. Rates based on measurements on suspended particles do not always agree with rates inferred from measurements made near the sediment–water interface of the actual ocean. By contrast, the freshwater dissolution rate of calcite has been well described by bulk rate measurements from a number of different laboratories, fit by basic kinetic theory, and well studied by atomic force microscopy and vertical scanning interferometry to document the processes at the atomic scale. In this review, we try to better unify our understanding of carbonate dissolution in the ocean via a relatively new, highly sensitive method we have developed combined with a theoretical framework guided by the success of the freshwater studies. We show that empirical curve fits of seawater data as a function of saturation state do not agree, largely because the curvature is itself a function of the thermodynamics. Instead, we show that models that consider both surface energetic theory and the complicated speciation of seawater and calcite surfaces in seawater are able to explain most of the most recent data. This new framework can also explain features of the historical data that have not been previously explained. The existence of a kink in the relationship between rate and saturation state, reflecting a change in dissolution mechanism, may be playing an important role in accelerating CaCO3 dissolution in key sedimentary environments. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Tides are changing worldwide at rates not explained by astronomical forcing. Rather, the observed evolution of tides and other long waves, such as storm surges, is influenced by shelf processes and changes to the roughness, depth, width, and length of embayments, estuaries, and tidal rivers. In this review, we focus on processes in estuaries and tidal rivers, because that is where the largest changes to tidal properties are occurring. Recent literature shows that changes in tidal amplitude have been ubiquitous worldwide over the past century, often in response to wetland reclamation, channel dredging, and other environmental changes. While tidal amplitude changes are sometimes slight (

Description: Monitoring Earth's energy imbalance requires monitoring changes in the heat content of the ocean. Recent observational estimates indicate that ocean heat uptake is accelerating in the twenty-first century. Examination of estimates of ocean heat uptake over the industrial era, the Common Era of the last 2,000 years, and the period since the Last Glacial Maximum, 20,000 years ago, permits a wide perspective on modern-day warming rates. In addition, this longer-term focus illustrates how the dynamics of the deep ocean and the cryosphere were active in the past and are still active today. The large climatic shifts that started with the melting of the great ice sheets have involved significant ocean heat uptake that was sustained over centuries and millennia, and modern-ocean heat content changes are small by comparison. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: In this article, we analyze the impacts of climate change on Antarctic marine ecosystems. Observations demonstrate large-scale changes in the physical variables and circulation of the Southern Ocean driven by warming, stratospheric ozone depletion, and a positive Southern Annular Mode. Alterations in the physical environment are driving change through all levels of Antarctic marine food webs, which differ regionally. The distributions of key species, such as Antarctic krill, are also changing. Differential responses among predators reflect differences in species ecology. The impacts of climate change on Antarctic biodiversity will likely vary for different communities and depend on species range. Coastal communities and those of sub-Antarctic islands, especially range-restricted endemic communities, will likely suffer the greatest negative consequences of climate change. Simultaneously, ecosystem services in the Southern Ocean will likely increase. Such decoupling of ecosystem services and endemic species will require consideration in the management of human activities such as fishing in Antarctic marine ecosystems.

Description: In the last few decades, numerous studies have investigated the impacts of simulated ocean acidification on marine species and communities, particularly those inhabiting dynamic coastal systems. Despite these research efforts, there are many gaps in our understanding, particularly with respect to physiological mechanisms that lead to pathologies. In this review, we trace how carbonate system disturbances propagate from the coastal environment into marine invertebrates and highlight mechanistic links between these disturbances and organism function. We also point toward several processes related to basic invertebrate biology that are severely understudied and prevent an accurate understanding of how carbonate system dynamics influence organismic homeostasis and fitness-related traits. We recommend that significant research effort be directed to studying cellular phenotypes of invertebrates acclimated or adapted to elevated seawater pCO2 using biochemical and physiological methods.

Description: Photosynthesis evolved in the ocean more than 2 billion years ago and is now performed by a wide range of evolutionarily distinct organisms, including both prokaryotes and eukaryotes. Our appreciation of their abundance, distributions, and contributions to primary production in the ocean has been increasing since they were first discovered in the seventeenth century and has now been enhanced by data emerging from the Tara Oceans project, which performed a comprehensive worldwide sampling of plankton in the upper layers of the ocean between 2009 and 2013. Largely using recent data from Tara Oceans, here we review the geographic distributions of phytoplankton in the global ocean and their diversity, abundance, and standing stock biomass. We also discuss how omics-based information can be incorporated into studies of photosynthesis in the ocean and show the likely importance of mixotrophs and photosymbionts.

Description: This narrative is a personal account of my evolution as a student of phytoplankton and the ocean. Initially I focused on phytoplankton nutrient physiology and uptake, later switching to photosynthetic physiology. Better models of photosynthesis naturally require a better understanding of spectral underwater light fields and absorption coefficients, which precipitated my involvement in the nascent field of bio-optical oceanography. Establishment of the now 34-year-old summer graduate course in ocean optics, which continues to attract students from around the globe, is a legacy of my jumping into optics. The importance of social interactions in advancing science cannot be underestimated; a prime example is how a TGIF gathering led to my immersion in the world of autonomous underwater vehicles for the past two decades of my career. Working with people who you like and respect is also critical; I believe collegial friendship played a major role in the great success of the 2008 North Atlantic Bloom Experiment.

Description: Implementation of marine conservation strategies, such as increasing the numbers, extent, and effectiveness of protected areas (PAs), can help achieve conservation and restoration of ocean health and associated goods and services. Despite increasing recognition of the importance of including aspects of ecological functioning in PA design, the physical characteristics of habitats and simple measures of species diversity inform most PA designations. Marine and terrestrial ecologists have recently used biological traits to assess community dynamics, functioning, and vulnerability to anthropogenic impacts. Here, we explore potential trait-based marine applications to advance PA design. We recommend strategies to integrate biological traits into ( a) conservation objectives (e.g., by assessing and predicting impacts and vulnerability), ( b) PA spatial planning (e.g., mapping ecosystem functions and functional diversity hot spots), and ( c) time series monitoring protocols (e.g., using functional traits to detect recoveries). We conclude by emphasizing the need for pragmatic tools to improve the efficacy of spatial planning and monitoring efforts. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: With the decline of reef-building corals on tropical reefs, sponges have emerged as an important component of changing coral reef ecosystems. Seemingly simple, sponges are highly diverse taxonomically, morphologically, and in terms of their relationships with symbiotic microbes, and they are one of nature's richest sources of novel secondary metabolites. Unlike most other benthic organisms, sponges have the capacity to disrupt boundary flow as they pump large volumes of seawater into the water column. This seawater is chemically transformed as it passes through the sponge body as a consequence of sponge feeding, excretion, and the activities of microbial symbionts, with important effects on carbon and nutrient cycling and on the organisms in the water column and on the adjacent reef. In this review, we critically evaluate developments in the recently dynamic research area of sponge ecology on tropical reefs and provide a perspective for future studies.

Description: Bays in coastal upwelling regions are physically driven and biochemically fueled by their interaction with open coastal waters. Wind-driven flow over the shelf imposes a circulation in the bay, which is also influenced by local wind stress and thermal bay–ocean density differences. Three types of bays are recognized based on the degree of exposure to coastal currents and winds (wide-open bays, square bays, and elongated bays), and the characteristic circulation and stratification patterns of each type are described. Retention of upwelled waters in bays allows for dense phytoplankton blooms that support productive bay ecosystems. Retention is also important for the accumulation of larvae, which accounts for high recruitment in bays. In addition, bays are coupled to the shelf ecosystem through export of plankton-rich waters during relaxation events. Ocean acidification and deoxygenation are a concern in bays because local extrema can develop beneath strong stratification.

Description: Much of the global cooling during ice ages arose from changes in ocean carbon storage that lowered atmospheric CO2. A slew of mechanisms, both physical and biological, have been proposed as key drivers of these changes. Here we discuss the current understanding of these mechanisms with a focus on how they altered the theoretically defined soft-tissue and biological disequilibrium carbon storage at the peak of the last ice age. Observations and models indicate a role for Antarctic sea ice through its influence on ocean circulation patterns, but other mechanisms, including changes in biological processes, must have been important as well, and may have been coordinated through links with global air temperature. Further research is required to better quantify the contributions of the various mechanisms, and there remains great potential to use the Last Glacial Maximum and the ensuing global warming as natural experiments from which to learn about climate-driven changes in the marine ecosystem.

Description: More than two-thirds of global biomass consists of vascular plants. A portion of the detritus they generate is carried into the oceans from land and highly productive blue carbon ecosystems—salt marshes, mangrove forests, and seagrass meadows. This large detrital input receives scant attention in current models of the global carbon cycle, though for blue carbon ecosystems, increasingly well-constrained estimates of biomass, productivity, and carbon fluxes, reviewed in this article, are now available. We show that the fate of this detritus differs markedly from that of strictly marine origin, because the former contains lignocellulose—an energy-rich polymer complex of cellulose, hemicelluloses, and lignin that is resistant to enzymatic breakdown. This complex can be depolymerized for nutritional purposes by specialized marine prokaryotes, fungi, protists, and invertebrates using enzymes such as glycoside hydrolases and lytic polysaccharide monooxygenases to release sugar monomers. The lignin component, however, is less readily depolymerized, and detritus therefore becomes lignin enriched, particularly in anoxic sediments, and forms a major carbon sink in blue carbon ecosystems. Eventual lignin breakdown releases a wide variety of small molecules that may contribute significantly to the oceanic pool of recalcitrant dissolved organic carbon. Marine carbon fluxes and sinks dependent on lignocellulosic detritus are important ecosystem services that are vulnerable to human interventions. These services must be considered when protecting blue carbon ecosystems and planning initiatives aimed at mitigating anthropogenic carbon emissions.

Description: Jellyfish have provided insight into important components of animal propulsion, such as suction thrust, passive energy recapture, vortex wall effects, and the rotational mechanics of turning. These traits are critically important to jellyfish because they must propel themselves despite severe limitations on force production imposed by rudimentary cnidarian muscular structures. Consequently, jellyfish swimming can occur only by careful orchestration of fluid interactions. Yet these mechanics may be more broadly instructive because they also characterize processes shared with other animal swimmers, whose structural and neurological complexity can obscure these interactions. In comparison with other animal models, the structural simplicity, comparative energetic efficiency, and ease of use in laboratory experimentation allow jellyfish to serve as favorable test subjects for exploration of the hydrodynamic bases of animal propulsion. These same attributes also make jellyfish valuable models for insight into biomimetic or bioinspired engineering of swimming vehicles. Here, we review advances in understanding of propulsive mechanics derived from jellyfish models as a pathway toward the application of animal mechanics to vehicle designs. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Over the past several decades, there has developed a community-wide appreciation for the importance of mixing at the smallest scales to geophysical fluid dynamics on all scales. This appreciation has spawned greater participation in the investigation of ocean mixing and new ways to measure it. These are welcome developments given the tremendous separation in scales between the basins, ?(107) m, and the turbulence, ? (10−2) m, and the fact that turbulence that leads to thermodynamically irreversible mixing in high-Reynolds-number geophysical flows varies by at least eight orders of magnitude in both space and time. In many cases, it is difficult to separate the dependencies because measurements are sparse, also in both space and time. Comprehensive shipboard turbulence profiling experiments supplemented by Doppler sonar current measurements provide detailed observations of the evolution of the vertical structure of upper-ocean turbulence on timescales of minutes to weeks. Recent technical developments now permit measurements of turbulence in the ocean, at least at a few locations, for extended periods. This review summarizes recent and classic results in the context of our expanding knowledge of the temporal variability of ocean mixing, beginning with a discussion of the timescales of the turbulence itself (seconds to minutes) and how turbulence-enhanced mixing varies over hours, days, tidal cycles, monsoons, seasons, and El Niño–Southern Oscillation timescales (years). Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Apex predators play pivotal roles in marine ecosystems, mediated principally through diet and nutrition. Yet, compared with terrestrial animals, the nutritional ecology of marine predators is poorly understood. One reason is that the field has adhered to an approach that evaluates diet principally in terms of energy gain. Studies in terrestrial systems, by contrast, increasingly adopt a multidimensional approach, the nutritional geometry framework, that distinguishes specific nutrients and calories. We provide evidence that a nutritional approach is likewise relevant to marine apex predators, then demonstrate how nutritional geometry can characterize the nutrient and energy content of marine prey. Next, we show how this framework can be used to reconceptualize ecological interactions via the ecological niche concept, and close with a consideration of its application to problems in marine predator research.

Description: My career spanned the revolution in understanding of the large-scale fluid ocean, as modern electronics produced vast new capabilities. I started in the days of almost purely mechanical instruments operated by seagoing scientists, ones not so different from those used more than a century earlier. Elegant theories existed of hypothetical steady-state oceans. Today, we understand that the ocean is a highly turbulent fluid, interacting over global scales, and it is now studied by large teams using spacecraft and diverse sets of self-contained in situ instrumentation. Mine was an accidental career: I was lucky to be in the right place at the right time. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: While the ocean has suffered many losses, there is increasing evidence that important progress is being made in marine conservation. Examples include striking recoveries of once-threatened species, increasing rates of protection of marine habitats, more sustainably managed fisheries and aquaculture, reductions in some forms of pollution, accelerating restoration of degraded habitats, and use of the ocean and its habitats to sequester carbon and provide clean energy. Many of these achievements have multiple benefits, including improved human well-being. Moreover, better understanding of how to implement conservation strategies effectively, new technologies and databases, increased integration of the natural and social sciences, and use of indigenous knowledge promise continued progress. Enormous challenges remain, and there is no single solution; successful efforts typically are neither quick nor cheap and require trust and collaboration. Nevertheless, a greater focus on solutions and successes will help them to become the norm rather than the exception. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Advances in technologies for molecular observation are leading to novel types of data, including gene, transcript, protein, and metabolite levels, which are fundamentally different from the types traditionally compared with microbial ecosystem models, such as biomass (e.g., chlorophyll a) and nutrient concentrations. A grand challenge is to use these data to improve predictive models and use models to explain observed patterns. This article presents a framework that aligns observations and models along the dimension of abstraction or biological organization—from raw sequences to ecosystem patterns for observations, and from sequence simulators to ecological theory for models. It then reviews 16 studies that compared model results with molecular observations. Molecular data can and are being combined with microbial ecosystem models, but to keep up with and take advantage of the full scope of observations, models need to become more mechanistically detailed and complex, which is a technical and cultural challenge for the ecological modeling community.

Description: Climate change affects ecological processes and interactions, including parasitism. Because parasites are natural components of ecological systems, as well as agents of outbreak and disease-induced mortality, it is important to summarize current knowledge of the sensitivity of parasites to climate and identify how to better predict their responses to it. This need is particularly great in marine systems, where the responses of parasites to climate variables are less well studied than those in other biomes. As examples of climate's influence on parasitism increase, they enable generalizations of expected responses as well as insight into useful study approaches, such as thermal performance curves that compare the vital rates of hosts and parasites when exposed to several temperatures across a gradient. For parasites not killed by rising temperatures, some simple physiological rules, including the tendency of temperature to increase the metabolism of ectotherms and increase oxygen stress on hosts, suggest that parasites’ intensity and pathologies might increase. In addition to temperature, climate-induced changes in dissolved oxygen, ocean acidity, salinity, and host and parasite distributions also affect parasitism and disease, but these factors are much less studied. Finally, because parasites are constituents of ecological communities, we must consider indirect and secondary effects stemming from climate-induced changes in host–parasite interactions, which may not be evident if these interactions are studied in isolation. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: The biogeochemical cycles of trace elements and their isotopes (TEIs) constitute an active area of oceanographic research due to their role as essential nutrients for marine organisms and their use as tracers of oceanographic processes. Selected TEIs also provide diagnostic information about the physical, geological, and chemical processes that supply or remove solutes in the ocean. Many of these same TEIs provide information about ocean conditions in the past, as their imprint on marine sediments can be interpreted to reflect changes in ocean circulation, biological productivity, the ocean carbon cycle, and more. Other TEIs have been introduced as the result of human activities and are considered contaminants. The development and implementation of contamination-free methods for collecting and analyzing samples for TEIs revolutionized marine chemistry, revealing trace element distributions with oceanographically consistent features and new insights about the processes regulating them. Despite these advances, the volume and geographic coverage of high-quality TEI data by the end of the twentieth century were insufficient to constrain their global biogeochemical cycles. To accelerate progress in this field of research, marine geochemists developed a coordinated international effort to systematically study the marine biogeochemical cycles of TEIs—the GEOTRACES program. Following a decade of planning and implementation, GEOTRACES launched its main field effort in 2010. This review, roughly midway through the field program, summarizes the steps involved in designing the program, its management structure, and selected findings.

Description: Compared with terrestrial ecosystems, marine ecosystems have a higher proportion of heterotrophic biomass. Building from this observation, we define the North Atlantic biome as the region where the large, lipid-rich copepod Calanus finmarchicus is the dominant mesozooplankton species. This species is superbly adapted to take advantage of the intense pulse of productivity associated with the North Atlantic spring bloom. Most of the characteristic North Atlantic species, including cod, herring, and right whales, rely on C. finmarchicus either directly or indirectly. The notion of a biome rests inherently on an assumption of stability, yet conditions in the North Atlantic are anything but stable. Humans have reduced the abundance of many fish and whales (though some recovery is underway). Humans are also introducing physical and chemical trends associated with global climate change. Thus, the future of the North Atlantic depends on the biome's newest species, Homo sapiens.

Description: Glacial–interglacial cycles have constituted a primary mode of climate variability over the last 2.6 million years of Earth's history. While glacial periods cannot be seen simply as a reverse analogue of future warming, they offer an opportunity to test our understanding of the response of precipitation patterns to a much wider range of conditions than we have been able to directly observe. This review explores key features of precipitation patterns associated with glacial climates, which include drying in large regions of the tropics and wetter conditions in substantial parts of the subtropics and midlatitudes. I describe the evidence for these changes and examine the potential causes of hydrological changes during glacial periods. Central themes that emerge include the importance of atmospheric circulation changes in determining glacial–interglacial precipitation changes at the regional scale, the need to take into account climatic factors beyond local precipitation amount when interpreting proxy data, and the role of glacial conditions in suppressing the strength of Northern Hemisphere monsoon systems.

Description: One major objective of aquatic microbial ecology is to understand the distribution of microbial populations over space and time and in response to environmental factors. Perhaps more importantly, it is crucial to quantify how those microbial cells affect biogeochemical processes of interest, such as primary production, nitrogen cycling, or the breakdown of pollutants. One valuable approach to link microbial identity to activity is to carry out incubations with stable-isotope-labeled substrates and then quantify the isotope incorporation by individual microbial cells using nanoscale secondary ion mass spectrometry (NanoSIMS). This review summarizes recent efforts in this field, highlights novel methods, describes studies investigating rare metabolisms as well as widespread microbial activity, and hopes to provide a framework to increase the use and capabilities of NanoSIMS for microbial biogeochemical studies in the future.

Description: Biogeochemical-Argo (BGC-Argo) is a network of profiling floats carrying sensors that enable observation of as many as six essential biogeochemical and bio-optical variables: oxygen, nitrate, pH, chlorophyll a, suspended particles, and downwelling irradiance. This sensor network represents today's most promising strategy for collecting temporally and vertically resolved observations of biogeochemical properties throughout the ocean. All data are freely available within 24 hours of transmission. These data fill large gaps in ocean-observing systems and support three ambitions: gaining a better understanding of biogeochemical processes (e.g., the biological carbon pump and air–sea CO2 exchanges) and evaluating ongoing changes resulting from increasing anthropogenic pressure (e.g., acidification and deoxygenation); managing the ocean (e.g., improving the global carbon budget and developing sustainable fisheries); and carrying out exploration for potential discoveries. The BGC-Argo network has already delivered extensive high-quality global data sets that have resulted in unique scientific outcomes from regional to global scales. With the proposed expansion of BGC-Argo in the near future, this network has the potential to become a pivotal observation system that links satellite and ship-based observations in a transformative manner.

Description: Experimental evolution and the associated theory are underutilized in marine microbial studies; the two fields have developed largely in isolation. Here, we review evolutionary tools for addressing four key areas of ocean global change biology: linking plastic and evolutionary trait changes, the contribution of environmental variability to determining trait values, the role of multiple environmental drivers in trait change, and the fate of populations near their tolerance limits. Wherever possible, we highlight which data from marine studies could use evolutionary approaches and where marine model systems can advance our understanding of evolution. Finally, we discuss the emerging field of marine microbial experimental evolution. We propose a framework linking changes in environmental quality (defined as the cumulative effect on population growth rate) with population traits affecting evolutionary potential, in order to understand which evolutionary processes are likely to be most important across a range of locations for different types of marine microbes.

Description: The geographic distributions of marine species are changing rapidly, with leading range edges following climate poleward, deeper, and in other directions and trailing range edges often contracting in similar directions. These shifts have their roots in fine-scale interactions between organisms and their environment—including mosaics and gradients of temperature and oxygen—mediated by physiology, behavior, evolution, dispersal, and species interactions. These shifts reassemble food webs and can have dramatic consequences. Compared with species on land, marine species are more sensitive to changing climate but have a greater capacity for colonization. These differences suggest that species cope with climate change at different spatial scales in the two realms and that range shifts across wide spatial scales are a key mechanism at sea. Additional research is needed to understand how processes interact to promote or constrain range shifts, how the dominant responses vary among species, and how the emergent communities of the future ocean will function.

Description: Microbes in marine sediments represent a large portion of the biosphere, and resolving their ecology is crucial for understanding global ocean processes. Single-gene diversity surveys have revealed several uncultured lineages that are widespread in ocean sediments and whose ecological roles are unknown, and advancements in the computational analysis of increasingly large genomic data sets have made it possible to reconstruct individual genomes from complex microbial communities. Using these metagenomic approaches to characterize sediments is transforming our view of microbial communities on the ocean floor and the biodiversity of the planet. In recent years, marine sediments have been a prominent source of new lineages in the tree of life. The incorporation of these lineages into existing phylogenies has revealed that many belong to distinct phyla, including archaeal phyla that are advancing our understanding of the origins of cellular complexity and eukaryotes. Detailed comparisons of the metabolic potentials of these new lineages have made it clear that uncultured bacteria and archaea are capable of mediating key previously undescribed steps in carbon and nutrient cycling. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Polysaccharides are major components of macroalgal and phytoplankton biomass and constitute a large fraction of the organic matter produced and degraded in the ocean. Until recently, however, our knowledge of marine polysaccharides was limited due to their great structural complexity, the correspondingly complicated enzymatic machinery used by microbial communities to degrade them, and a lack of readily applied means to isolate and characterize polysaccharides in detail. Advances in carbohydrate chemistry, bioinformatics, molecular ecology, and microbiology have led to new insights into the structures of polysaccharides, the means by which they are degraded by bacteria, and the ecology of polysaccharide production and decomposition. Here, we survey current knowledge, discuss recent advances, and present a new conceptual model linking polysaccharide structural complexity and abundance to microbially driven mechanisms of polysaccharide processing. We conclude by highlighting specific future research foci that will shed light on this central but poorly characterized component of the marine carbon cycle. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Salt marshes are recognized as valuable resources that are threatened by climate change and human activities. Better management and planning for these ecosystems will depend on understanding which marshes are most vulnerable, what is driving their change, and what their future trajectory is likely to be. Both observations and models have provided inconsistent answers to these questions, likely in part because of comparisons among sites and/or models that differ significantly in their characteristics and processes. Some of these differences almost certainly arise from processes that are not fully accounted for in marsh morphodynamic models. Here, we review distinguishing properties of marshes, important processes missing from many morphodynamic models, and key measurements missing from many observational studies. We then suggest some comparisons between models and observations that will provide critical tests and insights to improve our ability to forecast future change in these coastal landscapes.

Description: Ocean ecosystems are experiencing unprecedented rates of climate and anthropogenic change, which can often initiate stress in marine organisms. Symbioses, or associations between different organisms, are plentiful in the ocean and could play a significant role in facilitating organismal adaptations to stressful ocean conditions. This article reviews current knowledge about the role of symbiosis in marine organismal acclimation and adaptation. It discusses stress and adaptations in symbioses from coral reef ecosystems, which are among the most affected environments in the ocean, including the relationships between corals and microalgae, corals and bacteria, anemones and clownfish, and cleaner fish and client fish. Despite the importance of this subject, knowledge of how marine organisms adapt to stress is still limited, and there are vast opportunities for research and technological development in this area. Attention to this subject will enhance our understanding of the capacity of symbioses to alleviate organismal stress in the oceans.

Description: The interaction of coral reefs, both chemically and physically, with the surrounding seawater is governed, at the smallest scales, by turbulence. Here, we review recent progress in understanding turbulence in the unique setting of coral reefs—how it influences flow and the exchange of mass and momentum both above and within the complex geometry of coral reef canopies. Flow above reefs diverges from canonical rough boundary layers due to their large and highly heterogeneous roughness and the influence of surface waves. Within coral canopies, turbulence is dominated by large coherent structures that transport momentum both into and away from the canopy, but it is also generated at smaller scales as flow is forced to move around branches or blades, creating wakes. Future work interpreting reef-related observations or numerical models should carefully consider the influence that spatial variation has on momentum and scalar flux. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Millions of tons of oil are spilled in aquatic environments every decade, and this oil has the potential to greatly impact fish populations. Here, we review available information on the physiological effects of oil and polycyclic aromatic hydrocarbons on fish. Oil toxicity affects multiple biological systems, including cardiac function, cholesterol biosynthesis, peripheral and central nervous system function, the stress response, and osmoregulatory and acid–base balance processes. We propose that cholesterol depletion may be a significant contributor to impacts on cardiac, neuronal, and synaptic function as well as reduced cortisol production and release. Furthermore, it is possible that intracellular calcium homeostasis—a part of cardiotoxic and neuronal function that is affected by oil exposure—may be related to cholesterol depletion. A detailed understanding of oil impacts and affected physiological processes is emerging, but knowledge of their combined effects on fish in natural habitats is largely lacking. We identify key areas deserving attention in future research. Expected final online publication date for the Annual Review of Marine Science, Volume 13 is January 3, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: If we accept that a critical condition for plate tectonics is the creation and maintenance of a global network of narrow boundaries separating multiple plates, then to argue for plate tectonics during the Archean requires more than a local record of subduction. A case is made for plate tectonics back to the early Paleoproterozoic, when a cycle of breakup and collision led to formation of the supercontinent Columbia, and bimodal metamorphism is registered globally. Before this, less preserved crust and survivorship bias become greater concerns, and the geological record may yield only a lower limit on the emergence of plate tectonics. Higher mantle temperature in the Archean precluded or limited stable subduction, requiring a transition to plate tectonics from another tectonic mode. This transition is recorded by changes in geochemical proxies and interpreted based on numerical modeling. Improved understanding of the secular evolution of temperature and water in the mantle are key targets for future research. ▪ Higher mantle temperature in the Archean precluded or limited stable subduction, requiring a transition to plate tectonics from another tectonic mode. ▪ Plate tectonics can be demonstrated on Earth since the early Paleoproterozoic (since c. 2.2 Ga), but before the Proterozoic Earth's tectonic mode remains ambiguous. ▪ The Mesoarchean to early Paleoproterozoic (3.2–2.3 Ga) represents a period of transition from an early tectonic mode (stagnant or sluggish lid) to plate tectonics. ▪ The development of a global network of narrow boundaries separating multiple plates could have been kick-started by plume-induced subduction. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: At present, meteorites collected in Antarctica dominate the total number of the world's known meteorites. We focus here on the scientific advances in cosmochemistry and planetary science that have been enabled by access to, and investigations of, these Antarctic meteorites. A meteorite recovered during one of the earliest field seasons of systematic searches, Elephant Moraine (EET) 79001, was identified as having originated on Mars based on the composition of gases released from shock melt pockets in this rock. Subsequently, the first lunar meteorite, Allan Hills (ALH) 81005, was also recovered from the Antarctic. Since then, many more meteorites belonging to these two classes of planetary meteorites, as well as other previously rare or unknown classes of meteorites (particularly primitive chondrites and achondrites), have been recovered from Antarctica. Studies of these samples are providing unique insights into the origin and evolution of the Solar System and planetary bodies. ▪ Antarctic meteorites dominate the inventory of the world's known meteorites and provide access to new types of planetary and asteroidal materials. ▪ The first meteorites recognized to be of lunar and martian origin were collected from Antarctica and provided unique constraints on the evolution of the Moon and Mars. ▪ Previously rare or unknown classes of meteorites have been recovered from Antarctica and provide new insights into the origin and evolution of the Solar System. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Continuously operating global positioning system sites in the North Island of New Zealand have revealed a diverse range of slow motion earthquakes on the Hikurangi subduction zone. These slow slip events (SSEs) exhibit diverse characteristics, from shallow (1 year), less frequent (approximately every 5 years) SSEs in the southern part of the subduction zone. Hikurangi SSEs show intriguing relationships to interseismic coupling, seismicity, and tectonic tremor, and they exhibit a diversity of interactions with large, regional earthquakes. Due to the marked along-strike variations in Hikurangi SSE characteristics, which coincide with changes in physical characteristics of the subduction margin, the Hikurangi subduction zone presents a globally unique natural laboratory to resolve outstanding questions regarding the origin of episodic, slow fault slip behavior. ▪ New Zealand's Hikurangi subduction zone hosts slow slip events with a diverse range of depth, size, duration, and recurrence characteristics. ▪ Hikurangi slow slip events show intriguing relationships with seismicity from small earthquakes and tremor to larger earthquakes. ▪ Slow slip events play a major role in the accommodation of plate motion at the Hikurangi subduction zone. ▪ Many aspects of the Hikurangi subduction zone make it an ideal natural laboratory to resolve the physical processes controlling slow slip. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Groundwater is a crucial resource for current and future generations, but it is not being sustainably used in many parts of the world. The objective of this review is to provide a clear portrait of global-scale groundwater sustainability, systems, and resources in the Anthropocene to inspire a pivot toward more sustainable pathways of groundwater use. We examine groundwater from three different but related perspectives of sustainability science, natural resource governance and management, and Earth System science. An Earth System approach thus highlights the connections between groundwater and the other parts of the system and how these connections are impacting, or are impacted by, groundwater pumping. Groundwater is the largest store of unfrozen freshwater on Earth and is heterogeneously connected to many Earth System processes on different timescales. We propose a definition of groundwater sustainability that has a direct link with observable data, governance, and management as well as the crucial functions and services of groundwater. ▪ Groundwater is depleted or contaminated in some regions; it is ubiquitously distributed, which, importantly, makes it broadly accessible but also slow and invisible and therefore challenging to govern and manage. ▪ Regional differences in priorities, hydrology, politics, culture, and economic contexts mean that different governance and management tools are important, but a global perspective can support higher level international policies in an increasingly globalized world that require broader analysis of interconnections and knowledge transfer between regions. ▪ A coherent, overarching framework of groundwater sustainability is more important for groundwater governance and management than the concepts of safe yield, renewability, depletion, or stress. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Severe climatic and environmental changes are far more prevalent in Earth history than major extinction events, and the relationship between environmental change and extinction severity has important implications for the outcome of the ongoing anthropogenic extinction event. The response of mineralized marine plankton to environmental change offers an interesting contrast to the overall record of marine biota, which is dominated by benthic invertebrates. Here, we summarize changes in the species diversity of planktic foraminifera and calcareous nannoplankton over the Mesozoic–Cenozoic and that of radiolarians and diatoms over the Cenozoic. We find that, aside from the Triassic–Jurassic and Cretaceous–Paleogene mass extinction events, extinction in the plankton is decoupled from that in the benthos. Extinction in the plankton appears to be driven primarily by major climatic shifts affecting water column stratification, temperature, and, perhaps, chemistry. Changes that strongly affect the benthos, such as acidification and anoxia, have little effect on the plankton or are associated with radiation. ▪ Fossilizing marine plankton provide some of the most highly temporally and taxonomically resolved records of biodiversity since the Mesozoic. ▪ The record of extinction and origination in the plankton differs from the overall marine biodiversity record in revealing ways. ▪ Changes to water column stratification and global circulation are the main drivers of plankton diversity. ▪ Anoxia, acidification, and eutrophication (which strongly influence total marine fossil diversity) are less important in the plankton. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Jupiter is in the class of planets that we call gas giants, not because they consist of gas but because they were primarily made from hydrogen-helium gas, which upon gravitational compression becomes a metallic fluid.  Juno, in orbit about Jupiter since 2016, has changed our view: The gravity data are much improved, and the simplest interpretation of the higher order even harmonics implies that the planet may have a diluted central concentration of heavy elements.  Jupiter has strong winds extending to perhaps ∼3,000-km depth that are evident in the odd zonal harmonics of the gravity field. Jupiter's distinctive magnetic field displays some limited local structure, most notably the Great Blue Spot (a region of downward flux near the equator), and some evidence for secular variation, possibly arising from the winds. However, Juno is ongoing; it has not answered all questions and has posed new ones. ▪ Juno's mission reveals Jupiter's interior. ▪ A core exists but is diluted by hydrogen. ▪ The mission revealed wind depth and magnetic field. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: A remarkable diversity of plant-eating mammals known as South American native ungulates (SANUs) flourished in South America for most of the Cenozoic. Although some of these species likely filled ecological niches similar to those of modern hoofed mammals, others differed substantially from extant artiodactyls and perissodactyls in their skull and limb anatomy and probably also in their ecology. Notoungulates and litopterns were the longest-lived and most diverse SANU clades and survived into the Quaternary; astrapotheres went extinct in the late Miocene, whereas other SANU groups were restricted to the Paleogene. Neogene notoungulates were quite specialized in craniodental structure, but many were rather unspecialized postcranially; in contrast, litopterns evolved limb specializations early in their history while maintaining more conservative dentitions. In this article, we review the current understanding of SANU evolutionary relationships and paleoecology, provide an updated compilation of genus temporal ranges, and discuss possible directions for future research. ▪ South American native ungulates (SANUs) were a diverse, long-lived, and independent radiation of mammals into varied terrestrial plant-eater niches. ▪ We review origins, evolution, and paleoecology the major SANU clades: Notoungulata, Litopterna, Astrapotheria, Xenungulata, and Pyrotheria. ▪ At their peak, during the Eocene and Oligocene, more than 40 genera of native ungulates inhabited South America at any one time. ▪ SANUs ranged from

Description: As the world overheats—potentially to conditions warmer than during the three million years over which modern humans evolved—suffering from heat stress will become widespread. Fundamental questions about humans’ thermal tolerance limits are pressing. Understanding heat stress as a process requires linking a network of disciplines, from human health and evolutionary theory to planetary atmospheres and economic modeling. The practical implications of heat stress are equally transdisciplinary, requiring technological, engineering, social, and political decisions to be made in the coming century. Yet relative to the importance of the issue, many of heat stress's crucial aspects, including the relationship between its underlying atmospheric drivers—temperature, moisture, and radiation—remain poorly understood. This review focuses on moist heat stress, describing a theoretical and modeling framework that enables robust prediction of the averaged properties of moist heat stress extremes and their spatial distribution in the future, and draws some implications for human and natural systems from this framework. ▪ Moist heat stress affects society; we summarize drivers of moist heat stress and assess future impacts on societal and global scales. ▪ Moist heat stress pattern scaling of climate models allows research on future heat waves, infrastructure planning, and economic productivity. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Climate extremes threaten human health, economic stability, and the well-being of natural and built environments (e.g., 2003 European heat wave). As the world continues to warm, climate hazards are expected to increase in frequency and intensity. The impacts of extreme events will also be more severe due to the increased exposure (growing population and development) and vulnerability (aging infrastructure) of human settlements. Climate models attribute part of the projected increases in the intensity and frequency of natural disasters to anthropogenic emissions and changes in land use and land cover. Here, we review the impacts, historical and projected changes, and theoretical research gaps of key extreme events (heat waves, droughts, wildfires, precipitation, and flooding). We also highlight the need to improve our understanding of the dependence between individual and interrelated climate extremes because anthropogenic-induced warming increases the risk of not only individual climate extremes but also compound (co-occurring) and cascading hazards. ▪ Climate hazards are expected to increase in frequency and intensity in a warming world. ▪ Anthropogenic-induced warming increases the risk of compound and cascading hazards. ▪ We need to improve our understanding of causes and drivers of compound and cascading hazards. Expected final online publication date for the Annual Review of Clinical Psychology, Volume 16 is May 7, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: The sinking of organic matter to the deep ocean leaves extremely low concentrations of major and trace nutrients for photosynthetic organisms at the sunlit surface. As a result, marine phytoplankton make use of alternative sources of essential elements and have evolved to substitute some elements by others in various biochemical processes. A particularly intriguing example is that of Zn, which is used in many biochemical functions but is often depleted down to picomolar concentrations in surface seawater. Laboratory data show that many phytoplankton species are able to achieve high growth rates by replacing Zn with Cd or Co in cultures. One documented biochemical replacement occurs in some carbonic anhydrases that are used in the acquisition of inorganic carbon for photosynthesis. Field data show the existence of such enzymes in surface seawater and indicate a replacement of Zn by Cd and Co in the surface waters of the eastern tropical South Pacific. Those results point at interesting opportunities for future research. ▪ The dearth of essential elements in surface seawater has caused marine phytoplankton to substitute some trace metals by others in various biochemical processes. ▪ Many species can substitute Cd and/or Co for Zn as a metal center in carbonic anhydrase enzymes that are used in the acquisition of inorganic carbon for photosynthesis. ▪ Field data show the presence of such enzymes in the sea and indicate a replacement of Zn by Cd and Co in the surface upwelling waters of the eastern tropical South Pacific. ▪ New analytical and molecular tools provide opportunities to elucidate the unusual biochemistry of marine phytoplankton. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Seismology provides important constraints on the structure and dynamics of the deep mantle. Computational and methodological advances in the past two decades improved tomographic imaging of the mantle and revealed the fine-scale structure of plumes ascending from the core-mantle boundary region and slabs of oceanic lithosphere sinking into the lower mantle. We discuss the modeling aspects of global tomography including theoretical approximations, data selection, and model fidelity and resolution. Using spectral, principal component, and cluster analyses, we highlight the robust patterns of seismic heterogeneity, which inform us of flow in the mantle, the history of plate motions, and potential compositionally distinct reservoirs. In closing, we emphasize that data mining of vast collections of seismic waveforms and new data from distributed acoustic sensing, autonomous hydrophones, ocean-bottom seismometers, and correlation-based techniques will boost the development of the next generation of global models of density, seismic velocity, and attenuation. ▪ Seismic tomography reveals the 100-km to 1,000-km scale variation of seismic velocity heterogeneity in the mantle. Tomographic images are the most important geophysical constraints on mantle circulation and evolution. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Mars is the nearest planet that potentially harbors life and that can be explored by humans, so its history of water is of considerable importance. Water was abundant on early Mars but disappeared as Mars became the cold, dry planet we see today. Loss of water to space played a major role in the history of this water. Variability of components of the atmosphere that can drive escape has taken place on all timescales, from interannual to the 105-, 106-, and 〉107-year timescales of obliquity variations to the 4 billion-year timescale of large-scale climate evolution. These variations have had a major impact on the behavior of the atmosphere, climate, and water. They also make it difficult to evaluate quantitatively where the water has gone. Despite this uncertainty, the observed enrichment in the ratio of deuterium/hydrogen requires that loss to space has been substantial. ▪ Mars is the nearest planet that potentially harbors life and that can be explored by humans, so its history of water is important. ▪ The Mars atmosphere has varied on all timescales, from year to year to its 4 billion-year history, driving the evolution of water. ▪ Loss of water from the Martian atmosphere to space has been a major process in Mars’ atmospheric evolution. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 28, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Minoru Ozima describes important influences in his scientific life, from the trauma of World War II during adolescence to studying with such giants of Earth science as J. Tuzo Wilson. He benefited from international collaborations in helping to establish noble gas geochemistry as an important discipline that reveals much about the origin and evolution of our planet Earth. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 28, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Understanding the evolution and processes that shape our planet critically depends on the robustness of the absolute ages and process durations obtained from rocks and crystals. Two main aspects of time information on magmatic systems are currently at the forefront of new knowledge. The capacity to determine process durations on human timescales makes it possible to relate the magma dynamics below active volcanoes with the monitoring signals measured at the surface, thereby improving eruption hazards mitigation. The combination of precise in situ dating of accessory minerals and diffusion chronometry is unraveling the incremental growth of large silica-rich magma reservoirs over thousands to hundreds of thousands of years and illuminates the complex relationships between plutonic and volcanic systems. Further progress could be made by decreasing the volume of the analyzed crystals and the error of time determinations, addressing the crystal representativeness and sampling bias, and connecting the time information with physicochemical models of magmatic systems. ▪ Rock-forming minerals are time capsules of magmatic processes that occur on human timescales and can help to better anticipate volcanic eruptions. ▪ In situ dating of accessory minerals reveals that large magma reservoirs evolve through multiple thermal fluctuations of over tens to hundreds of thousands of years. ▪ Progress on conceptual models of magma storage and rejuvenation requires improved error analysis of timescales and representativeness of crystal populations. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 28, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: The influence of the continental lithosphere and its root (or keel) on the continental drift of Earth is a key element in the history of plate tectonics. Previous geodynamic studies of mantle flow suggested that the cratonic root is moderately mechanically coupled with the underlying mantle, and stable continental drift on Earth's timescales occurs when the effective viscosity contrast between the continental lithosphere and the underlying mantle is approximately 103. Both geodynamics and seismological studies indicate that mechanically weak mobile belts (i.e., orogenic or suture zones) that surround cratons may play a role in the longevity of the cratonic lithosphere over geologically long timescales (i.e., over 1,000 million years) because they act as a buffer region against the high-viscosity cratons. Low-viscosity asthenosphere, characterized by slow seismic velocities, reduces the basal drag force acting on the cratonic root, which may also contribute to the longevity of the cratonic lithosphere. ▪ The role of the continental lithosphere and its root on the continental drift is reviewed from recent geodynamic and seismological studies. ▪ The cratonic root is moderately mechanically coupled with the underlying mantle and deformed by mantle flow over geological timescales. ▪ Orogenic belts or suture zones that surround cratons act as a buffer to protect cratons and are essential for their longevity. ▪ Low-viscosity asthenosphere may reduce the basal drag acting on the cratonic root and also contribute to its stability and longevity. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 28, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: The martian surface preserves a record of aqueous fluids throughout the planet's history, but when, where, and even whether such fluids exist at the contemporary surface remains an area of ongoing research. Large water volumes remain on the planet today, but mostly bound in minerals or frozen in the subsurface, with limited direct evidence for aquifers. A role for water has been suggested to explain active surface processes monitored by orbital and landed spacecraft, such as gullies and slope streaks across a range of latitudes; however, dry mechanisms appear at least equally plausible for many active slopes. The low modern atmospheric density and cold surface temperatures challenge models for producing sufficient volumes of water to do the observed geomorphic work. The seeming ubiquity of salts in martian soils facilitates liquid stability but also has implications for the habitability of any such liquids. ▪ A thin modern atmosphere and low temperatures make pure liquid water unstable on the surface of modern Mars. ▪ Widespread salts could enhance liquid durability by lowering the freezing point and slowing evaporation. ▪ Dielectric measurements suggest active brines deep beneath the south pole and, in transient thin films, within shallow polar soils. ▪ Some characteristics of gullies, recurring slope lineae, and other active features challenge both current wet and dry formation models. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 28, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Pluto and Charon are strikingly diverse in their range of geologies, surface compositions, and crater retention ages. This is despite the two having similar densities and presumed bulk compositions. Much of Pluto's surface reflects surface-atmosphere interactions and the mobilization of volatile ices by insolation. Abundant evidence, including past and present N2 ice glacial activity, implies that Pluto has undergone substantial climate evolution. An ancient impact basin contains a massive, convectively overturning N2 ice reservoir, whose position and surrounding tectonics suggest a subsurface ocean. Aligned blades of methane ice hundreds of meters tall, found only at high altitude, likely cover much of Pluto's low latitudes and may be a consequence of obliquity variation driven volatile migration. Multikilometer-high possible cryovolcanic constructs and apparent fissure eruptions indicate relatively late endogenic activity on Pluto. Pluto's range of surface ages is extreme, whereas Charon's surface, while old, displays a large resurfaced plain and globally engirdling extensional tectonic network attesting to earlier endogenic vigor. ▪ The vast N2 ice sheet Sputnik Planitia controls Pluto's atmosphere and climate, comparable in importance with the role of Greenland and Antarctica on the climate of Earth. ▪ Spectacular evidence for erosion such as now-unoccupied glacial valley networks implies a vigorous early climate, and more widespread N2 ice glaciation, on Pluto. ▪ Geological activity on both bodies requires or required sustained internal heat release and suggests a past (Charon) or present (Pluto) ammoniated, subsurface ocean. ▪ The varieties of geologic experience witnessed on Pluto and Charon should play out among the many and varied dwarf planets of the Kuiper belt Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 28, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: The Laurentian Great Lakes are vast, spatially heterogeneous, and changing. Across these hydrologically linked basins, some conditions approach biogeochemical extremes for freshwater systems anywhere. Some of the biogeochemical processes operate over nearly as broad a range of temporal and spatial scales as is possible to observe in freshwater. What we know about the biogeochemistry of this system is strongly influenced by an intense focus on phosphorus loading, eutrophication, and partial recovery; therefore, some important biogeochemical processes are known in detail while others are scarcely described. These lakes serve as a life support system for tens of millions of people, and they generate trillions of dollars of economic activity. Many biogeochemical changes that have occurred have surprised us. Biogeochemistry affects how these lakes perform these functions and should be a higher research priority. ▪ The biogeochemical functioning of the Great Lakes affects tens of millions of people and trillions of dollars of economy, but our knowledge of their biogeochemistry is fragmentary. ▪ The history of environmental damage and recovery in the Great Lakes is long and includes many surprises. ▪ Large lakes such as the Great Lakes combine characteristics of small lakes and the world's oceans, making them worthy objects of study to advance fundamental understanding. ▪ The Great Lakes are understudied relative to their scale and importance. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 28, 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Hydrogen and deuterium isotopic evidence indicates that the source of terrestrial water was mostly meteorites, with additional influx from nebula gas during accretion. There are two Earth models, with large (7–12 ocean masses) and small (1–4 ocean masses) water budgets that can explain the geochemical, cosmochemical, and geological observations. Geophysical and mineral physics data indicate that the upper and lower mantles are generally dry, whereas the mantle transition zone is wetter, with heterogeneous water distribution. Subducting slabs are a source of water influx, and there are three major sites of deep dehydration: the base of the upper mantle, and the top and bottom of the lower mantle in addition to slabs in the shallow upper mantle. Hydrated regions surround these dehydration sites. The core may be a hidden reservoir of hydrogen under the large water budget model. ▪ Earth is a water planet. Where and when was water delivered, and how much? How does water circulate in Earth? This review looks at the current answers to these fundamental questions. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Higher boreal summer insolation in the early to middle Holocene drove thousands of years of summer warming across the Arctic. Modern-day warming has distinctly different causes, but geologic data from this past warm period hold lessons for the future. We compile Holocene temperature reconstructions from ice, lake, and marine cores around Greenland, where summer temperatures are globally important due to their influence on ice sheet mass balance, ocean circulation, and sea ice. Highlighting and accounting for some key issues with proxy interpretation, we find that much of Greenland experienced summers 3 to 5°C warmer than the mid-twentieth century in the early Holocene—earlier and stronger warming than often presumed. Warmth had dramatic consequences: Many glaciers disappeared, perennial sea ice retreated, plants and animals migrated northward, the Greenland Ice Sheet shrank rapidly, and increased meltwater discharge led to strong marine water stratification and enhanced winter sea ice in some areas. ▪ Summer air temperatures and open ocean temperatures around much of Greenland peaked in the early Holocene in response to elevated summer insolation. ▪ Peak summer air temperatures ranged from 3 to 5°C warmer than the mid-twentieth century in northwest and central Greenland to perhaps 1 to 2°C in south Greenland. ▪ Many differences between records can be explained by proxy seasonality, ice sheet elevation changes, vegetation analogs and lags, and the nearshore effects of ice sheet meltwater. ▪ Early Holocene warmth dramatically affected glaciers and the Greenland Ice Sheet; meltwater discharge, nearshore ocean salinity, and sea ice; and diverse flora and fauna. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Deep earthquakes behave like shallow earthquakes but must have fundamentally different physical processes. Their rupture behaviors, magnitude-frequency statistics, and aftershocks are diverse and imperfectly dependent on various factors, such as slab temperature, depth, and magnitude. The three leading mechanisms for deep earthquakes (i.e., transformational faulting, dehydration embrittlement, and thermal runaway) can each explain portions of the observations but have potentially fundamental difficulties explaining the rest. This situation calls for more serious consideration of hypotheses that involve more than one mechanism. For example, deep earthquakes may initiate by one mechanism, but the ruptures may propagate via another mechanism once triggered. To make further progress, it is critical to evaluate the hypotheses, both single- or dual-mechanism, under conditions as close to those of real slabs as possible to make accurate and specific predictions that are testable using seismic or other geophysical observations. Any new understanding of deep earthquakes promises new constraints on subduction zone structure and dynamics. ▪ Deep earthquakes display the complex structure and dynamics of subduction zones in terms of geometry, stress state, rheology, hydration, and phase changes. ▪ Phase transformation, dehydration, and thermal runaway are the leading mechanisms for deep earthquakes, but all have major gaps or fundamental difficulties. ▪ Deep earthquakes may involve dual-mechanism processes, as hinted at by the diverse rupture and statistic properties and the break of self-similarity. ▪ Further progresses would benefit from specific and testable predictions that consider realistic slab conditions with insights from geodynamics, petrology, and mineral physics. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Exoplanets with substantial hydrogen/helium atmospheres have been discovered in abundance, many residing extremely close to their parent stars. The extreme irradiation levels that these atmospheres experience cause them to undergo hydrodynamic atmospheric escape. Ongoing atmospheric escape has been observed to be occurring in a few nearby exoplanet systems through transit spectroscopy both for hot Jupiters and for lower-mass super-Earths and mini-Neptunes. Detailed hydrodynamic calculations that incorporate radiative transfer and ionization chemistry are now common in one-dimensional models, and multidimensional calculations that incorporate magnetic fields and interactions with the interstellar environment are cutting edge. However, comparison between simulations and observations remains very limited. While hot Jupiters experience atmospheric escape, the mass-loss rates are not high enough to affect their evolution. However, for lower-mass planets, atmospheric escape drives and controls their evolution, sculpting the exoplanet population that we observe today. ▪ Observations of some exoplanets have detected atmospheric escape driven by hydrodynamic outflows, causing the exoplanets to lose mass over time. ▪ Hydrodynamic simulations of atmospheric escape are approaching the sophistication required to compare them directly to observations. ▪ Atmospheric escape sculpts sharp features into the exoplanet population that we can observe today; these features have recently been detected.

Description: Recent progress in theoretical mineral physics based on the ab initio quantum mechanical computation method has been dramatic in conjunction with the rapid advancement of computer technologies. It is now possible to predict stability, elasticity, and transport properties of complex minerals quantitatively with uncertainties that are comparable to or even smaller than those attached in experimental data. These calculations under in situ high-pressure ( P) and high-temperature conditions are of particular interest because they allow us to construct a priori mineralogical models of the deep Earth. In this article, we briefly review recent progress in studying high- P phase relations, elasticity, thermal conductivity, and rheological properties of lower mantle minerals including silicates, oxides, and some hydrous phases. Our analyses indicate that the pyrolitic composition can describe Earth's properties quite well in terms of density and P- and S-wave velocity. Computations also suggest some new hydrous compounds that could persist up to the deepest mantle and that the postperovskite phase boundary is the boundary of not only the mineralogy but also the thermal conductivity. ▪ The ab initio method is a strong tool to investigate physical properties of minerals under high pressure and high temperature. ▪ Calculated thermoelasticity indicates that the pyrolytic composition is representative to the chemistry of Earth's lower mantle. ▪ Simulations predict new dense hydrous phases stable in the whole lower mantle pressure and temperature condition. ▪ Calculated lattice thermal conductivity suggests a heat flow across the core mantle boundary no greater than 10 TW. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Earthquake early warning (EEW) is the delivery of ground shaking alerts or warnings. It is distinguished from earthquake prediction in that the earthquake has nucleated to provide detectable ground motion when an EEW is issued. Here we review progress in the field in the last 10 years. We begin with EEW users, synthesizing what we now know about who uses EEW and what information they need and can digest. We summarize the approaches to EEW and gather information about currently existing EEW systems implemented in various countries while providing the context and stimulus for their creation and development. We survey important advances in methods, instrumentation, and algorithms that improve the quality and timeliness of EEW alerts. We also discuss the development of new, potentially transformative ideas and methodologies that could change how we provide alerts in the future. ▪ Earthquake early warning (EEW) is the rapid detection and characterization of earthquakes and delivery of an alert so that protective actions can be taken. ▪ EEW systems now provide public alerts in Mexico, Japan, South Korea, and Taiwan and alerts to select user groups in India, Turkey, Romania, and the United States. ▪ EEW methodologies fall into three categories, point source, finite fault, and ground motion models, and we review the advantages of each of these approaches. ▪ The wealth of information about EEW uses and user needs must be employed to focus future developments and improvements in EEW systems.

Description: The Proterozoic Eon was once regarded as the neglected middle half of Earth history. The name refers to early animals, but they did not appear until the eon (2.5–0.54 Ga) was nearly over. Eukaryotic cells and sexual reproduction evolved much earlier in the eon, as did chloroplasts. Molecular dioxygen, the presence of which altered the geochemical behavior of nearly every element essential to life, rose from negligible to near-modern levels, and then plummeted before rising fitfully again. Plate tectonics took on a modern form, and two supercontinents, Nuna and Rodinia, successively congregated and later dispersed. Climate regulatory failures, i.e., Snowball Earth, appear to be a uniquely Proterozoic phenomenon, having occurred twice in rapid succession near the end of the eon (from 717 to 660 Ma and from 650 to 635 Ma) and arguably once near its beginning (ca. 2.43 Ga). Dynamic sea glaciers covered Snowball Earth oceans from pole to pole, and equatorial sublimation drove slow-moving ice sheets on land. Ultimately, the gradual accumulation of CO2 triggered rapid deglaciation and transient greenhouse aftermaths. Physically based and geologically tested, Neoproterozoic Snowball Earth appears to have molecular legacies in ancient bitumens and modern organisms. This is the story of my love affair with an eon that is now a little less neglected.

Description: Low-mass planets have an extraordinarily diverse range of bulk compositions, from primarily rocky worlds to those with deep gaseous atmospheres. As techniques for measuring the masses of exoplanets advance the field toward the regime of rocky planets, from ultrashort orbital periods to Venus-like distances, we identify the bounds on planet compositions, where sizes and incident fluxes inform bulk planet properties. In some cases, the precision of measurement of planet masses and sizes is approaching the theoretical uncertainties in planet models. An emerging picture explains aspects of the diversity of low-mass planets, although some problems remain: Do extreme low-density, low-mass planets challenge models of atmospheric mass loss? Are planet sizes strictly separated by bulk composition? Why do some stellar characterizations differ between observational techniques? With the Transiting Exoplanet Survey Satellite ( TESS) mission, low-mass exoplanets around the nearest stars will soon be discovered and characterized with unprecedented precision, permitting more detailed planetary modeling and atmospheric characterization of low-mass exoplanets than ever before. ▪ Following the Kepler mission, studies of exoplanetary compositions have entered the terrestrial regime. ▪ Low-mass planets have an extraordinary range of compositions, from Earth-like mixtures of rock and metal to mostly tenuous gas. ▪ The TESS mission will discover low-mass planets that can be studied in more detail than ever before.

Description: In addition to their being vital components of mid- to high-latitude coastal ecosystems, salt marshes contain 0.1% of global sequestered terrestrial carbon. Their sustainability is now threatened by accelerating sea-level rise (SLR) that has reached a rate that is many times greater than the rate at which they formed and evolved. Modeling studies have been instrumental in predicting how marsh systems will respond to greater frequencies and durations of tidal inundation and in quantifying thresholds when marshes will succumb and begin to disintegrate due to accelerating SLR. Over the short term, some researchers believe that biogeomorphic feedbacks will improve marsh survival through greater biomass productivity enhanced by warmer temperatures and higher carbon dioxide concentrations. Increased sedimentation rates are less likely due to lower-than-expected suspended sediment concentrations. The majority of marsh loss today is through wave-induced edge erosion that beneficially adds sediment to the system. Edge erosion is partly offset by upland marsh migration during SLR. ▪ Despite positive biogeomorphic feedbacks, many salt marshes will succumb to accelerating sea-level rise due to insufficient mineral sediment. ▪ The latest multivariate marsh modeling is producing predictions of marsh evolution under various sea-level rise scenarios. ▪ The least well-known variables in projecting changes to salt marshes are suspended sediment concentrations and net sediment influx to the marsh. ▪ We are in the infancy of understanding the importance and processes of marsh edge erosion and the overall dynamicism of marshes. ▪ This review defines the latest breakthroughs in understanding the response of salt marshes to accelerating sea-level rise and decreasing sediment supply. ▪ Climate change is accelerating sea-level rise, warming temperatures, and increasing carbon dioxide, all of which are impacting marsh vegetation and vertical accretion.

Description: Flood basalts were Earth's largest volcanic episodes that, along with related intrusions, were often emplaced rapidly and coincided with environmental disruption: oceanic anoxic events, hyperthermals, and mass extinction events. Volatile emissions, both from magmatic degassing and vaporized from surrounding rock, triggered short-term cooling and longer-term warming, ocean acidification, and deoxygenation. The magnitude of biological extinction varied considerably, from small events affecting only select groups to the largest extinction of the Phanerozoic, with less-active organisms and those with less-developed respiratory physiology faring especially poorly. The disparate environmental and biological outcomes of different flood basalt events may at first order be explained by variations in the rate of volatile release modulated by longer trends in ocean carbon cycle buffering and the composition of marine ecosystems. Assessing volatile release, environmental change, and biological extinction at finer temporal resolution should be a top priority to refine ancient hyperthermals as analogs for anthropogenic climate change. ▪ Flood basalts, the largest volcanic events in Earth history, triggered dramatic environmental changes on land and in the oceans. ▪ Rapid volcanic carbon emissions led to ocean warming, acidification, and deoxygenation that often caused widespread animal extinctions. ▪ Animal physiology played a key role in survival during flood basalt extinctions, with reef builders such as corals being especially vulnerable. ▪ The rate and duration of volcanic carbon emission controlled the type of environmental disruption and the severity of biological extinction.

Description: Earthquakes occur by overcoming fault friction; therefore, quantifying fault resistance is central to earthquake physics. Values for both static and dynamic friction are required, and the latter is especially difficult to determine on natural faults. However, large earthquakes provide signals that can determine friction in situ. The Japan Trench Fast Drilling Project ( JFAST), an Integrated Ocean Discovery Program expedition, determined stresses by collecting data directly from the fault 1–2 years after the 2011 Mw 9.1 Tohoku earthquake. Geological, rheological, and geophysical data record stress before, during, and after the earthquake. Together, the observations imply that the shear strength during the earthquake was substantially below that predicted by the traditional Byerlee's law. Locally the stress drop appears near total, and stress reversal is plausible. Most solutions to the energy balance require off-fault deformation to account for dissipation during rupture. These observations make extreme coseismic weakening the preferred model for fault behavior. ▪ Determining the friction during an earthquake is required to understand when and where earthquakes occur. ▪ Drilling into the Tohoku fault showed that friction during the earthquake was low. ▪ Dynamic friction during the earthquake was lower than static friction. ▪ Complete stress drop is possible, and stress reversal is plausible. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: Interpretations of the tempo of mass extinctions and recoveries often rely on the distribution of fossils in a stratigraphic column. These interpretations are generally compromised when they are not based on a knowledge of marine ecological gradients and sequence-stratigraphic architecture. Crucially, last and first occurrences of species do not record times of extinction and origination. A face-value interpretation of the stratigraphic record leads to incorrect inferences of pulsed extinction, underestimates of the duration of mass extinction, and overestimates of local recovery times. An understanding of the processes of extinction and recovery is substantially improved by knowledge of the distribution of species along marine environmental gradients, interpreting sequence-stratigraphic architecture to show how those gradients are sampled through time, and sampling along regional transects along depositional dip. Doing so suggests that most ancient mass extinctions were substantially longer and local recoveries substantially shorter than generally thought. ▪ The concepts that let geologists find petroleum allow paleontologists to reinterpret ancient mass extinctions and their recoveries. ▪ Most ancient mass extinctions were longer than the fossil record suggests, lasting hundreds of thousands of years to a few million years. ▪ Ancient recoveries from mass extinctions were shorter than thought and likely overlapped with extinction during a period of turnover. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 48 is May 29, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Description: In this review, we address the current status of numerical modeling of the mantle transition zone and uppermost lower mantle, focusing on the hydration mechanism in these areas. The main points are as follows: ( a) Slab stagnation and penetration may play significant roles in transporting the water in the whole mantle, and ( b) a huge amount of water could be absorbed into the deep mantle to preserve the surface seawater over the geologic timescale. However, for further understanding of water circulation in the deep planetary interior, more mineral physics investigations are required to reveal the mechanism of water absorption in the lower mantle and thermochemical interaction across the core–mantle boundary region, which can provide information on material properties to the geodynamics community. Moreover, future investigations should focus on determining the amount of water in the early planetary interior, as suggested by the planetary formation theory of rocky planets. Moreover, the supplying mechanism of water during planetary formation and its evolution caused by plate tectonics are still essential issues because, in geodynamics modeling, a huge amount of water seems to be required to preserve the surface seawater in the present day and to not be dependent on an initial amount of water in Earth's system. ▪ Slab stagnation and penetration of the hydrous lithosphere are essential for understanding the global-scale material circulation. ▪ Thermal feedback caused by water-dependent viscosity is a main driving mechanism of water absorption in the mantle transition zone and uppermost lower mantle. ▪ The hydrous state in the early rocky planets remains to be determined from cosmo- and geochemistry and planetary formation theory. ▪ Volatile cycles in the deep planetary interior may affect the evolution of the surface environment.

Description: The low permeability of clays, shales, and other argillaceous lithologies makes them key controls of transport and deformation processes in the crust but is known for being challenging to characterize. As muds are modified by compaction and diagenesis to low-porosity shales, permeability can decrease by six or more orders of magnitude, but at large scales it is often dramatically and unpredictably increased by fractures, faults, and other features. Testing and inverse modeling show that petrophysical properties and the geological environment are dominant controls of clay and shale matrix permeability and its scale dependence. Active sedimentation and tectonism on continental margins cause large-scale permeability to vary with time, but in stable continent interiors it is unclear how regional permeability of argillaceous formations changes over time or, in most cases, what controls it. Although rarely considered, it is also unknown whether Darcian permeability adequately describes flow in clay-rich materials. ▪ Critical for problems in energy, water supply, waste isolation, and geologic hazards, clay and shale permeability remains problematic. ▪ Test data and inverse model analyses are beginning to reveal where and how permeability of clay and shale changes with scale. ▪ In clays and shales, causes of permeability scale effects, their time dependence, and even flow behavior continue to raise questions.

Description: Global Positioning System (GPS) instruments are routinely used today to measure crustal deformation signals from tectonic plate motions, faulting, and glacial isostatic adjustment. In parallel with the expansion of GPS networks around the world, several new and unexpected applications of GPS have been developed. For example, GPS instruments are now being used routinely to measure ground motions during large earthquakes. Access to real-time GPS data streams has led to the development of better hazard warnings for tsunamis, flash floods, earthquakes, and volcanic eruptions. Terrestrial water storage changes can be derived from GPS vertical coordinate time series. Finally, GPS signals that reflect on the surfaces below a GPS antenna can be used to measure soil moisture, snow accumulation, vegetation water content, and water levels. In the future, combining GPS with the signals from the Russian, European, and Chinese navigation constellations will significantly enhance these applications. ▪ GPS data are now routinely used to study the dynamics of earthquake rupture. ▪ GPS instruments are an integral part of warning systems for earth- quakes, tsunamis, flash floods, and volcanic eruptions. ▪ Reflected GPS signals provide a new source of soil moisture, snow depth, vegetation water content, and tide gauge data. ▪ GPS networks can sense changes in soil moisture, groundwater, and snow depth and thus can contribute to water resource assessments.

Description: The water mass transformation (WMT) framework weaves together circulation, thermodynamics, and biogeochemistry into a description of the ocean that complements traditional Eulerian and Lagrangian methods. In so doing, a WMT analysis renders novel insights and predictive capabilities for studies of ocean physics and biogeochemistry. In this review, we describe fundamentals of the WMT framework and illustrate its practical analysis capabilities. We show how it provides a robust methodology to characterize and quantify the impact of physical processes on buoyancy and other thermodynamic fields. We also detail how to extend WMT to insightful analysis of biogeochemical cycles.

Description: The CARIACO (Carbon Retention in a Colored Ocean) Ocean Time-Series Program station, located at 10.50°N, 64.66°W, observed biogeochemical and ecological processes in the Cariaco Basin of the southwestern Caribbean Sea from November 1995 to January 2017. The program completed 232 monthly core cruises, 40 sediment trap deployment cruises, and 40 microbiogeochemical process cruises. Upwelling along the southern Caribbean Sea occurs from approximately November to August. High biological productivity (320–628 g C m−2y−1) leads to large vertical fluxes of particulate organic matter, but only approximately 9–10 g C m−2y−1fall to the bottom sediments (∼1–3% of primary production). A diverse community of heterotrophic and chemoautotrophic microorganisms, viruses, and protozoa thrives within the oxic–anoxic interface. A decrease in upwelling intensity from approximately 2003 to 2013 and the simultaneous overfishing of sardines in the region led to diminished phytoplankton bloom intensities, increased phytoplankton diversity, and increased zooplankton densities. The deepest waters of the Cariaco Basin exhibited long-term positive trends in temperature, salinity, hydrogen sulfide, ammonia, phosphate, methane, and silica. Earthquakes and coastal flooding also resulted in the delivery of sediment to the seafloor. The program's legacy includes climate-quality data from suboxic and anoxic habitats and lasting relationships between international researchers.

Description: The use of unoccupied aircraft systems (UASs, also known as drones) in science is growing rapidly. Recent advances in microelectronics and battery technology have resulted in the rapid development of low-cost UASs that are transforming many industries. Drones are poised to revolutionize marine science and conservation, as they provide essentially on-demand remote sensing capabilities at low cost and with reduced human risk. A variety of multirotor, fixed-wing, and transitional UAS platforms are capable of carrying various optical and physical sampling payloads and are being employed in almost every subdiscipline of marine science and conservation. This article provides an overview of the UAS platforms and sensors used in marine science and conservation missions along with example physical, biological, and natural resource management applications and typical analytical workflows. It concludes with details on potential effects of UASs on marine wildlife and a look to the future of UASs in marine science and conservation.

Description: In this article, I use the Estimating the Circulation and Climate of the Ocean version 4 (ECCO4) reanalysis to estimate the residual meridional overturning circulation, zonally averaged, over the separate Atlantic and Indo-Pacific sectors. The abyssal component of this estimate differs quantitatively from previously published estimates that use comparable observations, indicating that this component is still undersampled. I also review recent conceptual models of the oceanic meridional overturning circulation and of the mid-depth and abyssal stratification. These theories show that dynamics in the Antarctic circumpolar region are essential in determining the deep and abyssal stratification. In addition, they show that a mid-depth cell consistent with observational estimates is powered by the wind stress in the Antarctic circumpolar region, while the abyssal cell relies on interior diapycnal mixing, which is bottom intensified.

Description: Archaea are ubiquitous and abundant members of the marine plankton. Once thought of as rare organisms found in exotic extremes of temperature, pressure, or salinity, archaea are now known in nearly every marine environment. Though frequently referred to collectively, the planktonic archaea actually comprise four major phylogenetic groups, each with its own distinct physiology and ecology. Only one group—the marine Thaumarchaeota—has cultivated representatives, making marine archaea an attractive focus point for the latest developments in cultivation-independent molecular methods. Here, we review the ecology, physiology, and biogeochemical impact of the four archaeal groups using recent insights from cultures and large-scale environmental sequencing studies. We highlight key gaps in our knowledge about the ecological roles of marine archaea in carbon flow and food web interactions. We emphasize the incredible uncultivated diversity within each of the four groups, suggesting there is much more to be done.

Description: Aquatic environments experiencing low-oxygen conditions have been described as hypoxic, suboxic, or anoxic zones; oxygen minimum zones; and, in the popular media, the misnomer “dead zones.” This review aims to elucidate important aspects underlying oxygen depletion in diverse coastal systems and provides a synthesis of general relationships between hypoxia and its controlling factors. After presenting a generic overview of the first-order processes, we review system-specific characteristics for selected estuaries where adjacent human settlements contribute to high nutrient loads, river-dominated shelves that receive large inputs of fresh water and anthropogenic nutrients, and upwelling regions where a supply of nutrient-rich, low-oxygen waters generates oxygen minimum zones without direct anthropogenic influence. We propose a nondimensional number that relates the hypoxia timescale and water residence time to guide the cross-system comparison. Our analysis reveals the basic principles underlying hypoxia generation in coastal systems and provides a framework for discussing future changes.