Annual Review of Earth and Planetary Sciences - Volume 49, 2021

The budget of reactive nitrogen (Nr; oxidized and reduced inorganic and organic forms of nitrogen) has at least doubled since the preindustrial era due to human activities. Excess Nr causes significant detrimental effects on many terrestrial and aquatic ecosystems; less is known about the impact on the open ocean. Nr deposition may already rival biological N2 fixation quantitatively and will likely continue to rise. However, it is unclear how much of the Nr currently deposited to the ocean is external in origin. Understanding the importance of ocean Nr emissions versus external Nr deposition is key to quantifying the influence of deposition on ocean biogeochemistry and climate. This article reviews our understanding of the impacts of Nr deposition on the open ocean and the emissions of Nr from the ocean, placing particular emphasis on stable isotopes as a tool to investigate the surface ocean–lower atmosphere Nr cycle and its variations over time.

• ▪   The ocean has a dynamic exchange of reactive nitrogen with the atmosphere and is not just a passive recipient of nitrogen pollution from land.
• ▪   Tracing anthropogenic nitrogen deposition to the ocean is a challenge due to overlapping geochemical signatures with other nitrogen inputs.
• ▪   However, studies suggest an imprint of external (anthropogenic) nitrogen deposition in the Mediterranean Sea and North Pacific Ocean.
• ▪   Climate change will impact nitrogen emissions from the ocean through warming, acidification, stratification, and changes in food webs.

Most tsunamis are generated by earthquakes, but in 1998, a seabed slump offshore of northern Papua New Guinea (PNG) generated a tsunami up to 15 m high that killed more than 2,200 people. The event changed our understanding of tsunami mechanisms and was the forerunner to two decades of major tsunamis that included those in Turkey, the Indian Ocean, Japan, and Sulawesi and Anak Krakatau in Indonesia. PNG provided a context to better understand these tsunamis as well as older submarine landslide events, such as Storegga (8150 BP); Alika 2 in Hawaii (120,000 BP), and Grand Banks, Canada (1929), together with those from dual earthquake/landslide mechanisms, such as Messina (1908), Puerto Rico (1928), and Japan (2011). PNG proved that submarine landslides generate devastating tsunamis from failure mechanisms that can be very different, whether singly or in combination with earthquakes. It demonstrated the critical importance of seabed mapping to identify these mechanisms as well as stimulated the development of new numerical tsunami modeling methodologies. In combination with other recent tsunamis, PNG demonstrated the critical importance of these events in advancing our understanding of tsunami hazard and risk. This review recounts how, since 1998, understanding of the tsunami hazard from submarine landslides has progressed far beyond anything considered possible at that time.

• ▪   For submarine landslide tsunamis, advances in understanding take place incrementally, usually in response to major, sometimes catastrophic, events.
• ▪   The Papua New Guinea tsunami in 1998, when more than 2,200 people perished, was a turning point in first recognizing the significant tsunami hazard from submarine landslides.
• ▪   Over the past 2 to 3 years advances have also been made mainly because of improvements in numerical modeling based on older tsunamis such as Grand Banks in 1929, Messina in 1908, and Storegga at 8150 BP.
• ▪   Two recent tsunamis in late 2018, in Sulawesi and Anak Krakatau, Indonesia, where several hundred people died, were from very unusual landslide mechanisms—dual (strike-slip and landslide) and volcanic collapse—and provide new motivations for understanding these tsunami mechanisms.
• ▪   This is a timely, state of the art review of landslide tsunamis based on recent well-studied events and new research on older ones, which provide an important context for the recent tsunamis in Indonesia in 2018.
  
  

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.

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 Cenozoic—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 ∼500 ppm in the Miocene, and falling further into the ice age world of the Plio–Pleistocene. Around two-thirds of Cenozoic CO2 drawdown is explained by an increase in the ratio of ocean 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.
  
  

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.

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 750 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 750 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.