Description: Four mud extrusions were investigated along the erosive subduction zone off Costa Rica. Active fluid seepage from these structures is indicated by chemosynthetic communities, authigenic carbonates and methane plumes in the water column. We estimate the methane output from the individual mud extrusions using two independent approaches. The first is based on the amount of CH4 that becomes anaerobically oxidized in the sediment beneath areas covered by chemosynthetic communities, which ranges from 104 to 105 mol yr− 1. The remaining portion of CH4, which is released into the ocean, has been estimated to be 102–104 mol yr− 1 per mud extrusion. The second approach estimates the amount of CH4 discharging into the water column based on measurements of the near-bottom methane distribution and current velocities. This approach yields estimates between 104–105 mol yr−1. The discrepancy of the amount of CH4 emitted into the bottom water derived from the two approaches hints to methane seepage that cannot be accounted for by faunal growth, e.g. focused fluid emission through channels in sediments and fractures in carbonates. Extrapolated over the 48 mud extrusions discovered off Costa Rica, we estimate a CH4 output of 20·106 mol yr− 1 from mud extrusions along this 350 km long section of the continental margin. These estimates of methane emissions at an erosional continental margin are considerably lower than those reported from mud extrusion at accretionary and passive margins. Almost half of the continental margins are described as non-accretionary. Assuming that the moderate emission of methane at the mud extrusions off Costa Rica are typical for this kind of setting, then global estimates of methane emissions from submarine mud extrusions, which are based on data of mud extrusions located at accretionary and passive continental margins, appear to be significantly too high.

Description: Shallow hydrothermal vents are of pivotal relevance for ocean biogeochemical cycles, including seawater dissolved heavy metals and trace elements as well as the carbonate system balance. The Kueishan Tao (KST) stratovolcano off Taiwan is associated with numerous hydrothermal vents emitting warm sulfur-rich fluids at so-called White Vents (WV) and Yellow Vent (YV) that impact the surrounding seawater masses and habitats. The morphological and biogeochemical consequences caused by a M5.8 earthquake and a C5 typhoon (“Nepartak”) hitting KST (12th May, and 2nd–10th July, 2016) were studied within a 10-year time series (2009–2018) combining aerial drone imagery, technical diving, and hydrographic surveys. The catastrophic disturbances triggered landslides that reshaped the shoreline, burying the seabed and, as a consequence, native sulfur accretions that were abundant on the seafloor disappeared. A significant reduction in venting activity and fluid flow was observed at the high-temperature YV. Dissolved Inorganic Carbon (DIC) maxima in surrounding seawater reached 3000–5000 µmol kg−1, and Total Alkalinity (TA) drawdowns were below 1500–1000 µmol kg−1 lasting for one year. A strong decrease and, in some cases, depletion of dissolved elements (Cd, Ba, Tl, Pb, Fe, Cu, As) including Mg and Cl in seawater from shallow depths to the open ocean followed the disturbance, with a recovery of Mg and Cl to pre-disturbance concentrations in 2018. The WV and YV benthic megafauna exhibited mixed responses in their skeleton Mg:Ca and Sr:Ca ratios, not always following directions of seawater chemical changes. Over 70% of the organisms increased skeleton Mg:Ca ratio during rising DIC (higher CO2) despite decreasing seawater Mg:Ca ratios showing a high level of resilience. KST benthic organisms have historically co-existed with such events providing them ecological advantages under extreme conditions. The sudden and catastrophic changes observed at the KST site profoundly reshaped biogeochemical processes in shallow and offshore waters for one year, but they remained transient in nature, with a possible recovery of the system within two years.

Description: A compilation of data on volumes and masses of evaporite deposits is used as the basis for reconstruction of the salinity of the ocean in the past. Chloride is tracked as the only ion essentially restricted to the ocean, and past salinities are calculated from reconstructed chlorine content of the ocean. Models for ocean salinity through the Phanerozoic are developed using maximal and minimal estimates of the volumes of existing evaporite deposits, and using constant and declining volumes of ocean water through the Phanerozoic. We conclude that there have been significant changes in the mean salinity of the ocean accompanying a general decline throughout the Phanerozoic. The greatest changes are related to major extractions of salt into the young ocean basins which developed during the Mesozoic as Pangaea broke apart. Unfortunately, the sizes of these salt deposits are also the least well known. The last major extractions of salt from the ocean occurred during the Miocene, shortly after the large scale extraction of water from the ocean to form the ice cap of Antarctica. However, these two modifications of the masses of H2O and salt in the ocean followed in sequence and did not cancel each other out. Accordingly, salinities during the Early Miocene were between 37‰ and 39‰. The Mesozoic was a time of generally declining salinity associated with the deep sea salt extractions of the North Atlantic and Gulf of Mexico (Middle to Late Jurassic) and South Atlantic (Early Cretaceous). The earliest of the major extractions of the Phanerozoic occurred during the Permian. There were few large extractions of salt during the earlier Palaeozoic. The models suggest that this was a time of relatively stable but slowly increasing salinities ranging through the upper 40‰'s into the lower 50‰'s. Higher salinities for the world ocean have profound consequences for the thermohaline circulation of the ocean in the past. In the modern ocean, with an average salinity of about 34.7‰, the density of water is only very slightly affected by cooling as it approaches the freezing point. Consequently, salinization through sea-ice formation or evaporation is usually required to make water dense enough to sink into the ocean interior. At salinities above about 40‰ water continues to become more dense as it approaches the freezing point, and salinization is not required. The energy-consuming phase changes involved in sea-ice formation and evaporation would not be required for vertical circulation in the ocean. The hypothesized major declines in salinity correspond closely to the evolution of both planktonic foraminifera and calcareous nannoplankton. Both groups were restricted to shelf regions in the Jurassic and early Cretaceous, but spread into the open ocean in the mid-Cretaceous. Their availability to inhabit the open ocean may be directly related to the decline in salinity. The Permian extraction may have created stress for marine organisms and may have been a factor contributing to the end-Permian extinction. The modeling also suggests that there was a major salinity decline from the Late Precambrian to the Cambrian, and it is tempting to speculate that this may have been a factor in the Cambrian explosion of life.

Description: Today, the ocean is characterized by pools of warm tropical–subtropical water bounded poleward and at depth by cold water. In the tropics and subtropics, the warm waters are floored at depth by the thermocline–pycnocline, which crops out on the ocean surface between the subtropical and polar frontal systems that form the poleward boundary. It is along and between the frontal systems that the thermocline waters enter the ocean interior. These frontal systems form beneath the maxima of the zonal component of the westerly winds. Today, the location of the westerly winds is stabilized by the persistent high-pressure systems at the polar regions produced by the ice cover of the Antarctic and sea-ice cover of the Arctic. The paleobiogeographic distribution of plankton fossils indicates that, prior to the Oligocene, the subtropical and polar frontal systems were not persistent features. Recent climate model experiments show that without perennial ice cover in the polar regions a seasonal alternation between high and low atmospheric pressure systems can occur. These seasonal alternations would force major changes in the location and strength of the westerly winds, preventing the development of the well-defined frontal systems that characterize the Earth today. Without the subtropical and polar frontal systems, the thermocline would be less well developed and the pycnocline could be dominated by salinity differences. Evidence from ocean drilling suggests that the glaciation of East Antarctica began at the Eocene–Oligocene boundary, but took time to spread over the entire continent. The presence of calcareous nannoplankton in the Arctic basin prior to the Oligocene and their absence thereafter suggests that the ice cover of the Arctic Ocean also developed at the Eocene–Oligocene boundary. Both events appear to be related to the development of the modern oceanic structure, but it remains uncertain whether the ocean changed in response to the development of ice covered polar regions or vice versa.

Description: Highlights • Silicoflagellates include ten Cenozoic genera, starting with Corbisema. • Rotated apical structures begin with Dictyocha byronalis in early Eocene. • Neogene rotation of Dictyocha apical bridge is an independent evolution. • Multi-windowed Stephanopsis are common from early Oligocene. • Paramesocena develops from Stephanopsis and is a possible ancestor of Octactis. Cenozoic silicoflagellate evolution led to ten known genera derived from two groups classified together in Corbisema that may have survived the end-Cretaceous extinction. These underwent rapid diversification and gave rise to at least five genera before the end of the Paleocene, including Dictyocha and Naviculopsis. Important silicoflagellate evolutionary events include the emergence of corner-aligned double skeleton configuration by the early Paleocene and the sinistral rotation of the Dictyocha byronalis apical bridge in early to middle Eocene that evolved into the apically-ringed silicoflagellate genera (e.g., Distephanopsis and Stephanocha). We interpret Paramesocena and Octactis as descended from Stephanocha, although their precise evolutionary paths remain uncertain. The earliest Octactis has thicker apical ring elements than modern O. pulchra, and is here described as the new species O. kosciuszkoi. New combinations are provided for Stephanocha antarctica and Dityocha octangulata. Overall, Cenozoic silicoflagellates show a trend towards more complicated apical geometries and smaller portals. These features enable silicoflagellate double skeletons to form near-spherical skeletal structures that support the external cell boundaries during mitosis. Silicoflagellates employ various strategies to hold double skeletons together and support the cell boundaries across the portal and window openings. Variability is an important feature of the silicoflagellate skeleton and occurs in two broadly different contexts. The first occurs for all silicoflagellate species, with occasional variant skeletal designs distinct from the predominant morphology. The second variability is associated with a single taxon, where multiple unusual skeletal shapes occur in a silicoflagellate plexus over a geologically narrow time interval and limited geographic extent.

Description: Highlights: • Silicoflagellates more common and diverse in late Eocene, lower in Oligocene. • Record of new species Corbisema tajmahalii from Late Eocene, with interpreted double skeleton (DS). • First known Bachmannocena DS, has corner-aligned configuration. • Observation of early evolution of Stephanocha raupii, transferred to Distephanopsis. Abstract Study of the South Pacific Ocean sediments from IODP Expedition 378, Holes U1553A and U1553B, shows 19 late Eocene to late Oligocene silicoflagellate species. The late Eocene includes Corbisema tajmahalii n. sp., observed over a ∼ 8.4 m interval and proposed as a new subzone within the Corbisema apiculata Zone. The new species is characterized by the presence of basal ring that has three sides and an arrowhead-shaped outline with the two minor-axis corners more closely spaced. This study has found the first known Bachmannocena double skeleton, in corner-aligned configuration that suggests evolutionary relationship with the Corbisema triacantha group of Cenozoic silicoflagellates. Also observed is the apparent early evolution of Stephanocha raupii at ∼29.6 Ma, derived from a skeletal diversity related to Distephanopsis crux, thus the species is here transferred to Distephanopsis (abbreviated as Ds.). Silicoflagellate skeletal abundance declines after the Eocene-Oligocene boundary, with generally low abundance except for two sediment samples immediately below the Ds. raupii n. comb. first appearance.