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Holocene oceanographic variability in the Subtropical Northeast Atlantic
No full textHighlights
• Early Holocene featured weak stratification and possibly year-round upwelling.
• Subsurface warming peaked and upwelling weakened during the Middle Holocene.
• Late Holocene marked by SST cooling, weaker NASTG, and seasonal upwelling.
• Proxies suggest changes in NEABW composition and corrosivity during the Holocene.
• Precession and remote forcings shaped complex monsoon–gyre–upwelling interactions.
Abstract
Cabo Verde hosts unique, highly biodiverse marine ecosystems that thrive on volcanic seamounts and island slopes. These ecosystems are shaped by distinct oceanographic dynamics, influenced by the southeastern edge of the North Atlantic Subtropical Gyre (NASTG) and by seasonal upwelling. To explore regional oceanographic variability over time, this study investigates Holocene (last 11.7 ka) sediments using multi-proxy palaeoenvironmental reconstructions from a short core retrieved from ∼ 4,400 m water depth off Cabo Verde.
During the Early Holocene, year-round upwelling, or an intensified Guinea Dome, may have inhibited the development of the strong summer stratification characteristic of the modern regional non-upwelling season. Despite humid conditions over the continent, sea surface temperatures (SSTs) remained relatively low during this subepoch, diverging from the present-day pattern in Northwest Africa, where the wet season is marked by weaker upwelling and higher SSTs. This oceanographic state was likely driven by precession-induced insolation changes associated with the precession minimum, which may have modified seasonal regional wind regimes and influenced broader atmospheric processes. Teleconnections related to transitional postglacial conditions and/or continental climate feedbacks, may also have played a role. The Middle Holocene, corresponding to the most humid conditions of this epoch in Northwest Africa, is characterized by reduced upwelling and an eastward expansion of the NASTG, inferred from warmer subsurface conditions at our study site. This interval also provides tentative evidence for enhanced input of North Atlantic Deep Water (NADW) into the Northeast Atlantic Bottom Water (NEABW). During the Late Holocene, intensified upwelling and a reduced influence of the NASTG, possibly due to a westward retraction of its eastern boundary, are suggested at our site, occurring under arid conditions in Northwest Africa.
These results highlight that, despite the overall climatic stability of the Holocene, oceanographic conditions off Cabo Verde experienced significant changes in seasonal upper ocean stratification, upwelling, subtropical gyre influence, and deep-water structure. Such insights improve our understanding of regional climate-ocean interactions, helping to refine climate models and improve predictions of ecosystem responses in this sensitive marine region
Decoding microbial diversity, biogeochemical functions, and interaction potentials in red sea hydrothermal vents
No full textThe effect of pre-eruptive fluid exsolution on the volatile budgets of large explosive eruptions in Central America: constraints from fluid inclusions and thermobarometry
No full textThe amounts of volatiles emitted from large Plinian eruptions are typically estimated using the difference between their concentration in silicate melt inclusions formed at depth, and their concentration in the partially degassed glassy groundmass of tephras (the so-called petrologic method). However, a pre-eruptive fluid phase coexisting with the magma prior to eruption may add significantly to the emission budgets. We have combined previously published chlorine emission data obtained by the petrologic method from seven Plinian eruptions along the Central American Volcanic Arc (CAVA) with new data obtained from magmatic fluid inclusions in the same samples. The presence of the magmatic fluid inclusions demonstrates the pre-eruptive criticality of these volcanic systems. The pre-eruptive magmatic fluid phase of silicic CAVA eruptions is water dominated, and contains on average 5 ± 3.5 mass% NaCl equivalents and 5 ± 4 mass% CO 2 with no systematic along-arc variations. The pressures obtained from the typical magmatic fluid inclusions range between 140 ± 30 and 170 ± 30 MPa for the various eruptions, which corresponds to minimum pre-eruptive water contents in the melts between 4.8 and 5.2 mass%. We consider a scenario where each magma coexists with between 1 and 5 volume % pre-eruptive fluid, which is erupted together with the magma, and thus adds on average 6 to 30 mass % “excess” Cl to the degassing budgets determined by the petrologic method, and by inference also represents minimum values for “excess” Br degassing. The high efficiency of Br for ozone destruction in the stratosphere is enhanced through interactions with sulfur aerosols present in an eruption column, causing average increases in stratospheric halogen loading, referred to as equivalent effective stratospheric chlorine (EESC), between 6 and 97% per eruption. Depending on the amount and composition of a pre-eruptive fluid phase, the estimated stratospheric loading of a Plinian eruption may thus be doubled compared to data from the petrologic method. Fluid inclusion data from other large eruptions may therefore be used to significantly revise the global emission budgets and the effects of stratospheric ozone destruction related to Cl and Br from large explosive eruptions
Origin of the King’s Trough Complex in the North Atlantic: Interplay between a transient plate boundary and the early Azores mantle plume
No full textThe King’s Trough Complex (KTC), a huge, canyon-like structure in the eastern North Atlantic consists of a ~500 km long series of NW-SE oriented basins and flanking ridges and cuts into a plateau of thickened oceanic crust that surrounds the 45°N geochemical anomaly at the Mid-Atlantic Ridge (MAR). Here we present geochemical data (including radiogenic isotopes), as well as new 40Ar/39Ar ages from magmatic rocks dredged from the eponymous King’s Trough and the smaller Peake and Freen Deeps adjoining to the east, together referred to as KTC, and from the Gnitsevich Seamounts located to the west. The samples can be divided into two groups: (1) King’s Trough and Gnitsevich Seamounts as well as cored rocks from the plateau that possess mainly alkali basaltic, geochemically enriched compositions overlapping with Azores Island lavas, and (2) tholeiitic lavas from the Peake and Freen Deeps, located just outside of the thickened plateau, that exclusively show depleted, normal MORB signatures. Ages of lavas dredged along the flanks of the King’s Trough progressively decrease from east to west and are systematically 3-5 m.y. younger than their surrounding oceanic crust suggesting that the sampled structures formed somewhat off-axis. Based on isotopic similarity, it is proposed that the 45°N anomaly at the MAR represents a remnant of a mantle plume, which formed the thickened plateau by plume-ridge interaction from ~53-55 Ma. A jump of the Eurasian-Iberian/African plate boundary to this region at ~37 Ma resulted in progressive opening of the KTC as a graben structure from E to W by oblique extension until ~24 Ma. The largest extension took place at its eastern end, outside of the area of plume-influenced, thickened crust, with the opening of the ultra-deep Peake and Freen Deeps resulting in high-degree, shallow decompression melting. Ages between 36.0 and 39.5 Ma obtained from this volcanism support this plate kinematic model. At ~20-23 Ma, the plate boundary relocated to the Azores-Gibraltar Fracture Zone further south, which caused deflection of the plume and formation of the Azores Plateau. The 45°N MAR melting anomaly is therefore regarded as a waning branch of the early Azores mantle plume
Hydrothermal Circulation During Continental Breakup: Evolution of a Subseafloor Stockwork Mineralization at the South China Sea Rifted Margin
No full textHydrothermal activity during continental breakup and early oceanic spreading is poorly understood due to limited samples. A basalt‐hosted hydrothermal mineralization, drilled during IODP Expedition 367/368 at the northern South China Sea at the continent‐ocean transition provides a unique opportunity to investigate this phenomenon. Detailed petrological and geochemical analyses, including pyrite trace elements reveal a diverse paragenetic sequence consisting of two main generations of hydrothermal alteration and veining. The first generation is marked by chloritization and disseminated pyrite mineralization, followed by quartz‐epidote‐pyrite veining with minor chalcopyrite and sphalerite precipitation. The second generation consists of early, sulfide‐free siderite‐ankerite veins, followed by the formation of ankerite‐dolomite veins containing pyrite, chalcopyrite, and rare calcite. These variations highlight the spatiotemporal complexity of this hydrothermal system. Pyrite trace element chemistry, particularly Co, Ni, and Cu enrichments, suggests peak fluid temperatures exceeding 250°C for all sulfide‐bearing generations. Pyrite trace elements further contribute to a better understanding of the nature of the underlying crust at this continent‐ocean transition. They infer a reaction of hydrothermal fluids with underlying basalt and syn‐rift sediments with no indication for interaction with continental or mantle material. The early silicate‐rich generation of chlorite‐quartz‐sulfide mineralization shows characteristics similar to other drilled hydrothermal systems, for example, TAG on the Mid‐Atlantic Ridge. However, abundant epidote and siderite, as observed here, have not been documented for modern seafloor stockwork zones. This indicates potentially important differences in the characteristics and formation conditions of hydrothermal mineralization formed during the onset of oceanic spreading and those formed at mature mid‐ocean ridges.
Plain Language Summary
This study describes a hydrothermal system formed by heated seawater circulating through a warm, young crust in the South China Sea. Here, the continent broke apart 32 million years ago, forming the ocean we see today. We collected samples to learn how these hot fluids formed minerals. Our study provides new insights into how hydrothermal circulation evolves as oceans begin to form. By analyzing their chemical compositions, we learned that the minerals precipitated in two phases: the first phase involved the formation of silica‐rich minerals chlorite, pyrite, quartz, and epidote. The second phase formed carbonate‐rich veins of siderite, ankerite, and dolomite with pyrite and chalcopyrite at the end of this phase. The chemical composition of the pyrite suggests high temperature fluids, higher than 250°C. The study also found that these fluids mainly reacted with basalt and sediments in the deep ocean crust, and not with continental or mantle rocks. While some characteristics of this hydrothermal system are similar to those found in large, older oceans like the Mid‐Atlantic Ridge, key differences, such as the presence of minerals such as epidote and siderite, indicate that hydrothermal activity during early ocean formation may differ from systems at mature ocean floor spreading centers.
Key Points
Hydrothermal stockwork mineralization, drilled from a continent‐ocean transition, is formed by hydrothermal activity
Several phases of cross‐cutting sulfide‐bearing hydrothermal veins reflect a multi‐stage evolution
The occurrence of siderite, ankerite, and epidote in veins differs from modern seafloor hydrothermal stockwork
The chromosomal genome sequence of the glass sponge Aphrocallistes beatrix Gray, 1858 and its associated microbial metagenome sequences
No full textWe present a genome assembly from an individual Aphrocallistes beatrix (Porifera; Hexactinellida; Hexasterophora; Sceptrulophora; Aphrocallistidae). The genome sequence is 100.50 megabases in span. Most of the assembly is scaffolded into 19 chromosomal pseudomolecules. The mitochondrial genome has also been assembled and is 18.0 kilobases in length. Several symbiotic bacterial genomes were assembled as MAGs, including Gammaproteobacteria and Nitropumilaceae (Archaea)
Deep-sea ecosystems of the North Atlantic Ocean: discovery, status, function and future challenges
No full textThe North Atlantic is an ocean basin with a diversity of deep-sea ecosystems. Here we provide a summary of the topography and oceanography of the North Atlantic including the Gulf of Mexico and Caribbean Sea, provide a brief overview of the history of scientific research therein, and review the current status of knowledge of each of 18 pelagic and benthic deep-sea ecosystems, with a particular focus on knowledge gaps. We analyse biodiversity data records across the North Atlantic and highlight spatial data gaps that could provide important foci for future expeditions. We note particular data gaps in EEZs of nations within and bordering the Caribbean Sea. Our data provide a baseline against which progress can be tracked into the future. We review human impacts caused by fishing, shipping, mineral extraction, introduction of substances, and climate change, and provide an overview of international, regional and national measures to protect ecosystems. We recommend that scientific research in the deep sea should focus on increasing knowledge of the distribution and the connectivity of key species and habitats, and increasing our understanding of the processes leading to the delivery of ecosystem services. These three pillars - distribution, connectivity, ecosystem function - will provide the knowledge required to implement conservation and management measures to ensure that any deep-sea development in the future is sustainable. Infrastructure and capacity are unevenly distributed and implementation of strategies that will lead to more equitable deep-sea science is required to ensure that essential science can be delivered
Environmental Filtering Drives Widespread Trait Convergence in Marine Demersal Ray‐Finned Fishes
No full textAim:
Understanding the processes that shape the distribution of biodiversity in the oceans is central for predicting and conserving ecosystems under global change. Although a vast literature exists on drivers of species diversity, the geographical patterns and drivers of ecosystem functioning, and in particular the traits that shape this functioning, remain relatively unexplored. We address this gap by testing the effects of environment, fishing pressure and evolutionary history on fish trait compositions across continental shelf seas using scientific trawl surveys.
Location:
Northern Hemisphere shelf seas.
Time Period:
1999–2021.
Major Taxa Studied:
Marine demersal ray-finned fishes.
Methods:
Here, we aggregate trawl and trait information (body size, habitat, reproduction, trophic ecology and growth) for 1164 demersal ray-finned fishes on continental shelf seas throughout the Northern Hemisphere to test the relative importance of environmental, evolutionary and anthropogenic drivers in shaping trait compositions. These patterns are tested across three different spatial scales (100 km2 to marine Ecoregions) using linear and non-linear models. We also compare trait compositions to expectations under null and neutral models.
Results:
Trait compositions throughout shelf seas are always positively related to environmental conditions but appear strongly associated with evolutionary history on the northeast Pacific shelves. Although fishing can alter individual traits and deplete populations, it shows no explanatory power in describing trait compositions. The majority (81%) of trait compositions are more similar than expected under neutral drift.
Main Conclusions:
We find that environmental filtering has strongly shaped the functional convergence of fish communities while, in contrast to expectations, phylogenetic conservatism across evolutionary lineages appears uniquely strong in the Pacific Ocean but less important in the Atlantic. The widespread role of environmental conditions in shaping fish traits highlights the potential sensitivity of community functioning to environmental and climate change and sheds new light on the potential for trait-based conservation strategies