Introduction to the marine environment from a geological perspective

PRATOLONGO, PAULA D.; PAN, JERÓNIMO; CUADRADO, DIANA G.

Marine Biology: a functional approach to the oceans and their organisms

CRC Press/Taylor & Francis Group

Lugar: Boca Ratón, FL; Año: 2022; p. 3 - 20

1. The age of the ocean is about 4,000 MY. Considered the largest habitat for life on Earth (97% of the biosphere by volume), oceans cover 71% of its surface and harbor 97.5% of its superficial water. The oceans produce about 40% of the annual global primary productivity, and their average salinity is 34.48.2. The oceanic crust is ~ 8 km-thick, and geologically younger (≤ 200 MY) in relation to continental crust. Ocean basins are formed by virtue of the oceanic crust being made up of basaltic material, which is denser than the granitic rocks that continental crusts are made of.3. The hypotheses of isostasy explains differences in thickness in Earth?s lithosphere. The lighter lithosphere floats over the denser underlying material of the asthenosphere. Isostatic equilibrium occurs where the crust and mantle settle into place in absence of disturbing forces. There are two main hypotheses (Airy?s and Pratt?s) for isostatic adjustment.4. The hypothesis of continental drift has been incorporated in the Theory of Plate Tectonics, which explains that continents move as part of lithospheric plates on top of the asthenosphere, as a result of the underlying convection cells in the upper mantle.5. Where two plates meet, there might be divergent boundaries in the case of two lithospheric plates being pulled apart from each other. For instance, at mid-ocean ridges, new oceanic crust and lithosphere is created by seafloor spreading; at zones of continent-to-continent rifting, divergent boundaries may cause new ocean basin to form. On the other hand, convergent boundaries occur where two plates slide toward each other to either form a subduction zone or a continental collision. Finally, in transform or conservative boundaries, two lithospheric plates slide along transform faults, where plates are neither created nor destroyed.6. The Wilson cycle explains the cyclical opening and closing of ocean basins. Active continental margins are associated with convergent or transform plate boundaries; a narrow continental shelf quickly drops off into the depths of the subduction trench. On the other hand, passive margins occur away from plate boundaries, tectonic activity is minimal, and there is sediment accumulation on the wider continental shelves.7. The evolution of life on Earth (~ 3.5 BY ago) and the invention of oxygenic photosynthesis by cyanobacteria led to an unprecedented increase in O2 ~ 2.4 BY ago (known as the Great Oxidation Event, GOE). This transition from a non-oxidizing atmosphere to one with free O2 has been well recorded in sedimentary sequences by the disappearance of banded-iron formations (controlled by the availability of ferrous Fe+2 in seawater), and their replacement by red beds (oxidized Fe+3). Some of the earlier life forms left conspicuous fossils known as stromatolites, which declined in abundance with the appearance of animal grazers, later followed by bioturbation and the Cambrian substrate revolution.8. The most recent period of repeated glaciations occurred during the Pleistocene Epoch, beginning about 2.6 MY ago and lasting until about 11,700 yr ago, coinciding with the start of the Holocene Epoch. The episodic sequence of glacial and interglacial periods throughout the Pleistocene is explained by Milankovitch s hypothesis for cyclical variations in Earth?s eccentricity, obliquity and precession.9. During glacial periods, large amounts of water are stored in ice sheets on land, lowering the mean global sea level; the sea level changes are called glacial-eustatic. After the Last Glacial Maximum (LGM), the melting of large ice sheets produced a net transfer of mass to the oceans, but also Earth?s crust was uplifted (?bounced?) in a process of glacio-isostatic adjustment.10. Estuaries are semi-enclosed coastal bodies of water with an open connection to the ocean, and within which seawater is diluted by land freshwater inputs. Estuaries adapted rapidly to the Holocene sea level rise.11. The geomorphologic classification of estuaries considers four different types: (1) coastal plain, (2) bar-built, (3) fjords, and (4) tectonically-produced estuaries. Additionally, estuaries may be classified according to the estuarine circulation created due to density differences between seawater and freshwater, into four major types: (1) salt-wedge, (2) partially-stratified, (3) vertically homogeneous, and (4) fjord estuaries.12. Deep-sea pelagic sediments, collectively called oozes, are mostly composed of fine-textured small shells of CaCO3 or SiO2 from once living organisms (hence, biogenous). At a certain depth (the lysocline), carbonatic shell dissolution becomes detectable. On the other hand, ~ 38% of the deep ocean floor is covered by terrigenous red clays. An ooze and a red clay are distinguished by the proportion of its biogenous component (30% or more for oozes).13. Scholars have divided the oceans into distinct divisions, as a means of better understanding their characteristics, and conceptualizing them. These divisions respond to geological, physical and chemical attributes.14. The most basic division considers benthic and pelagic environments. The former is sub-divided into the supralittoral, intertidal and sublittoral zones down to the shelf-break located at 200 m-depths; and the bathyal zone (200?3,000 m-depths), the abyssal plains? (3,000?6,000 m-depths) and the hadal zone (6,000?10,000 m-depths). Pelagic environments have both horizontal and vertical divisions. The neritic zone comprises the water masses over the continental shelf, while the oceanic zone is subdivided into different depth-layers (the most significant being the epipelagic zone from 0?200 m-depths).