10.1 Marine environments

Marine environments are diverse and complex, ranging from shallow coastal areas to the deep ocean. These habitats shape the distribution of marine life and influence sedimentary processes. Understanding marine environments is crucial for interpreting the fossil record and reconstructing past ecosystems.

Physical, chemical, and biological factors interact in marine settings, affecting sediment deposition and fossil preservation. From coastal zones to abyssal plains, each environment has unique characteristics that leave distinct signatures in the rock record, providing valuable insights into Earth's history.

Types of marine environments

• Marine environments encompass a wide range of aquatic habitats, from shallow coastal areas to the vast expanses of the open ocean
• Understanding the diversity and characteristics of these environments is crucial for interpreting the fossil record and reconstructing past ecosystems

Coastal vs open ocean

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• Coastal environments include areas near the shore, such as beaches, estuaries, and continental shelves, which are influenced by terrestrial processes and freshwater input
• Open ocean environments, also known as pelagic zones, are vast expanses of water that extend beyond the continental shelves and are characterized by their distance from land and greater water depths

Intertidal zones

• Intertidal zones are the areas between the high and low tide marks, exposed during low tide and submerged during high tide
• These zones are characterized by strong environmental gradients (temperature, salinity, and exposure to air) and are home to diverse communities of organisms adapted to these fluctuating conditions (barnacles, mussels, and seaweeds)

Estuaries and lagoons

• Estuaries are partially enclosed coastal bodies of water where freshwater from rivers and streams mixes with saltwater from the ocean, creating unique brackish environments
• Lagoons are shallow, protected bodies of water separated from the open ocean by barriers such as sandbars or coral reefs, often with limited water exchange and variable salinity

Continental shelves

• Continental shelves are the submerged edges of continents, extending from the shoreline to the shelf break, where the seafloor slopes steeply towards the deep ocean
• These shallow, relatively flat areas are often rich in nutrients and support diverse marine ecosystems, including important commercial fisheries

Deep sea environments

• Deep sea environments include the vast, dark, and cold regions of the ocean below the photic zone (200 m), where sunlight cannot penetrate
• These environments are characterized by high pressure, low temperatures, and unique adaptations of organisms to survive in these extreme conditions (bioluminescence, slow metabolism, and specialized feeding strategies)

Physical characteristics

• The physical properties of marine environments play a crucial role in shaping the distribution and diversity of marine life, as well as influencing sedimentary processes and fossil preservation
• Key physical characteristics include water depth, pressure, temperature, salinity, density, and ocean circulation patterns

Water depth and pressure

• Water depth varies greatly across marine environments, from shallow coastal areas to the deepest parts of the ocean (Mariana Trench, ~11,000 m)
• Pressure increases with depth at a rate of approximately 1 atmosphere (atm) for every 10 meters, leading to extreme pressures in the deep sea that influence the physiology and adaptations of marine organisms

Temperature variations

• Ocean temperatures range from near-freezing in polar regions to over 30°C in tropical surface waters, with a gradual decrease in temperature with increasing depth
• These temperature variations create distinct thermal zones (epipelagic, mesopelagic, and bathypelagic) and influence the distribution and metabolism of marine organisms

Salinity and density

• Salinity is the measure of dissolved salts in seawater, typically around 35 parts per thousand (ppt) in the open ocean, but can vary in coastal areas due to freshwater input and evaporation
• Density is determined by temperature and salinity, with colder and saltier water being denser and sinking, while warmer and fresher water is less dense and rises, driving ocean circulation patterns

Ocean currents and circulation

• Ocean currents are driven by wind patterns, density differences (thermohaline circulation), and the Earth's rotation (Coriolis effect), creating a global conveyor belt that redistributes heat, nutrients, and organisms
• Surface currents (Gulf Stream) and deep-water currents (Antarctic Bottom Water) play important roles in regional climate, productivity, and the dispersal of marine larvae and plankton

Sediment types and distribution

• Marine sediments can be classified as terrigenous (derived from land), biogenic (produced by organisms), or hydrogenous (precipitated from seawater)
• Sediment distribution is influenced by factors such as proximity to land, ocean currents, water depth, and biological productivity, with coarser sediments (sand and gravel) found in high-energy coastal environments and finer sediments (silt and clay) in deeper, calmer waters

Chemical characteristics

• The chemical properties of seawater, including dissolved gases, nutrients, pH, and redox conditions, play critical roles in marine ecosystem functioning, productivity, and fossil preservation
• These characteristics are influenced by physical processes, biological activity, and the input of organic matter from terrestrial and marine sources

Dissolved gases and nutrients

• Seawater contains dissolved gases, such as oxygen, carbon dioxide, and nitrogen, which are essential for marine life and biogeochemical cycles
• Nutrients, including nitrates, phosphates, and silicates, are crucial for primary productivity and are often limiting factors in marine ecosystems, with their availability influenced by upwelling, river input, and biological cycling

pH and alkalinity

• The pH of seawater is typically around 8.1, making it slightly alkaline due to the buffering capacity of dissolved carbonate and bicarbonate ions
• Ocean acidification, caused by the absorption of atmospheric CO2, can lower pH and impact calcifying organisms (corals and mollusks) by reducing the availability of carbonate ions for shell and skeleton formation

Redox conditions

• Redox (reduction-oxidation) conditions in marine sediments are determined by the availability of oxygen and the presence of organic matter
• Oxic conditions occur in well-oxygenated environments, while anoxic conditions develop in oxygen-depleted settings, such as stagnant basins or beneath the sediment-water interface, influencing the preservation of organic matter and the formation of specific mineral deposits (pyrite)

Organic matter input and preservation

• Organic matter in marine environments originates from the remains of marine organisms, terrestrial plant debris, and dissolved organic compounds
• The preservation of organic matter in marine sediments depends on factors such as sedimentation rate, oxygen availability, and the efficiency of microbial decomposition, with anoxic conditions and rapid burial favoring the preservation of organic-rich deposits (black shales and oil source rocks)

Biological aspects

• Marine environments host an incredible diversity of life, from microscopic plankton to large mammals, with organisms adapted to various ecological niches and environmental conditions
• Understanding the biological aspects of marine ecosystems is essential for interpreting fossil assemblages and reconstructing past environments and evolutionary histories

Primary productivity and nutrient cycling

• Primary productivity in the ocean is driven by photosynthetic organisms, such as phytoplankton and marine algae, which form the base of marine food webs
• Nutrient cycling involves the uptake, transfer, and regeneration of essential elements (carbon, nitrogen, and phosphorus) through biological processes, such as photosynthesis, respiration, and decomposition, and physical processes, like upwelling and mixing

Planktonic vs benthic organisms

• Planktonic organisms are those that drift or swim weakly in the water column, including phytoplankton (diatoms and dinoflagellates) and zooplankton (copepods and larvae of larger organisms)
• Benthic organisms live on or in the seafloor, such as sessile invertebrates (corals and sponges), burrowing animals (worms and clams), and bottom-dwelling fish, and play important roles in sediment mixing and nutrient cycling

Marine food webs and trophic levels

• Marine food webs describe the feeding relationships and energy transfer between organisms in an ecosystem, with primary producers at the base and consumers at higher trophic levels
• Trophic levels include primary producers, primary consumers (herbivores), secondary consumers (carnivores), and decomposers, with energy transfer efficiency decreasing at each step due to metabolic losses and incomplete consumption

Biodiversity patterns

• Marine biodiversity encompasses the variety of life in the ocean, including genetic diversity, species richness, and ecosystem diversity
• Biodiversity patterns are influenced by factors such as latitude (higher diversity in the tropics), water depth (decreasing diversity with increasing depth), and habitat complexity (higher diversity in structurally complex environments like coral reefs)

Adaptations to marine life

• Marine organisms have evolved a wide range of adaptations to cope with the challenges of living in aquatic environments, such as buoyancy control, osmotic regulation, and specialized feeding strategies
• Examples include streamlined body shapes for efficient swimming (sharks and dolphins), gas-filled swim bladders for buoyancy control (fish), and filter-feeding structures for capturing plankton (baleen whales and oysters)

Sedimentary processes

• Sedimentary processes in marine environments encompass the erosion, transport, deposition, and modification of sediments, which ultimately form the rock record that preserves fossils and provides insights into past environmental conditions
• Understanding these processes is crucial for interpreting the depositional settings, diagenetic history, and paleoecology of marine fossil assemblages

Clastic vs chemical sedimentation

• Clastic sedimentation involves the accumulation of rock and mineral fragments derived from the weathering and erosion of pre-existing rocks, with sediment grain size ranging from clay to boulders
• Chemical sedimentation occurs through the precipitation of minerals from seawater, such as calcium carbonate (limestone and dolomite) and evaporites (gypsum and halite), often mediated by biological processes or changes in environmental conditions

Sediment transport and deposition

• Sediment transport in marine environments is driven by currents, waves, and gravity, with particles moved as bedload (rolling and sliding along the seafloor) or suspended load (carried in the water column)
• Deposition occurs when the energy of the transporting medium decreases, allowing sediments to settle and accumulate, forming various sedimentary structures (ripples, cross-bedding, and graded bedding) that reflect the depositional conditions

Bioturbation and ichnofossils

• Bioturbation refers to the disturbance and mixing of sediments by the activities of organisms, such as burrowing, feeding, and locomotion, which can alter the original sedimentary fabric and geochemical gradients
• Ichnofossils are the fossilized traces of organism behavior, such as burrows (Thalassinoides), trails (Archaeonassa), and footprints, providing valuable information about the paleoecology and depositional environment

Diagenesis and lithification

• Diagenesis encompasses the physical, chemical, and biological changes that occur in sediments after deposition, including compaction, cementation, and recrystallization, which transform loose sediments into solid rock
• Lithification is the process by which sediments are converted into sedimentary rocks through compaction and cementation, with common cementing agents including calcite, silica, and iron oxides

Fossil preservation

• The preservation of fossils in marine settings depends on a complex interplay of biological, chemical, and physical factors that influence the potential for organism remains to be incorporated into the sedimentary record and resist destruction over geologic time
• Understanding the processes and biases in marine fossil preservation is essential for interpreting the paleontological record and reconstructing past ecosystems

Fossilization processes in marine settings

• Common fossilization processes in marine environments include permineralization (infiltration of minerals into pore spaces), carbonization (preservation of organic material as a thin film of carbon), and authigenic mineralization (precipitation of minerals on or within organism remains)
• Other processes, such as pyritization (replacement by iron sulfide minerals) and phosphatization (replacement by phosphate minerals), can lead to exceptional preservation of soft tissues in specific geochemical conditions

Biases in marine fossil record

• The marine fossil record is inherently biased due to differential preservation potential among organisms, with hard-bodied taxa (mollusks and brachiopods) more likely to be preserved than soft-bodied ones (jellyfish and worms)
• Other biases include temporal and spatial variations in sedimentation rates, diagenetic alteration, and the incompleteness of the stratigraphic record due to erosion and non-deposition

Exceptional preservation (Lagerstätten)

• Lagerstätten are sedimentary deposits that exhibit exceptional preservation of fossils, often including soft tissues, delicate structures, and complete organisms
• Examples of marine Lagerstätten include the Burgess Shale (Cambrian) and the Solnhofen Limestone (Jurassic), which provide unique insights into the diversity and ecology of ancient marine communities

Fossil assemblages and communities

• Fossil assemblages are groups of fossils found together in a particular stratigraphic horizon or locality, representing a snapshot of the organisms that lived and died in a specific environment and time
• By studying the composition, diversity, and ecological interactions within fossil assemblages, paleontologists can reconstruct ancient marine communities and interpret their paleoecology and depositional setting

Marine depositional environments

• Marine depositional environments are characterized by distinct physical, chemical, and biological conditions that influence sediment accumulation, fossil preservation, and the resulting sedimentary facies
• Recognizing and interpreting these environments in the rock record is crucial for reconstructing past sea-level changes, climate conditions, and paleogeography

Shoreline and beach deposits

• Shoreline and beach deposits form in the zone where land meets the sea, influenced by waves, tides, and coastal processes
• Characteristic features include well-sorted sand and gravel, parallel lamination, and cross-stratification, with common trace fossils such as Skolithos and Ophiomorpha, reflecting high-energy conditions and occasional subaerial exposure

Tidal flats and sabkhas

• Tidal flats are low-relief, intertidal areas that develop along coastlines with large tidal ranges, characterized by alternating layers of sand and mud deposited during tidal cycles
• Sabkhas are supratidal, evaporitic environments that form in arid coastal settings, characterized by the precipitation of evaporite minerals (gypsum and halite) and the presence of microbial mats and fenestral porosity

Reefs and carbonate platforms

• Reefs are wave-resistant structures built by the skeletal growth and accumulation of organisms such as corals, sponges, and algae, forming complex, three-dimensional habitats for diverse marine communities
• Carbonate platforms are broad, shallow-water areas where carbonate sediments accumulate, often surrounding reefs and characterized by a variety of depositional environments (lagoons, shoals, and tidal flats)

Submarine fans and turbidites

• Submarine fans are large, cone-shaped accumulations of sediment that form at the base of continental slopes, fed by turbidity currents (underwater avalanches of sediment-laden water)
• Turbidites are the sedimentary deposits of turbidity currents, characterized by graded bedding, sole marks, and a predictable vertical sequence of sedimentary structures (Bouma sequence) reflecting the waning flow conditions

Abyssal plains and pelagic sediments

• Abyssal plains are vast, flat, and deep (>4,000 m) regions of the ocean floor, covering a significant portion of the Earth's surface and characterized by slow sedimentation rates and fine-grained, clay-rich sediments
• Pelagic sediments accumulate in the open ocean, far from continental influences, and are composed primarily of the skeletal remains of planktonic organisms (foraminifera and coccolithophores) and wind-blown dust, providing valuable records of past ocean conditions and climate change

Marine biostratigraphy

• Biostratigraphy is the study of the distribution of fossils within sedimentary strata, using the presence and absence of specific taxa to establish relative ages, correlate rock units, and reconstruct past environments
• In marine settings, biostratigraphy relies on the evolutionary changes and stratigraphic ranges of various fossil groups, such as microfossils, invertebrates, and vertebrates

Index fossils and zonation

• Index fossils are species or genera with short stratigraphic ranges, wide geographic distributions, and easy identification, making them useful for defining and correlating biostratigraphic units
• Biostratigraphic zonation involves the division of sedimentary sequences into zones based on the presence or absence of specific index fossils, with each zone representing a distinct time interval and enabling correlation between different regions and basins

Microfossils in marine stratigraphy

• Microfossils, such as foraminifera, calcareous nannoplankton, and radiolarians, are particularly valuable for marine biostratigraphy due to their small size, abundance, and rapid evolutionary rates
• The study of microfossil assemblages and their stratigraphic distribution allows for high-resolution dating, correlation, and paleoenvironmental reconstruction, especially in deep-sea sediments where other fossil groups may be scarce

Chemostratigraphy and isotope ratios

• Chemostratigraphy involves the use of geochemical variations in sedimentary rocks, such as changes in elemental concentrations or isotope ratios, to establish stratigraphic correlations and reconstruct past environmental conditions
• Stable isotope ratios of carbon ($δ^{13}C$) and oxygen ($δ^{18}O$) in marine carbonates and organic matter provide insights into past ocean circulation, productivity, and global climate events, such as the Paleocene-Eocene Thermal Maximum (PETM)

Sequence stratigraphy and sea-level changes

• Sequence stratigraphy is a framework for analyzing and interpreting sedimentary successions in terms of changes in relative sea level, sediment supply, and accommodation space
• By recognizing key surfaces (sequence boundaries, transgressive surfaces, and maximum flooding surfaces) and stacking patterns of sedimentary facies, sequence stratigraphy enables the reconstruction of past sea-level fluctuations and the prediction of reservoir and source rock distribution in hydro