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Sea Level Change

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Sea Level Change Sea Level Change Outline Causes of sea level change Temporal (time) scales of sea level changes Climate-related sealevel change and oxygen isotope stages Eustatic and relative sea levels Evidence of sealevel change Sedimentary records of sealevel changes Causes of sea level change Why does the sea level change? Changes in the relative volumes of seawater and polar ice caps and ice sheets a) Formation and melting of ice caps e.g., if Antarctic ice cap and Greenland ice sheets melted completely the sealevel would rise 60-80 m. b) Thermal expansion of seawater 10ºC increase in seawater temperature would cause 10 m increase in sea level These processes are a function of climate: external earth process Changes in the volume of ocean basins containing seawater The ocean basins changing in size and shape: Increase in length of the ocean ridges and rate of seafloor spreading would decrease the volume of ocean basins, causing a sealevel rise. The main process is Plate Tectonics (internal forces) Temporal scale of sea level change Sealevel fluctuations occur on time scales of: Millions of years Thousands of years Diurnal variations: Tides and weather Tides: Diurnal variations is the result of gravitational attraction between Earth, Moon, Sun and rotation of Earth. The times and amplitude of tides at a locale are influenced by: • Alignment of Sun and Moon, • Pattern of tides in the deep ocean • The amphidromic (zero amplitude) systems of the oceans creating waves, tidal currents • The shape of the coastline and near-shore bathymetry Tides Neap tides occur during quarter Moon Spring tides occur during full and new Moon Tidal amplitude Mid-spring range: 14.5 m Extreme range: 16.3 m Amphidromic system and points These occur because of the Coriolis effect and interference within oceanic basins, seas and bays creating a wave pattern Tide amplitude indicated by color. The white lines are cotidal lines spaced at phase intervals of 30° (a little over 1 hr).The amphidromic points are the dark blue areas where the lines come together. Global changes Eustatic: affecting shelves via transgessions and regressions. a) Glacio-eustatic: originating from volume of ocean water due changes in volume of ice, b) Volume of ocean ridges: spreading rates causing changes in the average depth of the ocean basin. Regional changes: Regional tectonics: transgessions and regressions affecting a particular shelf areas Causes of regional changes: 1. Tectonic uplift or subsidence along continental margins 2. Loading and unloading of the continents by glaciers and water 3. Sediment loading causing the basins to subside Eustatic sea level (eustacy) Eustasy refers to global sea level independent of local factors. It is the measure of distance between sea surface and a fixed datum (e.g., centre of the earth). It is variable because of the tectonic behaviour of sedimentary fill (subsidenc and uplift). As discussed earlier, variations in eustacy is controlled by: a) Changes in the volume of seawater (climate control) b) Changes in volume of the ocean basins (tectonic control) Relative sea level This is the distance between sea surface and a local datum, such as the top of the basement rocks in a sedimentary basin. Relative sea level is controlled by: a) Eustasy b) Changes in elevation of continents and seafloor Relative sea level is useful term, as it does not imply that a particular mechanism is responsible for the sea level change, or that it is global in extent. It accounts for both local subsidence (or uplift) and eustatic changes in sea-level. Question: Explain a situation in a sedimentary basin whereby eustatic sea level is falling during a relative sea level rise. Eustasy and relative sea level Concept of relative sealevel Carbonate crust underlying black sulfidic sediment in NE Central Basin Water depth: The distance between sea-bed (sea floor) and sea surface (i.e., sea level). This term is not the same as relative sea level. Even if the basin subsidence and eustatic sea level are stable, water depth will be reduced as sediment fills the basin. Short-term changes: tide-gauge measurements Stable crust Global signal Subsidence due to water& HC extraction Tectonic uplift Glacial rebound Sealevel affects the shelf areas and coastal areas most because here: Waves, tides and currents are most active Productivity is great Sediments are associated with rapid nutrient cycling, life activities, gas exchange Base-line is located within here, causing erosion and deposition Flooded shelves absorb more sun light than exposed ones, adding heat to the global heat budget Submerged shelves have little chemical weathering which keeps the CO2 in the atmosphere Evidence of sealevel changes Sedimentary evidence: a) Isotopic evidence from benthic foraminifera b) Coastal and shelf-margin sedimentary facies changes Isotopic evidence Oxygen Isotope Stages Odd numbers: interglacial (warm) periods Even numbers: glacial (cold) periods Sealevel estimates from Oxygen Isotope δ18O Composition of foraminifera shells Measurement of past sealevel changes Oxygen isotopes Coastline maps Coastal sediments Oxygen isotopes 18 16 18 16 ( O/ O)sample- ( O/ O)std Oxygen isototope ratio: δ18O = ------------------------------------ x 1000 18 16 ( O/ O)std is a function of: 1. Ice volume: δ18O value increase with ice volume and, thus, sealevel fall (1 ‰ is equivalent to 100 m of sealevel change) 2. Salinity (evaporation-precipitation): δ18O values increase with salinity and evaporation, decrease with increased precipitation 3. Temperature: δ18O decrease with temperature (0.2 ‰ per 1ºC) Benthic forams are good recorders of the seawater oxygen isotope values On the basis of this we distinguish marine oxygen isotope stages Carbonate crustsCoastal as archives sedimentary of the cyclic changes evidence in the chemical composition of the fluids. NE Central Basin Nautile dive 661, June 2007 Erosion Erosion Erosion •Mineralogy •Isotopic composition Evidence of sealevel change: Seismic sections of shelf areas a) Shelf-crossing unconformities b) Ancient shorelines: Berms, onlapping sediments c) Wave-cut platforms and notches Vertical exag.: 4.3 x Onlapping sediment unit Wave-cut notch Prograding units Sedimentary records of sealevel change Fluid vents Cyclic changes in sedimentary sequences: Cold fuid venting, carbonate mound, bacterial mats and benthic fauna in SE Tekirdağ Basin climate controlled by astronomical (Milankovich) cycles R/V Le Atalante cruise, Marmarascarps project, Armijo et al. (2003) GAS HYDRATE IN THE MARMARA SEA at 660 m water depth Marnaut Cruise onboard L’Atalante Climate has changed in the geological past. These changes are recorded in Sedimentary sequences Cyclic sedimentation, controlled by climate Western Ridge Hydrocarbon Seeps Gas expulsion, oil droplets, brines, and gas hydrate Sequence stratigraphy Sequence stratigraphy is a concept used to explain the evolution of sedimentary environments in time and space. Sequence stratigraphy identifies packages of strata each of which was deposited during a cycle of relative sealevel change and/or changing sediment supply. The packages of strata are bounded by chrono- stratigraphical surfaces. These surfaces are: Unconformities formed during relative sealevel fall Flooding surfaces formed during relative sealevel rise. In this way sequence stratigraphy divides the sedimentary record into time- related genetic rock units, which are useful for stratigraphical correlation and prediction of sedimentary facies. It is also useful in determining the amplitude and rate of past changes in sealevel and in identifying sedimentary cycles of 10 ka to >50 Ma scale. Principles and controls of sedimentation Accommodation space: It is the available space for sediment deposition. It is controlled by changes in relative sealevel, which is in turn controlled by: a) Eustatic sealevel fluctuations b) Tectonic subsidence and uplift The geometry of sediments accumulated is controlled by the balance between a) Accommodation space b) Sediment supply If there is zero accommodation space the sediments will be transported to an area of positive accommodation space. The zero accommodation space areas are areas of non-deposition (sediment by-pass). If there is negative accommodation space, the previously deposited sediments will be eroded and deposited in in positive accommodation space areas. In this way sedimentary systems try to preserve and achieve an equilibrium profile. Equilibrium is achieved between the rate of sediment supply and rate of changes in accommodation space. If sediment supply increased at a higher rate than accommodation space, this would Result in regression and a shallowing-upward sequence. Fractured carbonate pavement with black sulphide patches colonized with bivalves, NE Central Basin Sedimentary recordMarnaut of sealevel cruise- Nautilechange: Dive sequence 661 stratigraphy Base level: This is the level below which sediment will be deposited and above which sediment will be eroded. The ideal equilibrium profile of a river system is an exponentially curved topographic gradient. Uplift of the source area will cause rivers to cut down and sediment to be removed. Similarly, a sealevel fall will also cause rivers to cut down and alter the equilibrium alluvial system profile. Filamentous bacterial mat, tube worms Shallow-marine equilibrium profile and various base levels R/V Le Atalante cruise, Marmarascarps project, Armijo et al. (2003) A parasequence: A small scale succession of relatively conformable beds or bed sets bounded by flooding surfaces.
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