Lecture 14 - Sedimentary notes PDF

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Lecture 14: Sea level change and sedimentationSea level measurements:- Can look at raised beachesand fossil cliff line- Can measure by tide staff- Tectonics can mess records upSea level definitionsEustatic sea level: is affected by ocean basin volume or by changes in water volume in ocean basins. Me...

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Lecture 14: Sea level change and sedimentation Sea level measurements: - Can look at raised beaches and fossil cliff line - Can measure by tide staff - Tectonics can mess records up Sea level definitions Eustatic sea level: is affected by ocean basin volume or by changes in water volume in ocean basins. Measure from centre of the earth to sea surface. Relative: affected by subsidence rates, sediment loading and eustatic level. Measure from crust rot sea surface Depth: affected by addition and removal of sediment. Measure ocean surface to top of sediment. This changes the most Causes of sea level change: - Miller diagram: there are 2 distinct factors that affect sea level change: short term, long term - Long term: Sea Floor spreading, sedimentation and continental collision - Short term: Ice, thermal expansion and groundwater/lakes Long term affects (tectonic causes): - Tectonic cycles are around rate of change 10m every 106 years - Sea floor spreading: if have a mid ocean ridge will displace water vertically and horizontally — sea level rises - 2 main cycles of sea floor spreading: Pangea rift — began during eustatic low and Pannotia (Vendian Supercontinent) rifts: during eustatic low - Continental collision — has the effect of lowering sea level — increases volume of ocean volume - Example is India colliding with Eurasian plate — crust is shortened but thickened — large scale folding and thrusting — take sediment out of the ocean — more ocean volume = lower sea level - Sedimentation — caused by erosion of continental piles — ocean basin volume to decrease — putting sediment back — raises sea level change - Sedimentation can fill up basins — causing a cone - In sea level highs — you see maximum carbonate production — get a greater abundance

Short term affects (tectonic causes): - Cycles are around 5x 106-107 years - Areas above hot mantle can be raised — which raises the sea level - If above cold areas — will sink down — lowering sea level - These are called Sloss cycles Very short term cycles: - Milankovitch cyclicity — cause sea level changes

-Orbital changes that change climate on a relatively short time scale -Rise and fall ice sheets

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16O is lighter than 18O — if water evaporates the 16O will be taken up more readily As temperature rises, more 16O evaporates into the atmosphere, enriching ocean water wrt 18O Water vapour precipitates as snow at the poles with 18O much reduced. The larger the icecaps, the higher the concentration of 18O in seawater. The higher the δ18O value in forams, the larger the volume of the icecaps. Evapotranspiration in plants produces 16O. 0.2‰ change in δ18O represent a temperature change of 1°C. A change from glacial to interglacial will result from a shift of 1.2‰. A 1‰ change in δ18O is equivalent to a 100m rise or fall in global sea level. More saline waters are enriched wrt to 18O as 16O evaporates more readily.

- Need to see how much fresh water has come in — rich in 16O, needs to be taken in account to raise 16O values

- If grow an ice sheet causes eustatic variation with an amplitude of 200m and rate of change of 20m ky-1

- Thermal expansion 10°C temperature rise will produce a eustatic rise of 10m - Desiccation can also affect sea level change, where a basin is completely evaporated, Miocene the Mediterranean dried out so water would have increased in other areas raising the sea level

- Desiccation events have an amplitude of 5-10m and occur at a rate of 10m ky-1. - Found footprints of mammals in the ocean and evaporite deposits of halite and gypsum Neotectonics: changes in crustal elevation can confuse Isostatic rebound: can also confuse with sea level...