I. Introduction

A. Position of the Shoreline Relative to the Edge of the Continent

1. the shoreline generally does not correspond to the edge of the continent

2. the shoreline is an ephemeral feature

a. as sea level changes the shoreline can move landward or seaward

3. the edge of the continent is a fixed feature established at the time of continental rifting

B. Importance of sea level

1. profound effect on coastal zone & continental shelves

a. profound effect on people

i. half the world's population lives at or near the coastal zone

ii. most marine resources taken from the shelf

2. the effects of sea level changes on the coastal zone & continental shelves is a complicated story

C. Eustatic Sea Level Changes vs. Trangressions & Regressions

1. Transgressions & Regressions are Landward & Seaward Movements of the Shoreline - in essence these represent local relative sea levels

2. Eustatic Sea Level is the Global Level of the Ocean Surface

3. Generally, transgressions are associated with rises in eustatic sea level, while regressions are associated with drops in eustatic sea level

a. BUT NOT ALWAYS

i. fast sedimentation can cause a regression during a eustatic sea level rise

ii. or, fast subsidence can cause a transgression during a eustatic sea level lowering

II. Causes of Sea Level Changes

A. Glacio-eustatic

1. Movement of water out of the oceans to build glacial ice sheets on the continents

a. eustatic lowering

b. at the last glacial maximum (18 ky), sea level was 100-130 m lower than present sea level (shoreline at shelf edge - near continent edge)

2. Melting of ice sheets moves water back into the oceans

a. eustatic rise

b. from 18-6 ky, the ice covering most of North America & Europe melted & sea level rose to its present level (more or less)

c. if the Greenland & Antarctic ice caps melted, sea level would rise 70 m

3. Rates - 1 cm/yr (100 m in ~10 ky = 104cm/104yr)

B. Tectono-eustatic

1. Movement of water onto or off of the continents because the volume of the ocean basins changes

2. Volume of the ocean basins changes because of the width of the MOR varies according to seafloor spreading rates

a. fast spreading rates generate wide ridges & decrease the average depth of the ocean basins

i. eustatic rise

b. slow spreading rates generate narrow ridges & increase the average depth of the ocean basins

i. eustatic lowering

3. Magnitude of change - 100 to 300 m

4. Rates - 1 cm/103yr (100 m in 10 my = 104cm/107yr)

C. Sedimento-eustatic

1. Deposition of sediment in the ocean basins also decreases ocean basin volume

2. Rates - deep ocean - 0.5 cm/103yr

- continental rise - 2.5 cm/103yr

- continental shelf- 5 cm/103yr

- average - 1 to 2 cm/103yr

D. Addition of Juvenile Water

1. H2O is a volatile being degassed from the mantle

2. Rate is relatively small

a. Earth differentiated early

b. 3He2 anomaly gives rate

E. Isostatic Effects

1. Vertical movement of the crust due to loads (sediment, ice or water)

F. Tectonic Erosion

1. Removal of crustal material at subduction zones

III. History of Sea Level Changes

A. How Are Sea Level Changes Documented

1. Position of old shorelines

2. Seismic stratigraphy

a. see description of the Vail technique in your text

b. essentially similar to point 1 above - but using relationships of seismic reflectors to establish old shorelines

3. Continental margin subsidence histories

a. passive (or Atlantic type) continental margins subside in a manner similar to that of oceanic crust, & for the same reasons

i. the continental crust is heated up during rifting, cools as it moves away from the rift & subsides as it cools

b. the observed rate of subsidence can be compared with the theoretical subsidence

c. differences between the observed & theoretical subsidence are due to sea level changes

4. d18O records

a. this is the ratio of 18O to 16O in CaCO3 comprising the shells of one-celled marine organisms

b. for the last 2.5 million years, the d18O composition of these shells has varied because large glacial ice sheets have alternately build up on North America & Europe, & then subsequently melted

c. the water used to make the ice is removed by evaporation fromthe ocean

d. evaporated water is enriched in 16O, & the ocean water remaining behind (and the CaCO3 precipitated from that water) is enriched in 18O

i. d18O values for enriched CaCO3 are more positive (or heavier) than d18O values for depleted CaCO3

e. the d18O variation through time is thus a proxy for the amount of ice stored on the continents & for sea level (see section II. A. above)

B. Quaternary Sea level Changes

1. Mostly glacio-eustatic

2. Late Quaternary

a. determined by radiocarbon dating of shoreline deposits at different depths on the continental shelf

i. oysters

ii. peat

iii. mastadon tusks

b. sea level was about as high as modern about 40-50 ky

i. as the ice advanced, sea level dropped

c. sea level was lowest from 25-18 ky

i. approximately 70-130 m below present

d. sea level rose rapidly from 18-6 ky

i. by 6 ky sea level was within 5-10 m of present sea level

e. isostatic effects complicate the picture

3. Early Quaternary

a. determined by Uranium-Thorium dating of uplifted coral terraces in New Guinea & Barbados & by the marine d18O record (see handouts 1&2 & your text; in particular figure 3 on the back of handout 1 is useful)

b. sea level lows (d18O maxima) at 18 ky, 135 ky, 275 ky, 350 ky, 425 ky, 550 ky, & 625 ky, & then the record gets fuzzy

i. sea level 100 to 150 m below present

c. sea level highs (d18O minima) at present, 125 ky, 250 ky, 325 ky, 400 ky, 525 ky, & 600 ky, & then the record gets fuzzy

i. sea level about the same as present or maybe 10-20 m lower

ii. note - the apparent sea level high from 40-50 ky based on 14C dating of shoreline deposits appears to be 40-50 m lower than present based on the U-Th dating of coral terraces & the marine d18O record; noone really knows the reason

C. Tertiary & Mesozoic Sea Level Changes (handouts 2, 3 & 4)

1. Mostly tectono-eustatic, although possibly some glacio-eustatic during Oligocene

2. Disagreement between Vail & Pitman for Cretaceous & Teriary)

a. Vail (based on seismic stratigraphy) says many eustatic sea level changes (see book & handout 3)

b. Pitman (based on ridge volume calculations) says continuous eustatic sea level drop (see handouts 3 & 4)

i. Pitman says that slowing & increasing the rate of eustatic sea level fall on a steadily subsiding passive continental margin (from which most of Vail's seismic data comes) can alternately cause transgressions & regressions to occur (see figure 4 on handout 3)

ii. so transgressions during Paleocene/Eocene & early Miocene caused by slowing of sea level fall

3. So sea level low at the start of the Mesozoic (about present level; it increased steadily to 150 to 300 m above present 85-90 my ago, & then has dropped throughout the late Cretaceous & Tertiary

D. Paleozoic Sea Level Changes (handout 3)

1. High during early Paleozoic, low during late Paleozoic

a. mostly tectono-eustatic

b. glacio-eustatic fluctuations during the Ordovician (during high sea level stand) & during the Pennsylvanian-Permian (during a low sea level stand)

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