Sea-level change
Topics:
1) Processes and Feedbacks
2) Time scales of sea-level change
3) sea-level and the geoid
4) land elevation changes
5) effects of atmospheric pressure, winds and oceans
currents
6) changes in the mass of ocean water
7) eustatic effects of changes in liquid water on land
8) changes in the volume of water without changes in
mass
9) volume of ocean basins
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Introduction
What’s so sacrosynct about sea-level elevation as a
datum?
- entirety of civilization within a Holocene highstand
(i.e., within last 10 ka)
- BUT, changing slightly today (e.g, as measured
with tide gauges)
- Predicted to rise by 0.5 m to 1.0 m in 100 years!
- highly unstable in both time and in space
- Major sea level fall @ 18ka (i.e., LGM)
- never constant
- how and why?
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1. PROCESSES CAUSING SEA-LEVEL CHANGE
What processes cause relative sea-level change at
any particular location?
1. Local/regional uplift &/or subsidence of land
2. Changes in atmospheric pressure, winds, currents
3. Change to ocean-water mass
4. Steric changes to ocean volume
5. Change in ocean-basin configuration
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2. TIMESCALES OF SEA-LEVEL CHANGE
What are the time scales of sea-level change?
- broad range of time scales (see Table 1)
short duration events
- wind-driven waves,
 seconds
long duration events
- opening/closing of new basins
 millions of years
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3. SEA-LEVEL ELEVATION vs the GEOID
Geoid
The figure of Earth considered as a sea-level surface
extended continuously through the continents.
A theoretically continuous surface that is everywhere
perpendicular to the direction of gravity
Question: Does the sea-surface elevation closely
mimic the geoid?
Answer: No
- tens of meters departure from ideal sphere
e.g., Pacific 20 cm higher than Atlantic b/c of
density differences
e.g., Cat Bay Bahamas 55cm higher than Miami
b/c of Gulf Stream set up
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If sea-level is not an inviolably scared reference, how
well /accurately can it be measured?
How is the record of sea-level change recorded in
rock?
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4. THE ELEVATION OF LAND:
Is land elevation static?
- glaciated land depressed by ice
- adjacent ice-free areas are uplifted
- previously ice covered areas still experiencing 10
mm/a uplift (isostatic rebound)
- forebulge in adjacent ice-free areas still
experiencing 1 mm/a subsidence
- localized subsidence due to groundwater/
hydrocarbon withdrawl
- coastal subsidence due to deposition/dewatering of
sediments
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- Tide-gauges located on a subsiding surface can
register a relative sea-level rise (RSLR) even if sealevel is falling globally!
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5. Local effects of atmospheric pressure,
winds/currents
- produces sub-meter, short-duration (decadal to
days) local changes in sea-level elevation
-e.g., 1 millibar (mb) pressure differentials produce
10 mm sea-surface elevation change
 semipermanent distribution of hi/low pressure
cells parallel equator
 highest = 1084 mb
 lowest = 870 mb
-e.g., wind stress can locally elevate/depress sea level
 steady state achieved within a few days
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6. CHANGES IN THE MASS OF OCEAN WATER
- assumed constant at < 10ka timescales
- juvenile waters introduced via volcanic eruptions
What’s the water-mass partition in the hydrological
cycle?
4 reservoirs
oceans (1370 x 106 km3)
ice (30 x 106 km3)
ground/surface waters (8 to 19 x 106 km3)
atmospheric moisture (0.01 x 106 km3)
 principal exchanges over past several million
years involved ocean-cyrosphere transfers
OIS5E
Northern hemisphere ice sheets contributed ~ 100 m
to relative sea-level rise
- Antarctic Ice Sheet reduction contributed ~25 m
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- + 70 m SLR if remaining polar ice sheets removed
- alpine glaciers melting would raise sea level by 1m
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7. EUSTATIC CHANGES IN LIQUID WATER ON
LAND
By how much would changes to distribution of liquid
water on land impact sea level?
- river-water volume is __________0.28 m SLE__
-lake-water volume equivalent to ____ 0.63 m SLE
- ground-water volume equivalent to __22 m SLE_
 1960 river diversion from Aral Sea
-> area decreased 40%
-> water level dropped 9 m
-> volume decreased 66%
-> 2mm sea-level rise if returned to ocean
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GROUND WATER?
How could climatic-change control groundwaterand sea-level?
 climatic cooling reduces precipitation
- decreased recharge lowers water table/lake levels
- net flux to ocean produces sea-level rise
 atmospheric warming (3 to 6°C) would
sufficiently increase atmospheric vapor to cause 7
mm sea-level fall
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CHANGE IN VOLUME OF AQUIFER?
How might land “denudation” affect the sea-level?
Net transfer of porous surface sediments to ocean
basins
- equivalent to decreasing size of aquifer above
sea level
- reduced amount of water can be held in near
surface aquifers…
Accretion of coastal plain…
Deposition of porous sediments during cratonic
transgressions…
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8. STERIC CHANGES TO OCEAN WATERS
What is the effect of changes the temperature/
salinity structure of the oceans?
 1°C increase of surface water (upper 0-500 m)
causes thermal expansion of 100 mm (i.e., sea-level
rise)
- rapid heat exchange between atmosphere and
upper ocean (months to decades)
- strong correlations to sea-level changes at
seasonal/annual timescale
 1°C warming of deep water (500-4000 m) causes
1 m sea-level rise
- heat transfer to deep ocean operates over long
time scales (>>millennia)
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- salinity content of global oceans presumably
constant over short time scales
- effects of salinity changes on global sea level
presumably are slight
- within thermocline, 1°C increase in temperature
accompanied by 0.1 ppt increase in salinity
- associated thermal expansion is 3X greater than
the sea-level fall associated with the
concommittant density increase
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Some important events in geologic history
- K/T impact may have rapidly raised water
temperature in upper 50 m by 5 to 10°C & by
several degrees in deep water
- sea-surface temperatures dropped by 12-14°C in
mid Eocene
- at 38 Ma, abyssal ocean filled with cold water
(4-5°C cooling within 100 ka (terminal Eocene
event)
- during the latest Messinian, global salinity
dropped by ~6%
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9. VOLUME OF OCEAN BASIN
How does change to ocean-basin configuration affect
sea-level elevation?
- can be greater than the effect of ice volume changes
- long-term effect
- variable spreading-center volume due to
- changes in spreading rate
- changes in ridge-crest length
e.g., w/i past 80 Ma, ocean-ridge volume
 decreased by 9.55 x 1016 m3
 produced 150 to 260 m RSL fall
- continental growth/destruction
 ocean-volume change
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