Coastal Geol week 2 - Ch 2 - Changing Sea Level
General intro - You can view sea level change as simple relative shift of ocean to
land, accomplished in a variety of ways….land can go up or down, ocean can go
up or down. If either changes relative to the other, you've got a sea level
change.
Typical vertical range of wave and current energy only = 20 m
10 m below MSL is typical calm weather wave base
10 m above MSL is high point of tide and storm activity
so a consequence is that a modest sea level rise can submerge a former coast,
and a modest sea level fall can leave a former coast high and dry
and that is what's happened over last 2 million yrs….during cold periods, ocean
water is taken up in glaciers & polar icecaps, sea level falls….during warm
periods, glaciers shrink and ocean basins fill with water
high to low sea level ranges can be 100 meters (300 feet) vertically,
"enough to build a whole series of coasts"
this chapter focuses upon climatic and tectonic factors influencing sea level, but
not tides….tides somewhat limited in range; daily flux causes a swath that
defines that one particular coastal zone; here, we're interested in shifts that lead
to development of new or different coastal zones
A. Tectonic activity
 Seismic activity (earthquakes) can change the landscape instantaneously,
like Alaskan eqk in 1964, mag 8.6, one of biggest ever recorded.

Land elevations changed 6 m in some areas, islands offshore uplifted as
much as 12 m (40 feet)…"in a matter of minutes"
Napier, New Zealand - similar story from a 1931 eqk.
 Areas formerly underwater now dry land, uplifted 2 m (6.6 feet)…"fish flopped
about and boats were stranded". This area now consists of farms and an
airport, not a bay.
Tectonic activity a fairly common agent in sea level change in those areas where
active converging margins are located…for example, east and west sides of
Pacific Ocean; eastern Mediterranean and into the Middle East.
Coas Geol wk 2 p.2
B. Climatic Fluctuation
This too is a big agent in sea level change. Surprising cycle from spring to fall
that shifts sea level 10 cm (4") to 30 cm (a foot). Pretty big swing…
 spring lowest (coming out of winter, not much runoff?),
 fall highest (spring and summer runoff has made it to the ocean?).
well, just a thought….
Davis correlates these patterns with sun's path, changing temp, wind pattern,
wind velocity, all conspire to change sea level in one area by "piling up" water,
while leaving a depression behind.
wind
mean sea level
Water
land
There are complications to this picture, when precip patterns are superimposed.
Bay of Bengal in India example - chg in sea level of 100 cm (over 3 feet) during
course of year….what would that do to Sandy Hook??
Another factor that changes sea level is volume change of ocean as it expands
due to warming (!) note the diagrams on p. 43.
Examples:
Annapolis in summer - Gulf Stream warms up Chesapeake Bay
Coast of Washington in summer cooled off by California current
Point here is, a whole bunch of factors can contribute and interact to create a
sea level in a certain place at a certain time….it is NOT a simple process.
Davis points out correlation seen by LaFond at Scripps after monitoring So. Calif
coast for years…
 warm temp raises sea level
 high sea level and high wave energy occurred in Winter (not summer)
 corresponding increase in coastal erosion
Coas Geol wk 2 p.3
Can't get through chapter without mentioning El Nino…warming trend every 4 to
7 years affects entire South Pacific - note fig on p. 45
Basic mechanics of El Nino typically, cold Peru current flows S to N along w. side of S. Amer. But El
Nino disrupts this pattern, displaces the cold current, placing warm water at
surface. This blocks ability for cool upwelling water to bring nutrients to surface,
biology patterns get disrupted. For example, fish either have to move or die.
Warm water also changes weather patterns: more precip, erosion, flooding, sea
level rise.
C. Regional Subsidence
2 factors that Davis focuses upon here that can cause subsidence on land  compaction
 fluid (e.g., groundwater) withdrawal
significant problem, even at human scale, in places like US Gulf Coast - relative
drop in land surface coupled with ocean level rise averages 10 mm/year change
(almost 1/2 " rise every year….this is VERY fast….think of this rate over 1000
years….500" = 40 feet change….you'd see huge portion of Gulf coast go
under….let's get some maps and do an exercise on this…..
the mechanics of compaction can be simplified to be straightforward…in deltas,
lots of mud with bound/trapped water in/on clay particles is deposited…not
uncommon to see clays with 80-90% original porosity, all water-filled
overlying sedimentary overburden weight eventually squeezes water out of clay,
it compacts:
this
to
this
Coas Geol wk 2 p.4
Some human factors have exacerbated the existing problems too - dams &
erosion control devices can block new sediment from coming in and helping to
"fill in" the gaps left by the older compacting sediment.
A particularly big problem is groundwater withdrawal….and oil and gas too.
Davis also cites "high-rise" city centers in Houston and New Orleans.
2 meters of subsidence recorded near Galveston, TX this century.
Davis speculates about next 50 years….50 yrs at 10mm/yr subsidence could
result in half-meter of sea level rise, this could submerge a huge area of fragile
coastal wetlands.
D. Isostatic Subsidence
Broader subsidence issues can impact sea level - for example, weight of
huge glacial ice sheets is interpreted to have pushed down land surface. This is
illustrative of general concept of isostasy - balance of forces that tend to depress
on one hand, elevate on the other hand. (hence term "isostatic rebound" to
describe how land rebounds when weight of ice is removed). Fig p. 48
Davis points to diverging coasts with fresh lithosphere that is cooling, contracting,
becoming thinner and denser, as excellent candidates for relative sea level rise.
(Red Sea, Gulf of California)
Also mentioned is concept that sedimentary loading of contin shelves can create
large piles of sediment that will cause subsidence.
E. Changes in ocean volume
Global (or eustatic) change in sea level - can come about in 3 ways:
1. add or subtract water
2. make ocean basins bigger or smaller
3. change ocean temperature (and change its volume)
we won't look at item 2., because it's plate- tectonics -driven
Coas Geol wk 2, p.5
rest of chapter focuses on volume change of water by either adding/subtracting it
or by increasing temperature.
1. Global temperature change
 Good point on how two factors work in concert when it comes to climate:
- global cooling causes bigger glaciers, less ocean water
- global cooling causes contraction of ocean water
and vice versa in terms of global warming

Another key point: it does NOT take dramatic temp change to produce
profound results….
EXAMPLE:
Last ice age produced big drop in sea level, 30% of N. Hemi land was icecovered, and yet mean annual temp was only 2-3o C (4-6o F) cooler than today.
If current trends continue, ice sheets could melt, inundate most sea ports…
Big changes in past have been over thousands of years, our records are recent,
no historical perspective.



Poss of human-influenced sea level rise is very real. We can see increase in
avg temp over last few hundred years, and it can be correlated with incr in
CO2 and other greenhouse gases since beginning of Industrial Revolution
Conceivable that sea level could rise "30 - 40 meters in a few centuries"
But don't get overly dramatic….there is much we don't know…be careful
about drawing inferences from limited data
2. Advance/retreat of ice sheets
from beginning of Quaternary (2 million yrs ago), looks like sea level "changed
more and faster than ever before". Quite a statement……
4 major periods of US glaciation:
Nebraskan
630,000 yrs ago culmination
Kansan
430,000
Illinoisan
150,000
Wisconsinan
20,000
But with help of isotope analysis, we speculate about more events now….
In principle, 16O/18O ratio changes in the CaCO3 of organisms can be used as
indicator of seawater temp at time an invertebrate pulls this into the shell.
Coas Geol wk 2. P6
Isotope changes in foraminifera shells show more complex history than just 4 big
advances.
Also, the foram cycles fit well with Milankovitch cycles, which are controlled by
Earth position relative to the sun (that's the theory..)
Davis does some calcs to come up with poss amt of sea level rise up to present
day, with some assumptions concerning Wisconsinan ice sheet extent and
thickness…Table p.56
Estimate - of an original 75 million km3 in Wisconsinan ice, about 45 has melted
and gone into the ocean. Estimated rise in sea level = 120 m (400 feet). This is
consistent with fossil control on continental shelves (e.g., horses, bears, other
mammals).
3.Continental Rebound
Isostatic rebound has occurred as ice mass is removed from continent.
Some still occurring - example: Scandinavia coastline currently rising 9 mm/yr
relative to sea level. Translates to 50-60m of uplift over 5000 years.
Result is a series of ancient shorelines (Fig p.58)
Cross-section through the picture:
SW
NE
Old shorelines
Great Lakes and other areas also show higher rates of rebound in north, where
thicker ice presumably was, and loaded crust more than in the south.
Coas Geol wk 2 - p.7
4. Holocene rise in sea level
Holocene has seen the most recent melting of ice, since the max ice advance at
20,000 yrs ago….big sea level rise since then. But tough to say how much
further it will go….
"most researchers place lowstand at 120m (400 ft) below current sea level"
they also suggest very rapid sea level rise from 20,000 to 7,000 yrs ago
(Fig p.59), at a rate of 10 mm/year (what we see in some places today)
at this point, glacial melting thought to have slowed, start seeing some stability in
terms of shoreline development; that is, transgression not so rapid, give waves a
chance to develop a shoreline
rapid rise
slow rise
Fast transgression, no stillstands
Several stillstands
5.Current and future sea level change
100 years of data from tidal gauges indicate that sea level is currently rising, at a
rapid rate, depending upon where you are. Fig p.61
note the 4 locations in the US, how they all have different curves a. New York - on the rise, but isostatic rebound of continent moderates the rate
b. Galveston, TX - very fast rise (lots of subsidence)
c. Astoria, OR - stable (continental rise appears to be matching sea level rise)
d. Juneau, AK - sea level falling (continent rising much faster than sea level)
Coas Geol wk 2 p.8
Fig p. 62 shows a range of rates of sea level rise (+) and even fall (-) in the US,
in cm/year (note the lack of a legend….a big no-no)
Davis indicates that most coasts around the world have experienced sea level
rise in last century, with one big exception - that of the Pacific Ocean. Fast
spread of East Pacific rise (expansion of ocean basin?), convergent boundaries
all around Pacific, combine to form relatively fast-rising continents, so sea level,
even if rising, isn't encroaching as rapidly in Pacific as in other areas
Disclaimer - we still don't have a lot of history to show what the long-term trends
really are….
There may be short-term cycles of both warming and cooling - for example, "Little
Ice Age" from 1450 to 1850 AD - reports from Europe of cool climate, growth of
glaciers, slight fall in sea level.
F. Implications for coastal environments
As citizens, we need to take a long-term view….coastal environments are
extremely rapidly changing in terms of geologic time, but they look somewhat
stable to us humans (about 3 meters of sea level rise during "period of significant
human occupation").
Remember that they are NOT stable….
Davis closes chapter with a stunning map Fig p. 65
Ganges-Brahmaputra delta if sea level rises 10 mm/yr for 50 years (500 mm =
1/2 meter) - incredible amount of the delta flooded, millions of people displaced
So consequences of sea level rise could be profound in areas with a lot of
subsidence…..