2000%20H216O

1
ice
2000 H216O
10 HD16O
10 H218O
land
Latitude increasing
2
ice
land
Latitude increasing
3
ice
land
Latitude increasing
4
ice
land
Latitude increasing
5
300 H216O
0 HD16O
0 H218O
ice
1700 H216O
10 HD16O
10 H218O
land
Latitude increasing
6
d18O of ice decreasing
300 H216O
ice
d18O of sea-water increasing
1700 H216O
10 HD16O
10 H218O
land
Latitude increasing
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C. Oxygen Isotope stratigraphy
1. The overwhelming conclusion from the studies,
which have been made of both planktic and
benthic species, is that similar isotopic
variations are recorded in all areas.
Because of the relatively short mixing time,
these nearly synchronous variations enable
correlations to be made between cores that
may be thousands of kilometer apart.
2. Warmer periods (interglacials and
interstadials) are assigned odd numbers (the
present interglacial being number 1) and
colder (glacial) periods are assigned even
numbers.
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3. The change in benthic d18O commonly recorded
between stage 5e and 5d is so large and so
rapid that it is almost impossible to
account for it only in terms of ice-sheet
growth. It seems likely that at least part of
this change reflects a rapid temperature
decline (of 1.5?C) in abyssal water
temperature. Subsequent changes in d18O (in stage
5c to 1) were then primarily the result of
changing ice volume on the continent. 4. It
should be emphasized that the isotopic signals in
ocean cores contain both a temperature and
an ice-volume component, which may not be
synchronous.
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(Shackleton Opdyke 1976)
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(Prell et al. 1986)
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D. d18O / Ice volume / Sea-level changes
1. Milankovitch hypotheses(1941) Glaciations
in the past were principally a function of
variations in the Earths orbital parameters, and
the resulting redistribution of solar
radiation reaching the earth. An important
signal which has been inspected for a
relationship between orbital perturbations and
climatic change is the marine core d18O
record, which reflects changes in
continental ice volume (principally Northern
Hemisphere).
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(a) Emiliani(1955, 1966) d18O maxima in
Caribbean and equatorial Atlantic cores
closely matched summer isolation minima at 65?N,
which was the latitude that Milankovitch
had considered critical for the growth of
continental ice sheets. (b) Broecker and Van
Donk(1970) They suggested revisions of
Emilianis timescale, but still concluded
that insolation changes were a primary factor
in continental glaciation.
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(c) Broecker et al. 1968, Mesolella et al. 1969,
Veeh Chappell 1970 Dates of coral
terrace formation, indicative of a former
higher sea level (lower global ice volume), were
shown to be closely related to times of
insolation maxima.
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(d) Hays et al.(1976) Three parameters were
studied d18O value in the foram
G.bulloides (an index of global, but primarily
Northern Hemisphere, ice volume) summer
sea-surface temperature (Ts) derived from
radiolaria-based transfer functions (an
index of sub-Antarctic temperatures) and
abundance variations of the radiolaria
C.davisiana (an index of Antarctic surface
water structure). These proxy records were
concentrated at frequencies corresponding
closely to those expected from an orbital
forcing function(100kyrs, 40-43kyrs, and
19.5-24kyrs).
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2. Chappell Shackleton(1986) (a) formal
assumption the deep ocean, at least in the
Pacific, is so cold that its temperature may be
regarded as constant. (b) V19-30 vs.
Huon Pennisula, New Guinea a discrepancy
has been noted. (c) the final time-scale of
V19-30 is developed by tuning the initial
record on the basis of its relationship to
orbital precession, obliquity and
eccentricity functions. (d) oxygen isotope
studies clearly associate reef VIIa (the
older) with substage 5e.
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(e) Before 130kyr the sea-level curve derived
from marine terraces is subject to larger
uncertainties because both the assigned
ages and assumed uplift rates become less
secure. (f) Within the past 130kyr the greatest
uncertainty relates to the reef IVb-IIIa
area where the isotopic record shows little
structure. (g) By plotting sea-level against 18O,
they found a cluster of points around zero
sea level and 3.4 corresponding to full
interglacial stage 1 and substage 5e.
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(h) A more probable explanation is that this
isotopic shift results from a temperature
effect on the isotopic composition of the
benthic foraminifera analysed. (i) They conclude
that deep waters in the Pacific Ocean were
1.5?C cooler in glacial and interstadial times
than in the short (10kyr duration)
interglacials of substage 5e and the present.
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3. Mechanisms of glaciation and deglaciation the
oceanic evidence Ruddinman and
McIntyre(1981) Ice sheet growth is favored
when Northern Hemisphere summer insolation
levels are low (due to orbital factors) but
oceanic temperatures at high latitudes are warm,
providing an abundant moisture source
adjacent to the relatively cool continents.
Strong thermal contrasts at the continental
margin help steer depressions towards the
developing ice sheets, thereby increasing the
local accumulation rate.
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(a) Heinrich events are attributed to
instabilities in the ice sheets once they
have grown to continental dimensions,
resulting in iceberg discharge. (b) Heinrich
events raises global sea level by 10-15m (c)
There must be strong and swift interactions
between the major ice sheets in both
hemisphere, in which the collapse of one
ice sheet raises sea level sufficiently to
destabilize those margins of the others where ice
advanced onto the shelves.
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4. A longer Perspective the Entire Brunhes (a)
Oxygen isotope values for interglacial extremes
are then compared with the Stage 1. (b)
The extremes of Stage 1, 5e, 9, 11 are
significantly lighter than Stage 7, 13,
15,17 and 19. During these interglacials
either some northern hemisphere ice must
have remained, or ocean deep water must have been
colder than they are today.
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(c) Although the planktonic values are more
scattered, the values for interglacial
Stage 7, 13, 15, 17 and 19 are indeed
systematically more positive than the extreme
values for Stages 1, 5, 9, and 11. This in
turns suggests that on slowly uplifting
coastlines where should be marked gap
between the Stage 11 and the much older
Stage 23. (d) Sachs(1973) suggested that this was
substantially the warmest interglacial in
the last million years. In DSDP Site 552A
in the North Atlantic, Stage 11 is represented
by the thickest section of nannofossil ooze
with the least ice-rafted contribution of
any of the interglacials in the last 2.5Ma.
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(e) Comparing each glacial extreme, without doubt
Stages 12 and 16 were more extreme than
Stage 2. Stage 6 perhaps marginally more
extreme. Stage 10 was perhaps marginally
less extreme than Stage 2. Stages 4, 8, 14,
and 18 were significantly less important. (f)
Amongst the planktonic data sets no consistent
pattern emerges. This is not surprising,
since temperature variations must have
played a part for many of the cores.