Transcript 2 - SOEST
Dendroclimatology
Dendroclimatology (relationship between annual
tree growth and climate) offers high resolution
paleoclimate reconstruction for most of the
Holocene
Huon pine - Lagarostrobos franklinii
A conifer endemic to Tasmania is recognized
as the longest living tree (or organism) known
A medium sized specimen growing on the
west coast of Tassie is estimated to be
about 10,000 years old
Lagarostrobos franklinii
Sequoiadendron
giganteum
Tree Rings
Cross section of temperate forest tree trunks
reveal alternation of light and dark bands
Seasonal growth increments consisting of
earlywood (light growth band from early part
of the growing season) and denser latewood (a
dark band produced towards the end of the
growing season)
Mean width of tree rings are a function of
tree species, tree age, soil nutrient availability
and a whole host of climatic factors
Dendroclimatologist must extract climatic
signals available in the tree-ring data from
remaining background "noise"
Tree Ring Banding
Splicing Tree Ring Records
Sampling Tree Rings
Climate Information from Trees
Tree growth can be limited directly or indirectly by some
climate variable
If the limitation can be quantified and dated,
dendroclimatology can be used to reconstruct some
information about past environments
Trees growing near the extremes of their ecological
niche are subject to climatic stresses – typically
moisture and temperature stress
Trees in semi-arid regions are frequently limited by
the availability of water
Dendroclimatic indicators reflect water
Trees growing near the latitudinal or altitudinal tree
line are frequently temperature limited
Dendroclimatic indicators reflect temperature
Extreme Ecological Niche
Sediments
Marine sediments accumulating in ocean basins can
indicate climate conditions in the surface ocean or
on the adjacent continents
Sediments are composed of both biogenic and
terrigenous materials
Biogenic components include planktonic and
benthic organisms
The nature and abundance of terrigenous
materials provides information about
continental weathering and the intensities and
directions of winds
Ocean sediment records have been used to
reconstruct climate change ranging from thousands
of years to tens of millions of years in the past
Biogenic Sediments
Calcareous or siliceous oozes
Three types of analysis of calcareous and
siliceous tests are typically used for climate
reconstruction
The oxygen isotopic composition of calcium
carbonate
The relative abundance of warm- and coldwater species
The morphological variations in particular
species resulting from environmental factors
Isotopic Composition of Shells
First general rule of isotope geochemistry
Heavy isotopes concentrate in the compound
where bond energy is strongest
When a mineral forms in water, heavy isotope
concentrates in the mineral
The isotopic composition is a function of
The isotopic composition of the water
The temperature of formation
• Fractionation decreases as temperature
increases
The d18O of Shells
Temperature dependent
T = 16.9 - 4.2 (dc - dw) + 0.13 (dc - dw)2
Isotopic variations in carbonates small
For modern analyses, dw can be measured
directly in ocean water samples; in fossil
samples, however, the isotopic composition
of sea water is unknown and cannot be
assumed to have been the same as it is
today
Glacial/Interglacial change in
Interglacial scenario: High sea-level
stand coupled with little ice at the
poles and relatively little storage of
16O in ice caps leads to relatively
sea-water rich in 16O. Calcareous
organisms living in the oceans will
incorporate more 16O in their
carbonate shells. Clouds contain high
proportion of the light isotope
because of it's higher vapor
pressure.
18O
Glacial scenario: Low seal level
stands with much polar ice will store
more 16O and thus sea water will
contain a higher proportion of 18O;
this proportion will be mirrored by
calcareous organisms that live and
fractionate this water when they
form their shell. Clouds contain high
proportion of the light isotope
because of it's higher vapor
pressure.
Trends in d18O
During glacial times
Sea water enriched in 18O
Surface water colder
d18O of planktonic calcareous
organisms more positive
During interglacial times
Sea water enriched in 16O
Surface water warmer
d18O of planktonic calcareous
organisms more negative
Constraining d18O of Seawater
Isotopic records of deep water organisms can
help
Bottom water temperatures (» -1°C to 2°C)
have changed little since glacial times
Therefore increases in the d18O of deep water
organisms mostly reflect changes in the
isotopic composition of the glacial ocean
Concluded that 70% of the changes in the
isotopic composition of surface dwelling
organisms was due to changes in the isotopic
composition of the oceans, and only 30% due to
temperature variations
Other Complications – Vital Effects
Unfortunately calcareous marine organisms
never took a course in chemical thermodynamics
They do not precipitate their shell in oxygen
isotope equilibrium with seawater
Calcareous organisms commonly display “vital”
isotope effects
For example, incorporation of metabolically
produced carbon dioxide
Vital isotope effects are not a problem if
They are known
They are constant
Other Biotic Climate Data
Climate reconstruction can be achieved by
studying
Relative abundances of species
Species assemblages
Morphological variations
Test coiling directions, either right-coiling
(dextral) or left-coiling (sinistral), reveal
proxy information about paleotemperatures of the oceans
Other variations include differences in test
size, shape and surface structure
Corals
Coral skeleton are colonies
composed of polyps
Symbiotic algae (zooxanthellae)
Zooxanthellae supply both with
food and oxygen
Food caught by the coral
supplies both with phosphorous
and nitrogen
Algae are crucial to calcium
carbonate deposition
Without algae corals unable to
produce substantial reef
structures
Complicates geochemical records
from corals
Coral Growth
Polyps are seated in aragonite
secreted by the epidermis
CaCO3 is deposited beneath
living tissue
Interconnected polyp
networks completely covers
the skeleton
Corals periodically encapsulate
a portion of their skeleton and
seal it off from contact with
sea water or living tissues
Over the course of years,
each polyp lifts itself
hundreds of times leaving
new skeleton behind
Annual Banding in Coral
Density of skeleton depends on coral
growth rate
Related to temperature and cloud
cover
Winter growth slow and skeleton is
dense (dark)
Spring and summer growth rapid
and skeleton is less dense (light)
Seasonal coral banding may be visible to
the naked eye or apparent in an x-ray
Age of corals determined by counting
bands
Uneven banding can reveal significant
weather events
Sample Collection
Hydraulic drill
used to collect a
core through the
coral
Cores taken to
coral's plane of
maximum growth
Coral Records of SST
d18O function of SST and
salinity (fresh water influx and
precipitation)
Close correspondence between
d18O and instrumental
measurements
Red spikes in d18O record
match up with red spikes in
the SST record
Coral d18O data nearly as
accurate as instrumental
data
Coral records can cover the
past 500-800 years
Instrumental records are only
available for the last 50-100
years
Long Records
Detailed records of d18O provide information on
SST and El Nino activity for last 350 years
Longer records obtained by splicing records
Other Coral Geochemical Proxies
Cd/Ca and Ba/Ca proxy for upwelling
Cd and Ba have nutrient-like distributions in
seawater and therefore are sensitive indicators
of vertical mixing -- Other proxies?
Terrigenous Material in Marine Environments
Weathering and erosion processes in different
climatic zones on continental land masses
produce characteristic inorganic products
Those products are transported to oceans (wind,
rivers or floating ice) and deposited on the sea
floor
Carry information about the climate of their
origin or transportation route, at the time of
deposition
Terrestrial detritus dilutes the relatively
constant influx of calcium carbonate
Most basic information is carbonate purity
Terrestrial Sediments
Several types of non-marine sediments can
provide relevant climatic information
Aeolian, glacial, lacustrine and fluvial
deposits are a function of climate
Often difficult to distinguish specific
causes of climatic change
Erosional features such as ancient
lacustrine or marine shorelines, or glacial
striae also reveal climatic information
Periglacial Features
Morphological features associated with continuous
(permafrost) or discontinuous (diurnal or seasonal
freezing) periods of sub-zero temperatures
Features such as fossil ice wedges; pingos;
sorted polygons; stone stripes; and periglacial
involutions
Climate reconstructions are subject to a fair
degree of uncertainty
The occurrence of periglacial activity can only
indicate an upper limit on temperatures
These features are difficult to date
Dating of associated sediments provides only a
maximum age estimate
Glacial Fluctuations
Glacier fluctuations result from changes in the
mass balance of glaciers
Glacial movements lag climate forcing
Different glaciers have different response
times to mass balance variations
Interpreting glacial movements in terms of
climate complex
Many combinations of climatic conditions
might correspond to specific mass balance
fluctuations
Temperature, precipitation (snowfall) and
wind speed are three main factors
Records of Glacial Movements
Record of glacial front movements is
derived from moraines
Piles of sediments carried by advancing
glaciers and deposited when they
retreat
Periods of glacial recession, and the
magnitude of recession, are harder to
identify
Repeated glacial movements can
destroy evidence from earlier
advances
Dating Glacial Movements
Dating glacial movements prone to considerable
error
Radiocarbon dates on organic material in soils
on moraines provides a minimum age for glacial
advance
Considerable time lag may exist between
moraine deposition and soil formation
Lichenometry (lichens) and tephrochronology
(lava flows) can sometimes be to date glacial
events
Reliability is restricted
Lake Level Fluctuations
In regions where surface water discharge (via rivers
and other waterways) is restricted to inland basins
Changes in the hydrological balance can provide
evidence for past climatic fluctuations
In land-locked basins, water loss is almost
entirely due to evaporation
• During times of positive water budgets
(wetter climates), lake levels rise and lakes
expand
• During times of negative water budgets
(drier climates), lake levels drop and the
aerial expanses recede
Lake studies particularly useful in arid or semi-arid
areas
Lake Titicaca, Altiplano, Andes
What can the lake level of high altitude lakes tell us about
oceanic circulation and atmosphere-oceanic interactions?
R/V Neecho, WHOI
Salar de Uyuni, Bolivia
World's largest salt flat contains a record of alternating wet/dry
periods on the Altiplano
Factors Affecting Lake Level
Factors affecting the rates of evaporation
include
Temperature, cloudiness, wind speed,
humidity, lake water depth and salinity
Factors influencing the rate of water
runoff include
Ground temperature, vegetation cover,
soil type, precipitation frequency,
intensity and type (i.e. rain, snow etc.),
slope gradients and stream sizes and
numbers
Identifying Lake Levels
Episodes of lake growth identified by
Abandoned wave-cut shorelines, beach
deposits, perched river deltas and
exposed lacustrine sediments
Episodes of lake retreat
Identified in lake sediment cores or by
paleosols and evaporites developed on
exposed lake bed
Stratigraphy, microfossil analysis and
geochemistry may be used to decipher lake
level history
Pollen Analysis
Pollen and spores accumulations
Record past vegetation
Changes in the vegetation of
an area can be due to
changes of climate
Pollen grains and spores form
ideal records
Extremely resistant to decay
Produced in huge quantities
Distributed widely from
their source
Can possess unique
morphological
characteristics
Problems with Pollen
Differences in pollen productivity and dispersion rates
pose significant problems
Relative abundances of pollen grains in a deposit
cannot be directly interpreted in terms of species
abundance
Calibration of pollen abundance and spatial
distribution to species frequency is necessary
Pollen is a wind-blown sediment
Accumulates on any undisturbed surface
Sediments containing fossil pollen include peat
bogs, lake beds, alluvial deposits, ocean bottoms
and ice cores
When pollen is deposited in water, differential
settling, turbulent mixing and sediment bioturbation
can bias record
Pollen Uses
Pollen analysis usually allow only qualitative
reconstructions
The climate was wetter/drier or
warmer/colder
Sometimes it is possible to quantify
climatic variations by the use of individual
indicator species rather than total pollen
assemblages
The occurrence of plants that may not
be abundant but which are limited by
specific climatic conditions
Sedimentary Rocks
Marine sediments >100 my subducted
If sediments uplifted and exposed, can be
used to reconstruct past climates
As sediments become progressively buried
undergo lithification and diagenesis
Geochemical proxies must take into
account chemical alteration
Record can be compressed
Climate Reconstruction – Rock Type
Rock type provides valuable information
Evaporites
Lithified salt deposits and evidence of dry
arid climates
Coals
Lithified organic matter and evidence of
warm, humid climates
Phosphates and cherts
Lithified siliceous and phosphate material
and evidence of ocean upwelling
Reef limestone
Lithified coral reef and evidence of warm
surface ocean conditions
Climate Reconstruction – Facies Analysis
Investigates how rock type changes over time
A formation consisting of a shale layer
interbedded between two sandstone layers
Evidence of a changing sea level
Potentially linked to climatic change (e.g.,
glacial ice formation)
Sedimentation rates, sediment grain
morphology and chemical composition
Provide information on the climatic
conditions at the time of parent rock
weathering
Biotic Indicators
Type and distribution of marine and
continental fossils within fossil-bearing
rocks
Principally limestones and mudstones,
occasionally sandstones
Microfossil type, abundance and
morphology
Paleotemperatures can sometimes be
derived from oxygen isotope analysis