Transcript Special events

CHEMICAL GEOLOGY
Global Change - Some Special Events
Andrea Koschinsky
Heinrich Events: Marine Record of Abrupt
Climate Changes in the Late Pleistocene
In the 1980s, Hartmut Heinrich
extracted samples at regular increments
from northeastern Atlantic sediment
cores. He counted the number of both
lithic and planktonic foraminifera shell
fragments. He found that the relative
number of foraminifera and lithics in
particular North Atlantic sediment cores
fluctuated in a surprising manner. Samples
like the on the upper slide contain nearly
all foraminifers.
Core sample sediments deposited during
Heinrich events are comprised almost
entirely of lithic fragments (lower slide).
Heinrich Events
Heinrich discovered that the dominant component of these sediments not
only differed substantially, but that the transitions between foraminiferadominated and lithic-dominated sediments were unexpectedly abrupt.
Foraminifera were dominant for long stretches of time, but lithic sediments
punctuated the sedimentological record in six spikes. These sudden changes
in sediments were termed Heinrich events H1 - H6.
The Heinrich events are also visible in X-rays of
sediment cores represented here as sharp
transitions between dark-colored (foraminiferadominated) and light-colored (lithic-dominated)
segments.
Heinrich Events
The graph shows how the lithic fraction would
oscillate in a hypothetical core extracted
from a continental shelf margin in the North
Atlantic.
The environmental conditions during sediment
deposition on the ocean floor changed rapidly
over time as the position and dynamics of the
large continental ice sheets shifted. All of the
scientific evidence points to Heinrich events
requiring a significant drop in sea surface
temperatures, a reduction in the flux of
planktonic foraminifera, and an extension of
the ice sheet onto the continental shelf,
where deposition of ice rafted detritus, (IRD)
occurred.
Heinrich Events
Probable differences in ice extent and sediment processes during Heinrich
events and the corresponding non-Heinrich intervals:
The key features of sediment delivery during Heinrich events are the
presence of icebergs, meltwater plumes and probably massive gravity flows
called turbidites. All of these processes, occurring within cold stadial events,
produce sediments with high lithic content and deposit ice rafted detritus.
Heinrich Events
In contrast, non-Heinrich events are distinguished by hemipelagic sediments
with high foraminifer content. They are formed at the margin between
coastal and pelagic environments and are deposited during warm interstadial
periods. Because Heinrich and non-Heinrich sediments formed in different
environmental conditions, these events tell us much about fluctuations in
climatic conditions in the North Atlantic over the last 60,000 years.
Processes by which ice
rafted detritus are
deposited
The ice rafted detritus that
ends up on the ocean floor
originates as basal sediments
(till) plucked from the
bedrock substrate at the
bottom of an ice sheet. The
basal sediments are
transported within ice
streams from the middle of
the ice sheet to calving ice
sheet margins that extend to
the shelf break. Once the
basal sediments are
transported to the ice sheet
margin, the material is either
transported as debris layers
within icebergs or released
into the ocean and
transported to the ocean
floor as debris slides or
turbidites.
Heinrich Events
Identification and composition of Heinrich
layers in sediment cores
The ice rafted detritus is comprised of sand
grains, pebbles and even stones that were
carried out onto the shelf margin and beyond
by iceberg. These lithic fractions have an
unusual composition, with a high
concentrations of light colored detrital
carbonate within the ice rafted detritus.
High IRD counts and abundances of detrital
carbonate within the IRD reinforce the
theory that Heinrich events are truly ice
rafting events and not merely indications of
reduced numbers of foraminifers.
In this image, note the entirety of the H-1 event
represented by the light-colored sediment in the
bottom half of the image. The black mottling within
the Heinrich event is probably due to bioturbation.
Heinrich
Events
Heinrich Events
Origin of detrital carbonate in
Heinrich layers
A high percentage of detrital
carbonate is a defining characteristic
of North Atlantic sediments
deposited during Heinrich events. The
Laurentide Ice Sheet (North
American Ice Sheet) scraped these
carbonates from the source regions
of Paleozoic limestones and dolomites
in eastern Canada and possibly
northwestern Greenland. These
carbonates were then carried to the
ice margin and deposited in well
defined areas of the North Atlantic
ocean via the processes explained
previously.
Heinrich Events
Heinrich Events
Records of Heinrich Events
Heinrich events have several
distinctive properties that allow them
to be distinguished over large
distances. The thickness of H-1 and
H-2 events (radiocarbon dated at
14.5 kyr and 20.5 kyr, respectively) in
each core were measured and plotted
on a map. Finally, lines of equal
thickness were drawn through the
map.The thickest sediments were
found to lie off the southern coast of
Baffin Island, right where the Hudson
ice stream flowed into the ocean. To
the south and northeast of this area,
the detrital carbonate sediments
became thin or even absent,
suggesting that the carbonate rich
layers were restricted to areas of
high IRD counts that amassed during
the last glaciation.
Heinrich Events
Records of Heinrich Events
Nearly a decade after Heinrich published his article, the proximal causes of
Heinrich events seem clear: different sedimentological environments created
by different glaciological conditions. But it is still unclear what factors and
forces ultimately lay behind these abrupt variations. And so scientists
continue working to unlock the mysteries of Heinrich events by obtaining more
cores, employing new analytical techniques, and seeking to uncover connections
between North Atlantic climatic variations and changes elsewhere in the
world.
Heinrich Events
Records of Heinrich Events
This slide shows data from core
ODP-609 indicating changes across
event H-4. The top graph shows the
inverse relationship between the
percentage of the planktonic
foraminifera N. pachyderma and
the percentage of lithic fragments.
The bottom graph represents
changes in the isotopic composition
of oxygen contained in N.
pachydermas' calcium carbonate
shells. This index of the relative
heaviness of oxygen isotopes, known
as d18O, demonstrates a decrease
of nearly 1.5 o/oo during H-4.
Heinrich Events
Records of Heinrich Events
The more negative d18O values during
Heinrich events is interpreted to be
evidence of plumes of extremely
fresh glacial meltwater which flowed
into the normally salty North
Atlantic.
Not only does the presence of
foraminifera N. pachyderma indicate
the deep southward invasion of polar
water during the time of an Heinrich
event, but the drop in salinities
indicated by the d18O measurements
was probably significant enough to
temporarily shut down thermohaline
circulation in the North Atlantic.
Heinrich Events
Variability in sediment properties
within Heinrich and non-Heinrich
sediments from same core
Researchers are taking a closer look
at variability within Heinrich events.
This is particularly effective at sites
close to former ice sheet margins,
such as the Hudson Strait where
Heinrich events can exceed 0.5 m in
thickness. Moreover, these thick
deposits were laid down during
relatively short spans of a few
hundred years. Not only were the
changes between Heinrich and nonHeinrich events abrupt, but
conditions within Heinrich events
could also vary to a great extent.
Heinrich Events
Correlations with Heinrich Events
Climatic variations as significant as
Heinrich events probably affected
climates and environments beyond the
regions of the North Atlantic where
Heinrich events appear in the
sedimentological record.
Variations in fossil pollen abundances
in sediment cores retrieved from
Lake Tulane in Florida, for instance,
seem to correlate well with variations
in lithic fragments from North
Atlantic cores like DSDP 609. The
peaks in the percentage of Pinus
(pine) pollen match the IRD spikes of
H-1 through H-5 events very closely.
Heinrich Events
Correlations with Heinrich Events
In the last several years, many scientists researching locations as varied as
the Nevada desert and the Greenland ice cap have reported events they
believe to be synchronous with Heinrich events.
Bond et al. (1993) documented correlations between Greenland ice cores and
Heinrich events.
They demonstrated that Heinrich events occurred at the termination of
bundled cooling cycles (Dansgaard-Oeschger cycles) that were documented in
Greenland ice cores, and confirmed that the cooling cycles and Heinrich
events were followed by abrupt periods of significant warming.
Research such as this investigates the formerly unrecognized relationships
between ice sheet behavior and ocean-atmosphere temperature changes.
Heinrich Events
Interpretation of the causes of Heinrich Events
The following models for the causes of Heinrich Events are discussed in the
literature:
 High-frequent Milankovitch cycles
 Earthquakes which were induced by the mass of the ice on the Earth’s
crust and the respective shear strain on the ice borders
 Internal instability of the glaciers and interactions with the North
Atlantic current
 Changes between stable and marginally stable modes of the North Atlantic
current; synchronization by stochastic resonance
Heinrich Events
Correlations with Heinrich Events
The Messinian Salinity Crisis
In August of 1970 the DSDP ship Challenger was positioned in the western
Mediterranean, south of the Balearic Islands, above almost 3000 m of water
depth. The geologists on board were looking for the source of a prominent
sub-sea-floor seismic feature called the M-reflector, and, to their great
surprise, they drilled into a thick layer of anhydrite - the first evidence of a
vast deposit of evaporite rocks extending across the Mediterranean.
The Messinian Salinity Crisis
Selenitic gypsum crystals growing
from a nucleation point. Miocene
Yesares Formation, Sorbas, Spain.
This evaporitic gypsum was
precipitated in response to rapid
desiccation of a marine basin as part
of the Messinian ‘Salinity Crisis’.
Messinian Halite mine
The Messinian Salinity Crisis
It is now widely accepted that these evaporites, which formed during the
Messinian (late Miocene) – between 5.96 and 5.33 m.y. - resulted from the
closure of the marine passages between the Atlantic and the Mediterranean,
and the subsequent (and repeated) complete (or near-complete) desiccation
of the Mediterranean Sea. This “Messinian Salinity Crisis” (MSC)
represents one of the most dramatic examples of base-level fluctuation
known in the geological record: an amplitude of perhaps 1-2 km within a
stage with a duration of less than 2 Myr.
The important question, which is still in debate, is the mechanism by which
the Mediterranean became isolated.
The dry and hot Mediterranean basin has been an area with a negative
precipitation-evaporation budget for millions of years. Without a significant
inflow of Atlantic Ocean water, the Mediterranean Sea cannot be sustained.
The Messinian Salinity Crisis
Three mechanisms have been proposed to explain the isolation of the
Mediterranean during the Messinian, including:
1) A 60 m global drop in sea level due to glaciation:
Insufficient to have closed all of the Late Miocene marine gateways
2) Horizontal shortening associated with crustal nappe movements:
unlikely because emplacement of crustal nappes in the Miocene had
already ceased at that time in this place
3) Tectonic uplift.
Evidence that sediments in the former marine gateways were uplifted in
the Late Miocene and Pliocene to their present elevations
The Messinian Salinity Crisis
Krijgsman, W., et al.,. 1999. Chronology, causes and progression of the Messinian
salinity crisis. Nature, 400 (6745), 652-655.
Abstract: The Messinian salinity crisis is widely regarded as one of the most dramatic
episodes of oceanic change of the past 20 or so million years. … elucidation of the
causes of the isolation - whether driven largely by glacio-eustatic or tectonic processes
- have been hampered by the absence of an accurate time frame. Here we present an
astronomically calibrated chronology for the Mediterranean Messinian age based on an
integrated high-resolution stratigraphy and 'tuning' of sedimentary cycle patterns to
variations in the Earths orbital parameters. We show that the onset of the MSC is
synchronous over the entire Mediterranean basin, dated at 5.96 +/- 0.02 million years
ago. Isolation from the Atlantic Ocean was established between 5.59 and 5.33 million
years ago, causing a large fall in Mediterranean water level followed by erosion (5.595.50 million years ago) and deposition (5.50-5.33 million years ago) of non-marine
sediments in a large 'Lago Mare' (Lake Sea) basin. Cyclic evaporite deposition is almost
entirely related to circum-Mediterranean climate changes driven by changes in the
Earth’s precession, and not to obliquity-induced glacio-eustatic sea-level changes. We
argue in favour of a dominantly tectonic origin for the MSC, although its exact timing
may well have been controlled by the 400-kyr component of the Earths eccentricity
cycle.
The
Messinian
Salinity
Crisis
Astronomical calibration of
Messinian pre-evaporite
sequences
(Krijgsman et al., 1999)
The Messinian Salinity Crisis
In a recent paper published in Nature (Duggen et al., 2003), researchers
cast doubt on all three of the proposed mechanisms, and suggest that a
significant change in the dynamics of subduction and in volcanism was
responsible for the closure of the waterways.
Miocene to Pleistocene aged volcanic rocks are present, both above and
below water, in the Alboran Basin of the westernmost
Mediterranean. Geochemical and isotopic data acquired by Duggen et al.
(2003) show two distinctive types of volcanism, including generally felsic
rocks typical of subduction, and more mafic rocks typical of a direct source
from the asthenosphere.
The felsic rocks are probably related to east-dipping subduction underneath
southern Spain and northern Morocco, a process that may still be active
(Gutscher et al., 2002). Duggen et al. suggest that westward migration (rollback) of the subduction zone could be responsible both for the change in
volcanic rock chemistry at close to 6.3 m.y., and also for significant uplift in
the area of the Betic and Rifean waterways.
The Messinian Salinity Crisis
NASA photograph of the
westernmost Mediterranean,
modified to show Atlantic–
Mediterranean marine gateways in
southern Spain and northwestern
Africa about 8 Myr ago, based on
the distribution of Upper Miocene
reef complexes and marine
sediments.
The Messinian Salinity Crisis
The proposed mechanism
of Duggen et al. (2003)
involves steepening of the
subducting oceanic slab,
and the flow of hot lowdensity asthenosphere
into where the slab had
been, up against the base
of the continental
crust. The presence of
this relatively buoyant
material could have
produced up to 1000 m of
uplift in the Betic and
Rifean corridor areas more than enough to close
the two waterways.
The Messinian Salinity Crisis
Trace-element and isotope ratios change during the MSC, consistent with the change
from subduction-related to intraplate-type magmatism (Duggen et al., 2003).
The Messinian Salinity Crisis
A composite model for uplift in the Alborán region. It illustrates that maximum
uplift (1 km) will occur on the continental margins, where the Late Miocene
marine gateways linking the Mediterranean Sea to the Atlantic Ocean were
located (Duggen et al., 2003).
The Messinian Salinity Crisis
During the terminal Messinian, after the MSC, huge fresh-water dilution
created brackish conditions of variable level of concentration in the deep
shallow-water Mediterranean basins. In a period that probably lasted
~1000–2000 yr, the entire Mediterranean was refilled by marine waters at
the onset of the early Pliocene so that homogeneous open-sea conditions
were established in the different basins.
According to Duggen et al. (2003), the flooding of the Mediterranean at the
end of the MSC may also have had a mantle-related cause. Westwardmigrating Late Miocene uplift may have also cause gravity-induced slumping
from the western margin of the Gibraltar arc into the Atlantic abyssal
plains, which may have allowed a new marine gateway to open at the Strait
of Gibraltar.
Traces of the MSC in Marine Precipitates
Position of the
Lion Seamount in
the influence zone
of the
Mediterranean
Outflow Water
(MOW) and of the
Tropic Seamount
out of the direct
MOW zone
(research cruise
SO 83)
(Koschinsky et al.,
1996)
Traces of the MSC in Marine Precipitates
0,0
1.6 my
Zn
Profile depth (mm)
10,0
Co
Cu
Mn
20,0
30,0
Ni
6.2 my
Fe
40,0
0,0
5,0
10,0
15,0
20,0
Concentration (%)
25,0
0,00
0,50
1,00
1,50
Concentration (%)
0,000
2,00
0,100
0,200
0,300
0,400
Concentration (%)
Chemical profile of a ferromganganese crust sample from Lion Seamount (1500 m water
depth) with ages for the peaks of indicated elements; the peak at ca. 6.2 million years
corresponds to the time of the MSC. (Koschinsky et al., 1996)
Traces of the MSC in Marine Precipitates
0
2
4
6
8
10
12
14 Age (Ma)
0
2
4
6
8
10
12
14
0
2
4
6
8
10
12
14 Age (Ma)
0
2
4
6
8
10
12
14
Variations of Pb- and Nd-isotopes as a function of age in a ferromanganese crust
sample from Tropic Seamount. The isotopic signatures of this crust are representative
of the North Atlantic Deep Water NADW.
(Abouchami et al., 1999)
Methane release as climate change trigger
Extreme perturbations of climate, toward either greenhouse warming or glaciation,
trigger collateral changes in sea-surface temperature, deep-water circulation,
biogeochemical cycling, community evolution, and extinction.
Greenhouse transients of significance occurred during the Late Paleocene Thermal
Maximum (at ~55 Ma) and the early Aptian Oceanic Anoxic Event (at ~120 Ma).
Catastrophic release of methane from dissociated gas hydrate may have accompanied
those events, and subsequent oxidation of the methane may have triggered oceanic
anoxia, wholesale changes in the fertility structure of the oceans, and abrupt changes
in alkalinity.
Methane release as climate change trigger
An intense period of global warming about 55.5 million years ago has been linked to a
massive release of methane, an event that killed many deep-sea species and enabled
terrestrial animals to flourish.
The warming, known as the "latest Paleocene thermal maximum," or Paleocene-Eocene
Thermal Maximum (PETM), occurred over a 10,000 to 20,000-year interval and
corresponds to the appearance of numerous mammals (including primates) and the
extinction or temporary disappearance of many deep-sea species.
The link between this warming period and the methane release is based on analysis of a
sediment core taken from the ocean floor.
According to the hypothesis, vast quantities of methane were stored as frozen gas
hydrate in the upper hundreds of m of continental slope sediments before the latest
Paleocene thermal maximum, during which ocean waters warmed by 7 to 14 degrees.
Scientists studied fossil wood deposits and identified a signal that indicates an unusual
level of light carbon in the Earth's atmosphere. The best explanation is that it comes
from methane - methane hydrate from ocean margin sediment.
Methane release as climate change trigger
Methane release as climate change trigger
The researchers believe massive volcanic eruptions during the Jurassic period initiated
global warming by spewing carbon dioxide and other greenhouse gases into the
atmosphere. Deep-sea currents also were affected.
Methane, freed from its suboceanic cage by warmer water, then used the oxygen in the
water or atmosphere to form carbon dioxide. In either case, it would have accelerated
global warming.
A number of important fossil groups disappeared at exactly that time. Hardest hit
were bottom-feeding clam-like organisms known as bivalves: An estimated 80 percent
of the species disappeared. Others affected included ostacods, belemnites and some
marine plants.
The event took place over a relatively short period. The release was estimated to be 20
percent of the present-day 14,000 billion tons of gas hydrate on the sea floor.
Methane release as climate change trigger
Recently, scientists provided the first astronomically-calibrated date of the PETM
(about 54.98 Ma) and a chronology for the event itself using cyclostratigraphy. It was
suggested that the onset of the event was not a simple one-step injection of CH4 but
rather a stepwise event. The return to initial conditions took place over about 120,000
years.
The Paleocene-Eocene Thermal Maximum is one of our best examples in the geologic
record of global warming caused by rapid injection of greenhouse gases. The ultimate
causes of these marked changes in climate and biota are, however, still poorly known. In
particular, we still have only a sketchy idea of what triggered the massive release of
greenhouse gases from the deep sea. Mechanisms ranging from asteroid impacts and
giant submarine land slides to volcanic eruptions have been proposed.
Methane release as climate change trigger
Why are “transient” climate events such as the PETM so important?
Our society is concerned with the fate of fossil fuel carbon that we are presently
adding to the atmosphere at a rate of 5 x 1014 mol C/yr.
While we have a considerable understanding of how the global carbon
cycle operates, we have no knowledge as yet of how a massive injection of fossil fuel
will perturb the global carbon cycle when the world is already warm.
Transient events such as the PETM provide the opportunity to study major upheavals
of the carbon cycle that are similar to the present day.
Methane release as climate change trigger
Stability conditions for gas hydrate
occurrences
Methane release as climate change trigger
Methane bound in hydrates amounts to approximately 3,000 times the volume of
methane in the atmosphere. Methane released as a result of landslides caused by a sealevel fall would warm the Earth, as would methane released from gas hydrates in Arctic
sediments as they become warmed during a sea-level rise. This global warming might
counteract cooling trends and thereby stabilize climatic fluctuation, or it could
exacerbate climatic warming and thereby destabilize the climate.
Gas hydrate
release in shelf
regions
Methane release as climate change
trigger
Gas hydrate release in Permafrost regions
Methane release as climate change trigger
The dissociation of gas hydrates during deglaciation has been linked to the ending of
ice ages during the last the last few millions of years. It was suggested that the
occurrence of large oceanic gas hydrate reservoirs are the factor limiting the severity
of ice ages. During formation of large polar ice sheets sea level falls, reducing the
pressure on the ocean margin gas hydrates. The shallower gas hydrate deposits become
unstable, and release methane into the atmosphere, which causes warming and the
ending of the ice age.
In addition, gas hydrate dissociation has been suggested as the cause for oceanic
anoxia and massive extinctions of marine biota at the end of the Permian: oxidation of
the methane within ocean waters could have used up a large part of the dissolved
oxygen in the oceans.
Oceanic Anoxic Events
The most extreme warm conditions in sedimentary records are indicated around the
Cenomanian-Turonian boundary from intermediate waters of the proto-Atlantic.
The entire water column warmed by up to as much as 15 to 19°C. This is warmer than
at any other time
during the Cretaceous
or Cenozoic and is
consistent with
Paleontological evidence
for extreme warmth in
the Arctic.
Oceanic Anoxic Events
Oceanic anoxic events (OAEs) are defined by intervals of enhanced deposition of
organic matter in marine environments. There were arguably between two and five
OAEs during the mid-Cretaceous. All recorded rapid changes in the carbon cycle
and/or were associated with major changes in marine biota.
Two of these events, the late early Aptian Selli Event (OAE-1a; about 120 Ma) and the
Cenomanian-Turonian Boundary Bonarelli Event (OAE-2; about 93.5 Ma) are the most
prominent and are characterized by the deposition of dark marls or shales enriched in
organic carbon. Enhanced preservation of organic matter during the OAE events
probably resulted from global expansion of the oxygen minimum zone.
While the cause(s) of these events is widely debated, most authors acknowledge a
complicated interplay between global warmth, increased surface water productivity
and/or deep-water stagnation.
OAE occurred when the world was extremely warm.
All authors agree that the OAEs were associated with major steps in climate
evolution because burial of excess organic carbon, by drawing down CO2, must have
had an influence on global temperatures.
Oceanic Anoxic Events
Excellent work has been done on the Selli Event (OAE-1a), with detailed stratigraphic
studies carried out on both land sequences and marine sequences. Most importantly, a
marked negative carbon isotope anomaly now has been found in both terrestrial and
marine realms, similar to the ones found during the PETM. This indicates that the
whole of the ocean-atmosphere system was influenced by changes in the global
carbon cycle.
Contrary to the PETM, the negative carbon isotope anomaly is superseded by an
abrupt positive d13C excursion. This sequence of carbon isotope stratigraphy suggests
that there was an initial phase of carbon release, possibly of mantle-derived CO2 or
by gas hydrate release, which gave rise to a distinct negative d13C excursion. This was
then followed by carbon burial enriched in 12C, possibly triggered by increased ocean
surface productivity, which gave rise to the subsequent positive d13C excursion.
Mechanism(s) responsible for the OAE-1a event have been linked to the Ontong JavaPacific “superplume” event, although gas hydrate release also is an option.
Oceanic
Anoxic
Events