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Natural Global Change - Some Special Events

Andrea Koschinsky

Heinrich Events: Marine Record of Abrupt Climate Changes in the Late

PleistoceneIn 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 foraminifera-dominated 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 (foraminifera-dominated) 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 EventsProbable 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 EventsIn 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.

Heinrich EventsProcesses by which ice rafted detritus are depositedThe 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

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.

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.

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 EventsRecords of Heinrich EventsHeinrich 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 EventsRecords of Heinrich EventsThis 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 18O, demonstrates a decrease of nearly 1.5 o/oo during H-4.

Heinrich EventsRecords of Heinrich EventsThe more negative 18O 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 18O measurements was probably significant enough to temporarily shut down thermohaline circulation in the North Atlantic.

Heinrich EventsVariability 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 non- Heinrich events abrupt, but conditions within Heinrich events could also vary to a great extent.

Heinrich EventsCorrelations 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.


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


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.


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

gateways2) Horizontal shortening associated with crustal nappe movements: unlikely because emplacement of crustal nappes in the Miocene

had already ceased at that time in this place3) Tectonic uplift. Evidence that sediments in the former marine gateways were

uplifted in the Late Miocene and Pliocene to their present elevations


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 (5.59-5.50 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.


Astronomical calibration of Messinian pre-evaporite sequences

(Krijgsman et al., 1999)


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 (roll-back) 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.


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 proposed mechanism of Duggen et al. (2003) involves steepening of the subducting oceanic slab, and the flow of hot low-density 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.


Trace-element and isotope ratios change during the MSC, consistent with the change from subduction-related to intraplate-type magmatism (Duggen et al., 2003).


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).


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. Westward-migrating 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)

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MAURITANIA

SPAIN

MADEIRA

CANARY IS.

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4000

MOW

Lion SeamountTropic Seamount

20°

30°

40°30°

20° 10°

20° 10°30°


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)


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)

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

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.


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.



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.


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.


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 carboncycle 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.


Stability conditions for gas hydrate occurrences


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 sea-level 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

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Gas hydrate release in Permafrost regions


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 EventsThe 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.


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 globalcarbon cycle.

Contrary to the PETM, the negative carbon isotope anomaly is superseded by an abrupt positive 13C 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 13C 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 13C excursion.

Mechanism(s) responsible for the OAE-1a event have been linked to the Ontong Java-Pacific “superplume” event, although gas hydrate release also is an option.


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