Australia: The Land Where Time Began
Palaeocene-Eocene Thermal Maximum (PETM) - "all part of a natural cycle" (aka Late Palaeocene Thermal Maximum (LPTM))
About 55-56 Ma, in the Early Eocene, there was a geologically brief period when the temperature of the water at the high latitudes and the deep oceans rose by 5-7o C. Geologically speaking, this was a sudden rise. It is indicated by an abrupt change in the carbon isotope ratios, the 13C dropped by 2.5-3 %, the 12C rose by the same amount. This indicates there was a massive and sudden input of 12C enriched carbon into the oceans or atmosphere, over a period of about 10,000 years. It took about 140,000 years for the balance to be restored to previous ratios. It is believed that it required an input at a rate comparable to that of the present rate of addition of carbon from human activity to achieve the amount of change over the short period that has been found. The belief that the amount of CO2 in the atmosphere remains relatively stable over time has been questioned as a result of the work on 13C/12C ratios that led to the finding of this sudden temperature change. It now seems there is a vast store of carbon, which is enriched in 12C, which may be intermittently added to the atmosphere over very short periods, geologically speaking.
The prime suspect is a large gas-hydrate capacitor (White, 2003), because it is the only known reservoir that is large enough and that is sufficiently enriched in 12C. The trigger for the release of 1000 gigatonnes of carbon from this store at the beginning of the LPTM is not known for certain. Whatever the trigger, increased deep ocean temperatures would be involved. A suggestion has been a period of explosive volcanism in the North Atlantic. Whatever the cause, deep ocean water that warmed by only a few degrees would lead to the release of the methane to the water, and then to the atmosphere, where the methane and its oxidation product CO2 would lead to greenhouse warming on a global scale.
The threshold at which the release of methane occurs is not known, but it appears to have been crossed, at least in limited areas, as methane has been observed bubbling to the surface in a number of places in the Arctic Ocean to the north and east of Siberia. A deposit of methane clathrate that is surprisingly large has been found on the Lord Howe Rise, off the east coast of Australia.
This event from a bit less than 56 million years ago might provide us with a glimpse of what might be happening with the climate now, as the atmospheric CO2 continues to rise and ice caps continue to melt. According to the author¹ this event from the distant past that provides a lab in which to study the consequences and effects of ”wholesale release of carbon" into the atmosphere over a very short time period. At the time of the PETM the earth was a hothouse and the indications are that the world is probably heading in the same direction as it did at the start of the PETM.
The available evidence has led scientists to believe the warming at the PETM coincided with the release of 4,000-7,000, and possibly up to 10,000 billion tonnes of carbon (in carbon dioxide and methane) into the atmosphere over a time period that may have been as short as 10,000 years, a proverbial "blink" in geological time, leading to a global average temperature rise of 6° C. It has been estimated that about 500 billion tonnes of carbon have been added to the atmosphere by the burning of fossil fuels, and continues to rise both by the increasing use of fossil fuel and the burning of tropical rainforests as is occurring in Indonesia and Malaysia, especially to expand their palm oil production, and in South America for other agricultural production. It has been estimated that more than 8 billion tonnes of carbon are being added to the atmosphere per year.
A common factor in all such events that are known appears to be a sudden rise in atmospheric carbon dioxide. There are a number of causes that have been proposed to explain this sudden leap in atmospheric carbon dioxide, 2 proposals that are not widely supported are a comet striking the Earth and the burning of the vast areas of peatland that is believed to have been present at that time, though estimates of the amount of biomass required to be burnt to produce the amount of carbon dioxide that entered the atmosphere at that time was more than 90 % of the total biomass of the world. The suggestion that volcanic activity was responsible had the same failing, not enough carbon dioxide; one model suggests that rising magma was an important component.
The original estimates of the amount of carbon in the clathrate deposits around the world was about 10,000 billion tonnes, higher than the combined amounts of carbon dioxide and methane in the atmosphere and much more than the total amount in fossil fuel reserves of the world. The amount in the clathrate deposits has since been revised down to about 2,000 billion tonnes of carbon, this new figure being less than the total amount of carbon in the known fossil fuel reserves, though remains 2.5 times the amount of carbon in the atmosphere.
According to the author² there is strong evidence for the involvement of gas hydrate in the sudden warming that occurred in the PETM. Giant submarine sediment slides along the continental margins, the western Atlantic in particular, that indicates the sort of widespread destabilisation expected when the solid hydrate decomposes into much greater volumes of gaseous methane is one line of evidence. Another line of evidence is the distinctive carbon signature that is associated with methane that is derived from gas hydrates, this carbon being depleted in 13C relative to the lighter 12C isotope, making it 12C enriched. Analysis of the carbon isotope composition of marine sediments that were deposited at the time can determine the very large volumes of carbon sourced from gas hydrate that were added during the PTEM. The existence of the Carbon Isotope Excursion (CIE) in ocean waters is revealed by such analysis, meaning that the isotope composition of the oceans underwent a sudden, significant change with respect to the relative proportions of the 3 isotopes of carbon. When the CIE is described as negative it indicates enrichment of the lighter 12C isotope. A negative CIE is identified that amounts to about 4 %, indicating that there was an addition of a significant amount of carbon, which was 13C-depleted relative to 12C, to the oceans and atmosphere in the Late Palaeocene.
According to the author² there are a couple of key questions that need to be answered to allow the acceptance of the gas hydrate role in the PETM as the main contributor, though the carbon isotope change was sudden at the PETM. Were the gas hydrates solely responsible for initiating the sudden warming in the end-Palaeocene event, and what triggered the sudden breakdown of the gas hydrates? The total amount of gas hydrate at the time of the PETM is presently thought to have been sufficient to explain all the additional carbon. The amount of gas hydrate at the present is believed to be insufficient to provide 2,000 billion tonnes of carbon, and it is thought the reserves of gas hydrate present at the time of the PETM would have been smaller than at present, thus requiring most of the carbon to have come foam another source. This suggests that at the time of the PETM rapid warming resulting from the high levels of CO2 in the atmosphere that was derived from a source yet to be identified causing the oceans to warm sufficiently to lead to the wholesale disassociation of the gas hydrate deposits in a positive feedback effect, resulting in large volumes of methane that would add to the warming that had already taken place, indicating that the breakdown of the gas hydrates was actually a response to the initial warming by other means that are yet to be determined. A study of the environmental conditions in the Late Palaeocene by Appy Sluijs, Utrecht University, Netherlands, supports the exacerbation of warming that was already underway. The temperatures of the oceans at the PETM were revealed by the study of single-celled organisms in the marine sediment. The results of this study indicated that the warming had begun several thousand years prior to the CIE that arose from the breakdown of gas hydrates, though the study didn't suggest a possible cause of the warming. Other research has pointed to the dramatic geological events that were occurring in the North Atlantic during the Late Palaeocene.
Pangaea began fragmenting in the Early Jurassic and it was then that the narrow proto-North Atlantic Ocean opened separating Laurasia in the north from Gondwana in the south. More than 100 My later the Atlantic was a wide ocean extending to the south that separated South America and much of North America, in the west, from Africa in the east by the Palaeocene. The North Atlantic was undergoing its final extension coincident with the PETM, opening northwards between Greenland and northern Europe. The mantle beneath this region was melting and poured vast quantities of lava across Baffin Island in Canada, Greenland and the Faeroes, and the northwest of Britain, in places the lava reached to more than 7 km in thickness, and in other places intruded into local rock and sediments. Estimates of the total volume of lava extruded range between 5 and 10 million cubic kilometres. It has been suggested by Mike Storey et al. of Roskilde University, Denmark that the initial warming of the PETM was caused by the prodigious amounts of 12C-enriched methane released as magma associated with the separation of Greenland from Europe heated and baked the sediments over much of the floor of the region prior to the tectonic activity. The PETM occurred shortly after the commencement of the tectonic activity, though the link is hypothetical until it is supported by evidence.
According to Bowen et al., (2015)3 the climate of the Earth warmed abruptly by 5-8oC during the Palaeocene-Eocene Thermal Maximum (PETM), about 55.5 Ma (Westerhold, Röhl & Laskar, 2012; McInerney & Wing, 2011). Associated with this warming was a massive addition of carbon to the ocean-atmosphere system, though estimates of the response of the Earth system to this perturbation are complicated by the estimates of the duration of carbon release which varies greatly, ranging from less than a year to tens of thousands of years. As well as the source of carbon, it is still being debated whether the release took place as a single injection or as several pulses (McInerney & Wing, 2011; Wright & Schaller, 2013; Cui et al., 2011). In this paper Bowen et al., present the results of their study of a new high-resolution carbon isotope record obtained in terrestrial deposits in the Bighorn Basin, Wyoming, USA, that spans the PETM and their interpretation of the record by the use of a carbon-cycle box model of the carbon-atmosphere-biosphere system. The record shows that 2 distinct carbon release events characterised the beginning of the PETM, and these events were separated by a period of recovery to background values. The model they used was found to require 2 discrete pulses of carbon released directly to the atmosphere at average rates that exceeded 0.9 Pg C yr-1, with the first pulse lasting less than 2,000 years. They therefore concluded that the PETM involved 1 or more reservoirs that were capable of carbon releases that were repeated and catastrophic, and that the rates at which the carbon was released during the PETM were more similar to those associated with modern anthropogenic emissions than has been suggested previously (Wright & Schaller, 2013; Cui et al., 2011).
Eocene hypothermal event – insight into greenhouse warming4
New findings from studies of palaeoclimate have provided some idea of the climate problems the world can look forward to if the atmospheric greenhouse gas concentrations continue rising at the present rate. As Bowen et al., say, the modern anthropogenic forcing of atmospheric chemistry is in line to provide an experiment in such change that was last matched in the early Palaeogene, more than 50 Ma, a time of catastrophic carbon release to the atmosphere that drove hyperthermal events that were abrupt and transient.
Research on the climate that existed during the Palaeocene-Eocene Thermal Maximum (PETM), which is the best documented of such events, at about 55 Ma, has significantly advanced since its discovery. Carbon additions to oceans and atmosphere during the PETM were at a similar magnitude to those expected to occur for the remainder of the 21st century. The event in the Palaeogene initiated global warming, biotic extinction events and migration, and fundamental changes in the carbon and hydrological cycles that transformed the early Palaeogene world.
It is demonstrated by the PETM that carbon cycle perturbation, even in times of global warmth and in a world free of ice, can trigger extreme, rapid changes in Earth systems. An array of changes in the atmosphere, hydrosphere, geosphere, and biosphere have been documented from the PETM, and insight has been provided by these studies into the temporal patterns and coupling of changes in the Earth systems that accompany massive release of carbon in a warm world.
Atmospheric temperatures are inferred to have been up by 5-9oC globally from ocean surface (see Zachos et al., 2005 and references therein) and terrestrial (e.g., Wing et al., 2005) from proxies during the PETM. Closely associated with warming was the release of between 1,500 and 4,500 gigatons of carbon to the ocean and atmosphere, with the result that there was a large, though poorly quantified increase of atmospheric carbon dioxide levels (Zachos et al., 2005). The PETM also affected moisture transport in the atmosphere, as is evidenced by indicators of terrestrial discharge along the margins of the continents (e.g., Crouch et al., 2003) and isotope records which suggest growing conditions were humid across the northern midlatitudes (Bowen et al., 2004). It is suggested by floral evidence from Wyoming that in that location the amounts of precipitation varied throughout the PETM (Wing et a., 2005).
During the PETM input to the ocean-atmosphere system is most likely, according to Bowen et al., to have come from geospheric reservoirs in sediments that were buried shallowly or the crust. Since it was proposed (Dickens et al., 1995) that a methane clathrate source that was destabilised as ocean temperatures increased, beginning in the Late Palaeocene, then rising to a maximum in the Early Eocene.
Alternative sources that were proposed include thermogenic methane that was produced during the placement of igneous plutons in the seafloor of the North Atlantic (Svensen et al., 2004) or the widespread burning of peat and coal (Kurtz et al., 2003). An important role in the sequestration of carbon during the later stages of the PETM was played by the geosphere, at a time when increased marine carbonate burial was driven by weathering feedbacks, which buffered and ultimately led to recovery of the carbon cycle from the PETM (Zachos et al., 2005).
Perturbations to the thermohaline circulation of the ocean are a potential consequence of future global warming, which Bowen et al. suggest may further change the global climate. It is therefore of great significance that indications are that ocean circulation changes occurred during the PETM.
During the PETM warming of the surface of the ocean was amplified at high latitudes by as much as 9oC relative to the temperatures at low latitudes of about 5oC, with the temperatures of the deep waters rising by 4-5oC globally (see Zachos et al., 2005, and references therein).
Gradient of sea surface temperatures or continental runoff of freshwater may have shifted the site of formation of deep water from a locus in the Southern Ocean to a locus in the sub-tropical latitudes or to high latitudes in the Northern Hemisphere (e.g., Kennett & Scott, 1991; Bice & Marotzke, 2002; Nunes & Norris, 2006), which drove warm water into the deep sea which would have driven the destabilisation of methane clathrate and further greenhouse warming (Bice & Marotzke, 2002).
Deep water flow during the PETM has been reconstructed by the use of geographic patterns of benthic foraminifera carbon isotope fraction (δ13C) values and to support a reversal of circulation during the PETM (Nunes &Norris, 2006). These data reflect water mass characteristics of intermediate and deep waters from about 1,000 to >3,000 m palaeodepth, and Bowen et al., suggest they may have been influenced by local regeneration locally. At the onset of the PETM varying degrees of dissolution are displayed by the sections that were investigated, which resulted in time gaps that make precise site-to-site correlation difficult if not impossible. Among other tools that can be used for the reconstruction of deep-water circulation are patterns of dissolution of undersea carbonates, oxygen content, and concentrations of neodymium isotope that are not consistent with circulation changes in the deep ocean at the beginning of the PETM, as inferred from δ13C records (e.g., Thomas et al., 2003).
At the PETM, global environmental perturbations are no less apparent in biotic records as those records that document the climate or carbon cycle. Associated with the dispersal of mammals among the continents of the Northern Hemisphere at the beginning of the PETM were lasting changes in the taxonomic composition and diversity, as well as transient reduction of body size of mammals (e.g., Clyde & Gingerich, 1998). In the mid-latitudes, floras of the PETM document range extensions to the north over hundreds to thousands of kilometres and intercontinental dispersal (Wing et al., 2005).
Among marine communities a complex array of responses were shown ranging from the loss of about 35-50 % of deep sea species of benthic foraminifera, the most severe extinction event in the last 90 million years (e.g., Thomas, 1998), to significant assemblage changes to other groups, though the changes were transient, including dominance of Apectodinium, a warm-water dinoflagellate (e.g., Crouch et al., 2003), rapid diversification of planktonic foraminifera (Kelly et al., 1998), and in shelf and open-ocean locations, shifts in trophic strategies of nannoplankton. Throughout the PETM overall patterns of productivity of terrestrial and marine ecosystems appear to have varied substantially (Bowen et al., 2004; Thomas, 1998).
Earth systems evolution in the PETM – a synthesis
A 3-stage perturbation and response associated with greenhouse gas release scenario through time is reflected in changes in individual Earth Systems and the interaction over time.
Phase I – Initiation
Decreases in global δ13C during the first about 15,000-30,000 years of the PETM are documented in carbon isotope records and indicate 1 or more rapid releases of 13C depleted carbon to the atmosphere-ocean system, which occurred within 1,000-2,000 years. Bowen et al., say it remains a critical question what triggered the PETM. In considering the PETM as a potential analogue to modern global change, it is important to understand whether the PETM was initiated as a feedback, i.e. as climate crossed a warming threshold, or as an event that was externally forced. The human-induced warming may also trigger a cascade of amplifying carbon cycle feedbacks if the carbon release of the PETM was a feedback. If an external forcing caused the carbon release of the PETM it might better be considered analogous to anthropogenic carbon release itself. The suggestion that multiple PETM-like events may have occurred in phase with orbital cycles has the potential to falsify hypotheses that link the PETM to singular forcing factors such as impacts of bolides or volcanic events (Lourens et al., 2005).
Abrupt changes that spanned the Earth systems, that included acidification of the oceans, rapid changes in the biota, terrestrial and marine, and the extinction of the benthic foraminifera, whatever the trigger of the PETM was. It is not completely understood what the linkages were between the perturbations of the carbon cycle and the synchronous changes of the biota – it may be that individual ecosystems responded directly to aspects of environmental change such as carbon addition, e.g., acidification of the ocean, raised partial pressures of CO2 and /or indirectly consequences of carbon release, such as rising temperatures, increased precipitation, nutrient supply changes and/or distribution.
Phase II – Alternate semi-stable state
According to Bowen et al. this phase was characterised by a distinct interval of about 60,000 years that began when δ13C values reached their minimum has been called the ‘body’ of the PETM. Continual increases in global temperature, oceanic and terrestrial δ13C values that were relatively stable, increased offsets between terrestrial and marine systems, slowly diluting acidity of the ocean, as biotic assemblages that include transient, often unique taxa, such as dinoflagellate cysts, benthic and planktonic foraminifera, and calcareous nannoplankton in the oceans; on the continents plants and mammals.
It is suggested by Bowen et al. in this paper that it is demonstrated by the body of the PETM that the response of the Earth systems to the initial PETM forcings was not a simple shift away from and recovery to equilibrium; rather, it was a shift to a semistable state that was fundamentally different (Bowen et al., 2004). Bowen et al. suggest that in some ways this may represent the global environmental future, making it critical that it be characterised. A model was proposed (Bowen et al., 2004) that suggested substantial ecosystem change during the body of the PETM, though additional focused studies are needed. An unexplained and characteristic feature of the body of the PETM is low δ13C values. Temporary stagnation of components of the carbon cycle during a time when seafloor preservation of carbonate was poor may be reflected in the stable isotope values (Zachos et al., 2005) and reduced export production of open-ocean carbonate (Thomas, 1998). If this proves to be the case, that continually rising global temperatures and changes to ecosystems during the body of the PETM may represent changes that are associated with a long-term lag in recovery of the carbon cycle from massive carbon release, which may be indicative of future patterns of global change.
Phase III – Recovery
In this phase the final about 70,000 years of the PETM, the Earth systems recovered in the earliest Eocene to a state that was similar in many ways to that of the Late Palaeocene. With regard to understanding how the systems of the Earth recover from perturbations of the carbon cycle and the degree to which lasting changes to the climate of the Earth and the biota and geochemical systems result from these events, the details of the recovery process are relevant. Dramatic increases in the rate of seafloor carbonate burial, falling global temperatures and a transition from biotic assemblages that are distinctive of the PETM to those that were typical of the Early Eocene were all included in recovery. A potential mechanism for restoring balance to the carbon cycle following massive release of carbon is represented by increased burial of marine carbonate; though it is important to understand how this process proceeded given the intervening 60,000 years of the body of the PETM during which carbonate burial was low.
The strongest evidence for lasting change that was induced by the PETM is provided by the biotic record. Communities of terrestrial mammals and benthic foraminifera from the earliest Eocene are widely different from their latest Palaeocene counterparts in species composition and ecological features. Bowen et al. suggest permanent environmental changes may be reflected in these differences, as may interactions among organisms that are brought together by changes of range, or the irreversibility of evolution and extinction.
The PETM has been shown by 15 years of study to be a case study of the broad impacts of massive perturbation of the carbon cycle in a time of globally warm climate. Further study promises to not only guide an understanding of the mechanisms of global change during the PETM, but to also illustrate connectivity among the Earth systems and patterns of change that could possibly characterise the future of the Earth.
4. Bowen, G. J., T. J. Bralower, M. L. Delaney, G. R. Dickens, D. C. Kelly, P. L. Koch, L. R. Kump, J. Meng, L. C. Sloan, E. Thomas, S. L. Wing and J. C. Zachos (2006). "Eocene hyperthermal event offers insight into greenhouse warming." Eos, Transactions American Geophysical Union 87(17): 165-169.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|