Various geochemical signals show that the Palaeocene-Eocene boundary (at 55 Ma) was a time of global warming superimposed on the general Cainozoic cooling from the ‘hothouse’ of the Cretaceous Period. Some also point to an enhanced ‘greenhouse’ effect driven by massive methane release from gas hydrates on the sea floor. Methane, a ‘greenhouse’ gas in its own right, oxidizes to CO2 in the atmosphere, transferring its carbon that eventually ends up in the shells of marine organisms. It is the carbon-isotope blip at the P-E boundary that points to methane as a source of the warming. Not only does it appear in the marine C-isotope record from foraminifera shells in cores, but also in the teeth of terrestrial mammals, which means that the carbon reservoirs of both atmosphere and seawater were globally changed. Using the magnitude of that signal allowed palaeoclimatologists to estimate the amount of methane released – about 1 500 billion tonnes. On a millennial scale, that is comparable to a rate of warming similar to that currently induced by human activities.
The P-E boundary marks the most dramatic biological changes since the mass extinction 10 million years before at the Cretaceous-Tertiary boundary. But its underlying control is sufficiently close to what is happening to climate now to form both an object lesson and a means of modelling what may happen if current emissions continue. One of the important aspects needing scrutiny is how such warming events come to an end. British and American oceanographers have taken a look at the P-E record in ocean sediment cores, and believe they have come up with an answer, at least in part (Bains, S., 2000. Termination of global warmth at the Palaeocene/Eocene boundary through productivity feedback. Nature, v. 407 14 September 2000, p. 171-174).
Most such studies focus on oxygen- and carbon-isotope records in the carbonate of foraminifera shells, revealing ups and downs in seawater temperature and volume of land ice, and of biological productivity and releases of ‘greenhouse’ gases. Unfortunately, neither isotopic record properly resolves the alternative contributions to variation. Santo Bains and colleagues add another parameter that helps resolve the influence of biological productivity in the oceans. Marine organisms, especially plankton, either precipitate barium sulphate (barite) in tiny crystals within their cells or induce its precipitation once they die and decay. Because barite is not prone to much change by later events on the sea floor, counting its crystals in marine cores is a reliable proxy for the varying abundance of plankton through time.
One strong possibility during major warming events is that ocean circulation becomes sluggish, perhaps stopping altogether. That slows the re-supply of nutrients to sunlit upper layers, and works to reduce photosynthetic life in the oceans. The barite record produced by Bains et al. shows the opposite for the P-E events. For about 40 000 years after the P-E event biogenic barite rose to more than twice its normal abundance. The ocean biosphere responded to the methane blurt by blooming. Why it did so is not yet clear, but such a spurt in drawing CO2 into living and dead and buried tissue would work to reverse the warming event. The barite peak coincides exactly with the oxygen- and carbon-isotope records’ features that signify temperature and the influence of isotopically light carbon from methane released by gas-hydrate breakdown. It might seem as if life did regulate climate in a geologically rapid manner following the P-E event, to the delight of Gaians. However, the control over biological productivity is ultimately nutrients, and life has little influence over their supply to the oceans. Among the possibilities for an essential nutrient bonanza, and increased circulation of the oceans is definitely ruled out during major warmings, are hugely increased rainfall to wash terrestrial sediments and dissolved matter into the oceans, and increased volcanism that would supply fine ash to the distant ocean surface.
Converging on an explanation for the end of a period of global warming is far from showing how this might be achieved for a warming induced by human activities. That might well prove eventually to be a life-or-death necessity for our species, bearing in mind that the P-E warming was a fatal crisis for many land mammals of the time.
See also: Schmitz, B., 2000. Plankton cooled a greenhouse. Nature, v. 407 14 September 2000, p. 143-144.
A new regular pulse in recent climate
Gerard Bond of the Lamont-Doherty Earth Observatory at Columbia University, Palisades, New York has taken his analysis of high-frequency climatic shifts in the last glaciation into the Holocene record. Previously, Bond had tried to make sense of the sharp fluctuations of the order of a few thousand years that are seen as gravel layers in the uppermost levels of sea-floor cores and in the oxygen isotope records of cores through the Greenlandic and Antarctic ice sheets. The first signs of short ups and downs in climate were the coarse layers first found by Hartmut Heinrich in the glacial part of the sea-floor record. Heinrich ascribed them to periodic releases of iceberg armadas as the ice sheets of the last glaciation became unstable. Bond’s latest work also focuses on Heinrich events, but he has used specific lithologies as markers rather than merely grain-size variations. In particular, hematite-stained quartzo-feldspathic materials seem likely to have come from altered rocks in east Greenland and Svalbard, far distant from the drill sites whose cores he has examined. The proportion of reddish grains varies systematically in the cores, some layers coinciding with Heinrich events, but there are many more. The layers appear roughly every 1500 years. This periodicity coincides with cycles of dust blown from the Sahara to form layers in cores from the west African coast, so whatever the pulses represent, they are global signals.
Interestingly, the cycles show little sign of change in the period after the melt back that signified the beginning of the Holocene interglacial. Behind the long-term climatic shifts in glacials and interglacials, that coincide with the 100, 41, 23 and 19 thousand year fluctuations in solar warming of the northern hemisphere, some other process must be put-putting in the background. The 1500 year cycles may stem from processes that shift heat in the oceans and atmosphere. A likely candidate is the production of deep currents by sea-ice formation in the northern North Atlantic. However, detailed calculations of tides suggest a similar pacing that might change the mixing of surface and deep water in the ocean conveyor system.
Whatever the driving force, this periodicity strikes a chord with emerging details of Holocene climate changes from lake-sediments studies and the historic record. One such recent cooling pulse that might have delivered icebergs to mid-latitudes in the North Atlantic was the Little Ice Age that peaked in the 17th century that saw prolonged stresses on the population of Europe, and major political changes that resulted from such events as the Peasants’ Revolt and repeated famines.
Source: Pearce, F. 2000. Feel the pulse. New Scientist, 2 September 2000, p. 30-33.