ScienceWeek

PALEOCLIMATE: A PAST WARM ARCTIC OCEAN

The following points are made by Christopher J. Poulsen (Nature 2004 432:814):

1) Various lines of evidence demonstrate that Earth's climate was much warmer during the Cretaceous period than it is today. Yet that evidence -- fossil plants and animals, sedimentary features, and geochemical indicators -- is sparse, spotty, and often inexact, making the magnitude and distribution of Cretaceous temperatures highly uncertain. Jenkyns et al[1] have produced a single new datum from one of the coldest spots on Earth's surface, a single new datum that clarifies the nature of extreme warmth in the Cretaceous, and reveals that the past was radically different from the present.

2) The new evidence for a warm Arctic climate was retrieved by drilling through an ice island drifting over the Alpha Ridge, one of three submarine ridges that divide the Arctic sea floor. Sediments deposited on the modern Arctic sea floor are oxidized, rich in silt and sand, and have only sparse evidence of planktonic life: with nearly 3 meters of sea ice capping its surface, the Arctic Ocean is not a hospitable environment for most organisms. In sharp contrast, the Late Cretaceous sediment from the Alpha Ridge consists of an ooze that is rich in diatoms (unicellular algae) and black mud that contains woody fragments, leaf cuticles, spores and pollen. The sediments are also notable for what they do not contain -- dropstones, the non-marine debris that fall from the base of icebergs. All in all, these sediments were evidently the product of fertile, ice-free marine waters.

3) Although the sedimentary evidence from the Arctic marine cores is suggestive of warm, ice-free conditions, it is a very imprecise thermometer. For this reason, paleo-oceanographers often draw on geochemical tools, such as the ratio of oxygen isotopes in carbonate shells from marine microorganisms, that can provide precise estimates of seawater temperature. Because of the absence of carbonate in Arctic Cretaceous sediments, traditional methods could not be used. So Jenkyns et al[1] turned to a novel method (called TEX86) for quantifying sea surface temperature. They examined the composition of lipids in the membranes of Crenarchaeota, floating marine microorganisms that range in size from 0.2 to 2.0 micrometers, and which have been found in sediments up to 112 million years old.

4) Earlier work demonstrated that the composition of organic compounds in the membranes of modern Crenarchaeota is highly correlated with seawater temperature and may well be a biological adaptation to the thermal state of the marine environment[2]. The TEX86 method has previously been tested on Cretaceous sediments from low latitudes, and the results are in good agreement with estimates of seawater temperature using oxygen isotopes[3]. Applying this new geochemical tool to 70-million-year-old organic material from the Alpha Ridge, Jenkyns et al[1] calculate that the average sea surface temperature was 15 °C. Sea water of similar temperature is presently found off (for example) Maryland and France, at latitudes between 35° N and 45° N. For a region blanketed in darkness for half of the year, the Arctic Ocean was astoundingly warm.[4,5]

References (abridged):

1. Jenkyns, H. C., Forster, A., Schouten, S. & Sinninghe Damsté, J. S. Nature 432, 888-892 (2004)

2. Schouten, S., Hopmans, E., Schefu, E. & Sinninghe Damsté, J. S. Earth Planet. Sci. Lett. 204, 265-274 (2002)

3. Schouten, S. et al. Geology 31, 1069-1072 (2003)

4. Tarduno, J. et al. Science 282, 2241-2244 (1998)

5. Morton, J. Fruits of Warm Climates (Creative Resources Systems, Miami, 1987)

Nature http://www.nature.com/nature

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PALEOCLIMATE: AN ANCIENT HYDROCARBON-DRIVEN GLOBAL WARMING

The following points are made by Gerald R. Dickens (Nature 2004 429:513):

1) The outstanding examples of intense global warming and massive greenhouse-gas emissions occurred during a brief episode, known as the "initial Eocene thermal maximum" (IETM), about 55 million years ago. Superimposed on already warm climates, Earth's surface temperatures soared by 5-10 deg C within a geological instant(1,2). At the same time, an enormous amount of carbon dioxide, apparently produced through oxidation of hydrocarbons, rapidly entered the global carbon cycle(3). Researchers have only reluctantly taken the IETM as an analogue for examining our planet's future, however, because direct evidence for the actual release of hydrocarbons, and the driving mechanism, has remained elusive.

2) There is seemingly incontrovertible evidence of the injection of huge amounts of organically derived CO2 during the IETM. Numerous isotope records, constructed using data from primary carbonate or organic matter, display an extraordinary drop in the ratio of C-13 to C-12 at that time, clearly signalling a perturbation of the entire carbon pool on Earth's surface(3). Given the coeval dissolution of deep marine carbonate(5), and the short duration of the isotope excursion (less than 20,000 years(1,5), at least 1500 gigatonnes (Gt) of carbon as CO2 must have suddenly been injected into the ocean or atmosphere from a C-13-deficient source(3). The only satisfactory explanation for this is the release and oxidation of carbon from a large reservoir of organic material (the biological generation of organic compounds preferentially incorporates 12C).

3) In several sediment sections from the north and central Atlantic Ocean, the IETM is marked by finely layered deposits devoid of carbonate(5). Combined with the isotope information, that suggests the addition of substantial amounts of methane to this ocean basin, and its oxidation, because methane oxidation consumes dissolved oxygen (which precludes fauna from turning the sediment over) and produces CO2 (which dissolves carbonate). The dual discoveries of the apparent existence of warm, deep waters at the IETM(1), and vast amounts of methane-containing gas hydrates -- ice-like crystals of gas and water -- along continental slopes, thus led to the following proposal(3): Like those of today, gas hydrates in marine sediments included large amounts of C-13-depleted biogenic methane; some change in conditions at the start of the IETM caused relatively warm water to sink in the oceans, and that warming resulted in the dissociation of the gas hydrate; the free methane thus released escaped from the sea floor, and was then oxidized to CO2 in the ocean or atmosphere.(4)

References (abridged):

1. Kennett, J. P. & Stott, L. D. Nature 353, 225-229 (1991)

2. Zachos, J. C. et al. Science 302, 1551-1554 (2003)

3. Dickens, G. R., O'Neil, J. R., Rea, D. K. & Owen, R. M. Paleoceanography 10, 965-971 (1995)

4. Svensen, H. et al. Nature 429, 542-545 (2004)

5. Bralower, T. J. et al. Geology 25, 963-966 (1997)

Nature http://www.nature.com/nature

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PALEOCLIMATE: SEAWATER TEMPERATURE AND GLOBAL COOLING

The following points are made by Roger Francois (Nature 2004 428:31):

1) Since the beginning of the Tertiary, 65 million years ago, the Earth has experienced a long-term cooling trend(1). The reasons invoked to explain the trend are complex, involving changes in major ocean currents and continental topography, the amount of solar energy reflected by the Earth, concentrations of atmospheric CO2, and continental glaciations. In contrast, Sigman et al(2) highlight a simple yet largely overlooked factor, which may have contributed significantly to cooling during the past 3 million years or so. It is based on the temperature-dependent density of sea water, and the interplay with the other main contributor to density, salinity.

2) One of the principal determinants of the Earth's mean temperature is the level of atmospheric CO2, which regulates its radiative balance. One possible contributor to global cooling during the Tertiary is thus a reduction of atmospheric CO2 concentration with time(3). Such a reduction would have resulted from a change in the balance between CO2 emitted by volcanoes and ocean ridges, and CO2 uptake by chemical weathering of the continental crust, which between them control the amount of carbon present in the ocean, on continents, and in the atmosphere on a timescale of millions of years. A variety of oceanic processes determine the partitioning of CO2 between atmosphere and ocean, and produce higher-frequency variability in atmospheric CO2 on timescales of 10^(4) to 10^(5) years. Particularly well documented are cyclical variations, which occurred during the past 500,000 years in concert with variations in Earth's orbital parameters and which resulted in lower atmospheric CO2 during glacial periods(4).

3) One of the leitmotifs that are used to try to explain this lower glacial CO2 is stratification in the upper ocean at high latitudes. Some 20 years ago, this mechanism was identified as a particularly effective means of sequestering carbon in the deep sea at the expense of the atmosphere(5). In the modern ocean, there is extensive vertical mixing at high latitudes, particularly in the cold waters surrounding Antarctica. This mixing returns to the atmosphere some of the CO2 that accumulates in the deep sea from the decay of sinking organic matter produced in the sunlit surface waters. Density stratification of the upper ocean in these regions would prevent such return and keep CO2 sequestered in the deep sea. This finding prompted an extensive search in the sedimentary record for evidence that such stratification happened during the recurring ice ages of the past 500,000 years. Some compelling but not universally accepted evidence for glacial stratification in the Antarctic emerged from this work, but with no clear consensus about the mechanism responsible.

4) Sigman et al(2) have also found evidence for enhanced ocean stratification at high latitudes starting 2.7 million years ago, a period coinciding with the initiation of glaciation in the Northern Hemisphere. That evidence comes from the past record of the activity of diatoms, one of the main members of the phytoplankton, whose cell walls are made from opal (a mineral composed of amorphous silica). Sigman et al(2) report a dramatic decrease in opal accumulation rates in the sediment deposited in the North Pacific and the Antarctic Ocean at that time, which they interpret as a reduction in opal production by diatoms and in the phytoplankton productivity of both regions. They also measured the nitrogen isotopic composition of bulk sediment, and the results suggest that these fewer diatoms used either a higher (in the North Pacific) or a similar (in the Antarctic) fraction of the nitrate nutrients supplied to the sunlit layer by vertical mixing.

References (abridged):

1. Zachos, J. et al. Science 292, 686-693 (2001)

2. Sigman, D. M., Jaccard, S. L. & Haug, G. H. Nature 428, 59-63 (2004)

3. Pearson, P. N. & Palmer, M. R. Nature 406, 695-699 (2000)

4. Petit, J. R. et al. Nature 429, 429-436 (1999)

5. Sarmiento, J. L. & Toggweiler, J. R. Nature 308, 621-624 (1984)

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