The most favoured means whereby the weak fluctuation in solar radiation due to the Milankovich-Croll Effect become amplified to affect climate’s ups and downs is the switching on and off of thermohaline circulation in the North Atlantic Ocean. The key to such ocean circulation is formation today of dense, cold brine through sea-ice formation around Iceland. To set circulation in motion, however, depends on these brines being able to move southwards, which they do now in a sea-floor channel between Shetland and the Faeroe Islands. When the North Atlantic began to open, this route was blocked by a ridge between Greenland and Shetland, buoyed up by residual warmth in the lithosphere from volcanic activity at the Iceland plume.
It is important to assess when the Shetland-Faeroe “gateway” formed, so that the effects of thermohaline circulation on pre-glacial climate can be assessed. Petroleum exploration using high-resolution seismic reflection profiles and drilling has resolved this particular issue. Geologists and geophysicists from Exxon and Cardiff University have found signs that sediment drift dragged by such a deep flow began in the early Oligocene (about 35 Ma ago) (Davies, R. et al. 2001. Early Oligocene initiation of North Atlantic Deep Water formation. Nature, v. 410, p. 917-920). The evidence takes the form of multiple, moat-like erosion surfaces down to the base of sediment fill between the Faeroes and Shetland, shown superbly by the seismic data. Drilling shows that these signs of deep-water flow stop abruptly in Early Oligocene sediments.
Astrology and ice
The early Oligocene marked the onset of serious ice cover on Antarctica, and it shows as a dramatic increase in d18O values in the ocean-floor record of benthic forms – lighter 16O had been trapped in land ice. That may or may not be a coincidence with the finding about the start of North Atlantic thermohaline flow in the previous item. A lesser, but still dramatic increase marks the Oligocene-Miocene boundary, suggesting further growth of the Antarctic ice sheet, which is not so readily matched empirically. Detailed study of the isotopic “blip” at this time by a team from the Universities of California, Cambridge and South Florida (Zachos, J.C. et al. 2001. Climate response to orbital forcing across the Oligocene-Miocene boundary. Science, V. 292, p. 274-278) suggests that it related to a remarkable coincidence in the astronomical record of solar heating.
Round 23 Ma ago, the orbital eccentricity dropped almost to zero – Earth’s orbit would have been circular – at the same time as its axial tilt became very stable, the one reinforcing the climatic effect of the other. The isotopic “blip” coincides exactly with the coincidence. The detailed record also shows very clearly that minor fluctuations in climate at that time were in step with the 400 and 100 ka periods in the eccentricity variations, and with those of 41 ka that relate to changes in axial tilt. If nothing else, these results confirm that it is unnecessary to turn to extraterrestrial influences over climate other than those which are predictable from Milankovich’s theory (see Impacts and human evolution, above).
Additional source: Kerr, R.A. 2001. An orbital confluence leaves its mark. Science, v. 292, p. 191.
Start of Pleistocene environmental change in tropical Africa
Pollen records from an ODP core drilled off the Congo estuary provide a record of the fluctuation in the monsoon of western tropical Africa (Dupont, L.M. et al. 2001. Mid-Pleistocene environmental change in tropical Africa began as early as 1.05 Ma. Geology, v. 29, p. 1195-198). Before 1.05 Ma there is little sign of a glacial-interglacial pulse in the fluctuation of vegetation in the Congo Basin. Thereafter, ups and downs in pollen from various vegetation groups correlate well with the benthic foram oxygen-isotope time series. However there are a few surprises.
Conventional wisdom is that Africa experienced drying during glacial epochs, rain forest expanding during interglacials. In the Congo basin, grasses and savannah trees increased during interglacials while mountain trees fell in their influence, up to 600 ka. This suggests the opposite trend of warm, dry interglacials and cool, humid conditions during glacial periods, similar to the record for tropical South America. In the later Pleistocene, the fluctuation switched to that indicated by fluctuating lake levels throughout the continent. The pollen variations are backed up by variations in dinoflagellate cysts, which show that discharge from the Congo dropped during interglacials. The other surprise is that the onset of astronomically paced environmental change in west Africa predated the change to a 100 ka domination of global climate, and the increase in amplitude of changes in land-ice volume at 900 ka by a hundred thousand years. Dupont et al. suggest that the changes in albedo in tropical West Africa in response to vegetation changes could have had an influence on global climate when the fluctuations began.
As well as being interesting in terms of climate change, the new data throw doubt on the hypothesized link between climate in Africa and pulses of migration of early human species, such as H. ergaster and H. erectus. There were fluctuations in humidity in the earlier Pleistocene, but they show no link to global climate change. So, it seems unwise simply to look to the Milankovich forcing as a pacemaker in early human affairs.
A Late-Jurassic methane “gun”
Massive releases of methane from gas hydrate layers beneath the ocean floor, and its subsequent oxidation to carbon dioxide have been implicated in major climatic and oceanographic changes in the mid-Jurassic, Cretaceous and Palaeocene. They can be detected by drops in the 13C content of marine carbonates, caused by the “light” carbon trapped in biogenic methane. All those known also correlate with evidence for climatic warming.
The Swiss Jura mountains are a repository of great thicknesses of Jurassic carbonates, whose ammonite faunas allow fine stratigraphic division. Between 157 and 156 Ma (late Middle Oxfordian) there is a major negative excursion in d13C whose duration was as short as 180 ka (Padden, M. et al. 2001. Evidence for Late Jurassic release of methane from gas hydrate. Geology, v. 29, p. 223-226). The Swiss-French geochemists who discovered the anomaly believe that the release may have linked to opening of the ocean gateway that connected currents between Tethys and the easter Pacific oceans through what is now the Atlantic.