Terrestrial Palaeoecology and Global Change

216 Valentin A. Krassilov. Terrestrial Palaeoecology
would have resulted in a less straightforward correlation, with a periodic mixing of the
Boreal and Tethys waters over transcontinental seaways, such as the Turgay Strait.
A climatic effect of isostatic sea-level fluctuations is mediated by deep-water circulation.
With a convergence of isostatically compensated oceanic and continental lithosphere
(shallow oceans), the thermocline is depressed by influx of warm waters from
epeiric seas while the psychrospheric circulation is further restricted by oceanographic
barriers. Conversely, with their divergence (deep oceans), the psychrospheric circulation
is enhanced, bringing cold deep-water masses to sea surface at the equator.
Downwelling–upwelling system. Deep-water circulation is a major heat transfer
agent coupled with atmospheric circulation via its effects on SST, water vapour and
wind velocities. The oceanic downwelling/upwelling system is also a major regulator of
biotic productivity and atmospheric CO 2
.
Hence all factors affecting deep water production, such as the Milankovitch cycles
of polar insolation (VII.2.1), their related ice volume fluctuations, Heinrich events and
dilution of surface waters by meltwaters (presently reducing NADW: Mikolajewicz et
al., 1997) or heat transfer by oceanic currents (in particular, the Gulf Stream, presently
increased by warm water passing through the Isthmus of Panama: Haug & Tiedemann,
1998) are of profound climatic significance.
Deep-water production correlates with equatorial upwellings, in turn enhancing biotic
productivity of surface waters. A concomitant increase in monsoon index stimulates
terrestrial productivity and contributes to fertilization of oceanic surface waters by the
nutrients fluxed in with the terrestrial runoff. So a positive feedback loop is set bringing
about rapid climatic changes. A decrease in deep water circulation would entail a reverse
sequence of climate-generating events, with the tropical monsoons weakened,
terrestrial biotic production falling, and an additional amount of CO 2
released to the
atmosphere.
A coupling of continental ice volume and deep-water circulation is manifestly expressed
over the millennial – about 1500 years – Dansggaard–Oeschger (D-O) cycles
(Hansen & Takahashi, 1984; Dansgaard et al., 1993) and their bundles known as the
Bond cycles about 5–10 k.y. (Bond et al., 1993) terminated by the ice surge (Heinrich)
events (HE). The cycle is prominent over the fluctuations of the East Greenland ice
sheet, the concomitant planktic δ 18 O events, and the epizodic deposition of ice-rafted
debris marking the HE.
Temperature fluctuations over the D-O cycles in the northern North Atlantic greatly
exceed their planetary effects (Bond et al., 2001; Shindell et al., 2001). The cycle starts
with a rapid warming followed by a moderate cooling and then by an abrupt cooling.
NADW production is shut off, or slowed down, by a meltwater injection into the East
Greenland Current to be rapidly reactivated after the HE. The D-O cool stadials correspond
to the North Pacific warm phases while the East Asian monsoons are intensified
during the D-O warm phases. Such antithetic correlation indicates a prevalence of oceanic
teleconnection, via the NADW reduction–upwelling turnoff, over the atmospheric