Geoscience Reference
In-Depth Information
At that time, sea surface temperatures near the North Pole increased from about 18
°
C
to more than 23
C (Sluijs et al.,
2006
). The transition from the Palaeocene and early
Eocene warm 'greenhouse' world to a colder world with sea-ice and icebergs in the
Arctic Ocean was underway from middle Eocene times onwards (Moran et al.,
2006
).
Marine sediment cores from the Lemonosov ridge in the Arctic Ocean show the first
evidence of ice-rafted debris in the middle Eocene 45 Ma ago, with fresh surface
waters present by around 49 Ma (Moran et al.,
2006
). The earliest cooling in the
Arctic is coeval with that in the Antarctic, suggesting that global rather than regional
factors were responsible for the initial cooling. Eldrett et al. (
2007
) found abundant
evidence of deposition of macroscopic ice-rafted debris in the Norwegian-Greenland
Sea sediments between 38 and 30 Ma, and concluded that isolated glaciers were
probably present on Greenland in the late Eocene to early Oligocene. The presence of
the freshwater fern
Azolla
in Eocene Arctic Ocean waters is also indicative of pulses
of fresh surface water discharge from the Arctic Ocean to the south, some as far as
the North Sea (Brinkhuis et al.,
2006
).
Stickley et al. (
2009
) found abundant sea-ice dependent fossil diatoms (
Synedropsis
spp.) in middle Eocene marine sediments from the Arctic Ocean. They concluded that
the establishment of sea-ice in this region took place in two stages: an initial phase
of sporadic sea-ice formation in marginal shelf areas starting around 47.5 Ma ago,
followed 0.5 Ma later by seasonal sea-ice formation offshore in the central Arctic.
They stressed the need to distinguish between sea-ice formation and evidence for ice-
rafted debris from the land, noting that sea-ice affects ocean-atmosphere exchanges,
while land-ice affects sea level and ocean acidity.
Migrations of Arctic marine organisms (dinoflagellates) and changes in the
°
18
O
composition of benthic forams indicate strong mid-Oligocene cooling of surface water
in the Arctic at around 27.1 Ma, synchronous with Antarctic ice sheet growth and
sea level lowering (Van Simaeys et al.,
2005
). The opening of Fram Strait in the
early Miocene allowed the free flow of deep water from the Arctic to the North
Atlantic and the inception of the global thermohaline circulation system (Jakobsson
et al.,
2007
). This was a precursor to the progressive late Pliocene cooling of the
North Atlantic (Loubere,
1988
), which culminated in the high-latitude accumulation
of snow and ice in North America and the onset of major North American glaciation
at around 2.5 Ma. The Pliocene cooling was relatively rapid; Early Pliocene fossil
plants, mammal bones and beetles on Ellesmere Island in northern Canada indicate
that summer temperatures were 10
δ
°
C higher and winter temperatures were 15
°
C
higher than they are today (Tedford and Harington,
2003
).
20.6 Cenozoic desiccation of North America and its impact on the biota
Flannery (
2001
) has provided a useful overview of changes in the Cenozoic flora and
fauna of North America caused by changes in regional climate and topography, as
