Climatic and tectonic controls on Triassic dryland terminal fluvial system architecture, central North Sea - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 49

or warming (Hornung et al ., 2007; Kozur &
Bachmann, 2010) is debated. The Tethyan and
Muschelkalk oxygen isotopic record (Fig. 18;
Korte et al ., 2005) shows a clear correspondence
between lighter values, the general timing of plu-
vial phases and the expansion of the Skagerrak
Formation fluvial systems. These lighter values
potentially reflect warmer sea surface tempera-
tures, but will also have been influenced by a vari-
ety of additional mechanisms including enhanced
freshwater input and pH (Korte et al ., 2005).
However, there remains the general impression of
warmer, albeit highly variable (Nützel et al ., 2010),
temperatures during the pluvial dominated
Anisian to Carnian episodes, but with substantial
cooling to long term stability through the Norian
(Fig. 18). This stability is reflected in the
comparatively monotonous terrestrial and marine
successions of the Mercia Mudstone Group,
Keuper and Hauptdolomit formations and cooling
in the latitudinal restriction of Late Triassic reefs
(Kiessling, 2010). However, whilst volcanism
may explain some Ladinian and Carnian pluvial
episodes when Tethyan magmatism was active,
it remains to be proven that there is in all cases a
direct correspondence between volcanic episodes
and pluvialisation. In particular, the more global
Spathian drying and Carnian wettening episodes
are likely to have been driven by more widespread
changes than localised volcanic episodes on the
margins of Tethys (e.g. eruption of the large igne-
ous province of Wrangellia; Furin et al ., 2006).
Also, the influence of the Muschelkalk Sea on
the local climate is also evidenced by the onset
of wetland environments in the upper Julius
Mudstone-lower Joanne Sandstone member inter-
val during a time of general aridity (Fig. 18). In
addition to volcanism, tectonic events in the
Tethys Sea may have altered marine circulation
patterns, with the Carnian closure of Palaeotethys
possibly allowing greater influx of cooler oceanic
waters into a more open Neotethys, thereby alter-
ing temperature gradients across the region and
contributing to the apparent stability and arid
nature of the Norian interval.
Studies of Holocene climate change offer some
clues as to the likely contributing variables which
may have influenced Triassic climate, although
this post-Pleistocene period was still marked by
icehouse conditions and a markedly different con-
tinental configuration to the Triassic Pangaean
greenhouse situation described here. The Holocene
record is one of relatively complex climate change

as a result of strongly non-linear feedback processes
(cf. Su & Neelin, 2005) between solar insolation
(as the Earth reached perihelion during the north-
ern hemisphere summer), changing oceanic and
atmospheric circulation (albeit partly driven by
fluctuating polar ice volumes) and surface albedo
(resulting from the changing distribution of peren-
nial water bodies and vegetation cover). Arid and
pluvial episodes in the equatorial regions appear
to have had a strong component of orbital forcing,
which resulted in fluctuating solar insolation and
latitudinal migration of the extent of the summer
monsoon. Arid episodes correspond to cooler
periods when monsoon systems weakened (Prell
& Kutzbach, 1987), evaporation was lower from
cooler oceans and thermal convection over tropi-
cal landmasses reduced (Mayewski et al ., 2004).
In addition, increased sea surface temperature
gradients resulted in strengthened trade winds,
which inhibited the inflow of moist ocean air into
some continental regions (Schefuβ et al ., 2005;
Brook et al ., 2007).
In the Triassic a reduced temperature gradient
during warming episodes may also have resulted
in a weakening of the north-easterly trade winds,
allowing inflow of moist Tethyan air masses into
the central North Sea to maintain water availabil-
ity in the dry season (Fig. 20), which combined
with enhanced flood magnitude in the wet season
as the monsoon intensified to maintain year-round
moisture. However, the Tethyan monsoon might
also be expected to have generated substantial flu-
vial transport off the Variscan massifs into the
Southern Permian Basin, which does not always
appear to have been the case. Reinhardt & Ricken
(2000) and Vollmer et al . (2008) report a detecta-
ble monsoon influence in the Keuper Formation,
with evidence of intensification in the Milankovitch
frequency band, indicating that a monsoon was
maintained and influenced deposition within
Carnian-Norian playa in the Southern Permian
Basin. The absence of significant sand transport
off these massifs when a monsoon was present
could be attributed to a loss of relief across this
formerly mountainous region (cf. Lelue & Hartley,
2010), coupled with a lack of significant thermal
convection so close to the Tethyan margin, or
could be an indication that monsoon precipi-
tation from Tethys was not sufficient to generate
large sand-transporting fluvial systems (which
would call into question whether the Skagerrak
Formation was also the product of enhanced
monsoon precipitation). The regional distribution