Geology Reference
In-Depth Information
6.5.4
Climate Changes and Human
Infl uences
that period due to the Pleistocene glacio-marine sand
stock that was emplaced previously at the seaward edge
of the estuary. When considering the cases of the Seine
or Mont St Michel, prior to becoming tide-dominated
after 6,500 years BP, these estuaries were probably
characterized by moderate- to high-energy wave dynam-
ics, meso- to macrotidal range, but overall by a low vol-
ume of available marine sediment. This fundamentally
explains why the transgressive systems tract in both
estuaries is made of fi ne-grained organic-rich facies that
was deposited as aggrading tidal fl ats along an open
coast, rather than as tide-dominated estuarine or either
wave-dominated sediment successions.
Relative sediment supply increased as soon as sea-
level rise slowed down, allowing tidal currents to stock
sediment and construct a tide-dominated estuary. In the
case of the rocky coast tide-dominated estuary of
Vilaine, the very low sediment supply is interpreted as
the main factor responsible for the late position of the
maximum fl ooding surface, compared for instance with
the Seine or Mont St Michel tide-dominated estuaries.
Two additional aspects should be considered about
sediment supply. Firstly, in most tide-dominated estuary
case studies, sediments are dominantly siliciclastic. The
Mont St Michel estuary is an exception with mixed sili-
ciclastic/carbonate sediments. Carbonate production, as
represented by bivalves and red algae, is high in this
siliciclastic-sediment-starved rocky area of the English
Channel (the so-called 'Norman-Breton Gulf'). This
additional carbonate sediment source could explain why
the highstand systems tract in the Mont St Michel tide-
dominated estuary is particularly well developed.
Secondly, major estuaries and deltas can deliver during
river fl oods a signifi cant amount of fi ne-grained sedi-
ment to the proximal shelf.
This is the case for example with the Yangtze delta
that delivers large amounts of fi ne-grained sediment to
the adjacent Qiantang estuary. Along the French
Atlantic coasts (Bay of Biscay), fi ne-grained sediments
are delivered to the shelf by the Gironde and Loire
estuaries (Chaumillon et al.
2008
) . These 'estuary'-
borne sources are known to contribute signifi cantly in
supplying sediment to adjacent smaller estuaries such
as the Charente estuary-Marenns-Oléron Bay located
north the Gironde (Chaumillon and Weber
2006
) or the
Vilaine tide-dominated estuary located north the Loire
estuary. These additional sources partly explain the
mud-dominated character of the infi ll of these systems
(Menier et al.
2010
; Allard et al.
2010
) .
Lastly, climate change also should be considered as an
important controlling factor of estuary infi ll since it
directly governs river-borne sediment supply. More
importantly, with respect to tide-dominated estuary
infi ll is that climate change has an impact on wave
dynamics at the mouth. It is suggested for instance that
periodic enhanced storminess episodes resulted in suc-
cessive wave-dominated coarse-grained facies in the
tide-dominated infi ll of the Seine estuary (Sorrel et al.
2009
) and Vilaine estuary (Sorrel et al.
2010
) . Such an
enhanced storminess episode is believed to be respon-
sible for the destruction around 3,000 years BP of the
coastal barrier that formed on the margin of the Seine
estuary (Tessier et al.
2010a
) . The 3,500-2,500 years
BP period is indeed recognized all along the coasts of
northern Europe as a period of climatic deterioration
(cf. review in Sorrel et al.
2009
) .
After the barrier destruction, the resulting smoothed
shape of the Seine estuarine entrance would have trig-
gered the passage to full tidal conditions throughout
the estuary. It is proposed that a similar event occurred
at about 3,400 years BP in the Bay of Fundy (Shaw
et al.
2010
). In relation probably with enhanced storm
action, the coastal barrier located at the mouth of the
Minas Basin was destroyed, provoking a sudden tidal
expansion in the Bay of Fundy and consequently the
passage, as it is recorded in the sedimentary infi ll, from
lagoonal-mesotidal to macrotidal environment. The
environmental change was so fast that it gave rise to an
old aboriginal legend. Middle to late Holocene millen-
nial climate changes have probably infl uenced signifi -
cantly the Mont St Michel estuary behaviour since
they have also caused periodic destruction of marginal
barriers (Billeaud et al.
2009
) .
Lastly, it is worth noting that many tide-dominated
and mixed-energy estuaries along the French coasts
experienced since about 1,000 years a signifi cant
increase in fi ne-grained sedimentation. This is the case
in particular of the Vilaine estuary (Menier et al.
2010
;
Sorrel et al.
2010
) and the Charente estuary-Marennes-
Oléron Bay system (Billeaud et al.
2005
; Poirier et al.
2009
; Allard et al.
2010
). This mud supply is believed
to originate mainly from the catchments where land
use changes, in relation with deforestation and agricul-
ture development, have dramatically enhanced soil
erosion. Such a phenomenon has probably been
