Water Lecture 4- Water in Mediterranean Landscapes Flashcards
Discuss the geological history of the Mediterranean.
Between ~6 Ma and 5.3 Ma, a slab of subducting
lithosphere broke off, creating isostatic uplift around
the Strait of Gibraltar
* The uplift closed off the Mediterranean from the
Atlantic (at the time via narrow seaways in the
Baetic and Rif areas)
* Large natural experiment in evaporation as
discharge from rivers and precipitation could not
keep up with the evaporative demand* (temps were 3-5 degrees higher)
* → Messinian (last part of Miocene epoch) Salinity crisis in the Mediterranean
What is the drawdown process?
Meijer and Krijgsman (2005) estimated how long it would take to balance
evaporation with inflow from rivers and precipitation to reach a balance
with no sea-water input – a mass balance approach
Estimate from modern river inflows that freshwater input is ~6,000 m3 a-1
Evaporation decreases as area decreases
So drawdown of the Mediterranean would only have taken 5 – 8 ka
Salinity also increases dramatically over this time
Once salinity is high enough, evaporites (salts including gypsum and
halite) are precipitated. Total deposit depth = 1500 m in W and 3500 m in
E basins.
The low stage conditions are still debated. Meijer and Krijgsman’s (2005)
analysis suggests 2500 m of drawdown
What is the equation for evaporation?
Precipitation + river discharge = evaporation
What do Gvirstman et al (2022) suggest about the drawdown process?
But geomorphic analysis of the Nile River canyon by Gvirstzman et al (2022) suggests only 600 m of drawdown, and salt deposition under 1+ km of water
How did Garcia-Castallanos et al. (2009) predict water returned to Gibraltar falls?
Garcia-Castellanos et al. (2009) looked at dynamic feedbacks in incision at the Gibraltar Sill, and suggested flooding discharges three orders of magnitude larger than the Amazon, and that the entire basin could refill in about
two years!
How did Meijer and Krijgsman (2005) predict water returned to Gibraltar falls?
Meijer and Krijgsman (2005) used a static model assuming current inflow rates, in which refilling occurred in 2 – 3 ka.
Discuss Zanclean (first part of the Pliocene starting around 5.3 Ma) megaflood deposits.
Micallef et al. (2018) used seismic data to
identify significant sediment deposits, with
a maximum thickness of 760–860 m and
volume of 1430–1620 km3
They interpret the sediments as being due to
erosion of the Sicily sill by flows estimated
to be similar to those at the Straits of
Gibraltar (up to 45 m s−1 and 108 m3 s−1)
→ Mediterranean-wide megaflood at end of
Messinian Salinity Crisis
Why is it important to understand past events for modern landscapes?
- They change the boundary conditions for understanding current landscape evolution.
E.g. incised canyons. Carving of Gibraltar Sill is deep enough to prevent the Mediterranean being cut off during Quaternary glaciations - They produced basins filled with evaporites, marls or alluvial sediments that are weakly consolidated and thus erode very actively as badlands
- They show that high magnitude but low frequency events significantly change the behaviour of the landscape, and that contingent (historical) events are important
Discuss the Mediterranean climate.
General climate is warm dry summers and relatively wet winters
Mild maritime climate in W, to more continental climate in E
Also strong N-S gradients 70-80% of rainfall from depressions in October to March (in winter or in spring and autumn)
Variation in number of raindays, as well as
total precipitation
Is mediterranean high or low pressure in summer?
High pressure
Has intense rainfall increased in the Mediterranean?
Yes with around 50mm or more in less than 1 hour
Discuss dry season in the Mediterranean.
Dry season increases from the NW to the SE
How do plans adapt to water deficits?
Either they avoid stress (by growing in more favourable locations) …
… Or they adapt to it by:
- Having small (and/or thick and/or hairy and/or waxy) leaves
- Shedding leaves in times of stress
- Having open canopies with non-overlapping leaves
- Rolling leaves
- Sun tracking
- Salt secretion
E.g. Holm oak/holly oak vs white oak
What happens to vegetation as water availability decreases?
The ability to support biomass decreases and vegetation becomes patchy.
