ess w8 l2 pt 1 - past landscape responses

terrestrial sphere

Climate cycles – past:

•   Climate controls landscape evolution through:

•    Land surface temperature and rainfall affects processes and rates

•    Sea level affects energy and form of rivers, deltas and shelves

 

•    Changes in sea level over time

•    Shows how theres been a significant change in warmth of earth – wsrmest time = ecocene optimum

• Up to 12* warmer than today

•     End of extended interglacial

• Focusing on the red for this lecture

• Sums up different drivers than run the glacial cycles

• Has been lots of change sparticularly last million years – how landscpa responsds is driven by the red processes

 

  125m rise of sea level

Quaternary climate change:

•   Shows extent of the ice sheet

•   Especially north America/ then northern england

Model for post glacial rebound:

•     The glacial sheets reuslte din crust being pushed downwards due to its weight

•    When weight removes – the material flows backwards

• Process takes tens of thousands of years as moves slowly

• Effects most dramatic in poles but significant fluxing elsewhere – particularl biggest rivers. Changing of a few metres makes a bog difference as theyr eon a low gradient

Global glacial-isostatic adjustment: some regions rise, others fall:

•    Global map of vertical movement from provided by GPS surveys (avoiding areas of active tectonic deformation)

•    Tallest bars approx. 1cm/ year

•   Blue arrows – many places where things are sinking due to: ice melted – where did water go (12m equivalent went into the oceans – adds lots of weight. Pushes down oceanic crust and the masisve weight makes lower lqatitudes an dequitoiral  sink duw to additional ocean mass around). And as asthenosphere migrated to where weight moved – come from ages where there wasn’t a change – results in sinking

•    All this adjusting impacts the fluvial processes

Temperature changes: last glacial maximum vs now:

•   Also difference of December- feb to June-august

•   There were regions of tropics that were warmer compare dto today – come areas got a bit colder

•   Greatest impact in poles           

• Local changes important to pattern in rainfall

• Africa – early Holocene compared to today

• Based on sediment cores around Africa

•    How wet it was based on sediment deposits

•    Shows larger chunks of sahara desert/ saha – were much wetter than now

•   These wetter environment weres recorded in art

Global vegetation change:

   During last glacial max – much less vegetation – impacts the changes

Landscape evolution:

Shorter term deviations from longer term progressive landscape lowering

 

WM Davis ‘cycle of erosion’:

• Shows steady change in the model but more usual to have interruptions in the process

• Terraces – periods hwere the rivers were degrading – with step ups – shows past floodplains

•     If take the davis view of a steady way – if zoom in can see its not a smooth line – there will be hiccups – come down quickly/ slow down etc.

• See there’s depositions events – steady states – erosional event (whole system erodes down a but more ) – there are cycles in the process

River terraces:

•    River terraces are formed by alternating periods of erosion and deposition (cut and fill)

• Initial valley then increase in sediment beyond what it can carry - so deposition

•      2^nd phase – cut donw futher and more degraded

•   Multiple incision events

River long profile evolution:

•     Occurs due to lane equation

• Concave shape as sediment discharge declines more rapidly than water discharge

•    Ultimately the processes lead to periods of aggradation and degradation

• Two conditions of the ystsme lead to river channel either coming up (seidmnet increase. Degradation inverse – brings everyting donw but aggradation makes it steeper

• The process construct a lot of landscape features

Glacial cycles and long profile change:

• During glacial periods upland glaciers scour the landscape and supply abundant coarse material to pr-glacial braided rivers

• This sediment enters storage within braidplain deposits – and this is then remobilised during interglacials

•   During interglacials – warmer – starts to have incisions and terraces

•   New Zealand – lower sea levels but very long continental shelf – as have lots of space

•   Remobilisation involves the cutting of a trench – sediment cut from the trench is deposited downstream as a fan

Fluvial knickpoints (waterfalls or rapids):

•   Very sharp section of the profile

• Dramatic examples = waterfall

• Knickpoints repsrsent how channel longprofiles change

• Doesn’t change immediately

• Migration of steep portion migrating upstream

•   Actions occurs on the steep part. – erosion occuirng.

• How the processes adjust themselves over time

• Started 20,000 years ago

•    Erosional profile making its way up to main channel and the tributaries to the fltter area

•    This erosion is driven by increased local erosion potential

• Positive feedback mechanism

Environmental archives:

   Do landscapes and sediment deposits provide a reliable archive of environmental change?

Floodplains as of archives of environmental change:

• Sediment deposits provide a record of past environmental conditions

•     To make use of such deposits we must understand the relationships between environmental change and landscape response

•   Particularly with types of dating e.g. radiocarbon

•   Work out how old portions of the floodplain are

• Understand how changing landscape reflects conditions fo how it formed

How should we interpret landforms in order to reconstruct past environmental change:

Is this evidence for changing water and sediment supply?

Laboratory study of fan evolution:

•      Supply a consistent known amount of sediment

• Dyed water red – more red= deeper water

• Work out depoth of flow across the fan

• As fan surface progresses over time – infilled the space

• Sediment deposited in the little channels concentrating the flow into one deep channel

Process-form feedbacks on evolving fans:

•    Move down fan from top to bottom – sometime there’s water everywhere (maximum width) but there’s a slight decrease in width of surface that wet as move down the fan

• Flow is increasingly concentrated in to 20% or less of area

• Flow width depends on whether or not flow deposits. Sediment and expands the fan

•   Implications for landscape interpretation?

  Incisions process driven by fan taking up all availbel deposition spaces. If have fluctuations will get terrace features and an incised fan too

Causes of fan entrenchment:

•    Paraglacial decline in sediment supply

•     Product of difference between slope and river sediment transport regimes

• Process-form feedbacks

• Autogenic behaviour from internal form

Equifinality:

              When different processes or environmental conditions can produce the same landform

Summary:

•   Climate cycles affects ice volume, sea level, and isostatic response, along with local precipitation and vegetation

• Glacial cycles and associated eustatic and isostatic responses have impacts on sea level, coastline position and coastal landforms

•    Past changes in climate and vegetation drove changes in hydrological processes, erosion and sediment delivery to rivers

• River terraces represent former long-profile (valley floor) positions and provide evidence of phases of aggradation and incision

•    Landscape rejuvenation (incision) can also be driven by tectonic uplist and relative base level change (including sea level), creating knickpoints

• Landforms and deposits that result can emerge either from external forces of environmental change or internal (autogenic) system responses to geometric constraints or process-form feedbacks. Models are a way to explore these different scenarios to best interpret environmental archives