Low Latitude - Issues and Key Themes Flashcards
The Quaternary Period
glacial-interglacial framework
high latitude cooling = steeper meridional temperature gradient = H.C. intensification and wind speeds enhanced = lower sea levels (Anderson et al., 2010)
general rules for climate oscillations
higher latitudes = temp change w/ ice growth/retreat
lower latitudes = precipitation change w/humidity or aridity
vostok ice core identified the LGM as
110-11.5ka (Petit et al., 1999)
LGM was around 20ka
triggered by solar forcing
climate models forced latitudinal LGM and there was a global temp drop of 5.8°C while l.l. did not change in temp (Schenider von Deimling et al., 2006)
proxies indicate a 4°C temp change (Annan and Hargreaves, 2013)
sea levels dropped 120m (Waelbroech et al., 2002) = 45% reduction in precipitation rates (van der Hammen and Hooiemestra, 2000)
Issues with the LGM
l.l. -> not covered by ice, 1/3 of global land covered, SH = ice free
maximum extent variable across the globe e.g. West Antarctica (29-33ka) other regions around 26.5ka (Clark et al., 2009)
glacial-aridity hypothesis (Sarnthein, 1978)
increased dust plumes (Harmattan) in sediment core taken of western coastline of N.A = increased aridity during glacial periods due to an enhancement of the p.g. (Sarnthein, 1978)
controversy of the glacial-aridity hypothesis
spatial complexity to patterns
dust in icre cores = glacial foreland erosion not deserts (Machowal et al., 1999).
BUT -> trace dust sources via geochemical means e.g. analysing radiogenic Sr-Nd isotopes = dust in Greenland cores from N. Mongolia and Chinese Loess Plateau (Ujvari et al., 2015)
support for the glacial-aridity hypothesis
glacial periods = aeolian deposits across Australia (Hesse, 1994) and the south atlantic (Stuut, 2004)
glacial periods = more dust in EPICA and Vostok ice cores (Lambert et al., 2018)
BIOME3 dust flux modelling = 20x more dust in the last glacial section of cores (Mahowald et al., 1999)
l.l. change in the LGM
lower obliquity caused the LGM
sea ice extent over Antarctic Sea increased -> enhanced p.g. -> intensified westerlies and led to more moisture advected from Atlantic Ocean into S. Africa = more precipitation (Chase and Meadows, 2007) -> but no precipitation across eastern S. Africa
Amazonian Forest Birds (Haffer, 1969) -> early devising of the Tropical Refugia Theory
Aridity expanded during glacial periods.
Led to wet forest being replaced by savannah e.g. Amazon Basin and Congo Basin -> tropical rainforest pushed to isolated mountain locations = geographic isolation and speciation/genetic drift.
Sand dunes replaced savannah regions.
Further enhanced by tectonic processes e.g. Uplift of Andes end of Tertiary prior to Quaternary
evidence for the Tropical Refugia Theory
analysed Roondonia (S.A.) that there was savanna pollen, but tropical rainforest pollen is located above (van der Hammer, 2000)
criticisms of the Tropical Refugia Theory
little evidence of savannah, and the extremity of the hypothesis was over exaggerated (Haberle and Maslin, 1999)
speciation occurred prior to the Quaternary (Ribas et al., 2012)
C-isotope analysis of soils = no C4 plant (savannah-favouring) evidence where C3 plants are located (Freycon et al., 2010)
landbridges = facilitated the movement of species
Sunda Shelf = indonesia to Malaysia
Isthmus of Panama = N. America to S. America
(Lomolino et al., 2016)
limitations of pollen
not a reliable proxy in drylands due to oxidation (Thomas and Burrough, 2012)
africa proxy records
pollen in terrestrial regions (Scott et al., 2012) and in marine regions e.g. ‘Site 1078C’ off the coast of Angola (Dunpont et al., 2008)
geochemical data from leaf wax in Lake Tanganyika sediment cores E. Africa (Tierney et al., 2010)
LGM = 32% decrease in lake volume, 4-5°C temp change
caused by changes to the Indian Ocean Monsoon + SSTs
geoproxies = geomorphological features
inferences are harder due to diagenesis e.g. sediment reshaped after deposition (Chase and Meadows, 2007)
multivariate calibration-functions = pollen records in S. Africa (Scott et al., 2012).
lower temps during HS1 and HS2 -< increased humidity during the YD across S. Africa but not uniform (Scott et al., 2012)
Start of Holocene = precipitation increased over N.E. Africa due to procession weakening the SSTs in the Indian Ocean -> lagged across central S. Africa as ITCZ was displaced southward
lower latitude insolation changes influence the net radiation budget = temp changes also important at low latitudes e.g. African Humid Period and THC
LGM -> THC (AMOC slowed) = warmer temperatures were trapped at lower latitudes (Broecker, 1998)
African Humid Period (14.8ka-5.5ka) -> intensification of the African monsoon due to 8% increase in orbital forcing
procession = perihelion + boreal summer
more northerly position of rainbelt = 40% increase in precipitation led to vegetation + lakes over Sahara (deMenocal et al., 2000)
end driven by vegetation shifts -> modelled by CLIMBER 2 (deMenocal et al., 2000)
African Humid Periods
20 humid periods across N. Africa over 0.8 million yrs (Armstrong et al., 2023)
Site 658C Cap Blanc (offshore west of N. Africa -> dust supplied by the Sahara (deMenocal et al., 2000)
HadCM3B model = simulates the humid period well
amplitude of AHPs = high variability -> eccentricity on ice sheets and a suppression of the monsoon formation through cooling effects
Lake Tanganyika -> geochemical leaf wax proxy = overflowed during AHP (Tierney et al., 2010).
AHP -> not in S. Africa but lake levels were higher (Burrough and Thomas, 2013)
Arabian Humid Period?
subtropical rain belt extended over Arabia (Woor et al., 2022)
evidence in Nafud Desert, Saudi Arabia (Parton et al., 2018) and grassland in Arabia (Dinies et al., 2015)
