Paleoaltimetry. Approaches and Challenges

Transcription

1 Paleoaltimetry Approaches and Challenges

2 Both height and time are important puberty Brian Staveley (mun.ca)

3 Uplift rates constrain mechanisms of mountain building Ehlers and Poulson 2009

4 Uplift chronology constrains the relationship between orogeny, CO2, and global climate Uplift of Bolivian Altiplano? Tibetan 5km Zachos et al. 2001

5 Most of the Earth is within several km of sea level so paleoaltimetry needs to resolve km-scale changes in altitude NOAA/NCAR

6 Current paleoaltimetry approaches exploit decreases in T and P with altitude Leaf shape Paleophysiology Stable isotopes Bubble sizes in basalt flows Leaf stomatal density Cosmogenic nuclides Altitude -5.5 C/km (wet) -10 C/km (dry) Temperature -12%/km Pressure (air density)

7 Bubble sizes in basalts vary with overhead pressure PbottomVbottom = PtopVtop Pressure due to lava Vtop Patm + ρgh = Vbottom Patm Find vesicular basalts from low and high altitudes Pick basalts with simple emplacement histories Count bubbles/sizes using X-Ray tomography (CT scans) Sahagian et al. 2002

8 Bubble sizes in basalts vary with overhead pressure Modal size 2.36 mm mm3 Patm = 0.63 atm 4093 m (measured) vs m (actual) Sahagian et al. 2002

9 Plants adapt to low CO2 by making more stomata Higher CO2 MacElwain 2004 Lower CO2 CO2 Sugano 2010 Find fossil leaves from different altitudes Calculate stomatal density Assume a stomata-altitude relationship based on modern flora

10 Production of cosmogenic nuclides increases with altitude Simple U. Glasgow Pick nuclides with good targets and decay lifetimes Constrain exposure age Constrain depth history of rock Calculate excess nuclide production 40 N scaled Normalized production rate Riihimaki and Libarkin 2007

11 Current paleoaltimetry approaches exploit decreases in T and P with altitude Leaf shape Paleophysiology Stable isotopes Bubble sizes in basalt flows Leaf stomatal density Cosmogenic nuclides Altitude -5.5 C/km (wet) -10 C/km (dry) Temperature -12%/km Pressure (air density)

12 Plant community composition changes with altitude because of the temperature gradient Biomes in the Santa Catalina Sky Islands (Brusca and Moore 2013)

13 Therefore: Leaf shape distribution changes with altitude Sample a large number of fossil leaves Screen all leaves according to shape Assume that ancient plants adapt like modern plants Perform multivariate statistical analysis Wolfe 1995

14 Isotope altimetry: Heavy isotopes rain out first (18O/16O)rain > (18O/16O)cloud δ18orain > δ18ocloud δ18orain,1 > δ18orain,2 > δ18orain,3 δdrain,1 > δdrain,2 > δdrain,3

15 Rain becomes depleted in heavy isotopes (18O, D) as clouds ascend up a mountain slope δ18o < 0 δd < 0 δ18o = 0 δd = 0 δ18o << 0 δd << 0 δ18o <<< 0 δd <<< 0 Sample records of precipitation (e.g., pedogenic carbonates, hydrous silicates, leaf waxes, fossil teeth) Use theory that integrates aridity/t/altitude effects on precipitation Account for mineral growth temperature Modified from Cin-Ty s presentation

16 A 1-D isotope fractionation model captures the basic effects of altitude on isotope composition of precipitation ( ) ( ) 18O 16O cond = 18O 16O init f(α 1) Most sensitive range Rowley and Garzione 2007

17 but hydrology conspires against it! ( ) ( ) 18O 16O cond = 18O 16O init f(α 1) Most VARIABLE range Rowley and Garzione 2007

18 Water source and rainout amount can change with topography Some uncertainties Amount effect Re-evaporation Evapotranspiration Air-mass path Ehlers and Poulson 2009

19 Despite the uncertainties, water-isotope altimetry places quantitative constraints on orogen evolution Rowley and Garzione 2007

20 Current paleoaltimetry approaches exploit decreases in T and P with altitude Leaf shape Paleophysiology Stable isotopes Bubble sizes in basalt flows Leaf stomatal density Cosmogenic nuclides Altitude -5.5 C/km (wet) -10 C/km (dry) Temperature -12%/km Pressure (air density)

21 Isotope altimetry: Heavy isotopes concentrate in minerals at low temperatures 0.8 /km Equilibrium isotope fractionation ( ) Calcite Mineral growth temperatures Altitudes? Smectite Kaolinite must divorce temperature and hydrologic effects on δ18o Goethite Precipitation (simplified) Poage and Chamberlain 2001

22 Clumping of heavy isotopes (e.g., 13C 18O) is sensitive to temperature and not hydrology Proportional enrichment of 13C 18O bonds Precision = 2 5 C (Depends on who you ask) Growth temperatures Growth-water δ18o Altitudes Eiler 2011

23 Temperature and isotope lapse rates are complementary constraints on paleoaltitude km km km Temperature and δ18owater vary together as expected from altitude changes Ghosh et al. 2006

24 Fundamental limitations: Sample availability and analytical uncertainties Leaf shape (>1 km) Paleophysiology (?) Stable isotopes (0.5 1 km) Bubble sizes in basalt flows (0.5 km) Leaf stomatal density (0.5 km) Cosmogenic nuclides (>1 km) Altitude -5.5 C/km (wet) -10 C/km (dry) Temperature -12%/km Pressure (air density)

25 How did the Andes rise? Gradually, or in bursts? Ehlers and Poulson 2009

26 How did Andean uplift affect climate and CO2? Uplift of Bolivian Altiplano? Tibetan 5km Zachos et al. 2001

27 Topographical forcing of atmospheric circulation altered precipitation patterns and climate Eastern Altiplano Convective storms form Poulson et al. 2010

28 Andean uplift may have occurred heterogeneously Southern Altiplano uplifted first North-Central Altiplano: 7 Myr later Rapid rise supports delamination mechanism (loss of lower lithosphere) km/myr Garzione et al. 2014

29 Does the field need new approaches? Approach Advantages Disadvantages Status Leaf shape Large datasets Complicated to apply Paleo-correlations unknown Largely abandoned Leaf stomata Climateindependent Limited data/samples Lacking quantitative theory Not yet applied Basalt bubbles Robust theory Sample-limited Requires simple emplacement Used when available Cosmogenic nuclides Climateindependent Very sensitive to burial depth Need independent exposure age Not yet applied Isotope hydrology (δ18o, δd) Theory-based Sample variety Climate-sensitive Catchment-sensitive Possible diagenetic overprints Works best with climate model Widely used Clumped isotopes (Δ47) Robust theory Can be combined with isotope hydrology Possible diagenetic overprints Widely used Altimetry requires high precision Works best with climate model Additional methods: Atmospheric thermodynamics, thermochronology