Evoluzione Sistema Terra 2004/2005 Introduzione al problema della variazione di livello marino medio isostasia e movimen

1. Evoluzione Sistema Terra2004/2005Introduzione al problema della variazione di livello marino medio isostasia e movimenti crostali verticali Carla Braitenberg Dipartimento Scienze della Terra, Università di Trieste, Via Weiss 1, 34100 Trieste Berg@units.it Tel +39-040-5582258 fax +39-040-575519

2. Program-Exercizes on PC: • Scope of exercize: familiarize with flexure response of crust. • Load: bathymetry • The flexure model is tested trough the observed gravity field. Procedure: Take Bouguer anomaly over sea. This field is representative of crustal thickness variations. Invert field by downward continuation-you obtain first approximation of Moho.

3. Introduction • An important question is: how big is the influence of mankind and industrialisation on climate evolution • Necessary: separation of the man-induced effect from those effects independent of man, which may be termed “natural” • Mean sea level (MSL): tightly tied to the conditions of the global earth climate. Strongly dependent on: • mass exchange between ice-sheets and ocean water • Thermal expansion

4. Introduction 2 • Impact of sea level rise: • Increased erosion of beaches • Model of Bruun (1962): beaches erode on the order of 50-200 times the increase of sea level. • Example Ocean City, Maryland: erosion 150 times. From tide gauges: increase of sea level is 3.5 mm/yr • Erosion/decade=150 * 3.5mm/yr*10yr=5m/decade • Major damage for storms and inundations • Flooding of low lying flatlands

5. Introduction 3 • Measurement of today’s MSL changes through: • Tide-gauges. Time interval: 102 yr • Local measurement • Satellite altimetry. Time interval: 10 yr • Global measurement • Geomorphology/geological inferences: 105 yr • Local measurement

6. MSL changes in recent 300 yrs. Tide gauge measurements. Observations have been corrected for postglatial isostatic movements. (Lambeck und Chappell, 2001)

7. Short period MSL variations: • The measurement is influenced by: • tides • Sea currents • Temperature • Local subsidence/emergence: tectonic, isostatic, anthropogen • Climatic influences

8. Introduction 4 • Present global increase observed with tide-gauges over last 100 yrs is estimated to 2 mm/yr (Douglas et al., 2001) • Question: is this increase significative? Is it a fluctuation? Does it comply with an extrapolation of the variations over previous centuries/millennia? • Necessary: knowledge of MSL variations in the past: • How big were the variations? • Are variations local and/or global? • Knowledge on geographic distribution of variations

9. Intro 5 • Find driving mechanism for the variations • What other parameters correlate with the MSL variations • Explain past MSL variations in order to predict today’s variations • Be able to detect whether today’s MSL increase is accelerated with respect to model.

10. MSL variations in geologic history • Time scale of milions of years • Results from “seismic sequence stratigraphy”. Combination of local and global variations. Greatest oscillation: connected with plate tectonics (Hallam, Annu. Rev. Earth Planet. Sci., 12, 205, 1984,(Lambeck und Chappell, 2001)

11. Time scale of 140 000 years: Huon Peninsula Papua New Guinea. Dating of cores from coral reefs. MSL in m. OIS: Oxygen Isotope Stage

12. Time scale of 140 000 years: Huon Peninsula • Measurement: dating of cores from coral riffs • Area subject to tectonic uplift. Therefore MSL of LGM (Last Glacial maximum) is in 30-40 m depth. • Compare to mediterranean: in stable areas this level is at 120 m depth • Growing of corals: tied to typical water depth (several cm to several m, depending on species). Therefore index of MSL, Dating with 14C method.

13. Time scale of 140 000 years: Huon Peninsula • Properties of MSL: • Variations due to mass exchange between ice cover and sea water • Glatial: MSL Min Interglatial: MSL max • Curves affected by local effects

14. Geographic differences in the MSL variation(Lambeck, Chappell 2001)

15. Description of geographic differences • Ångermann- river sediments now in 200 m r.p.s.l. • Transgression of sea: change from fresh water to marine sediments • Regression: inverse • Time scale from dating of sediments or counting seasonal Varves • S-England: transition from fresh-water to estuarine deposition • In situ tree-stumps: give upper margin to MSL

16. Description of geographic differences • Sunda Shelf: flooding of shelf • Barbados: Fossil corals: age-height relation • Dating: Carbon or Uranium series methods • North Queensland: Micro-atoll-formation of corals. Today same corals live in 10 cm depth relative to minimum sea level.

17. Geographic differences- Description • Classification of observed areas: • Central area of former ice-sheet: Ångermann, Hudson Bay • Marginal areas of ice-sheet or area of small ice-sheets: Åndoya • Medium latitudes, broad area that confined to ice-sheet: South England. The same: Mediterranean, Atlantic coast of SA, Gulf of Mexico. • Areas far from ice-sheet-margin: Barbados, Sunda Shelf • Most observations regard time after LGM: older traces were cancelled by: • A) rising MSL after LGM • B) advancing ice-sheet before LGM

18. Conservation the signatures • In areas with uplift: older signatures conserved if now above MSL • Example: Huon Peninsula- Papua New Guinea: coral rift up to 1000 m a.s.l. Huon Sequence is an important record for MSL • Glatial ice model: • Known: geographical extent of ice-sheet in N-Europe and N-America • Not clear: extent in E-Siberia and in Shelf-Areas • Not clear: thickness and evolution of ice-sheet before LGM

19. How to recover evolution of ice-sheet • Method to determine volume of ice-sheet: • From the observation of MSL and used as constraints • Consider: vertical crust movements due to • Isostasy • Tectonics

20. Isostatic Model: local equilibrium (Airy and Pratt) and regional equilibrium (Flexural Isostasy) • Airy: variation of crustal thickness function of topography • Pratt: variation of crustal density function of topography

21. Airy and Pratt Isostatic models

22. Examples • In Airy approximation: consider subsidence (r) of crust below iceload of thickness (h): Maximum icethickness at LGM in Scandinavia and N-America estimated to max 2000-2500 m (Lambeck and Chappell, 2001). result: r about 600-760 m

23. Deformation due to the ice-load

24. example • Airy isostasy: calculate uprising of crust (r) in the case of a MSL lowstand : With a measured sealevel change of 120 m, the value of r is about 50 m. The value corrected for the hydro-isostatic effect would then be: It should be borne in mind, that the value calculated with the Airy-approximation is generally over-estimated.

25. Differential sea level change at stable coast and in ocean basin

26. Regional equilibrium (Flexural Isostasy)Model of the flexure of a thin plate

27. Regional equilibrium (Flexural Isostasy) • Flexural rigidity: Typical values: E= 1011 N/m2  = 0.25

28. Regional equilibrium (Flexural Isostasy) • Insert the expression for p and q: • Solution of the equation: k= wavenumber We set: We obtain:

29. Regional equilibrium (Flexural Isostasy) An arbitrary topography can be expressed as the sum of sine-functions (Fourier-Transformation) The flexure of the plate is then:

30. Regional equilibrium (Flexural Isostasy) • k= wave number • W(k)= FT(w(x)) H(k)= FT (h(x)) • To the same result one can arrive by applying the Fourier Transform to the above equation:

31. Transition to local compensation: With very low flexural rigidity or for very small wave numbers (long wavelengths) the regional isostasy goes over into local Airy type compensation: With very high flexural rigidity or for very great wave numbers (short wavelengths) the loading does not deform the plate.

32. Flexure of the plate by point-like loading Te=1,3,5,10,15,25,40 km

33. Properties of the flexure of the plate: • Below the load: maximum downward flexure • In the limiting areas of the load: flexural bulge • With decreasing elastic thickness of the plate: • greater amplitude of flexure • smaller wavelength of flexure • At great distances from load: no effect

34. Mean sea level MIS 5.5 along italian coast • Lambeck et al., 2004

35. Loading with volcanic masses Parameter: Elastic thickness Laureanda: Patrizia Maraini

36. Deformation along the coast • Curves for different Te (km)

37. Glacial-hydro-isostatic model of Lambeck

38. Glatial-hydro-isostatic model of Lambeck

39. Model-parameters • Mantle: divided in 3 spherical shells • Elastic lithosphere with effective elastic thickness H1 • Upper mantle: from base of lithosphere to 670 km depth. Medium viscosity um • Lower mantle: Medium viscosity lm

40. Ice-equivalent MSL From the observations of MSL recover the ice-equivalent MSL variations. Necessary: isostatic correction

41. Some examples of geological markers • Categories: • Biological, Sedimentological, Erosional, archaeological • Morpho-stratigraphic markers: • Notches • Lagoonal sedimentary facies • Fossil beaches and terraces

42. Some examples of geological markers(courtesy Dr. F. Antonioli) • Biological markers: the living habits must be tied to a certain range from the sea level surface • Vermetids: reef building species of Gastropods. Reefs are submerged during high tide, exposed during low tide • Lithophaga: bivalves living in calcareous rock. 90% live in the upper 2m of sea.

43. Some examples of geological markers(courtesy Dr. F. Antonioli) • Speleothems with marine overgrowth:on speleothems in flooded caves, alternation of speleothem growth and encrustments of colonies of marine worm Serpula massiliensis can be found. The worm forms calcitic tubes. • Mediterranean: Strombus bubonus. Gastropod now living in tropical seas. Lived in Medit. only during the Last Interglacial (124 000 ± 2000 ys BP). A marker for this event in the Medit.

44. Some examples of geological markers(courtesy Dr. F. Antonioli) Core samples: analyzed for biological markers. For example fluvial and marine environments are detected Beach rock: shoreline deposits cemented by calcitic-magnesitic or aragonitic-carbonates in or near the intertidal zone, at the interface of freshwater-marine phreatic flow Coral reefs: height-age dependence of core sample. Coral algea: algae that form features similar to coral. Living habitat in tidal range.

45. Example for Vermetid reef (Dr. F. Antonioli, ENEA)

46. Example for vermetid dendropoma(Dr. F. Antonioli, ENEA)

47. Example for present marine notch.Dark part: coralline algal rim. Max 1.90 m from base of algal rim top of notch, Maxwell coast, Barbados (Dr. F. Antonioli, ENEA)

48. Grotta Argentaorla -22 m (Antonioli)

49. S. Vito Lo Capo, Vermetide Reef(Antonioli)

50. Sea level curve with italian archeological markers (Antonioli e Leoni, 1998, Il Quaternario).