Using Fossil Coral Reefs from the Seychelles to Understand Past Sea-Level Rise

Transcription

1 Using Fossil Coral Reefs from the Seychelles to Understand Past Sea-Level Rise Karen Vyverberg, University of Florida Belinda Dechnik Andrea Dutton Jody Webster Dan Zwartz

2 Future sea-level rise projections IPCC, 2013

3 Motivating Questions: How do the large polar ice sheets respond to sustained global warming? Greenland What is the pattern of global sea-level rise during a warming period? NASA Antarctica What can we predict about future sea-level rise? NASA

4 What is the pattern of global sea-level rise during a warming period? Shallow-water reefs are sensitive recorders of sea-level change Examine fossil reefs for evidence of abrupt sealevel change coralworlddvi.com

5 Elevation Reconstructing Sea Level Time

6 Elevation Elevation Reconstructing Sea Level Time Time

7 Searching for evidence of past sea-level change Sedimentary evidence in fossil coral reefs Shifts in coral communities Unconformities Freshwater exposure The Last Interglacial (LIG) period: 129, ,000 years ago Relatively abundant sea-level records Warmer temperatures especially at the poles Global sea level 6-9 m higher than today Sea-level Oscillations Abrupt changes in global mean sea level (GMSL)

8 LIG Sea Level Reconstructions Bahamas Corals W. Australia Corals Barbados Corals Global Corals Beach deposits Mediterranean d 18 O Pl. Forams Good agreement that: Significant Antarctic contribution Up to 3.5 m from Greenland ~ 1 m from mountain glaciers and thermal expansion One global ice volume history

9 The Multi-Proxy Approach Using fossil corals from ~125,000 years ago to reconstruct a global sea-level curve Field Work Outcrop Description Stratigraphy Coralgal Assemblages/Facies Elevation Surveying Sampling Drill cores or hammer INFORMS Analytical Work X-Ray Diffraction (XRD) Degree of aragonite preservation Thin Sections Cement type and generation Stable Isotopes U/Th Dating

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12 Narrowing the paleo-water depth estimate

13 Narrowing the paleo-water depth estimate Coral Assemblage Key Components Palaeoenvironmental Interpretation

14 Narrowing the paleo-water depth estimate Coral Assemblage Key Components Palaeoenvironmental Interpretation Assemblage A (Goniastrea) Massive/sub-massive and encrusting Goniastrea sp. (G. minuta; G. aspera; G. retiformis). G. retiformis is most common. Shallow water reef environment; subtidal sandy reef flats at 0-2 m depth (Taylor, 1968), and especially in the intertidal zone (Veron, 1986). Assemblage B (Faviid/Stylophora) Assemblage C (Siderastrea/Pavona) Assemblage D (Porites/Faviid) Massive/sub-massive Favites sp. and Favia sp. Stylophora sp. and massive/sub-massive Cyphastrea sp. are common. Rare massive Platygyra sp. and Porites sp. Encrusting plates of Siderastrea sp. and Pavona sp. with less common platy Favites sp. and Millepora sp. Encrusting Porites sp. and Faviids with extensive coralline algae. Broad paleo-depth; Stylophora sp. may limit to environment to protected lagoons and lower reef slopes (Veron, 1986), but Lewis (1969) observed Stylophora sp. in 1-2 m water depth on the reef crest. Favia sp. are also common in the reef crest/reef front in as low as 1-2 m depth (Taylor, 1968). Shallow reef environment or sandy lagoon; subtidal reef flat at 1-2 m low tide water depth (Taylor, 1968 and Veron, 1986). Reef edge/reef front community in <6m water depth (Taylor, 1968).

15 Petrography Results Fine grained sediment = peloidal micrite Lagoonal in the modern Seychelles V P Coralline algae associated with vermetid gastropods Association of these two biota in the modern Seychelles observed by several authors in the uppermost subtidal zone CCA P CCA V CCA V P P V

16 Petrography Results Cements: Stratigraphy and Type a. Calcite cements adjacent to geopetal structures b. Calcite vadose cements c. Adjacent calcite and aragonite a. b. c.

17 Core Logs Core D Core A Core B Core E Core C

18 Sequence of Fabrics 6. Two generations of cements 5. Sediment 4. Cements 3. Surface (black or red) 2. Encrusters (CCA/forams) CCA Homotrema - Carpenteria 1. Framework

19 Similar Sequence of Fabrics in the Bahamas Jones and Hunter, PALAIOS, Fine sediment and/or microbialite 2. Encrusters (CCA/Homotrema/Carpenteria) 1. Framework Regressive Sequence Fig. 7 Jones and Hunter, PALAIOS, 1991

20 Sequence of Fabrics 6. Two generations of cements 5. Sediment 4. Cements 3. Surface (black or red) 2. Encrusters (CCA/forams) CCA Homotrema - Carpenteria 1. Framework

21 14 d 18 O vs d 13 C of La Digue Hand-Samples and Cores d 13 C ( VPDB) Micrite CEMENTS Large Pendant Cement 1st Gen Cement 2nd Gen Cement CORAL- DWELLING BARNACLES Fossil Trevathana sp. Modern Trevathana sp. Fossil host coral of Trevathana sp. REEF BUILDERS Corals Coralline Algae d 18 O ( VPDB)

22 d 13 C ( VPDB) d 18 O vs d 13 C of La Digue Hand-Samples and Cores Location of sampled cements at 7.52 m above MLWS in Core C from Site 7. Large Pendant Cement Second Generation First Generation Micrite CEMENTS Large Pendant Cement 1st Gen Cement 2nd Gen Cement CORAL- DWELLING BARNACLES Fossil Trevathana sp. Modern Trevathana sp. Fossil host coral of Trevathana sp REEF BUILDERS Corals Coralline Algae d 18 O ( VPDB)

23 Barnacle/Micrite Layer

24 1 mm Pyrgomatids = coral-dwelling barnacles Trevathana sp. Malay and Michonneau, 2014

25 Barnacle/Micrite Layer What we know: Pyrgomatids = coral-dwelling barnacles 1. The host coral was stressed 2. Host coral almost completely dissolved 3. Appear recrystallized 4. Opercular plates are intact 5. Between or capping reef framework units

26 Reef Units in Outcrop Reef Framework Transition Layer Assemblage A Assemblage B Intertidal/ 0-2 m 1-2 m water depth Reef Framework Transition Layer Reef Framework

27 Outcrop Architecture Reef Framework Transition Layer Reef Framework Transition Layer Reef Framework

28 Elevation Timing of transitions Reef Framework Transition Layer Reef Framework Transition Layer Curieuse Reef Framework Assemblage B Rubble Assemblage B (Dutton et al., QSR 2015) m ±0.4 ka Time La Digue Shallow Corals Rubble Shallow Corals N m ±0.5 ka Upper timing bound on rubble Concurrent with a global drop in sea level

29 Summary Three episodes of reef growth punctuated by two interruptions At least one sea-level oscillation; concurrent with a global drop Sea level was unstable during the last interglacial Ice sheets may behave dynamically in future warm temperatures Acknowledgements P. Samson at PetroSeychelles Seychelles National Parks Authority Ministry of Environment and Energy Seychelles Bureau of Standards

30 Lignumvitae Key Botanical State Park Windley Key Fossil Reef Geological State Park The Florida Keys

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34 Summary Three episodes of reef growth punctuated by two interruptions At least one sea-level oscillation; concurrent with a global drop Sea level was unstable during the last interglacial Ice sheets may behave dynamically in future warm temperatures Acknowledgements P. Samson at PetroSeychelles Seychelles National Parks Authority Ministry of Environment and Energy Seychelles Bureau of Standards