Guest Blog: How Speleothems Are Used To Determine Past Climates?

About author: Alex Graham is an undergraduate student at University of Newcastle, Australia. He is interested in Geology as a whole but his major interests include fluvial processes, karst systems and ocean science. During his visit to New Zealand, he has obeserved the glow worms in Waitomo Caves and spelunking in Nikau Caves.

Speleothems, more commonly known as stalactites or stalagmites, consist of calcium carbonate (calcite or aragonite) crystals of various dimensions, ranging from just a few micrometers to several centimetres in length, which generally have their growth axis perpendicular to the growth surface. Speleothems are formed through the deposition of calcium carbonate minerals in karst systems, providing archives of information on past climates, vegetation types and hydrology, particularly groundwater and precipitation. However, they can also provide information on anthropogenic impacts, landscape evolution, volcanism and tectonic evolution in mineral deposits formed in cave systems.

Stalagmite Formation

Rainfall containing carbonic acid weathers the rock unit (generally either limestone or dolomite) and seeps into the cracks, forming caverns and karst systems. The groundwater, percolating through such cracks and caverns, also contains dissolved calcium bicarbonate. The dripping action of these groundwater droplets is the driving force behind the deposition of speleothems in caves.

Core drilling of an active stalagmite in Hang Chuot cave.

Speleothems are mainly studied as paleoclimate indicators, providing clues to past precipitation, temperature and vegetation changes over the past »500,000 years. Radioisotopic dating of speleothems is the primary method used by researchers to find annual variations in temperature. Carbon isotopes (d^13C) reflect C3/C4 plant compositions and plant productivity, where increased plant productivity may indicate greater amounts of rainfall and carbon dioxide absorption. Thus, a larger carbon absorption can be reflective of a greater atmospheric concentration of greenhouse gases. On the other hand, oxygen isotopes (d^8O) provide researchers with past rainfall temperatures and quantified levels of precipitation, both of which are used to determine the nature of past climates.

Stalactite and stalagmite growth rates also indicate the climatic variations in rainfall over time, with this variation directly influencing the growth of ring formations on speleothems. Closed ring formations are indicative of little rainfall or even drought, where-as wider spaced ring formations indicate periods of heavy rainfall or flooding. These ring formations thus enable researchers to potentially predict and model the occurrence of future climatic patterns, based off the atmospheric signals extrapolated from speleothems. Researchers also use Uranium –Thorium radioisotopic dating, to determine the age of speleothems in karst formations. Once the layers have been accurately dated, researchers record the level of variance in groundwater levels over the lifetime of the karst formation. Hydrogeologists specialise in such areas of quantitative research. As a result, speleothems are widely regarded as a crucial geological feature that provide useful information for researchers studying past climates, vegetation types and hydrology.


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10 of the Best Learning Geology Videos of 2017

Following are the best videos of 2017.
Some of the videos are part of our Live Virtual Field Tours project and Video Lecture Series while some videos were reposted by us.

1. Live from Kamokuna Ocean Entry, Big Island of Hawaii 

                                   
2. Live from Tucson Gem Show, Tucson, Arizona

                                            

3. The landslide of Maierato, Vibo Valentia, Calabria, Italy 

4. A double terminated Quartz being pulled from a pocket in the Alps. 

5. An incredible footage of a Flash flood

6. Live from Kaibab Limestone, South Rim, Grand Canyon

                                               

7. Earthquakes and the Richter scale with Fabiana from Geologia da Terra

8. Learn all about Actinolite with Chad keel

9. Soil Erosion 

10. Live from Mount Hood with Andrew Dunning of BetterGeology

Huge thanks to all who contribute videos to us and thanks to everyone for watching! :)

Want to contribute? Read guidelines here.

Basics of Basin Analysis

·         A sedimentary basin is an area in which sediments have accumulated during a particular time period at a significantly greater rate and to a significantly greater thickness than surrounding areas.

·         A low area on the Earth’s surface relative to surroundings e.g. deep ocean basin (5-10 km deep), intramontane basin (2-3 km a.s.l.)

·         Basins may be small (kms2) or large (106+ km2)

·         Basins may be simple or composite (sub-basins)

·         Basins may change in size & shape due to:

1.      erosion

2.      sedimentation

3.      tectonic activity

4.      eustatic sea-level changes

·         Basins may overlap each other in time

·         Controls on Basin Formation

1.      Accommodation Space,

a.       Space available for the accumulation of sediment

b.      T + E = S + W T=tectonic subsidence E= Eustatic sea level rise S=Rate of sedimentation W=increase in water depth

2.      Source of Sediment

a.       Topographic Controls

b.      Climate/Vegetation Controls

c.       Oceanographic Controls (Chemical/Biochemical Conditions)

·         The evolution of sedimentary basins may include:

1.      tectonic activity (initiation, termination)

2.      magmatic activity

3.      metamorphism

4.      as well as sedimentation

·         Axial elements of sedimentary basins:

1.      Basin axis is the lowest point on the basement surface

2.      Topographic axis is the lowest point on the depositional surface

3.      Depocentre is the point of thickest sediment accumulation

·         The driving mechanisms of subsidence are ultimately related to processes within the relatively rigid, cooled thermal boundary layer of the Earth known as the lithosphere. The lithosphere is composed of a number of tectonic plates that are in relative motion with one another. The relative motion produces deformation concentrated along plate boundaries which are of three basic types:

1.      Divergent boundaries form where new oceanic lithosphere is formed and plates diverge. These occur at the mid-ocean ridges.

2.      Convergent boundaries form where plates converge. One plate is usually subducted beneath the other at a convergent plate boundary. Convergent boundaries may be of different types, depending on the types of lithosphere involved. This result in a wide diversity of basin types formed at convergent boundaries.

3.      Transform boundaries form where plates move laterally past one another. These can be complex and are associated with a variety of basin types.

·         Many basins form at continental margins.

Using the plate tectonics paradigm, sedimentary basins have been classified principally in terms of the type of lithospheric substratum (continental, oceanic, transitional), the position with respect to a plate boundary (interplate, intraplate) and the type of plate margin (divergent, convergent, transform) closest to the basin.

·         Plate Tectonic Setting for Basin Formation

1.      Size and Shape of basin deposits, including the nature of the floor and flanks of the basin

2.      Type of Sedimentary infill

·         Rate of Subsidence/Infill

·         Depositional Systems

·         Provenance

·         Texture/Mineralogy maturity of strata

3.      Contemporaneous Structure and Syndepositional deformation

4.      Heat Flow, Subsidence History and Diagenesis

·         Interrelationship Between Tectonics - Paleoclimates - and Eustacy

1.      Anorogenic Areas------>

·         Climate and Eustacy Dominate

2.      Orogenic Areas--------->

Sedimentation responds to TectonismPlate Tectonics and Sedimentary Basin

   Types

SB = Suture Belt

RMP = Rifted margin prism

S C = Subduction complex

FTB = Fold and thrust belt

RA = Remnant arc
Wilson Cycle about opening and closing of ocean basins and creation of continental crust.

Structural Controls on Sedimentary Systems in Basins Forming:

Stage 1: Capacity < Sediment

Fluvial sedimentation

Stage 2: Capacity = Sediment

Fluvial lacustrine Transition

Stage 3: Capacity > Sediment

Water Volume > excess capacity

Shallow-water lacustrine sedimentation

Stage 4: Capacity >> Sediment

Water volume = excess capacity

Deep-water lacustrine sedimentation

Stage 5: Capacity > Sediment

Water volume < excess capacity

Shallow-water lacustrine sedimentation     

Contributed by:

Rehan.A Farooqui

M.Sc Geology,,

University of Karachi.