Using geochemical proxies to trace sediment sources Karin Block & Annika Johansson

Columbia University

What processes at the surface control sedimentation?

• To understand how elements move through the earth system, how climate change affects living things, how resources are managed (i.e., energy) we need to understand:– How surface events are recorded in sediments

at continental margins (e.g., weathering).– What happens when sediments are deposited

and how they appear in the rock record.

• The chemistry of sediments can be used to trace these processes.

proxy (noun): 1. A measurement of one physical quantity that is used as an indicator of the value of another. 2. the authority to represent another entity.

Since we cannot directly measure the Earth’s past using instruments we can use chemical proxies to understand a variety of geological processes.

Sediment sequences do not directly record climatic parameters (temperature, rainfall, atmospheric pressure, seasonality, etc.)

They can only be inferred from proxies which must usually be calibrated empirically against modern data or other geologic variables. Image credit:

http://www.glossary.oilfield.slb.com/DisplayImage.cfm?ID=209

Examples of geochemical proxies in marine sediments

• Stable isotopes: Oxygen isotope values of air trapped in ice core bubbles are used as a paleothermometer and tell us how much water was bound up in glaciers.

• K/Al: Potassium is highly soluble and readily leached by chemical weathering while Al is extremely insoluble and is essentially conserved as source rock is converted to clays. Therefore lower K/Al values indicate strong chemical weathering.

• Ba: in deep-sea sediment barium is a geochemical proxy for productivity as it allows distinction between preservation and productivity.

• Radiogenic isotopes: 87Sr/86Sr in marine sediments serve as a proxy for silicate weathering and determination of provenance (e.g. from the Himalayas to the Bay of Bengal)

What do Sr isotopes tell us?

• The ultimate composition of any sediment is influenced by a host of primary and secondary factors such as the lithologies of the source terranes, climate, depositional environment and diagenesis. However, diagenesis of aluminosilicates does not generally result in equilibration of Sr isotopes between the sediment and seawater. Thus the isotopic composition of strontium can be used as an indicator of geologic provenance.

Rubidium-Strontium Decay

87Rb87Rb

87Sr87Sr

e-

(272 keV)

-

• 87Rb undergoes beta decay to produce daughter isotope 87Sr.

• 86Sr is a stable isotope used to reference how much radiogenic strontium is in the mineral or rock relative to non-radiogenic strontium.

• At the time of formation, minerals have the ratio of 87Sr to 86Sr isotopes.

• With time, Rb decays and more 87Sr accumulates in the mineral.

The half-life of 87Rb is 4.88 x 1010 years.

87Sr/86Sr

Ganges-Brahmaputra DeltaThe range of Sr isotope values in the sediments of ocean basins reflect the various sources and ages of the originally eroded material. One of the world’s largest deltas is formed where the Ganges and Brahmaputra Rivers coalesce in the Bay of Bengal. Sediments carried by the Ganges-Brahmaputra river system and deposited in the Bengal delta have their main sources in rock outcrops of the Himalayas and Tibetan Plateau. These high altitude areas formed as the result of the collision of the Indian landmass with the Eurasian plate. The major rock units of this region have characteristic Sr isotopic signatures traceable to the marine sediments that show a strong continental source of crustal rocks with high Sr isotope ratios.

87Sr/86Sr at the Ganges-Bramaputra Delta

References

Data from SedDB: http://www.seddb.org

GeoMapApp: http://www.geomapapp.org

Plateau headwaters image: Copyright © Michael Collier and

SeaWifs Bay of Bengal and Brahmaputra courtesy:

NASA, Visible Earth