Environmental isotopes

They are primarily used as tracers to see how things move around within the ocean-atmosphere system, within terrestrial biomes, within the Earth's surface, and between these broad domains.

[1] An example of a mass dependent process is the fractionation of water as it transitions from the liquid to gas phase.

Water molecules with heavier isotopes (18O and 2H) tend to stay in the liquid phase as water molecules with lighter isotopes (16O and 1H) preferentially move to the gas phase.

By comparison, stable isotopes do not undergo radioactive decay, and their fixed proportions are measured against exponentially decaying proportions of radioactive isotopes to determine the age of a substance.

This mixes between water masses, but it also decays over time, reducing the amount of 14C in the region.

This leads to an understanding of how the oceans (along with the atmosphere) transfer heat from the tropics to the poles.

This also helps deconvolve circulation effects from other phenomena that affect certain tracers such as radioactive and biological processes.

In the Pacific Ocean, the gyres still form, but there is comparatively very little large scale meridional (North-South) movement.

14C is predominantly produced in the upper atmosphere and from nuclear testing, with no major sources or sinks in the ocean.

This 14C from the atmosphere becomes oxidized into 14CO2, allowing it to enter the surface ocean through gas transfer.

[4] Deviations from this expected value are indicative of other processes that affect the delta ratio of radiocarbon, namely radioactive decay.

The only records of these times that we have are buried in rocks, sediments, biological shells, stalagmites and stalactites, etc.

The isotope ratios in these samples were affected by the temperature, salinity, circulation of the ocean, precipitation, etc.

It is another stable isotope of oxygen along with oxygen-16, and its incorporation into water and carbon dioxide/carbonate molecules is strongly temperature dependent.

This can be done by comparing δ18O in calcium carbonate shells in sediment cores to these records to match large scale changes in the temperature of the Earth.

There are some processes that mix water from different times into the same depth in the ice core, such as firn production and sloped landscape floes.

Lisiecki and Raymo (2005) used measurements of δ18O in benthic foraminifera from 57 globally distributed deep sea sediment cores, taken as a proxy for the total global mass of glacial ice sheets, to reconstruct the climate for the past five million years.

These are called Milankovitch cycles, and these are related to eccentricity, obliquity (axial tilt), and precession of Earth around its axis.

Koutavas et al. (2006) used δ18O of G. ruber foraminifera to study the El Niño–Southern Oscillation (ENSO) and it's variability through the mid-Holocene.

[6] By isolating individual foram shells, Koutavas et al. were able to obtain a spread of δ18O values at a specific depth.

Koutavas et al. found that ENSO was much less variable in the mid Holocene (~6,000 years ago) than it is currently.

Particles from these rocks come into the ocean through weathering by rivers, meaning that this strontium isotope ratio is related to the weathering ion flux coming from rivers into the ocean.

Richter and Turekian have done work on this, finding that over glacial-interglacial timescales (105 years), the 87Sr/86Sr ratio varies by 3*10−5.

[8] Uranium has many radioactive isotopes that continue emitting particles down a decay chain.

This makes it simple to determine the age of a sample based on the various ratios of radioactive isotopes that exist.

One way uranium isotopes are used is to date rocks from millions to billions of years ago.

Zircon incorporates uranium and thorium atoms into its crystal structure, but strongly rejects lead.

The decay of uranium is thus also isotropic, but the daughter isotopes react differently.

Thorium is readily scavenged by particles, leading to rapid removal from the ocean into sediments.

[9] By contrast, 231Pa is not as particle-reactive, feeling the circulation of the ocean in small amounts before settling into the sediment.