The oceans are important repositories of global carbon on Earth. The biological pump (BP) plays an important role in this storage. It is principally driven by the gravitational sinking of organic rich particles from the upper ocean and transfers Gigatons of carbon to its interior every year. This process therefore influences the level of atmospheric carbon dioxide and the global climate system. Atmospheric carbon dioxide levels are significantly lower than would be if BP did not exist.
Accurately evaluating the carbon export flux is crucial for estimating the magnitude of the BP and predicting its response to global climate change. Quantifying the strength (i.e., magnitude) of the global BP is essential to understand the Earth’s carbon cycle and the global temporal evolution of the atmospheric CO2 concentration, enhanced by anthropogenic releases. One of the key parameters to quantify the action of the BP is the downward flux of particulate organic matter, mainly in the form of carbon
The measurement of the downward carbon flux is challenging and thus complementary approaches to estimate it are usually combined to estimate the carbon flux. The use of the disequilibrium of radioactive pairs, especially Th-234 - U-238, is one of the most popular methods.
The Th-234 - U-238 radioactive pair has been extensively used to evaluate downward carbon fluxes in the upper ocean, but also the fluxes of other trace elements. It can be also used to evaluate other parameters, such as the efficiency with which carbon is exported from the surface ocean by means of the biological pump, the remineralization of organic carbon, or the particle sinking velocity.
Since the 70's, a large number of vertical profiles of Th-234 have been collected using a variety of sampling instruments and analytical strategies that have changed along years. Here, we compile the most extensive global oceanic data set of Th-234 measurements, including all the Th-234 data in the published literature as well as non-published data, up to 2020, in our open data repository.
The Method
How and why we use thorium-234 in the ocean:
Uranium-238 (half-life = 4.5 109 years) is naturally dissolved in the ocean and decays into Thorium-234 (half-life = 24.1 days). Due to their contrasted radioactive half-lives, these two elements eventually reach what is called the secular equilibrium, i.e., both radionuclides have the same activity concentrations. However, when the euphotic zone is sampled, it is generally found a deficit of Th-234 in relation to U-238. This is because U-238 is soluble and remains dissolved in water while Th-234 is highly particle-reactive so it is rapidly scavenged onto particles as they sink. The measured disequilibrium of U-238 and Th-234 in the shallower depths (0 - 500m) of the water column can be integrated at the specific depths where POC flux aims to be obtained to calculate Th-234 downward flux either by assuming steady state or by applying a non-steady state correction to describe the temporal situation.
The great success of the technique is that the Th-234 downward flux can be converted to the downward flux of any element of interest, such as carbon, phosphorus or nitrogen, by using the ratio of this element to Th-234 measured in the sinking particles. The most common use of the Th-234 flux is to estimate the downward flux of Particulate Organic Carbon (POC). Additionally, when other components such as Particulate Inorganic Carbon (PIC), Biogenic Silica (BSi) or trace metals are measured in the sinking particles their equivalent fluxes can be also obtained.
This way, when sampling a station, discrete water samples (typically 2 L) are collected at different depths using a stainless-steel sampling rosette equipped with Niskins bottles and a CTD Seabirds sensor package. Samples are measured for total and/or dissolved and particulate Th-234. U-238 is also measured at the same depths using ICP-MS or estimated by using any of the U-238–salinity relationship available. Sinking particles are also collected in discrete depths using varied methods (e.g., stand-alone pumps or sediment traps) and the Th-234 content in the particles is also measured, together with the POC content and other additional parameters (e.g., PIC).
These Th-234 – U-238 data can be also used to analyze sinking particles remineralization (i.e., conversion of sinking to non-sinking particles) or the velocity of the sinking particles.