Sedimentary organic matter: an endless source of information - Part 1

By Juliana Leonel

English edit by Carla Elliff

*post originally published in Portuguese on December 5, 2020

If the thermometer was only invented in the beginning of the 18th century, how is it possible that there are studies that talk about sea surface temperature thousands of years ago?

For an oceanographer, who works in marine geochemistry, sediment is much more than simple particles of organic and inorganic matter. When I look at marine sediment, I see a wealth of preserved information that is the key (or one of the keys) to studying the past history not only of the ocean, but of our planet. For example, from information stored in marine sediments we know that during the Cretaceous (approximately 90 million years ago) the temperature of the ocean (and the atmosphere) was much warmer than it is today. Moreover, the amount of dissolved oxygen was very low and a large amount of organic matter produced was preserved in the sediments and, later, gave rise to important oil basins. It was also through studies of marine sediment that we know that the Earth has gone through cycles of glaciation and interglaciation over the last 2 million years. We know when the Isthmus of Panama closed and when the polar ice caps formed in Antarctica.

"But how is this possible?" you might be asking yourself.

These studies are possible thanks to the use of different tools, ranging from the study of the composition of organic matter preserved in marine sediments to studies of the occurrence/composition of microfossils (such as foraminifera, diatoms, etc.) preserved in the sediment.  

Regarding sedimentary organic matter, let's first understand where it comes from, where it goes and how it survives this "journey"...

The organic matter (OM) present in marine sediments comes mainly from two sources: a) terrestrial plants (mainly higher plants); and b) phytoplankton organisms. In the first case, it is allochthonous organic matter (produced in an external environment) and in the second case, it is autochthonous OM (produced in the environment itself). OM goes beyond leaf and algae remains, it is made up of several groups of compounds, such as proteins, carbohydrates, lipids, lignin, among others. Not only the quantity of each of these components, but also the molecules that constitute them, will depend on factors such as: source of the OM (higher plants with C3 metabolism, higher plants with C4 metabolism, phytoplankton, bacteria, etc.) and the characteristics of the environment during its formation (temperature, salinity, oxygen availability, etc.).

Differences in the composition of terrestrial and marine organic matter.

Once in the water column, the particulate organic matter will sediment and during this process it may be remineralized, that is, it will be degraded until it returns to its inorganic constituents (such as inorganic carbon, nitrogen and phosphorus). However, a small portion will reach the sediment intact or undergoing only minor changes. It is important to emphasize that even in the sediment, organic matter can be remineralized. Under current environmental conditions, it is estimated that, on average, of the total OM produced in the photic zone, only 1% will reach the marine sediment and only 0.1% will be preserved.

Although it represents a small portion of all organic particles in the marine environment, sedimentary organic matter contains important information about its source and the conditions in which it was produced and preserved. To retrieve this information, researchers collect samples of surface sediments or sediment cores that are sliced ​​(usually every 1 or 2 cm) and dated (using radioisotopes, which are unstable isotopes that, in search of stability, decay producing other isotopes and releasing energy in the form of radiation). Once the age of each of these layers is known, the organic matter is analyzed and its components identified and quantified.

Sediment cores by Juliana Leonel with CC BY-SA 4.0 license.

Ok. But how do I use sedimentary OM to find out the sea surface temperature of thousands of years ago? 

Among the various components of sedimentary OM, there is a group of compounds called alkenones, which are long-chain ketones (37 to 39 carbons) containing 2 to 4 unsaturations (= double bonds). Produced by two species of coccolithophorids, Emiliana huxleyi and Gephyrocapsa oceanica, the former of which is widely distributed in the oceans, these compounds are lipids that resist degradation processes throughout their sedimentation in the water column and are part of the sedimentary OM.

Emiliana huxleyi 

Source: (2011) PLoS Biology Issue Image | Vol. 9(6) June 2011. PLoS Biol 9(6): ev09.i06 with CC license.

Alkenones with 37 carbons and 2 and 3 unsaturations are used as biomarkers to assess sea surface temperature (SST), as they are produced in different proportions depending on the temperature of the water in which the organism is found. To maintain the stability of cell membranes, as the temperature increases, these organisms produce more alkenones with a greater number of unsaturations. Knowing this, researchers created an index, the Uk'37, which is a ratio between alkenones with 2 and 3 unsaturations and which allows calculating the temperature of the water where the alkenones were formed.

In addition to alkenones, there are other geochemical tools that can be used to study ocean temperatures in the past, such as Glycerol dialkyl glycerol tetraethers (produced by archaebacteria and affectionately called by the acronym GDGTs) and the ratio between stable oxygen isotopes (δ18O) or the ratio between magnesium and calcium (Mg/Ca) in the calcareous shells of foraminifera. 

Organic matter is an incredible tool and helps us understand a lot about Earth's past and present, so I'll talk more about the topic in a future post.

#JulianaLeonel #MarineScience #OrganicMatter #Paleoceanography #Paleothermometer #Alkenones #MarineGeochemistry