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Laura Roberts-Artal

Laura Roberts Artal is the Outreach and Dissemination Manager at The Water Innovation Hub (University of Sheffield). Laura also volunteers as the Associate Director of Communications for Geology for Global Development. She has also held a role in industry as Marketing Manager for PDS Ava (part of PDS Group). Laura was the Communications Officer at the European Geosciences Union from the summer of 2014 to the end of 2017. Laura is a geologist by training and holds a PhD in palaeomagnetism from the University of Liverpool. She tweets at @LauRob85.

Becoming a Ghost Buster: What triggers sapropel formation?

As I touched upon in our first post, we can use the magnetic properties of minerals in sediments (and other environmental materials) to understand changes in environmental and climatic conditions. This is known as environmental magnetism. The basic idea is to identify links between the magnetic properties of a material and environmental conditions and depositional processes. This approach is not as modern as you might think and was first used back in 1926! Understanding and characterising how and in what quantities magnetic minerals form can give key indicators of past climates. A great example of how useful this tool can be for understanding rates of deposition, geochemical conditions and past climate changes was shown in our post last week. Magnetic iron oxides and sulphides can form and be dissolved in deep-sea sediments, depending on the geochemical conditions and this can be used to identify ghost sapropels, (Langereis & Dekkers, 1999). Combining evidence for change in the magnetic signature of sediments and changes in composition can indicate changes in the climatic or tectonic setting in which the material was being deposited.

Magnetostratigraphy as a dating tool

In addition, magnetostratigraphy can be used to date sedimentary (and volcanic) sequences. It is, essentially, a correlation technique, which aided by independent isotopic ages, can be used to date a sedimentary section or core. The Global Magnetic Polarity Time Scale (GMPTS) is used for this . The direction of the field recorded in a stratum can be normal or reversed and this will coincide with a known normal or reversed chron of a given age. In the 1950s the first GMPTS, of sorts, was established using the sea floor magnetic anomaly patterns (the familiar bar code type outline seen spreading away from the North Atlantic Ridge). However, it was later on, when each reversal was accurately dated using the astronomical polarity time scale (APTS ) that magnetostratigraphic became a valuable chronology tool. The rates of sediment accumulation within a sequence or core can also be established by plotting the age of the each reversal versus the stratigraphic level at which the reversal is found, giving deposition rates in meters per million years.

What triggered sapropel formation?

We explained in our last post some of the proxy techniques capable of distinguishing sapropels from the background sediment matrix. A dramatic environmental change must have been necessary to create the depositional conditions suitable for their formation. Investigating the nature of this change has been a key task for palaeoceanographers over several decades!

The critical condition for sapropel formation was deep water anoxia (water depleted in oxygen) within the eastern Mediterranean Sea. So, periodically, dissolved oxygen was unable to reach the deeper sea because the water column was vertically stratified; in other words, surface water had quite low salinity while the deeper waters were highly saline (Rossignol-Strick et al. 1982). An influx of freshwater to the Mediterranean would have decreased the salinity of its surface waters; scientists have therefore posed the questions “from where did this increased flow of freshwater come and what was its trigger?” A number of hypotheses have been put forward over recent decades, all of which invoke a strong link between climate and sapropel formation, although the primary trigger has been more widely debated.

The darker sapropelic layer is clearly visible in this photographic core log. Photograph used with the kind permission of Dr Mike Rogerson, University of Hull.

The darker sapropelic layer is clearly visible in this photographic core log. Photograph used with the kind permission of Dr Mike Rogerson, University of Hull.

The original hypothesis from E. Olausson (1961) proposed massive volumes of meltwater from Eurasian ice-sheets entered the Mediterranean from the north at the beginning of warm interglacials. While this has undoubtedly occurred periodically through the Quaternary, improved dating (using magnetostratigraphy as described above, for example) indicates a mismatch between the timing of sapropel formation and meltwater influx.

More recent research linked the formation of sapropels to enhanced solar insolation. Insolation refers to the amount of solar radiation reaching an area of the Earth’s surface, which varies through the day, annually and on longer timescales (i.e., Milankovitch cycles). Over these longer timescales, it appears phases of insolation maxima during the Northern Hemispheric summer caused stronger monsoons to form over northern Africa, bringing more intense rainfall. As a result, flow in the River Nile (and likely in other rivers draining into the Mediterranean from North Africa that have since dried up) was greatly increased, delivering substantial volumes of freshwater.

The most widely accepted hypothesis today expands on the Nile freshwater hypothesis and suggests warmer sea surface temperatures (due to higher insolation) occurred simultaneously (Emais et al. 2003).  Together, the influx of freshwater and warmer sea surface temperatures were sufficient to create an upper water layer of sufficiently low density to interrupt circulation and create the oxygen-poor conditions at the sea bottom necessary for the formation of sapropels.


Olausson, E. (1961) Studies in deep-sea cores. Reports of the Swedish Deep-Sea Expedition, 1947-1948, v.8, Sediment cores from the Mediterranean Sea and Black Sea. [d1]


How to Find Ghosts in Sediment Cores

Hi! And welcome to Geology Jenga and our very first blog post :)!

As this is our first post, we have picked a topic that integrates elements of both our research interests. In this initial post, we aim to give a flavour of the science that will be covered on our blog in the future, provide an introduction to the techniques we employ in our PhD’s and hopefully provide some useful insight into how scientists interested in reconstructing past environments operate.


Photographed sections of core 17X, Site 970. Taken from the ODP Proceedings Volume 160. The dark bands are the organic-rich sapropels within a carbonate-rich sediment matrix

The Integrated Oceanic Drilling Programme (IOPD) has extracted numerous long sediment cores from marine basins all over the globe. Striking changes in the colour, texture or chemical composition of material in these sediment cores often tells a story of major environmental change. As a case study, we will investigate some stratigraphic records from the Mediterranean Sea. These cores contain sediments accumulated over the past several hundred thousand years (spanning much of the Quaternary) and provide a near-continuous record of environmental changes in the region.

Of particular interest to oceanographers are a series of dark, black bands rich in organic matter identified in sediment cores from the Mediterranean Sea, and at nearby terrestrial sites, termed ‘sapropels’. They are instantly recognisable as they are found within a sediment matrix rich in carbonate and much lighter in colour, sometimes grey or white (as seen in these photographic logs of core 17X, Site 970). Their re-occurrence suggests a repeated, dramatic shift in local environmental conditions. Numerous important papers have been published on these in recent decades, such as Rossignol-Strick et al. (1982) in Nature.

It appears that periods of stratification of the water column in the Mediterranean trigger the formation of sapropels (Rossignol-Strick et al. 1982), as highly saline water at the bottom of the Mediterranean are depleted in oxygen, thereby preserving the organic matter as it settles out at the sea floor. Identifying the mechanism which would cause the cyclical nature of sapropel formation has been challenging and a follow-up blog article will examine various proposed hypotheses in more detail. These include pulses of Eurasian ice-sheet meltwater, increased rainfall around the Mediterranean or flood-rich phases of the Nile River.

Here, we will focus on laboratory techniques scientists employ to distinguish these sapropelic layers in a sediment core. Colour is not always an effective proxy as it can be altered significantly since the time of deposition. In the case of sapropels, the organic matter may oxidise through time, leaving no visual imprint in the extracted sequence – ‘ghost sapropels’.

Geochemical measurements can help with this problem. Concentrations of the element Barium (Ba) can be used as a tracer as they appear to respond to biological productivity. Measurements across sapropel layers show enriched Ba (Moller et al. 2012), indicating elevated productivity during these time periods. Titanium (Ti) is a minerogenic element often used as a proxy of intense weathering and transport of silt and fine-sand particles and concentrations of Ti appear to decrease across the sapropelic layers, suggesting reduced sediment flux from land to the marine basin. In addition, the proportion of the sediment made up of organic carbon (OC) varies between 2 and 25% by weight in the sapropels, which vastly exceeds OC of the carbonate sediments above and below. Planktonic foraminifera (often Globigerinoides ruber) have been subjected to oxygen isotopic analysis (δ18O) and it appears that, during periods of sapropel deposition, isotopic ratios are significantly depleted related to decreased salinity of the surface waters in the Mediterranean (Emais et al. 2003).


Magnetic susceptibility (mass specific χ; 10-6 m3 kg-1) vs Ba/Al ratio for samples from Core ET99-M11 (from Vigliotti et al. (2011)). Samples at the bottom of the sapropel indicate that significant magnetic dissolution took place. A moderate increase in the Ba/Al ratio suggests higher sensitivity of the magnetic parameters to the environmental changes occurring during sapropel deposition.

It’s not only geochemical measurements that can help identify ghost sapropels, those that are present even when the lithological signatures are missing. Magnetic parameters can be used to indicate paleoredox conditions (low oxygen levels) in the water column. The process of magnetic minerals accumulating in the sedimentary column is known as the acquisition of a depositional remanent magnetisation (see this page from the Institute of Rock Magnetism, Minnesota, about the different types of remanence held in rocks). Rock magnetic parameters show a very distinct signature with respect to sapropels as shown by Dekkers et al. (1994) and Langereis et al. (1997). Just above a sapropel it is not uncommon to find that magnetic minerals precipitate, whilst just below it, magnetic minerals tend to dissolve. This can be tested for by measuring low values of anhysteretic remanent magnetization[SJM1]  (ARM, which is imparted in the laboratory by slowly reducing a peak alternating field to zero while at the same time applying a constant DC). The above being true, you would expect very low values of ARM at the base of sapropels, as shown by Dekkers et al., (1994).  The measurement of magnetic susceptibility can also be used as an indicator for the presence of ‘ghost sapropels’. Susceptibility plotted against ratios of Br/Al concentrations are indicative of the top and base of sapropels as show in this diagram by Vigliotti et al. (2011).

The use of magnetic measurements in understanding sapropels and reconstructing past environments is not restricted to the study of the rock magnetic minerals in the sediment; magnetostratigraphy can be used to establish the age of the cores and the total time span sampled by the recovered sediments where dating of sediment cores via traditional means is problematic. I hope to cover some of those aspects in a bit more detail in our next post.

We hope you’ve enjoyed our first post; we will take a more detailed look at proposed mechanisms for sapropel formation in a follow-up post (now uploaded here!) to appear on the blog shortly.  Stay tuned for more posts on: palaeoenvironmental reconstructions and how geophysics and geomagnetism in particular can contribute to our understanding of those; cutting edge research in the fields of Achaean geology, geophysics, geomagnetism from Laura! Dan will report on research being conducted by Quaternary scientists working in glacial, marine, fluvial or lacustrine settings to better understand past environmental changes.

It is worth noting that all ODP Proceedings are in the public domain and not copyrighted; if you would like more information, it is worth browsing the ODP website for their Mediterranean Reports (e.g., Volume 160). In the meantime, here are the references we’ve incorporated:

Dekkers, M.J. et al. (1994) Fuzzy c-means cluster analysis of early diagenetic effects on natural remanent magentisation in a 1.1 Myr piston core from the Central Mediterranean. Phys.Earth planet.Inter., v.85, p.155-171.


Emais, K.C. et al. (2003) Eastern Mediterranean surface water temperatures and δ18O composition during deposition of sapropels in the late Quaternary. Palaeoceanography, v. 18(1), 1005


Langerair, C.G. et al (1997) Magnetostratigraphy and astronomical calibration of the last 1.1Myr from an eastern Mediterranean piston core and dating short events in the Brunhes.

Geophysical Journal International, v. 129, p. 75-94.


Moller, T. et al., (2012) Sedimentology and geochemistry of an exceptionally preserved last interglacial sapropel S5 in the Levantine Basin (Mediterranean Sea). Marine Geology, v.291-294, p.24-38


Rossingol-Strick, M. et al. (1982) After the deluge: Mediterranean stagnation and sapropel formation. Nature, v. 295, p. 105-110


Vigliotti, L.A. et al. (2011). Magnetic properties of the youngest sapropel S1 in the Ionian and Adriatic Sea: inference for the timing and mechanism of sapropel formation. Italian Journal of Geosciences, v.130, p. 106-118