Organic-carbon-rich sediments: benthic foraminifera as bio-indicators of depositional environments

Abstract

Abstract. Fjords have been described as hotspots for carbon burial, potentially playing a key role within the carbon cycle as climate regulators over multiple timescales. Nevertheless, little is known about the long-term fate of the carbon that may become stored in fjordic sediments. One of the main reasons for this knowledge gap is that carbon arriving on the seafloor is prone to post-depositional degradation, posing a great challenge when trying to discriminate between an actual change in the carbon deposition rate and post-depositional carbon loss. In this study, we evaluate the use of modern benthic foraminifera as bio-indicators of organic carbon content in six voes (fjords) on the west coast of Shetland. Benthic foraminifera are known to be sensitive to changes in organic carbon content in the sediments, and changes in their assemblage composition therefore reflect synchronous variations in the quantity and quality of carbon reaching the seafloor. We identified four environments based on the relationship between benthic foraminiferal assemblages and organic carbon content in the sediments: (1) land-locked regions influenced by riverine and/or freshwater inputs of organic matter, namely the head of fjords with a restricted geomorphology; (2) stressed environments with a heavily stratified water column and sediments rich in organic matter of low nutritional value; (3) depositional environments with moderate organic content and mild or episodic current activity; and (4) marginal to coastal settings with low organic content, such as fjords with an unrestricted geomorphology. We conclude that foraminifera potentially provide a tool to disentangle primary organic carbon signals from post-depositional degradation and loss of organic carbon because of their environmental sensitivity and high preservation potential in the sedimentary record.

Response of inner shelf benthic foraminiferal community to different concentrations of dissolved oxygen under laboratory culture experiment

“…The low foraminiferal abundance at the highest oxygen concentration is attributed to the difference in organic matter and predatory pressure (Figure 4(B)). Increased benthic foraminiferal abundance in a region is often attributed to the high organic carbon content of the sediments (Cappelli et al, 2019; Chaturvedi et al, 2000; Singh et al, 2021). The prevalence of macrobenthic communities also adversely affects benthic foraminifera by inducing high predatory pressure (Enge et al, 2016; Levin, 2003).…”

Understanding Role of Foraminifera in Environmental Studies: A Review

“…Sediment is a key component that controls ecological functions, food production and many other processes at the Earth's surface. Benthic foraminifera are sensitive to changes in sediment texture (Zallesa et al 2014, Abuzied et al 2016) and organic carbon content (Cappelli et al 2019). Foraminiferal densities increased towards the deeper continental shelf zones, correlated with an increase in fine-grained sediment and total organic matter, according to Martins (2019), whereas low density of foraminifera is found in shallow stations due to sediment instability and lack of food quality associated with organic matter.…”

“…Current energy and water depth influence grain size distribution and sediment sorting (Lidz et al 1965). Cappelli (2019) found that the small size fraction (63-150mm) has the most diagnostic species and the greatest species diversity and richness, providing a more robust statistical foundation for high-resolution environmental reconstructions. Under oxygenated conditions, a group of foraminifera including Caronia silvestrii, Epistominella vitrea and Acostata mariae were strongly affected by labile organic matter in samples collected from the Po river outlet, northern Adriatic Sea, demonstrating habitat distribution by migration towards the sediment-water interface (Ernst et al 2005).…”

Understanding the Role of Terrestrial and Marine Carbon in the Mid‐Latitude Fjords of Scotland

“…Initial examination of these characteristics indicated that smaller fjards were not comparable to the larger fjords. The disparity between these types of systems has previously been observed in Scotland, where the fjards of the Shetland Islands were categorized as distinct systems separate from the fjords of the mainland and Hebridian Islands (Lo Giudice Cappelli et al, 2019;, therefore for the purposes of the study the six fjards (Figure 4) were excluded from the data mining exercise.…”