What is it about?
In the Azores, waves attacking sea cliffs produce dark sand that is moved offshore by waves and currents. At the same time, marine organisms, in particular #molluscs, create shells that break up, producing carbonate sand. Waves agitate the seabed, mixing of the carbonate sand with the dark sand from the cliffs, diluting it. We show how this #dilution leads to a change in the carbonate content of the sediment. If the movement ("flux") of the sand can be measured, it is then possible to work out the rate at which the carbonate sand was created and how that rate varies with depth or distance. Samples and geophysical data from the shelf of a volcanic island in the #Azores (Santa Maria) are used to reveal that growth of shells is most rapid further away from the coast and into the middle of the shelf. The molluscs mainly generating the shells, which seem to prefer the quieter environment where wave movements are less intense.
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Photo by Majid Gheidarlou on Unsplash
Why is it important?
Where water movements are moderate, working out the rate at which carbonates are produced is easy from how rapidly they accumulate over time (depth-age relationships). However, this is not so easy in hydrodynamically active environments, where the particles end up being deposited away from where they were produced (otherwise, how do you associate a given mass of carbonate with a particular growth location?). The dilution method provides one way to tackle this issue. Our results suggest that molluscs, which dominate the sand near Santa Maria Island, grow more rapidly in deeper water where the agitations of the sediment by waves are less frequent and vigorous. In this temperate environment, sunlight is less important for the growth of organisms than it is in tropical environments.
Perspectives
Our equation (figure above) relates rate of change of carbonate content C to the #production A and sediment flux Q. The idea of investigating dilution by carbonates occurred to me while teaching dilution gauging of streams to undergraduates in the Pennine Hills of the UK. Dilution gauging involves injecting a tracer into a stream (we typically use a small amount of common salt) and measuring the concentration downstream. After manipulating the results, we quantify water discharge. A high discharge leads to the salt being transported more quickly and hence it is detected over a shorter period of time. The method is not exactly analogous to the carbonate mixing problem, though it prompted the idea of using a tracer. The equations presented in our article may be useful for other problems in sedimentology where a conservative component of a sediment acts as a tracer when it is diluted by another component.
Dr Neil C. Mitchell
University of Manchester
Read the Original
This page is a summary of: An equation for deriving spatial variations in carbonate production rates from sediment deposition rates and dilution: Application to Santa Maria Island, Azores, Journal of Sedimentary Research, November 2022, Society for Sedimentary Geology,
DOI: 10.2110/jsr.2022.006.
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