Organic Matter Redox State Driven by Specific Sources in Mangrove Sediments: A Case Study from Peruvian Ecosystems

Alexander Pérez,Gerardo Cebrián-Torrejón,Christian J Sanders,Noemí Montoya,Joan Piquero-Cilla,Wilson Machado,Antonio Doménech-Carbó

doi:10.3390/jmse9121438

Abstract

In order to determine the organic matter redox state in relation to specific sources in mangrove sediments, two 60 cm-long sediment cores were collected from mangrove-covered and mudflat zones within a mangrove forest in Peru. Sediment subsamples from these cores were analyzed to determine δ13C values and C:N ratios, whereas two redox indices, namely, electrochemical (fEAOM) and spectroscopical (A1650/A3400) indices, were taken from a previous study and correlated with the geochemical indices obtained from this work. These indices may provide accurate information on sedimentary organic matter diagenesis by oxidative processes through its redox state. The results show that the electrochemical index (fEAOM) and the spectroscopical index (A1650/A3400) for mangrove-covered sediments exhibited a positive correlation with δ13C values and a negative correlation with C:N molar ratios. These correlations suggest that the more labile sedimentary organic matter derived from non-terrestrial sources is in a more oxidized state than that derived from mangrove vegetation. However, this was not valid for mudflat zones, where non-significant correlations between geochemical indices were observed. Furthermore, the results suggest that the redox state of the organic matter deposited over time is dependent on source mixing influences, being better preserved in the presence of mangrove-derived organic matter.

Highlights

The role of mangrove ecosystems in the coastal carbon cycle has been increasingly recognized, as these ecosystems sequester and accumulate large quantities of organic carbon [1,2]
The redox state of sedimentary organic matter plays an essential role in geochemical processes, where compounds containing quinones, hydroquinones, aromatic groups and others functional groups are involved in the preservation of organic carbon [15,16,17]
There have been no previous initiatives to determine the influence of sedimentary organic matter sources on the redox state of organic matter using electrochemical and spectroscopic techniques on mangrove-covered and mudflat sediments

Summary

Introduction

The role of mangrove ecosystems in the coastal carbon cycle has been increasingly recognized, as these ecosystems sequester and accumulate large quantities of organic carbon [1,2]. Mangrove sediments are estimated to contribute between 15 and 25% of the coastal blue carbon burial [1,9], derived from autochthonous and allochthonous organic matter production [7,10]. These autochthonous and allochthonous organic matter sources are frequently assessed through well-established techniques, including elemental and stable isotope measurements in coastal sediments [3,10]. There have been no previous initiatives to determine the influence of sedimentary organic matter sources on the redox state of organic matter using electrochemical and spectroscopic techniques on mangrove-covered and mudflat sediments. An initial attempt using these techniques was based on the relationship between electrochemical and spectroscopical proxies of the redox state and total organic matter content in mangrove sediments [12]

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