Characterized by high levels of terrestrial organic carbon inputs, estuaries and coastal marshes are among the most productive ecosystems on earth and significantly impact the global carbon cycle. Unfortunately, rates of natural organic matter (NOM) degradation in these environments are difficult to quantify directly due to the complex interaction between microbial respiration processes and abiotic reactions in these sediments, yet estuaries and marshes are considered both net sources and sinks of carbon. Typically carbon remineralization rates are determined by measuring total (TOU) and diffusive (DOU) oxygen uptake fluxes assuming oxygen is the ultimate oxidant. This assumption, however, requires any reduced metabolites produced during microbial respiration to be reoxidized by oxygen. In this study, voltammetric sensors were used to measure terminal electron acceptors or their reduced by-products. By simultaneously considering oxygen as well as anaerobic respiration accepting processes, this study demonstrates that oxygen does not function as the ultimate oxidant in coastal marine sediments due to precipitation and burial of reduced species. Furthermore, the biogeochemistry of coastal sediments is typically investigated ex situ after collection of sediment cores. However, coastal sediments are subject to complex subsurface hydrological forcing that cannot be accounted for with ex situ measurements. Consequently, in situ approaches are required to better understand the impact of physical processes on sediment biogeochemistry, and two novel in situ voltammetric systems were developed as part of this research. First, a new autonomous benthic lander equipped with a benthic chamber to measure TOU fluxes with a high temporal resolution and a potentiostat and micromanipulator to simultaneously acquire voltammetric depth profiles of the main redox species in pore waters was deployed in a pristine river-fed estuary to characterize the seasonal variability of coastal sediment biogeochemistry and examine the impact of riverine discharge on carbon remineralization processes. Simultaneously, a new electrochemical analyzer equipped with a solar and wind power charging system to ensure continuous monitoring capability and a VHF radio to transmit data was operated remotely via the internet from the Georgia Tech campus to investigate the dynamic coupling between hydrological, chemical, and biological processes in intertidal marsh sediments. Finally, new microelectrodes were deployed in microbial mats to examine the chemical and biological oxidation of sulfide with submillimeter resolution. Typically, only biological processes are considered to oxidize sulfide in these environments. Depth profiles during diel studies were able to demonstrate the formation of thiosulfate as an intermediate oxidation product of sulfide oxidation, suggesting that the chemical oxidation of sulfide is much more prevalent than previously recognized when compared to biological oxidation. Overall, using a novel in situ sampling technique with high temporal resolution, these studies confirm that biogeochemical processes in coastal sediments vary seasonally. More importantly, these studies also reveal that estuarine sediments are significantly influenced by riverine discharge, demonstrate that the biogeochemical response of these sediments to natural perturbations is rapid, and indicate that respiration processes in continental shelf sediments are controlled by a combination of temperature, supply of inorganic and organic substrates, and hydrological processes, which has important implications regarding the effect of climate change on the biogeochemical cycling of carbon in these environments.