USING ²²²Rn TO ANALYZE THE TIDAL PUMPING INFLUENCE ON CARBON EXPORTATION IN AN AMAZONIAN ESTUARY ACROSS NEAP–SPRING TIDAL CYCLES

USING ²²²Rn TO ANALYZE THE TIDAL PUMPING INFLUENCE ON CARBON EXPORTATION IN AN AMAZONIAN ESTUARY ACROSS NEAP–SPRING TIDAL CYCLES

NORONHA FILHO, FRANCISCO ÁUREO^a*, MEIRA, VINICIUS L.^b, PACHECO, KAUÃ M.^{c e},FURTADO, LUAN V. da S.^d, CHAVES, LUANA V. da S. ^a, PANTOJA, CARLOS D. S.^d, MATOS, CHRISTIENE R. L. de^e, KÜTTER, VINICIUS T.^a, BERREDO, JOSÉ F. ^e, ABRIL, GWENÄEL Y.^b

^a GRADUATE PROGRAM IN GEOLOGY AND GEOCHEMISTRY-FEDERAL UNIVERSITY OF PARÁ (PPGG-UFPA)

^b GRADUATE PROGRAM IN GEOSCIENCE-FLUMINENSE FEDERAL UNIVERSITY (PPG-UFF)

^c FACULTY OF GEOLOGY-FEDERAL UNIVERSITY OF PARÁ (FAGEO-UFPA)

^d FACULTY OF OCEANOGRAPHY-FEDERAL UNIVERSITY OF PARÁ (FAOC-UFPA)

^e EMILIO GOELDI PARAENSE MUSEUM (MPEG)

*[email protected]

Abstract

This study investigates the influence of tidal pumping on carbon dynamics and submarine groundwater discharge (SGD) in the Marapanim estuary, located on the Amazonian coast of Brazil. Continuous high-resolution monitoring was conducted across neap and spring tides, assessing partial pressure of carbon dioxide (pCO₂), salinity, pH, water depth, and radon-222 (²²²Rn) concentrations in surface and pore waters. Results show that spring tides enhance benthic exchange and groundwater inputs, reflected in higher pCO₂ and ²²²Rn concentrations and greater variability in salinity and pH. Conversely, neap tides exhibit more stable conditions with diminished porewater influence. A strong inverse correlation between salinity and ²²²Rn, alongside significantly elevated radon levels in pore waters, confirms the role of SGD in modulating estuarine biogeochemistry. These findings emphasize the importance of tidal modulation in controlling carbon fluxes and groundwater–surface water interactions in tropical estuarine systems.

Keywords: CO₂, blue carbon cycle, submarine groundwater discharge, environmental geochemistry, climate change.

Introduction

The blue carbon is currently object of research throughout the world, specifically the mangroves, which play an important role in the biogeochemical cycle of the carbon and his budget. In the Amazon, beyond the rain forest, there are the mangrove forests cycling, storing and releasing the carbon in known superior rates than the any other adjacent forest (Adame et al, 2024).

However, the studies about blue carbon in coastal areas are necessary to establish a more accurate and conclusive carbon budget. Therefore, the influence of diverse factors in the carbon dynamics storage in tropical estuaries and mangroves must be considered to improve the understanding of that important cycle and his influence in the climate dynamics (Young et al, 2021).

This research analyzes the influence of the tidal pumping in the carbon exportation using ²²²Rn isotope in the estuary at Marapanim, Pará, on the Amazonian coast.

Experimental

The study was conducted in the Marapanim River estuary, located in Pará State, in the Eastern Amazon. Fieldwork involved continuous 25-hour monitoring campaigns carried out aboard a small vessel during both spring tide (11–12 May 2025) and neap tide (17–18 May 2025). Measurements were recorded at fixed intervals for each parameter: ²²²Rn every 15 minutes, pCO₂, physicochemical parameters (temperature, salinity, and pH), and water depth every minute. Radon-222 and pCO₂ were measured using a RAD8 detector and a LI-COR infrared gas analyzer, respectively, both integrated into a closed-loop equilibrator system that allowed the water–gas concentration to reach equilibrium. Temperature, salinity, and pH were measured with a Hanna HI98195 multiparameter probe, and water depth was recorded using a HOBO pressure sensor.

Results and Discussion

The results show that diel variations in pCO₂, salinity, pH, ²²²Rn, and water depth in the Marapanim estuary are strongly influenced by tidal regime (Table 1; Fig. 1 A and B). Spring tides exhibit greater fluctuations in all parameters compared to neap tides. pCO₂ peaks during low tide, particularly in the daytime, and is inversely related to salinity and pH, indicating the influence of biological processes and groundwater input. The tidal range is wider during spring tides, and higher ²²²Rn levels at low tide and night further support the role of submarine groundwater discharge (Polsenaere et al., 2023).

Table 01. Summary statistics of the parameters pCO₂, salinity, pH, ²²²Rn activity, and depth in surface and pore waters of the Marapanim estuary during spring and neap tidal cycles.

There is a strong inverse relationship between salinity and ²²²Rn, indicating that lower salinity zones are associated with higher radon levels, typical of groundwater influence (Fig. 2 A and B). The Kruskal-wallis test statistically confirms that ²²²Rn concentrations in pore water are significantly higher than in surface waters (p < 0.001), reinforcing the role of radon-222 as a tracer of submarine groundwater discharge in the estuary (Akhand et al., 2022).

The summary statistics support the patterns observed (Table 01). Spring tides showed higher average concentrations and greater variability in pCO₂ and ²²²Rn, along with lower salinity and pH values compared to neap tides. In contrast, pore water was marked by consistently high salinity and exceptionally elevated ²²²Rn levels, while pH remained slightly lower. Depth variations were similar between tidal phases, but pore water samples were taken from a fixed subsurface level (Chen et al., 2021).

F
igure 1. Diel variation of pCO₂, salinity, pH, ²²²Rn, and water depth in surface waters of the Marapanim estuary, Pará, Brazil, during: (A) Neap tide and (B) Spring tide.

F igure 2. Salinity versus radon-222 (²²²Rn) in pore (black squares) and surface waters (dark blue squares) from the Marapanim estuary. (A) Negative correlation between salinity and ²²²Rn. (B) Statistical comparison of ²²²Rn concentrations across water types using Shapiro-Wilk and Kruskal-Wallis tests.

Conclusions

Tidal dynamics play a key role in driving biogeochemical variability in the Marapanim estuary. Spring tides enhance physical mixing, benthic exchange, and submarine groundwater discharge, leading to higher surface water concentrations of pCO₂ and ²²²Rn, along with greater variability in salinity and pH. In contrast, neap tides create more stable conditions with reduced groundwater influence. The strong inverse correlation between salinity and ²²²Rn, and the significantly elevated radon levels in pore waters, underscore the importance of groundwater inputs in shaping the estuary’s chemical profile and highlight ²²²Rn as an effective tracer of these interactions.

Acknowledgements

The authors thank PPGG-UFPA, CAPES, MPEG, PPG-UFF AND TROPECOS.